EDP Sciences logo
Subscriber Authentication Point
Search
Menu
Home All issues Volume 698 (May 2025) A&A, 698 (2025) A321 Full HTML
Open Access
Issue
A&A
Volume 698, May 2025
Article Number A321
Number of page(s) 13
Section Astrophysical processes
DOI https://doi.org/10.1051/0004-6361/202452230
Published online 23 June 2025

© The Authors 2025

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.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1. Introduction

A white dwarf (hereafter WD) is a common endpoint of the stellar evolution for > 90% of main-sequence stars. The cataclysmic variable (CV) is a binary system composed of a WD primary and a low-mass main-sequence star (Warner 1995), and the orbital period is usually less than one day. The secondary fills its Roche-lobe, and hence the CVs are known as a binary system operating an accretion process. The CVs are usually observed with a transient optical outburst (dwarf nova) due to the disk instability and show a steady X-ray emission due to the accretion onto the WD surface. Because of its large population, the CVs are one of the major X-ray sources in the Galaxy.

Increasing X-ray population sources provide an opportunity to carry out identification for new CVs by cross-matching the X-ray source catalogs and optical source catalogs. An X-ray survey for the CVs is important for identifying the magnetic CVs (e.g., Lutovinov et al. 2020), for which the optical surveys alone may be difficult and inefficient for identification because of their lack of frequent outburst behavior. The increasing in the CVs identified in X-rays will overcome the bias of the optical survey in the population of the CVs. Moreover, identifying the X-ray-emitting CVs will help us to exclude the CV candidates from the list of new types of compact X-ray sources, such as the emission from the isolated black hole in the Galaxy (Kimura et al. 2021).

Dedicated studies for identifying the X-ray-emitting CVs have been conducted in previous studies. For example, Drake et al. (2014) searched for CVs from the Catalina Real-time Transient Survey and identified 855 CV candidates. They cross-matched their CVs with the ROSAT source catalog and found 42 X-ray-emitting CVs or their candidates. Takata et al. (2022) cross-matched the X-ray sources in the catalogs (ROSAT, Swift, and XMM-Newton) and the Gaia sources that are located in the bridge region between the main sequence and the WD’s cooling sequence in the Hertzsprung-Russell diagram. They determined the orbital characteristics for eight X-ray CV candidates using the photometric light curves in the optical bands. Galiullin et al. (2024a) searched for new CVs in Chandra Source Catalog v2.0, cross-matching with Gaia data release 3 (DR3). They report 14 new CV candidates and confirm the CV nature of four CV candidates in the follow-up optical observations. These results show that the conduct of a joint X-ray and optical study is a useful tool for finding new CVs and their candidates.

The extended Roentgen Survey with an Imaging Telescope Array (hereafter eROSITA) and Mikhail Pavlinsky Astronomical Roentgen Telescope–X-ray Concentrator (ART-XC telescope) of the Spectrum-Roentgen-Gamma (SRG) mission began a new all-sky survey in 2019 (Sunyaev et al. 2021). Zaznobin et al. (2022) identified three X-ray-emitting CVs using eROSITA and the ART-XC telescope with optical follow-up observations. Schwope et al. (2022) reported the discovery of an eclipsing polar, eRASSt J192932.9–560346/Gaia21bxo, which was identified through SRG/eROSITA and Gaia transient surveys. They also confirmed the orbital period of this polar to be ∼92.51 minutes. Muñoz-Giraldo et al. (2023) used the XMM-Newton and eROSITA observations to confirm the X-ray emissions from three candidates of the period-bounce CVs (V379 Vir, SDSS J151415.65+074446.5, and SDSS J125044.42+154957.4) with a luminosity of LX ∼ 1029 erg s−1, suggesting the presence of the accretion process in these binary systems. Rodriguez (2024a) conducted a follow-up optical observation for SRGeJ045359.9+622444, which is a new eclipsing AM CVn system, discovered from joint eROSITA and Zwicky Transient Facility (hereafter ZTF, Masci et al. 2018) observations to identify the CVs and confirmed the orbital period of ∼55.08 minutes. Finally, Galiullin et al. (2024b) also utilized the joint eROSITA and ZTF search and discovered an eclipsing CV (SRGeJ041130.3+685350). These studies demonstrate the potential of eROSITA observations to identify new X-ray-emitting CVs.

The X-ray source catalogs of the observation by the SRG observatory enable us to carry out an extensive search for new CVs and their candidates. Rodriguez et al. (2023) cross-matched the Final Equatorial Depth Survey (eFEDS) sources of eROSITA and the photometry catalog of the ZTF observation, and they identified two new polars. Schwope et al. (2024a) also used the catalog of the eFEDS survey and identified 26 CVs through the follow-up observation by the Sloan Digital Sky Survey. Sazonov et al. (2024) reported the catalog of 469 Galactic sources measured by the SRG/ART-XC telescope, in which 192 sources belong to previously identified CVs or the candidates of CVs. Rodriguez et al. (2025) cross-matched eROSITA-DE Data Release 1 (DR1) and Gaia DR3 to identify X-ray-emitting CVs located within 150 pc. They estimate that the number density of the CVs in the solar neighborhood is ρN ∼ (3.7 ± 0.7)×10−6 pc−3. Finally, Schwope et al. (2024b) reported a characteristic of ∼400 CVs and its candidates selected from eROSITA-DE DR1, which significantly increases the population of CVs identified by the X-ray observations.

The sky survey of eROSITA is about 25 times more sensitive than the previous ROSAT all-sky survey (Predehl et al. 2021) and covers the whole Galaxy. With its high sensitivity level and large sky region, the eROSITA survey offers a new opportunity to identify numerous X-ray-emitting CVs. In this study, therefore, we aim to search for the X-ray-emitting CVs and their candidates in the eROSITA source catalog. We selected the eROSITA sources by cross-matching with the Gaia sources that are potential candidates of CVs based on the color–magnitude diagram. This cross-matching process is similar to that of a study carried out by Rodriguez et al. (2025). In our study, we further searched for a periodic signal in photometric data of the selected targets taken by ZTF and The Transiting Exoplanet Survey Satellite (TESS, Ricker et al. 2015). We also studied how the eROSITA survey enhances identification of the X-ray-emitting CVs. In Sect. 2, we describe the data reduction and method used to search for the periodic modulation in the optical light curve. In Sect. 3, we present the results of the survey for the X-ray-emitting CVs. We summarize our results in Sect. 4.

2. Sample selection and analysis process

In this section, we present the sample selection and process used to identify the candidates of the X-ray-emitting CVs. An outline of the process is listed below.

  1. Selection of the Gaia sources from the bridge region between the main sequence and the WD’s cooling sequence in the Hertzsprung-Russell diagram (Sect. 2.1).

