New EGRET/COMPTEL studies of globular clusters.

Details New EGRET/COMPTEL studies of globular clusters. New EGRET/COMPTEL studies of globular clusters.

Dec 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26as..120c.255m&link_type=abstract

Astronomy and Astrophysics Supplement, v.120, p.255-260

Astronomy and Astrophysics

Astrophysics

4

Pulsars: General, Globular Clusters: Individual: Ngc 6397, Globular Clusters: Individual: Ngc 6752, Gamma Rays: Observations

Scientific paper

We present results of our Phase 2 program of observations of two nearby globular clusters, NGC 6397 and NGC 6752, with EGRET and COMPTEL to search for the integrated emission from their populations of millisecond pulsars, LMXBs (in quiescence), and CVs. Upper limits were derived for each cluster that are a factor of 2 more sensitive than in the general cluster survey of Michelson et al. (1994ApJ...435..218M) using EGRET. Our COMPTEL limits are the first reported for the 1-30MeV band for these clusters and are comparably sensitive to the EGRET limits for an assumed Crab-like spectrum yet can also constrain source spectra if they break at energies below 30MeV. These limits are better than the COMPTEL limits recently reported for 47 Tuc by O'Flaherty et al. (1995A&A...297L..29O). The lowest upper limits, for NGC 6397, restrict the integrated luminosity to be below ~10^35^ergs/s in either the COMPTEL or EGRET energy bands. A serendipitous source, GRO J1718-60, was detected (4.1σ) with EGRET, near (but not coincident with) NGC 6397. Although this source would be consistent with the nearby (1.7°) pulsar J1704-60, the published spindown luminosity is too low for it to be a plausible counterpart. Constraints on the typical gamma-ray luminosity for millisecond pulsars and quiescent LMXBs are derived and discussed.

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