© The Authors 2025

1. Introduction

In recent years, increasing attention has been devoted to the study of diffuse radio sources in galaxy clusters. Indeed, with the advent of sensitive and high spatial resolution radio telescopes – such as the Low Frequency Array (LOFAR), the Karl G. Jansky Very Large Array (JVLA), the upgraded Giant Metrewave Radio Telescope (uGMRT), and MeerKAT – the window onto the Universe at low frequencies has revealed a growing number of such sources (e.g., Giovannini & Feretti 2000; Feretti et al. 2012; van Weeren et al. 2019; Knowles et al. 2022). Several classification schemes have been proposed to better understand the diverse category of diffuse sources. Here we consider one of the most recent ones by van Weeren et al. (2019), which includes radio halos (giant and mini), cluster radio shocks, and revived active galactic nuclei (AGN) fossil plasma sources (e.g., phoenices, gently re-energized tail).

The origin of diffuse radio emission in galaxy clusters remains an open question. The relativistic electron populations that should be responsible for the extended synchrotron emission in clusters typically have t_age ≤ 10⁸ yr, which implies a diffusion length scale on the order of ≈10² pc (e.g., Bagchi et al. 2002). This suggests that extended emission on scale of hundreds of kiloparsec cannot be attributed to relativistic electrons accelerated at a single location in the intracluster medium (ICM), a challenge known as the slow diffusion problem. To explain this, the electrons must either undergo reacceleration or be produced in situ (Jaffe 1977). While progress has been made in understanding the mechanisms behind the formation of radio halos (e.g., Gitti et al. 2002; Brunetti & Jones 2014; Cuciti et al. 2021, 2023) and cluster radio shocks (e.g., van Weeren et al. 2011; Molnar & Broadhurst 2017; Botteon et al. 2020; Jones et al. 2023), the origin of revived AGN plasma sources is still less clear. This is primarily due to the limited number of known examples, but also to the presence of the radio AGNs in the brightest cluster galaxy (BCG), which can further complicate their study.

Active galactic nuclei in galaxy clusters are a natural source of relativistic electrons that age over time due to radiative losses. Therefore, they may provide the seed particles for the formation of diffuse radio emission (e.g., de Gasperin et al. 2017). In particular, it has been proposed that phoenices trace old radio plasma emitted from past episodes of AGNs activity (e.g., van Weeren et al. 2019). Shock waves moving in the ICM can compress the seed relativistic electrons, boosting their momentum and, in the presence of magnetic fields, producing synchrotron emission characterized by a steep spectral index through adiabatic compression (Enßlin & Gopal-Krishna 2001). However, direct observational evidence for a connection between shock waves and phoenices is still missing; therefore their formation scenario remains somewhat uncertain. The spectral index¹ distribution across these sources is irregular without clear common trends (Cohen & Clarke 2011) and with global values α ≤ −1.5 (e.g., Cohen & Clarke 2011; de Gasperin et al. 2015). Only a few polarization studies have been performed so far of these sources (Slee et al. 2001), finding low polarization levels (2–35%). The elongated and filamentary morphologies are the most common for phoenices (e.g., Slee & Roy 1998; Slee et al. 2001), with sizes ≤300 − 400 kpc. Recently, a new formation scenario has been proposed for phoenices, in which the relativistic plasma expelled by AGNs may interact with the gas which is sloshing in the cluster gravitational potential well, as observed in Abell 2657 (Botteon et al. 2024). This scenario, in which sloshing motions are able to disrupt AGN-plasma lobes and form phoenices was studied in simulations done by ZuHone et al. (2021).

Cold fronts were first discovered in galaxy clusters by Markevitch et al. (2000), where they were interpreted as contact discontinuities in pressure equilibrium, distinct from shocks. These features are characterized by a sharp temperature jump across the front, with lower temperatures in regions of a higher density and higher temperatures in regions of a lower density, while maintaining nearly constant pressure. Subsequent studies revealed that cold fronts are ubiquitous in many galaxy clusters, including those considered to be dynamically relaxed (e.g., Markevitch & Vikhlinin 2007, Ghizzardi et al. 2010). A leading interpretation for the origin of cold fronts in dynamically relaxed clusters involves the sloshing mechanism, where the ICM undergoes oscillations within the gravitational potential well of the cluster, triggered by a minor merger with a large impact parameter (Ascasibar & Markevitch 2006). These oscillations produce distinctive features in X-ray images, such as spiral-like structures or staggered arcs, depending on the viewing angle and the angular momentum generated. The typical sizes of these structures are on the order of ≈10² kpc (Zuhone & Roediger 2016). The perturber initiating the sloshing process, often a smaller cluster or galaxy group, experiences ram pressure stripping as it moves through the ICM. This interaction can lead to the removal of gas from the perturber, forming an extended gas structure along its motion, and in later stages, possibly resulting in the complete depletion of its gas content (Ascasibar & Markevitch 2006). Additionally, cold fronts have been associated with mini radio halos, particularly in cool-core clusters, linking sloshing-induced turbulence to the reacceleration of relativistic electrons in the cluster core (Mazzotta & Giacintucci 2008).

The galaxy cluster Abell 795 (hereafter A795, z = 0.1374, Rines et al. 2013), located at RA 09:24:05.3, Dec +14:10:21.5 (J2000), represents a relevant case of study in this context. The first dedicated X-ray study of A795 was performed by Ubertosi et al. (2021), who analyzed Chandra observations and classified it as a weak cool-core cluster. They detected sloshing signatures in the form of two cold fronts at ≈60 kpc and ≈180 kpc from the BCG, and also explored the potential feedback on the ICM from the radio BCG, which had been previously classified as a Fanaroff-Riley (Fanaroff & Riley 1974) Type 0 radio galaxy (Baldi et al. 2015, 2019). Furthermore, the analysis of archival GMRT data at 150 MHz at 25″ resolution revealed a slightly resolved, extended radio structure spanning approximately 200–300 kpc with a complex morphology. Ubertosi et al. (2021) proposed this extended emission as a candidate mini-halo based on its steep (α ≤ −1.5) spectral index and shape, as well as its association with sloshing, but it remained to be confirmed by deeper multifrequency radio observations.

Recently, Kadam et al. (2024) analyzed archival GMRT observations at both 150 MHz (the same examined by Ubertosi et al. 2021) and 325 MHz finding extended radio emission with an ultra-steep spectral index (α = −2.71 ± 0.28) that they classified as a radio mini-halo. Furthermore, their reanalysis of archival Chandra data confirmed the spiral-shaped morphology of the ICM in A795.

In this work, we analyzed new observations of A795 performed with the JVLA at 1.5 GHz, as well as archival GMRT data at 325 MHz (the same studied by Kadam et al. 2024), and present the first in-depth individual study of archival XMM-Newton observations of this cluster.

The paper is organized as follows: Sect. 2 describes the data reduction procedures for the JVLA, GMRT, and XMM-Newton observations. In Sect. 3, we analyze the radio and X-ray morphology of A795, focusing on the extended radio emission, the sloshing features in the intracluster medium, and the identification of a candidate galaxy group. The spectral analysis of both radio and X-ray data, including the thermodynamic properties of the ICM and the candidate group, is detailed in Sect. 4. In Sect. 5, we discuss the implications of our findings, focusing on the sloshing-induced features, the possible interaction between A795 and the candidate galaxy group, and the classification of the extended radio emission as a potential radio phoenix. Finally, we summarize our conclusions in Sect. 6.

The cosmology adopted is H₀= 73 km/s/Mpc, Ω_m = 0.3, Ω_Λ = 0.7, which gives a conversion scale factor of 2.43 kpc/arcsec at the redshift of A795.

2. Data reduction

2.1. Radio data: JVLA and GMRT

We analyzed new JVLA observations of A795 (Project code 22B-196, PI: Ubertosi) performed with the C configuration at L band (1–2 GHz) for a total time of 1 hour. The source 3C 147 was used as the flux density and bandpass calibrator, while J0854+2006 was used as the phase calibrator. The data were calibrated in CASA v.6.4.4 (CASA Team 2022) using standard data reduction techniques for continuum calibration. We also performed two rounds of phase self-calibration and one round of phase and amplitude self-calibration, which significantly reduced noise levels across the field.

