SOAR spectroscopic follow up of ASASSN-21br -- a possible AM CVn outburst?

ATel #14405; E. Aydi, J. Strader, L. Chomiuk, A. Kawash, K. V. Sokolovsky (MSU), K. Z. Stanek, C. S. Kochanek (OSU), and B. J. Shappee (Univ. of Hawaii)
on 18 Feb 2021; 23:43 UT
Credential Certification: Elias Aydi (eaydi@saao.ac.za)

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We report on spectroscopic follow up of the optical transient ASASSN-21br, which was discovered by the All-Sky Automated Survey for SNe (ASAS-SN, Shappee et al. 2014, ApJ, 788, 48) on 2021-02-13.12UT. On 2021-02-17.3, we obtained a low-resolution spectrum using the Goodman spectrograph (Clemens et al. 2004, SPIE, 5492, 331) on the 4.1 m SOAR telescope, covering a wavelength range of 4000-7800 A. The spectrum shows strong emission lines of He I and He II, with very weak H-Balmer lines. We also detect several lines, which identifications are not certain (see list below). The FWHMs of the strongest He I lines are around 1600 km/s. The FHWM of He II at 4686 A is around 4100 km/s, but the line might be blended with neighboring lines. The strong He emission lines and the near-absence of H-Balmer lines in the spectrum is reminiscent of the spectra of AM Canum Venaticorum (AM CVn) cataclysmic variable systems.

The ASAS-SN data show a quick rise to a peak magnitude of g = 13.6, in around 1 day, from a limiting magnitude > 17.1. Then the light curve shows a slow decline by 0.4 magnitude over 2 days, before dropping to g = 16.7 in another 2 days. The full ASAS-SN light curve, shows a possible outburst in March 2016. The spectrum and light curve evolution suggest that ASASSN-21br could be an AM CVn outburst, but other possibilities are not to be ruled out. Taichi Kato also suggested that the system might be an AM CVn based on the rapid decline in the light curve (vsnet-alert 25412), which might show further re-brightening similar to the case of the recently discovered AM CVn ASASSN-21au (see ATel #14390). We encourage follow up observations in all bands to determine the definitive nature of this outburst.

The latest ASAS-SN light curve (Kochanek et al. 2017, PASP, 129, 4502): https://asas-sn.osu.edu/sky-patrol/coordinate/e8ca2819-ba21-49d3-bc68-30ff60832279 The full ASAS-SN light curve: https://asas-sn.osu.edu/sky-patrol/coordinate/801f8652-e3c6-4b6d-bffc-60abf115ab43

Unidentified lines: strong line at 5170 A (possibly Fe II), relatively weak lines at 5680 and 6156 A (possibly N II or Na I), broad line at 6357 A (likely blended Si II 6341/6370 A).

This ATel is based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, Inovações e Comunicações do Brasil (MCTIC/LNA), the U.S. National Science Foundation's National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).