The Birthplace of Low-Mass X-ray Binaries: Field Versus Globular Cluster Populations

Recent Chandra studies of low-mass X-ray binaries (LMXBs) within early-type galaxies have found that LMXBs are commonly located within globular clusters of the galaxies. However, whether all LMXBs are formed within globular clusters has remained an open question. If all LMXBs formed within globular clusters, the summed X-ray luminosity of the LMXBs in a galaxy should be directly proportional to the number of globular clusters in the galaxy regardless of where the LMXBs currently reside. We have compared these two quantities over the same angular area for a sample of 12 elliptical and S0 galaxies observed with Chandra and found that the correlation between the two quantities is weaker than expected if all LMXBs formed within globular clusters. This indicates that a significant number of the LMXBs were formed in the field, and naturally accounts for the spread in field-to-cluster fractions of LMXBs from galaxy to galaxy. We also find that the "pollution" of globular cluster LMXBs into the field has been minimal within elliptical galaxies, but there is evidence that roughly half of the LMXBs originally in the globular clusters of S0 galaxies in our sample have escaped into the field. This is due to higher globular cluster disruption rates in S0s resulting from stronger gravitational shocks caused by the passage of globular clusters through the disks of S0 galaxies that are absent in elliptical galaxies.Comment: To appear in ApJ, 1 October 2005, v631 2 issue, 9 pages, 3 figures, typos and a few minor issues correcte

Hard X-ray Emission from the M87 AGN Detected with NuSTAR

M87 hosts a 3-6 billion solar mass black hole with a remarkable relativistic jet that has been regularly monitored in radio to TeV bands. However, hard X-ray emission \gtrsim 10keV, which would be expected to primarily come from the jet or the accretion flow, had never been detected from its unresolved X-ray core. We report NuSTAR detection up to 40 keV from the the central regions of M87. Together with simultaneous Chandra observations, we have constrained the dominant hard X-ray emission to be from its unresolved X-ray core, presumably in its quiescent state. The core spectrum is well fitted by a power law with photon index Gamma=2.11 (+0.15 -0.11). The measured flux density at 40 keV is consistent with a jet origin, although emission from the advection-dominated accretion flow cannot be completely ruled out. The detected hard X-ray emission is significantly lower than that predicted by synchrotron self-Compton models introduced to explain emission above a GeV.Comment: 5 pages, 4 figures, updated to better match the published version in the Astrophysical Journal Letters. A minor typo in the published version (angular scale should be 1 arcsec = 78 pc instead, no result of the paper is affected) is fixed her