Early NuSTAR and Swift X-ray detection of Nova Cassiopeiae 2021 = V1405 Cas = PNV J23244760+6111140

ATel #14530; K. V. Sokolovsky, E. Aydi, L. Chomiuk, A. Kawash, J. Strader (MSU), K. Mukai (NASA/GSFC), K.-L. Li (NCKU), A. Babul, J. L. Sokoloski (Columbia), J. D. Linford (NRAO), N. Ikonnikova, K. E. Atapin, A. A. Belinski, A. V. Dodin, N. A. Maslennikova, K. A. Postnov, S. A. Potanin, B. S. Safonov, N. I. Shatsky, A. M. Tatarnikov (SAI MSU), K. L. Page (U. Leicester), O. V. Maryeva (ASU and SAI MSU)
on 8 Apr 2021; 16:01 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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V1405 Cas (Nova Cas 2021, PNV J23244760+6111140) was discovered on t0 = 2021-03-18.4236 UT by Y. Nakamura and confirmed spectroscopically as a He/N-type nova (CBET #4945, ATel #14471, #14472, #14476, #14478, #14482). Its position 23:24:47.745 +61:11:14.82 +/-0.1" J2000 measured with the 0.6m SAI Moscow State University telescope (Kislovodsk, Russia) coincides with a previously known variable star CzeV3217 (possibly a novalike misclassified as W UMa-type binary) at the Gaia distance of 1740 pc (Bailer-Jones et al. 2018, AJ, 156, 58).

NuSTAR performed a 40ks DDT observation of V1405 Cas between 2021-03-20.9548 (t0 + 2.5 days) and 2021-03-21.6111. We found a faint X-ray source at the nova position (4 and 5 sigma detection with the focal plane modules A and B, respectively). The source spectrum can be fit by a kT= 1.8 +/-0.6 keV thermal plasma with unconstrained absorption and the unabsorbed flux in the NuSTAR 3.5-78.0 keV band of 8.8x10^-14 erg/cm^2/s corresponding to an intrinsic luminosity of 3.2x10^31 erg/s (2.7x10^32 erg/s if extrapolated to 0.3-78.0 keV).

Swift/XRT observed V1405 Cas for 0.3ks on 2021-03-24.064 (t0 + 5.6 days) and detected an X-ray source at the nova position with the net count rate of 0.043 +/-0.013 cts/s. The 0.3-10 keV spectrum is consistent with kT= 0.7 +/-0.3 keV thermal plasma assuming nH= 3.768x10^21 cm^-2 estimated from the nH to optical reddening relation (ATel #14476; Guver & Ozel 2009, MNRAS, 400, 2050).

We obtained optical spectroscopy of V1405 Cas with the 2.5m SAI MSU telescope (Kislovodsk, Russia) on 2021-03-19.762, 2021-03-23.050, 2021-04-01.718 using the Transient Double-beam Spectrograph (Dodin et al. 2020, AstL, 46, 429). The spectral resolution R=1200-2200 for the blue and red arms of the spectrograph together covering the range 3500-7500 A. We also obtained two spectra covering 8400-8870 A range on 2021-03-25 and 2021-04-04 with the Perek 2m telescope of Astronomical Institute (Ondrejov, Czech Republic). The spectra show P Cygni profiles of Balmer and He I lines, with absorption troughs at velocities of around -1200 km/s, relative to the rest velocities of the lines. The spectra are consistent with a nova that did not reach optical peak.

We expect that as the nova approach the peak, the high excitation He I lines fade while lower ionization Fe II lines dominate the spectrum. A broader spectral components may emerge when the nova reach optical peak, originating in fast winds/outflows. This anticipated optical peak is driven by shocks - the product of collision between the different outflows (Munari et al. 2017, MNRAS, 469, 4341; Aydi et al. 2020, ApJ, 905, 62). These shocks may lead to gamma-ray emission.

We found V1405 Cas an order of magnitude less luminous in the NuSTAR band compared to YZ Ret (ATel #13900) and two orders of magnitude less luminous than V906 Car (Sokolovsky et al. 2020, MNRAS, 497, 2569) and V5855 Sgr (Nelson et al. 2019, ApJ, 872, 86) - the three gamma-ray bright classical novae previously observed by NuSTAR 10 to 57 days past eruption. Although only a few photons from V1405 Cas were detected by Swift/XRT, this detection suggests the intrinsic absorption is not important, in stark contrast to the early NuSTAR spectra of YZ Ret, V906 Car and V5855 Sgr. The origin of these relatively unabsorbed X-rays in a nova, that may have not reached its peak (according to the optical spectra), is currently unclear.

X-ray and optical spectra of V1405 Cas