The Neutron Star Population in M28: A Joint Chandra/GBT Look at Pulsar Paradise

We present the results of a deep study of the neutron star (NS) population in the globular cluster M28 (NGC 6626), using the full 330 ks 2002-2015 ACIS data set from the Chandra X-ray Observatory and coordinated radio observations taken with the Green Bank Telescope (GBT) in 2015. We investigate the X-ray luminosity (L _X ), spectrum, and orbital modulation of the seven known compact binary millisecond pulsars in the cluster. We report two simultaneous detections of the redback PSR J1824-2452I (M28I) and its X-ray counterpart at L _X = [8.3 ± 0.9] × 10³¹ erg s^-1. We discover a double-peaked X-ray orbital flux modulation in M28I during its pulsar state, centered around pulsar inferior conjunction. We analyze the spectrum of the quiescent NS low-mass X-ray binary to constrain its mass and radius. Using both hydrogen and helium NS atmosphere models, we find an NS radius of R = 9.2-11.5 km and R = 13.0-17.5 km, respectively, for an NS mass of 1.4 M _⊙ (68% confidence ranges). We also search for long-term variability in the 46 brightest X-ray sources and report the discovery of six new variable low-luminosity X-ray sources in M28.