HW Boötis: an enigmatic cataclysmic variable star

2015 July 20

Jeremy Shears, David Boyd, Graham Darlington, Ian Miller, Roger Pickard, Gary Poyner, Richard Sabo, Mike Simonsen & William Stein

We present the 13-year lightcurve of HW Boo between 2001 May and 2014 May. We identified 12 outbursts, which typically lasted 2 to 5 days, with an amplitude of 2.7 to 3.6 magnitudes. Time resolved photometry during two outbursts revealed small hump-like structures which increased in size as the outburst progressed, reaching a peak-to-peak amplitude of ~0.8 mag. They occurred on timescales of 15 min to an hour, but did not exhibit a stable period. Similar irregular hump-like variations of 0.1 to 0.8 magnitudes, at intervals of 7 to 30 minutes, were also detected during quiescence. We discuss whether HW Boo might be a dwarf nova of the SU UMa family or an Intermediate Polar, but further observations are needed to support classification.

Introduction
Cataclysmic variables (CVs) are semi-detached binary stars in which a white dwarf primary accretes material from a secondary star via Roche lobe overflow. The secondary is usually a late-type main-sequence star. In the absence of a significant white dwarf magnetic field, material from the secondary is processed through an accretion disc before settling on the surface of the white dwarf. In CVs with low to intermediate mass transfer rates, dwarf nova outbursts with amplitudes of 2–8 mag and durations of days to weeks are observed. These outbursts are thought to be caused by a thermal instability in the accretion disc.
In cases where the white dwarf has a stronger magnetic field, the inner portion of the disc becomes truncated and the accreting material follows the magnetic field lines and falls onto the poles of the white dwarf. Such systems are called Intermediate Polars (IPs). IPs also exhibit short-term variability, mostly associated with turbulence in the accretion system. (Continued…)