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Issue
A&A
Volume 690, October 2024
Article Number A142
Number of page(s) 6
Section Astrophysical processes
DOI https://doi.org/10.1051/0004-6361/202451018
Published online 04 October 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1. Introduction

HESS J1702-420 is a very high-energy (VHE) complex detected by the High Energy Spectroscopic System (H.E.S.S.) in the TeV energy band during the first Galactic plane survey campaign (Aharonian et al. 2006a). The complex was further studied in Aharonian et al. (2008), which reported the extended morphology of the source and the first measurements of its spectral parameters.

Recent H.E.S.S. observations (Abdalla et al. 2021) suggest that the morphology of the complex is changing with energy. While at low energies (≲2 TeV) the source is clearly characterised by a diffuse morphology, at the highest energies (≳10 TeV) the morphology of the source is consistent with a point-like one. Such an energy-dependent morphology is consistent with a superposition of a point-like source HESS J1702-420A with a hard ΓA = 1.53 ± 0.2 power law spectrum and a much softer ΓB = 2.62 ± 0.2 extended (∼0.3°) source HESS J1702-420B with an elliptical morphology.

Peculiarly, no direct counterparts of these sources were detected at lower energies. The dedicated analysis of 10–900 GeV Fermi/LAT data reported in Abdalla et al. (2021) resulted only in upper limits on the fluxes of both extended and point-like components. XMM-Newton observations of the HESS J1702-420 region did not result in a detection of an X-ray counterpart of the HESS J1702-420A source, but allowed Giunti et al. (2022) to put constraints on its flux. We note that the size of HESS J1702-420B exceeds the field of view (FoV) of XMM-Newton, which prevented Giunti et al. (2022) from constraining this source.

While, as was suggested by Abdalla et al. (2021), HESS J1702-420A and HESS J1702-420B can have an independent origin, in our recent paper (Aharonian et al. 2023) we suggested that both sources are manifestations of a single phenomenon. To explain the TeV emission from HESS J1702-420 complex, we proposed a model in which VHE protons are accelerated in (or close to) the point-like source HESS J1702-420A and injected into a dense surrounding neutral hydrogen cloud with a characteristic size of ∼0.3° and density of n0 ∼ 100 cm−3. The observed emission originate from pion-decay process and observed morphological changes correspond to the transition in the protons’ propagation regime. The low-energy protons are propagating in a strongly diffusive regime, and consequently characterised by a quasi-isotropic angular momenta distribution. The pion decay emission from protons propagating in this regime result in an extended TeV source HESS J1702-420B. On the contrary, the high-energy protons are propagating in a (quasi) ballistic regime and characterised by a delta-function like distribution of angular momenta. In this regime, the observed γ-ray emission is characterised by a point-like morphology. A similar model was proposed earlier by Chernyakova et al. (2011) to describe the spectral behaviour of the Galactic centre in the GeV energy band.

To explain the observed spatial and spectral behaviour of HESS J1702-420A and HESS J1702-420B sources simultaneously, Aharonian et al. (2023) proposed that the region is characterised by a diffusion coefficient, D(Ep) = D0(Ep/1 TeV)β, which is essentially suppressed at low energies in comparison to the interstellar medium (D0 ∼ 1026 cm2/s at 1 TeV) but with a strong energy dependence (β ≳ 1) that results in the propagation of the highest-energy protons (E ≥ 100 TeV) in the ballistic regime. The detected fluxes of gamma rays require a powerful proton accelerator with an injection rate at the level of Q0 ∼ 1038(n0/100 cm−3)−1 erg/s.

The secondary electrons produced as a result of the pion-decay process simultaneously with TeV photons are expected to emit synchrotron radiation, which is potentially detectable in the X-ray band. This radiation is expected to be characterised by a hard (Γ ∼ 2) X-ray spectrum and a flux of F0 ∼ (2..3)⋅10−13 erg/cm2/s at 1 keV for a broad range (10 − 100 μG) of considered magnetic fields in the region (Aharonian et al. 2023). The spatial scale of the emission is expected to significantly exceed the size of the HESS J1702-420 complex and be of a degree scale for preferable (∼kpc) distances to this complex.

