V. Bosch-Ramon

On the formation of TeV radiation in LS 5039

The recent detections of TeV gamma-rays from compact binary systems show that relativistic outflows (jets or winds) are sites of effective acceleration of particles up to multi-TeV energies. In this paper, we discuss the conditions of acceleration and radiation of ultrarelativistic electrons in LS 5039, the gamma-ray emitting binary system for which the highest quality TeV data are available. Assuming that the gamma-ray emitter is a jet-like structure, we performed detailed numerical calculations of the energy spectrum and light curves accounting for the acceleration efficiency, the location of the accelerator, the speed of the emitting flow, the inclination angle of the system, as well as specific features related to anisotropic inverse Compton (IC) scattering and pair production. We conclude that the accelerator should not be deep inside the binary system unless we assume a very efficient acceleration rate. We show that within the IC scenario both the gamma-ray spectrum and flux are strongly orbital phase dependent. Formally, our model can reproduce, for specific sets of parameter values, the energy spectrum of gamma-rays reported by HESS for wide orbital phase intervals. However, the physical properties of the source can be constrained only by observations capable of providing detailed energy spectra for narrow orbital phase intervals (Δ(Φ) << 0.1).

Understanding the very-high-energy emission from microquasars

Microquasars are X-ray binaries with relativistic jets. These jets are powerful energy carriers — thought to be fed by accretion — which produce nonthermal emission at different energy bands. The processes behind the bulk of the nonthermal emission in microquasars may be of leptonic (synchrotron and inverse Compton) and hadronic (proton–proton interactions, photomeson production, and photodisintegration) nature. When leptonic, the fast particle cooling would allow one to obtain relevant information about the properties close to the accelerator, like the radiation and the magnetic field energy densities, and the acceleration efficiency. When hadronic, the extreme conditions required in the emitter would have strong implications for the physics of jets and their surroundings. The very-high-energy part of the spectrum, i.e. > 100 GeV, is a good energy range to explore the physics behind the nonthermal radiation in these compact variable sources. In addition, this energy range, when taken together with lower energy bands, is a key piece for constructing a comprehensive picture of the processes occurring in the emitter. Until recently, the very-high-energy range was hard to probe due to the lack of sensitivity and spatial and spectral resolution of previous instrumentation. Nowadays, however, powerful gamma-ray instruments are operating and the quality of their observations is allowing one, for the first time, to start to understand the production of high-energy emission in microquasars. To date, several galactic sources showing extended radio emission — among them at least one confirmed microquasar, Cygnus X-1 — have shown a TeV signal. All of them show complex patterns of spectral and temporal behavior. In this work, we discuss the physics behind the very-high-energy emission in Cygnus X-1, and also in the other two TeV binaries with detected extended outflows, LS 5039 and LS I +61 303, pointing out relevant aspects of the complex phenomena occurring in them. We conclude that the TeV emission is likely of leptonic origin, although hadrons cannot be discarded. In addition, efficient electromagnetic cascades can hardly develop since even relatively low magnetic fields suppress them. Also, the modeling of the radiation from some of the detected sources points to them as extremely efficient accelerators and/or having the TeV emitter at a distance from the compact object of about ~ 1012 cm. Finally, we point out that the role of a massive and hot stellar companion, due to its strong photon field and wind, cannot be neglected when trying to understand the behavior of microquasars at high and very high energies. The complexity of microquasars precludes straightforward generalizations to a whole population, and are better studied presently on a source-by-source basis. The new and future gamma-ray instrumentation will imply a big step further in our understanding of the processes in microquasars and gamma-ray-emitting binaries.

Study of the Spectral and Temporal Characteristics of X-Ray Emission of the Gamma-Ray Binary LS 5039 with Suzaku

We report on the results from Suzaku broadband X-ray observations of the galactic binary source LSxa05039. The Suzaku data, which have continuous coverage of more than one orbital period, show strong modulation of the X-ray emission at the orbital period of this TeV gamma-ray emitting system. The X-ray emission shows a minimum at orbital phase ~0.1, close to the so-called superior conjunction of the compact object, and a maximum at phase ~0.7, very close to the inferior conjunction of the compact object. The X-ray spectral data up to 70xa0keV are described by a hard power law with a phase-dependent photon index which varies within Γ 1.45- 1.61. The amplitude of the flux variation is a factor of 2.5, but is significantly less than that of the factor ~8 variation in the TeV flux. Otherwise the two light curves are similar, but not identical. Although periodic X-ray emission has been found from many galactic binary systems, the Suzaku result implies a phenomenon different from the standard origin of X-rays related to the emission of the hot accretion plasma formed around the compact companion object. The X-ray radiation of LSxa05039 is likely to be linked to very high energy electrons which are also responsible for the TeV gamma-ray emission. While the gamma rays are the result of inverse Compton (IC) scattering by electrons on optical stellar photons, X-rays are produced via synchrotron radiation. Yet, while the modulation of the TeV gamma-ray signal can be naturally explained by the photon-photon pair production and anisotropic IC scattering, the observed modulation of synchrotron X-rays requires an additional process, the most natural one being adiabatic expansion in the radiation production region.

