The gamma-ray emitting region of the jet in Cyg X-3

Abstract

We study models of the gamma-ray emission of Cyg X-3 observed by Fermi. We calculate the average X-ray spectrum during the gamma-ray active periods. Then, we calculate spectra from Compton scattering of a photon beam into a given direction by isotropic relativistic electrons with a power-law distribution, both based on the Klein-Nishina cross section and in the Thomson limit. Applying the results to scattering of stellar blackbody radiation in the inner jet of Cyg X-3, we find that a low-energy break in the electron distribution at a Lorentz factor of ~ 300--1000 is required by the shape of the observed X-ray/gamma-ray spectrum in order to avoid overproducing the observed X-ray flux. The electrons giving rise to the observed \g-rays are efficiently cooled by Compton scattering, and the power-law index of the acceleration process is ~ 2.5--3. The bulk Lorentz factor of the jet and the kinetic power before the dissipation region depend on the fraction of the dissipation power supplied to the electrons; if it is ~ 1/2, the Lorentz factor is ~ 2.5, and the kinetic power is ~ 10^38 erg/s, which represents a firm lower limit on the jet power, and is comparable to the bolometric luminosity of Cyg X-3. Most of the power supplied to the electrons is radiated. The broad band spectrum constrains the synchrotron and self-Compton emission from the gamma-ray emitting electrons, which requires the magnetic field to be relatively weak, with the magnetic energy density < a few times 10^-3 of that in the electrons. The actual value of the magnetic field strength can be inferred from a future simultaneous measurement of the IR and gamma-ray fluxes.Comment: MNRAS, in press, 13 page

Formation of recollimation shocks in jets of high-mass X-ray binaries

“…Since P j ∼ P cr in both cases, the shock location is close to the maximum possible position of z ∼ a. This, in fact, agrees with the analyses of Dubus et al (2010) and Zdziarski et al (2012b) for Cyg X-3, based on modelling of the γ-ray emission. In Cyg X-1, the formation of a recollimation shock at z < ∼ a may thus explain an apparent departure of the structure of the jet from the standard model of Blandford & Königl (1979), see Heinz (2006), Zdziarski (2012) and Zdziarski et al (2012a).…”

“…We then obtain P cr ≃ 6.6 × 10 37 erg s −1 . This is similar to, or somewhat below, the power estimate of Zdziarski et al (2012b) Thus, a strong recollimation shock may occur in the hard-state jet of Cyg X-1 and the soft state of Cyg X-3 during periods of strong γ-ray emission. Since P j ∼ P cr in both cases, the shock location is close to the maximum possible position of z ∼ a.…”

“…We adopṫ M w ≃ 7 × 10 −6 M ⊙ yr −1 (see Zdziarski et al 2013 and references therein), v w ≃ 1.6×10 8 cm s −1 (van Kerkwijk et al 1996), v j ≃ 0.5c (Zdziarski et al 2012b;Dubus et al 2010) and a = 2.4 × 10 11 cm (assuming the total mass of 13M ⊙ , Zdziarski et al 2013). We then obtain P cr ≃ 6.6 × 10 37 erg s −1 .…”

“…The condition formulated by Dubus et al (2010) was then applied to Cyg X-3 by Zdziarski et al (2012b), who concluded from it that a recollimation shock can be formed only if the initial jet half-opening angle, α 0 , is rather large, > ∼ 30…”

Synergies in extragalactic and Galactic jet research

“…The large opacities to gamma-ray photon propagation ensure efficient cascading in close binaries. Compact binaries with early-type stars such as Cygnus X-1 (Orellana et al 2007Romero et al 2010, Bednarek & Giovannelli 2007 and Cygnus X-3 (Cerutti et al 2011, Zdziarski et al 2012b are very likely to sustain cascade production during gamma-ray flares.…”