In addition, in this work we employed GMRT observations at 325 MHz (Observation 4715, January 14, 2010), with 4.2 hours of time on source. We processed the archival GMRT observations using the Source Peeling and Atmospheric Modeling pipeline (SPAM; Intema et al. 2009). SPAM performs the standard data reduction and calibration steps (bandpass and gain calibration, direction-independent self-calibration), as well as direction-dependent self-calibration. We used the calibrator 3C 286 to set the flux density scale and determine the bandpass shape in SPAM.

The data were imaged in CASA using the tclean task, exploring various weighting schemes and deconvolution algorithms to optimize the results. We compared robust = 0.5, which balances angular resolution and sensitivity, with robust = 2 (natural weighting), which maximizes sensitivity at the expense of angular resolution. As expected, the natural-weighted image achieved the lowest RMS noise level (e.g., at 1.5 GHz, natural weighting yields an RMS noise of 27 μJy/beam, compared to 35 μJy/beam for the briggs = 0.5 image). We found the “mtmfs” algorithm (Rau & Cornwell 2011) to be the most effective deconvolution method, as it accounts for variations in spectral index across the field and for emission on different spatial scales.

The errors on JVLA and GMRT flux densities were computed using the formula

$$\begin{array}{c}\hfill \mathrm{\Delta }S=\sqrt{{({\sigma }_c·S)}^2+{(\mathrm{RMS}·\sqrt{n_{\mathrm{beam}}})}^2}\\ \hfill ,\end{array}$$(1)

where n_beam is the number of beams in the source area and σ_c is the systematic uncertainty in the flux density calibration, assumed to be 5% for JVLA (Condon et al. 1998) and 8% for GMRT (Basu et al. 2012).

2.2. X-ray data: XMM-Newton

We analyzed an archival XMM-Newton data set (Observation ID: 0827031001) with an exposure time of 34 ks. The source A795 was observed with the European Photon Imaging Camera (EPIC), which consists of two MOS (Turner et al. 2001) cameras and one pn (Strüder et al. 2001) camera. The dataset was reduced using the X-COP pipeline, which was specifically designed for the analysis of X-COP observations (Tchernin et al. 2016; Ghirardini et al. 2019; Eckert et al. 2020). This pipeline performs data processing using the Extended Source Analysis Software (ESAS, Snowden et al. 2008), which is integrated into SAS v.16.1, starting from the observation data file (ODF) through to the 2D spectral analysis results.

After examining the light curves with the mos-filter and pn-filter tasks, we found no soft proton flares, allowing us to use the entire observation duration. We then applied the pn-spectra and mos-spectra tasks to the event files to produce images and spectra of the source. The pn-back and mos-back tasks were used to compute the particle background images and spectra. EPIC images (0.5 – 2 keV) were obtained by merging the pn and MOS data, and point sources were removed using the ewavelet task.

For the surface brightness analysis, we used pyproffit (Eckert et al. 2020), a python package specifically designed for the morphological analysis of cluster of galaxies. Spectral fitting was performed with Xspec v.12.13.0.

3. Radio and X-ray morphology of A795

In this section, we thoroughly analyze the morphology of A795 in the radio and X-ray bands. First, we present the JVLA and GMRT images of the radio sources in the field of A795. Then, we examine the X-ray surface brightness distribution of the ICM in A795. Figure 1 shows a composite optical, radio, and X-ray image of A795.

Fig. 1. Composite optical, radio, and X-ray image of A795. The optical image in the background is from the DESI survey. Blue corresponds to the XMM-EPIC (0.5–2 keV) exposure-corrected image, while yellow and red correspond to the GMRT 325 (restoring beam of 15″× 15″) MHz and JVLA 1.5 GHz (restoring beam of 12.8″× 10.4″) respectively.

3.1. The radio view

The final GMRT 325 MHz and JVLA 1.5 GHz images are presented in Fig. 2. These images were produced with robust = 0.5, while those obtained with robust = 2 are shown in Fig. A.1. These images display the central region of A795, where the radio AGN in the BCG coincides with the flux density peak. Consistent with previous work (Ubertosi et al. 2021; Kadam et al. 2024), we confirm the detection of two extended structures: the emission surrounding the BCG and extended emission towards the southwest direction (hereafter referred to as the SW blob; see the dashed circle in Fig. 2). Both components appear more extended at 325 MHz than at 1.5 GHz. The emission surrounding the BCG has a largest linear size (LLS) of ≈170 kpc in the 1.5 GHz data, while it reaches ≈217 kpc in the 325 MHz image. Similarly, the SW blob, located ≈201 kpc from the cluster center, exhibits a LLS of ≈90 kpc at 1.5 GHz but extends to ≈110 kpc at 325 MHz.

Fig. 2. Radio images of the central radio source in A795 obtained with briggs = 0.5. Left panel: GMRT 325 MHz image; the restoring beam of 15″ × 15″ is shown in gray. RMS is 520 μJy/beam. Right panel: JVLA 1.5 GHz image; the restoring beam of 12.8″ × 10.4″ is shown in gray. RMS is 35 μJy/beam. In both panels, the contours are drawn at 3, 6, 12, 24, 48 × RMS and the dotted circle marks the SW blob.

Measuring the flux density of the extended emission requires a careful subtraction of the cospatial radio emission from the AGN in the BCG. The radio emission from the central AGN is unresolved at 325 MHz and 1.5 GHz (as expected given its classification as a FR0 radio galaxy, see Torresi et al. 2018, Ubertosi et al. 2021). Therefore, to isolate the AGN emission, we produced an image of the point sources only, by restricting the uv-range during imaging to 7–22 kλ. The lower bound corresponds to scales of 30″, which is twice the beam FWHM, and is set to avoid the inclusion of any extended emission. Images at 1.5 GHz and 325 MHz were created by imposing the above uv-range and by selecting robust = 0.5. The results are shown in Fig. A.2. Using this method, we measured the flux densities at 325 MHz and 1.5 GHz corresponding to the unresolved core of the central AGN. Then, we obtained the flux densities of the extended emission by subtracting the point-source flux density from the total flux density measured in the images obtained from the full uv-range. In Table 1 we summarize the flux density values and LLS of the region used to measure the total flux density (within 3 RMS contour) of the central extended emission, the BCG and the SW blob.

3.2. X-ray morphological analysis

The left panel in Fig. 3 shows the exposure-corrected image of A795 in the 0.5–2 keV energy band, which was Gaussian-smoothed with a kernel radius of 3 pixels. The X-ray surface brightness is centrally peaked and shows a southeast extension at ≈600 kpc from the center.

Fig. 3. XMM-EPIC (0.5–2 keV) exposure-corrected image Gaussian-smoothed with a kernel radius of 3 pixels shown on top, and below it, the optical image from the DESI survey. The cyan circle marks the position of an additional extended X-ray source (separate from the ICM of A795), see Sect. 3.2.2. The point sources detected are marked with white circles in left panel of Fig. 8. Top left panel: wide field view of A795. Top right panel: zoom-in on the extended X-ray source within the cyan circle in the left panel. The white contours are from the JVLA 1.5 GHz data (same as in right panel of Fig. 2). Bottom left panel: optical image from the DESI survey at the location of the extended X-ray source. The green squares mark the positions of four galaxies with similar spectroscopic redshift (from Rines et al. 2013), which are reported on the image. The light red contours are from the JVLA 1.5 GHz data (same as in right panel of Fig. 2). In each panel, the cyan circle has a radius of 1.5′, roughly corresponding to 200 kpc at the redshift of A795 (z = 0.1374).

