Swift-XRT observations of V1674 Her 10 months after eruption: the modulation remains
ATel #15317; K. L. Page (U. Leicester), A. P. Beardmore (U. Leicester), S. Starrfield (ASU), R. M. Wagner (OSU) and C. E. Woodward (U. Minnesota)
on 8 Apr 2022; 15:06 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
V1674 Her (Nova Her 2021; TCP J18573095+1653396) erupted on 2021 June 12,
initially discovered by Seidji Ueda on June 12.537 UT (CBET #4976), though
an earlier detection on June 12.1903 UT was then reported by ASASSN (ATel #14710). Munari,
Valisa & Dallaporta (ATel #14704) classified the source
spectroscopically as a classical nova, approaching naked-eye brightness,
with Wagner et al. finding it to be a neon nova (ATel #14746). The source
was subsequently well-observed across the EM spectrum, from radio to
gamma-rays (ATel #14705, #14707, #14710, #14713, #14718, #14720, #14723,
#14731, #14736, #14737, #14747, #14758, #14798, #14824, #14835, #14856;
Woodward et al., 2021, ApJ, 922, L10; Drake et al. 2021, ApJ, 922, L42).
No neutrinos were detected by IceCube, however (ATel #14713).
In 2022 April, optical spectroscopy suggested V1674 Her had returned to
quiescence (ATel #15312). Prompted by this, a Neil Gehrels Swift
Observatory ToO was requested, to determine the current state of the X-ray
emission. A 4 ks observation was performed on April 7 (day 299 after
eruption), finding a 0.3-10 keV X-ray count rate of 0.171 +/- 0.006 count
s-1; this is higher than the previous observation on 2021
October 15/16, when the corresponding count rate was 0.094 +/- 0.005
count -1.
The latest X-ray spectrum is still soft, requiring a two component fit.
Following Drake et al. (2021), we fitted the XRT spectrum with a optically
thin APEC component to account for the data above about 1 keV, and a BB
with absorption edges to model the softer emission. This gave the
following parameters: APEC kT > 12 keV; BB kT = 100 +/- 10 eV, with
absorption edges at 0.74 keV and 0.87 keV being significant. This is a
similar BB temperature as that found by Drake et al. once the X-rays had
faded from peak, and the count rate had plateaued around 0.1 count
s-1.
Examination of the 0.3-10 keV X-ray light curve shows it is still strongly
modulated at the 8.36 min period seen both prior to and during the nova
outburst (e.g., ATel #14720, #14776, #14856; Drake et al. 2021). Although
affected by the window function imposed by Swift's 95 min orbital period,
an e-folding period search reveals the strongest signal at 501.38 s, with
a preliminary one sigma uncertainty estimate of 0.25 s. When folded at
this period a modulation fraction of 93-98 per cent is seen, with a duty
cycle (pulse width / period) of 30-35 per cent.
We plan to continue low-cadence monitoring of V1674 Her with Swift,
and thank Brad Cenko, his deputy PIs, and the science operations
team for their help in performing the observations.
