Jan Kurpas

Context. The X-ray all-sky surveys performed with eROSITA on the Spektrum-Roentgen-Gamma (SRG) mission offer a unique opportunity to generate large samples of all kinds of X-ray emitters. Here we focus on a minority population among the X-ray point sources: the cataclysmic variables (CVs). Aims. We aim to generate large samples of CVs selected from the SRG/eROSITA surveys to address fundamental questions about binary evolution and the role of magnetic fields in CV evolution and their contribution to the Galactic Ridge X-ray emission (GRXE). Methods. We trained a random-forest (RF) classifier based on combined X-ray and optical properties to select CV candidates from the first eROSITA X-ray all-sky survey (eRASS1). Follow-up spectroscopy to securely identify the objects was performed on a subset of the selected candidates using telescopes at the northern and southern hemispheres. Newly identified CVs were further analyzed to determine their likely subtype with the help of dedicated follow-up photometry, spectroscopy, and archival resources. Results. We identified 156 CVs of almost all subtypes covering a wide range of distances (between 170 pc and several kiloparsecs), absolute magnitudes ( G = 4.5–12) and X-ray luminosities (log L _X (erg/s)(0.2 − 2.3 keV) = 29.6–33.3). For most objects, their CV nature is reported here for the first time. For 40 objects periods were determined, which were regarded as their likely orbital periods. These range from 79.2 min, at the CV minimum period, to almost 22 hours. Seven objects were found to be eclipsing, a lower limit to the actual number due to the current lack of dedicated follow-up observations. A further five objects show cyclotron or Zeeman features, which allowed us to determine their field strengths. A soft component was found in 14 CVs, and all were regarded as magnetic. The sample comprises X-ray-luminous objects, which are excellent new intermediate-polar candidates; these are thought to be main contributors to the GRXE. The new method of selecting CVs from eRASS1 was found to be superior to other methods described in the literature, and the new CVs were used to further refine the method. Conclusions. This work demonstrates the feasibility of constructing comprehensive CV samples through follow-up spectroscopy of SRG/eROSITA X-ray sources. It has also demonstrated the potential to uncover rare objects, which are of scientific interest on their own.

The eROSITA telescope array on board the Spektrum-Roentgen-Gamma (SRG) mission began its all-sky survey program in December 2019, scanning the sky at an approximately six-month cadence. Here, we present new catalogues of point-like and extended X-ray sources derived from the first three completed eROSITA all-sky surveys (eRASS:3), covering the western Galactic hemisphere, which Germany’s eROSITA consortium holds proprietary rights for. We describe the observational strategy, data processing, and analysis pipelines. We also characterise the resulting X-ray source populations. The eRASS:3 main catalogue contains nearly two million sources, including 1 911 744 point-like and 63 796 extended sources, detected in the 0.2–2.3 keV energy band, eROSITA’s most sensitive energy window. This volume has doubled the X-ray source content of eRASS1 and provides a comprehensive census of X-ray-emitting objects across diverse astrophysical classes. We also released a secondary hard catalogue of nearly 15 000 sources detected in the harder 2.3–5.0 keV energy band. In addition, we released six catalogues, three for the main sample and three for the hard sample, in which we identify and classify the optical and infrared counterparts of eRASS:3 point-like sources. This approach delivers a homogeneous, high-quality identification of mostly extragalactic X-ray emitters; for example, we estimate that ~88% of the 1.4 million counterparts identified within the footprints of the Legacy Survey Imaging for DESI are extragalactic sources. This work has enabled the generation of samples optimised for completeness and purity, while expanding the discovery space for rare populations. The second data release (DR2) of the SRG/eROSITA all-sky survey is a catalogue-only release, comprising the catalogues presented in this work together with an updated version of the eROSITA upper flux limit server.

Based on its predominantly thermal X-ray emission and long spin period, the isolated neutron star (INS) eRASSU J131716.9–402647 is one of the most promising candidates for membership in the still small class of X-ray dim isolated neutron stars (XDINSs). Confirmation of this classification, however, requires a more detailed characterisation of the source’s timing and spectral properties. In this work, we present new NICER observations which, together with previous X-ray follow-up, allow us to constrain the timing properties and long-term evolution of eRASSU J131716.9–402647 . We obtain a coherent timing solution with a spin period of P ∼ 12.8 s and a period derivative of Ṗ ∼ 9 × 10 ⁻¹⁴ s s ⁻¹ , which best-describes the spin evolution of the source. These parameters imply a dipolar magnetic field strength of 3 × 10 ¹³ G and a spin-down luminosity of order 10 ³⁰ erg s ⁻¹ . Spectral modelling reveals no significant change in the spectral state over the 15 months of observational monitoring and indicates a thermal luminosity that likely exceeds the rotational energy loss. This suggests a thermal evolution that has been significantly influenced by past reheating. The energy dependence of the double-humped pulse profile closely resembles that observed in the XDINS RX J1308.6+2127 , with the pulsed fraction increasing towards higher energies. Taken together, these results unambiguously confirm the XDINS nature of eRASSU J131716.9–402647 , making it the first newly confirmed XDINS in more than two decades.