  2. Cross-matching the selected Gaia sources and eROSITA catalog (Sect. 2.2).

  3. Timing analysis of the light curves measured by ZTF and TESS observations (Sects. 2.3 and 2.4).

  4. Evaluation of the possibility of other types of X-ray sources (Sects. 3.4 and 3.5).

2.1. Gaia data

The Gaia DR3 provides photometric data for about 1.8 billion sources in the G band, and for roughly 1.5 billion sources in the GBP and GRP bands. Additionally, it includes astrometric information such as positions, parallaxes, proper motions, and so on (Rimoldini et al. 2023). We downloaded the data from the Gaia Archive1 using astroquery (Ginsburg et al. 2019).

It is known that many CVs are located in the region between the main sequence and the WD’s cooling sequence on the Hertzsprung-Russell diagram. We therefore selected the Gaia source by limiting (i) a color to 0.5 < GBP − GRP < 1.5 – where GBP and GRP are blue and red magnitudes defined by the Gaia photometric system, respectively; and (ii) a magnitude of 9 < MG < 12, as indicated in Fig. 1. We limited the search region to obtain a manageable sample size. We applied the following additional conditions in order to select clean samples (Lindegren et al. 2018):

  1. parallaxovererror > 5

  2. photbpmeanfluxovererror > 8

  3. photrpmeanfluxovererror > 10

  4. astrometric_excess_noise < 1

  5. phot_bp_rp_excess_factor < 2.0+0.06*power(photbpmeanmag-photrpmeanmag,2)

  6. photbprpexcessfactor > 1.0+0.015*power(photbpmeanmag-photrpmeanmag,2)

  7. visibilityperiodsused > 5.

We obtain a list of 121 488 Gaia sources.

thumbnail Fig. 1.

Hertzsprung-Russell diagram with Gaia sources. The red square indicates the region from which we selected the Gaia sources.

2.2. Cross-matching with eROSITA catalog

The eROSITA-DE Data Release 1 (DR1) comprises data from the first six months of the SRG/eROSITA all-sky survey (hereafter eRASS1) and includes 930 203 sources located at the Galactic hemisphere (Merloni et al. 2024). We cross-matched the sources in the eROSITA catalog and the 121 488 Gaia sources that are described in Sect. 2.1. We selected the closest Gaia source located within a 10″ radius, which is the typical angular resolution of the eROSITA observation (Predehl et al. 2021; Merloni et al. 2024), from the center of the Gaia sources. We also checked the distribution of the angular separation between the eROSITA position and the Gaia position for the identified CVs presented in Schwope et al. (2024b). As indicated in Fig. 2, more than 90% of the sources have an angular separation under 10″. Our cross-matching process selected 264 Gaia/eROSITA sources.

thumbnail Fig. 2.

Distribution of angular separation between eROSITA position and Gaia position for the identified CVs. The data were taken from Schwope et al. (2024b).

We cross-matched the selected Gaia/eROSITA sources with the SIMBAD astronomical database2 (Wenger et al. 2000), the International Variable Star Index 3 (hereafter VSX), the Open Cataclysmic Variable Catalog 4 (hereafter OCVC), and the Ritter Cataclysmic Binaries Catalog 5 (hereafter RK, Ritter & Kolb 2003), which list numerous CVs and CV candidates. We divided the selected Gaia/eROSITA sources into two categories: (i) identified CVs, which are sources listed as CVs in SIMBAD, VSX, OCVC and RK databases; and (ii) others, which are referred to as CV candidates in this paper (Table 1). We note that many VSX-listed sources are discovered by an optical transient survey; for example, the Catalina Real-Time Transient Survey (Drake et al. 2009) and All-Sky Automated Survey for SuperNovae (Shappee et al. 2014). Hence, our CV candidates listed in VSX are more likely CVs. Additionally, some CV-candidate sources are listed as WDs in the primary or secondary category of the SIMBAD database, suggesting they are accreting WD candidates.

Table 1.

Cross-matching for X-ray catalogs with 121 488 Gaia sources.

We note that in our cross-matching process with 121 488 Gaia sources, two eROSITA sources have multiple (two) Gaia sources located within a 10″ search radius. 1eRASS J170454.8–291337, which is categorized as a CV candidate, has potential Gaia counterparts DR3 6029819936903977856 and 6029819936840722048, which are separated by 7.53″ and 7.69″, respectively, from the eROSITA source. The latter Gaia source is a young stellar object candidate in the SIMBAD catalog. Consequently, we cannot exclude the possibility that this eROSITA source is the X-ray counterpart of this Gaia source. 1eRASS J180018.7–353311 (CV) also has two counterparts, Gaia DR3 4041983178121822976 and 4041983178121844224, which are separated by 4.60″ and 5.22″ respectively, both of them corresponding to TCP J18001854–3533149, a CV identified in VSX. Since Gaia DR3 4041983178121844224 is closer to TCP J18001854–3533149, we anticipate this Gaia source as the counterpart of this eROSITA source. In addition, if we cross-match our eROSITA sources with all Gaia DR3 sources, 153 eROSITA sources have multiple Gaia sources with, on average, six possible counterparts; the maximum is 37 Gaia sources. Hence, despite the fact that the expected X-ray luminosity and the optical-to-X-ray-flux ratios of most CV candidates selected in this study are consistent with identified CVs (Section 3.4), multiwavelength follow-up observations are necessary to confirm their nature.

2.3. ZTF observation

One of the characteristics of the CVs is the orbital modulation in the optical photometric light curve caused by an eclipse of one star by another or the elliptical shape of the companion star or the heating of the surface of the companion star by irradiation from the WD or disk. Another characteristic is outbursts (dwarf nova) due to an instability of the accretion disk of nonmagnetic CVs. We focused on searching for CV candidates by identifying the possible signal of the orbital modulation in the optical light curves and confirmed the binary nature of our CV candidates. To perform a timing analysis, the cadence of the Gaia observations are insufficient to search for the periodic modulation in the light curves. In this study, therefore, we used the photometric light curves taken by ZTF6 (Masci et al. 2018) and/or TESS7 (Ricker et al. 2015).

We cross-matched the selected eROSITA/Gaia sources with the ZTF DR-21 objects and searched for a periodic modulation in the photometric light curve. We created a Lomb-Scargle periodogram (hereafter LS, Lomb 1976) using the python package LombScargle in astropy (Astropy Collaboration 2022). We evaluated the false-alarm probability of the periodic signal with the methods of Baluev (2008) and using the bootstrap of VanderPlas (2018). We find that the ZTF observations for most of our targets provide only several hundred data points, and the quality of the light curve is insufficient to evaluate the periodic signal. Additionally, a significant portion of our targets are located outside the field of view of the ZTF observations. The ZTF observation only covers 50 sources among ∼180 CV candidates, in which 34 sources have enough (> 200) data points for a timing analysis. Hence, conducting a detailed search for periodic signals using ZTF data is challenging for most cases. For period obtained from ZTF/TESS observations of CV candidates presented in this study, we denote the uncertainty of the frequency estimated from the Fourier resolution of the observation (namely, the inverse of the time span covered by the observation).

