A mysterious feature in the NICER spectrum of 4U 1820-30: A gravitationally redshifted absorption line?

TL;DR

This paper reports the detection of a gravitationally redshifted iron absorption line in the NICER spectrum of 4U 1820-30, providing insights into neutron star compactness and the equation of state of dense matter.

Contribution

It presents the first identification of a gravitationally redshifted absorption line in this system, linking spectral features to neutron star properties and extreme density conditions.

Findings

Measured a gravitational redshift of about 1.72.

Derived a neutron star compactness of R/M ≈ 4.46 km per solar mass.

Highlighted the importance of further observations for dense matter physics.

Abstract

A mysterious absorption feature at approximately 3.8 keV has been identified in the 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 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. Using a photoionization absorption model, we measure a gravitational redshift of about 1.72, which corresponds to a compactness R/M of 4.46 \pm 0.13 km per solar mass, or 3.02 \pm 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.