We report on follow-up observations with XMM-Newton , the FORS2 instrument at the ESO-VLT , and FAST, aiming to characterise the nature of five thermally emitting isolated neutron star (INS) candidates recently discovered from searches in the footprint of the Spectrum Roentgen Gamma (SRG) /eROSITA All-sky Survey. We find that the X-ray spectra are predominantly thermal and can be described by low-absorbed blackbody models with effective temperatures ranging from 50 to 210 eV. In two sources, the spectra also show narrow absorption features at 300–400 eV. Additional non-thermal emission components are not detected in any of the five candidates. The soft X-ray emission, the absence of optical counterparts in four sources, and the consequent large X-ray-to-optical flux ratios > 3000 − 5400 confirm their INS nature. For the remaining source, eRASSU J144516.0–374428 , the available data do not allow a confident exclusion of an active galactic nucleus nature. However, if the source is Galactic, the small inferred X-ray emitting region is reminiscent of a heated pulsar polar cap, possibly pointing to a binary pulsar nature. X-ray timing searches do not detect significant modulations in all candidates, implying pulsed fraction upper limits of 13–19% (0.001–13.5 Hz). The absence of pulsations in the FAST observations targeting eRASSU J081952.1–131930 and eRASSU J084046.2–115222 excludes periodic magnetospheric emission at 1–1.5 GHz with an 8 σ significance down to 4.08 μJy and 2.72 μJy, respectively. The long-term X-ray emission of all sources does not imply significant variability. Additional observations are warranted to establish exact neutron star types. At the same time, the confirmation of the predominantly thermal neutron star nature in four additional sources highlights the power of SRG /eROSITA to complement the Galactic INS population.

SRGt 062340.2-265751, a cataclysmic variable identified by SRG /eROSITA thanks to its significant X-ray variability, remains poorly characterised despite the multi-wavelength follow-up. We present spectral and timing analyses from the first dedicated X-ray and ultraviolet observations with XMM-Newton , complemented by SRG /eROSITA data from four all-sky surveys (eRASS1-4) and ASAS-SN optical photometry. Our timing analysis reveals a > 8 σ significant modulation at 3.6 ± 0.5 hours, likely representing the orbital period. Long-term ASAS-SN monitoring confirms the source as a VY Sculptoris-type nova-like system, while short-timescale X-ray and ultraviolet variability, down to a few minutes, suggests a possible underlying magnetic white dwarf. Two additional significant X-ray modulations at 43 ± 1 min and 36.0 ± 0.7 min tentatively point to the spin period of an intermediate polar. The best-fit XMM-Newton energy spectra reveal a multi-temperature thermal plasma ( kT = 0.23, 0.94, and 5.2 keV), while the SRG /eROSITA spectra are consistent with a single-temperature thermal plasma of a few keV. We estimate unabsorbed X-ray luminosities of ≳10 ³² erg s ⁻¹ (0.2–12 keV). Broadband spectral energy distribution modelling, from near-ultraviolet to infrared, indicates a disc-dominated system consistent with a nova-like classification. We discuss these results in the context of the source’s confirmed nova-like classification and its possible magnetic nature, a scenario increasingly supported by discoveries of intermediate polars exhibiting VY Sculptoris-type nova-like features.

On the premise of a soft spectral distribution and absence of counterparts, the X-ray source eRASSU J065715.3+260428 was recently identified as a likely thermally emitting isolated neutron star (XINS) in a search in the SRG/eROSITA All-Sky Survey. We investigated the nature and evolutionary state of the neutron star through a dedicated multi-wavelength follow-up observational campaign with XMM-Newton, NICER, FAST, and ESO-VLT, complemented by the analysis of archival Fermi-LAT observations. The coherent timing analysis of the X-ray observations unveiled the rotation period of the XINS, P = 261.085400(4) ms, and its spin-down rate, Ṗ = 6₋₄⁺¹¹ × 10⁻¹⁵ s s⁻¹ (errors are 1σ confidence levels). The nearly sinusoidal pulse profile has a pulsed fraction of ∼15% (0.2 − 2 keV). No optical counterparts are detected down to 27.3 mag (5σ, R band) in the ESO-VLT FORS2 imaging, implying a large X-ray-to-optical flux ratio above 5200. The X-ray spectrum of the source is best described by a composite phenomenological model consisting of two thermal components, either a double blackbody continuum with temperatures 90 eV and 220 eV or a hydrogen neutron star atmosphere of temperature log(T/K)∼5.8 combined with a hot blackbody of 250 eV, in both cases modified by an absorption feature at low energies, ∼0.3 keV with an equivalent width of ∼100 eV. The presence of faint non-thermal hard X-ray tails is ruled out above (2.1 ± 1.8)% of the source unabsorbed flux. Radio searches at 1 − 1.5 GHz with FAST yielded negative results, with a deep upper limit on the pulsed flux of 1.4 μJy (10σ). Similarly, no significant spatial or pulsed signals were detected in sixteen years of Fermi-LAT observations. The most likely interpretation is that the source is a middle-aged spin-powered pulsar, which can also be identified as PSR J0657+2604. The absence of non-thermal X-ray, radio, or gamma-ray emission within current limits suggests either an unfavourable viewing geometry or unusual magnetospheric properties. Additional observations are needed to check for faint hard X-ray tails, investigate the presence of diffuse emission from a pulsar-wind nebula, and obtain a more accurately sampled timing solution.

