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Chinese astronomers investigated the spectral and temporal properties of a black hole X-ray binary system known as SLX 1746–331, during its recent outburst. Results of the study, in The Astrophysical Journal Letters, could help us better understand the nature of this system and its behavior.

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 (LMXBs) and high-mass X-ray binaries (HMXBs).

SLX 1746–331 is a transient black hole LMXB at a distance of about 35,200 light years, discovered in 1985. After a 13-year-long period of quiescence, it entered an outburst in 2023, which peaked on March 14.

The recent outburst of SLX 1746–331 was a good occasion for a team of astronomers, led by Jing-Qiang Peng of the Chinese Academy of Sciences (CAS), to perform a detailed study of this source. For this purpose, they employed China's Insight-HXMT satellite, as well as NASA's Neutron star Interior Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR).

"We study the spectral and temporal properties of the black hole X-ray transient binary SLX 1746–331 during the 2023 outburst with NICER, NuSTAR, and Insight-HXMT observations," the paper reads.

The observations found that during the 2023 outburst, SLX 1746–331 remained mostly in a soft state with no state transition observed, even at a luminosity 500 times lower than the peak luminosity of the outburst. However, a significant reflection component in three NuSTAR observations has been observed, where the disk is most likely illuminated by itself in the inner part.

In general, it turned out that the entire 2023 outburst was dominated by the disk emissions, and the thermal spectrum showcases a correlation of the disk unabsorbed flux with the disk inner temperature, until a luminosity of more than two magnitudes is lower than the maximum of the outburst.

The collected data indicate that the 2023 outburst of SLX 1746–331 differs from those of normal black hole transient X-ray binaries, given that instead of a low-hard state toward a soft state, the source stayed around the innermost stable circular orbit (ISCO) during the whole outburst.

The observations also found that the spin and inclination of SLX 1746-331 are approximately 0.85 and 53 degrees, respectively. The mass of the compact object was measured to be about 5.5 solar masses, while the inner disk radius was estimated to be 5.8–11.7 km.

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