The neutron star population in M28: a joint Chandra/GBT look at pulsar paradise

TL;DR

This study combines deep X-ray and radio observations to analyze the neutron star population in globular cluster M28, revealing new pulsar behaviors, spectral properties, and variable X-ray sources, advancing understanding of neutron star characteristics in dense stellar environments.

Contribution

The paper provides the first simultaneous X-ray and radio detection of a redback pulsar in M28, and constrains neutron star radii using atmosphere models, offering new insights into neutron star properties and binary interactions.

Findings

Detection of simultaneous X-ray and radio signals from PSR J1824-2452I.

Discovery of double-peaked X-ray orbital flux modulation in the pulsar.

Constraints on neutron star radius using atmosphere models.

Abstract

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 dataset 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 (Lx), spectrum, and orbital modulation of the 7 known compact binary millisecond pulsars (MSPs) in the cluster. We report two simultaneous detections of the redback PSR J1824-2452I (M28I) and its X-ray counterpart. 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 neutron star low-mass X-ray binary to constrain its mass and radius. Using both hydrogen and helium NS atmosphere models, we find a NS radius of R = 9.5-11.5 km and R = 13.5 - 16.7 km, respectively, for a neutron star mass of 1.4 Msun. 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.