SRG/eROSITA discovery of a bright supersoft X-ray emission from the classical nova AT 2018bej in the Large Magellanic Cloud
ATel #13545; L. Ducci, L. Ji (all IAAT, Tuebingen), F. Haberl, A. Rau (all MPE), M. Sasaki, J. Wilms, I. Kreykenbohm, P. Weber (all ECAP/FAU), K. Werner, A. Santangelo (all IAAT, Tuebingen), C. Maitra, S. Carpano, J. Buchner (all MPE), A. Schwope (AIP), V. Suleimanov, V. Doroshenko (all IAAT, Tuebingen)
on 6 Mar 2020; 11:08 UT
Credential Certification: Lorenzo Ducci (lorenzo.ducci@unige.ch)
We report the discovery by the eROSITA instrument onboard of the Russian/German Spektrum-Roentgen-Gamma
of a bright supersoft source consistent with the position of the LMC classical nova AT 2018bej/ASAS-SN 18jj (ATel #11610).
AT 2018bej was discovered on 2018 May 3rd by the ASAS-SN survey (ATel #11610).
As part of its all-sky survey, eROSITA started scanning over the position of AT 2018bej
on 2020 February 12 with visits every 4hr of ~40s duration. The coverage of this sky region is still ongoing.
A bright new X-ray source was found at:
RA(J2000) = 06:26:21.58
Dec(J2000) = -69:45:49.5
with an estimated uncertainty of 10 arc-seconds radius (including systematics). This position is consistent with AT 2018bej.
A preliminary analysis of all the eROSITA scanning observations obtained between Feb 12th and Feb 27th
shows that the 0.3-1 keV spectrum can be described by an absorbed blackbody with temperature of
about 30 eV and a column density of ~2.4×1021 cm-2. The source flux is
~1.72×10-11 erg/cm2/s in the 0.3-1 keV band (corresponding to an
absorbed luminosity of ~5.14×1036 erg/s, assuming a distance of 50 kpc).
An alternative fit with an absorbed non local thermodynamic equilibrium (NLTE) atmosphere
model (see: http://astro.uni-tuebingen.de/~rauch/TMAF/flux_H-Ca.html )
for a star with metallicity Z=0.1 times solar and surface gravity log g = 9
gives a best fit value for the temperature of about 43 eV and a bolometric luminosity
of about 3.5×1037 erg/s.
The residuals of the fit indicate the potential existence of an additional
harder spectral component.
The soft X-ray emission observed with eROSITA is consistent with the
supersoft X-ray source (SSS) state following classical nova outbursts,
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.
The eROSITA 0.3-1keV lightcurve is constant on long time scales, indicating that the SSS state started already prior to the onset of the eROSITA observations.