In order to investigate the global surface brightness profile of A795, we extracted the profile in pyproffit using concentric circular annuli with logarithmically spaced binning starting from a radius 6″, with an outer radius of 13.5′ (1.97 Mpc). As the center of the annuli, we selected the position of the X-ray peak (RA, Dec = 09:24:05.69, +14:10:26.09) after confirming its agreement with the X-ray peak found in the Chandra data from Ubertosi et al. (2021). The Chandra data also revealed an offset of ≈7″ (18 kpc) between the X-ray peak and the position of the BCG (RA, Dec = 09:24:05.30, +14:10:21.51). We fitted the XMM surface brightness profile of A795 using both a spherical single β model (Cavaliere & Fusco-Femiano 1976) and a spherical double β model (Mohr et al. 1999) with linked β, see left panel in Fig. 4. The green line in the plot represents the particle background profile, which is subtracted from the source one, and the lower panel shows the residuals from the best-fit model. The results of the fits are summarized in Table 2. The double β model provides a lower reduced χ² value than the single β model. Therefore, we kept the double β as the best model. We note that, in the analysis by Ubertosi et al. (2021), a single β model was found to provide a better fit to the Chandra data. This discrepancy could be due to the smaller field of view (FOV) of the Chandra observation, with a radius of 2.78′ (405 kpc), that may have prevented a reliable characterization of the outer β model component.

Fig. 4. XMM-EPIC (0.5–2 keV) surface brightness profiles of the galaxy cluster A795 and the candidate galaxy group. Black points are the surface brightness measurements, while the green line represents the particle background. The lower panel shows the residuals from the best-fit model. Left panel: A795 profile fitted with a double β model. The best-fit parameters are reported in Table 2. Right panel: Surface brightness profile of the extended X-ray source 7.36′ northwest of A795, that we interpret as a candidate galaxy group at a similar redshift of A795 (see Sect. 3.2.2 and Fig. 3), fitted with a single β model. The best-fit parameters are reported in Table 2 (last row).

3.2.1. The southeast ICM extension: A possible cold front

In left panel of Fig. 3, an asymmetric elongation of the X-ray emission toward the southeast direction can be observed. To highlight the asymmetry of the ICM, we obtained a residual image (left panel of Fig. 5) by subtracting the best-fit double β model from the exposure-corrected XMM image of A795. The residuals reveal a spiraling structure extending in the southeast direction, which appears to follow the previously detected sloshing motions. Two bright opposite regions are visible at the center of the cluster, which correspond to the cold fronts identified by Ubertosi et al. (2021) from Chandra data (green regions in left panel of Fig. 5).

Fig. 5. Morphological analysis of the large-scale surface brightness asymmetries in A795, see Sect. 3.2.1. Left panel: XMM-EPIC (0.5–2 keV) residual image Gaussian-smoothed with a kernel radius of 9 pixels. The four regions mark the sectors used to extract the surface brightness profiles shown in the right panel. The inner and outer radii of the annulus correspond to 190 kpc and 1 Mpc from the X-ray peak, respectively. The two cold fronts detected by Ubertosi et al. (2021) are marked in green. Right panel: XMM-EPIC (0.5–2 keV) surface brightness profile of the four sectors. The blue points represent the SE sector which shows the excess.

With the aim of confirming the presence of the southeast asymmetric elongation of the ICM, we extracted the surface brightness profile in four elliptical sectors (each 90°-wide, axis ratio b/a = 0.83) centered on the X-ray peak. We focused on the radial range r > 290 kpc (outside the inner two cold fronts found by Ubertosi et al. 2021) to r ≈ 1 Mpc. The resulting sectors are SE, NW, NE and SW, and are shown in left panel of Fig. 5. The right panel shows the SB profiles. The SE sector clearly shows an excess in surface brightness relative to the other directions, particularly from 2′ (292 kpc) to 6′ (875 kpc). Given the spiral structure of the ICM and the presence of two inner cold fronts within the innermost ≈200 kpc of the cluster, it is possible that the southeastern asymmetric surface brightness excess represents a third, outer cold front. To verify this possibility, we considered a 3D broken power-law density model, defined in pyproffit as

$$\begin{array}{c}\hfill F(r)=\{\begin{array}{cc}{r}^{-{\alpha }_1}\hfill & \text{if}r<r_j\hfill \\ \frac{1}{\text{jump}}·r^{-{\alpha }_2}\hfill & \text{if}r\ge r_j\hfill \end{array}\\ \hfill ,\end{array}$$(2)

where “jump” defines the density jump across the front, α₁ and α₂ are the slopes before and after the jump, and r_j is the distance of the jump from the center. This density model is projected along the line of sight and fitted to the observed surface brightness profile. The best-fit parameters are presented in Table 3. The jump parameter is consistent with unity, suggesting that this surface brightness excess cannot be classified as a contact discontinuity. Varying the surface brightness profile extraction region did not produce significant changes in the results. It is possible that, given the large distance of the excess from the center of the cluster (≈650 kpc), deeper XMM observations would be necessary to verify the presence of a mild surface brightness discontinuity. The most cluster-center distant cold fronts detected so far in galaxy clusters are summarized in Mirakhor et al. (2023). At distances larger than 500 kpc only five discontinuities are known. This highlights the difficulty in detecting such features at considerable distances from the center. However, it is not certain that all large-scale surface brightness excesses associated with sloshing should end with a cold front. Simulations predict the presence of surface brightness excesses at large distance from the cluster center but do not necessarily predict the formation of cold fronts in these regions (see Sect. 4.2 and Sect. 4.5 in Rossetti et al. 2013).

3.2.2. Detection of a candidate galaxy group

By inspecting the exposure-corrected image, upper left panel in Fig. 3, a region of enhanced, diffuse X-ray emission (above the surrounding background level) can be seen at ≈7.36′ northwest of the center of A795, as marked by the cyan dotted region. We searched for known celestial objects around the approximate position of the X-ray diffuse emission in the Simbad archive², finding a possible association with the radio galaxy NVSS J092352+141657 (RA, Dec = 09:23:52.75, +14:16:58.61). This galaxy has a redshift of z = 0.141, which is similar to that of A795 (z = 0.1374). In right panel of Fig. 3 the radio counter-part of this source is shown using the JVLA 1.5 GHz contours, while in the lower panel, we show an overlay of the 1.5 GHz radio contours with the DESI survey optical image. This optical image reveals four galaxies (including the optical host of NVSS J092352+141657) within 1.5′(≈200 kpc) from the center of the diffuse X-ray emission, with similar spectroscopic redshifts (from Rines et al. 2013). To confirm the presence of extended X-ray emission at this position, we extracted the surface brightness profile from the XMM data using concentric circular annuli with linearly spaced binning of 6″, with an outer radius of 1.5′ (218.7 kpc), centered on the radio galaxy NVSS J092352+141657.

Subsequently, we fitted the profile with a single β model (see right panel in Fig. 4) and the best-fit parameters are summarized in Table 2 (last row). The low reduced-χ² value suggests that the single β model is a good fit. The obtained β value, β = 0.53 ± 0.17, is close to the expected range for galaxy clusters or groups (β ≈ 2/3, e.g., Jones & Forman 1984), strongly suggesting that the detected X-ray emission is produced by thermal plasma³, such as the ICM or intragroup medium (IGrM). This, along with the detection of an elliptical galaxy with extended radio lobes (see right panel of Fig. 3) at the center of the X-ray extended emission, provides further evidence supporting the classification of this object as a galaxy group. There are no previous mentions in the literature of galaxy groups present in the analyzed region. To better characterize the properties of this system, we present in Sect. 4.3 a spectral analysis of its X-ray emission.

Furthermore, given the location of this candidate galaxy group in the proximity of a sloshing galaxy cluster, A795, we further explore in Sect. 5.1 any possible signs of past dynamical interactions between the two systems.

4. Spectral analysis

In this section, we presents the radio and X-ray spectral analysis. We first focus on the spectral index measurements of the different radio components (the radio active BCG, the extended radio source) at the center of A795 detected in JVLA and GMRT data. We then report the results of the X-ray spectral analysis of the ICM of A795, as well as of the X-ray emission from the candidate galaxy group ≈1 Mpc northwest of the cluster.

4.1. Radio spectral index analysis

We measured the spectral index of the different sources by combining flux density measurements from the JVLA at 1.5 GHz and the GMRT at 325 MHz. The derived flux density values are shown in Table 1 and were measured from the images in Fig. 2, obtained with robust = 0.5. The resulting spectra are shown in Fig. 6, with the values of α displayed in the bottom left corner.

