Spectroscopic and Photometric Periods of Six Ultracompact Accreting Binaries

TL;DR

This study analyzes six ultracompact accreting binary systems using spectroscopy and photometry, revealing new orbital periods, donor star detection, and outburst behaviors, advancing understanding of these compact binaries.

Contribution

It provides the first spectroscopic orbital period for SDSS J1505+0659, detects the donor star via infrared excess, and reports unique outburst patterns in several systems.

Findings

SDSS J1505+0659 has a 67.8 min orbital period, longer than previously thought.

Shortest orbital period of 52.1 min found in CRTS J1028-0819.

ASASSN-14ei shows increased outburst rate post-superoutburst.

Abstract

Ultracompact accreting binary systems each consist of a stellar remnant accreting helium-enriched material from a compact donor star. Such binaries include two related sub-classes, AM CVn-type binaries and helium cataclysmic variables, in both of which the central star is a white dwarf. We present a spectroscopic and photometric study of six accreting binaries with orbital periods in the range of 40--70 min, including phase-resolved VLT spectroscopy and high-speed ULTRACAM photometry. Four of these are AM CVn systems and two are helium cataclysmic variables. For four of these binaries we are able to identify orbital periods (of which three are spectroscopic). SDSS J1505+0659 has an orbital period of 67.8 min, significantly longer than previously believed, and longer than any other known AM CVn binary. We identify a WISE infrared excess in SDSS J1505+0659 that we believe to be the first direct detection of an AM CVn donor star in a non-direct impacting binary. The mass ratio of SDSS J1505+0659 is consistent with a white dwarf donor. CRTS J1028-0819 has an orbital period of 52.1 min, the shortest period of any helium cataclysmic variable. MOA 2010-BLG-087 is co-aligned with a K-class star that dominates its spectrum. ASASSN-14ei and ASASSN-14mv both show a remarkable number of echo outbursts following superoutbursts (13 and 10 echo outbursts respectively). ASASSN-14ei shows an increased outburst rate over the years following its superoutburst, perhaps resulting from an increased accretion rate.