SRG discovery of a bright, strongly variable, nova-like CV: SRGt 062340.2-265715

ATel #14219; A. Schwope (AIP), A. Semena (IKI), C. Maitra (MPE), D. A.H. Buckley (SAAO), A. Kawka (Curtin University), O. König (ECAP/FAU), A. Lutovinov (IKI), I. Mereminskiy (IKI), A. Miller-Jones (Curtin University), A. Rau (MPE), L. Townsend (SAAO/SALT), J. Wilms (ECAP)
on 25 Nov 2020; 22:30 UT
Credential Certification: Axel Schwope (aschwope@aip.de)

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In the course of the ongoing second all-sky survey, both instruments onboard the Russian/German Spektrum-Roentgen-Gamma Mission (SRG) noticed on 2020 October 12th a bright transient at the eROSITA position RA(J2000):06:23:40.2 (95.91754 deg) DEC(J2000):-26:57:15 (-26.96425 deg) with a positional uncertainty of 1.6 arcsec (1sigma, 0.6” statistical, 1.5” systematical). The Mikhail Pavlinsky ART-XC telescope detected the source at position RA(J2000): 06:23:39.7 DEC(J2000): -26:57:53 with an uncertainty of 15 arcsec (90%). The object was discovered at a mean flux of 1.5e-11 erg/cm^2/s (4-12 keV, effective exposure 25 s) with ART-XC (Pavlinsky et al. 2020, in prep.) but was below the detection threshold during the first survey (f< 1e-11 erg/cm^2/s, effective exposure 21 s). With eROSITA (Predehl et al. 2020, arXiv:2010.03477) the source was discovered in both surveys, in eRASS1 (2020 April 8th) with a mean count rate of 1.38 +- 0.11 ct/s (total exposure 228 s) and in eRASS2 (2020 October 12th) with 5.84+-0.24 ct/s (total exposure 208 s). The joint eROSITA/ART-XC spectrum obtained in October 2020, fitted with a thermal plasma model, gives a temperature kT ~7 keV and a 0.5-2.0 keV flux of 3.8e-12 erg/cm^2/s. The same model applied to the eRASS1 data provided kT ~2-3 keV, at a flux (0.5-2.0 keV) of 8.5e-13 erg/cm^2/s. During eRASS2 the object displayed 90% variability of its X-ray flux between successive scans (separated by 4 hrs). The position coincides with the ROSAT and Swift sources 1RXS J062339.8-265744, 2SXPS J062339.9-265751, and the optical transient ZTF19aaabzuh, respectively. Two short snapshots measurements were reported during XMM-Newton slews on 2003 April 1st and 2006 October 2nd with fluxes of (2.3+/-0.8) e-12 erg/cm^2/s and (5.0+/-0.8) e-12 ergs/cm^2/s within 0.2-2 keV for a power law model, respectively (xmmuls.esac.esa.int/hiligt). The transient coincides with the Gaia object 2899766827964264192 with an apparent G-band magnitude of 12.46 and BP-RP=0.0028 mag. At the estimated distance of 495 +/- 8 pc (Bailer-Jones et al. 2018, AJ 156:58) the absolute magnitude is G=4.0. CRTS and ZTF archival data show pronounced optical variability with an amplitude of up to 1 mag. Single-epoch low- and high-resolution spectra obtained with ANU/WiFeS (2020 October 28th) and SALT/HRS (2020 October 31st) show a blue continuum with broad Balmer absorption lines and narrow emission lines of H-Balmer, HeI, HeII, and the Bowen blend CIII/NIII between 4635 and 4655 Angstrom. The emission lines are symmetric with FWHM of 5.5 Angstrom. The object is tentatively classified as a non-magnetic, nova-like (NL) cataclysmic variable. The classification is based on its position in the color-magnitude diagram, its X-ray spectrum, the X-ray to optical flux ratio, and the appearance of the optical spectrum. The blue continuum and the absorption lines likely originate from an optically thick accretion disk, while the emission lines stem from the optically thin parts of the disk with possible contributions from the irradiated donor star. The very high degree of X-ray variability is somewhat unexpected. In CVs of the NL-type X-rays are thought to originate from the accretion disk boundary layer (Mukai 2017, PASP 129:062001) usually a fairly constant source, possibly with stochastic higher frequency flickering. Follow-up observations at X-ray and optical wavelengths are encouraged. In the optical, time-resolved optical photometry and spectroscopy is needed to ascertain the classification, and to determine the orbital period which is expected to be longer than 4 hours. Variability measurements at all wavebands are needed to uncover the variability timescales and the physical origin of the observed large-amplitude X-ray variability.