Two new bursting neutron star low-mass X-ray binaries: Swift J185003.2-005627 and Swift J1922.7-1716

TL;DR

This study identifies two new neutron star low-mass X-ray binaries through analysis of Swift BAT triggers, confirming their nature as thermonuclear X-ray burst sources and characterizing their properties and behaviors.

Contribution

First identification and detailed analysis of two new neutron star low-mass X-ray binaries based on Swift BAT data and follow-up observations.

Findings

Both sources exhibit thermonuclear X-ray bursts with durations of ~7 minutes.

Estimated distances are less than 3.7 kpc and 4.8 kpc for the two sources.

Sources have comparable outburst luminosities of ~10^35-10^36 erg/s.

Abstract

We discuss the origin of two triggers of Swift's Burst Alert Telescope (BAT) that occurred in 2011. The triggers were identified with Swift J185003.2-005627, a previously unknown X-ray source, and the known but unclassified X-ray transient Swift J1922.7-1716. We investigate the BAT data and follow-up observations obtained with the X-ray and ultraviolet/optical telescopes to demonstrate that both triggers are consistent with thermonuclear X-ray bursts. This implies that both sources are neutron star low-mass X-ray binaries. The total duration of ~7 minutes and estimated energy output of ~(3-7)E39 erg, fall in between that of normal and intermediately long X-ray bursts. From the observed peaks of the X-ray bursts, we estimate a distance of <3.7 kpc for Swift J185003.2-005627 and <4.8 kpc for Swift J1922.7-1716. We characterize the outburst and quiescent X-ray properties of the two sources. They have comparable average outburst luminosities of ~1E35-1E36 erg/s, and a quiescent luminosity equal to or lower than ~2E32 erg/s (0.5-10 keV). Swift J185003.2-005627 returned to quiescence ~20 d after its BAT trigger, while Swift J1922.7-1716 appears to exhibit long accretion outbursts that last several months to years. We identify a unique counterpart for Swift J1922.7-1716 in the ultraviolet/optical data. Finally, we serendipitously detected a flare lasting ~500 s from an uncataloged X-ray/optical object that we tentatively classify as a flaring M-dwarf.