Evidence that short period AM CVn systems are diverse in outburst behaviour

TL;DR

This study analyzes 15 years of photometric data from eight AM CVn systems, revealing diverse outburst behaviors similar to hydrogen-rich dwarf novae, including super outbursts, standstills, and normal outbursts, with implications for theoretical models.

Contribution

It demonstrates that AM CVn systems in the accretion disc instability region are not homogeneous and exhibit a variety of outburst behaviors previously unrecognized.

Findings

AM CVn systems show diverse long-term optical behaviors.

Presence of normal outbursts as precursors to super outbursts in KL Dra.

Different systems resemble Z Cam, SU UMa, and nova-like types.

Abstract

We present results of our analysis of up to 15 years of photometric data from eight AM CVn systems with orbital periods between 22.5 and 26.8 min. Our data has been collected from the GOTO, ZTF, Pan-STARRS, ASAS-SN and Catalina all-sky surveys and amateur observations collated by the AAVSO. We find evidence that these interacting ultra-compact binaries show a similar diversity of long term optical properties as the hydrogen accreting dwarf novae. We found that AM CVn systems in the previously identified accretion disc instability region are not a homogenous group. Various members of the analysed sample exhibit behaviour reminiscent of Z Cam systems with long super outbursts and standstills, SU UMa systems with regular, shorter super outbursts, and nova-like systems which appear only in a high state. The addition of TESS full frame images of one of these systems, KL Dra, reveals the first evidence for normal outbursts appearing as a precursor to super outbursts in an AM CVn system. Our results will inform theoretical modelling of the outbursts of hydrogen deficient systems.