Chandra X‐Ray Observatory Observations of the Globular Cluster M28 and Its Millisecond Pulsar PSR B1821−24

Abstract

We report here the results of the first Chandra X- Ray Observatory observations of the globular cluster M28 ( NGC 6626). We detect 46 X- ray sources, of which 12 lie within 1 core radius of the center. We show that the apparently extended X- ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources, for which we determine the X- ray luminosity function down to a limit of about 6 x 1030 ergs s(-1). We measure the radial distribution of the X- ray sources and fit it to a King profile finding a core radius of r(c), x approximate to 1 1". We measure for the first time the unconfused phase- averaged X- ray spectrum of the 3.05 ms pulsar B1821 - 24 and find that it is best described by a power law with photon index Gamma similar or equal to 1.2. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar's polar cap if the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5 - 8.0 keV band is approximate to 3. 5 x 10(-13) ergs s(-1) cm(-2). We present spectral analyses of the five brightest unidentified sources. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low- mass X- ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T-eff(infinity) = 90(-10)(+30) eV and the radius R-NS(infinity) = 14.5(-3.8)(+6.9) km. In addition to the resolved sources, we detect fainter, unresolved X- ray emission from the central core. Using the Chandra- derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.