2.4. TESS observation

Because of the space-based observation, TESS covers a large area of the sky and provides the photometric data for the region around most of the targets examined in this study. One sector of the TESS observations provides a light curve of about 27 days, which enabled us to carry out a detailed investigation for the periodic modulation of a timescale of one day. Moreover, the different sectors of the TESS observations have covered one sky region, enabling us to distinguish true periodic signal from a spurious one. On the other hand, the angular resolution covered by one pixel is on the order of 20″. With such an insufficient angular resolution, one pixel may contain several optical sources. Consequently, even when a periodic signal is detected in the photometric light curve corresponding to a pixel that includes our target, we cannot definitively attribute the signal to the target itself.

For each target, we examined all available TESS light-curve files and/or full-frame images (FFIs). We utilized the TESS analysis tool Lightkurve (Cardoso et al. 2018) to analyze the FFIs, and cut the image into 10 × 10 pixels (Fig. 3). We extracted the light-curve information of each pixel to search for the periodic modulation in the LS periodogram: for FFIs, we were unable to extract the meaningful light curves from some pixels due to the insufficient data quality (e.g., NAN flux value). To enhance the likelihood that the observed signal is associated with our target, we applied the following criteria:

  1. The LS power should be at least double the power associated with a false alarm probability (FAP) of 0.01. This will ensure that the signal stands out significantly against the background noise.

  2. The signal is only detected from pixels around the target (shaded region in Fig. 3). This may avoid the possibility of the detected signal originating from either the contamination of a nearby brighter source or the spurious signal.

thumbnail Fig. 3.

TESS full-frame images from sector 39 for region around Gaia DR3 5822540653269409408. The LS periodogram was created for each pixel in the figure, and the periodic signal only detected pixels around the target (indicated by red square) is considered as a possible signal from the target.

Figures 3 and 4 present an example of the identification of a CV candidate using TESS data. Figure 3 shows the TESS FFI from sector 39 centered on the source Gaia DR3 5822540653269409408, which is selected as an optical counterpart of 1eRASS J155030.1–654403. For each pixel in the figure, we created an LS periodogram in a frequency range of 1–100 day−1, which typically corresponds to the period of the observed CVs. In the LS periodogram for the light curve extracted from the central region in FFIs, we identify a strong period signal at ∼3.13(3) day−1, as shown in Fig. 4, with an LS power level that is twice as high; that is, corresponding to FAP = 0.01. We also identify the periodic signal using FFIs from sectors 12, 65, and 66, suggesting this periodic signal is likely related to an astrophysical source. If the periodic signal is indeed related to the Gaia source, the orbital signal of the accreting WD candidates will be the most natural explanation, since the Gaia source is selected from the bridge region between the main sequence and the WD’s cooling sequence in the H-R diagram (Fig. 1). We classify this source as a CV candidate, since no record of this source is found in the SIMBAD, VSX, OCVC, and RK databases. The parallax measured by Gaia suggests the distance from Earth to this source is 570 pc. The eROSITA observation suggests that the luminosity in 0.2 − 8.0 keV of this source is ∼1.6 × 1030 erg s−1, which is also a typical value for CVs. Finally, if we cross-match the eROSITA source with all Gaia sources, the 1eRASS J155030.1–654403 has five Gaia sources within a radius of 10″. None of those Gaia sources are listed in SIMBAD or other databases used in this study.

thumbnail Fig. 4.

LS periodogram (left panel) and folded light curve (right panel) with TESS data for Gaia DR3 5822540653269409408, which was selected as the counterpart of 1eRASS J155030.1–654403. The dashed black line and the dashed-dotted black line are (FAP = 0.01) determined by the methods of Baluev (2008) and the bootstrap of VanderPlas (2018), respectively. The LS diagram reveals an ∼3.13(3) day−1 modulation and its first harmonic. Two period cycles of the light curve are presented for clarity.

We note that each target is usually covered by several sectors with different readout times of the TESS observations. This helps us to distinguish a true signal from aliasing signals caused by the effect of the readout time. If the periodic signal is only confirmed at one sector, as described below, we cannot discriminate between true signal and the aliasing signal. In such cases, we report a longer one, which is usually on the order of hours and is closer to the typical orbital period of the CVs. We find that several sources only show the significant periodic signal at one sector and no signal in other sectors. One possibility related to such a transient periodic signal is a superhump, which is a periodic variation in the emission observed from an eccentric disk after the outburst of CVs (Warner 1995). For example, three sectors of the TESS observations cover the region of Gaia 5710755475028251776/1eRASS J075656.3–231557, but only data of sector 34 shows significant modulation, with a period of ∼0.0892(4) days. This period is likely period of the superhump after the outburst happened in early 2021. CBA Extremadura Observatory observations carried out on 2021 February 28 also confirm the period of the superhump of 0.0970 ± 0.0027 days8, which is close to the value of the TESS observation. Since the superhump period typically varies by a few percent compared to the orbital period, we consider that the observed modulation could indicate the actual orbital period.

2.5. Other X-ray observations

We also used the X-ray catalogs of the XMM-Newton (DR13, Webb et al. 2020), Swift (Second Swift-XRT Point Source (2SXPS) Catalog, Evans et al. 2020), and ROSAT (Second ROSAT all-sky survey (2RXS) source catalog, Boller et al. 2016). The error on the position of the XMM-Newton observations can be smaller than a few arcseconds for on-axis observations, but it exceeds 10″ for off-axis observations. The typical position errors of the 2SXPS catalog and 2RXS catalog are 5.6″ and greater than 7″, respectively. To maintain consistency with the cross-matching process of the eROSITA catalog, we selected the X-ray sources located within 10″ of the center of Gaia’s position. By cross-matching with the Gaia sources, we selected 109 XMM-Newton sources, 111 Swift sources, and 69 ROSAT sources. Out of these, 44 from XMM-Newton, 91 from Swift, and 59 from ROSAT are identified CVs (Table 1). As we expected, the eROSITA observation alone can identify more identified CVs and their candidates compared to other X-ray observations.

3. Results

We selected 121 488 Gaia sources that are selected according to the conditions described in Sect. 2.1. Table 1 shows the results of the cross-matching for four X-ray catalogs with the selected Gaia sources. For example, we selected 264 eROSITA sources as the potential X-ray counterparts, which included 173 identified CVs and 91 CV candidates. Avoiding duplication, we took 444 X-ray sources from the four catalogs as potential counterparts to our selected Gaia sources. For the CV candidates, we analyzed ZTF and TESS data to search for the periodic signal and present number of the sources with detections of the period signal in Table 1. For the eROSITA sources, for example, we identified the periodic signal from 40 sources categorized as CV candidates. Among the 40 sources of CV candidates, the periodic signal of four Gaia DR3 sources (5994940674888088832, 6286158119187026688, 5293314439454159488, and 4835425609500365184) are reported in the VSX database.

