Multi-wavelength follow up of the very fast Nova Herculis 2021 (TCP J18573095+1653396)

ATel #14710; E. Aydi, K. V. Sokolovsky, L. Chomiuk, J. Strader, A. Kawash (MSU), K. L. Page (U. Leicester), C. Boussin (ARAS Group), N. Ikonnikova, K. E. Atapin, A. A. Belinski, M. A. Burlak, A. V. Dodin, N. A. Maslennikova, K. A. Postnov, S. A. Potanin, B. S. Safonov, N. I. Shatsky, A. M. Tatarnikov (SAI Moscow State University), S. Korotkiy (Ka-Dar Obs./Astrovert), K. Z. Stanek, C. S. Kochanek (OSU), and B. J. Shappee (Univ. of Hawaii)
on 14 Jun 2021; 15:15 UT
Credential Certification: Elias Aydi (aydielia@msu.edu)

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We report on multi-wavelength spectroscopic and photometric follow up of the very fast Nova Herculis 2021 (TCP J18573095+1653396), which was discovered by Seiji Ueda (Kushiro, Hokkaido, Japan) on 021-06-12.5484 UT and classified spectroscopically as a classical nova by Munari et al. (ATel#14704). The nova was also detected as a GeV gamma-ray source by Fermi-LAT (ATels #14705 and #14707).

On 2021-06-12.93/13.01 low-resolution spectra were obtained using the 2.5-m SAI Moscow State University telescope (Kislovodsk, Russia) and by C. Boussin (Chateau-Thierry, France), respectively. Less than a day later, on 20201-06-13.84, we obtained a low-resolution spectrum using the 2.5-m SAI telescope. The first spectra taken around 9 hours after discovery show P Cygni profiles of Balmer, He I, and Fe II. The absorption troughs of the P Cygni profiles are at blue-shifted velocities between 3000 and 3500 km/s, which is consistent with the results reported in Munari et al. (ATel#14704). The first spectra are consistent with that of a classical nova near optical peak and reflect the velocities of the slow ejection flow (see, e.g., Aydi et al. 2020, ApJ, 905, 62). The later spectrum, taken less than a day later, shows significant changes and is dominated by broad emission lines of the same species with shallow blue-shifted absorptions. The FWZI of the Balmer lines is > 11000 km/s and the troughs of the absorption features are at blue-shifted velocities of around 5000 km/s. At this stage the spectrum is consistent with that of a classical nova after optical peak and the larger velocities of the emission lines reflect that of the fast ejection flow. This is in agreement with the fast evolution of the light curve and the detection of gamma-ray emission by Fermi-LAT, which is likely due to the interaction between these different flows (Aydi et al. 2020, ApJ, 905, 62)

We used the 0.6-m SAI telescope on 2021-06-13.88 to perform photometry (V=7.98 +/-0.01, B-V=0.56 +/-0.02; APASS ensemble) and measure the position of the nova 18:57:30.98 +16:53:39.5 +/-0.1" J2000 relative to UCAC3 stars in the field. We also report on time-series photometry obtained by The New Milky Way wide-field survey (see table below). The All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014, ApJ, 788, 48) detected the nova on 2021-06-12.1903 (8.4 hours before discovery) at g = 16.62. The last observation of the field without a detection was taken on 2021-06-10.9660, which places the start of the eruption between these two dates. Based on the multi-band photometry and visual measurements reported to the AAVSO, the nova reached its optical peak around 2021-06-12.9 and declined by 2 magnitudes from peak in less than a day (t2 < 1 day). The very rapid rise and decline, as well as the large velocities observed in the optical spectra make N Her 2021 one of the fastest novae observed to date.

The Neil Gehrels Swift Observatory observed the nova for 1.9ks on 2021-06-13.51. No X-ray counterpart of the nova was detected by the XRT as the (grade 0) count rate of 0.01 cts/s at the position of the nova is consistent with the expected level of optical loading in the photon counting mode. Assuming kT= 2 keV thermal plasma emission and HI column density of 2.94x10^21 cm^-2 (Kalberla et al. 2005 A&A, 440, 775) this corresponds to the unabsorbed 0.3-10.0 keV flux upper limit of 10^-12 ergs/cm^2/s.

 
JD         CV 
2459378.4031 6.51 
2459378.4060 6.51 
2459378.4078 6.42 
2459379.2660 7.73 
2459379.2838 7.72 
2459379.3015 7.71 
2459379.3195 7.71 
2459379.3375 7.75 
2459379.3557 7.76 
2459379.3737 7.75 
2459379.3913 7.79 
2459379.4101 7.80 
2459379.4321 7.82 
2459379.4572 7.82 
2459379.4763 7.85 
2459379.4936 7.89 
2459379.5059 7.90