Super-soft X-ray emission of Nova Reticuli 2020

ATel #14043; Kirill Sokolovsky, Elias Aydi, Laura Chomiuk, Adam Kawash, Jay Strader (MSU), Koji Mukai (NASA/GSFC), Kwan-Lok Li (NCKU), Stella Kafka (AAVSO), Marina Orio (INAF Padova and U. Wisconsin), Kim Page, Andy Beardmore (U. Leicester)
on 27 Sep 2020; 07:01 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The outburst of Nova Ret 2020 (MGAB-V207, YZ Ret; ATel #13867, #13874) was identified by R. H. McNaught on 2020-07-15.590 UT (CBET #4811). Pre-discovery images by M. A. Phillips show the nova peaking on 2020-07-11.76 at 3.7mag (CBET #4812) while ASAS-SN (Shappee et al. 2014, ApJ, 788, 48; Kochanek et al. 2017, PASP, 129, 104502) data indicate the eruption started on 2020-07-08.171. The nova was detected at GeV band by Fermi/LAT (ATel #13868) and at hard X-rays by NuSTAR (ATel #13900).

Swift/XRT observed Nova Ret 2020 on 14 epochs between 2020-07-16 and 2020-09-25 for a total exposure of 32.3ks in the Windowed Timing (WT) mode and 11.2ks in the Photon Counting (PC) mode. The WT mode was preferred to reduce optical loading; we also restrict our analysis to Grade 0 events only. The X-ray emission from the nova is detected starting from 2020-08-04 with the 0.3-10 keV count rate ~0.02 cts/s. The 2.0-10 keV region (least affected by optical loading and background problems from trailing charge) may be fit by the thermal plasma model with kT ~ 4 keV. The XRT count rate starts rising around 2020-09-05 peaking at 2.87 +/-0.03 cts/s on 2020-09-15.38. The rise is due to soft photons indicating the onset of the super-soft source (SSS) phase. The latest observation on 2020-09-25.63 found the source at 1.83 +/-0.03 cts/s.

The Swift/XRT spectrum obtained on 2020-09-15.38 can be fit with a combination of kT ~ 30 eV black body and kT ~ 4 keV optically thin emission behind the absorbing column of n_H = 2x10^21 cm^-2, exceeding the total Galactic value in the direction of the source (1.18x10^20 cm^-2; Kalberla et al. 2005 A&A, 440, 775). The unabsorbed 0.3-10 keV flux of the model is 3x10^-9 ergs/cm^2/s. The two emission components are usually interpreted as shock-heated plasma (the hard component) and the white dwarf atmosphere heated by the ongoing nuclear burning (soft black-body-like emission - super-soft source), see e.g. Orio (2012, BASI, 40, 333); Mukai (2017, PASP, 129:062001). The use of TMAP white dwarf atmosphere model (Rauch 2003, A&A, 403, 709) leads to a higher kT of the soft component and lower n_H value compared to the black body model.

The AAVSO visual lightcurve shows a dramatic slow down in the rate of decline that occurred on 2020-08-06.9, coincident with the onset of the X-ray SSS emission. The optical decline rate has changed from the initial (time to decline by 2 magnitudes) t_2= 15.1 days to t_2= 153.6. The visual magnitude of the nova is 8.7 on 2020-09-25.09.

We thank the Neil Gehrels Swift Observatory team and PI, Brad Cenko, for scheduling these ToO observations and the AAVSO observers for their valuable contributions.