An explanation for long flares from extragalactic globular cluster X-ray sources

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Abstract

Repeatedly flaring X-ray binaries have recently been discovered in NGC 4697 by Sivakoff and collaborators. We show that these flares can be explained as the result of eccentric binaries in globular clusters which accrete more rapidly at the periastron than during the rest of the binary orbit. We show that theoretical time-scales for producing eccentricities and circularizing the binaries are consistent with what is needed to produce the observed population of flaring sources, although the circularization time-scales are highly uncertain on both observational and theoretical grounds. This model makes two clear theoretical predictions: (i) the flares should be seen to be strictly periodic if adequate sampling is provided and that periodicity should be of approximately 15 h; and (ii) this class of flaring behaviour should be seen only in globular cluster sources and predominantly in the densest globular clusters. We also test the model for producing eccentricities through fly-bys of a third star near the binary in a globular cluster against a much larger data base of millisecond pulsar observations than has been used in past work and find that the theoretical cross-sections for producing eccentricity in binaries are in reasonable agreement with most of the data, provided that the pulsar ages are about 4 × 10 9 yr.

Original languageEnglish
Pages (from-to)971-976
Number of pages6
JournalMonthly Notices of the Royal Astronomical Society
Volume364
Issue number3
DOIs
StatePublished - Dec 11 2005

Keywords

  • Binaries: close
  • Galaxies: star clusters
  • Stellar dynamics
  • X-rays: binaries

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