3.1. CVs covered by four catalogs

Among the selected Gaia sources in this study, there are four X-ray-emitting CVs found across four catalogs, as presented in Table 2. CD Ind (EUVE J2115–58.6) is a magnetic CV identified by Craig (1996). Schwope et al. (1997) confirmed it as polar, but the WD’s spin is not synchronized with the binary orbit. U Gem is a dwarf nova with eclipse features (Abril et al. 2020; Arnold et al. 1976). PM J07068+0324 (PBC J0706.7+0327) was detected with Swift/BAT and is classified as polar due to its optical features, such as a variation of the emission-line synchronized with the orbital phase (Parisi et al. 2014; Halpern & Thorstensen 2015). A spectroscopic observation conducted by the 2.4 m Hiltner telescope from the MDM Observatory finds an orbital period of 0.070907(11) days. We reconfirmed this period, finding 0.0709(4) days, with the TESS observation. AR Sco is an eclipsing CV, emitting across the electromagnetic spectrum from radio-to-X-ray wavelength bands, with an orbital period of 3.56 h. The brightness of the emissions varies with a period of 1.97 minutes, which is interpreted as the WD’s spin period (Marsh et al. 2016).

Table 2.

CVs found in all four catalogs.

3.2. Search for periodic modulation with ZTF data

Due to the insufficient quality of the light curve and the limited field of view of the ZTF observation, a detailed search for the periodic signal using ZTF data is challenging for most of our targets. Among the CV candidates in Table 1, we identify six sources with the periodic modulation in the ZTF data, as summarized in Table 3. The identified periods are consistent with the value found in TESS data and/or reported in the VSX database, as indicated in Col. 7 of Table 3.

Table 3.

ZTF sources with the periodic modulation in CV candidates.

Among the six sources in Table 3, the eROSITA observations cover three sources. We identify two sources (Gaia DR3 3208641703155603584 and 406056486024031872) that are not listed in SIMBAD/VSX/OCVC/RK databases and four sources for which their periodic modulations have not been reported in the VSX. In Table 3, the eROSITA catalog does not contain three ZTF sources. For two sources (2SXPS J014816.9+510117 and 2SXPS J200023.5+091547), the current eROSITA catalog does not cover their source positions. For HE 1436–2137, the X-ray flux level, ∼2 × 10−14 erg cm−2 s−1, may be too low to detect with a half-year eROSITA observation (see Sect. 3.6).

3.3. Search for periodic modulation with TESS data

Most of the periodic signals reported in this study were found using the TESS data, which cover about 156 sources of our CV candidates. One issue in identifying the source as a CV candidate is that due to the limited spatial resolution of the TESS observation, we cannot state whether the signal is related to the orbital motion, especially if there is no report of the periodicity in the VSX database. Hence, we evaluated the reliability of the detected signal being related to the orbital period of the binary by analyzing the light curves of the identified CVs using TESS data. We analyzed the TESS data for the CVs (i) selected from the eROSITA source catalog reported in Schwope et al. (2024b) and (ii) selected from the four X-ray catalogs in our study (the sources in Col. 3 of Table 1).

Schwope et al. (2024b) present characteristics of the CVs selected from eROSITA-DE DR1. We checked the TESS data of 401 CVs and CV candidates after removing sources categorized as the symbiotic stars, listed in their list. The results of the timing analysis of the TESS data are summarized in the second line of Table 4. Among 401 sources, 140 exhibit TESS periods that align with those listed in the VSX database (NP0 in Table 4), while nine sources display the strongest signal corresponding to the first harmonics (NP1). We have six sources (Noth) from which the periodic signal confirmed with the TESS data is different from that reported in the VSX database. We also obtain Nnon = 38 sources that have a periodic signal in the TESS light curves, but are not recorded in the VSX database. For those 38 sources with the period detection, we also checked the RK catalog and found the information of the periods for the six sources, among which the periods of the five sources are consistent with the periods detected in the TESS data. Our results would suggest that the TESS data are useful for identifying the orbital period of the CVs.

Table 4.

Results of timing analysis with TESS light curves for identified CVs.

The results for the identified CVs selected from our X-ray lists are summarized in the third to sixth columns in Table 4. For example, our eROSITA source list contains 173 identified CVs and confirms the orbital period from NP0 = 30 sources, or the strongest harmonics signal from NP0 = 12 sources. We also find possible signal of the orbital variation from Nnon = 35 sources. We can see that the distribution patterns of the sources in Table 4 are roughly consistent across the five source lists. Although the direct application of the distribution to the CV candidates may be crude, we anticipate that if the periodic signals are confirmed in the TESS data, the most likely explanation is the orbital period, with the first harmonic being a secondary possibility.

Figure 5 shows the observed period for the CVs and CV candidates selected by cross-matching with the eROSITA sources. In the left panel of the figure, we can see that the CVs of our list typically exhibit a period between 0.1 days below the so-called period gap of the CVs (Spruit & Ritter 1983; Garraffo et al. 2018) and 0.05 days, which is known as the minimum orbital period in the standard binary evolution of CVs (Ritter 2010). These observed periods are typical values for non-magnetic CVs or polar ones (see Takata et al. 2022, Figure 18). As the right panel of Fig. 5 shows, the CV candidates of our list also have a similar period distribution to the identified CVs. Since our CV candidates are selected from the bridge region between the WD cooling sequence and the main sequence in the H-R diagram, it is more likely that the origin of their periodic signal is related to the orbital period of accreting WD candidate systems.

thumbnail Fig. 5.

Periodic signal from CVs (102 sources, filled circles) and CV candidates (40 sources, stars) selected by cross-matching with eROSITA catalog. Left: Gaia G-band magnitude versus observed period of modulation in ZTF and/or TESS light curves. Right: Distribution of observed periods.

Table 5.

List of six sources with low X-ray luminosities.

3.4. Optical and X-ray properties

The eROSITA catalog provides the observed X-ray flux in five distinct energy bands: P1 = 0.2 − 0.5 keV, P2 = 0.5 − 1.0 keV, P3 = 1.0 − 2.0 keV, P4 = 2.0 − 5.0 keV, and P5 = 5.0 − 8.0 keV energy bands. Using the information of the fluxes of eROSITA and the parallax of the corresponding Gaia sources, we estimated the luminosity in 0.2–8.0 keV bands by assuming an isotropic radiation. Figure 6 plots the estimated X-ray luminosity with the observed period (left panel) or the Gaia G-band magnitudes (right panel) for the selected eROSITA sources with a detection of the periodic modulation in ZTF/TESS data. The symbols marked by the circle and star correspond to the identified CVs and CV candidates, respectively. We can see in the figure that majority of the sources have a luminosity in the range of LX ∼ 1030 − 32 erg s−1, which is characteristic of typical X-ray-emitting CVs.

thumbnail Fig. 6.

Left: Period and estimated X-ray luminosity in the eROSITA energy bands. Right: G-band absolute magnitude and estimated X-ray luminosity. The symbols of the circles and stars correspond to the CVs and CV candidates respectively. The six sources with lower X-ray luminosities, indicated by the numbering, are presented in Table 5.