Alternatively, the sources could have an independent origin with the TeV emission produced by a population of relativistic electrons present in the region and emitting inverse Compton (IC) in a soft-photon radiation field (Abdalla et al. 2021; Giunti et al. 2022). A search of the X-ray counterpart of a point-like HESS J1702-420A source for a model of this type was performed by Giunti et al. (2022) with XMM-Newton. This study did not result in a significant detection of a point-like X-ray counterpart, but allowed the magnetic field in the vicinity of HESS J1702-420A to be constrained at ≲1.5 μG. At the same time, the narrow FoV of XMM-Newton did not allow Giunti et al. (2022) to perform searches for the counterpart of the diffuse source HESS J1702-420B.

In this work, we have performed a search for an extended emission around the HESS J1702-420A position, an expected X-ray counterpart of the HESS J1702-420 complex. For these purposes, we utilised the DR1 dataset (Merloni et al. 2024) of the eROSITA satellite (Predehl et al. 2021), in which the data was taken within the first six months of the eROSITA operation. The publicly available data fully covers the western part of the Galactic hemisphere (Merloni et al. 2024), and thus is perfectly suitable for searches of the large-scale diffuse emission.

2. Data analysis

For the analysis, we utilised all publicly available eROSITA DR1 data (data processing version 010), analysed it with eSSAS4DR1 and heasoft-6.31, and performed spectral analysis with XSPEC v.12.13.0c software. We selected for the analysis 19 eROSITA sky tiles1 overlapping with a 5°-radius circle around HESS J1702-420A’s position.

In the initial step of our analysis2, we build the count map of the analysed region in the 0.3–10 keV energy band with the help of evtool routine. The corresponding map is shown in the left panel of Fig. 1 overlaid with green and magenta ellipses illustrating the positions and spatial extent in the TeV band of HESS J1702-420A and HESS J1702-420B sources, respectively. Aiming in searches for the potentially largely spatially extended (suggested to be of a degree-scale Aharonian et al. 2023) emission from the X-ray counterpart(s) of these sources, for the analysis we explicitly masked all bright X-ray sources detected in the region. Namely, we performed a search of sources detected at up to a level of ≳3σ above the local background with the help of erbox and created a mask of the region, masking all sources above the detection likelihood threshold equal to five with the help of erbackmap routines. The count map of the analysed region with the applied exclusion mask is shown in the right panel of Fig. 1.

thumbnail Fig. 1.

eROSITA count map of HESS J1702-420 region (in galactic coordinates). Green and magenta ellipses in both panels indicate the positions and TeV sizes of HESS J1702-420A and HESS J1702-420B sources. The right panel shows the region with the bright nearby X-ray sources masked for the searches of the diffuse emission from the X-ray counterpart of the HESS J1702-420 TeV complex. The dashed yellow circle of 3°-radius is centred on the HESS J1702-420A position and illustrates the maximal radius of the ON region used for the searches. The magenta truncated annulus in the right panel illustrates the shape and extent of the corresponding OFF region.

We note that there are no visible indications of the presence of a large spatial-scale residual emission3 centred in the vicinity of HESS J1702-420 in Fig. 1. In order to justify this qualitative statement, we performed a dedicated spectral analysis, aiming either to detect the extended emission in the region or to put constraints on its flux.

2.1. Hadronic model: Secondary electrons’ synchrotron emission

In this subsection, we explicitly assume that the X-ray emission originates from the synchrotron emission of the secondary electrons produced in the hadronic model Aharonian et al. (2023). In this case, the expected X-ray signal is characterised by a hard spectrum, dN/dE  ∝  E−2 (Aharonian et al. 2023), and the analysis described below was performed for this spectral index.

