Swift detection of supersoft emission in V1716 Sco
ATel #16069; K. L. Page (U. Leicester) & N. P.M. Kuin (UCL/MSSL)
on 1 Jun 2023; 15:27 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
Subjects: Ultra-Violet, X-ray
V1716 Sco (PNV J17224490-4137160; Nova Sco 2023) was discovered by Pearce
on 2023-04-20.678 UT (CBET #5245), with pre-discovery images from ASAS-SN
showing the nova to be saturated on 2023-04-20.410 UT (ATel #16018), and
was classified as a classical nova by Walter & Pearce (ATel #16003).
Optical spectroscopy monitoring has since been performed (ATels #16004,
#16006, #16007, #16018, #16036).
The nova was also detected by Fermi/LAT on 2023-04-21 UT (ATel #16002),
with upper limits only from a neutrino search with IceCube (ATel #16019).
Hard X-rays were detected by NuSTAR on 2023-04-21.89 UT, with the spectrum
being consistent with a heavily absorbed thermal plasma (ATel #16018).
The earliest Swift-XRT observation, on 2023-04-21.628 UT, found no X-ray
source at the location of the nova (ATel #16005). Swift monitoring then continued with
observations every few days. The first X-ray detection occurred on
2023-05-01, at a count rate of 0.016 +0.004/-0.003 count s-1
(0.3-10 keV; grade 0 events); as with the earlier NuSTAR detection, the
spectrum could be modelled as a heavily absorbed thermal component. The
X-ray count rate subsequently rose until 2023-05-07, after which it
plateaued, staying in the range of ~0.05-0.09 count s-1 until
2023-05-28.
The latest observation, on 2023-05-31, revealed an additional soft
component, indicating the start of the supersoft source (SSS) phase. Given
the relatively faint X-ray emission (count rate of 0.078 +/- 0.008 count
s-1), the temperature of the SSS component cannot be well
constrained. Using an atmosphere grid, we can only place an upper limit of
kT < 7e5 K (60 eV). The underlying optically-thin thermal plasma has a
temperature of kT = 3.3 +5.9/-1.3 keV, and both components are absorbed
with NH = (1.1 +0.5/-0.4) x 1022 cm-2.
Swift-UVOT photometry has been mainly collected using the uvw1 and uvw2
filters, showing a fading UV source. On 2023-04-21 the uvw1 data were
saturated, with a limit of uvw1 < 11.01; the most recent observation
shows a uvw1 magnitude of 13.49 +/- 0.04.
Swift-UVOT spectra have also been taken throughout. On 2023-05-28 a weak
O III 3130 Å line appeared, which on 2023-05-31 became a strong
presence. This line is pumped by the He II FUV and indicates the
emergence of a hotter component in the far-UV. The near-UV continues
to show substantial absorption. [Ne V] 3350/3426 Å was detected for the first time on 2023-05-31.
Further Swift monitoring is planned. We thank the Swift PI for approving these
observations, and the Swift operations team for implementing them.