Figure 6 also shows six sources, indicated by the numbering, which exhibit an X-ray luminosity below LX ∼ 1030 erg s−1. To investigate possible contamination from the other X-ray sources, Fig. 7 presents the X-ray-to-optical flux ratio (Fx/Fopt) as a function of the optical color (left panel), or the X-ray color (right panel) for the eROSITA sources with the detection of the periodic modulation. Rodriguez (2024b) proposes an empirical boundary to distinguish the accreting compact binary systems from the X-ray-emitting active stars. We find that the six sources with lower X-ray luminosities are clustered around the boundary, which is shown by the dashed line in Fig. 7. The light-curve profiles with the ZTF/TESS data and the characteristics of those six sources are presented in Fig. A.1 and Table 5, respectively. In Table 5, we can see that one source is an eclipsing binary in VSX, and three sources are categorized as WDs in SIMBAD. Thus, the most probable explanation of those six sources is the eclipsing binary with the companion star’s activity likely producing the observed X-ray emission. Nevertheless, since other X-ray emission processes, such as a low accretion rate, cannot be entirely ruled out in this study, deeper observations would be needed to classify those sources conclusively.

Galiullin et al. (2024a) also proposes a boundary (solid line in the left panel of Fig. 7) to select a pure sample of CVs. We can see that most of our CV candidates are located around or above this boundary, consistently with the properties of the identified CVs. 24 CV candidates are located above this boundary, suggesting their periodic modulations in ZTF/TESS data are probably related to the orbital periods. Figure 8 presents the flux ratio (FX/Fopt) and the optical color (Gbp − Grp) of the CV candidates without periodic signals. We found 31 sources located above the boundary to select pure sample of CV candidates. Their locations suggest they are promising CV candidates, but further observations are needed to confirm their nature due to the lack of periodic signals.

thumbnail Fig. 7.

Ratio of X-ray flux (FX) in 0.2–8.0 keV energy bands to optical flux (Fopt) versus hardness of the optical bands (left panel) or the X-ray bands (right panel). The F(P1) and F(P4) represent the observed fluxes in P1 = 0.2 − 0.5 keV bands and P4 = 2.0 − 5.0 keV bands, respectively. The circles and stars correspond to the identified CVs and CV candidates for eROSITA sources with the detection of the periodic signal in ZTF/TESS data. The solid and dashed lines in the left panel are taken from Galiullin et al. (2024a) and Rodriguez (2024b), respectively. The six sources enclosed by the black circle are low-luminosity candidates corresponding to those in Fig. 6 and Table 5. The second source is missing from the right panel as its F(P4) is not measured, while the third source is missing because neither F(P1) nor F(P4) has been measured.

thumbnail Fig. 8.

Same as for Fig. 7, but the CV candidates are without detection of the periodic signal in ZTF/TESS data.

3.5. Differentiating these from other compact object binary systems

Although the detection of the periodic signal and estimated X-ray luminosity of most of the CV candidates are consistent with those of known CVs, the current photometric study may still contain a contamination from other types of compact object binary systems, such as active Galactic nuclei (AGNs) and high-mass or low-mass X-ray binaries. As pointed out by Schwope et al. (2024a), the CVs and AGNs show similar optical and X-ray flux ratios, and it may be difficult to distinguish between two candidates based on the color-color diagram. We therefore cross-matched our list of eROSITA sources with the Gaia DR3 AGN catalog9 (Gaia Collaboration 2022) and obtain no matches. Consequently, AGN contamination would be minimal. To effectively distinguish between these two types of X-ray sources, a detailed analysis of the X-ray spectral properties will be necessary. Additionally, an investigation of the variability patterns could help distinguish between the CVs and AGNs: CVs have short-term periodic variations with orbital motions typically shorter than half a day, while AGNs usually show long-term variability from days to years (Ulrich et al. 1997; Smith 2006).

The Galactic high-mass and low-mass X-ray binaries hosting a neutron star or black hole may be another main source of contamination in the X-ray bands. In the optical bands, however, the CVs usually have a Gaia magnitude lower than that of the high-mass binary system. Moreover, the X-ray binaries usually show a higher X-ray luminosity (> 1033 erg s−1) than that of the CVs. Figure. 9 illustrates the differences in the X-ray properties of the eROSITA observations among three types of binary systems; we cross-matched the eROSITA catalog with X-ray binary catalogs1011 (Fortin et al. 2023, 2024), using the limit of the angular separation of 10″. As shown in the figure, the high-mass and low-mass X-ray binaries generally exhibit higher X-ray luminosity (> 1033 erg s−1) compared to CVs. Additionally, the hardness ratio (log F(P1)−log F(P4)) increases with decreasing luminosity in the eROSITA bands. For CVs, on the other hand, the hardness ratio is less dependent on the luminosity. The figure indicates that our eROSITA sources identified as CV candidates are more consistent with the CVs rather than high-mass or low-mass X-ray binaries.

thumbnail Fig. 9.

Luminosity and hardness relation measured by eROSITA. The triangles and inverted triangles correspond to the high-mass and low-mass X-ray binaries, respectively. The circles and stars correspond to the CVs and CV candidates of eROSITA, respectively.

3.6. Comparison among X-ray catalogs

As expected and demonstrated in Table 1, eROSITA can observe the X-ray emission from more identified CVs and CV candidates compared to other X-ray observations. To illustrate the flux sensitivity of each catalog, Fig. 10 presents the observed X-ray fluxes of the identified CVs (upper panel) and CV candidates (lower panel) selected from the eROSITA (circles), XMM-Newton (stars), and Swift (triangles). We removed the ROSAT sources as (i) most CVs selected from ROSAT can be found in other catalogs, and (ii) the number of CV candidates is much lower than in other catalogs. The three catalogs provide the energy flux in different energy bands: 0.2–8.0 keV bands of eROSITA, 0.2–12.0 keV bands of XMM-Newton and 0.3–10 keV bands of Swift. Despite these differences, those fluxes will represent the order of magnitude of the soft X-ray emission from the system.

thumbnail Fig. 10.

Flux distribution of CVs (upper panel) and CV candidates (bottom panel) in eROSITA (filled circles), XMM-Newton (stars), and Swift (triangles).

As illustrated in the upper panel of Fig. 10, the typical fluxes of identified CVs are on the order of 10−14 − 10−11 erg cm−2 s−1, and the distributions of the three observations are similar to each other. Notably, about 120 CVs from the eROSITA catalog are not found in the other three catalogs. This demonstrates that the eROSITA observations are more efficient in discovering the X-ray emission from the CVs and encourages us to carry out a target observation for further in-depth study (see also Schwope et al. 2024b). Despite eROSITA being the main contributor to our CV-candidate list, it does not cover about 64 (out of 65) and 18 (out of 20) CV candidates selected from the XMM-Newton and Swift catalogs, respectively. This is mainly because those sources are located at the Galactic hemisphere that is not covered by the current eROSITA catalog.

