Bright radio emission from Nova Cassiopeiae 2021 = V1405 Cas

ATel #15150; Kirill Sokolovsky, Elias Aydi, Laura Chomiuk, Adam Kawash, Jay Strader (MSU), Aliya-Nur Babul, Jennifer Sokoloski (Columbia), Amy Mioduszewski, Justin Linford (NRAO), Koji Mukai (NASA/GSFC), Kwan-Lok Li (NCKU), Tim O'Brien (JBCA/Manchester), Michael Rupen (NRC)
on 2 Jan 2022; 05:45 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

Tweet

The classical nova V1405 Cas was discovered on 2021-03-18.4236 UT by Y. Nakamura (CBET #4945, ATel #14471, #14472, #14476, #14478, #14482, #14530, #14577, #14614, #14620, #14622, #14665, #14704, #14794). For seven month V1405 Cas stayed near its peak optical brightness showing a series of flares with the brightest one reaching V=5.1 according to the AAVSO photometry. Around 2021-11-01 the nova started a steady decline (ATel #15093) and on 2021-12-14 was detected as a super-soft X-ray source (ATel #15111).

We follow V1405 Cas with the Karl G. Jansky Very Large Array (VLA) from 2021-06-10 (ATel #14731), prompted by the GeV detection of the nova (ATel #14658). V1405 Cas displays an inverted radio spectrum with flux densities gradually increasing with time at all frequencies, reaching at our latest observing epoch 2021-12-20:

 
# f(GHz) F(mJy) eF(mJy)   
 2.6     1.110  0.037 
 3.4     1.679  0.023 
 5.1     3.333  0.019 
 7.0     6.356  0.019 
13.7    23.685  0.043 
16.5    32.958  0.043 
31.1   105.930  0.120 
34.9   128.630  0.130 

A power law fit results in a positively defined spectral index of 1.86 +/-0.02. The source is resolved at 31.1 and 34.9 GHz according to the SNR-adjusted resolution criterion of Kovalev et al. (2005, AJ, 130, 2473) having a Gaussian FWHM size of ~0.07 arcsec.

The flux densities above 10 GHz were measured by fitting a circular Gaussian (at 31.1 and 34.9 GHz) or point source model to the uv-data and applying phase-only self-calibration. No self-calibration was applied at lower frequencies (due to more stable phases, fainter target and the presence of other sources within the field of view) - the flux densities were obtained from peaks of naturally-weighted CLEAN images of the nova. We used 3C147 to set the absolute flux density scale (Perley & Butler, 2017, ApJS, 230, 7) with expected uncertainties of 15% at 35 GHz and 5% at lower frequencies. The source J2339+6010 (TXS 2336+598) was used as the phase calibrator. From the 31.1 and 34.9 GHz data we measure the position of the radio emission peak (relative to the phase calibrator) at 23:24:47.7325 +61:11:14.599 J2000. The expected uncertainty is about 0.010 arcsec dominated by the phase calibration errors.

The measured flux densities make V1405 Cas one of the brightest radio novae. Among the classical novae well observed in radio only V5668 Sgr (2015), V1369 Cen (2013), V1974 Cyg (1992) and QU Vul (1984) (Chomiuk et al., 2021, ApJS, 257, 49) as well as V959 Mon (2012) (Chomiuk et al. 2014, Nature, 514, 339) reached 100 mJy above 20 GHz (all having inverted spectra). The red giant donor recurrent nova RS Oph reached ~90 mJy at both high and low frequencies during its 2021 eruption (ATel #14886), while another embedded nova V407 Cyg peaked at lower flux densities (Chomiuk et al. 2012, ApJ, 761, 173).

The VLA radio spectrum of V1405_Cas on 2021-12-20.