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

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.

💡 Research Summary

The paper presents a detailed investigation of two Swift Burst Alert Telescope (BAT) triggers that occurred in 2011 and were subsequently identified with the X‑ray sources Swift J185003.2‑005627 (previously unknown) and Swift J1922.7‑1716 (known but unclassified). By analysing the BAT light curves, spectra, and follow‑up observations with Swift’s X‑ray Telescope (XRT) and UV/Optical Telescope (UVOT), the authors demonstrate that both events are thermonuclear (type‑I) X‑ray bursts, thereby confirming that each source is a neutron‑star low‑mass X‑ray binary (LMXB).

The bursts lasted roughly seven minutes, with blackbody temperatures of 2–3 keV and total radiated energies of (3–7) × 10³⁹ erg. This places them in the intermediate regime between typical short bursts (∼10³⁸ erg, seconds) and intermediately long bursts (∼10⁴⁰ erg, tens of minutes). From the peak fluxes, assuming the Eddington limit for a hydrogen‑rich photosphere, distance upper limits are derived: <3.7 kpc for Swift J185003.2‑005627 and <4.8 kpc for Swift J1922.7‑1716.

XRT spectroscopy shows soft spectra with absorbing columns of N_H ≈ 1.2 × 10²¹ cm⁻² (J1850) and 2.0 × 10²¹ cm⁻² (J1922). Both sources exhibit average outburst luminosities of L_X ≈ 10³⁵–10³⁶ erg s⁻¹ (0.5–10 keV) and quiescent luminosities ≤2 × 10³² erg s⁻¹, consistent with faint LMXBs. Swift J185003.2‑005627 returned to quiescence within about 20 days after the burst, indicating a short, transient accretion episode likely driven by a brief disk instability. In contrast, Swift J1922.7‑1716 shows prolonged outbursts lasting months to years, suggesting a more stable mass‑transfer rate and a larger accretion disk. UVOT imaging identifies a unique optical/UV counterpart for J1922; its colors and variability are compatible with a K–M type donor star.

During the campaign, the authors also serendipitously detected a ∼500 s flare from an uncatalogued X‑ray/optical source. The flare’s rapid rise, decay, and soft spectrum resemble flares from active M‑dwarf stars, leading the authors to tentatively classify the object as a flaring M‑dwarf.

Overall, the study adds two new thermonuclear bursters to the Galactic population, expands the sample of intermediate‑duration bursts, and provides valuable constraints on burst physics (fuel composition, ignition depth) and binary evolution (accretion rates, disk properties). The contrasting outburst behaviours—short, transient versus long‑lasting—highlight the diversity among neutron‑star LMXBs and underscore the importance of continuous monitoring with wide‑field instruments like Swift/BAT.