Using the AstroSat spacecraft, Indian astronomers have investigated a low-mass X-ray binary known as GX 3+1. The study provided more insights into the properties of GX 3+1 and detected a thermonuclear burst from this source. The findings are reported in a paper published June 15 on arXiv.org.

Generally, X-ray binaries are composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a 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 may exhibit transient outbursts during which an increase in X-ray luminosities is observed. Some of these outbursts are characterized as type I X-ray bursts鈥攖hermonuclear explosions taking place on the surface layers of neutron stars.

Detected in 1964, GX 3+1 is a persistently bright X-ray binary source that was classified as an LMXB of atoll subtype with a soft spectrum of about 2-10 keV. First type I burst from GX 3+1 was detected in 1983 and since then this source was found to be a very active X-ray burster, with bursting activity inspected by numerous studies.

However, many properties of this LMXB still remain uncertain. That is why a team of astronomers led by Ankur Nath of Tezpur University in India decided to inspect GX 3+1 with AstroSat's Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT).

"In this work, we report the AstroSat observation of the bright atoll source, the LMXB GX 3+1. The light curve obtained from the LAXPC 20 [one of the three LAXPC counters] instrument indicated the presence of a thermonuclear burst feature of type I," the researchers wrote in the paper.

As noted in the study, the observations detected one type I thermonuclear burst lasting approximately 15 seconds. A drop in count rate in the light curve of the X-ray burst was found when the narrow energy bands become harder. The burst turned out to be the brightest in the 5鈥�8 keV energy band with a count rate higher than the softest band (3鈥�5 keV). Moreover, it was found that the burst decayed faster at higher energies, what seems to suggest that the temperature is decreasing as the burst evolves.

By analyzing the data, the astronomers noticed a double-peak feature in the burst at higher energies (8鈥�12 keV, 12鈥�20 keV). This was a relatively quick event, lasting about two seconds, within which the burst exhibited the double peak, indicating a radius expansion phase.

"Based on our time-resolved spectroscopy, we claim that the detected burst is a photospheric radius expansion (PRE) burst," the researchers concluded.

The collected data allowed the team to estimate the Eddington luminosity of GX 3+1, which was found to be at a level of 287 undecillion erg/s. The study also put more constraints on the distance to this system, finding that it is located most likely some 30,300 light years away.

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