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Title:
Gamma-Ray Variability from Wind Clumping in High-Mass X-Ray Binaries with Jets
Authors:
Owocki, S. P.; Romero, G. E.; Townsend, R. H. D.; Araudo, A. T.
Affiliation:
AA(Bartol Research Institute, Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA ), AB(Inst. Argentino de Radioastronomía (CCT La Plata, CONICET), C.C.5, 1894 Villa Elisa, Buenos Aires, Argentina ; Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina ), AC(Bartol Research Institute, Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA ; Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling Hall, 475 N Charter Street, Madison, WI 53706, USA), AD(Inst. Argentino de Radioastronomía (CCT La Plata, CONICET), C.C.5, 1894 Villa Elisa, Buenos Aires, Argentina ; Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina )
Publication:
The Astrophysical Journal, Volume 696, Issue 1, pp. 690-693 (2009). (ApJ Homepage)
Publication Date:
05/2009
Origin:
IOP
ApJ Keywords:
binaries: general, gamma rays: theory, stars: winds, outflows
DOI:
10.1088/0004-637X/696/1/690
Bibliographic Code:
2009ApJ...696..690O

Abstract

In the subclass of high-mass X-ray binaries known as "microquasars," relativistic hadrons in the jets launched by the compact object can interact with cold protons from the star's radiatively driven wind, producing pions that then quickly decay into gamma rays. Since the resulting gamma-ray emissivity depends on the target density, the detection of rapid variability in microquasars with Gamma-Ray Large Area Space Telescope and the new generation of Cherenkov imaging arrays could be used to probe the clumped structure of the stellar wind. We show here that the fluctuation in gamma rays can be modeled using a "porosity length" formalism, usually applied to characterize clumping effects. In particular, for a porosity length defined by h ≡ ell/f, i.e., as the ratio of the characteristic size ell of clumps to their volume filling factor f, we find that the relative fluctuation in gamma-ray emission in a binary with orbital separation a scales as \sqrt{h/\pi a} in the "thin-jet" limit, and is reduced by a factor 1/\sqrt{1 + \phi a/2 \ell } for a jet with a finite opening angle phi. For a thin jet and quite moderate porosity length h ≈ 0.03a, this implies a ca. 10% variation in the gamma-ray emission. Moreover, the illumination of individual large clumps might result in isolated flares, as has been recently observed in some massive gamma-ray binaries.
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