The CV candidates in the bottom panel of Fig. 10 contain 91 sources from eROSITA, 65 from the XMM-Newton, and 20 from Swift. Out of the 65 XMM-Newton sources and 20 Swift sources, only one XMM-Newton source and two Swift sources are found in the eROSITA catalog, and about half of the sources are located at the Galactic hemisphere that is not covered by the current eROSITA catalog. We find in the figure that the flux distributions of the eROSITA sources and the Swift sources are similar to those for the CVs (FX ∼ 10−14 − 10−11 erg cm−2 s−1). For XMM-Newton sources (the symbol with stars), on the other hand, a certain fraction have a flux level below 10−14 erg cm−2 s−1. From Fig. 10, we may draw two conclusions. First, the large population of X-ray-emitting CV candidates having a flux of FX ∼ 10−14 − 10−11 erg cm−2 s−1 were recently confirmed by eROSITA due to its larger sky region. Second, the XMM-Newton observations indicate a significant population of the CV candidates having a flux level below ∼1014 erg cm−2 s−1. So, eROSITA observations with more exposure time have the potential to find more X-ray-emitting CV candidates with a flux level below ∼1014 erg cm−2 s−1. Since the current eROSITA catalog is the result of the operation in the first six months, it is expected that the future operation will further increase the population of the X-ray-identified CVs and CV candidates.

3.7. X-ray-emitting and accreting WD binaries within 100 pc of Earth

A significant fraction of accreting WD binaries are expected to be X-ray emitters due to either (i) the accretion process, as seen in CVs, or (ii) hydrogen or helium burning on the surface, as in super-soft sources with a luminosity of 1037 − 38 erg s−1. The new eROSITA all-sky survey presents a valuable opportunity to explore the Galactic distribution of the accreting WD candidates. Figure. 11 shows the distribution of the distance for 121 488 Gaia sources (solid line) and 444 X-ray sources (dashed line), respectively.

thumbnail Fig. 11.

Distribution of source distances for 121 488 Gaia sources (solid histogram) selected in Sect. 2.1 and for 444 Gaia sources that may be associated with X-ray sources (dashed histogram).

Due to the high completeness level of the accreting WD samples within 100 pc of the Gaia observations (Hallakoun et al. 2024), we investigate our targets located within 100 pc of Earth. Among 121 488 Gaia sources in our study, 32 sources are located within 100 pc of Earth and they are listed in Table B.1. We find that seven sources among them exhibit possible X-ray counterparts and summarize their characteristics in Table 6. Three systems, U Gem, VW Hyi and AR Uma, are identified CVs. The remaining four sources exhibit an X-ray luminosity under 1030 erg s−1, and three of them have already been mentioned as low-luminosity X-ray sources in Table 5.

As shown in Table 6, there are two X-ray-emitting, accreting WD binaries, but there are no records in the eROSITA catalog. First, UZ Sex is the closest binary system in our targets, but its X-ray flux level of ∼10−14 erg cm−2 s−1 measured by XMM-Newton is the lowest, as Table 4 indicates. Hence, the current eROSITA catalog does not include UZ SeX, likely due to the limited sensitivity level of the current survey. Second, AR UMa, which is an identified CV, has a short orbital period with ∼0.0805(4) days and it has not been recorded in the four X-ray catalogs used in this study. However, Remillard et al. (1994) reported AR UMa as an X-ray source with a luminosity level of ∼4 × 1032 erg s−1, and the source has been recorded in the ROSAT Source Catalog of Pointed Observations with the High Resolution Imager (Team 2000). In addition, a recent target observation of AR UMa by XMM-Newton (Obs.IDn: 0884870101 and PI: Schwope, Axel) clearly shows a bright X-ray emission that modulates with the orbital period, suggesting AR UMa is a polar-type magnetic CV. Unfortunately, the current eROSITA catalog, which records the X-ray sources located in the western Galactic hemisphere (Merloni et al. 2024), does not cover the sky region around the source.

Table 6.

List of X-ray-emitting and accreting WD binaries within 100 pc of Earth in this study.

Among 32 sources located within 100 pc of Earth, we confirm the X-ray emissions from seven sources. In Table B.1, on the other hand, there are several sources (e.g., GD 245, UCAC4 135-002106, etc.) that have an orbital period shorter than one day, but no X-ray emissions have been recorded in the catalogs. This may be because the sources were not covered by the previous observations. Another possibility is that the source is an eclipsing binary system rather than a CV, and the binary system has a weak or nonexistent accretion process. For cases such as UCAC4 135-002106 and UCAC4 293-078484, for example, the eROSITA catalog inverses the source regions, suggesting their X-ray fluxes are on the order of or below ∼10−14 erg cm−2 s−1. For other cases, such as GD 245, the current eROSITA catalog does not cover their sky regions. Hence, the results of the continuous operation and source list of the all sky of the eROSITA observation are needed to obtain a more complete list of the X-ray-emitting, accreting WD binaries within 100 pc of Earth.

4. Summary

The sky survey of eROSITA offers a new opportunity to identify numerous X-ray-emitting CVs. In this study, therefore, we searched for candidates of the X-ray-emitting CVs by cross-matching four X-ray catalogs (eROSITA, XMM-Newton, Swift, and ROSAT) with Gaia’s sources. We demonstrate how the eROSITA survey is more efficient in searching for X-ray-emitting CVs. We selected 264 sources, including 176 identified CVs and 91 CV candidates from the eROSITA catalog. Among 91 CV candidates, we identified a periodic signal from 40 sources in ZTF and/or TESS photometric light curves. The distribution of the period and the expected X-ray luminosity are consistent with those of the identified CVs (Figs. 5 and 6).

Most of our CV candidates with the detection of the periodic modulation are distinguished from the active stars on the color-color diagram using the X-ray and optical observations (Fig. 7). On the other hand, we also find six sources whose X-ray luminosity is LX < 1030 erg cm−2 s−1, which is lower than that of typical CVs (Fig. 6). This suggests that those sources are candidates of the eclipsing binary rather than CVs (Table 5). The X-ray emission from AGNs may be another source of contamination. Although a cross-matching between the list of our eROSITA sources and AGN candidates provides a null result, further studies of the X-ray properties (e.g., spectrum and modulation) are necessary to differentiate them from X-ray-emitting CVs and AGNs. We also demonstrate that the CV candidates are differentiated from the low-mass and high-mass X-ray binaries (Fig. 9).

Due to its larger sky region, the number of CVs and CV candidates selected from eROSITA catalogs (264 sources) is much higher than from XMM-Newton (109), Swift (111), and ROSAT (69), as presented in Table 1. It is found that while the eROSITA sources selected in this study have a bigger flux than 10−14 erg cm−2 s−1, the XMM-Newton observations indicate a certain population of the CV candidates with a flux below 10−14 erg cm−2 s−1 (Fig. 10). This suggests that future eROSITA observations will increase the population of the CVs and CV candidates identified by the X-ray bands.

Among 121 488 Gaia sources selected in this study, 32 are located within 100 pc of Earth, and seven exhibit X-ray emissions. Three sources are identified CVs, and another four are likely eclipsing binary systems. The five sources can be found in the eROSITA catalog, and three of them cannot be found in the other three catalogs. On the other hand, the current eROSITA catalog is missing some X-ray-emitting, accreting WD binaries located within 100 pc of Earth that have a flux level lower than the current sensitivity level of the survey (e.g., UZ Sex). Moreover, the current eROSITA catalog only covers one Galactic hemisphere. Hence, the continuous eROSITA survey and the catalog covering the whole sky region will provide a more comprehensive understanding for the population of the X-ray-emitting, accreting WD binaries.