For the spectral analysis, we selected an ‘ON’ region of a certain radius, RON (see Tab. 1), centred on the position of HESS J1702-420A (ℓ = 344.15°, b = −0.15°) and corresponding to the possible size of the X-ray counterpart of the TeV source HESS J1702-420. For the estimations of the background flux, we utilised the ‘OFF’-region of the ‘truncated annulus’ shape. The borders of the OFF-region are given by an annulus centred on the HESS J1702-420A position with the inner radius Rin = RON and outer radius ROFF = 1.7 RON. Aiming to minimise the impact of possible background variations due to somewhat different average galactic latitudes of ON and OFF regions, we additionally cut the OFF-region annulus at minimal and maximal galactic latitudes of the ON region. The characteristic shapes of ON and OFF regions are illustrated in the right panel of Fig. 1 with a dashed yellow circle and magenta truncated annulus, respectively. The exposures of the spectra vary from ∼0.3 ksec for RON = 0.07° to ∼10 ksec for RON = 3° region.

Table 1.

Best-fit parameters of the astrophysical background model used to describe the data within ON and OFF regions (see text for details).

The widely used ‘ON-OFF’ approach for the estimation of the presence of the signal in the ON region relies on the assignment of the OFF-region spectrum as a background for the ON-spectrum and subsequent fitting of the resulting residual spectrum with a certain model. However, such an approach relies strongly on the assumption of an absence of variations in the parameters of the background emission in ON and OFF regions. Such an assumption, however, could fail if ON and/or OFF regions are significantly extended. In this case, the astrophysical background emission in ON and OFF regions could be characterised by somewhat different parameters of the medium (e.g. neutral hydrogen column density and/or temperature of the interstellar plasma), resulting in a biased (over- or under-subtracted) residual ON-OFF spectrum.

We demonstrate the presence of such an issue with the ON-OFF analysis of the HESS J1702-420 vicinity for the regions with a characteristic scale of ≳1°-radius. The left panel of Fig. 2 illustrates the ON-OFF (background subtracted) spectrum for the RON = 0.5°-radius size of the ON region (top sub-panel) and residuals with respect to the best-fit absorbed power law model of the signal ( tbabs*powerlaw in terms of XSPEC4 with an index fixed to 2). The residual spectrum is statistically consistent with 0, does not demonstrate any energy-depended features, and can be used to derive the unbiased upper limits on the normalisation of the signal’s model. The right panel of Fig. 2 shows a similar ON-OFF spectrum for RON = 1°. In this case, the spectrum is characterised by a wiggle-like structure with a strong over-subtraction at ≲2 keV energies and under-subtraction in the energy range of ∼2 − 3 keV. The ON-OFF spectrum shown is thus unsuitable for unbiased estimations of the strength of the signal. We argue that the observed wiggle-like structure is connected to variations in the neutral hydrogen density and/or the interstellar plasma temperature between ON and OFF regions; in other words, on ∼1° spatial scales.

thumbnail Fig. 2.

ON-OFF (background subtracted) eROSITA spectra of 0.5°-radius (left panel) and 1°-radius (right panel) regions centred on HESS J1702-420A with the residuals with respect to the best-fit absorbed power law model (bottom sub-panels). The right panel demonstrates the bias of the subtraction procedure due to the changes in nH between ON and OFF regions, see text for details and Fig. 3.

Thus, to derive robust constraints on the flux of the possible spatially extended source present in the region, instead of the ON-OFF approach we adopted a background-modelling technique that has been widely used; for example, in dark matter searches (see e.g. Thorpe-Morgan et al. 2020; Malyshev et al. 2022). Namely, instead of considering a difference in ON and OFF spectra, we focussed on joint spectral modelling of these quantities. We propose to model spectra of ON and OFF regions with a sum of the astrophysical and instrumental background models.

In the model of the astrophysical background, we included the following components: (i) a power law with the spectral index fixed to 2 (representing the possible signal from the X-ray counterpart of HESS J1702-420); (ii) low-temperature plasma (kT ∼ 0.2 keV, representing the contribution from the plasma local to the Solar System; (iii) high-temperature plasma (kT ∼ 0.8 keV, representing the hot plasma in the Milky Way); (iv) a power law with the index fixed to 1.4 (representing the contribution from the cosmic X-ray background De Luca & Molendi 2004). We additionally corrected the described model for interstellar neutral hydrogen absorption. In terms of XSPEC, the model is given by the apec + tbabs(powerlaw + apec + powerlaw) model. We note that the apec model component not convolved with the interstellar absorption represents the low-temperature plasma local to the Solar System.

