Swift-XRT and optical observations of V1405 Cas during the super-soft X-ray phase

ATel #16876; K. L. Page (Univ. of Leicester) & S. N. Shore (Univ. of Pisa)
on 25 Oct 2024; 15:08 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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On 2024 October 23 (1315 days post-eruption), Swift performed an observation of V1405 Cas (Nova Cas 2021; PNV J23244760+6111140). Compared to the previous observation on 2023 July 23, the X-ray emission is about a factor of three brighter, with a grade 0-12 count rate of 0.121 +/- 0.006 count s^-1 (cf. 0.038 +/- 0.004 count s^-1 15 months earlier). This increase in count rate appears as a soft component below 0.7 keV, and can be parameterised with a blackbody of temperature kT = 35 +10/-11 eV, together with an underlying optically thin plasma of kT = 4 +16/-2 keV, both absorbed by a column, N_H, of (2.4 +2.2/-1.1)x10²¹ cm^-2. In 2023 July, the spectrum could be fitted simply with an optically-thin component of kT > 3.2 keV and consistent N_H (< 2.0x10²¹ cm^-2). Over this same interval, the UV emission has continued to fade, from m2 = 12.49 +/- 0.02 to 13.18 +/- 0.02.

An optical (from about 3900 to 7000 Å) low resolution (R ~ 800) spectrum obtained on 2024 October 14 shows [Fe VII] 6086 as the strongest emission line with the same profile as in 2023. Virtually all other transitions are coronal lines from [Fe V] and higher ionization transitions with full width zero intensity of order 2000 km s^-1.

We note that, while Page et al. (2021) reported in ATel #15111 that V1405 Cas was detected as a faint super-soft source in 2021 December, it seems likely that the small number of photons detected is the reason the source appeared soft at the time; subsequent Swift observations during 2022 and 2023 did not show an obvious soft component.

A low-cadence Swift monitoring campaign has been approved, for which we thank the Swift PI and deputies. We also thank Forrest Sims (ARAS) for obtaining the optical spectrum.