Spectroscopic confirmation of PGIR21fjn/AT2021kgk as a classical nova outburst

ATel #14574; R. Soria (UCAS/Sydney), X. Liang (UCAS), J. Soon (ANU), K. De (Caltech), M. M. Kasliwal (Caltech), G. Da Costa (ANU), T. Nordlander (ANU), R. M. Lau (ISAS/JAXA), M. Hankins (Caltech), J. Sokoloski (Columbia), M. Ashley (UNSW), A. Babul (Columbia), V. Karambelkar (Caltech), J. Jencson (Arizona), A. Moore (ANU), E. O. Ofek (Weizmann), M. Sharma (Columbia), T. Travouillon (ANU) on behalf of the Palomar Gattini-IR team
on 26 Apr 2021; 05:11 UT
Credential Certification: Roberto Soria (rsoria@physics.usyd.edu.au)

Tweet

On 2021-04-22 (20:34:48.8 UTC) we obtained an optical spectrum of the highly reddened transient PGIR21fjn/AT2021kgk that was discovered by the Palomar Gattini-IR survey (ATel #14567). The spectrum was taken with the Yunnan Faint Object Spectrograph and Camera, mounted on the Lijiang 2.4m telescope (Yunnan Observatories, Chinese Academy of Science), using grism Number 8, which covers the wavelength range 5100-9600 Ang with a resolution of about 4000 (1.5 Ang/pixel). The total exposure time was 2700 s and the effective airmass was 1.13. Images taken from the same telescope immediately after the spectral observations show that PGIR21fjn/AT2021kgk had an apparent brightness of (r = 17.5 +/- 0.1) Vegamag, i = (14.0 +/- 0.1) Vegamag.
Earlier in the same night (2021-04-22 18:12:55.5 UTC) we obtained another spectrum of the same source with Wide-Field Spectrograph on the Australian National University's 2.3m telescope at Siding Spring Observatory (exposure time: 180 s; gratings: B3000/R3000; wavelength range: 3500-9500 Ang).
The line measurements given below are derived from the Lijian telescope spectrum, but the results from the two calibrated spectra are perfectly consistent.
We find a clear P-Cygni profile at Halpha, consistent with what was reported in ATel #14570. Gaussian deblending shows a peak of the absorption component at (6535.0 +/- 0.2) Ang and a peak of the emission component at (6558.1 +/- 0.1) Ang, for a velocity separation of (1060 +/- 5) km/s. The FWHM of the absorption component is (23 +/- 1) Ang, and FWHM = (49 +/- 1) Ang for the emission component. The EWs are (23 +/- 1) Ang and (-64 +/- 1) Ang for absorption and emission components, respectively.
Another prominent P-Cyg line is OI 7774, for which we fit a velocity separation of (1120 +/- 110) km/s between absorption and emission peaks. The fluorescent line OI 8446 is weaker than the non-fluorescent OI 7774 (intensity ratio of about 1/4), but is also clearly detected with a P-Cyg profile, velocity separation of (850 +/- 150) km/s. Other clearly identifiable P-Cyg lines are NI 8747, OI 9118, OI 9261,9266, OI 9399 (for the last one, we infer a velocity separation of (1500 +/- 100) km/s). In addition, we see emission from a likely blend of OI 8233 and NI 8242, and from the Ca II triplet (CaII 8498,8452,8662). NaI 8183,8195 are seen in absorption. High reddening prevents an investigation of the lines bluewards of Halpha.
Overall, the spectrum is consistent with a young classical nova outburst, which peaked at J = 9.1 Vegamag around April 22-23 and has now started to decline, based on the most recent Palomar Gattini-IR data. We encourage further spectroscopic follow-up for this young and rapidly evolving nova.