Spectro-photometric follow up of the outbursting AM CVn system ASASSN-21br

TL;DR

This paper presents spectroscopic and photometric observations of the outbursting AM CVn system ASASSN-21br, revealing its outburst duration, brightness dip, and potential orbital period, contributing to understanding these rare systems and their gravitational wave signals.

Contribution

It provides detailed observational data on ASASSN-21br, increasing the sample of known AM CVn systems and offering insights into their outburst behavior and orbital characteristics.

Findings

Outburst lasted about three weeks with a 4-day brightness dip.

Photometry suggests a 36.65-minute period, likely the orbital or superhump period.

Spectroscopy did not reveal periodicity in He I lines, possibly due to disc origin.

Abstract

We report on spectroscopic and photometric observations of the AM CVn system ASASSN-21br, which was discovered in outburst by the All-Sky Automated Survey for Supernovae in February 2021. The outburst lasted for around three weeks, and exhibited a pronounced brightness dip for $\approx$ 4 days, during which the spectra showed a sudden transition from emission- to absorption-line dominated. Only $\approx$ 60 AM CVn systems with derived orbital periods are found in the Galaxy, therefore increasing the sample of AM CVn systems with known orbital periods is of tremendous importance to (1) constrain the physical mechanisms of their outbursts and (2) establish a better understanding of the low-frequency background noise of future gravitational wave surveys. Time-resolved photometry taken during the outburst of ASASSN-21br showed modulation with a period of around 36.65 minutes, which is likely the superhump or orbital period of the system. Time-resolved spectroscopy taken with the Southern African Large Telescope did not show any sign of periodicity in the He I absorption lines. This is possibly due to the origin of these lines in the outbursting accretion disc, which makes it challenging to retrieve periodicity from the spectral lines. Future follow up spectral observations during quiescence might allow us better constrain the orbital period of ASASSN-21br.