In the model of the instrumental background, we included a broken power law and three narrow Gaussian lines at energies of ∼0.9 keV (Ne IX line), ∼1.8 keV (Si Kα/Si Kβ lines), and ∼6.4 keV (Fe Kα line) (see e.g. Malyshev et al. 2022 for a summary of the typical XMM-Newton instrumental lines). The instrumental model was not convolved with eROSITA’s effective area.

To derive the constraints on the flux of the extended source, we simultaneously modelled the spectra of ON and OFF regions with a sum of described models. We additionally required that (i) the temperature of the local plasma be identical for ON and OFF regions, and (ii) the flux of the astrophysical components of the model in ON and OFF regions be proportional to the not-masked area of the analysed region (based on the BACKSCALE keyword of the corresponding spectra). The flux from the possible X-ray counterpart of HESS J1702-420 was explicitly set to 0 in the OFF region. The characteristic spectra of ON (red points and curves) and OFF (black points and curves) regions, along with the proposed best-fit models and residuals with respect to these models, are shown in Fig. 3.

thumbnail Fig. 3.

eROSITA spectrum of 1° region. Red points and curve illustrate the spectrum and best-fit model for the ON region; black points and curves the same for the OFF region. The bottom panel illustrates residuals with respect to the selected models.

The best-fit values of the key parameters of astrophysical model for ON and OFF regions are summarised in Tab. 1. This table summarises the temperature of the plasma local to the Solar System (kTsol); temperatures of the interstellar plasma in ON and OFF regions (kTON and kTOFF, respectively); neutral hydrogen column density for ON and OFF regions (nH, ON and nH, OFF, respectively); and the limit on the flux of an extended source with the dN/dE  ∝  E−2 spectrum at 1 keV energy. The flux limit corresponds to the 2σ (∼95% c.l.) statistical limit and was calculated with the error 4.0 XSPEC command. The RON and ROFF columns indicate the radius of the ON region and the outer radius of the OFF region, respectively.

For the relatively small radii of the ON region, the statistics of the data does not allow firm measurement of the plasma temperatures and/or interstellar absorption. In these cases, we fixed corresponding parameters to the values indicated in Table 1 based on the values observed at larger RON. For the smallest RON = 0.07° corresponding to the size of the H.E.S.S. point spread function, the statistics of the data does not allow any firm measurement of the neutral hydrogen absorption. In this case, we explicitly considered two nH values – 0.7 ⋅ 1022 cm−2, indicated by the best-fit nH values at larger RON, and 1.7 ⋅ 1022 cm−2, suggested by the mean galactic nH value in the direction of HESS J1702-420.5 For this RON value, we indicate in the Table 1 two values for the flux limit and use in what follows the conservative limit.

The described models fit the data with the reduced χ2 ≲ 1 for all RON < 2°. For larger radii, we noticed a worsening of the fit, possibly originating from the presence of multi-temperature and/or variable nH sub-regions within the analysed area. To derive robust constraints in these cases, we considered a ‘systematics-driven approach’. We increased the systematics (added in quadratures) up to the level required to make the reduced χ2 of the fit be 1. The characteristic values of systematics in this case were ∼5% (see ‘Systematic’ column of Tab. 1).

The derived upper limits on the flux at 1 keV of the extended source centred on the position of HESS J1702-420A as a function of the radius of the source are shown in Fig. 4. The square red points indicate results from the ON-OFF analysis performed at R < 0.5°, where the bias caused by different plasma temperatures or nH values in ON and OFF regions can be neglected. Blue circle points present the results from the background modelling approach and correspond to the flux limit values indicated in Tab. 1. The dashed green line illustrates the characteristic flux of the extended source expected from the Aharonian et al. (2023) model. We note also a weak, statistically insignificant detection at a level of ∼1.5 σ of the signal at RON ∼ 0.7°, which somewhat weakened the flux limits at these radii. We note that the dedicated analysis limited only to cameras not affected by a light leak6 results in very similar upper limits on the flux of the extended source.

thumbnail Fig. 4.

eROSITA limits on HESS J1702-420 flux at 1 keV based on eROSITA-DR1 data. The green line illustrates the model-predicted flux from Aharonian et al. (2023).

