NICER observations of Nova Ret 2020

ATel #14067; Posted by Songpeng Pei (Padova University), Marina Orio (University of Wisconsin and INAF-Padova), Keith Gendreau (NASA-Goddard), Andrej Dobrotka, Pavol Besak (Slovak University of Technology in Bratislava), Ehud Behar (Technion), Gerardo Juan Manuel Luna (CONICET-University of Buenos Aires), Joanna Mikolajewska (Nicolaus Copernicus Astronomical Center of the PAS), Jan-Uwe Ness (ESA-ESAC)
on 6 Oct 2020; 21:20 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Marina Orio (orio@astro.wisc.edu)

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Nova Ret 2020 (MGAB-V207, YZ Ret; see ATels #13867, #13868, #13874, #14043, #14048) was observed with the Neutron Star Interior Composition Explorer (NICER) during 29.71 hours starting on 2020-09-28 at 15:25:44 UT, for a total exposure time of about 20 kiloseconds, in 20 continuous time intervals of about 1000 s each. The average count rate was 107.40 ± 0.07 counts/s and the average count rate of the single uninterrupted intervals of exposure varied by up to 8%. The spectrum in the 0.24-10 keV range of NICER was very soft, peaking at 0.3 keV and with very few counts above 0.6 keV. It is consistent with a white dwarf atmosphere with effective temperature in the 500,000-550,000 K range, and column density lower than N(H)=3 x 10²⁰ cm^-2, which indicates no large intrinsic absorption in the nova shell (the Galactic column density in the direction of the nova is N(H)=1.2-1.3 x 10²⁰ cm^-2 , Bekhti et al. 2016 ) . Variations in the continuum during the observation may indicate that an additional component of thermal plasma, originating in the ejecta, is perhaps superimposed on the stellar continuum and varies in time as the nova outflow continues. The absorbed flux in the best spectral fit we obtained is about 1.2 x 10^-10 erg/cm²/s and the absorbed flux is 2.9 x 10^-10 erg/cm²/s. Since the distance of the nova has been determined to be 2.7^+0.4_-0.3 kiloparsec thanks to the GAIA parallax ( Bailer-Jones et al. 2018 ), the absolute luminosity in the X-ray range is about 2.5 x 10³⁵ erg/s. The X-ray luminosity of a hydrogen burning post-nova white dwarf at effective temperature around 500,000 K is almost as large as the bolometric luminosity, which in model calculations turns out to be always of the order of 10³⁸ erg/s ( Yaron et al., 2005). This difference seems to imply that we are observing only a small portion of the white dwarf surface, or Thomson scattered radiation, while most of the surface is hidden, either by ejecta that are not uniformly distributed in space and optically thick to X-rays, or by an accretion disk like in nova U Sco ( Ness et al. 2012 , Orio et al. 2013 ). We did not detect any modulations of the X-ray flux with a periodicity of tens of seconds, as were measured in transient phases in other novae and supersoft X-ray sources (which, we note, mostly were at higher effective temperature than we derived here for Nova Ret 2020; see Ness et al. 2015 , Page et al. 2020 ). NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.