[vsnet-campaign-dn 15372] ASASSN-21br (ATel)

[Taichi Kato]

   Please don't stop your observation!  Nightly snapshots
are important.  Once it is seen brighter, high-cadence
observations are of utmost importance.
ASASSN-21au = ZTF20acyxwzf is currently behaving very well
with beautiful humps (superhumps or early superhumps?).
We can expect the same for ASASSN-21br once it brightens
again.

===

ATEL #14405                                                          ATEL #14405

Title:  SOAR spectroscopic follow up of ASASSN-21br -- a possible AM
                CVn outburst?
Author: E. Aydi, J. Strader, L. Chomiuk, A. Kawash, K. V. Sokolovsky
                (MSU), K. Z. Stanek, C. S. Kochanek (OSU), and B. J. Shappee (Un
iv.
                of Hawaii)
Queries:        aydielia at msu.edu
Posted: 18 Feb 2021; 23:43 UT
Subjects:Binary, Cataclysmic Variable, Star, Transient, Variables

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-bc68-30ff608322
79