Gamma-ray Flares from Red Giant/Jet Interactions in Active Galactic Nuclei

Non-blazar active galactic nuclei (AGNs) have been recently established as a class of gamma-ray sources. M87, a nearby representative of this class, shows fast TeV variability on timescales of a few days. We suggest a scenario of flare gamma-ray emission in non-blazar AGNs based on a red giant (RG) interacting with the jet at the base. We solve the hydrodynamical equations that describe the evolution of the envelope of an RG blown by the impact of the jet. If the RG is at least slightly tidally disrupted by the supermassive black hole, enough stellar material will be blown by the jet, expanding quickly until a significant part of the jet is shocked. This process can render suitable conditions for energy dissipation and proton acceleration, which could explain the detected day-scale TeV flares from M87 via proton-proton collisions. Since the radiation produced would be unbeamed, such an event should be mostly detected from non-blazar AGNs. They may be frequent phenomena, detectable in the GeV-TeV range even up to distances of ~1 Gpc for the most powerful jets. The counterparts at lower energies are expected to be not too bright. M87, and nearby non-blazar AGNs in general, can be fast variable sources of gamma-rays through RG/jet interactions.

On the interaction of microquasar jets with stellar winds

Context. Strong interactions between jets and stellar winds at binary-system, spatial-scales could occur in high-mass microquasars. Aims. We study here, mainly from a dynamical but also a radiative point of view, the collision between a dense stellar wind and a mildly relativistic hydrodynamical jet of supersonic nature. Methods. We have performed numerical two-dimensional simulations of jets, with cylindrical and planar (slab) symmetry, crossing the stellar-wind material. From the results of the simulations, we derive estimates of the particle acceleration efficiency, using firstorder, Fermi-acceleration theory, and provide insight into the possible radiative outcomes. Results. We find that, during jet launching, the jet head generates a strong shock in the wind. During and after this process, strong recollimation shocks can occur due to the initial overpressure of the jet with its environment. The conditions in all these shocks are convenient to accelerate particles up to ∼TeV energies, which can lead to leptonic (synchrotron and inverse Compton) and hadronic (proton-proton) radiation. In principle, the cylindrical jet simulations show that the jet is stable, and can escape from the system even at relatively low power. However, when accounting for the wind ram pressure, the jet can be bent and disrupted for power < 10 36 erg s −1 .

Electromagnetic radiation initiated by hadronic jets from microquasars in the ISM

Microquasars are potential candidates to produce a non-negligible fraction of the observed galactic cosmic rays. The protons accelerated at the jet termination shock interact with the interstellar medium and may produce detectable fluxes of extended emission at different energy bands: high-energy and very high-energy γ-rays produced by neutral pion-decay, synchrotron and bremsstrahlung emission in a wide energy range generated by the secondary electrons produced by charged pion-decay. We discuss the association between this scenario and some of the unidentified EGRET sources in the galactic plane.

Gamma rays from cloud penetration at the base of AGN jets

Context. Dense and cold clouds seem to populate the broad line region surrounding the central black hole in AGNs. These clouds could interact with the AGN jet base and this could have observational consequences. Aims. We want to study the gamma-ray emission produced by these jet-cloud interactions, and explore under which conditions this radiation would be detectable. Methods. We investigate the hydrodynamical properties of jet-cloud interactions and the resulting shocks, and develop a model to compute the spectral energy distribution of the emission generated by the particles accelerated in these shocks. We discuss our model in the context of radio-loud AGNs, with applications to two representative cases, the low-luminous Centaurus A, and the powerful 3C 273. Results. Some fraction of the jet power can be channelled to gamma-rays, which would be likely dominated by synchrotron self-Compton radiation, and show typical variability timescales similar to the cloud lifetime within the jet, which is longer than several hours. Many clouds can interact with the jet simultaneously leading to fluxes significantly higher than in one interaction, but then variability will be smoothed out. Conclusions. Jet-cloud interactions may produce detectable gamma-rays in non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A, and steady in the case of powerful objects of FR II type.

