Jan Robrade

Context. γ Cas stars are Oe or Be stars that exhibit bright and hard X-ray emission. HD 45314 belonged to this category, but lost its γ Cas characteristics as its circumstellar disc started to fade away. Aims. The successive steps of the disc dissipation and its subsequent ongoing rebuilding provide insight into the physical processes at work. Methods. The star was monitored in optical spectroscopy, optical photometry, and X-ray spectroscopy to follow its variability on timescales of hours to years. Emission line properties were measured on the spectra and compared with optical brightness and colours. Time series of magnitudes, colours, line equivalent widths, and violet over red peak intensity ratios were analysed with Fourier methods to uncover possible periodicities. Results. For five years, HD 45314 exhibited magnitude and line strength oscillations on a timescale of 230 d. It then declined towards a nearly disc-free stage and is now slowly rebuilding its disc. During these phases, the violet over red peak intensity ratio exhibited a modulation on a timescale of about 1190 d. Over the whole campaign, prominent short-term photometric variations, notably at a frequency of 3.369 d ⁻¹ , were probably due to β Cep-type pulsations. The amplitude of these pulsations was stronger during phases of overall brightness changes. In parallel, the X-ray spectrum switched to a low state at the onset of the oscillation phase and has remained in this low state since then. However, some residual hard emission is observed. Conclusions. HD 45314’s long-term optical variations bear resemblance to the predictions of smooth particle hydrodynamics calculations, although a close look does reveal some inconsistencies. Unlike some other γ Cas stars, where the X-ray emission remained nearly constant despite major changes of the circumstellar disc, HD 45314’s X-ray properties changed dramatically as the disc began to dissipate. These differences might arise from a wider orbital separation between the Oe star and its putative white dwarf companion or from the ablation of the disc by the radiation field of the Oe star.

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 . The F-, G-, K-, and M-type stars are the most abundant stellar population in the Milky Way and are expected to contribute to its diffuse X-ray emission. Yet their intrinsic average X-ray spectrum remains poorly constrained due to their faint X-ray luminosities, leaving their collective role in the X-ray background of the Milky Way uncertain. Aims . We aim to derive a distance-normalised average X-ray spectrum for nearby M dwarfs and F, G, and K stars and to characterise their ensemble spectral properties. Methods . We analysed the volume-complete sample of M dwarfs (M0-M6) and F, G, and K stars within 10 pc of the Sun using data from the eROSITA all-sky survey aboard the Spectrum-Roentgen-Gamma (SRG) mission (eRASS:4). Individual spectra were normalised by exposure and distance and then stacked to produce representative averages. From these spectra, we derived characteristic X-ray luminosities, which are helpful for estimating the contribution of late-type stars to the soft, diffuse X-ray emission of the Galaxy. Results . The distance-normalised emission measures yield an average X-ray luminosity of (2.6 ± 0.1) × 10 ²⁷ erg/s for M-type stars, and (15 ± 3) × 10 ²⁷ erg/s for F-, G-, and K-type stars in 0.2-2.0 keV. The 10-pc average spectra could be well described by a sum of three and two thermal models. The fitted temperatures and abundances remain consistent across M-star sub-groups, while early-M stars are, surprisingly, less luminous on average than mid-to-late-M types. These results offer new insights into the collective X-ray properties of nearby stars in the volume-complete context, and provide motivation to further explore the link with the unresolved soft X-ray background of the Galaxy.

The rotation-activity connection is the standard paradigm for interpreting chromospheric and coronal activity in late-type stars, namely, stars with outer convection zones. This paradigm states that activity increases with decreasing rotation period until a saturation limit is reached. By scaling rotation periods with the convective turnover time via the Rossby number, Ro, saturation is expected to occur at a universal value across all spectral types. In our paper, we systematically investigate the relationship between rotation and activity as measured though X-ray emission for a large sample of late-type stars to test the universal applicability of this paradigm. To this end, we utilized TESS short-cadence space photometry to determine the rotation periods for late-type stars identified in the eROSITA all-sky survey. This combined dataset provides rotation and X-ray measurements for 14 004 stars, representing a sample size increase of more than an order of magnitude compared to previous studies. Our results show that F-type stars do not reach the “classical” saturation limit of L _X /L _bol ≈ 10 ⁻³ , instead exhibiting significantly lower L _x / L _bol values. We find that the convective turnover times derived from this sample closely agree with theoretical computations, supporting the idea that Rossby number-activity relations hold for all late-type main sequence stars. The lower level of activity in earlier spectral types (e.g., F-type and late A-type stars) is a physical consequence of their short convective turnover times, which prevent them from rotating rapidly enough to ever reach the saturation regime. We demonstrate that a simple model incorporating our derived turnover times versus color can successfully reproduce the observed characteristics of the eROSITA X-ray activity distribution, as measured by the L X /L _bol ratio and Gaia BP-RP color.

