A Mysterious Feature in the NICER Spectrum of 4U 1820-30: A Gravitationally Redshifted Absorption Line?

A mysterious absorption feature at approximately 3.8 keV has been identified in the Neutron star Interior Composition Explorer (NICER) spectrum of the low-mass X-ray binary system 4U 1820-30. We interpret this feature as a gravitationally redshifted iron absorption line. This interpretation is supported by the temporal proximity of the NICER observation to the detection of a carbon superburst—a long and intense thermonuclear flash on the neutron star’s surface—by the X-ray monitor MAXI, suggesting that the presence of the line is associated with this rare and extreme event. From the inferred redshift of the absorption line, the compactness of the neutron star can be derived. Fitting this feature with a photoionization absorption model, we measure a redshift of 1 + z ≃ 1.72, which implies a neutron star compactness of R/M = 4.46 ± 0.13 km/M⊙ or 3.02 ± 0.09 in dimensionless units. This unique feature highlights the importance of further observations and detailed modelling, offering promising insights into the equation of state of matter under extreme density conditions.