The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608-52

TL;DR

This study investigates how accretion states affect spectral measurements of neutron star properties in 4U 1608-52, revealing state-dependent biases that impact radius and mass estimates from X-ray burst data.

Contribution

It demonstrates the influence of accretion states on burst spectral properties and provides constraints on neutron star radius and distance, highlighting the importance of accretion context in measurements.

Findings

Hard-state bursts align with NS atmosphere models.

Soft-state bursts show inconsistent spectral normalization.

Lower limit on neutron star radius is 13 km for certain masses.

Abstract

Spectral measurements of thermonuclear (type-I) X-ray bursts from low mass X-ray binaries have been used to measure neutron star (NS) masses and radii. A number of systematic issues affect such measurements and have raised concerns as to the robustness of the methods. We present analysis of the X-ray emission from bursts observed from 4U 1608-52 at various persistent fluxes. We find a strong dependence of the burst properties on the flux and spectral hardness of the persistent emission before burst. Bursts occurring during the low-accretion rate (hard) state exhibit evolution of the black body normalisation consistent with the theoretical predictions of NS atmosphere models. However, bursts occurring during the high-accretion rate (soft) state show roughly constant normalisation, which is inconsistent with the NS atmosphere models and therefore these bursts cannot be easily used to determine NS parameters. We analyse the hard-state burst to put the lower limit on the neutron star radius in 4U 1608-52 of 13 km (for masses 1.2-2.4 M$_\odot$). The best agreement with the theoretical NS mass-radius relations is achieved for source distances in the range 3.1-3.7 kpc. We expect that the radius limit will be 10 per cent lower if spectral models including rapid rotation are used instead.