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Using NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), Indian astronomers have observed an X-ray binary system designated SXP 138. Results of the NuSTAR observations, March 26 on thearXiv pre-print server, yield important information regarding the behavior of this system.

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).

Be/X-ray binaries (Be/XRBs) are the largest subgroup of HMXBs. These systems consist of Be stars and, usually, neutron stars, including pulsars. Observations have found that most of these systems showcase weak persistent X-ray emission that is interrupted by outbursts lasting several weeks.

SXP 138 is a Be/XRB in the Small Magellanic Cloud (SMC) containing a pulsar with a spin period of 138 seconds. The orbital period of the system was found to be approximately 125 days. Moreover, previous observations have found that SXP 138 also displays a superorbital period of about 1,000 days, which could be caused by stochastic changes in the accretion disk.

A team of astronomers led by Soham Pravin Sanyashiv of the Indian Institute of Science Education and Research Kolkata, took a closer look at SXP 138 and its periodicity using NuSTAR hard X-ray telescopes—FPMA and FPMB.

The observations allowed Sanyashiv's team to get more insights into the spin evolution, the nature of the pulse profile, and the spectral characteristics of SXP 138.

By analyzing the light curve of SXP 138, it turned out that the pulsar spin period increased from 140.69 to 140.85 seconds between August 2016 and August 2017. This suggests that the source is in the propeller regime in which the magnetospheric radius exceeds the corotation radius, preventing efficient accretion onto the neutron star surface.

The pulse profile of SXP 138 exhibits a complex structure. In most of the observations, two high peaks with positive and two secondary peaks with negative normalized intensity are distinguished.

The astronomers explain that the high peaks point to pencil beam emission from asymmetric hotspots, while the two secondary peaks indicate a complex radiative transfer.

The spectral analysis of SXP 138 found that it requires both blackbody and power-law components.

"With enhanced accretion, the blackbody temperature rises and the power-law index decreases, likely due to inner disk heating and accretion column formation," the authors of the paper conclude.

The scientists add that future observations of SXP 138 and similar binaries should be focused on tracking long-term spin variations and investigating spectral state transitions. This could advance our knowledge of the accretion processes in such systems.

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