Spectroscopic and photometric confirmation of ASASSN-21br as an AM CVn system
ATel #14421; M. Motsoaledi (UCT/SAAO), E. Aydi (MSU), D. A. H. Buckley (SAAO), J. Strader, L. Chomiuk, A. Kawash, K. V. Sokolovsky (MSU), P. A. Woudt (UCT), K. Z. Stanek, C. S. Kochanek (OSU), and B. J. Shappee (Univ. of Hawaii)
on 23 Feb 2021; 20:06 UT
Credential Certification: Elias Aydi (eaydi@saao.ac.za)
Subjects: Optical, Binary, Cataclysmic Variable, Star, Transient, Variables
We report on spectroscopic and photometric follow up of ASASSN-21br, a candidate AM CVn (ATel #14405), which was discovered by the All-Sky Automated Survey for SNe (ASAS-SN, Shappee et al. 2014, ApJ, 788, 48) on 2021-02-13.12 UT. After a very rapid outburst lasting for less than 3 days, followed by a sudden dip, the system re-brightened and is still in outburst as of 2021-02-23.07 (see latest ASAS-SN light curve below). This pattern of a precursor followed by a main outburst is identical to the behavior of the recently discovered AM CVn ASASSN-21au (ATel #14390).
We obtained a low-resolution spectrum of ASASSN-21br using the Goodman spectrograph (Clemens et al. 2004, SPIE, 5492, 331) on the 4.1m SOAR telescope, covering a wavelength range of 4000-7800 A. We also obtained several low resolution phase-resolved spectra with the Robert Stobie Spectrograph (RSS; Burgh et al. 2013, SPIE, 4841, 1463; Kobulnicky et al. 2003, 4841, 1634) mounted on the 11m Southern African Large Telescope, covering the range of 4400-6400 A. The spectra are taken over the nights of 2021-02-19 and 20, during the re-brightenning phase. The spectra show a blue continuum with several absorption lines of He I and He II, but no emission lines, in contrast to the emission line dominated spectrum obtained during the precursor phase (ATel #14405). The current spectra are typical of AM CVn superoutbursts.
We also obtained high-cadence photometry using the Sutherland High Speed Optical Camera (SHOC; Coppejans, R. et al., 2013, PASP, 125, 976-988) mounted on the 1m telescope at SAAO. The data were taken on the nights of 2021-02-19, 20, 21, and 22, spanning around 2 hours each night, with an exposure of 5 or 10 seconds in the g-band. A Lomb-Scargle (Scargle 1982 ApJ, 263, 835) periodogram of the detrended light curve shows a period of 38.6 minutes with aliases at around 37.6 and 39.6 minutes. The periods persist regardless of the choice of the detrending technique (we tried both polynomial and piecewise linear). This period is consistent with the superhump periods observed in AM CVn systems during superoutbursts.
This ATel is based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministerio da Ciencia, Tecnologia, Inovacoes e Comunicacoes 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).
The latest ASAS-SN Sky Patrol light curve (Kochanek et al. 2017, PASP, 129, 4502)
