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

We report here the results of the first Chandra X-Ray Observatoryobservations of the globular cluster M28 (NGC 6626). 46 X-ray sources aredetected, of which 12 lie within one core radius of the center. We show thatthe apparently extended X-ray core emission seen with the ROSAT HRI is due tothe superposition of multiple discrete sources for which we determine the X-rayluminosity function down to a limit of about 6xE30 erg/s. For the first timethe unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 ismeasured and found to be best described by a power law with photon index ~ 1.2.Marginal evidence of an emission line centered at 3.3 keV in the pulsarspectrum is found, which could be interpreted as cyclotron emission from acorona above the pulsar's polar cap if the the magnetic field is stronglydifferent from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightestunidentified sources is presented. Based on the spectral parameters of thebrightest of these sources, we suggest that it is a transiently accretingneutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrumwith a hydrogen neutron star atmosphere model yields the effective temperatureT_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty =14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter,unresolved X-ray emission from the central core of M28. Using theChandra-derived positions, we also report on the result of searching archivalHubble Space Telescope data for possible optical counterparts.Comment: Accepted for publication in ApJ; 22 pages, 8 figures, 5 table