GRO J1744-28 is among the most puzzling neutron star X-ray binaries known in our Galaxy. How could the companion star have possibly reduced to such its current small size without spinning the neutron star up to very high rotation rate or without strongly reducing its magnetic field? High-resolution Chandra X-ray observations may have solved this puzzle.

After remaining dormant for nearly 20 years, GRO J1744-28 suddenly exhibited a new accretion outburst in 2014. This provided the excellent opportunity to study this extraordinary X-ray binary with modern X-ray telescopes. Using high-resolution Chandra Grating Spectroscopic observations, we found evidence for X-ray absorption lines that could arise from a dense wind blowing off the accretion disk. Such winds are commonly seen for black hole X-ray binaries, although it is not established yet whether similarly strong winds can also form in the accretion disks that surround neutron stars.

If the absorption features in GRO J1744-28 indeed origin in a disk wind, then energetic considerations suggest that this wind may carry away a considerable amount of matter that is being transferred from the companion star. Therefore, a considerable amount of matter may be lost from the binary without accreting onto the neutron star. This could potentially solve the long-standing puzzle of how to reconcile the very low mass of the companion star in GRO J1744-28 with the slow rotation period and high magnetic field of the neutron star primary.

Degenaar, Miller, Harrison, Kennea, Kouveliotou, Younes 2014, ApJ Letters 796, L9: High-resolution X-Ray Spectroscopy of the Bursting Pulsar GRO J1744-28

Paper link: ADS

Artist impression of an X-ray binary with a strong disk wind.
Credit: NASA’s Goddard Space Flight Center / Chandra X-ray observatory / M. Weiss