Recent observations in X-rays and gamma-rays of nearby Fanaroff-Riley type I (FR I) radio galaxies have raised the question of the origin of the emission detected in the termination structures of ...their jets. The study of these structures can give information on the conditions for particle acceleration and radiation at the front shocks. In addition, an evolutionary scenario can help to disentangle the origin of the detected X-ray emission in young FR I sources, like some gigahertz peaked spectrum active galactic nuclei. This work focuses on the nature and detectability of the radiation seen from the termination regions of evolving FR I jets. We use the results of a relativistic, two-dimensional numerical simulation of the propagation of an FR I jet, coupled with a radiation model, to make predictions for the spectra and light curves of the thermal and non-thermal emission at different stages of the FR I evolution. Our results show that under moderate magnetic fields, the synchrotron radiation would be the dominant non-thermal channel, appearing extended in radio and more compact in X-rays, with relatively small flux variations with time. The shocked jet synchrotron emission would dominate the X-ray band, although the shocked interstellar/intracluster media thermal component alone may be significant in old sources. Inverse Compton (IC) scattering of cosmic microwave background photons could yield significant fluxes in the GeV and TeV bands, with a non-negligible X-ray contribution. The IC radiation would present a bigger angular size in X-rays and GeV than in TeV, with fluxes increasing with time. We conclude that the thermal and non-thermal broad-band emission from the termination regions of FR I jets could be detectable for sources located up to distances of a few 100 Mpc.
Context. Binary systems containing a massive star and a non-accreting pulsar present strong interaction between the stellar and the pulsar winds. The properties of this interaction, which largely ...determine the non-thermal radiation in these systems, strongly depend on the structure of the stellar wind, which can be clumpy or strongly anisotropic, as in Be stars. Aims. We study numerically the influence of inhomogeneities in the stellar wind on the structure of the two-wind interaction region. Methods. We carried out for the first time axisymmetric, relativistic hydrodynamical simulations, with Lorentz factors of ~6 and accounting for the impact of instabilities, to study the impact in the two-wind interaction structure of an over-dense region of the stellar wind. We also followed the evolution of this over-dense region or clump as it faces the impact of the pulsar wind. Results. For typical system parameters, and adopting a stellar wind inhomogeneity with a density contrast χ ≳ 10, clumps with radii of a few percent of the binary size can significantly perturb the two-wind interaction region, pushing the two-wind interface to ≲40% of the initial distance to the pulsar. After it is shocked, the inhomogeneity quickly expands and is disrupted when it reaches the smallest distance to the pulsar. It eventually fragments, being advected away from the binary system. The whole interaction region is quite unstable, and the shocked pulsar wind can strongly change under small perturbations. Conclusions. We confirm the sensitive nature of the two-wind interaction structure to perturbations, in particular when the stellar wind is inhomogeneous. For realistic over-dense regions of the stellar wind, the interaction region may shrink by a factor of a few, with the shocked flow presenting a complex spatial and temporal pattern. This can lead to strong variations in the non-thermal radiation.
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.
Gamma-ray-loud X-ray binaries are binary systems that show non-thermal broadband emission from radio to gamma rays. If the system comprises a massive star and a young non-accreting pulsar, their ...winds will collide producing broadband non-thermal emission, most likely originated in the shocked pulsar wind. Thermal X-ray emission is expected from the shocked stellar wind, but until now it has neither been detected nor studied in the context of gamma-ray binaries. We present a semi-analytic model of the thermal X-ray emission from the shocked stellar wind in pulsar gamma-ray binaries, and find that the thermal X-ray emission increases monotonically with the pulsar spin-down luminosity, reaching luminosities of the order of 1033 erg s--1. The lack of thermal features in the X-ray spectrum of gamma-ray binaries can then be used to constrain the properties of the pulsar and stellar winds. By fitting the observed X-ray spectra of gamma-ray binaries with a source model composed of an absorbed non-thermal power law and the computed thermal X-ray emission, we are able to derive upper limits on the spin-down luminosity of the putative pulsar. We applied this method to LS 5039, the only gamma-ray binary with a radial, powerful wind, and obtain an upper limit on the pulsar spin-down luminosity of ~6 X 1036 erg s--1. Given the energetic constraints from its high-energy gamma-ray emission, a non-thermal to spin-down luminosity ratio very close to unity may be required.
