DIFFUSE X-RAY EMISSION IN GLOBULAR CLUSTER CORES

Journal Article · Sat Aug 01 04:00:00 EDT 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/700/2/1233; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)· OSTI ID:21313785

Hui, C Y ^[1]; Cheng, K S ^[2]; Taam, Ronald E ^[3]

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1. Max-Planck Institut fuer Extraterrestrische Physik, 85741 Garching bei Muenchen (Germany)
2. Department of Physics, University of Hong Kong, Pokfulam Road (Hong Kong)
3. Department of Physics and Astronomy, Northwestern University, 2131 Tech Drive, Evanston, IL 60208 (United States)

The unresolved X-ray emission in the cores of 10 globular clusters hosting millisecond pulsars is investigated. Subtraction of the known resolved point sources leads to detectable levels of unresolved emission in the core region of M28, NGC 6440, M62, and NGC 6752. The X-ray luminosities in the 0.3-8 keV energy band of this emission component were found to lie in the range {approx}1.5 x 10{sup 31}erg s{sup -1} (NGC 6752) to {approx}2.2 x 10{sup 32} erg s{sup -1} (M28). The lowest limiting luminosity for X-ray source detections amongst these four clusters was 1.1 x 10{sup 30} erg s{sup -1} for NGC 6752. The spectrum of the unresolved emission can be fit equally well by a power law, a thermal bremsstrahlung model, a blackbody plus power law, or a thermal bremsstrahlung model plus blackbody component. The unresolved emission is considered to arise from the cumulative contribution of active binaries, cataclysmic variables, and faint millisecond pulsars with their associated pulsar wind nebulae. In examining the available X-ray data, no evidence for any pulsar wind nebular emission in globular clusters is found. It is shown that the X-ray luminosity contribution of a faint source population based on an extrapolation of the luminosity function of detected point sources is compatible with the unresolved X-ray emission in the cores of NGC 6440 and NGC 6752. Adopting the same slope for the luminosity function for M62 as for NGC 6440 and NGC 6752 leads to a similar result for M62. For M28, the contribution from faint sources in the core can attain a level comparable with the observed value if a steeper slope is adopted. The characteristics on the faint source population as constrained by the properties of the unresolved X-ray emission are briefly discussed.

• OSTI ID: 21313785
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 700; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

99 GENERAL AND MISCELLANEOUS
BREMSSTRAHLUNG
EMISSION
EXTRAPOLATION
KEV RANGE 01-10
LUMINOSITY
NEBULAE
NEUTRONS
POINT SOURCES
PULSARS
SPECTRA
STARS
X-RAY SOURCES