Secondary emission behind the radio outflows in gamma-ray binaries?

Several binary systems consisting of a massive star and a compact object have been detected above 100 GeV in the Galaxy. In most of these sources, gamma-rays show a modulation associated to the orbital motion, which means that the emitter should not be too far from the bright primary star. This implies that gamma-ray absorption will be non negligible, and large amounts of secondary electron-positron pairs will be created in the stellar surroundings. In this work, we show that the radio emission from these pairs should be accounted for when interpreting the radio spectrum, variability, and morphology found in gamma-ray binaries. Relevant features of the secondary radio emission are the relatively hard spectrum, the orbital motion of the radio peak center, and the extended radio structure following a spiral-like trajectory. The impact of the stellar wind free-free absorption should not be neglected.Comment: 6 pages, 4 figures / presented as a contributed talk in HEPRO II, Buenos Aires, Argentina, October 26-30 2009 / accepted for publication in Int. Jour. Mod. Phys.

Non-thermal emission from standing relativistic shocks: an application to red giant winds interacting with AGN jets

Galactic and extragalactic relativistic jets have rich environments that are full of moving objects, such as stars and dense clumps. These objects can enter into the jets and generate shocks and non-thermal emission. We characterize the emitting properties of the downstream region of a standing shock formed due to the interaction of a relativistic jet with an obstacle. We focus on the case of red giants interacting with an extragalactic jet. We perform relativistic axisymmetric hydrodynamical simulations of a relativistic jet meeting an obstacle of very large inertia. The results are interpreted in the framework of a red giant whose dense and slow wind interacts with the jet of an active galactic nucleus. Assuming that particles are accelerated in the standing shock generated in the jet as it impacts the red giant wind, we compute the non-thermal particle distribution, the Doppler boosting enhancement, and the non-thermal luminosity in gamma rays. The available non-thermal energy from jet-obstacle interactions is potentially enhanced by a factor of $\sim 100$ when accounting for the whole surface of the shock induced by the obstacle, instead of just the obstacle section. The observer gamma-ray luminosity, including the flow velocity and Doppler boosting effects, can be ~300(g/10)^2 times higher than when the emitting flow is assumed at rest and only the obstacle section is considered, where g is the jet Lorentz factor. For a whole population of red giants inside the jet of an AGN, the predicted persistent gamma-ray luminosities may be potentially detectable for a jet pointing to the observer. Obstacles interacting with relativistic outflows, for instance clouds and populations of stars for extragalactic jets, or stellar wind inhomogeneities in microquasar jets and in winds of pulsars in binaries, should be taken into account when investigating the non-thermal emission from these sources.Comment: 7 pages, 6 figures, version after proofs to appear in Astronomy & Astrophysic

Clumpy stellar winds and high-energy emission in high-mass binaries hosting a young pulsar

High-mass binaries hosting young pulsars can be powerful gamma-ray emitters. The stellar wind of the massive star in the system is expected to be clumpy. Since the high-energy emission comes from the pulsar-star wind interaction, the presence of clumps can affect the spectrum and variability of this radiation. We look for the main effects of the clumps on the two-wind interaction region and on the non-thermal radiation. A simple analytical model for the two-wind interaction dynamics was developed accounting for the lifetime of clumps under the pulsar-wind impact. This time plays a very important role with regard to the evolution of the clump, the magnetic field in the clump-pulsar wind interaction region, and the non-radiative and radiative cooling of the non-thermal particles. We also computed the high-energy emission produced at the interaction of long-living clumps with the pulsar wind. For reasonable parameters, the clumps will induce small variability on the X-ray and gamma-ray radiation. Sporadically, large clumps can reach closer to the pulsar increasing the magnetic field, triggering synchrotron X-ray flares and weakening other emission components like inverse Compton. The reduction of the emitter size induced by clumps also makes non-radiative losses faster. Stellar wind clumps can also enhance instability development and matter entrainment in the shocked pulsar wind when it leaves the binary. Growth limitations of the clumps from the wind acceleration region may imply that a different origin for the largest clumps is required. The large-scale wind structures behind the observed discrete absorption components in the UV may be the source of these large clumps. The presence of structure in the stellar wind can produce substantial energy-dependent variability and should not be neglected when studying the broadband emission from high-mass binaries hosting young pulsars.Comment: 8 pages, 4 figures, accepted for publication in Astronomy and Astrophysics (minor corrections after proofs

Studying the interaction between microquasar jets and their environments

In high-mass microquasars (HMMQ), strong interactions between jets and stellar winds at binary system scales could occur. In order to explore this possibility, we have performed numerical 2-dimensional simulations of jets crossing the dense stellar material to study how the jet will be affected by these interactions. We find that the jet head generates strong shocks in the wind. These shocks reduce the jet advance speed, and compress and heat up jet and wind material. In addition, strong recollimation shocks can occur where pressure balance between the jet side and the surrounding medium is reached. All this, altogether with jet bending, could lead to the destruction of jets with power $<10^{36} \rm{erg/s}$. The conditions around the outflow shocks would be convenient for accelerating particles up to $\sim$TeV energies. These accelerated particles could emit via synchrotron and inverse Compton (IC) scattering if they were leptons, and via hadronic processes in case they were hadrons.Comment: 4 pages. Contribution to the proceedings of High Energy Phenomena in Relativistic Outflows, held in Dublin, Ireland, September 24-28, 200

Formation of large-scale magnetic structures associated with the Fermi bubbles

The Fermi bubbles are part of a complex region of the Milky Way. This region presents broadband extended non-thermal radiation, apparently coming from a physical structure rooted in the Galactic Centre and with a partly-ordered magnetic field threading it. We explore the possibility of an explosive origin for the Fermi bubble region to explain its morphology, in particular that of the large-scale magnetic fields, and provide context for the broadband non-thermal radiation. We perform 3D magnetohydrodynamical simulations of an explosion from a few million years ago that pushed and sheared a surrounding magnetic loop, anchored in the molecular torus around the Galactic Centre. Our results can explain the formation of the large-scale magnetic structure in the Fermi bubble region. Consecutive explosive events may match better the morphology of the region. Faster velocities at the top of the shocks than at their sides may explain the hardening with distance from the Galactic Plane found in the GeV emission. In the framework of our scenario, we estimate the lifetime of the Fermi bubbles as $2\times10^6$ yr, with a total energy injected in the explosion(s) $> 10^{55}$ ergs. The broadband non-thermal radiation from the region may be explained by leptonic emission, more extended in radio and X-rays, and confined to the Fermi bubbles in gamma rays.Comment: 5 pages, 4 figures, accepted for A&

On the interaction of jets with stellar winds in XRBs

We present the first three-dimensional simulations of the evolution of a microquasar jet inside the binary-star system. The aim is to study the interaction of these jets with the stellar wind from a massive companion and the possible locations of high-energy emission sites. We have simulated two jets with different injection power in order to give a hint on the minimum power required for the jet to escape the system and become visible in larger scales. In the setup, we include a massive star wind filling the grid through which the jet evolves. We show that jets should have powers of the order of $10^{37}\rm{erg/s}$ or more in order not to be destroyed by the stellar wind. The jet-wind interaction results in regions in which high energy emission could be produced. These results imply the possible existence of a population of X-ray binaries not detected in the radio band due to jet disruption inside the region dominated by the stellar wind.Comment: Published in Proceedings of High Energy Phenomena in Relativistic Outflows II, held in Buenos Aires, 26-30 October 2009, ed. G. Romer