Guwahati: Researchers of the Indian Institute of Technology Guwahati (IITG) and Indian Space Research Organisation (ISRO) have made a groundbreaking discovery while studying the first known Galactic Ultraluminous X-ray (ULX) emitting pulsar – ‘Swift J0243.6+6124.’
Their research shows that the polarisation of X-rays emitted by this pulsar is significantly lower than expected, challenging existing theories of emitted radiations from these astronomical bodies.
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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 magnetised neutron star, emitting pulses of X-rays.
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ULX sources 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 polarised 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 polarised 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 polarisation of X-rays from ‘Swift J0243.6+6124’ was much lower than expected, around three per cent.
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Speaking about the research, Prof Santabrata Das of IITG’s Department of Physics said,“The discovery of lower polarisation 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 polarisation of X-rays plays a big role in this. The unexpected low polarisation means our current understanding of these magnetic fields and X-rays needs to be updated,” Prof Das added.
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.