Low-Mass X-Ray Binary MAXI J1421-613 Observed by MAXI GSC and Swift XRT

TL;DR

This paper reports the discovery and analysis of a new low-mass X-ray binary, MAXI J1421-613, using MAXI GSC and Swift XRT data, revealing its spectral properties, distance estimate, and potential historical activity.

Contribution

First detailed analysis of MAXI J1421-613's spectral behavior and distance estimation, linking it to past X-ray sources and providing insights into its outburst characteristics.

Findings

Spectral hardness remained stable during the outburst.

XRT spectra fit thermal Comptonization of disk blackbody emission.

Estimated maximum distance of 7 kpc based on X-ray burst peak flux.

Abstract

Monitor of All sky X-ray Image (MAXI) discovered a new outburst of an X-ray transient source named MAXI J1421-613. Because of the detection of three X-ray bursts from the source, it was identified as a neutron star low-mass X-ray binary. The results of data analyses of the MAXI GSC and the Swift XRT follow-up observations suggest that the spectral hardness remained unchanged during the first two weeks of the outburst. All the XRT spectra in the 0.5-10 keV band can be well explained by thermal Comptonization of multi-color disk blackbody emission. The photon index of the Comptonized component is $\approx$ 2, which is typical of low-mass X-ray binaries in the low/hard state. Since X-ray bursts have a maximum peak luminosity, it is possible to estimate the (maximum) distance from its observed peak flux. The peak flux of the second X-ray burst, which was observed by the GSC, is about 5 photons cm$^{-2}$ s$^{-1}$. By assuming a blackbody spectrum of 2.5 keV, the maximum distance to the source is estimated as 7 kpc. The position of this source is contained by the large error regions of two bright X-ray sources detected with Orbiting Solar Observatory-7 (OSO-7) in 1970s. Besides this, no past activities at the XRT position are reported in the literature. If MAXI J1421-613 is the same source as (one of) them, the outburst observed with MAXI may have occurred after the quiescence of 30-40 years.