High-Resolution X-Ray Spectroscopy of the Bursting Pulsar GRO J1744-28

TL;DR

This study uses high-resolution X-ray spectroscopy to analyze the accretion environment of the bursting pulsar GRO J1744-28, revealing a truncated disk, a strong magnetic field, and a fast disk wind, providing insights into neutron star accretion physics.

Contribution

First high-resolution spectral analysis of GRO J1744-28 revealing disk truncation, magnetic field strength, and disk wind characteristics.

Findings

Disk is truncated at ~85 GM/c^2.

Magnetic field estimated at (2-6)×10^10 G.

Detected fast disk wind with outflow velocity ~7500-8200 km/s.

Abstract

The bursting pulsar GRO J1744-28 is a Galactic low-mass X-ray binary that distinguishes itself by displaying type-II X-ray bursts: brief, bright flashes of X-ray emission that likely arise from spasmodic accretion. Combined with its coherent 2.1 Hz X-ray pulsations and relatively high estimated magnetic field, it is a particularly interesting source to study the physics of accretion flows around neutron stars. Here we report on Chandra/HETG observations obtained near the peak of its bright 2014 accretion outburst. Spectral analysis suggests the presence of a broad iron emission line centered at E_l ~ 6.7 keV. Fits with a disk reflection model yield an inclination angle of i ~ 52 degrees and an inner disk radius of R_in ~ 85 GM/c^2, which is much further out than typically found for neutron star low-mass X-ray binaries. Assuming that the disk is truncated at the magnetospheric radius of the neutron star, we estimate a magnetic field strength of B ~ (2-6)E10 G. Furthermore, we identify an absorption feature near ~6.85 keV could correspond to blue-shifted Fe xxv and point to a fast disk wind with an outflow velocity of v_out ~ (7.5-8.2)E3 km/s (~0.025c-0.027c). If the covering fraction and filling factor are large, this wind could be energetically important and perhaps account for the fact that the companion star lost significant mass while the magnetic field of the neutron star remained strong.