Jaco Brink

Context. During their secular evolution, cataclysmic variable stars (CVs) evolve from long to short orbital periods ( P _orb ) before reaching a minimum near P _orb ∼ 80 min. Beyond this stage, the donor departs from thermal equilibrium or becomes progressively degenerate, driving the system toward longer P _orb . CVs that have evolved past this evolutionary turning point are known as period-bouncers (PBs). Despite predictions that 40−80% of all CVs should be PBs, only 3−25% of the observed CV population is actually composed of PBs. This discrepancy could possibly be attributed to their intrinsic low luminosities due to low mass-transfer rates. Aims. We aim to investigate the evolutionary status of 213 SRG/eROSITA-selected PB candidates through spectroscopic follow-up and time-domain observations. The sample also includes 19 previously confirmed PBs, which serve as benchmarks for evaluating the candidates. Methods. We confirmed CVs among the PB candidates through the identification of Balmer emission lines in follow-up optical spectra from Sloan Digital Sky Survey-V (SDSS-V). Additionally, archival photometric data from multiple time-domain surveys, including Zwicky Transient Facility (ZTF), Asteroid Terrestrial-impact Last Alert System (ATLAS), All-Sky Automated Survey for Supernovae (ASAS-SN), and Catalina Real-time Transient Survey (CRTS), were used to construct long-term light curves and search for dwarf-nova outbursts. By fitting hydrogen-rich atmosphere models to the SDSS-V spectra, we were able to estimate the effective temperature ( T _eff ) and secular mass accretion rate (⟨ Ṁ ⟩) of the WDs. We also measured the Balmer decrements, used as diagnostics of the physical conditions of the accretion disc, to assess whether they are consistent with known PBs. In addition, we analysed archival two-minute cadence light curves from the Transiting Exoplanet Survey Satellite (TESS) to determine P _orb for a subset of systems, and compiled multi-wavelength photometry data to construct and model spectral energy distributions (SEDs), which we used to infer approximate donor spectral types. Finally, we identified magnetic systems through Zeeman splitting in the Balmer lines and estimated their mean photospheric magnetic field strengths. Results. We report the confirmation of 24 new CVs, comprising 18 systems identified through Balmer emission lines and 8 through outburst activity, with two systems independently identified by both methods. We constructed the SEDs for 14 of these systems, yielding donor T _eff < 1850 K in 13 cases and T _eff ≲ 2350 K for the remaining system. Our spectroscopic analysis of the new CVs observed with SDSS-V indicates that they are consistent with a PB nature, potentially increasing the population of confirmed PBs by ∼50%. Conclusions. Our results suggest that a substantial fraction of the PB population may remain hidden in WD catalogues. The combination of X-ray selection with spectroscopic and time-domain observations has led to the identification of additional systems that are very likely PBs. This strategy emerges as a powerful means to uncover these elusive systems, offering a systematic approach toward resolving the long-standing discrepancy between theoretical predictions and the observed PB population.

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.

Context. We report on optical spectroscopic and photometric follow-up observations of the eROSITA discovered transient SRGt 062340.2-265751 and show that it displays the characteristics of a nova-like cataclysmic variable (CV), with possible indications of being a magnetic system. Aims. We try to put better constraints on the classification of SRGt 062340.2-265751 using optical time-resolved spectroscopic and photometric observations to find any periodicities in the system. From these periodicities we can classify the CV sub-type that it belongs to. Methods. Time-resolved photometric and spectroscopic observations were obtained at the South African Astronomical Observatory (SAAO), using the 1.9 m and the two 1.0 m telescopes. High-resolution spectra were also taken using the Southern African Large Telescope (SALT), to study the morphology of the emission lines seen in the system. Archival photometric data was also analysed in this study, including ASAS-SN, CRTS, and TESS observations. Results. Spectroscopic observations revealed a very low amplitude, K ∼ 14 km s ⁻¹ , in the radial velocity of the H β and H γ emission lines, suggesting that the system is likely observed at a low inclination angle. High-speed photometric observations revealed highly stochastic variability, characteristic of many magnetic cataclysmic variable systems. A probable 3.645 ± 0.006 hour orbital period was found by applying Lomb-Scargle period analysis to the H β and H γ emission line radial velocities. A 24.905 ± 0.065 min period was found from photometric observations, which we associate with the white dwarf spin. However, it was also found that the photometry revealed multiple periodicities from night to night. TESS observations in three sectors did not reveal any of the periodicities found from ground-based observations, but did show a prominent period in only one sector, which might be attributed to a positive superhump period. These multiple periodicities as well as the He II λ 4686 and Bowen blend emission lines seen in the spectra indicate that SRGt 062340.2-265751 is likely a nova-like CV, and might belong to the VY Scl sub-type.

