The end of the super-soft X-ray phase of Nova V3890 Sgr

ATel #13137; K. L. Page, A. P. Beardmore, J. P. Osborne (U. Leicester), N. P.M. Kuin (MSSL/UCL), J. U. Ness (ESA/ESAC), M. Orio (INAF Padova and University of Wisconsin-Madison), K. V. Sokolovsky (MSU) and S. Starrfield (ASU)
on 27 Sep 2019; 06:08 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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Following the most recent eruption of the recurrent Nova V3890 Sgr (first reported on 2019 Aug 27.87 by A. Pereira), the Neil Gehrels Swift Observatory has been monitoring the progress of the outburst (ATels #13050, #13072, #13084, #13104), noting the start of the supersoft source (SSS) phase on 2019 Sep 5.25 (8.39 days after eruption).

After an initial increase in the SSS temperature from ~30 eV on day 8.4 to ~70 eV on day 13.2 [assuming a simplified blackbody parameterisation; while temperatures/luminosities from a BB model may be under/over-estimated respectively, Henze et al. (2011, A&A, 533, A52) have shown there is a strong correlation with temperatures estimated using BB fits and those from model atmospheres], a gradual cooling trend has since been followed. After a period of significant variability (ATELs #13104, #13124), the X-ray count rate began to decrease steadily from day 20. Between days 25 and 29, the count rate has remained close to ~1 count s^-1, with the flux declining only by 7% over this interval.

Indeed, from day 26.18 onwards, the spectrum shows no evidence for SSS emission, and can be simply modelled with an optically thin plasma component, as was the case before the emergence of the SSS phase (ATel #13050). The temperature of this thin plasma component is, however, now much cooler; on day 28.8, we find kT = 1.6 +/- 0.1 keV (cf 7.9 +2.3/-1.6 keV on day 8.8; ATel #13050), and much less absorbed, with a column of (1.7 +0.6/-0.5) x 10²¹ cm^-2 [cf (3.4 +/-0.3) x 10²² cm^-2 on day 8.8).

The UVOT uvm2 magnitude (central wavelength 2246 A) has faded continuously since the first observation on day 3.1, though at different rates. The decay shows a plateau during the time of the X-ray SSS phase (as often seen in recurrent novae; e.g., Schaefer et al., 2010, ApJS, 187, 275); following the plateau, the rate of decay steepened around the same time the SSS emission disappeared.

The short duration (SSS component detected only between days 8.4 and 26.2), and high peak temperature (~70 eV), of the SSS phase suggests a massive (> 1.2 M_sun) white dwarf (Wolf et al., 2013, ApJ, 777, 136), as expected for a recurrent nova.

Further observations with Swift are planned, and we thank the Swift PI, Brad Cenko, for approving the monitoring, as well as the Swift planning and operations teams for their continuing support.