Speaker
Description
We report the detection of a mysterious absorption feature at approximately 3.8 keV in the Neutron star Interior Composition Explorer (NICER) spectrum of the ultracompact low-mass X-ray binary 4U 1820–30. We interpret this feature as a gravitationally redshifted iron absorption line produced in the vicinity of the neutron-star surface. This scenario is supported by the temporal proximity of the NICER observation to the detection, by the MAXI X-ray monitor, of a carbon superburst from the same source, suggesting that the line may be associated with the extreme physical conditions triggered by this rare and energetic thermonuclear event.
We model the feature with a photoionized absorption component and measure a redshift of (1+z \simeq 1.72). If the line originates close to the neutron-star surface, this redshift directly constrains the stellar compactness, yielding (R/M = 4.46 \pm 0.13) km (M_{\odot}^{-1}), corresponding to (R/M = 3.02 \pm 0.09) in dimensionless units. Such a high redshift provides a direct probe of the strong-gravity regime around the neutron star and offers valuable constraints on its mass-radius relation.
This unique feature highlights the diagnostic power of high-quality X-ray spectroscopy in probing the equation of state of ultra-dense matter. Future observations of superbursts and detailed radiative-transfer modelling will be crucial to confirm the origin of the line and to establish gravitationally redshifted absorption features as robust tools for neutron-star compactness measurements.