SRG/eROSITA detection of Nova V1708 Scorpii in its super-soft phase

ATel #14057; Ole Koenig (ECAP/FAU), Arne Rau, Frank Haberl (all MPE), Jakob Stierhof, Ingo Kreykenbohm, Philipp Weber, Joern Wilms (all ECAP/FAU)
on 1 Oct 2020; 09:40 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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We report the detection of super-soft X-ray emission from TCP J17234205-3103072 = Nova V1708 Scorpii during the ongoing second all-sky survey (eRASS2) of the eROSITA instrument on-board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission. The source entered the eROSITA field of view for the first time on MJD 59116.79 (UTC 2020-09-24 18:57) generating an independent alert by the eROSITA Near-Real Time Analysis (NRTA) transient search system. It was detected also in the following 6 scans over the position, each separated by ~4h and with an exposure of ~40s per visit. eROSITA scanned the last time over the source location on MJD 59117.79.

The source was detected as a bright X-ray emitter showing a mild decay from ~23 cnt/s to 19 cnt/s in the 0.2-5 keV band during the period of observations. The exception was an observation on MJD 59117.29 where the count rate had dropped to ~11 cnt/s.

The X-ray spectra are best fit with an absorbed black body plus powerlaw model with variable abundances (Asplund et al., 2009, ARA&A 47, 1). The best-fit black body temperature is 76)(+5/-6) eV with Ne abundance at 4.9+/-0.6 times the Solar value (90% CL). The required equivalent neutral hydrogen column density of 1.78(+0.18/-0.13)x10^22 cm^-2 is in excess of the Galactic foreground value of ~5.4x10^21 cm^-2. The hard spectral component can be described by a powerlaw of slope 2.4(+0.8/-0.7) and contributes to ~30% of the flux in the 0.2-5 keV band.

The averaged 0.2-5 keV flux derived using the best-fit model was 1.83 x 10^-11 erg/cm^2s. No source was detected during the first all-sky survey (eRASS1) in March 2020. Observations performed as part of the ROSAT survey (ObsID 932242) provide an historical upper limit of 4.9 x 10^-13 erg/cm^2/s (http://xmmuls.esac.esa.int/hiligt/).

The obtained spectral parameters are consistent with the observations of novae in the super-soft phase, i.e., a black body temperature <100 eV and high intrinsic absorption (see e.g., Schwarz et al., 2011, ApJS 197, 2, 31). The high neon abundance could point to a ONeMg white dwarf. We speculate that the flux decrease seen one of the observations originates from an eclipse by a companion. The nova was first detected in the optical on September 8th by Kojima (Tsumagoi, Gunma-ken, Japan) with follow-up observations and analysis reported by Aydi et al. 2020 (ATel #14015), Mroz & Udalski 2020 (ATel #14017), and Banerjee 2020 (ATel #14028). This suggests a short turn-on time of ~16d and a likely short decay time (Henze et al., 2013, A&A 563, A2, 20). Fast X-ray follow-up is therefore highly encouraged.