Data availability

Tables of identified CVs and CV candidates found in this study are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/698/A321.

Acknowledgments

We express our thanks to the referee for his/her comments and suggestions, which have significantly improved our manuscript. We appreciate Dr. A.D. Schwope for providing the list of CVs selected from the eROSITA catalog. We are grateful to Drs A.K.H. Kong, J.Mao, X. Hou, K.K. Li, L.C.-C. Lin and K.L. Li for useful discussion for the CVs. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France and the International Variable Star Index (VSX) database, operated at AAVSO, Cambridge, Massachusetts, USA. This work is based on data from eROSITA, the soft X-ray instrument aboard SRG, a joint Russian-German science mission supported by the Russian Space Agency (Roskosmos), in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL) and its subcontractors, and is operated by NPOL with support from the Max Planck Institute for Extraterrestrial Physics (MPE). The development and construction of the eROSITA X-ray instrument was led by MPE, with contributions from the Dr. Karl Remeis Observatory Bamberg & ECAP (FAU Erlangen-Nuernberg), the University of Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP), and the Institute for Astronomy and Astrophysics of the University of Tübingen, with the support of DLR and the Max Planck Society. The Argelander Institute for Astronomy of the University of Bonn and the Ludwig Maximilians University at Munich also participated in the science preparation for eROSITA. X.X.W. and J.T. are supported by the National Key Research and Development Program of China (grant No. 2020YFC2201400) and the National Natural Science Foundation of China (grant No. 12173014).

References

  1. Abril, J., Schmidtobreick, L., Ederoclite, A., & López-Sanjuan, C. 2020, MNRAS, 492, L40 [NASA ADS] [CrossRef] [Google Scholar]
  2. Arnold, S., Berg, R., & Duthie, J. 1976, ApJ, 206, 790 [Google Scholar]
  3. Astropy Collaboration (Price-Whelan, A. M., et al.) 2022, ApJ, 935, 167 [NASA ADS] [CrossRef] [Google Scholar]
  4. Baluev, R. V. 2008, MNRAS, 385, 1279 [Google Scholar]
  5. Boller, T., Freyberg, M., Trümper, J., et al. 2016, A&A, 588, A103 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  6. Cardoso, J. V. d. M., Hedges, C., Gully-Santiago, M., et al. 2018, Astrophysics Source Code Library [record ascl:1812.013] [Google Scholar]
  7. Craig, N. 1996, IAU Circ., 6297, 2 [Google Scholar]
  8. Drake, A., Djorgovski, S., Mahabal, A., et al. 2009, ApJ, 696, 870 [NASA ADS] [CrossRef] [Google Scholar]
  9. Drake, A., Gänsicke, B., Djorgovski, S., et al. 2014, MNRAS, 441, 1186 [NASA ADS] [CrossRef] [Google Scholar]
  10. Evans, P., Page, K., Osborne, J., et al. 2020, ApJS, 247, 54 [Google Scholar]
  11. Fortin, F., García, F., Bunzel, A. S., & Chaty, S. 2023, A&A, 671, A149 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  12. Fortin, F., Kalsi, A., García, F., Simaz-Bunzel, A., & Chaty, S. 2024, A&A, 684, A124 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  13. Gaia Collaboration 2022, VizieR Online Data Catalog: I/358 [Google Scholar]
  14. Galiullin, I., Rodriguez, A. C., El-Badry, K., et al. 2024a, A&A, 690, A374 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  15. Galiullin, I., Rodriguez, A. C., Kulkarni, S. R., et al. 2024b, MNRAS, 528, 676 [NASA ADS] [CrossRef] [Google Scholar]
  16. Garraffo, C., Drake, J., Alvarado-Gomez, J., Moschou, S., & Cohen, O. 2018, ApJ, 868, 60 [NASA ADS] [CrossRef] [Google Scholar]
  17. Ginsburg, A., Sipőcz, B. M., Brasseur, C., et al. 2019, AJ, 157, 98 [NASA ADS] [CrossRef] [Google Scholar]
  18. Hallakoun, N., Shahaf, S., Mazeh, T., Toonen, S., & Ben-Ami, S. 2024, ApJ, 970, L11 [Google Scholar]
  19. Halpern, J. P., & Thorstensen, J. R. 2015, AJ, 150, 170 [NASA ADS] [CrossRef] [Google Scholar]
  20. Kimura, S. S., Murase, K., & Mészáros, P. 2021, Nat. Commun., 12, 5615 [Google Scholar]
  21. Lindegren, L., Hernández, J., Bombrun, A., et al. 2018, A&A, 616, A2 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  22. Lomb, N. R. 1976, Astrophys. Space Sci., 39, 447 [Google Scholar]
  23. Lutovinov, A., Suleimanov, V., Luna, G. J. M., et al. 2020, New Astron. Rev., 91, 101547 [Google Scholar]
  24. Marsh, T., Gänsicke, B., Hümmerich, S., et al. 2016, Nature, 537, 374 [NASA ADS] [CrossRef] [Google Scholar]
  25. Masci, F. J., Laher, R. R., Rusholme, B., et al. 2018, PASP, 131, 018003 [Google Scholar]
  26. Merloni, A., Lamer, G., Liu, T., et al. 2024, A&A, 682, A34 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  27. Muñoz-Giraldo, D., Stelzer, B., de Martino, D., & Schwope, A. 2023, A&A, 676, A7 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  28. Parisi, P., Masetti, N., Rojas, A., et al. 2014, A&A, 561, A67 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  29. Predehl, P., Andritschke, R., Arefiev, V., et al. 2021, A&A, 647, A1 [EDP Sciences] [Google Scholar]
  30. Remillard, R., Schachter, J., Silber, A., & Slane, P. 1994, ApJ, 426, 288 [Google Scholar]
  31. Ricker, G. R., Winn, J. N., Vanderspek, R., et al. 2015, J. Astron. Telescopes Instrum. Syst., 1, 014003 [Google Scholar]
  32. Rimoldini, L., Holl, B., Gavras, P., et al. 2023, A&A, 674, A14 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  33. Ritter, H. 2010, Mem. Soc. Astron. It., 81, 849 [NASA ADS] [Google Scholar]
  34. Ritter, H., & Kolb, U. 2003, A&A, 404, 301 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  35. Rodriguez, A. 2024a, in The Golden Age of Cataclysmic Variables and Related Objects - VI, 17 [Google Scholar]
  36. Rodriguez, A. C. 2024b, PASP, 136, 054201 [NASA ADS] [CrossRef] [Google Scholar]
  37. Rodriguez, A. C., Kulkarni, S. R., Prince, T. A., et al. 2023, ApJ, 945, 141 [NASA ADS] [CrossRef] [Google Scholar]
  38. Rodriguez, A. C., El-Badry, K., Suleimanov, V., et al. 2025, PASP, 137, 014201 [NASA ADS] [CrossRef] [Google Scholar]
  39. Sazonov, S., Burenin, R., Filippova, E., et al. 2024, A&A, 687, A183 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  40. Schwope, A. D., Buckley, D. A. H., O’Donoghue, D., et al. 1997, A&A, 326, 195 [NASA ADS] [Google Scholar]
  41. Schwope, A., Buckley, D. A., Malyali, A., et al. 2022, A&A, 661, A43 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  42. Schwope, A., Kurpas, J., Baecke, P., et al. 2024a, A&A, 686, A110 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  43. Schwope, A., Knauff, K., Kurpas, J., et al. 2024b, A&A, 690, A243 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  44. Shappee, B. J., Prieto, J., Grupe, D., et al. 2014, ApJ, 788, 48 [NASA ADS] [CrossRef] [Google Scholar]
  45. Smith, R. C. 2006, Contemp. Phys., 47, 363 [Google Scholar]
  46. Spruit, H. C., & Ritter, H. 1983, A&A, 124, 267 [NASA ADS] [Google Scholar]
  47. Sunyaev, R., Arefiev, V., Babyshkin, V., et al. 2021, A&A, 656, A132 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  48. Takata, J., Wang, X., Kong, A., et al. 2022, ApJ, 936, 134 [NASA ADS] [CrossRef] [Google Scholar]
  49. Team R. S. 2000, VizieR On-line Data Catalog: IX/28A [Google Scholar]
  50. Ulrich, M.-H., Maraschi, L., & Urry, C. M. 1997, ARA&A, 35, 445 [NASA ADS] [CrossRef] [Google Scholar]
  51. VanderPlas, J. T. 2018, ApJS, 236, 16 [Google Scholar]
  52. Warner, B. 1995, Cambridge Atmospheric Space Science Series, 28 [Google Scholar]
  53. Webb, N. A., Coriat, M., Traulsen, I., et al. 2020, A&A, 641, A136 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  54. Wenger, M., Ochsenbein, F., Egret, D., et al. 2000, A&AS, 143, 9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  55. Zaznobin, I., Sazonov, S., Burenin, R., et al. 2022, A&A, 661, A39 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]