Fig. 6. Radio spectra of the three radio sources detected at the center of A795, see Sect. 3.1. In the bottom left corner, the spectral index α values are listed.

The BCG is an active radio galaxy and has a spectral index α_BCG = −1.18 ± 0.14, consistent with previous results at higher (5 GHz and 8.4 GHz, Ubertosi et al. 2021) and lower (150 MHz and 325 MHz, Kadam et al. 2024) frequencies, who reported spectral indeces of −0.93 ± 0.09 and −1.08 ± 0.26, respectively. The spectra we obtained for the extended emission (α_Ext. = −2.24 ± 0.13) and the SW blob (α_SWb = −2.10 ± 0.13), reported in Table 4, are significantly steeper. For comparison, Kadam et al. (2024) measured a spectral index α = −2.71 ± 0.28 for the extended emission⁴. Such steep spectra are consistent with a scenario in which the extended emission originates from the reacceleration of populations of relativistic electrons. We further explore this aspect in Sect. 5.2.

Additionally, we computed the radio power of the extended emission at 1.5 GHz as

$$\begin{array}{c}\hfill P_{1.5\mathrm{GHz}}=4\pi D_L^2{S}_{1.5\mathrm{GHz}}{(1+z)}^{-(\alpha +1)}\\ \hfill ,\end{array}$$(3)

where the factor (1+z)^{−(α + 1)} accounts for the K-correction. The luminosity distance of A795 is D_L = 671 ± 49 Mpc. Based on the 1.5 GHz flux density of the extended emission in Table 1, the resulting radio power is P_1.5 GHz(Ext.) = 1.89 ± 0.34 ⋅ 10²³ W/Hz. We also computed the radio power of the SW blob using α_SWb = −2.10 ± 0.13 and the value of S_1.5 GHz from Table 1, obtaining P_1.5 GHz(SWblob) = 0.63 ± 0.12 ⋅ 10²³ W/Hz. The radio power values are summarized in Table 4.

In order to study the spectral index 2D distribution, we generated spectral index maps by combining the 1.5 GHz and 325 MHz radio images. To generate this, we used the CASA task immath, which calculates the spectral index values pixel by pixel. To perform this operation effectively, the two images must be perfectly aligned and have matching restoring beam sizes and uvrange. Different uvranges result in an unequal sampling of angular scales at the two frequencies involved. This produces a bias in the computation of α. For this reason, we combined radio maps obtained by forcing a common uvrange of 0.2 – 22 kλ, with a circular restoring beam of radius 15″. Figure A.3 shows a wide-FOV spectral index map of A795, while Fig. 7 shows a zoom-in of the central region (as well as the spectral index error map). It can be seen how the spectral index steepens moving from the center of the emission, corresponding to the position of the BCG, toward the external regions. The SW blob shows a relatively uniform α distribution with steep values α≤ −2.

Fig. 7. Spectral index map (left) and spectral index error map (right) measured at 325 MHz and 1.5 GHz of A795 zoomed on the central extended radio emission, with JVLA 1.5 GHz contours, defined in Fig. 2.

4.2. X-ray spectral analysis

To comprehensively characterize the thermodynamic properties of the galaxy cluster A795, we conducted an X-ray spectral analysis of the ICM. We adopted the approach of creating a background model rather than subtracting the background from the data, following, for example, Leccardi & Molendi (2008) and Snowden et al. (2008). This differs from typical X-ray data analyses, where the background is subtracted. In the latter case, data taken from sky regions without sources called blank sky, are adapted and subtracted from the target observation. The issue with the subtraction method is the uncertainty regarding the background values that are subtracted, as they are derived from different sky regions and taken at different times. The method of creating a background model allows for greater control over its parameters, which are then measured on the observation of the source itself. To fully characterize the background model, it is necessary to take into account all possible sources of detection. In the band 0.7–10 keV the background spectral components can be categorized into two groups: the sky background and the particle background. The sky background includes three components: (i) the Cosmic X-ray Background (CXB), which represents the integrated emission from all extragalactic sources in the X-ray band; (ii) the Local Hot Bubble (LHB), a region of ionized X-ray emitting gas surrounding the Sun; and (iii) the Galactic Halo (GH) component. The particle background, also referred to as the non X-ray background (NXB), comprises high-energy cosmic ray particles of galactic origin (typically around GeV energies), and soft protons.

To study the global thermodynamic properties of A795, we employed a circular region centered on the X-ray peak, with a radius of R₅₀₀⁵ = 7.4′ (1.02 Mpc). R₅₀₀ was estimated from the total mass M₅₀₀ value found in the Planck catalog, using Eq. (9) of Planck Collaboration XX (2014). For the background, we selected an annulus centered on the X-ray peak, with an inner radius corresponding to R₂₀₀ = 9.6′ (1.33 Mpc⁶), beyond which the cluster’s emission is minimal, and an outer radius delineated by the limit of the XMM field of view, ≈15′ (2.19 Mpc). The spectral extraction regions are shown in left panel of Fig. 8.

The extracted spectra were binned so that every bin contained 20 counts. We first modeled the background region. This enables obtaining the best-fit parameters for the various components of the background, which are then used in calculating the source spectrum. For a detailed description of the background model components refer to works such as Ghirardini et al. (2019) and Rossetti et al. (2024).

For every thermal model and photoelectric absorption model employed in this work, we used the table of abundances of Asplund et al. (2009). The best-fit parameters for the sky background model are summarized in Table 5. The model used for the cluster A795, which contains the model for the sky background, is (apec₁+phabs*(apec₂+powerlaw+apec₃)) in XSPEC. The apec₃ accounts for the cluster absorbed thermal emission and it has abundance, temperature and normalization let free to vary. The best-fit parameters for A795’s thermal emission are shown in Table 6. Right panel in Fig. 8 shows the spectrum of A795 fitted with its model. The red and black data points are measured by MOS1 and MOS2, and the green ones by pn. The energy range ≈1.2–2 keV is excluded for the three instruments due to the presence of strong fluorescence emission lines. For the pn, we also removed the range ≈6.8–9 keV.

Fig. 8. Spectral analysis of A795. Left panel: XMM-EPIC (0.5–2 keV) surface brightness image Gaussian-smoothed with a kernel radius of 3 pixels. The green region was used for the global spectral analysis of the cluster A795, while the magenta one for the background. The white circles mark the position of the excluded sources and the cyan region signs the position of A795-B, see Sect. 4.3. Right panel: XMM-EPIC spectrum of A795 within the green region of the left panel, fitted with a thermal component and the background model. The red and black lines represent the models for MOS1 and MOS2 while the green one for pn. The lower panel shows the residuals from the best-fit curve. The best-fit parameters for the cluster thermal component are summarized in Table 6.

The abundance Z = 0.37 ± 0.02 Z_⊙ and the temperature kT = 3.87 ± 0.12 keV (4.49 ± 0.14 ⋅ 10⁷ K) are consistent with values typically found in galaxy clusters (Böhringer & Werner 2010). The global temperature is lower with respect to that reported in Ubertosi et al. (2021), kT = 4.63 ± 0.12 keV, measured from Chandra data. It is important to mention that there exists a systematic difference in temperature measurement between analyses using Chandra/ACIS and XMM-EPIC data (full band), that is, EPIC data yielding 16% lower global temperatures at 5 keV (Schellenberger et al. 2015). The difference observed here is consistent with this 16% offset, further supporting the systematic nature of the discrepancy. In addition to this, other factors may contribute to this temperature difference, including: the spectrum extraction region of Chandra data had a radius of 2.78′ (405.8 kpc), while the region used for the XMM analysis has a radius of 7.36′ (1.02 Mpc); the model used for the absorption employed by Ubertosi et al. (2021) was tbabs in XSPEC, while in this work we used phabs; the column density value used for the absorption component is different since they used only the atomic hydrogen, while in our analysis also the molecular hydrogen was taken into account, from Bourdin et al. (2023). The column density parameter used in this analysis was fixed at the value N_H = 4.35 ⋅ 10²⁰ cm⁻².

