STUDYPHYSGRAYBIN · Theoretical and phenomenological study of the physics of gamma-ray binaries

Theoretical and phenomenological study of the physics of gamma-ray binaries

The 'Theoretical and phenomenological study of the physics of gamma-ray binaries' (STUDYPHYSGRAYBIN) project had as its goal to study the high-energy processes in gamma-ray binaries through a combination of phenomenological and theoretical modelling, making use of the available data and proposing new observations. The aim was to help: (a) distinguish between jet and wind-dominated sources; (b) identify the acceleration and emission region(s); (c) determine the role of environmental factors on the radiation. All three questions have been approached and produced interesting results (see below); this has been carried out as planned, using three complementary approaches: semi-analytical modelling (Refs 3, 4, 8, 9, 12), hydrodynamical simulations (4, 6, 10, 14), Monte-Carlo techniques for radiation transport (7) and multiwavelength observations (15, 16, 17, 18). The multidisciplinary aspects of the project are indicated by the additional numerous works on active galactic nuclei in which similar approaches have been applied (1, 2, 5, 11, 12, 13). This project is probably amongst the few that combine analytical, numerical and observational studies of sources simultaneously and in an intensive way, taking profit also from the similarities between galactic and extragalactic sources. The main results of the work carried out during the project can be summarised as: - Particle acceleration should be very effective, with an energy transfer to accelerated particles likely higher than 10 %, and acceleration rates only comparable to the most efficient accelerators, like Crab (perhaps the most notorious gamma-ray object). - Adiabatic losses, very difficult to avoid in gamma-ray binaries, are likely to have an influence on the spectral and temporal behaviour of the non-thermal emission. Hydrodynamical studies show that in high-mass microquasars, and in high-mass binaries hosting a young pulsar, the role of the stellar wind is extremely important: in high-mass microquasars, the expected wind inhomogeneities are likely to have a strong impact on the propagation of the jet, with accompanying shocks due to flow recollimation and disturbances caused by medium inhomogeneity. In colliding star / pulsar winds, the orbital motion is going to play a very important role in the processes that lead to the formation of relativistic particles. Not only a double bow-shaped shock between stars will form, but also shocks will be produced by the bending of the shocked structure as orbital motion proceeds. Given the observational constraints and the mentioned hydrodynamical studies, the high-energy emission in gamma-ray binaries may be explained by several (very likely more than one) emitting sites, located in the vicinity of the binary system. The low-energy radiation, in particular in radio, is on the other hand likely the product of many small dissipation processes occurring in an extended region. In addition, radiation reprocessing, which can play an important role at high energies modifying the X-ray and gamma-ray spectra through synchrotron and inverse Compton respectively, can also produce secondary radio emission that will be added to that produced in the plasma flowing away from the binary system. Fortunately, despite its complexity, the morphology of radio emission can help to probe, through high-resolution radio observations, the extended emitter, so the results of the project are and will be subject to thorough observational testing. Note that radio very long baseline interferometry (VLBI) observations may be the best probe, together with the detection of pulsations, to disentangle the nature of the compact object in unclear cases. The analytical and numerical studies of microquasar jets, and star / pulsar outflows, when interacting on large scales with the environment seem to indicate that these outflows will terminate producing shocks and turbulent motions that could accelerate particles. These accelerated particles could explain some observations of binary systems showing radiation on large (parsec) scales. It is noteworthy, however, that the complex interplay of microquasars jets or pulsar winds, the wind of the companion star, and the motion through the Galaxy, may lead the interaction structure to disrupt. This would yield a whole phenomenology that could be studied by forthcoming, more sensitive, instrumentation with different energy coverage. Despite active galactic nuclei lacking orbital motion and a stellar companion, it seems clear that the interactions of their jets with the environment on small scales may have an impact, from the dynamical and the radiative point of view, as strong