A team of scientists from the Indian Institute of Technology Kanpur, Inter-University Centre for Astronomy and Astrophysics (IUCAA) Pune, and Ashoka University has made a significant breakthrough using data from India's first astronomical observatory, AstroSat. Their research, published in Astrophysical Journal, reveals new insights into the internal structure of neutron stars, some of the densest objects in the universe.

Neutron stars are formed from the collapsed cores of massive stars and pack more than the Sun's mass into a sphere just 10 kilometers across. This extreme density creates a powerful gravitational field and leads to a complex, poorly understood, equation of state, the relationship between pressure and density within the star.

The research team utilized data from AstroSat's LAXPC instrument, a domestically developed marvel of Indian engineering, to study X-rays emitted by the binary star system 4U 1728-34. This system comprises a neutron star accreting matter from a companion star.
The authors analyzed the X-ray data from the binary star system 4U 1728-34 and found several cases in which QPO triplets were observed. They discovered that the frequencies of these QPO triplets do not remain fixed; rather, they evolve continuously with time, maintaining a specific relationship with each other. Using this relationship, they found that the observed QPOs are best interpreted in terms of the three oscillations predicted by Einstein's general theory of relativity (GTR), i.e., the orbital motion, the precession of the perihelion, and the Lense-Thirring precession. 

Furthermore, they found that the observed relationship has a sensitive dependence on the mass, moment of inertia, and the equation of state of the neutron star. Hence, it can be used to probe these parameters in greater detail, which has not been possible in the past.

The research team includes Kewal Anand (Ph.D. Scholar, IIT Kanpur), Ranjeev Misra (Senior Professor, IUCAA), J. S. Yadav (Visiting Professor, IIT Kanpur; Retired Professor, TIFR Mumbai; Ex-PI LAXPC onboard AstroSat), Pankaj Jain (Professor, IIT Kanpur; Head of the SPASE Department, IIT Kanpur), Umang Kumar (Ph.D. Scholar, Ashoka University), and Dipankar Bhattacharya (Professor and Head of the Physics Department, Ashoka University; Ex-Senior Professor, IUCAA).

Pankaj Jain, Head of the SPASE Department at IIT Kanpur, commented on the findings: "This discovery advances our understanding of neutron stars and also opens new avenues for exploring the fundamental principles of physics in extreme environments. The insights gained from this study will have a lasting impact on astrophysics and related fields."
The study paves the way for a deeper understanding of these fascinating celestial objects and the fundamental laws governing the universe.

Please find link to the published paper here: https://doi.org/10.3847/1538-4357/ad410c