Spectroscopic classification of PGIR21fjn/AT2021kgk as a fast He/N nova with IRTF + SpeX

ATel #14647; K. De (Caltech), M. Ashley (UNSW), R. Soria (UCAS/Sydney), M. Kasliwal (Caltech), R. M. Lau (ISAS/JAXA), M. Hankins (Caltech), J. Sokoloski (Columbia), A. Babul (Columbia), V. Karambelkar (Caltech), J. Jencson (Arizona), A. Moore (ANU), E. O. Ofek (Weizmann), M. Sharma (Columbia), J. Soon (ANU), T. Travouillon (ANU) on behalf of the Palomar Gattini-IR team
on 20 May 2021; 19:37 UT
Credential Certification: Kishalay De (kde@astro.caltech.edu)

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The highly reddened Galactic transient PGIR21fjn/AT2021kgk was discovered by the Palomar Gattini-IR survey (ATel #14567). The source was classified as a young classical nova outburst with follow-up optical spectroscopy (ATel #14570, #14572, #14574, #14578). We obtained a near-infrared spectrum of the source using IRTF + SpeX on UT 2021-05-16, as a part of program 2021A083 (PI: De). The data were obtained in the SXD mode with a 0.5" slit, for a total integration time of 12 minutes. The source is clearly detected from a wavelength of 0.73 to 2.5 micron, brightening by a factor of ~ 5 in flux from the shortest to longest wavelengths, consistent with the high reddening. The reduced spectrum shows strong emission lines of H at the Pa Gamma, Pa Beta, Br Gamma and the Br 10-19 series; He I at 1.08 micron, 2.05 micron; and O I at 1.13 and 1.32 micron. The H lines exhibit a complex profile with at least three peaks and an overall FWHM of 2200 km/s. We do not detect emission features of C I, which are typically associated with Fe II novae in the NIR (Banerjee & Ashok 2012). Based on continued photometric coverage from regular survey operations of Palomar Gattini-IR, the nova reached a peak J mag of ~ 9.1 mag on UT 2021-04-21 before exhibiting a monotonic decline to J ~ 12.0 mag on UT 2021-05-19. We measure a t_2 of ~ 11 days and t_3 ~ 27 days. Overall, the spectrum and light curve are consistent with a fast, highly obscured He/N nova.