2.2. Leptonic model: Primary electrons’ synchrotron emission

As an alternative to the hadronic model described in Aharonian et al. (2023), we considered a simple leptonic model. In this model, the observed diffuse TeV emission from HESS J1702-420B is explained as an IC emission of a power law population of electrons producing VHE emission on the Cosmic Microwave Background photons. The emission in the X-ray band in this model corresponds to the synchrotron emission from the same population electrons. The emission of the discussed model components was calculated with the naima v. 0.10.0 model (Zabalza 2015), which uses the approach of Aharonian et al. (2010), Khangulyan et al. (2014) to calculate synchrotron and IC components, respectively.

We derived the parameters of the relativistic electron population by fitting the described IC radiation model to the H.E.S.S. data on HESS J1702-420B. We found that this population is characterised by a relatively soft spectrum of Γe = 3.85 ± 0.15 and the total energy in electrons above 1 TeV is We = 2.6 ⋅ 1046 erg (see Fig. 5). The corresponding X-ray spectrum is characterised by an index of Γ ∼ 2.4 and a normalisation proportional to the square of the strength of the magnetic field in the region.

thumbnail Fig. 5.

H.E.S.S. spectrum (green points) and eROSITA flux upper limit for the HESS J1702-420B-shape region (blue circle, upper limit) and for the 1.5°-radius disc-shape (red square, upper limit) region. The components of the leptonic model are shown with solid (IC) and dash-dotted (synchrotron) blue lines. The synchrotron component is shown for the 95% c.l. excluded value of the magnetic field (∼1.5 μG); see Sec. 2.2 for the details. The components of the Aharonian et al. (2023) model are shown with solid (π0-decay) and dash-dotted (synchrotron emission from secondary electrons) red lines. The shaded region illustrates the expected scatter of X-ray fluxes in the Aharonian et al. (2023) model for B = 10 − 100 μG.

Assuming that the X-ray signal originates from a region identical to the spatial shape of HESS J1702-420B source7, we performed an analysis similar to the one described in Sec. 2.1. Given the relatively large size of the signal-extraction region, we focussed on the background modelling approach. Namely, we extracted the background spectrum from the elliptic-annulus region with the inner boundaries coinciding with the signal extraction region. The outer boundary of the background region was defined as an ellipse a factor of 1.5 larger than the signal region.

We found that, similar to Sec. 2.1, the data is well fitted by the described model with zero-level systematic uncertainty. The 95% c.l. upper limit on the flux at 1 keV for the discussed spectral index is ∼2.5 ⋅ 10−4 ph/cm2/s or ∼4 ⋅ 10−13 erg/cm2/s (red upper limit in Fig. 5). This upper limit on the flux corresponds to the 95% upper limit on the magnetic field in the considered region of B ≲ 1.5 μG.

To estimate the potential impact of systematic uncertainties on this value, we considered an alternative model in which the IC TeV emission is produced on an interstellar radiation field background Porter et al. (2008) (for R = 8 kpc ISRF model). We found that for this model the upper limit on the magnetic field is somewhat relaxed to B ≲ 1.7 μG. We note that for the models suggesting higher density of the soft photons’ background, one can expect even more relaxed limits on the magnetic field.

An additional source of systematic uncertainty could arise from the systematic uncertainty on the flux level of HESS J1702-420B measured by H.E.S.S., which could reach ∼20% (Aharonian et al. 2006b). This uncertainty corresponds to an additional ∼10% systematic uncertainty on the derived limit on the magnetic field in the region, putting it at a level of ≲2 μG.