Context. The environment of high-mass X-ray binaries can be characterized either by the supernova remnant that forms these systems or by the wind from the companion massive star. These regions should ...be tenuous but very hot and surrounded by a dense and cold shocked ISM shell. The interaction between the jet and such a complex medium, also affected by the system proper motion, can lead to very different jet termination structures. Aims. The evolution of the jet termination regions during the life of a high-mass microquasar is simulated to improve our present understanding of these structures. Also, the evolving emission characteristics are modeled to inform potential observational campaigns of this class of object. Methods. We performed 2D numerical simulations of jets propagating in different scenarios, corresponding to different epochs after the formation of the high-mass X-ray binary, using the code Ratpenat. We also made simple estimates of the nonthermal emission that could be produced in the jet termination regions. Results. We find that, in the way through the hot and tenuous medium of the shocked wind/supernova ejecta, the jet suffers recollimation shocks in which it loses part of its thrust and ends in a strong shock inflating a hot cocoon. The jet head propagates with a speed that is similar to the medium sound speed, until it eventually reaches the denser and colder shocked ISM and the unperturbed ISM later on. In these last stages of evolution, the jet is significantly slowed down and can be disrupted. For relatively old sources, the microquasar peculiar velocity becomes important, leading to complete jet destruction. Extended nonthermal radiation can be generated in the jet termination regions, and hard X-rays and TeV photons are the wavelengths best suited for observing these structures.
Context.Massive stars form in dense and massive molecular cores. The exact formation mechanism is unclear, but it is possible that some massive stars are formed by processes similar to those that ...produce the low-mass stars, with accretion/ejection phenomena occurring at some point of the evolution of the protostar. This picture seems to be supported by the detection of a collimated stellar wind emanating from the massive protostar IRAS 16547-4247. A triple radio source is associated with the protostar: a compact core and two radio lobes. The emission of the southern lobe is clearly non-thermal. Such emission is interpreted as synchrotron radiation produced by relativistic electrons locally accelerated at the termination point of a thermal jet. Since the ambient medium is determined by the properties of the molecular cloud in which the whole system is embedded, we can expect high densities of particles and infrared photons. Because of the confirmed presence of relativistic electrons, inverse Compton and relativistic Bremsstrahlung interactions are unavoidable. Aims.We aim to make quantitative predictions of the spectral energy distribution of the non-thermal spots generated by massive young stellar objects, with emphasis on the particular case of IRAS 16547-4247. Methods.We study the high-energy emission generated by the relativistic electrons which produce the non-thermal radio source in IRAS 16547-4247. We also study the result of proton acceleration at the terminal shock of the thermal jet and make estimates of the secondary gamma rays and electron-positron pairs produced by pion decay. Results.We present spectral energy distributions for the southern lobe of IRAS 16547-4247, for a variety of conditions. We show that high-energy emission might be detectable from this object in the gamma-ray domain. The source may also be detectable in X-rays through long exposures with current X-ray instruments. Conclusions.Gamma-ray telescopes such as GLAST, and even ground-based Cherenkov arrays of new generation can be used to study non-thermal processes occurring during the formation of massive stars.
Context. LS 5039 is an X-ray binary that presents non-thermal radio emission. The radiation at ~5 GHz is quite steady and optically thin, consisting of a dominant core plus an extended jet-like ...structure. There is a spectral turnover around 1 GHz, and evidence of variability on timescales of 1 yr at 234 MHz. Aims. We investigate the radio emitter properties using the available broadband radio data, and assuming two possible scenarios to explain the turnover: free-free absorption in the stellar wind, or synchrotron self-absorption. Methods. We use the relationships between the turnover frequency, the stellar wind density, the emitter location, size and magnetic field, and the Lorentz factor of the emitting electrons, as well as a reasonable assumption regarding the energy budget, to infer the properties of the low-frequency radio emitter. Also, we put this information in context with the broadband radio data. Results. The location and size of the low-frequency radio emitter can be restricted to $\ga$few AU from the primary star, its magnetic field to ~3 $\times$ 10-3–1 G, and the electron Lorentz factors to ~$10{-}100$. The observed variability of the extended structures seen with VLBA would point to electron bulk velocities $\ga$3 $\times$ 108 cm s-1, whereas much less variable radiation at 5 GHz would indicate velocities for the VLBA core $\la$108 cm s-1. The emission at 234 MHz in the high state would mostly come from a region larger than the dominant broadband radio emitter. Conclusions. We suggest a scenario in which secondary pairs, created via gamma-ray absorption and moving in the stellar wind, are behind the steady broadband radio core, whereas the resolved jet-like radio emission would come from a collimated, faster, outflow.
Context. Cygnus X-1 and LS 5039 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 Cygnus X-1 and LS 5039 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 Cygnus X-1 and LS 5039, 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 Cygnus X-1 and LS 5039, 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 >1012 cm from the compact object. Conclusions. Our results strongly suggest that the TeV emitters in Cygnus X-1 and LS 5039 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 (Cygnus X-1 and LS 5039; microquasar scenario) or the region between the pulsar and the primary star (LS 5039; standard pulsar scenario).
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 phase-dependent, and some variability modulated by the orbital period is predicted.
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