as in galactic jets. The physics underlying the high-energy radiation of both types of jets should share important points in common. From these results, the conclusions are: - The acceleration mechanism should operate at its limits, in the sense of both energy gain and energy injection into the non-thermal particles, to explain the gamma-ray observations (3, 9). - The broadband emitter is unlikely to be a smooth quasi-homogeneous region but rather a complex extended structure, whose morphology can be studied at least in radio (7, 8, 10, 14). - The jets and outflows present in these systems will end somewhere and their modes of termination should have specific observable consequences, providing information about the flow propagation and the surrounding medium (4, 6, 8). - Galactic and extragalactic jets share important physics that can be studied using the same numerical and phenomenological tools (1, 2, 5, 11, 12, 13). This investigation has hinted at strategies to tackle the open problems, but also seems to leave more open than close questions. However, the main conclusion of the project is precisely that the research of gamma-ray binaries has to widen beyond one-zone models and schematic interpretations of data. This, together with the wealth of growing data (of which the project has participated 15, 16, 17, 18) will approach the field to its maturity. This is to benefit not only the gamma-ray binary community but also those who work in intersecting fields, which in fact comprehends significant parts of astrophysics because of the multidisciplinary topic. References (1) Gamma rays from cloud penetration at the base of AGN jets, Araudo, A. T., Bosch-Ramon, V., Romero, G. E., Astronomy and Astrophysics, vol. 522, p. 97, 2010 Nov 01 (2) Gamma-ray Flares from Red Giant/Jet Interactions in Active Galactic Nuclei, Barkov, M. V.,Aharonian, F. A. , Bosch-Ramon, V.,The Astrophysical Journal, vol. 724, p. 1517, 2010 Dec 01 (3) On the origin of correlated X-ray/VHE emission from LS I +61 303, Zabalza, V., Paredes, J. M., Bosch-Ramon, V., Astronomy & Astrophysics, vol. 527, p. 9, 2011 (4) The termination region of high-mass microquasar jets, Bosch-Ramon, V., Perucho, M., Bordas, P., Astronomy & Astrophysics, vol. 528, p. 89, 2011 (5) The evolution of the large-scale emission in Fanaroff-Riley type I jets, Bordas, P., Bosch-Ramon, V., Perucho, M., Monthly Notices of the Royal Astronomical Society, vol. 412, p. 1229, 2011 (6) The termination region of high-mass microquasar jets (corrigendum), Bosch-Ramon, V., Perucho, M., Bordas, P., Astronomy & Astrophysics, vol. 532, p. 1., 2011 (7) Monte Carlo Simulations of Radio Emitting Secondaries in gamma-Ray Binaries, Bosch-Ramon, V., Khangulyan, D., Publications of the Astronomical Society of Japan, vol. 63, p. 1023, 2011 (8) Large-scale flow dynamics and radiation in pulsar gamma-ray binaries, Bosch-Ramon, V., Barkov, M. V., Astronomy & Astrophysics, vol. 535, p. 20, 2011 (9) Thermal X-Ray Emission from the Shocked Stellar Wind of Pulsar Gamma-Ray Binaries, Zabalza, V., Bosch-Ramon, V., Paredes, J. M.,The Astrophysical Journal, vol. 743, p. 7, 2011 (10) 3D simulations of microquasar jets in clumpy stellar winds, Perucho, M., Bosch-Ramon, V., Astronomy & Astrophysics, vol. 539, p. 57, 2012 (11) Clouds and red giants interacting with the base of AGN jets, Bosch-Ramon, V., Perucho, M., Barkov, M. V., Astronomy & Astrophysics, vol. 539, p. 69, 2012 (12) Fermi I particle acceleration in converging flows mediated by magnetic reconnection, Bosch-Ramon, V., Astronomy & Astrophysics, vol 542, 125, 2012 (13) Interpretation of the flares of M87 at TeV energies in the cloud-jet interaction scenario, Barkov, M. V., Bosch-Ramon, V., Aharonian, F. A., Astrophysical Journal, vol 755, 170, 2012 (14) Simulations of stellar/pulsar wind interaction along one full orbit, Bosch-Ramon, V., Barkov, M. V., Khangulyan, D., Perucho, M., Astronomy & Astrophysics, vol 72, 843 2012 (15) Magic Constraints on gamma-ray Emission from Cygnus X-3, Aleksic et al. (MAGIC collaboration+Bosch-Ramon), Astrophysical Journal, vol. 721, p. 843, 2010 (16) A Search for Very High Energy Gamma-Ray Emission from Scorpius X-1 with the Magic Telescopes, Aleksic et al. (MAGIC collaboration+Bosch-Ramon), Astrophysical Journal Letters, vol. 735, p. 5, 2011 (17) Detection of the Gamma-Ray Binary LS I +61°303 in a Low-flux State at Very High Energy Gamma Rays with the MAGIC Telescopes in 2009, Aleksic et al. (MAGIC collaboration+Bosch-Ramon) Astrophysical Journal, vol. 746, p. 80, 2012 (18) Detection of VHE gamma rays from HESS J0632+057 during the 2011 February X-ray outburst with the MAGIC Telescopes, Aleksic et al. (MAGIC collaboration+Bosch-Ramon), The Astrophysical Journal Letters, 754:L10 (6pp), 2012

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