Discovery of Thermonuclear Type-I X-ray Bursts from the X-ray binary MAXI J1807+132

TL;DR

This paper reports the discovery of thermonuclear Type-I X-ray bursts from MAXI J1807+132, confirming it as a neutron star low-mass X-ray binary through detailed spectral analysis and burst characteristics.

Contribution

First detection of Type-I X-ray bursts from MAXI J1807+132, establishing its nature as a neutron star LMXB with detailed burst properties and implications for burst physics.

Findings

Detected three Type-I X-ray bursts during NICER observations.

Confirmed neutron star nature of MAXI J1807+132 through burst characteristics.

Observed a ~1.6 sec pause during burst rise, similar to other sources.

Abstract

MAXI J1807+132 is a low-mass X-ray binary (LMXB) first detected in outburst in 2017. Observations during the 2017 outburst did not allow for an unambiguous identification of the nature of the compact object. MAXI J1807+132 was detected in outburst again in 2019 and was monitored regularly with NICER. In this paper we report on five days of observations during which we detected three thermonuclear (Type-I) X-ray bursts, identifying the system as a neutron star LMXB. Time-resolved spectroscopy of the three Type-I bursts revealed typical characteristics expected for these phenomena. All three Type-I bursts show slow rises and long decays, indicative of mixed H/He fuel. We find no strong evidence that any of the Type-I bursts reached the Eddington Luminosity; however, under the assumption that the brightest X-ray burst underwent photospheric radius expansion, we estimate a <12.4kpc upper limit for the distance. We searched for burst oscillations during the Type-I bursts from MAXI J1807+132 and found none (<10% amplitude upper limit at 95% confidence level). Finally, we found that the brightest Type-I burst shows a ~1.6sec pause during the rise. This pause is similar to one recently found with NICER in a bright Type-I burst from the accreting millisecond X-ray pulsar SAX J1808.4-3658. The fact that Type-I bursts from both sources can show this type of pause suggests that the origin of the pauses is independent of the composition of the burning fuel, the peak luminosity of the Type-I bursts, or whether the NS is an X-ray pulsar.