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Absorption of high-energy gamma rays in Cygnus X-3

Abstract : Context. The microquasar Cygnus X-3 was detected at high energies by the gamma-ray space telescopes AGILE and Fermi. The gamma-ray emission is transient, modulated with the orbital period and seems related to major radio flares, i.e. to the relativistic jet. The GeV gamma-ray flux can be substantially attenuated by internal absorption with the ambient X-rays.
Aims: We examine quantitatively the effect of pair production in Cygnus X-3 and put constraints on the location of the gamma-ray source.
Methods: Cygnus X-3 exhibits complex temporal and spectral patterns in X-rays. During gamma-ray flares, the X-ray emission can be approximated by a bright disk black-body component and a non-thermal tail extending in hard X-rays, which is possibly related to a corona above the disk. We calculate numerically the exact optical depth for gamma rays above a standard accretion disk. Emission and absorption in the corona are also investigated.
Results: GeV gamma rays are significantly absorbed by soft X-rays emitted from the inner parts of the accretion disk. The absorption pattern is complex and anisotropic. Isotropization of X-rays caused by Thomson scattering in the companion-star wind tends to increase the gamma-ray opacity. Gamma rays from the corona suffer from strong absorption by photons from the disk and cannot explain the observed high-energy emission, unless the corona is unrealistically extended.
Conclusions: The lack of an absorption feature in the GeV emission indicates that high-energy gamma rays should be located at a minimum distance ~108-1010 cm from the compact object. The gamma-ray emission is unlikely to have a coronal origin.
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https://hal-insu.archives-ouvertes.fr/insu-03625123
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Submitted on : Wednesday, March 30, 2022 - 4:57:53 PM
Last modification on : Monday, May 16, 2022 - 8:20:25 AM

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B. Cerutti, G. Dubus, J. Malzac, A. Szostek, R. Belmont, et al.. Absorption of high-energy gamma rays in Cygnus X-3. Astronomy and Astrophysics - A&A, EDP Sciences, 2011, 529, ⟨10.1051/0004-6361/201116581⟩. ⟨insu-03625123⟩

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