4.3. Spectral analysis of the candidate galaxy group

The X-ray spectral analysis of the candidate group named A795-B required a modification of the spectral fitting model, since the emission from the outer regions of the cluster A795 constitutes an additional background component in the group region. The steps taken are as follows:

• Background region: We used the same annular region employed in the A795 background analysis for the A795-B background extraction (see the left panel of Fig. 8). The best-fit parameters for the background components are reported in Table 5.

• Cluster Emission: To account for the A795 emission at the candidate group’s location, we added a thermal component to the source model. We derived the parameters for this thermal component by fitting the spectrum of an annular region centered on the X-ray peak of A795. The inner radius of the annulus was 5.85′ (852 kpc), while the outer radius was 8.33′ (1.21 Mpc), encompassing A795-B (see left panel of Fig. 9). During the extraction, the emission from A795-B was excluded. The best-fit parameters for this annular region were kT = 2.53 ± 0.12 keV, Z = 0.17 ± 0.07 Z_⊙, and normalization norm = 1.03 ± 0.21 in units of APEC XSPEC normalization per arcmin².

• Group Emission: We extracted the spectrum of A795-B from a circular region centered on the associated radio galaxy, with a radius of 1.5′ (218.7 kpc; see left panel of Fig. 9). This radio galaxy lies 7.36′ (1.02 Mpc) from the cluster center, consistent with the R₅₀₀ of A795.

• Spectral Fitting: We modeled the A795-B spectrum in XSPEC as: (apec₁ + phabs * (apec₂ + powerlaw + apec₃+ apec₄)), where apec₃ accounts for A795’s thermal emission at the group’s location, and apec₄ represents the thermal emission of the group itself.

Fig. 9. Spectral analysis of A795-B, see Sect. 4.3. Left panel: XMM-EPIC (0.5–2 keV) surface brightness image Gaussian-smoothed with a kernel radius of 3 pixels. The yellow annular region was used for the spectral extraction of the cluster component, while the green circle marks the position of R500 = 1.02 Mpc of A795. The cyan region was used for the spectral extraction of A795-B. The white circles represent the point sources excluded. Right panel: XMM-EPIC spectrum of A795-B fitted with a thermal component and the background model accounting for the cluster emission. The red and black lines represent the models for MOS1 and MOS2 while the green one for pn. The lower panel shows the residuals from the best-fit curve. The best-fit parameters for A795-B’s thermal component are summarized in Table 6.

The spectrum is shown in the right panel of Fig. 9, and A795-B’s best-fit parameters are summarized in Table 6. The measured temperature of the system, kT = 1.02 ± 0.08 keV (1.18 ± 0.08 ⋅ 10⁷ K), agrees with expectations for galaxy groups (Bower & Balogh 2004). This, along with the surface brightness profile compatible with a β model, provides further confirmation that A795-B is a galaxy group.

The norm parameter is defined as

$$\begin{array}{c}\hfill \mathit{norm}=\frac{{10}^{-14}}{4\pi {[D_A(1+z)]}^2}{\displaystyle \int n_e}{n}_p\mathrm{d}V\\ \hfill ,\end{array}$$(4)

where D_A is the angular distance from the source, n_e and n_p are the electron and proton number densities, respectively, and V is the volume of the emitting region. The norm parameter enables the calculation of the IGrM electron number density, which can be used to estimate its total mass. However, it is important to note that the norm value, in this case, is derived from a spectrum that is not deprojected, meaning that the electron density may be overestimated due to projection effects. By reverting Eq. (4) and considering n_e = 1.2 n_p (e.g., Gitti et al. 2012), it is possible to calculate the average electron number density n_e in the group volume as

$$\begin{array}{c}\hfill n_e=\sqrt{\frac{{10}^{14}(4\pi ·\text{norm}·{[D_A(1+z)]}^2)}{0.83V}}\\ \hfill ,\end{array}$$(5)

where V is the volume of the region used for the spectrum extraction of the group. Assuming a radius of 1.5′ (218.7 kpc) from the morphological analysis, z = 0.141 and D_A = 516 Mpc we get n_e = 2.7 ± 0.6 ⋅ 10⁻⁴ cm⁻³, which is consistent with typical IGrM mass estimates for galaxy groups (Sun 2012). The total IGrM density can then be expressed as ρ_IGrM = 1.92 μ ⋅ n_e ⋅ m_p, where μ = 0.6 is the mean molecular weight and m_p = 1.67 ⋅ 10⁻²⁴ g is the proton mass. Finally, a simple estimate of the IGrM mass is obtained from M_IGrM = ρ_IGrM ⋅ V, leading to M_IGrM (r ≤ 218 kpc) = 3 ± 1 ⋅ 10¹¹ M_⊙, which is a result typically expected for galaxy groups (Sun et al. 2009).

The total mass of galaxy clusters or groups can be estimated using the hydrostatic equilibrium assumption. The latter is based on the balance between gravity and pressure:

$$\begin{array}{c}\hfill \mathrm{\nabla }{P}_{\text{gas}}=-{\rho }_{\text{gas}}\mathrm{\nabla }\varphi \\ \hfill ,\end{array}$$(6)

where P_gas is the gas pressure, ρ_gas the gas density, and ϕ the gravitational potential. Under the assumption of spherically symmetric gas distribution and considering that P_gas = ρ_gaskT/μm_p, it is possible to derive the expression for the mass profile M(r) inside the radius r as

$$\begin{array}{c}\hfill M_{\mathrm{tot}}(<r)=-\frac{\mathit{rkT}}{\mu m_p}(\frac{\mathrm{d}ln{\rho }_{\text{gas}}(r)}{\mathrm{d}ln(r)}+\frac{\mathrm{d}lnT(r)}{\mathrm{d}ln(r)})\\ \hfill ,\end{array}$$(7)

where μ is the mean molecular weight, m_p is the proton mass, and k is the Boltzmann constant. Since the single β model was used to fit the X-ray surface brightness of A795-B, the total mass can be recovered analytically and expressed by the formula

$$\begin{array}{c}\hfill M_{\mathrm{tot}}(<r)=\frac{k{r}^2}{G\mu m_p}[\frac{3\beta rT}{r^2+r_c^2}-\frac{\mathrm{d}T}{\mathrm{d}r}]\\ \hfill .\end{array}$$(8)

Substituting the single β model best-fit parameters from Table 2 (last row), the gravitational constant G = 6.67 ⋅ 10⁻⁸ cm³/g/s², the Boltzmann constant k = 1.38 ⋅ 10⁻¹⁶ erg/K, r = 1.5′ (218.7 kpc), T = 4.49 ⋅ 10⁷ K and assuming a constant temperature profile ($\frac{\mathit{dT}}{\mathit{dr}}=0$) we obtained the gravitational mass of A795-B M_tot (r ≤ 218 kpc) = 7 ± 3 ⋅ 10¹² M_⊙. This result is consistent with predictions for galaxy groups (Tully 1987). As a note of caution, we point out that the above M_IGrM and M_tot were derived within a fixed radius of 218 kpc (the extent of the group in X-rays, see Fig. 4, right), rather than within a scaled radius (e.g., R₅₀₀), and thus the comparison with reference values for groups is only approximate.

5. Discussion

The radio analysis of A795 with the new JVLA data at 1.5 GHz and the archival GMRT image at 325 MHz revealed the presence of an extended synchrotron source with elongated morphology, surrounding the central BCG. We discuss the classification and origin of this emission in Sect. 5.2.

The new XMM-EPIC (0.5–2 keV) surface brightness analysis revealed two extended features. The first is an arc-shaped surface brightness excess in the southeast region that appears to follow the sloshing spiral previously detected by Ubertosi et al. (2021) (see the next section for a comprehensive analysis). The second is an extended source located northwest of the cluster center. The JVLA image at 1.5 GHz reveals an extended radio galaxy coincident with this feature. The surface brightness profile well fitted by a single β model and the presence of thermal IGrM at temperature kT = 1.02 ± 0.08 keV (1.18 ± 0.08 ⋅ 10⁷ K), measured for the first time from the spectral analysis, corroborate the hypothesis that this object can be identified as a galaxy group, that we named A795-B. In the next section, we explore whether A795-B could be the perturber responsible for initiating the sloshing motion.