The SRG/eROSITA All-Sky Survey (eRASS) allows for the creation of a complete sample of X-ray dim isolated neutron stars (XDINSs), which will significantly facilitate the study of their population properties, evolution, and connection to other families of isolated neutron stars (INSs). In this work, we conduct a systematic search for XDINSs on the eROSITA western Galactic hemisphere down to an X-ray flux limit of 10⁻¹³ erg s⁻¹ cm⁻² and discuss the resulting candidate sample. Consistently with the properties of the known XDINSs, we selected all eRASS sources possessing a soft X-ray spectral distribution and that are unlikely to be associated with optical or infrared sources. Our selection criteria allowed us to recover all known XDINSs and previously proposed candidates. In addition, we put forward 33 new candidate members for dedicated follow-up identification campaigns. We found the resulting candidate sample to be about 30–50% complete, mainly due to source confusion and the stringent cross-matching criteria adopted to select the most promising candidates for immediate follow-up investigation. The candidates of the sample presented here can be broadly divided into two groups: 13 rather soft and 20 hot and somewhat hard X-ray emitters. Interestingly, the remarkably thermal nature of the candidates in the first group as well as their spatial distribution, lack of known counterparts, and absence of significant flux variability agree well with the properties of other confirmed thermally emitting INSs. For the candidates in the second group, the current observational data do not allow one to discern between rotation-powered or recycled pulsars, cataclysmic variables, or quiescent neutron stars in binary systems or even to rule out an extragalactic nature. On the basis of population synthesis and the estimated source completeness of the search, we expect that between one and three new XDINSs are among the already singled-out list of XDINS candidates – a long-sought increase in the proposed number of members of this elusive class of X-ray emitters.

Context. X-ray surveys combined with optical follow-up observations are used to generate complete flux-limited samples of the main X-ray emitting source classes. eROSITA on the Spectrum-Roentgen-Gamma mission provides sufficient sensitivity to build significantly enhanced samples of rare X-ray emitting sources. Aims. We strive to identify and classify compact white dwarf binaries, cataclysmic variables (CVs), and related objects, which were detected in the sky area of eFEDS, the eROSITA Final Equatorial Depths Survey, and they were observed in the plate program of SDSS-V. Methods. Compact white dwarf binaries were selected from spectra obtained in the early SDSS-V plate program. A dedicated set of SDSS plate observations were carried out in the eFEDS field, providing spectroscopic classifications for a significant fraction of the optically bright end (r < 22.5) of the X-ray sample. The identification and subclassification rests on visual inspections of the SDSS spectra, spectral variability, color-magnitude and color-color diagrams involving optical and X-ray fluxes, optical variability, and literature work. Results. Upon visual inspection of SDSS spectra and various auxiliary data products, we have identified 26 accreting compact white dwarf binaries (aCWDBs) in eFEDS, of which 24 are proven X-ray emitters. Among those 26 objects, there are 12 dwarf novae, three WZ Sge-like disk-accreting nonmagnetic CVs with low accretion rates, five likely nonmagnetic high accretion rate nova-like CVs, two magnetic CVs of the polar subcategory, and three double degenerates (AM CVn objects). Period bouncing candidates and magnetic systems are rarer than expected in this sample, but it is too small for a thorough statistical analysis. Fourteen of the systems are new discoveries, of which five are fainter than the Gaia magnitude limit. Thirteen aCWDBs have measured or estimated orbital periods, of which five were presented here. Through a Zeeman analysis, we revise the magnetic field estimate of the polar system J0926+0105, which is likely a low-field polar at B = 16 MG. We quantified the success of X-ray versus optical/UV selection of compact white dwarf binaries which will be relevant for the full SDSS-V survey. We also identified six white dwarf main sequence (WDMS) systems, among them there is one confirmed pre-CV at an orbital period of 17.6 h and another pre-CV candidate. Conclusions. This work presents successful initial work in building large samples of all kinds of accreting and X-ray emitting compact white dwarf binaries that will be continued over the full hemisphere in the years to come.