The slowly rising X-ray flux of Nova V5668 Sgr (N Sgr 2015 no. 2)
ATel #7953; K. L. Page (U. Leicester), N. P.M. Kuin (UCL/MSSL), J. P. Osborne (U. Leicester), G. J. Schwarz (AAS)
on 26 Aug 2015; 16:55 UT
Credential Certification: Kim Page (firstname.lastname@example.org)
Subjects: Ultra-Violet, X-ray, Nova
Nova PNV J18365700-2855420, also known as Nova Sgr 2015 No. 2 and
as V5668 Sgr, was discovered on 2015 March 15.634 at mag
6.0 (CBET #4080). Swift observations began 2.6 days later, on
March 18. The new nova was optically very bright, requiring that the
Swift-UVOT initially be blocked, and the XRT to be operated in
Windowed Timing (WT) mode to reduce optical loading. (The
1-dimensional WT mode has a frame-time of 1.76 ms, minimising the
charge liberated by optical photons and thus its effects on X-ray
measurement; see http://www.swift.ac.uk/analysis/xrt/optical_loading.php). WT
mode observations continued until the
optical magnitude had decreased sufficiently, to around magnitude
8, on 2015 June 16; no X-ray source was detected in WT mode.
After this time, PC mode could be reliably used.
The first X-ray detection of the nova occurred on 2015 June 18, 95
days after the outburst, at a count rate of (6.1 +2.5/-1.9) x
10-3 count s-1. The initial X-ray source was relatively hard and very absorbed; fitting a spectrum for the data obtained between June 18 and July 3 (days 95 - 112) with an absorbed optically-thin APEC component gives kT = 1.3 +0.5/-0.3 keV, and a high column of NH = (4.7 +1.6/-1.2) x 1022 cm-2. After this time, the X-ray source softened and slowly brightened, reaching a level
of 0.060 +/- 0.005 count s-1 on 2015 August 23 (day
161). A spectrum formed from the data collected between August 16 and 23 (days 153-161) is much less absorbed, with NH = (0.4 +/- 0.1) x 1022 cm-2, and kT = 3.4 +1.4/-0.8 keV. The differences between the spectra are almost entirely due to the decrease of the absorption column.
On 2015 June 21, using photometry from the readout streak,
UVOT measured a magnitude of uvm2 = 8.5 +/- 0.2. The UV emission
reached a minimum two weeks later, with uvw2 = 11.44 +/- 0.07 on
July 3. Since then the brightness has slowly increased to
uvw2 = 10.85 +/- 0.10 on August 18.
The optical emission was also seen to rebrighten over this
same time interval, evidently as the nova recovered after a dust
extinction event (ATel #7643, #7862)
We thank the Swift PI and mission operations team for their ongoing support.