ISDC Data Centre for Astrophysics

Measuring neutron star masses by gravitational deflection

Neutron stars are fundamental to probe the physics of matter at the highest density, where even atom nuclei are broken into quarks. New stellar systems, discovered by INTEGRAL, including a neutron star orbiting a massive companion, allow to weight the neutron star in an original manner, by measuring the gravitational deflection of the wind stream generated by the companion star. The gas stream is so dense that it completely absorbs the emission of the neutron star, even X-rays. Only gamma-rays telescopes, such as INTEGRAL, can detect them.

Manousakis, Walter and Blondin, 2012, A&A 547, 20 | Press Release

ABSTRACT

A population of obscured supergiant High Mass X-ray Binaries (sgHMXBs) has been discovered by INTEGRAL. X-ray wind tomography of IGR J17252-3616 inferred a slow wind velocity to account for the enhanced obscuration. The main goal of this study is to understand under which conditions high obscuration could occur. We have used an hydrodynamical code to simulate the flow of the stellar wind around the neutron star. A grid of simulations was used to study the dependency of the absorbing column density and of the X-ray light-curves on the model parameters. A comparison between the simulation results and the observations of IGR J17252-3616 provides an estimate on these parameters. We have constrained the wind terminal velocity to 500-600 km/s and the neutron star mass to 1.75 - 2.15 Msun. We have confirmed that the initial hypothesis of a slow wind velocity with a moderate mass loss rate is valid. The mass of the neutron star can be constrained by studying its impact on the accretion flow.

The stream of stellar wind is bended by the gravity of the neutron star

This movie shows the density of the stellar wind emitted by the companion star (visible on the bottom) and the formation of a high density stream of gas around the neutron star. The stream is opaque to X-rays, its effect can be observed measuring the absorption of X-rays along the orbit of the neutron star around its companion.
Credits: ISDC/A. Manousakis

Comparison of observational results and hydrodynamic simulations
The figure shows the X-ray absorbing column density as a function o orbital phase simulated for a range of neutron star masses. The absorbing column density can be measured with an X-ray satellite. The points have been obtained for a specific source discovered by INTEGRAL. The match between the observations and the predictions of the simulations allow to constrain the mass of the neutron star.
Credits: ISDC/A. Manousakis