Nova V5668 Sgr (N Sgr 2015 no. 2) enters the super-soft X-ray phase
ATel #8054; K. L. Page (U. Leicester), N. P.M. Kuin (UCL/MSSL), A. P. Beardmore (U. Leicester), J. P. Osborne (U. Leicester), G. J. Schwarz (AAS)
on 14 Sep 2015; 13:38 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
Subjects: Ultra-Violet, X-ray, Nova
V5668 Sgr (PNV J18365700-2855420, N Sgr 2015 no. 2) was first seen as an
X-ray source on 2015 June 18, 95 days after the outburst (ATEL #7953). A
slow brightening of the X-ray flux continued for just over 2 months, with
a Swift-XRT count rate of ~0.06 count s-1 being measured on
August 23 (day 161) as reported in ATEL #7953.
A week later, on August 30 (day 168), the source was seen to have
brightened considerably, to a level of ~0.4 count s-1, with the
increase in count rate being caused by the appearance of a new, soft
component, below ~0.7 keV, showing that the nova had entered the supersoft
source phase. After a short interval of variability (the XRT count rate
decreased between days 168 and 171, falling from ~0.4 to ~0.1 count
s-1, before re-brightening again), the X-ray flux has been
steadily increasing, reaching 10 count s-1 on September 12
(day 181). During this same time interval, the X-ray emission has been
softening; parametrising the emission with a simple BB component suggests
kT ~ 30-40 eV, with the softening caused by the increase in flux of this component, rather than a decrease in temperature. There is little change in the underlying optically thin component described in ATEL #7953.
The UV grism spectrum is dominated by very strong emission lines of C, N, and O II and III. The brightness in uvm2 = 9.0 +/- 0.15 mag over the past ten days, as derived from the readout streak.
Swift monitoring of the nova in both the X-ray and UV bands continues, and
we thank the Swift PI and mission operations team for their ongoing support.
