Catalog of outbursts of neutron star LMXBs

TL;DR

The paper addresses the need for robust statistics on outburst recurrence in neutron star LMXBs by compiling a comprehensive catalog of 86 transient NS LMXBs (87 including Cir X-1) identified via bursts, pulsations, and public lightcurves. It combines the MINBAR burst catalog with recent transients and accreting millisecond pulsars, and imposes clear outburst criteria of a >10x rise in flux and $L_X>10^{34}$ erg s$^{-1}$, using spectral conversions to estimate peak luminosities in the $2-10$ keV band. The study identifies 109 new outbursts (with significant contributions from MXB 1730-335 and GRS 1747-312) and documents notable cases such as XMMU J174716.1-281048 and AX J1754.2-2754, while also highlighting detection efficiency since 1996 and the large fraction of systems with few observed events. The results have important implications for understanding binary evolution and the population of faint X-ray binaries near the Galactic Center, and provide a valuable resource for future population studies and monitoring of NS LMXBs.

Abstract

Many X-ray binaries are transiently accreting. Having statistics on their recurrence times is helpful to address questions related to binary evolution and populations, as well as the physics of binary systems. We compile a catalog of known outbursts of 87 transient neutron star (identified through bursts or pulsations) low-mass X-ray binaries, until mid-2025. Most outbursts are taken from the literature, but we also identify some outbursts from public X-ray monitoring lightcurves. We find 109 outbursts not previously identified in the literature; most are from the frequent transients GRS 1747-312 and the Rapid Burster MXB 1730-335, though we suspect that two outbursts from Liller 1 may be from another transient, besides the Rapid Burster. We also find new outbursts for 10 other systems, and verify substantial quiescent intervals for XMM J174457-2850.3, XMMU J174716.1-281048, and AX J1754.2-2754. Outburst detection has been relatively efficient since 1996 for outbursts above $F_X$(2-10)$=3\times10^{-10}$ ergs/s/cm$^2$. While several systems have many known outbursts, 40 of the 87 systems we track have zero or one recorded outburst between 1996 and 2023. This suggests that many faint Galactic Center X-ray binaries may be neutron star X-ray binaries, though we cannot completely rule out the proposition that most neutron star X-ray binaries undergo frequent outbursts below all-sky monitor detection limits.