Chandra X-ray Observatory Observations of the Globular Cluster M28 and Its Millisecond Pulsar Psr B1821−24

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 one 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 6×1030 erg s−1. We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius of rc,x ≈ 11. We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821−24 and find it is best described by a power law with photon index Γ ≃ 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 the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5–8.0 keV band is ≈ 3.5×10−13 ergs s−1 cm−2. We present spectral analyses of the 5 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 = 90 +30 −10 eV and the radius R ∞ NS = 14.5 +6.9 −3.8 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. Subject headings: globular clusters:general — globular clusters:individual (M28) — stars:neutron — x-ray:stars — binaries:general — pulsars:general — pulsars:individual (PSR B1821−24)