Potential Detection of Gravitational Redshift Signature in Neutron Star GRS 1747-312
Researchers analyzing AstroSat X-ray data from neutron star low-mass X-ray binary GRS 1747-312 report a potential emission line at ~4.2 keV, which they interpret as an iron K-alpha line gravitationally redshifted from 6.4 keV at the neutron star surface. The observed redshift of z~0.6 is consistent with theoretical predictions for a non-spinning 1.4 solar-mass neutron star with a 10 km radius. If confirmed, the detection would provide a direct measurement of gravitational redshift and place meaningful constraints on the neutron star equation of state.
A team of astronomers has conducted a broadband spectral analysis of the neutron star low-mass X-ray binary (LMXB) GRS 1747-312 using approximately 40 kiloseconds of AstroSat observational data collected during the decay phase of the source's 2017 outburst. The analysis reveals a potential emission line centered at 4.19 keV with a line flux of 13×10⁻⁵ erg s⁻¹ cm⁻², which the authors propose originates from reflection off the neutron star surface. The line's energy corresponds to a gravitational redshift of z~0.6 applied to the neutral iron K-alpha line at 6.4 keV, a value consistent with what is expected from a canonical 1.4 solar-mass, 10 km radius non-spinning neutron star. The continuum spectrum was modeled using thermal Comptonization of blackbody emission, with a disc reflection component accounting for a broader iron line at ~6.4 keV and a hot plasma model (APEC) describing narrow lines below 2 keV. Archival Swift observations at higher source flux levels yielded only upper limits to the line flux of less than 30×10⁻⁵ erg s⁻¹ cm⁻², consistent with but not confirming the AstroSat detection. The paper has been accepted for publication in the Journal of High Energy Astrophysics, though the authors themselves characterize the detection as tentative and note that confirmation would enable strong constraints on the neutron star mass-to-radius ratio and the equation of state of dense nuclear matter.
What's missing
The authors do not discuss alternative physical interpretations for a line at this energy beyond gravitational redshift of iron K-alpha, nor do they address potential instrumental calibration uncertainties in AstroSat's SXT/LAXPC detectors at this energy.
What different sources said
- arXiv astro-phCenter
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