Two New Bursting Neutron Star Low-mass X-Ray Binaries: Swift J185003.2-005627 and Swift J1922.7-1716

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 Swift's 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) × 10³⁹ 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 ~= 10^35-36 erg s^-1, and a quiescent luminosity equal to or lower than ~= 2 × 10³² erg s^-1 (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 detect a flare lasting ~= 500 s from an uncataloged X-ray/optical object that we tentatively classify as a flaring M-dwarf.