IIT Guwahati & ISRO Studies Uncover Unexpectedly Low Polarization in Galactic Ultraluminous X-ray Pulsar, Challenging Existing Theories

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.