3. Results and discussion

In this work, we present the results of the search for a possible X-ray counterpart of the HESS J1702-420 complex (Abdalla et al. 2021). In the TeV band, it is characterised by a peculiar energy-dependent morphology and could be described as a superposition of two sources - the point-like HESS J1702-420A and the significantly extended (∼0.3°) HESS J1702-420B. These sources demonstrate hard Γ ∼ 1.5 (HESS J1702-420A) and soft Γ ∼ 2.5 (HESS J1702-420B) power law spectra up to a few tens of TeV without cut-off indications.

According to the model proposed by Aharonian et al. (2023), the morphology and spectral behaviour of the HESS J1702-420 complex can be explained in terms of hadronic emission from the protons accelerated in the HESS J1702-420A source and propagating through the surrounding medium in an energy-dependent regime. The synchrotron emission from the secondary electrons produced in π0-decay is characterised by a hard Γ ∼ 2 spectrum (see Fig. 5) and could be potentially detected in the keV energy band. Aharonian et al. (2023) suggested that the characteristic spatial size of the X-ray counterpart of HESS J1702-420 significantly exceeds the TeV-band size of the HESS J1702-420B source and reach degree-scale values.

As an alternative to the Aharonian et al. (2023) model, we considered a simple leptonic model in which the observed diffuse TeV emission originate from an IC emission of the population of relativistic electrons on a soft radiation field present in the region. In this case, the X-ray emission is expected to be characterised by a softer slope of Γ ∼ 2.4 (see Fig. 5) and the flux level to be defined by the strength of the magnetic field in the region.

In our work, we performed a search for a counterpart in the eROSITA DR1 dataset covering the first six months of data collection. Our analysis indicates that HESS J1702-420 is located in a relatively crowded X-ray region (see Fig. 1) characterised by degree-scale gradients of the interstellar plasma temperature and/or neutral hydrogen column densities (see Tab. 1). Similarly to previous XMM-Newton observations (Giunti et al. 2022), we did not find point-like X-ray counterparts of HESS J1702-420A and HESS J1702-420B sources within TeV error-ellipses8 and focussed on searches for the largely extended emission in this region.

To constrain the hadronic model of Aharonian et al. (2023), we performed a search for a hard spectrum (Γ = 2), disc-like spatial morphology sources centred on the position of HESS J1702-420A. Our searches did not result in a significant detection of the emission in the 0.3–10 keV energy band for a source radius varying between 0.07° and 3°. Fig. 4 shows the derived flux limits at 1 keV as a function of the radius of the source. The red points illustrate the limits derived within the widely used ON-OFF, ‘background subtraction’ approach, while the blue ones stand for the background modelling method. The ON-OFF approach at larger radii could result in biased limits (see Fig. 2 and Sec. 2 for the details). The dashed green line in Fig. 4 indicates the expected flux level of the HESS J1702-420 counterpart predicted by the Aharonian et al. (2023) model.

The derived limits are significantly weaker than the flux level expected from Aharonian et al. (2023) model. This does not allow us to firmly confirm or reject the proposed model with the current eROSITA data. While this work is based only on the first six months of eROSITA data (DR1 data-release), about 2 years of eROSITA data will become public at the end of 2025.9 With the same all-sky monitoring strategy, this will increase the available exposure by a factor of four, and in the case of absent systematics could allow an improvement in the presented limits by a factor of two. Although this improvement is not significant enough to probe the flux range predicted by Aharonian et al. (2023), it could help to detect the source, if the model flux were underestimated by a factor of a few. An order-of-magnitude improvement of the current limits, required to reach the flux level from a few-degrees-scale source predicted by Aharonian et al. (2023), could be achieved (assuming only statistical uncertainty) with an exposure ∼100 times longer (see Fig. 5). Namely, such an improvement would require ∼106 s long observations with eROSITA or similar missions.

We note also that our analysis indicates variations in the neutral hydrogen density along the analysed region. The density is highest close to the HESS J1702-420A position with a trend of decreasing at larger distances (i.e. larger RON and ROFF radii; see Tab. 1). This is consistent with the Aharonian et al. (2023) model, suggesting that the TeV emission from the HESS J1702-420B source is connected with a dense molecular cloud ∼0.3° in size surrounding the VHE protons’ accelerator, HESS J1702-420A.

