P. Bordas

Binaries with the eyes of CTA

The binary systems that have been detected in gamma rays have proven very useful to study high-energy processes, in particular particle acceleration, emission and radiation reprocessing, and the dynamics of the underlying magnetized flows. Binary systems, either detected or potential gamma-ray emitters, can be grouped in different subclasses depending on the nature of the binary components or the origin of the particle acceleration: the interaction of the winds of either a pulsar and a massive star or two massive stars; accretion onto a compact object and jet formation; and interaction of a relativistic outflow with the external medium. We evaluate the potentialities of an instrument like the Cherenkov telescope array (CTA) to study the non-thermal physics of gamma-ray binaries, which requires the observation of high-energy phenomena at different time and spatial scales. We analyze the capability of CTA, under different configurations, to probe the spectral, temporal and spatial behavior of gamma-ray binaries in the context of the known or expected physics of these sources. CTA will be able to probe with high spectral, temporal and spatial resolution the physical processes behind the gamma-ray emission in binaries, significantly increasing as well the number of known sources. This will allow the derivation of information on the particle acceleration and emission sites qualitatively better than what is currently available.

The termination region of high-mass microquasar jets

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.

The evolution of the large-scale emission in Fanaroff–Riley type I jets

Recent observations in X-rays and gamma-rays of nearby FRI 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 FRI sources, like some Gigahertz Peaked Spectrum AGNs. This work focuses on the nature and detectability of the radiation seen from the termination regions of evolving FRI jets. We use the results of a relativistic, two-dimensional numerical simulation of the propagation of an FRI jet, coupled with a radiation model, to make predictions for the spectra and lightcurves of the thermal and non-thermal emission at different stages of the FRI 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 ISM/ICM thermal component alone may be significant in old sources. Inverse Compton scattering of CMB 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 broadband emission from the termination regions of FRI jets could be detectable for sources located up to distances of a few 100 Mpc.

THE PUZZLING JET AND PULSAR WIND NEBULA OF IGR J11014-6103

IGR J11014-6103 is a hard X-ray source discovered by INTEGRAL. Follow-up X-ray and radio observations revealed an elongated pulsar wind nebula formed by a neutron star escaping supersonically its parent supernova remnant SNR MSH 11-61A. The pulsar also emits highly collimated jets extending perpendicularly to the direction of motion. The jet has a continuous helical structure extending up to more than 10 parsecs. IGR J11014-6103 is a laboratory to study jet ejection in the wind of a pulsar and to constrain the core collapse supernova mechanism responsible for the observed pulsar kick velocity in excess of 1000 km/s.

RADIATION FROM THE INTERACTION OF MICROQUASARS WITH THE ISM

Microquasars (MQs) are X-ray binary systems that display relativistic radio jets. These objects constitute a suitable laboratory for testing high energy astrophysical processes still not well understood, such as those present when jets interact with the interstellar medium (ISM). Focusing on the study of the nonthermal contribution from cocoon and bow-shock regions, we explore, under different ISM densities and ages of the jet source, the possibility to detect MQ jet termination regions. We conclude that emission from these regions may be faint, but still detectable in the radio, X-ray, and gamma-ray bands.

Discovery of gamma-ray emission from the extragalactic pulsar wind nebula N157B with the High Energy Stereoscopic System

We present the significant detection of the first extragalactic pulsar wind nebula (PWN) detected in gamma rays, N157B, located in the large Magellanic Cloud (LMC). Pulsars with high spin-down luminosity are found to power energised nebulae that emit gamma rays up to energies of several tens of TeV. N157B is associated with PSRJ0537-6910, which is the pulsar with the highest known spin-down luminosity. The High Energy Stereoscopic System telescope array observed this nebula on a yearly basis from 2004 to 2009 with a dead-time corrected exposure of 46 h. The gamma-ray spectrum between 600 GeV and 12 TeV is well-described by a pure power-law with a photon index of 2.8 \pm 0.2(stat) \pm 0.3(syst) and a normalisation at 1 TeV of (8.2 \pm 0.8(stat) \pm 2.5(syst)) \times 10^-13 cm^-2s^-1TeV^-1. A leptonic multi-wavelength model shows that an energy of about 4 \times 10^49erg is stored in electrons and positrons. The apparent efficiency, which is the ratio of the TeV gamma-ray luminosity to the pulsars spindown luminosity, 0.08% \pm 0.01%, is comparable to those of PWNe found in the Milky Way. The detection of a PWN at such a large distance is possible due to the pulsars favourable spin-down luminosity and a bright infrared photon-field serving as an inverse-Compton-scattering target for accelerated leptons. By applying a calorimetric technique to these observations, the pulsars birth period is estimated to be shorter than 10 ms.

Jet/medium interactions at large-scales

High energy emission can be produced in the interaction sites of both galactic and extragalactic jets with the surrounding medium. We have developed a radiative model that accounts for the continuous injection of relativistic electrons in the forward, reverse and recollimation shocks developed in the shell, cocoon and reconfinement interaction regions, respectively. We also performed hydrodynamical simulations to establish the physical properties in both galactic and extragalactic systems. The resulting non-thermal emission is studied assuming different values for the jet power, the external mass density and the source age for both FR I galaxies and galactic microquasars. The obtained fluxes are compared to current instrument sensitivities at radio, X-ray and gamma-ray bands.

