V1405 Cas is now a faint Super-Soft Source
ATel #15111; K. L. Page (U. Leicester), S. Starrfield (ASU) , U. Munari (INAF-Padova), C. E. Woodward (U. Minnesota) & R. M. Wagner (LBTO and OSU)
on 14 Dec 2021; 18:12 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
V1405 Cas (Nova Cas 2021; PNV J23244760+6111140) was discovered on 2021 March 18.4236 by Yuji Nakamura (CBET #4945), and subsequently classified as a classical nova (ATels #14471, #14472, #14478, #14476, #14482). The nova has since been monitored and detected across the electromagnetic spectrum (ATels #14577, #14614, #14615, #14622, #14665, #14731, #14794). While ATel #14530 announced an early X-ray detection by the Neil Gehrels Swift Observatory, more detailed analysis suggests this is likely an artifact caused by the optical brightness of the nova that time. The source was detected in harder X-rays by NuSTAR 2.5 days after the initial discovery, however (ATel #14530).
Following the announcement by Munari et al. in ATel #15093 that V1405 Cas had ended its ~7 month optical plateau and was showing a steady fading, a Swift Target of Opportunity observation was performed on December 14 (270.7 days after the discovery), to check for softer X-ray emission. A faint X-ray source was found, at a count rate of 0.011 +0.003/-0.002 count s-1 (single pixel events only, to mitigate continuing optical loading from the still-bright optical/UV source). Too few counts were detected in this short, ~1 ks observation to perform a spectral fit, but all photons were ≲1 keV, suggesting the source is in the super-soft phase.
We thank the Swift PI and deputies for approving this observation. Further
monitoring is planned.