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.

Context. Accurately accounting for the Active Galactic Nucleus (AGN) phase in galaxy evolution requires a large, clean AGN sample. This is now possible with SRG/eROSITA, which completed its first all-sky X-ray survey (eRASS1) on June 12, 2020. The public Data Release 1 (DR1, Jan 31, 2024) includes 930,203 sources from the western Galactic hemisphere. Aims. The data enable the selection of a large AGN sample and the discovery of rare sources. However, scientific return depends on accurate characterisation of the X-ray emitters, requiring high-quality multi-wavelength data. This paper presents the identification and classification of optical and infrared counterparts to eRASS1 sources. Methods. Counterparts to eRASS1 X-ray point sources were identified using Gaia DR3, CatWISE2020, and Legacy Survey DR10 (LS10) with the Bayesian NWAY algorithm and trained priors. Sources were classified as Galactic or extragalactic via a machine-learning model combining optical/IR and X-ray properties, trained on a reference sample. For extragalactic LS10 sources, photometric redshifts were computed using C IRCLEZ . Results. Within the LS10 footprint, all 656,614 eROSITA/DR1 sources have at least one possible optical counterpart; ∼570 000 are extragalactic and likely AGN. Half are new detections compared to AllWISE, Gaia, and Quaia AGN catalogues. Gaia and CatWISE2020 counterparts are less reliable, due to the survey’s shallowness and the limited amount of features available to assess the probability of being an X-ray emitter. In the Galactic plane, where the overdensity of stellar sources also increases the chance of associations, using conservative reliability cuts, we identified approximately 18 000 Gaia and 55 000 CatWISE2020 extragalactic sources. Conclusions. We have released three high-quality counterpart catalogues – plus the training and validation sets – as a benchmark for the field. These datasets have many applications, but in particular, they empower researchers to build AGN samples tailored for completeness and purity, accelerating the hunt for the Universe’s most energetic engines.

CXOU J110926.4−650224 is a candidate transitional millisecond pulsar (tMSP) with X-ray and radio emission properties reminiscent of those observed in confirmed tMSPs in their X-ray ‘subluminous’ disc state. We present the results of observing campaigns that, for the first time, characterise the optical and near-infrared variability of this source and establish a connection with the mode-switching phenomenon observed in X-rays. The optical emission exhibited flickering activity, frequent dipping episodes where it appeared redder, and a multi-peaked flare where it was bluer. The variability pattern was strongly correlated with that of the X-ray emission. Each dip matched an X-ray low-mode episode, indicating that a significant portion of the optical emission originates from nearly the same region as the X-ray emission. The near-infrared emission also displayed remarkable variability, including a dip of 20 min in length during which it nearly vanished. Time-resolved optical spectroscopic observations reveal significant changes in the properties of emission lines from the disc and help infer the spectral type of the companion star to be between K0 and K5. We compare the properties of CXOU J110926.4−650224 with those of other tMSPs in the X-ray subluminous disc state and discuss our findings within the context of a recently proposed scenario that explains the phenomenology exhibited by the prototypical tMSP PSR J1023+0038.

