SRG/eROSITA catalogue of X-ray active SDSS dwarf galaxies

Abstract

We present a sample of 99 dwarf galaxies (M* < 109.5 M⊙) with X-ray activity in their central regions. The sample was obtained from a match of the SRG/eROSITA X-ray catalogue in the eastern galactic hemisphere with the MPA-JHU SDSS catalogue. The obtained matches were cleaned rigorously with the help of external optical catalogues to increase the purity of the sample. This work is the largest study of this kind – X-ray activity in ≈85 per cent of matched dwarfs was not reported before. The majority of X-ray active dwarfs are identified as star-forming galaxies. However, the X-ray luminosity of 82 objects cannot be explained by the collective emission of X-ray binaries, rendering them strong candidates for dwarf galaxies with an active accreting black hole in their centre. We find that the fraction of AGN among dwarf galaxies drops from ∼2 · 10−2 at LX ∼ 1039 erg s–1 to ∼(2 − 4) · 10−4 at LX ∼ 1041 erg s–1 and increases with the stellar mass of the host galaxy. We serendipitously discovered sources with unexpected properties. We report on a tidal disruption event (TDE) candidate in a dwarf galaxy, a massive black hole in a dwarf galaxy with a soft thermal spectrum, a luminous dwarf galaxy with an obscured X-ray spectrum and a few other peculiar sources. We found three Ultra-luminous X-ray (ULX) source candidates and a sample of X-ray bright galaxy pairs, in four of which both members shine in X-rays.

Delayed and Fast-rising Radio Flares from an Optical and X-Ray-detected Tidal Disruption Event in the Center of a Dwarf Galaxy

“…No significant change in the spectral features, e.g., continuum and emission lines, is observed before and after the transient event, indicating that either any optical signatures had faded by the time of our follow-up observation, or there is no associated emission-line echo. The latter scenario seems consistent with nondetection of a mid-infrared echo from the WISE light curve (Bykov et al 2024).…”

“…In a more realistic scenario, if the viscous time (t visc ) is not negligible (the time it takes material to accrete), and t visc  t circ , the flare is prolonged at the expense of reduced peak luminosity (e.g., Guillochon & Ramirez-Ruiz 2015). These predicted accretion properties seem not at odds with the long rise to peak time and relatively low peak luminosity (L 0.3−2 keV ∼ 5.5 × 10 42 erg s −1 ) observed with eROSITA (Bykov et al 2024). The blackbody radius inferred from the eROSITA observations is R bb ∼ 2 × 10 11 cm, comparable to the Schwarzschild radius (R s = 2GM BH /c 2 ) for a BH mass of ≈10 6 M e .…”

“…The first flux rising epoch (MJD = 58285) was set to be the discovery time of the optical flare, which is on 2018 June 16. Using this discovery time and the occurrence time of the X-ray brightening from Bykov et al (2024), we estimated a more precise time delay between the optical and the X-ray flares to be ≈590 days. We measure an offset between the transient position reported in the ZTF observations during the optical flare (R.A. = 02 h 33 m 46 9308, decl.…”

“…This may suggest that the multiband flares of AT 2018cqh could be due to active galactic nucleus (AGN) activity. However, this possibility seems unlikely based on the X-ray properties (Bykov et al 2024). In addition, the rapid (duration of <1 yr) optical flare can also rule out the normal AGN variability, as it is rarely seen in the light curves of AGNs (Drake et al 2011;Kankare et al 2017;Zhang et al 2022).…”

“…SRGe J023346.8−010129 was reported as a candidate TDE in a dwarf galaxy (z = 0.048) by SRG/eROSITA (Bykov et al 2024) on 2019 November 13. Prior to the X-ray discovery, the object had also been named as an optical transient by the Gaia Alerts Team (Gaia18bod) on 2018 June 16 but was not classified.…”