eRASSt_J093641.9+111347: Discovery of an ultra-soft nuclear transient in a FRII radio galaxy at z=0.12
ATel #14668; I. Grotova, Z. Liu, A. Malyali, A. Rau, A. Merloni (all MPE)
on 28 May 2021; 17:45 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)
Subjects: X-ray, AGN, Transient, Tidal Disruption Event
On 2021 May 13th, the eROSITA instrument (Predehl et al. 2021) aboard the Russian/German Spektrum-Roentgen-Gamma (SRG) mission (Sunyaev et al. 2021) discovered a new ultra-soft X-ray source designated as eRASSt_J093641.9+111347 and located at:
Ra(J2000) = 09:36:41.99
Dec(J2000) = +11:13:50.96
with an estimated positional uncertainty of ~5" radius (incl. systematics).
The eROSITA X-ray spectrum is best fit by an absorbed blackbody model with kT=70 +/- 6 eV and N_H=(1.6 +/-0.5)e20 cm^{-2}, using the Bayesian X-ray Analysis spectral fitting software (Buchner+14). The inferred N_Hs is consistent with the Galactic foreground value along the line of sight. The corresponding 0.2-2.3keV flux is ~8.7e-13 erg/cm^2/s, ~13x times brighter than in the 3-sigma upper limit derived from the previous eROSITA scans over the position taken in May and November 2020 (upper limit from ROSAT is less constraining).
Follow-up DDT observations with NICER were performed on 26-27th May 2021. Fitting the currently available quick look data with an absorbed blackbody model over the 0.4-1keV energy band suggests that the source has not changed significantly since the eROSITA discovery.
The eROSITA position coincides with SDSS J093641.99+111350.9, a galaxy with a spectroscopic redshift of z=0.127. The archival SDSS spectrum does not show any signature of nuclear activity. Similarly, the WISE color of the galaxy (WISEA J093642.00+111351.0) of W1-W2=0.15mag is inconsistent with an actively accreting radiatively efficient AGN. In contrast, the galaxy was classified as a FRII radio galaxy in van Velzen et al. (2015) based on its strong radio symmetrical jets detected in the VLA FIRST Survey.
At the redshift of z=0.12, the eROSITA flux corresponds to an 0.2-2keV luminosity of ~3e43 erg/s. The ultra-soft X-ray spectrum, and optical properties of the host galaxy make the source a very good candidate for a new eROSITA Tidal Disruption Event (TDE). The association with a FRII galaxy on the other hand, could also suggest that we witness the onset of an AGN. Further multi-wavelength follow-up is strongly encouraged to disentangle the nature of the event.
We are grateful to the NICER PI K. Gendreau for quickly approving and scheduling the DDT observations.
