Type I thermonuclear X-ray bursts detected by MAXI J1807 + 132

An international team of astronomers studied a binary X-ray system known as MAXI J1807 + 132, using the NICER instrument aboard the International Space Station (ISS). Now they report the detection of three bursts of type I thermonuclear X-rays from this source. The discovery is reported in a document published November 20 on arXiv.org.

X-ray binaries consist of a normal star or a white dwarf that transfers mass to a compact neutron star or black hole. Based on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXB) and high-mass X-ray binaries (HMXB).

LMXBs can exhibit transient bursts during which an increase in X-ray brightness is observed. Some of these bursts are characterized as type I X-ray bursts, thermonuclear bursts that take place on the surface layers of neutron stars.

MAXI J1807 + 132 is an X-ray binary that was detected during its 2017 explosion by the Monitor of All-sky X-ray X-ray Image Gas Slit Camera (MAXI / GSC) on the ISS. In September 2019, another period of explosive activity of this source began. Follow-up observations of MAXI J1807 + 132 suggested that it is an LMXB with a neutron star (NS) as its primary object.

Now, a new study published by a team of researchers led by Arianna C. Albayati of the University of Southampton, UK, confirms the NS LMXB scenario for MAXI J1807 + 132. Using the Interior Composition Explorer (NICER) neutron star, they observed the system between September 16 and November 29, 2019 and identified three bursts of type I thermonuclear X-rays.

“NICER observed MAXI J1807 between September 16 and November 26, 2019, generating a total of 47 ObsIDs. We searched all available data for the X-ray bursts; here we report the five observations around the time of detection of three X-ray bursts, “the astronomers wrote in the paper.

The newly detected bursts were designated B1, B2 and B3. B2 occurred approximately 21.3 hours after B1, while B3 occurred nearly 24 hours after B2. The hardness ratios of all three bursts were observed to plot similar profiles to the light curves, increasing with increasing burst and decreasing with decay.

All three X-ray bursts have a rise time of about four seconds and show long decay tails, lasting over a minute. Astronomers explained that such slow growth and long decay suggest a hydrogen-rich fuel at ignition, which is likely the result of the accretion of a mixed hydrogen / helium fuel.

B1 is the brightest Type I thermonuclear X-ray burst of the three reported in the study. Observations found a pause in this burst, lasting about 1.6 seconds during the ascent. Based on this finding and comparing it with the results of other studies, the astronomers concluded that there is no link between detecting double peak profiles and detecting a pause on the climb.

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