AstroSat SXT observations of V3890Sgr

ATel #13102; K. P. Singh (IISER Mohali), V. Girish (ISRO Hq, Bengaluru), G. C. Anupama (IIA, Bengaluru), M. Pavana (IIA, Bengaluru)
on 12 Sep 2019; 10:44 UT
Credential Certification: G.C. Anupama (gca@iiap.res.in)

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We observed the recurrent nova V3890 Sgr from 2019 September 5, 01:37:55 UT to Sept 6, 20;19:27 UT with the AstroSat Soft X-ray Telescope (SXT) continuously, getting a useful exposure time of 40 ksec. Our preliminary analysis shows that the source was almost steady with total count rate of 1.5cps in the 0.3 - 7.0 keV for the first 18 hours, after which flaring episodes were noticed at count rate of 1.9cps to 2.2cps and the level of persistent emission also found to increase to 1.6cps. The flares and the increase are found to be confined to the soft energy band of 0.3-0.7 keV.

We extracted spectra for 6 different times and fitted them simultaneously with the model "tbabs*(bbody+vapec)" in the xspec program, using the abundance table "aspl", and varying the column density, normalization, temperatures of blackbody and "vapec" plasma model, independently for each time selection, while abundances of all elements in "vapec" were tied together. The column density varied between (3.5-7.3) x 10²¹ cm^-2 and the normalisation of the bbody component showed that the observed flux in the 0.3-0.7 keV energy band changed from 3.9x10^-12 ergs cm^-2 s^-1 to 5.7x10^-11 ergs cm^-2 s^-1 during the brightest flare observed towards the end of the observation while the black body temperature (kT) remained almost steady in the range of 34 to 46 eV. The plasma model temperature and the observed intensity in the hard band of 0.7-10.0 keV varied in the range of 4.0-2.5 keV and 9.6-10.7x10^-11 ergs cm^-2 s^-1 respectively, but required ~1/3rd solar abundances for all elements except for Ne that required a large supersolar (~6 times solar) abundance. Introducing a sharp and deep absorption edge at 0.59 keV, common to all spectra, led to a dramatic improvement in the fit but significantly altered the temperatures and column densities. Our observations confirm the entry of the source into supersoft stage reported earlier (Atel # 13084).

Follow-up observations with the AstroSat are in progress.

We thank the ISSDC, Bengaluru for scheduling these observations at a short notice. We thank the SXT Payload Operation Centre (POC) at TIFR for quick pipeline processing and providing the level2 data.