SRG/eROSITA discovery of the classical nova AT2021jiu as a bright super-soft X-ray source

ATel #14927; Iuliia Grotova (MPE), Christian Kirsch (ECAP), Arne Rau (MPE), Ole König (ECAP), Ingo Kreykenbohm (ECAP), Mirko Krumpe (AIP), Georg Lamer (AIP), Zhu Liu (MPE), Adam Malyali (MPE), Axel Schwope (AIP), Jörn Wilms (ECAP)
on 21 Sep 2021; 13:34 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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On 2021 September 17th, the near real time analysis of data from of the eROSITA instrument (Predehl et al. 2021, A&A 647, A1) aboard the Russian/German Spektrum-Roentgen-Gamma (SRG) mission (Sunyaev et al. 2021, arXiv:2104.13267) discovered a bright super-soft X-ray source designated as eRASSt_J170907.9-373041 and located at:

R.A.(J2000) = 17:09:08.04 (257.28350deg)
Dec.(J2000) = -37:30:40.3 (-37.51119deg)

with an estimated positional uncertainty of ~2" radius (incl. systematic uncertainty).

The X-ray position is consistent with AT 2021jiu (PNV J17091000-3730500, ASASSN-21fh) discovered by Stanek & Kochanek following its large amplitude (>8mag in g-band) optical outburst in April 2021 (https://www.wis-tns.org/object/2021jiu ) and spectroscopically classified as Fe II classical nova (Joshi et al., ATel #14544).

As part of its regular 4th all-sky survey, eROSITA scanned over the position of AT 2021jiu 8 times on 2021 September 17th with ~40s long visits separated by 4hr. During this period, the X-ray flux remained constant with the average value of ~9 cts/s.

The eROSITA spectrum is well described by an absorbed blackbody model with kT=40.5 +/- 0.3 eV and equivalent hydrogen column N_H=(8.3 +/-0.1)e20 cm^-2, using the Bayesian X-ray Analysis spectral fitting software (Buchner+14). The corresponding absorbed 0.2-2.3keV flux is (1.77e+/- 0.04)e-11 erg/cm^2/s, ~500x times brighter than in the 3-sigma upper limit from eROSITA's previous visit to the location on 2021 March 18th (one month prior to the optical discovery).

The X-ray properties suggest that eROSITA detected the super-soft X-ray source (SSS) state of AT2021jiu, as seen in other classical novae (see, e.g., Henze et al. 2014, A&A 563, A2, and references therein). During this state, the X-ray emission is produced by stable hydrogen burning on the surface of the white dwarf.

AT 2021jiu was also observed with the 7-channel optical/near-IR imager GROND mounted at the MPG 2.2m telescope at ESO's La Silla observatory (Greiner et al. 2008 PASP, 120, 405). Observations were performed on 2021 September 20th 00:55 UT under unfavorable seeing conditions (~2.5-3”). We measured the following AB magnitudes, calibrated against SkyMapper (gri) and 2MASS (JHK):

g’ = 16.56 +/- 0.03
r’ = 16.05 +/- 0.03
i’ = 15.12 +/- 0.04
J = 14.03 +/- 0.10
H = 14.05 +- 0.11
K = 12.45 +/- 0.10

We thank A. Hempel & P. Eigenthaler for their excellent support for obtaining the GROND observations.