Our analysis performed for the leptonic model did not lead to the detection of X-ray emission in the spatial region coinciding in shape with the HESS J1702-420B source. We estimated the magnetic field strength in the region to be B ≲ 2 μG (including possible systematic uncertainties, which is in broad agreement with the characteristic strength of the interstellar magnetic field (Jansson & Farrar 2012). We note that the derived value is also consistent with the value of the magnetic field for the point-like source HESS J1702-420A derived by Giunti et al. (2022) (B ≲ 1.5 μG).

We argue that further advances in our understanding of possible X-ray emission from HESS J1702-420 could be reached, either with broad-FoV, large-effective-area future missions such as Athena/WFI (Meidinger et al. 2020), or with Msec-long observational campaigns with current-generation instruments (XMM-Newton or eROSITA) that will allow at least an order-of-magnitude improvement in the presented upper limits based on ksec-long observations of eROSITA.

3

Note, that the diffuse shell-like structure at (ℓ,b) = (347.3° , − 0.5° ) is a known GeV/TeV SNR HESS J1713.7-3946 (Aharonian et al. 2007).

4

With “wilms” abundances (Wilms et al. 2000).

7

An ellipse, centred at (ℓ,b) = (344.29° , − 0.15° ), semi-axes (0.32° ,0.2° ), and rotation angle of 157°.

8

The closest sources are located ∼0.1° and ∼0.25° away from HESS J1702-420B ellipse and can be associated with SNR G344.7–00.1 and HMXB OAO 1657–41, respectively.

Acknowledgments

The authors acknowledge support by the state of Baden-Württemberg through bwHPC.

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All Tables

Table 1.

Best-fit parameters of the astrophysical background model used to describe the data within ON and OFF regions (see text for details).

In the text

All Figures

thumbnail Fig. 1.

eROSITA count map of HESS J1702-420 region (in galactic coordinates). Green and magenta ellipses in both panels indicate the positions and TeV sizes of HESS J1702-420A and HESS J1702-420B sources. The right panel shows the region with the bright nearby X-ray sources masked for the searches of the diffuse emission from the X-ray counterpart of the HESS J1702-420 TeV complex. The dashed yellow circle of 3°-radius is centred on the HESS J1702-420A position and illustrates the maximal radius of the ON region used for the searches. The magenta truncated annulus in the right panel illustrates the shape and extent of the corresponding OFF region.
In the text
thumbnail Fig. 2.

ON-OFF (background subtracted) eROSITA spectra of 0.5°-radius (left panel) and 1°-radius (right panel) regions centred on HESS J1702-420A with the residuals with respect to the best-fit absorbed power law model (bottom sub-panels). The right panel demonstrates the bias of the subtraction procedure due to the changes in nH between ON and OFF regions, see text for details and Fig. 3.
In the text
thumbnail Fig. 3.

eROSITA spectrum of 1° region. Red points and curve illustrate the spectrum and best-fit model for the ON region; black points and curves the same for the OFF region. The bottom panel illustrates residuals with respect to the selected models.
In the text
thumbnail Fig. 4.

eROSITA limits on HESS J1702-420 flux at 1 keV based on eROSITA-DR1 data. The green line illustrates the model-predicted flux from Aharonian et al. (2023).
In the text
thumbnail Fig. 5.

H.E.S.S. spectrum (green points) and eROSITA flux upper limit for the HESS J1702-420B-shape region (blue circle, upper limit) and for the 1.5°-radius disc-shape (red square, upper limit) region. The components of the leptonic model are shown with solid (IC) and dash-dotted (synchrotron) blue lines. The synchrotron component is shown for the 95% c.l. excluded value of the magnetic field (∼1.5 μG); see Sec. 2.2 for the details. The components of the Aharonian et al. (2023) model are shown with solid (π0-decay) and dash-dotted (synchrotron emission from secondary electrons) red lines. The shaded region illustrates the expected scatter of X-ray fluxes in the Aharonian et al. (2023) model for B = 10 − 100 μG.
In the text

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