Appendix A: Light curves of six sources with low X-ray luminosities

Figure A.1 presents the TESS light curves of the six CV candidates with low X-ray luminosities (LX < 1030 erg s−1), as present in Fig. 7 and Table 5.

thumbnail Fig. A.1.

Phase-folded light curves for six low-luminosity sources

Appendix B: List of sources within 100 pc of Earth

Table B.1 provides list of the selected Gaia sources located within 100 pc of Earth, based on their parallax. The fifth column shows the name of the WDs or CVs indicated in the SIMBAD and/or VSX databases. The sixth column summarizes the observed period of the modulation in the TESS light curves or the period reported in the VSX database.

Table B.1.

List of selected Gaia sources within 100 pc of Earth

All Tables

Table 1.

Cross-matching for X-ray catalogs with 121 488 Gaia sources.

In the text
Table 2.

CVs found in all four catalogs.

In the text
Table 3.

ZTF sources with the periodic modulation in CV candidates.

In the text
Table 4.

Results of timing analysis with TESS light curves for identified CVs.

In the text
Table 5.

List of six sources with low X-ray luminosities.

In the text
Table 6.

List of X-ray-emitting and accreting WD binaries within 100 pc of Earth in this study.

In the text
Table B.1.

List of selected Gaia sources within 100 pc of Earth

In the text

All Figures

thumbnail Fig. 1.

Hertzsprung-Russell diagram with Gaia sources. The red square indicates the region from which we selected the Gaia sources.
In the text
thumbnail Fig. 2.

Distribution of angular separation between eROSITA position and Gaia position for the identified CVs. The data were taken from Schwope et al. (2024b).
In the text
thumbnail Fig. 3.

TESS full-frame images from sector 39 for region around Gaia DR3 5822540653269409408. The LS periodogram was created for each pixel in the figure, and the periodic signal only detected pixels around the target (indicated by red square) is considered as a possible signal from the target.
In the text
thumbnail Fig. 4.

LS periodogram (left panel) and folded light curve (right panel) with TESS data for Gaia DR3 5822540653269409408, which was selected as the counterpart of 1eRASS J155030.1–654403. The dashed black line and the dashed-dotted black line are (FAP = 0.01) determined by the methods of Baluev (2008) and the bootstrap of VanderPlas (2018), respectively. The LS diagram reveals an ∼3.13(3) day−1 modulation and its first harmonic. Two period cycles of the light curve are presented for clarity.
In the text
thumbnail Fig. 5.

Periodic signal from CVs (102 sources, filled circles) and CV candidates (40 sources, stars) selected by cross-matching with eROSITA catalog. Left: Gaia G-band magnitude versus observed period of modulation in ZTF and/or TESS light curves. Right: Distribution of observed periods.
In the text
thumbnail Fig. 6.

Left: Period and estimated X-ray luminosity in the eROSITA energy bands. Right: G-band absolute magnitude and estimated X-ray luminosity. The symbols of the circles and stars correspond to the CVs and CV candidates respectively. The six sources with lower X-ray luminosities, indicated by the numbering, are presented in Table 5.
In the text
thumbnail Fig. 7.

Ratio of X-ray flux (FX) in 0.2–8.0 keV energy bands to optical flux (Fopt) versus hardness of the optical bands (left panel) or the X-ray bands (right panel). The F(P1) and F(P4) represent the observed fluxes in P1 = 0.2 − 0.5 keV bands and P4 = 2.0 − 5.0 keV bands, respectively. The circles and stars correspond to the identified CVs and CV candidates for eROSITA sources with the detection of the periodic signal in ZTF/TESS data. The solid and dashed lines in the left panel are taken from Galiullin et al. (2024a) and Rodriguez (2024b), respectively. The six sources enclosed by the black circle are low-luminosity candidates corresponding to those in Fig. 6 and Table 5. The second source is missing from the right panel as its F(P4) is not measured, while the third source is missing because neither F(P1) nor F(P4) has been measured.
In the text
thumbnail Fig. 8.

Same as for Fig. 7, but the CV candidates are without detection of the periodic signal in ZTF/TESS data.
In the text
thumbnail Fig. 9.

Luminosity and hardness relation measured by eROSITA. The triangles and inverted triangles correspond to the high-mass and low-mass X-ray binaries, respectively. The circles and stars correspond to the CVs and CV candidates of eROSITA, respectively.
In the text
thumbnail Fig. 10.

Flux distribution of CVs (upper panel) and CV candidates (bottom panel) in eROSITA (filled circles), XMM-Newton (stars), and Swift (triangles).
In the text
thumbnail Fig. 11.

Distribution of source distances for 121 488 Gaia sources (solid histogram) selected in Sect. 2.1 and for 444 Gaia sources that may be associated with X-ray sources (dashed histogram).
In the text
thumbnail Fig. A.1.

Phase-folded light curves for six low-luminosity sources
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.