XMMU J181227.8–181234: a new ultracompact X-ray binary candidate

Abstract

We report the discovery of Type I (thermonuclear) X-ray bursts from the transient source XMMU J181227.8–181234 = XTE J1812–182. We found seven X-ray bursts in Rossi X-ray Timing Explorer observations during the 2008 outburst, confirming the source as a neutron star low-mass X-ray binary. Based on the measured burst fluence and the average recurrence time of 1.4|$^{+0.9}_{-0.5}$| h, we deduce that the source is accreting almost pure helium (X ≤ 0.1) fuel. Two bursts occurred just 18 min apart; the first short waiting time bursts observed in a source accreting hydrogen-poor fuel. Taking into consideration the effects on the burst and persistent flux due to the inferred system inclination of 30 ± 10°, we estimate the distance to be 14 ± 2 kpc, where we report the statistical uncertainty but note that there could be up to |$20{{\ \rm per\ cent}}$| variation in the distance due to systematic effects discussed in the paper. The corresponding maximum accretion rate is 0.30 ± 0.05 times the Eddington limit. Based on the low hydrogen content of the accreted fuel and the short average recurrence time, we classify the source as a transient ultracompact low-mass X-ray binary.