5.1. Sloshing in A795

Evidence of sloshing at the center of A795 was previously reported using Chandra observations (Ubertosi et al. 2021), which revealed two cold fronts located at 59.6 ± 0.3 kpc and 178.2 ± 2.2 kpc from the cluster center (see also Kadam et al. 2024 for consistent results). With XMM-EPIC data we discovered a surface brightness arc-shaped excess reaching ≈ 650 kpc from the center, that seems to follow the sloshing spiral (see left panel in Fig. 5). However, this excess could not be classified as a density discontinuity since it did not show a significant edge in the SB profile. Such features have been qualitatively reproduced in simulations of galaxy clusters and groups, and can further constrain the geometry of sloshing (e.g., Roediger et al. 2011, 2012).

A similar configuration was observed in the galaxy group NGC 5044, where an X-ray surface brightness excess at 110 kpc from the center, which could not be confirmed as a proper third cold front, was found following the spiral structure traced by two inner cold fronts (Gastaldello et al. 2013). In the case of A795, the surface brightness excess is far more distant from the center (≈650 kpc). As noted earlier, only five discontinuities have been identified at distances beyond 500 kpc from the center of their host cluster center (Mirakhor et al. 2023). This underscores the challenges associated with resolving these morphological substructures in the peripheral regions of galaxy clusters, primarily due to the rapid outward decline in surface brightness. Additionally, simulations indicate that large-scale surface brightness excesses, while common in sloshing systems, do not always coincide with the formation of cold fronts at such distances from the cluster center (Rossetti et al. 2013).

The galaxy group we detected, A795-B, could be the candidate perturber for originating the sloshing motion. Simulations of gas sloshing in galaxy clusters, such as the one conducted by Ascasibar & Markevitch (2006), show how as the perturber falls in the gravitational potential well, its outer gas is stripped and forms a comet-shaped tail. Observational confirmations of this scenario can be found in the galaxy cluster RXJ1347.5-1145 (Johnson et al. 2012), one of the few objects for which the perturber galaxy group could be identified. This galaxy group exhibited a low gas content, likely indicating that a previous passage near the cluster center had depleted its gas. We therefore consider the possibility that A795-B is the perturber of A795. Using the redshift of the group (z = 0.141) and the average redshift of the cluster (z = 0.1374), we can measure the line of sight component of the peculiar velocity of A795-B with respect to A795, finding a reasonable value of v ≈ 1200 km/s (the velocity dispersion of A795 is ≈770 km/s, see Rines et al. 2013). The group has a gravitational mass M_tot (r ≤ 218 kpc) = 7 ± 3 ⋅ 10¹² M_⊙ and presents IGrM with mass M_IGrM (r ≤ 218 kpc) = 3 ± 1 ⋅ 10¹¹ M_⊙, values that are in line with the typical gas fraction f_gas for galaxy groups (Sun 2012). However, the gas fraction alone is insufficient to rule out a past interaction, as seen, for instance, in Ichinohe et al. (2015). Specifically, their study of the galaxy cluster Abell 85 reveals that it is undergoing a double merger, with one of the two infalling substructures exhibiting an X-ray bright core. Moreover, we did not detect any sign of gas stripping forming a comet-shaped tail around the group. This could be attributed to the exposure time of the XMM dataset analyzed in this work, projection effects, or a combination of both.

For completeness, we comment on the velocity distribution of member galaxies in A795. Past studies have shown that the spatial and kinematical distribution of member galaxies can be useful to reveal the presence of substructures (e.g., Girardi & Biviano 2002; Gastaldello et al. 2013; Brienza et al. 2022; Benavides et al. 2023). The identification of member galaxies of A795 has been performed by Rines et al. (2013), who presented spectroscopic redshifts for 179 member galaxies of the cluster. Starting from their published catalog of spectroscopic redshifts, we plot in Fig. 10 the redshift and velocity distribution of the member galaxies. The histogram of the spectroscopic redshifts (left panel) reveals several signs of dynamical disturbances. First, the member galaxies are distributed in a clearly bimodal distribution ranging between 0.132 ≤ z ≤ 0.142. A simple fit with a double Gaussian distribution yields a 99.4%-significance improvement with respect to a single Gaussian. One component is found to be located at z = 0.1342 ± 0.0003 (blue lines in Fig. 10, left), with σ_v = 240 ± 77 km/s (δ_z = 0.0008 ± 0.0003), and the other is located at z = 0.1388 ± 0.0002 (red lines in Fig. 10, left), with σ_v = 582 ± 58 km/s (δ_z = 0.0019 ± 0.0002). Interestingly, the member galaxies belonging to the blueshifted component and those belonging to the redshifted component are spatially coincident (Fig. 10, right). This is consistent with a possible line-of-sight minor merger occurring in A795. Second, the BCG of A795 is at v_rel = −500 km/s with respect to the average of A795. This configuration is typical of sloshing galaxy clusters whose potential well has been perturbed (e.g., Coziol et al. 2009), setting the BCG, the galaxies, and the ICM in relative motion. Notably, the BCG of A795 and the central galaxy of A795-B lie beyond ±1σ_v of the two Gaussians shown in Fig. 10 (left). Although a complete determination of the number and spatial distribution of substructures in A795 is beyond the scope of this work, the simple plots in Fig. 10 already demonstrate the presence of velocity substructures in the cluster, as well as their likely alignment with the line of sight. Therefore, while A795-B is a candidate perturber, there are other structures within 10 Mpc of the center of A795 that might have set the sloshing oscillation of the ICM in motion.

Fig. 10. Velocity distribution of member galaxies in A795. Left panel: histogram of the spectroscopic redshift (bottom x-axis) for 179 member galaxies of A795, measured and reported in Rines et al. (2013). The top x-axis shows the peculiar velocity relative to the average redshift of A795, that is z = 0.1374. Vertical dotted lines mark the central galaxy of A795-B (yellow), the average of A795 (red), and the BCG of A795 (green). The black solid line is the best-fit with a double Gaussian of the histogram, with the two components shown as blue and red dashed lines. The vertical dashed-dotted lines show the central velocity of each gaussian, while the shaded colored area represents the 1σ width of each Gaussian. Right panel: spatial distribution in RA and Dec of the member galaxies of A795 from Rines et al. (2013), color coded based on their peculiar velocity. Galaxies represented by red (blue) circles have peculiar velocity within ±1σ from the corresponding average of z = 0.1388 (z = 0.1342) shown in the left panel. Black + signs represent galaxies whose peculiar velocity falls outside the ±1σ range of both Gaussians in the left panel. The spatial position of the BCG of A795 and of the central galaxy of A795-B is highlighted, and both are plotted as with a black + sign.

5.2. A candidate radio phoenix

In a recent study of A795, Kadam et al. (2024) analyzed archival GMRT data at 150 MHz and 325 MHz and detected the extended radio emission with a complex shape at the center of the cluster. They interpreted this emission as a candidate mini-radio halo, even though they measured an ultra-steep spectral index of −2.71 ± 0.28. This value differs from the typical range for these type of sources, which is −1.4 < α < −1.1 (Giovannini et al. 2009). Moreover, they noted that the complex morphology poses challenges to the mini-radio halo interpretation, as these sources exhibit a rounder shape (Gitti et al. 2015).

In this work, we favor a different interpretation. The observed ultra-steep values of the spectral index (α_Ext., α_SWb ≈ −2.2), the complex shape (see Fig. 2 and Fig. A.1 and the cospatiality with the BCG, which is a radio-loud AGN (P_1.5 GHz = 4.31 ± 0.68 ⋅ 10²⁴ W/Hz), align with the scenario of a revived AGN plasma source. These sources are characterized by steep radio spectral indeces, with values α ≤ −1.5 (de Gasperin et al. 2015), and complex morphologies predicted by simulations (Enßlin & Brüggen 2002). We also note that phoenices are expected to exhibit high-frequency spectral curvature, and not only a simple power-law spectrum (Enßlin & Gopal-Krishna 2001). Specifically, this occurs because adiabatic compression revives fossil radio emission by shifting the original radio spectrum to higher frequencies. The original spectrum is already steep and curved due to the radiative losses the plasma experienced before the compression occurred. In the future, it would be desirable to obtain four or more flux density measurements to probe any spectral curvature of the radio source in A795.

