NICER detection of two thermonuclear (Type I) X-ray bursts establishes that MAXI J1807+132 hosts a neutron star

MAXI J1807+132 is an X-ray transient first discovered by MAXI/GSC in March 2017 (ATel #10208). Optical spectroscopy (ATel #10221) suggests the source is a low-mass X-ray binary. On 2019 September 10, MAXI/GSC detected a new outburst (ATel #13097). NICER has been regularly monitoring the outburst (ATel #13139), which shows strong variability on timescales of hours to days in the form of reflares (ATel #13173).

On 2019 October 28 at 14:50 UTC, NICER detected a sudden increase in flux, followed by an exponential decay. Results from time-resolved spectroscopy are consistent with this event being a thermonuclear (Type I) X-ray burst from the source, indicating that the compact object in this system is a neutron star.

A second Type I X-ray burst was detected about a day later, on October 29 at 12:10 UTC. Each burst had a duration of approximately 10 seconds and peak count rates of 2000 and 1000 cps in the 0.4-10 keV band, respectively. The bolometric peak flux of the first burst was measured to be 1.0x10^{-8} erg/s/cm^2 from time-resolved spectroscopy, with a peak blackbody temperature of 1.7 keV.

Assuming an Eddington-limited peak flux and solar composition, we derive an upper limit of the distance to the source of 13 kpc. The first burst occurred during the rise of a reflare and the second burst occurred near the peak of the most recent reflare. The persistent source count rates were 57 and 138 cps in the 0.4-10 keV band, respectively.

Assuming that there is no other source within the NICER 30 arcmin^2 FOV, the detection of these bursts demonstrates that MAXI J1807+132 harbors a neutron star. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.