The evolution of the large-scale emission in Fanaroff-Riley type I jets: The emission in evolving FR I jets

Recent observations in X-rays and gamma-rays of nearby FRI 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 FRI sources, like some Gigahertz Peaked Spectrum AGNs. This work focuses on the nature and detectability of the radiation seen from the termination regions of evolving FRI jets. We use the results of a relativistic, two-dimensional numerical simulation of the propagation of an FRI jet, coupled with a radiation model, to make predictions for the spectra and lightcurves of the thermal and non-thermal emission at different stages of the FRI 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 ISM/ICM thermal component alone may be significant in old sources. Inverse Compton scattering of CMB 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 broadband emission from the termination regions of FRI jets could be detectable for sources located up to distances of a few 100 Mpc.

Microquasar interaction with the surrounding medium

The high kinetic energy outflowing in the jets of microquasars is delivered to the surrounding interstellar medium. This energy input can cause the formation of bow shocks and cocoons that may be detectable from radio to gamma-ray energies. Evidences or hints of emission from jet/medium interactions have been reported for some sources, but little has been done regarding the theoretical modeling of the resulting non-thermal emission. We have developed an analytical model based on those successfully applied to extragalactic sources or the interaction of AGN jets with their surroundings. Focusing on the adiabatic phase of the growing structures, we give estimations of the expected luminosities through synchrotron, relativistic Bremsstrahlung and inverse Compton processes. We conclude that the interaction structures may be detectable at radio wavelengths, while extreme values for the jet kinetic power, the source age and the medium density are required to make the emission at high and very high energies detectable.Context. The interaction of microquasar jets with their environment can produce non-thermal radiation as is the case for extragalactic outflows impacting on their surroundings. Observational ev idences of jet/medium interactions in galactic microquasars have been collected in the past years, although few theoretical work has been done regarding the resulting non-thermal emission. Aims. In this work we investigate the non-thermal emission produced in the interaction between microquasar jets and their environment, and the physical conditions for its production. Methods. We have developed an analytical model based on those successfully applied to extragalactic sources. The jet is taken to b e a supersonic and mildly relativistic hydrodynamical outflow . We focus on the jet/shocked medium structure when being in its adiabatic phase, and assume that it grows in a self-similar way. We calculate the fluxes and spectra of the radiation produced via syn chrotron, Inverse Compton and relativistic Bremsstrahlung processes by electrons accelerated in strong shocks. A hydrodynamical simulation is also performed to further investigate the jet interaction w ith the environment and check the physical parameters used in the analytical model. Results. For reasonable values of the magnetic field, and using typica l values for the external matter density, the non-thermal pa rticles could produce significant amounts of radiation at di fferent wavelengths, although they do not cool mainly via radiative channels but through adiabatic losses. The physical conditions of the analytical jet/medium interaction model are consistent with those found in the hydrodynamical simulation. Conclusions. Microquasar jet termination regions could be detectable at radio wavelengths for current instruments sensitive to∼ arcminute scales. At X-rays, the expected luminosities are moderate, although the emitter is more compact than the radio one. The source may be detectable by XMM-Newton or Chandra, with 1‐10 arcseconds of angular resolution. The radiation at gamma-ray energies may be within the detection limits of the next generation of satellite and ground-based instruments.

Microquasar's interaction with the surrounding medium

The high kinetic energy outflowing in the jets of microquasars is delivered to the surrounding interstellar medium. This energy input can cause the formation of bow shocks and cocoons that may be detectable from radio to gamma-ray energies. Evidences or hints of emission from jet/medium interactions have been reported for some sources, but little has been done regarding the theoretical modeling of the resulting non-thermal emission. We have developed an analytical model based on those successfully applied to extragalactic sources or the interaction of AGN jets with their surroundings. Focusing on the adiabatic phase of the growing structures, we give estimations of the expected luminosities through synchrotron, relativistic Bremsstrahlung and inverse Compton processes. We conclude that the interaction structures may be detectable at radio wavelengths, while extreme values for the jet kinetic power, the source age and the medium density are required to make the emission at high and very high energies detectable.Context. The interaction of microquasar jets with their environment can produce non-thermal radiation as is the case for extragalactic outflows impacting on their surroundings. Observational ev idences of jet/medium interactions in galactic microquasars have been collected in the past years, although few theoretical work has been done regarding the resulting non-thermal emission. Aims. In this work we investigate the non-thermal emission produced in the interaction between microquasar jets and their environment, and the physical conditions for its production. Methods. We have developed an analytical model based on those successfully applied to extragalactic sources. The jet is taken to b e a supersonic and mildly relativistic hydrodynamical outflow . We focus on the jet/shocked medium structure when being in its adiabatic phase, and assume that it grows in a self-similar way. We calculate the fluxes and spectra of the radiation produced via syn chrotron, Inverse Compton and relativistic Bremsstrahlung processes by electrons accelerated in strong shocks. A hydrodynamical simulation is also performed to further investigate the jet interaction w ith the environment and check the physical parameters used in the analytical model. Results. For reasonable values of the magnetic field, and using typica l values for the external matter density, the non-thermal pa rticles could produce significant amounts of radiation at di fferent wavelengths, although they do not cool mainly via radiative channels but through adiabatic losses. The physical conditions of the analytical jet/medium interaction model are consistent with those found in the hydrodynamical simulation. Conclusions. Microquasar jet termination regions could be detectable at radio wavelengths for current instruments sensitive to∼ arcminute scales. At X-rays, the expected luminosities are moderate, although the emitter is more compact than the radio one. The source may be detectable by XMM-Newton or Chandra, with 1‐10 arcseconds of angular resolution. The radiation at gamma-ray energies may be within the detection limits of the next generation of satellite and ground-based instruments.