Among the key questions concerning radio phoenices, the most intriguing concerns the mechanism responsible for the reacceleration of the aged plasma emitted by the central AGN. Adiabatic compression has been proposed as a plausible explanation, attributed to the effects of shock passage. This mechanism effectively re-energizes electrons, leading to synchrotron emission characterized by a steep radio spectral index (Enßlin & Gopal-Krishna 2001). However, a connection between the diffuse emission and shocks waves is missing, as in the case of A795 no shock front was detected.

Another reacceleration scenario was proposed by Botteon et al. (2024) for the case of Abell 2567. The detected diffuse radio emission was cospatial with the sloshing spiral at the center of the cluster (see their Fig. 5). Comparison with tailored numerical simulations (see also ZuHone et al. 2021) supported the idea that the relativistic AGN plasma may have been compressed, and re-energized by the sloshing motion. The diffuse emission in A795 does not show, as in the case of Abell 2567, a perfect cospatiality with the enhancement in surface brightness caused by the sloshing (see Fig. 11), especially in the case of the SW blob, but this does not exclude a possible interaction (see the case of IC1860, Gastaldello et al. 2013). Projection effects further complicate the picture, since they represent a possible bias for the system configuration.

Fig. 11. XMM-EPIC (0.5–2 keV) residual image Gaussian-smoothed with a kernel radius of 9 pixels of A795’s central region with GMRT 325 MHz contours (left) and JVLA 1.5 GHz contours (right), defined in Fig. 2.

If sloshing motion were responsible for the reacceleration, it is expected that tangential motions would stretched magnetic fields along the sloshing spiral. As a result, the presence of polarized emission is predicted due to the alignment of magnetic field lines. Only a few polarization measurements have been performed so far of these sources (Slee et al. 2001), finding low polarization levels. However, since phoenices are typically located near the cluster center, as in the case of A795, Faraday depolarization could significantly reduce the observed polarization fraction, especially at low frequencies. A polarization analysis of the diffuse emission detected in A795 at frequencies higher than 1.5 GHz could serve as a topic for future studies.

6. Conclusions

In this work we presented the analysis of new JVLA 1.5 GHz observations, along with archival GMRT 325 MHz and XMM-Newton data, of the galaxy cluster A795. The main results are summarized below.

• The X-ray morphological analysis based on XMM data was conducted on scales larger than those covered with Chandra (Ubertosi et al. 2021, Kadam et al. 2024), allowing us to study A795 out to r₂₀₀ = 10.5′ (1.53 Mpc) (see Sect. 4.2). A double β model was found to provide the best-fit to the surface brightness profile, with best-fit parameters of β = 0.59 ± 0.03, r_c, 1 = 59.8 ± 6.7 kpc and r_c, 2 = 317.8 ± 34.5 kpc (see Table 2, middle row). We determined the global properties of the thermal ICM in A795 through spectral analysis (see Sect. 4.2), measuring a temperature kT = 3.87 ± 0.12 keV and abundance Z = 0.37 ± 0.02 Z_⊙ (see Table 6).

• The X-ray surface brightness analysis revealed an arc-shaped, azimuthally asymmetric excess extending to ≈650 kpc from the center (see left panel in Fig. 5), which appears to follow the known sloshing spiral (see Sect. 3.2.1). Deeper future XMM observations may determine whether this excess corresponds to a large-scale cold front.

• The XMM image revealed an extended source located northwest of the cluster center at a projected distance of ≈7.36′ (1.02 Mpc) from A795’s center (see Fig. 3), centered on the radio galaxy NVSS J092352+141657 (z = 0.141). Its X-ray surface brightness profile is well-fit by a β model with best-fit parameters β = 0.52 ± 0.17 and r_c = 28.2 ± 13.1 kpc (see Table 2). We identified this source as a candidate galaxy group. Extended radio emission has been found from the JVLA map at 1.5 GHz, associated with the elliptical galaxy at the center of the group (see right panel in Fig. 3). The X-ray spectrum supports the galaxy group interpretation, being well described by a thermal gas with temperature of kT = 1.02 ± 0.08 keV. We named it A795-B.

• We examined whether A795-B may have acted as the perturber that triggered the sloshing motion in A795 (see Sect. 5.1). The peculiar velocity between the group and A795 (v ≈ 1200 km/s) is consistent with a high-speed core passage, but the gas fraction of the group and the lack of a clear cometary IGrM morphology may argue against a past passage close to A795’s core. We conclude that A795-B is a plausible perturber, although further investigation is needed. The optical distribution of member galaxies of A795 shows evidence for sub-halos aligned along the line of sight, supporting the unrelaxed nature of A795.

• The new JVLA 1.5 GHz map confirmed the presence of extended emission surrounding the BCG of A795, with largest linear size ≈170 kpc, along with a sub-component extending in the southwest direction, named the SW blob, at a distance ≈200 kpc from the center (see right panel in Fig. 2). This emission was also detected in GMRT archival data at 325 MHz, with an extension of ≈217 kpc (see left panel in Fig. 2). We measured ultra-steep spectral indices of α_Ext. = −2.24 ± 0.13 for the extended emission (excluding the BCG) and α_SWb = −2.10 ± 0.13 for the SW blob (see Table 4). These ultra-steep indeces, combined with the elongated complex morphology and cospatiality with the radio-loud AGN present in the BCG, suggest that the extended emission of A795 is a radio phoenix (see Sect. 5.2). The reacceleration mechanism remains unclear, as the radio emission shows only a partial spatial match with the sloshing spiral in the ICM.

Future observations in polarization of the candidate radio phoenix may shed further light on this peculiar system and determine wether the magnetic field lines are aligned with the sloshing spiral – providing evidence for sloshing-related reacceleration – or not. In addition, higher-frequency radio observations could test wether the spectrum exhibits curvature above GHz frequencies, which would support an adiabatic compression scenario. Moreover, deeper XMM observations could enable the study of large-scale sloshing motion as well as any potential cometary morphology of the IGrM in A795-B.

Acknowledgments

We thank the referee for their clear revision of this manuscript. FU and MG acknowledge support from the research project PRIN 2022 “AGN-sCAN: zooming-in on the AGN-galaxy connection since the cosmic noon”, contract 2022JZJBHM_002 – CUP J53D23001610006. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.

References

Appendix A: Radio images

In this section we present three different views of the radio emission in A795. The first, Fig. A.1, shows the central diffuse emission at 325 MHz and 1.5 GHz using natural weighting. In Fig. A.2 are presented the images at 325 MHz and 1.5 GHz that we used to measured the BCG flux, by excluding the contribution of extended sources. Figure A.3 shows the wide field spectral index map of A795.

Fig. A.1. Radio images of the central radio source in A795 obtained with briggs = 2. Left panel: GMRT 325 MHz image; the restoring beam of 23″ × 19″ is shown in gray. RMS is 450 μJy/beam. Right panel: JVLA 1.5 GHz image; the restoring beam of 16.2″ × 12.5″ is shown in gray. RMS is 27 μJy/beam. In both panels, the contours are drawn at 3, 6, 12, 24, 48 × RMS and the dotted circle marks the SW blob.

Fig. A.2. Radio images of the central source in A795 with a cut at baselines < 7kλ made to select only the BCG emission. Left panel: GMRT 325 MHz image obtained with briggs = 0.5 of the central region of A795. The restoring beam of 7.8″ × 6.3″ is shown in gray. RMS is 480 μJy/beam. Right panel: JVLA 1.5 GHz image obtained with briggs = 0.5. The restoring beam of 9.1″ × 7.8″ is shown in gray. RMS is 54 μJy/beam. In both panels, the contours are at levels 3, 6, 12, 24, 48 RMS.

