Discovery of the new X-ray transient MAXI J1807+132: a Candidate of a Neutron Star Low-mass X-ray binary

TL;DR

This paper reports the discovery and multi-wavelength follow-up of the new X-ray transient MAXI J1807+132, suggesting it is likely a neutron star low-mass X-ray binary based on spectral and flux correlation analyses.

Contribution

First identification and detailed multi-wavelength characterization of MAXI J1807+132 as a candidate neutron star LMXB.

Findings

X-ray flux peaked at ~10 mCrab and faded over 10 days

Spectral analysis shows a low-temperature blackbody plus a hard power-law

Optical flux decline correlates with X-ray flux decrease

Abstract

We report on the detection and follow-up multi-wavelength observations of the new X-ray transient MAXI J1807+132 with the MAXI/GSC, Swift, and ground-based optical telescopes. The source was first recognized with the MAXI/GSC on 2017 March 13. About a week later, it reached the maximum intensity ($\sim$10 mCrab in 2-10 keV), and then gradually faded in $\sim$10 days by more than one order of magnitude. Time-averaged Swift/XRT spectra in the decaying phase can be described by a blackbody with a relatively low temperature (0.1-0.5 keV), plus a hard power-law component with a photon index of $\sim$2. These spectral properties are similar to those of neutron star low-mass X-ray binaries (LMXBs) in their dim periods. The blackbody temperature and the radius of the emission region varied in a complex manner as the source became dimmer. The source was detected in the optical wavelength on March 27-31 as well. The optical flux decreased monotonically as the X-ray flux decayed. The correlation between the X-ray and optical fluxes is found to be consistent with those of known neutron star LMXBs, supporting the idea that the source is likely to be a transient neutron star LMXB.