Context . Radio emission from magnetically active stars arises primarily from non-thermal processes and provides information complementary to their high-energy X-ray emission. Recent advances in sensitive, wide-field radio and X-ray surveys have enabled the identification of larger samples of active stars across the Galaxy. Aims . We aim to identify and characterise radio and X-ray-emitting stars in the Galactic plane by combining MeerKAT radio data with soft X-ray observations, and to assess their consistency with the canonical Güdel-Benz relation, which states that the thermal coronal emission observed in soft X-rays is correlated with the non-thermal gyrosynchrotron emission detected in the radio. Methods . We cross-matched compact radio sources from the SARAO MeerKAT Galactic Plane Survey with soft X-ray counterparts from the ROSAT All-Sky Survey and the first release of the SRG/eROSITA all-sky survey, the eRASS1. We investigated their radiobrightness temperatures and computed radio and X-ray luminosities to test for consistency with the Güdel-Benz relation. Results . We identify 137 stellar sources with both radio and X-ray detections. Their radio-brightness temperatures, T _B , range from 10 ⁷ to 10 ¹² K, with the exception of two outliers: AXJ1600.9-5142 ( T _B = 4.8 ± 1.5 × 10 ¹² K) and HD 124831 ( T _B = 8 ± 1 × 10 ⁶ K). The remaining sources are consistent with emission from incoherent gyrosynchrotron processes. The sample generally lies below the canonical Güdel-Benz relation, with the offset predominantly driven by enhanced radio luminosities at 1.3 GHz relative to the canonical relation derived at 5 GHz. Conclusions . Our results suggest that the classical Güdel-Benz relation represents an upper envelope rather than a tight correlation for active stars detected at 1.3 GHz. In addition, the eROSITA detections show that early-type stars are systematically below the log( L _X / L _bol )~ −3 relation, which is usually seen for stars of a later spectral type.

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.

With its first All-Sky Survey (eRASS1), the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) mission has offered an unprecedented, comprehensive view of the variable X-ray sky. Featuring enhanced sensitivity, broader energy coverage, and improved resolution compared to prior surveys, the eRASS1 Data Release 1 (DR1) catalogue underwent a variability analysis, and in this paper, we performed an advanced variability analysis focusing on a substantial subset of 128 669 sources, all exhibiting a net count exceeding ten. We performed multiple variability tests, utilising conventional normalised excess variance (NEV), maximum amplitude variability (AMP), and Bayesian excess variance methods (bexvar). The analysis focused on binned light curves; specifically, employing one eroday (a great circle scan with a duration of 4 hours) binning of the German part of the first eROSITA all-sky survey (eROSITA-DE) data, i.e., the source sample covers only half of the sky. Within the 128 669 DR1 sources with light curves, our research pinpointed 808 light curves that show hints of variability according to the AMP test, and 298 according to the NEV test. However, after applying suitable thresholds, 90 (123) sources were found to be significantly variable according to the AMP (NEV) tests. In addition, 1342 sources are considered variable according to the Bayesian test bexvar. The total number of unique sources is 1709, and they form the catalogue of variable sources released with this paper. We cross-matched with existing X-ray catalogues and identified 258, 318, 598, and 120 sources in 4XMM DR13, 2SXPS, 2RXS, and CSC2.1, respectively. Only 27 sources overlap across all catalogues, while 882 are new X-ray detections from eROSITA DR1. About 70% are coronal stars, 5% are Quasi-Stellar Objects, and 1.6% are normal galaxies. We further subclassified 18 sources as LMXBs, 11 as HMXBs, and 14 as bright stars. In this paper, we analyse the variability of eRASS1 sources on a timescale of only a few days. To study the physics of variable sources, we need more deeply pointed observations with other X-ray missions or at least the final depth of the eRASS: 8 observations. The timescale of the eRASS1 observations is not representative of the timescales of the expected upcoming eRASS catalogues. A substantial 52% of the eRASS1 variable sources were first discovered with eROSITA. The DR1 variability catalogue is excellent for follow-up observations with telescopes such as XMM-Newton, Chandra, or Swift.

Giant stars can be magnetically active. A prominent example is HD 251108, which exhibited a giant flare recorded on 7 November 2022 at X-ray and optical wavelengths. We report on a spectroscopic campaign monitoring this star from November 2022 to March 2024, starting with the late phases of the giant flare and accumulating 149 medium-resolution spectra with the TIGRE telescope. HD 251108 shows periodic variations in radial velocity (RV), which we interpret as being caused by the star being part of a binary system, yet the photospheric lines do not show any obvious signs of a second component. Similar periodic changes are found in the effective temperature and in the equivalent widths (EWs) of various chromospheric emission lines with the same period as observed in RV, but with a phase shift. We find these effects could be caused by a plage rotating with the star if the star is rotating synchronously. Since the star shows strong Hα and even He I infrared triplet emission outside the flare, we find it to be hyperactive, even though the star only has moderate lithium abundance of A(Li) = 1.28 dex. We further investigated archival TESS observations of HD 251108 and found that flares with energy releases in excess of 10³⁸ erg at optical wavelengths are not unusual. Finally, considering the November 2022 event in a broader context, we argue that this event likely released more than 10⁴⁰ erg at optical wavelengths.

Stellar activity can be observed at different wavelengths in a variety of different activity indicators. We investigated the correlation between coronal and chromospheric emissions by combining X-ray data from stars detected in the eROSITA all-sky surveys (eRASS1 and eRASS:5) with Ca II infrared triplet (IRT) activity indices as published in the third Gaia data release (Gaia DR3). We specifically studied 24 300 and 43 200 stellar sources with reliable Ca II IRT measurement and X-ray detection in eRASS1 and eRASS:5, which is by far the largest stellar sample available so far. The largest detection fraction is obtained for highly active sources and stars of a late spectral type, while F-type and less active stars (as measured in the Ca II IRT) remain mostly undetected in X-rays. Also, the correlation is the strongest for late-type sources, while F-type stars show a rather weak correlation between the X-ray to bolometric flux ratio and the Ca II IRT activity index. The relation between the X-ray and Ca II IRT surface fluxes changes with the fractional X-ray flux without showing two separated branches as described in previous studies. For fast rotators, both activity indicators saturate at a similar Rossby number and the X-ray to bolometric flux ratio decreases faster than the IRT index for slower rotating stars. As a consequence, the ratio between X-ray and IRT fluxes is constant in the saturation regime and decreases for slow rotators.