Query : 2010A&A...519A..81C

2010A&A...519A..81C - Astronomy and Astrophysics, volume 519, A81-81 (2010/9-1)

Modeling the three-dimensional pair cascade in binaries. Application to LS 5039.


Abstract (from CDS):

LS 5039 is a Galactic binary system emitting high and very-high energy gamma rays. The gamma-ray flux is modulated on the orbital period and the TeV lightcurve shaped by photon-photon annihilation. The observed very-high energy modulation can be reproduced with a simple leptonic model but fails to explain the flux detected by HESS at superior conjunction, where gamma rays are fully absorbed. The contribution from an electron-positron pair cascade could be strong and prevail over the primary flux at superior conjunction. The created pairs can be isotropized by the magnetic field, resulting in a three-dimensional cascade. The aim of this article is to investigate the gamma-ray radiation from this pair cascade in LS 5039. This additional component could account for HESS observations at superior conjunction in the system. A semi-analytical and a Monte Carlo method for computing three-dimensional cascade radiation are presented and applied in the context of binaries. The cascade is decomposed into discrete generations of particles where electron-positron pairs are assumed to be confined at their site of creation. Both methods give similar results. The Monte Carlo approach remains best suited to calculation of a multi-generation cascade. Three-dimensional cascade radiation contributes significantly at every orbital phase in the TeV lightcurve, and dominates close to superior conjunction. The amplitude of the gamma-ray modulation is correctly reproduced for an inclination of the orbit of ≃40°. Primary pairs should be injected close to the compact object location, otherwise the shape of the modulation is not explained. In addition, synchrotron emission from the cascade in X-rays constrains the ambient magnetic field to below 10G. The radiation from a three-dimensional pair cascade can account for the TeV flux detected by HESS at superior conjunction in LS 5039, but the very-high energy spectrum at low fluxes remains difficult to explain in this model.

Abstract Copyright:

Journal keyword(s): radiation mechanisms: non-thermal - stars: individual: LS 5039 - X-rays: binaries - gamma rays: general

Simbad objects: 2

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Number of rows : 2
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2024
1 V* V479 Sct HXB 18 26 15.0561548880 -14 50 54.247274304 12.02 12.23 11.27 11.04   ON6V((f))z 573 2
2 HD 226868 HXB 19 58 21.6757355952 +35 12 05.784512688 9.38 9.72 8.91 8.42   O9.7Iabpvar 4349 0

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