Swift detection of super-soft X-ray emission from V1674 Her

ATel #14747; K. L. Page (U. Leicester), M. Orio (U. Wisconsin, INAF-Padova), K. V. Sokolovsky (MSU) and N. P.M. Kuin (UCL/MSSL)
on 2 Jul 2021; 16:05 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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V1674 Her (Nova Her 2021; TCP J18573095+1653396) was initially discovered on 2021 Jun 12.537 UT by Seiji Ueda (CBET 4976), though data from the All-Sky Automated Survey for Supernovae (ASAS-SN) revealed an earlier detection, on June 12.1903 UT (ATel #14710). Munari, Valisa and Dallaporta spectroscopically classified the new source as a nova close to naked-eye brightness (ATel #14704), while Wagner et al. (ATel #14746) determined it to be a neon nova.

The Neil Gehrels Swift Observatory began observing V1674 Her on 2021 June 13.5, 1.3 days after the ASAS-SN detection (which we consider as T0 here), initially finding no X-ray detection (ATel #14710). The early Swift UV observations were reported by Kuin et al. in ATel #14736. The first X-ray emission was seen on June 14.41 (day 2.22), where a faint source was found, with a Swift-XRT count rate of 0.058 +0.013/-0.012 count s^-1 (single pixel grade 0 events only, to mitigate contamination by optical loading).

Swift monitoring continued, with observations occurring every 1-3 days until June 26 (day 14.2), during which time the X-ray emission could be modelled by an absorbed optically thin component, consistent with shock emission, with the absorption decreasing over time.

The following observation, on July 1 (day 18.9) showed an increase in count rate by about a factor of ten (1.31 +/- 0.10 count s^-1 compared with 0.121 +/- 0.010 count s^-1 on June 26), caused by the appearance of a new soft component below 1 keV -- the super-soft source phase had begun. This soft component can be approximated by a blackbody of kT = 37 +5/-6 eV and an absorbing column N_H = (4.2 +1.5/-1.0) x 10²¹ cm^-2.

Continuing observations with Swift are planned. We thank the Swift PI, Brad Cenko, and his deputies, Regina Caputo and Josh Schlieder, for approving the monitoring, as well as the Swift planning and operations teams for their continuing support.