We report the identification and subsequent examination of a polar-type cataclysmic variable named XMM J152737.4−205305.9 newly discovered with the X-ray Multi-Mirror Mission (XMM-Newton). This discovery was made by matching the XMM-Newton data archive with the cataclysmic variable candidate catalog provided by Gaia Data Release 3 (DR3). The use of X-ray photometry led to the identification of two distinct dips that exhibit a recurring pattern with a precise period of 112.4 (1) min in two XMM-Newton observations made one year apart. The data obtained from photometry provided by the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey consistently indicate the presence of mass-accretion states that differ by up to 2 mag. Following the optical data, the Spectrum Roentgen Gamma(SRG)/eROSITA All Sky Survey observed the system at two different X-ray levels, which may imply different accretion states. Following these observations, the low-resolution spectrum obtained using SALT spectroscopy exposes the prominent hydrogen Balmer and helium emission lines, strongly supporting the categorization of this system as a polar-type magnetic cataclysmic variable. The XMM-Newton observations conducted at various X-ray levels reveal a consistent pattern of a deep dip-like feature with a width of ≈9.1 min. This feature implies the presence of an eclipse in both observations. According to Gaia data, the object is located at a distance of 1156₋₃₃₉⁺⁷²⁰ pc, and its X-ray luminosity lies within the L_X = (3 − 6)×10³¹ erg s⁻¹ range.

X-ray quasi-periodic eruptions (QPEs) are a novel addition to the group of extragalactic transients. With only a select number of known sources, and many more models published trying to explain them, we are so far limited in our understanding by small number statistics. In this work, we report the discovery of two further galaxies showing QPEs, hereafter named eRO-QPE3 and eRO-QPE4, with the eROSITA X-ray telescope on board the Spectrum Roentgen Gamma observatory, followed by XMM-Newton, NICER, Swift-XRT, SALT (z = 0.024 and z = 0.044, respectively), and ATCA observations. Among the properties in common with those of known QPEs are: the thermal-like spectral shape in eruption (up to kT ∼ 110 − 120 eV) and quiescence (kT ∼ 50 − 90 eV) and its evolution during the eruptions (with a harder rise than decay); the lack of strong canonical signatures of active nuclei (from current optical, UV, infrared and radio data); and the low-mass nature of the host galaxies (logM_* ≈ 9 − 10) and their massive central black holes (logM_BH ≈ 5 − 7). These discoveries also bring several new insights into the QPE population: (i) eRO-QPE3 shows eruptions on top of a decaying quiescence flux, providing further evidence for a connection between QPEs and a preceding tidal disruption event; (ii) eRO-QPE3 exhibits the longest recurrence times and faintest peak luminosity of QPEs, compared to the known QPE population, excluding a correlation between the two; (iii) we find evidence, for the first time, of a transient component that is harder, albeit much fainter, than the thermal QPE spectrum in eRO-QPE4; and (iv) eRO-QPE4 displays the appearance (or significant brightening) of the quiescence disk component after the detection of QPEs, supporting its short-lived nature against a preexisting active galactic nucleus. These new properties further highlight the need to find additional QPE sources to increase the sample size and draw meaningful conclusions about the intrinsic population. Overall, the newly discovered properties (e.g., recent origin and/or transient nature of the quiescent accretion disk; lack of correlation between eruption recurrence timescales and luminosity) are qualitatively consistent with recent models that identify QPEs as extreme mass-ratio inspirals.

The Swift Deep Galactic Plane Survey (DGPS) is a Swift Key Project consisting of 380 tiled pointings covering ∼40 deg² of the Galactic plane between longitude 10 < ∣l∣ < 30 deg and latitude ∣b∣ < 0.5 deg. Each pointing has a 5 ks exposure, yielding a total of 1.9 Ms spread across the entire survey footprint. Phase I observations were carried out between 2017 March and 2021 May. The survey is complete to depth L _X > 10³⁴ erg s⁻¹ to the edge of the Galaxy. The main survey goal is to produce a rich sample of new X-ray sources and transients, while also covering a broad discovery space. Here, we introduce the survey strategy and present a catalog of sources detected during Phase I observations. In total, we identify 928 X-ray sources, of which 348 are unique to our X-ray catalog. We report on the characteristics of sources in our catalog and highlight sources newly classified and published by the DGPS team.