NuSTAR Reveals New Insights into Mysterious X-ray Binary SXP 138

[frankerkanol]

Indian astronomers using NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) have conducted a detailed study of the X-ray binary system SXP 138. Their findings, published on March 26 via the arXiv pre-print server, shed new light on the dynamic behavior and characteristics of this intriguing system.

X-ray binaries are stellar systems where a normal star or white dwarf transfers mass to a compact object such as a neutron star or black hole. These systems are categorized based on the mass of the donor star into low-mass (LMXBs) and high-mass (HMXBs) X-ray binaries. The Be/X-ray binaries (Be/XRBs), which involve a Be star and typically a neutron star, represent the largest subgroup within HMXBs. These systems are known for their weak, persistent X-ray emissions punctuated by outbursts lasting several weeks.

SXP 138, located in the Small Magellanic Cloud, is a Be/XRB system that hosts a pulsar spinning every 138 seconds. It has an orbital period of about 125 days and shows a superorbital period of roughly 1,000 days, likely driven by random changes in its accretion disk.

Led by Soham Pravin Sanyashiv of the Indian Institute of Science Education and Research Kolkata, the research team used NuSTAR’s FPMA and FPMB telescopes to examine the system’s periodic behavior and spectral properties in greater detail.

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Their analysis revealed that the pulsar's spin period increased from 140.69 to 140.85 seconds between August 2016 and August 2017, suggesting the system is in a “propeller regime.” In this state, the magnetospheric radius is larger than the corotation radius, effectively halting accretion onto the neutron star's surface.

The pulse profile of SXP 138 shows a complex pattern, typically featuring two dominant peaks with positive intensity and two secondary peaks with negative intensity. These features likely result from pencil-beam emission produced by asymmetric hotspots and more intricate radiative processes.

Spectral modeling indicated that both blackbody and power-law components are necessary to describe the emission from SXP 138. As accretion intensifies, the blackbody temperature rises and the power-law index decreases, possibly due to inner disk heating and the formation of an accretion column.

The team emphasizes the importance of continued monitoring of SXP 138 and similar X-ray binaries to track long-term spin behavior and study spectral transitions. Such efforts could deepen our understanding of the accretion mechanisms operating in these exotic stellar systems.