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

## TL;DR

This paper reports the discovery of thermonuclear X-ray bursts from XMMU J181227.8-181234, confirming it as an ultracompact neutron star low-mass X-ray binary with helium-rich accretion and short burst recurrence times.

## Contribution

It identifies XMMU J181227.8-181234 as a new ultracompact X-ray binary candidate based on burst analysis and system properties.

## Key findings

- Detected 7 X-ray bursts confirming neutron star binary nature.
- Estimated distance of 14±2 kpc with systematic uncertainties.
- Classified as a transient ultracompact low-mass X-ray binary.

## 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 7 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$^{+0.9}_{-0.5}$ hr, we deduce that the source is accreting almost pure helium ($X \leq 0.1$) fuel. Two bursts occurred just 18 minutes 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 $30\pm{10}$ degrees, we estimate the distance to be $14\pm{2}$ kpc, where we report the statistical uncertainty but note that there could be up to $20\%$ variation in the distance due to systematic effects discussed in the paper. The corresponding maximum accretion rate is $0.30\pm0.05$ 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.