The 0.7-10 keV energy spectrum of Swift J0243.6+6124 obtained from the NICER observation on MJD 58065 (ID 1050390115) near the peak of the X-ray burst. The second, third and Fourth panels from the top show the evolution of the residuals after continuum adjustment and subsequent addition of a 1 keV, 6–7 keV Fe line complex and an edge feature, respectively. Credit: Chhotaray et al., 2024.
Analyzing data from the Neutron Star Interior Composition Explorer (NICER), astronomers at the Physical Research Laboratory (PRL) in Ahmedabad, India and elsewhere have carried out a detailed study of the timing and spectrum X-rays from an ultraluminous X-ray pulsar designated Swift J0243.6+6124. Study results, presented January 26 on the preprint server arXivprovide important information on the behavior of this pulsar.
Ultraluminous X-ray (ULX) sources are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths. Although they are less luminous than active galactic nuclei, they are more constantly luminous than any known stellar process.
Astronomers generally think that because of their brightness, most ULXs are black holes. However, recent observations have shown that some ULXs exhibit coherent pulsations. These sources, known as ultraluminous X-ray pulsars (ULXPs), are neutron stars that are generally less massive than black holes. The list of known ULPs is still relatively short; thus, the study of objects in this class is essential for researchers exploring the universe with X-rays.
Swift J0243.6+6124 was discovered by NASA’s Swift spacecraft during its powerful X-ray burst in 2017-2018. It is considered the first galactic ULX due to its intense X-ray luminosity reaching up to the order of a duodecillion erg/s.
Previous studies of Swift J0243.6+6124 have shown that it hosts a neutron star with a pulsation period of 9.8 seconds and a companion (donor) star of spectral type O9.5Ve. The system has a relatively short orbital period of around 28 days.
A team of astronomers led by PRL’s Birenda Chhotaray conducted a long-term study of Swift J0243.6+6124, focusing primarily on its explosion activity between 2017 and 2023. Their timing and spectral analysis allowed them to to better understand the behavior of this ULXP. during different light phases.
NICER observations revealed a luminosity-dependent break in the power density spectrum of Swift J0243.6+6124, suggesting a change in accretion dynamics with mass accretion rate. Additionally, the data indicate quasi-periodic oscillations (QPOs) in a specific brightness range.
Between June and September 2023, Swift J0243.6+6124 entered an explosion phase during which the neutron star exhibited a rotating state and variations in its momentum profile. Spectral analysis detected two luminosity-dependent transitions at luminosities of approximately 75 and 210 undecillion erg/s in continuum parameters. These results highlight three distinct accretion modes during the giant explosion of 2017-2018.
The researchers also detected in the spectrum of Swift J0243.6+6124 a soft black body component with a temperature of 0.08 to 0.7 keV. It appears that this component underwent a discontinuous transition as the source evolved from a sub-Eddington state to a super-Eddington state.
More information:
Birendra Chhotaray et al, Long-term study of the first galactic ultraluminous X-ray source Swift J0243.6+6124 using NICER, arXiv (2024). DOI: 10.48550/arxiv.2401.15058
Journal information:
arXiv
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