Swift detection of super-soft X-ray emission from V3890 Sgr

ATel #13084; K. L. Page, A. P. Beardmore, J. P. Osborne (U. Leicester), M. Orio (INAF Padova and University of Wisconsin-Madison), K. V. Sokolovsky (MSU) and M. J. Darnley (LJMU)
on 6 Sep 2019; 16:34 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Tweet

The 2019 eruption of the recurrent nova V3890 Sgr (discovered by A. Pereira on 2019 Aug 27.87) has been followed by many observers (ATels #13047, #13059, #13060, #13062, #13069, #13081, #13083), and regularly monitored by the Neil Gehrels Swift Observatory (ATels #13072, #13050) between 2019 Aug 28.44 (0.54 days post-discovery) and Sep 5.99 (9.12 days post-discovery). Note that the actual eruption time was within 0.5 days of the discovery, with the last pre-outburst observation performed by R. Stubbings on 2019 Aug 27.521.

Initially the X-ray spectrum could be fitted with a heavily absorbed, optically-thin thermal plasma component (ATel #13050). The spectrum was found to soften as the absorbing column decreased by an order of magnitude from about 3x10²² cm^-2 on day 0.5 to 3x10²¹ cm^-2 on day 8, together with a cooling of the plasma temperature (~8 keV to ~3 keV over the same interval). However, in the spectrum obtained on 2019 Sep 5.25 (day 8.39), a bump at low energies suggested the possible emergence of an additional soft component. The following spectrum, obtained on Sep 5.99 (day 9.12) confirmed the start of the super-soft source (SSS) emission. This new soft emission can be modelled with an atmosphere grid model of kT = 62 +/- 1 eV [(7.2 +/- 0.1)x10⁵ K]). The optically thin plasma has a temperature of kT = 2.7 +0.3/-0.2 keV, and both components are absorbed by a column of N_H = (3.2 +/- 0.4)x10²¹ cm^-2. The net count rate (0.3-10 keV; single pixel grade 0 events) for this spectrum is 3.96 +/- 0.06 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.11x10^-10 (1.33x10^-9) erg cm^-2 s^-1.

While not the earliest detection of a SSS (V745 Sco became super-soft around 4 days after eruption; Page et al., 2015, MNRAS, 454, 3108), the appearance of the soft component between days 8 and 9 suggests a high mass white dwarf. We note that RS Oph, a similar system to V3890 Sgr, first showed SSS emission on day 26 (Osborne et al., 2011, ApJ, 727, 124).

V3890 Sgr is now too close to the Moon for Swift observations until 2019 Sep 09 at 21:24 UT, unfortunately precluding the following of the very early super-soft emission as it rises to peak. Further monitoring is planned once the source emerges from this observing constraint.

We thank the Swift PI, Brad Cenko, for approving the monitoring campaign, as well as the Swift planning and operations teams for their continuing support.