Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750−27

Abstract

The Be/X-ray transient GRO J1750−27 exhibited a type-II (giant) outburst in 2015. After the source transited to quiescence, we triggered our multi-year Chandra monitoring programme to study its quiescent behaviour. The programme was designed to follow the cooling of a potentially heated neutron-star crust due to accretion of matter during the preceding outburst, similar to what we potentially have observed before in two other Be/X-ray transients, namely 4U 0115+63 and V 0332+53. However, unlike for these other two systems, we do not find any strong evidence that the neutron-star crust in GRO J1750−27 was indeed heated during the accretion phase. We detected the source at a rather low X-ray luminosity (∼1033 erg s−1) during only three of our five observations. When the source was not detected it had very low-luminosity upper limits (< 1032 erg s−1; depending on assumed spectral model). We interpret these detections and the variability observed as emission likely due to very low-level accretion onto the neutron star. We also discuss why the neutron-star crust in GRO J1750−27 might not have been heated while the ones in 4U 0115+63 and V 0332+53 possibly were.