Compact white-dwarf binaries in the combined SRG/eROSITA/SDSS eFEDS survey

Aims. Compact white-dwarf binaries are 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 are 12 dwarf novae, three WZ Sge-like disk-accreting non-magnetic CVs with low accretion rates, five likely non-magnetic high accretion rate novalike 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 quantify 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 identify six white-dwarf main-sequence (WDMS) systems, among them one confirmed pre-CV at an orbital period of 17.6 hours and another pre-CV candidate.Comment: Revised version, submitted to A&A, original submitted Oct 30, 202

The SRG/eROSITA all-sky survey: First X-ray catalogues and data release of the western Galactic hemisphere

The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930 000 entries detected in the most sensitive 0.2- 2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3- 5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We present methods to identify and flag potential spurious sources in the catalogues, which we applied for this work, and we tested and validated the astrometric accuracy via cross-comparison with other X-ray and multi-wavelength catalogues. We show that the number counts of X-ray sources in eRASSl are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F05- 2 keV &gt; 5 × 10-14 erg s-1 cm-2, we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1- 2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided

A new Bowen fluorescence flare and extreme coronal line emitter discovered by SRG/eROSITA

The nuclear X-ray transient eRASSt J012026.5-292727 (J012026 hereafter) was discovered in the second SRG/eROSITA all-sky survey (eRASS2). The source appeared more than one order of magnitude brighter than the eRASS1 upper limits (peak eRASS2 0.2-2.3 keV flux of 1.14 x 10(-12) erg cm(-2) s(-1)) and with a soft X-ray spectrum (photon index of Gamma = 4.3). Over the following months, the X-ray flux started decaying and demonstrated significant flaring activity on both short (hour) and long (year) timescales. By inspecting the multiwavelength light curves of time-domain wide-field facilities, we detected a strong mid-infrared flare, which evolved over two years, and a weaker optical counterpart, with possible hints of a rise > 3 years prior to the X-ray discovery. Follow-up optical spectroscopy revealed transient features, including redshifted Balmer lines (FWHM of similar to 1500 km s(-1)), strong Fe II emission, He II, Bowen fluorescence lines, and high-ionization coronal lines such as [Fe X] and [Fe XIV]. One spectrum displayed a triple-peaked H beta line, consistent with emission from a face-on elliptical accretion disk. The spectroscopic features and the slow evolution of the event place J012026 within the nuclear-transient classifications of Bowen fluorescence flares (BFFs) and extreme coronal line emitters (ECLEs). BFFs have been associated with rejuvenated accreting supermassive black holes, although the mechanism triggering the onset of the new accretion flow is yet to be understood, while ECLEs have been linked to the disruption and accretion of stars in gas-rich environments. The association of J012026 with both classes, combined with the X-ray, multiwavelength, and spectroscopic information, supports the idea that the BFF emission could be, at least in some cases, triggered by tidal disruption events (TDEs) perturbing gaseous environments. The observed short- and long-term X-ray variability, uncommon in standard TDEs, adds complexity to these families of nuclear transients. These results highlight the diverse phenomenology of nuclear accretion events and demonstrate the value of systematic X-ray surveys, such as eROSITA and Einstein Probe, for uncovering such transients and characterizing their physical origin

Searching for supermassive black hole binaries within SRG/eROSITA-De I. Properties of the X-ray selected candidates

Context. Supermassive black hole binaries (SMBHBs) with separations on (sub)pc scales represent one of the latest stages of hierarchical galaxy assembly. However, many of these objects are hidden behind large columns of gas and dust at the centers of galaxies and are difficult to detect. In these systems, accretion is expected to take the form of two individual accretion disks around the individual black holes, fed by a larger circumbinary disk. The X-ray and UV emission in these systems are predicted to vary regularly on timescales that are comparable to that of the orbital period of the binary. Aims. This is the first of a series of papers where we continue to search for and characterize SMBHB candidates based on quasiperiodic light curves from the soft X-ray instrument eROSITA on board the Spectrum-Roentgen-Gamma (SRG) observatory and extensive X-ray follow-up. Methods. We searched the multi-epoch SRG/eROSITA all-sky surveys for extragalactic sources that show an "up-down-up-down" or "down-up-down-up" profile (from scan to scan) in their 0.2-2.3 keV flux light curves. We selected sources where the "up" and "down" flux levels vary by at least 3 sigma. The "down" states were also allowed to correspond to nondetections. We excluded stellar objects, blazars, and radio-loud active galactic nuclei (AGNs) via Gaia DR3 parallaxes and proper motions. We also carried out a visual inspection of the images drawn from the Legacy Survey DR10 and the SIMBAD database. Results. We compiled a sample of 16 sources that are suitable for X-ray follow-up campaigns given their brightness and significant variability between bright and faint SRG/eROSITA flux levels. We triggered extensive Swift-XRT and NICER monitoring campaigns on the best SMBHB candidates to confirm or discard their tentative periodicities. Optical spectroscopic observations confirmed the nuclear and extragalactic nature of 15/16 objects and enabled single-epoch SMBH mass measurements and BPT classifications of the dominant ionization in the host galaxy. Our most promising candidate, eRASSt J0530-4125, exhibits X-ray quasi-periodic variability with a typical time scale of one year in the observed frame. By stacking the X-ray observations of each source in our sample, we found that 14 out of the 15 sources can be modeled by a power law with a photon index ranging from Gamma similar to 1.8-2.8. Based on our selection, we estimate an optimistic upper limit on the fraction of SMBHB candidates to be similar to 0.05 per galaxy. We emphasize that further observational evidence is needed to confirm the SMBHB nature of our sources