Leptonic secondary emission in a hadronic microquasar model

Context. It has been proposed that the origin of the very high-energy photons emitted from high-mass X-ray binaries with jet-like features, so-called microquasars (MQs), is related to hadronic interactions between relativistic protons in the jet and cold protons of the stellar wind. Leptonic secondary emission should be calculated in a complete hadronic model that includes the effects of pairs from charged pion decays inside the jets and the emission from pairs generated by gamma-ray absorption in the photosphere of the system. Aims. We aim at predicting the broadband spectrum from a general hadronic microquasar model, taking into account the emission from secondaries created by charged pion decay inside the jet. Methods. The particle energy distribution for secondary leptons injected along the jets is consistently derived taking the energy losses into account. The spectral energy distribution resulting from these leptons is calculated after assuming different values of the magnetic field inside the jets. We also compute the spectrum of the gamma-rays produced by neutral pion-decay and processed by electromagnetic cascades under the stellar photon field. Results. We show that the secondary emission can dominate the spectral energy distribution at low energies (∼1 MeV). At high energies, the production spectrum can be significantly distorted by the effect of electromagnetic cascades. These effects are phasedependent, and some variability modulated by the orbital period is predicted.

High-energy emission from jet-clump interactions in microquasars

Context. High-mass microquasars are binary systems consisting of a massive star and an accreting compact object from which relativistic jets are launched. There is considerable observational evidence that winds of massive stars are clumpy. Individual clumps may interact with the jets in high-mass microquasars to produce outbursts of high-energy emission. Gamma-ray flares have been detected in some high-mass X-ray binaries, such as Cygnus X-1, and probably in LS 5039 and LS I+61 303. Aims. We predict the high-energy emission produced by the interaction between a jet and a clump of the stellar wind in a high-mass microquasar. Methods. Assuming a hydrodynamic scenario for the jet-clump interaction, we calculate the spectral energy distributions produced by the dominant non-thermal processes: relativistic bremsstrahlung, synchrotron and inverse Compton radiation, for leptons, and for hadrons, proton-proton collisions. Results. Significant levels of emission in X-rays (synchrotron), high-energy gamma rays (inverse Compton), and very high-energy gamma rays (from the decay of neutral pions) are predicted, with luminosities in the different domains in the range � 10 32 -10 35 erg s −1 . The spectral energy distributions vary strongly depending on the specific conditions. Conclusions. Jet-clump interactions may be detectable at high and very high energies, and provide an explanation for the fast TeV variability found in some high-mass X-ray binary systems. Our model can help to infer information about the properties of jets and clumpy winds by means of high-sensitivity gamma-ray astronomy.

The magnetic field and the location of the TeV emitter in Cygnus X-1 and LS 5039

Context. Cygnusxa0X-1 and LSxa05039 are two X-ray binaries observed at TeV energies. Both sources are compact systems, contain jet-like (radio) structures, and harbor very luminous O stars. A TeV signal has been found around the superior conjunction of the compact object in both objects, when the highest gamma-ray opacities are expected. Aims. We investigate the implications of finding TeV emission from Cygnusxa0X-1 and LSxa05039 around the superior conjunction, since this can give information on the system magnetic field and the location of the TeV emitter. Methods. Using the very high-energy spectra and fluxes observed around the superior conjunction in Cygnusxa0X-1 and LSxa05039, we compute the absorbed luminosity that is caused by pair creation in the stellar photon field for different emitter positions with respect to the star and the observer line of sight. The role of the magnetic field and electromagnetic cascading are discussed. For the case of inefficient electromagnetic cascading, the expected secondary synchrotron fluxes are compared with the observed ones at X-ray energies. Results. We find that, in Cygnusxa0X-1 and LSxa05039, either the magnetic field in the star surroundings is much smaller than the one expected for O stars or the TeV emitter is located at a distance >10 12 xa0cm from the compact object. Conclusions. Our results strongly suggest that the TeV emitters in Cygnusxa0X-1 and LSxa05039 are located at the borders of the binary system and well above the orbital plane. This would not agree with those models for which the emitter is well inside the system, like the innermost-jet region (Cygnusxa0X-1 and LSxa05039; microquasar scenario) or the region between the pulsar and the primary star (LSxa05039; standard pulsar scenario).