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

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
e-mail: A.roucoescorial@uva.nl
² Department of Physics and Astronomy, Wayne State University, 666 W. Hancock St, Detroit, MI 48201, USA
³ Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA 94602, USA
⁴ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 Utrecht, The Netherlands

Received: 26 September 2018
Accepted: 2 July 2019

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.