eRASSt J082337+042303: A bright, ultra-soft, high-amplitude transient in the direction of 2MASX J08233674+042300
ATel #13712; A. Malyali, A. Rau, R. Arcodia, Th. Boller, S. Carpano, F. Haberl, A. Merloni, K. Nandra, J. Buchner, T. Liu, M. Salvato (all MPE), J. Wilms, O. Koenig (both ECAP/FAU), M. Krumpe, G. Lamer (both AIP)
on 7 May 2020; 17:31 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)
Subjects: Optical, X-ray, AGN, Transient, Tidal Disruption Event
During its first all-sky survey, the eROSITA instrument aboard SRG detected a new, ultra-soft X-ray source on 2020-04-28, designated as eRASSt J082337+042303, and located at:
RA(J2000)= 08:23:36.8
Dec(J2000)= +04:23:03.4
with an estimated positional uncertainty of 1.4â radius (68% confidence interval). The X-ray position coincides with the low-redshift galaxy 2MASX J08233674+0423027 (photo_z~0.045+/-0.014) whose WISE colour of W1-W2=0.08mag is inconsistent with a typical AGN.
The X-ray spectrum can be fitted by a blackbody with kT=85+/-6 eV and absorbed with Galactic N_H=2.2e20 cm^-2, or alternatively with an absorbed powerlaw of slope 3.89+/-0.13 (uncertainties on spectral parameters approximate 68% credible regions, and spectra were analysed using the software BXA, Buchner et al. 2014). The source was not detected above 2.3keV. The 0.2-2 keV flux of ~1.5E-12 erg/s/cm^2 is 90x brighter than the 0.2-2keV 3-sigma upper flux detection limit of 1.7e-14 erg/s/cm^2 from an XMM pointed observation in 2015 (the source was previously non-detected in ROSAT and XMM). The upper flux detection limits were computed using the Upper Limit Server (http://xmmuls.esac.esa.int/upperlimitserver/ ), assuming an absorbed blackbody spectral model with kT=100eV and N_H=3e20 cm^-2.
The large amplitude flaring, soft X-ray spectrum, host galaxy properties, and the implied X-ray luminosity in the 0.2-2keV energy range of ~8e42 erg/s, would make the source a strong tidal disruption event (TDE) candidate.
This interpretation, however, is challenged by ZTF observations over the last 450 days, which have shown large amplitude nuclear variability of the ZTF transient AT2019avd/ ZTF19aaiqmgl (https://lasair.roe.ac.uk/object/ZTF19aaiqmgl/) associated with 2MASX J08233647+0423027. Between 2019-02-24 and 2020-01-01, the host nucleus' g-band magnitude decayed nearly monotonically from 17.13+/-0.09 to 20.08+/-0.20mag, followed by a re-brightening to 18.58+/-0.13 on the 2020-05-03. The eROSITA observation occurred close in time to the second peak. This behaviour is atypical for a standard TDE, but it is also unusual behaviour for AGN variability (especially when considering the ultra-soft X-ray emission).
eROSITA scanned over the source four times (with four hours between each visit) on 2020 April 28th, and there was no strong variability observed between scans. eROSITA will return to this part of the sky again only 6 months later. Further multi-wavelength follow-up is strongly encouraged.
