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)
Subjects: Optical, X-ray, Nova
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.