A Stellar Investigation: The Power of NuSTAR

In an exciting breakthrough, a team of Indian astronomers has harnessed the power of NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) to delve into the enigmatic X-ray binary system known as SXP 138. Their findings, recently shared on the arXiv pre-print server, are set to transform our understanding of these fascinating cosmic phenomena.

What Are X-Ray Binaries?

X-ray binaries are unique celestial systems where a normal star or white dwarf feeds mass into a compact neutron star or black hole. Astronomers classify these binaries into low-mass (LMXBs) and high-mass (HMXBs) categories, depending on the mass of the companion star. Among the HMXBs, Be/X-ray binaries (Be/XRBs) stand out, typically featuring Be stars alongside neutron stars—often pulsars.

Spotlight on SXP 138: A Cosmic Mystery in the Small Magellanic Cloud

Located in the Small Magellanic Cloud (SMC), SXP 138 is a notable Be/XRB that hosts a pulsar with a rapid spin period of 138 seconds. The system exhibits an orbital period close to 125 days and intriguingly showcases a superorbital period of about 1,000 days, likely influenced by unpredictable changes in its accretion disk.

Diving Deeper: Insights from NuSTAR Observations

Under the leadership of Soham Pravin Sanyashiv from the Indian Institute of Science Education and Research Kolkata, the astronomers conducted a thorough investigation into SXP 138's behavior using NuSTAR’s sophisticated hard X-ray telescopes—FPMA and FPMB. This focused analysis yielded vital insights into the pulsar's spin evolution, pulse profile, and spectral characteristics.

Revealing the Pulsar's Secrets: Spin Changes Uncovered!

Their light curve analysis revealed a captivating detail: between August 2016 and August 2017, the pulsar's spin period increased from 140.69 to 140.85 seconds. This shift indicates that SXP 138 occupies a 'propeller regime' where the magnetosphere's radius surpasses the corotation radius, thereby inhibiting efficient mass accretion onto the neutron star.

A Complex Pulse Profile: What Do the Peaks Mean?

The pulse profile of SXP 138 presents an intriguing complexity, characterized by two prominent peaks of positive intensity and two smaller peaks of negative intensity. The astronomers suggest that the high peaks are the result of pencil beam emissions from asymmetric hotspots, while the secondary peaks reveal a sophisticated radiative transfer process.

Spectral Analysis: An Intriguing Composition Emerges!

Spectral investigations have shown that SXP 138’s emissions comprise both blackbody and power-law components. The researchers highlight that as accretion intensifies, the blackbody temperature increases while the power-law index declines, possibly due to the heating of the inner disk and the formation of an accretion column.

Looking Ahead: Future Observations Could Illuminate More!

The scientists emphasize the importance of future observations of SXP 138 and similar binaries. By tracking long-term spin variations and exploring spectral state transitions, we could unlock even more mysteries about the intricate processes governing accretion in these extraordinary systems.