Fig. A.3. Wide field spectral index map of A795. The white ellipse marks the position of the extended radio emission cospatial with the BCG. The cyan dashed circle signs an additional extended X-ray source (separate from the ICM of A795), see Sec. 3.2.2.

All Tables

All Figures

	Fig. 1. Composite optical, radio, and X-ray image of A795. The optical image in the background is from the DESI survey. Blue corresponds to the XMM-EPIC (0.5–2 keV) exposure-corrected image, while yellow and red correspond to the GMRT 325 (restoring beam of 15″× 15″) MHz and JVLA 1.5 GHz (restoring beam of 12.8″× 10.4″) respectively.
In the text

Fig. 2. Radio images of the central radio source in A795 obtained with briggs = 0.5. Left panel: GMRT 325 MHz image; the restoring beam of 15″ × 15″ is shown in gray. RMS is 520 μJy/beam. Right panel: JVLA 1.5 GHz image; the restoring beam of 12.8″ × 10.4″ is shown in gray. RMS is 35 μJy/beam. In both panels, the contours are drawn at 3, 6, 12, 24, 48 × RMS and the dotted circle marks the SW blob.

In the text

Fig. 3. XMM-EPIC (0.5–2 keV) exposure-corrected image Gaussian-smoothed with a kernel radius of 3 pixels shown on top, and below it, the optical image from the DESI survey. The cyan circle marks the position of an additional extended X-ray source (separate from the ICM of A795), see Sect. 3.2.2. The point sources detected are marked with white circles in left panel of Fig. 8. Top left panel: wide field view of A795. Top right panel: zoom-in on the extended X-ray source within the cyan circle in the left panel. The white contours are from the JVLA 1.5 GHz data (same as in right panel of Fig. 2). Bottom left panel: optical image from the DESI survey at the location of the extended X-ray source. The green squares mark the positions of four galaxies with similar spectroscopic redshift (from Rines et al. 2013), which are reported on the image. The light red contours are from the JVLA 1.5 GHz data (same as in right panel of Fig. 2). In each panel, the cyan circle has a radius of 1.5′, roughly corresponding to 200 kpc at the redshift of A795 (z = 0.1374).

In the text

Fig. 4. XMM-EPIC (0.5–2 keV) surface brightness profiles of the galaxy cluster A795 and the candidate galaxy group. Black points are the surface brightness measurements, while the green line represents the particle background. The lower panel shows the residuals from the best-fit model. Left panel: A795 profile fitted with a double β model. The best-fit parameters are reported in Table 2. Right panel: Surface brightness profile of the extended X-ray source 7.36′ northwest of A795, that we interpret as a candidate galaxy group at a similar redshift of A795 (see Sect. 3.2.2 and Fig. 3), fitted with a single β model. The best-fit parameters are reported in Table 2 (last row).

In the text

Fig. 5. Morphological analysis of the large-scale surface brightness asymmetries in A795, see Sect. 3.2.1. Left panel: XMM-EPIC (0.5–2 keV) residual image Gaussian-smoothed with a kernel radius of 9 pixels. The four regions mark the sectors used to extract the surface brightness profiles shown in the right panel. The inner and outer radii of the annulus correspond to 190 kpc and 1 Mpc from the X-ray peak, respectively. The two cold fronts detected by Ubertosi et al. (2021) are marked in green. Right panel: XMM-EPIC (0.5–2 keV) surface brightness profile of the four sectors. The blue points represent the SE sector which shows the excess.

In the text

	Fig. 6. Radio spectra of the three radio sources detected at the center of A795, see Sect. 3.1. In the bottom left corner, the spectral index α values are listed.
In the text

	Fig. 7. Spectral index map (left) and spectral index error map (right) measured at 325 MHz and 1.5 GHz of A795 zoomed on the central extended radio emission, with JVLA 1.5 GHz contours, defined in Fig. 2.
In the text

Fig. 8. Spectral analysis of A795. Left panel: XMM-EPIC (0.5–2 keV) surface brightness image Gaussian-smoothed with a kernel radius of 3 pixels. The green region was used for the global spectral analysis of the cluster A795, while the magenta one for the background. The white circles mark the position of the excluded sources and the cyan region signs the position of A795-B, see Sect. 4.3. Right panel: XMM-EPIC spectrum of A795 within the green region of the left panel, fitted with a thermal component and the background model. The red and black lines represent the models for MOS1 and MOS2 while the green one for pn. The lower panel shows the residuals from the best-fit curve. The best-fit parameters for the cluster thermal component are summarized in Table 6.

In the text

Fig. 9. Spectral analysis of A795-B, see Sect. 4.3. Left panel: XMM-EPIC (0.5–2 keV) surface brightness image Gaussian-smoothed with a kernel radius of 3 pixels. The yellow annular region was used for the spectral extraction of the cluster component, while the green circle marks the position of R500 = 1.02 Mpc of A795. The cyan region was used for the spectral extraction of A795-B. The white circles represent the point sources excluded. Right panel: XMM-EPIC spectrum of A795-B fitted with a thermal component and the background model accounting for the cluster emission. The red and black lines represent the models for MOS1 and MOS2 while the green one for pn. The lower panel shows the residuals from the best-fit curve. The best-fit parameters for A795-B’s thermal component are summarized in Table 6.

In the text

Fig. 10. Velocity distribution of member galaxies in A795. Left panel: histogram of the spectroscopic redshift (bottom x-axis) for 179 member galaxies of A795, measured and reported in Rines et al. (2013). The top x-axis shows the peculiar velocity relative to the average redshift of A795, that is z = 0.1374. Vertical dotted lines mark the central galaxy of A795-B (yellow), the average of A795 (red), and the BCG of A795 (green). The black solid line is the best-fit with a double Gaussian of the histogram, with the two components shown as blue and red dashed lines. The vertical dashed-dotted lines show the central velocity of each gaussian, while the shaded colored area represents the 1σ width of each Gaussian. Right panel: spatial distribution in RA and Dec of the member galaxies of A795 from Rines et al. (2013), color coded based on their peculiar velocity. Galaxies represented by red (blue) circles have peculiar velocity within ±1σ from the corresponding average of z = 0.1388 (z = 0.1342) shown in the left panel. Black + signs represent galaxies whose peculiar velocity falls outside the ±1σ range of both Gaussians in the left panel. The spatial position of the BCG of A795 and of the central galaxy of A795-B is highlighted, and both are plotted as with a black + sign.

In the text

	Fig. 11. XMM-EPIC (0.5–2 keV) residual image Gaussian-smoothed with a kernel radius of 9 pixels of A795’s central region with GMRT 325 MHz contours (left) and JVLA 1.5 GHz contours (right), defined in Fig. 2.
In the text

Fig. A.1. Radio images of the central radio source in A795 obtained with briggs = 2. Left panel: GMRT 325 MHz image; the restoring beam of 23″ × 19″ is shown in gray. RMS is 450 μJy/beam. Right panel: JVLA 1.5 GHz image; the restoring beam of 16.2″ × 12.5″ is shown in gray. RMS is 27 μJy/beam. In both panels, the contours are drawn at 3, 6, 12, 24, 48 × RMS and the dotted circle marks the SW blob.

In the text

Fig. A.2. Radio images of the central source in A795 with a cut at baselines < 7kλ made to select only the BCG emission. Left panel: GMRT 325 MHz image obtained with briggs = 0.5 of the central region of A795. The restoring beam of 7.8″ × 6.3″ is shown in gray. RMS is 480 μJy/beam. Right panel: JVLA 1.5 GHz image obtained with briggs = 0.5. The restoring beam of 9.1″ × 7.8″ is shown in gray. RMS is 54 μJy/beam. In both panels, the contours are at levels 3, 6, 12, 24, 48 RMS.

In the text

	Fig. A.3. Wide field spectral index map of A795. The white ellipse marks the position of the extended radio emission cospatial with the BCG. The cyan dashed circle signs an additional extended X-ray source (separate from the ICM of A795), see Sec. 3.2.2.
In the text