GUWAHATI, : In a collaborative effort, Indian Institute of Technology Guwahati and Indian Space Research Organisation researchers have made a groundbreaking discovery while studying the first known Galactic Ultraluminous X-ray emitting pulsar, Swift J0243.6+6124. Their research shows that the polarization of X-rays emitted by this pulsar is significantly lower than expected, challenging existing theories of emitted radiations from these astronomical bodies.
Neutron stars are formed when a massive star exhausts its fuel and collapses under its own gravity, compressing its core and transforming protons and electrons into neutrons. If the core's mass is between one and three times that of the sun, the collapse halts, creating a neutron star. These incredibly dense objects are the most compact known, with a mass like the sun but condensed to the size of a city. Many neutron stars are observed as pulsars, emitting regular pulses of radiation as they spin. X-ray pulsars are a specific type of neutron star in binary systems, where a companion star orbits a highly magnetized neutron star, emitting pulses of X-rays.
Ultraluminous X-ray sources (ULXs) are bright X-ray emitters from nearby galaxies that were once thought to be intermediate-mass black holes. However, some are now believed to be pulsars because they emit regular pulses. Swift J0243.6+6124, detected by NASA's Swift spacecraft during a strong X-ray outburst in 2017–2018, was identified as the first Galactic ULX pulsar due to its exceptional X-ray brightness.
Researchers worldwide have been investigating the nature of polarized X-ray emissions from Swift J0243.6+6124. The ISRO and IITG scientists focused on this source during its active period in 2023, using NASA's Imaging X-ray Polarimetry Explorer (IXPE) to detect the polarized X-rays emission for the first time. They also combined data from the Neutron Star Interior Composition ExploreR (NICER) and the Nuclear Spectroscopic Telescope Array (NuSTAR) missions. This approach enabled them to gain a deeper understanding of the energy-dependent characteristics of the X-ray pulses from Swift J0243.6+6124.
The scientists found that the polarization of X-rays from Swift J0243.6+6124 was much lower than expected, around 3%.
Highlighting the impact of this research findings, Dr. Anuj Nandi added, "The IXPE mission's unique capabilities made it possible to detect low polarization in X-rays from the first known Galactic ULXPs. Notably, this low polarization appears to vary with the emitted pulses."
Speaking about the research, Prof. Santabrata Das, Department of Physics, IIT Guwahati, said, "The discovery of lower polarization in the X-rays from Swift J0243.6+6124 is important because it makes us rethink how these stars work. Neutron stars in binary systems have very strong magnetic fields that direct matter from a nearby star to their poles. This process affects the X-rays we see because the magnetic field influences how the X-rays behave. The polarization of X-rays plays a big role in this. The unexpected low polarization means our current understanding of these magnetic fields and X-rays needs to be updated."
This surprising result challenges current theories and raises new questions for further exploration. It also opens new opportunities for studying similar X-ray sources within our galaxy and beyond.
The details of this research have been published in The Astrophysical Journal Letters, co-authored by Dr. Santabrata Das, Dr. Anuj Nandi, and researchers Mr. Seshadri Majumder, Ms. Rwitika Chatterjee, and Mr. Kiran M. Jayasurya. The paper may be accessed at https://doi.org/10.3847/2041-8213/ad67e5.
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Indian Institute of Technology Guwahati, established in 1994, completed 25 years of glorious existence in 2019. At present, the Institute has eleven departments, nine interdisciplinary academic centres and five schools covering all the major engineering, science, healthcare, management, and humanities disciplines, offering BTech, BDes, BSc(Hons), MA, MDes, MTech, MSc, MS(R), MBA and PhD programmes. The institute offers a residential campus to 439 faculty members and more than 8,600 students at present.
IIT Guwahati has retained the 7th position among the best engineering institutions of the country and 9th position in both ‘Overall’ and ‘Research’ Categories in the ‘India Rankings 2023’ declared by the National Institutional Ranking Framework (NIRF) of the Union Ministry of Education.
IIT Guwahati has achieved rank 87 in the SDG 9 (Industry, Innovation, and Infrastructure) globally in the Times Higher Education (THE) Impact Rankings 2024 and is the only academic institution in India that occupied a place among the top 100 world universities – under 50 years of age – ranked by London-based Times Higher Education (THE) in 2014 and continues to maintain its superior position even today in various International Rankings. IIT Guwahati gained rank 32 globally in the ‘Research Citations per Faculty’ category and overall 344 rank in the QS World University Rankings 2025.
Among other frontier areas of research and innovation, IIT Guwahati is working towards augmenting critical science research initiatives in Genomics, Developmental Biology, Health Care and Bioinformatics and Flexible Electronics, among numerous other areas.
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