Discovery of the New X-Ray Transient MAXI J1807+132: A Candidate of a Neutron Star Low-mass X-Ray Binary

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 maximum intensity (∼10 mCrab in 2-10 keV), and then gradually faded in ∼10 days by more than one order of magnitude. Time-averaged Swift/X-ray Telescope 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 ∼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.