Multifrequency Behavior of Microquasars in the GeV--TeV era: A review

📝 Abstract

Microquasars are X-ray binaries that present non-thermal radio jets. Efficient particle acceleration can take place in different regions of the jets of microquasars. The accelerated particles can emit gamma-rays via leptonic or hadronic processes, with a complex spectral and temporal behavior. The jet termination region can be also an efficient non-thermal emitter, as well as, in high-mass microquasars, the region of the binary system outside the jet. In this work, I briefly describe the physics behind the non-thermal emission observed in microquasars at different scales, focusing in the GeV and TeV bands.

💡 Analysis

Microquasars are X-ray binaries that present non-thermal radio jets. Efficient particle acceleration can take place in different regions of the jets of microquasars. The accelerated particles can emit gamma-rays via leptonic or hadronic processes, with a complex spectral and temporal behavior. The jet termination region can be also an efficient non-thermal emitter, as well as, in high-mass microquasars, the region of the binary system outside the jet. In this work, I briefly describe the physics behind the non-thermal emission observed in microquasars at different scales, focusing in the GeV and TeV bands.

📄 Content

arXiv:1106.2059v1 [astro-ph.HE] 10 Jun 2011 Mem. S.A.It. Vol. 75, 282 c⃝SAIt 2008 Memorie della Multifrequency Behavior of Microquasars in the GeV–TeV era: A review V. Bosch-Ramon1 Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland e-mail: valenti@cp.dias.ie Abstract. Microquasars are X-ray binaries that present non-thermal radio jets. Efficient particle acceleration can take place in different regions of the jets of microquasars. The accelerated particles can emit gamma-rays via leptonic or hadronic processes, with a com- plex spectral and temporal behavior. The jet termination region can be also an efficient non-thermal emitter, as well as, in high-mass microquasars, the region of the binary system outside the jet. In this work, I briefly describe the physics behind the non-thermal emission observed in microquasars at different scales, focusing in the GeV and TeV bands. Key words. X-ray: binaries – Gamma-rays: theory – Radiation mechanism: non-thermal

1. Introduction Microquasars are X-ray binaries with non- thermal jets (e.g. Mirabel & Rodr´ıguez 1999; Rib´o 2005), being called high-mass micro- quasars when hosting a massive star, and low-mass microquasars otherwise. The energy powering the non-thermal emission in micro- quasars can be either of accretion or black- hole rotation origin. The magnetic and kinetic power is channelled through a jet launched from the inner regions of the accretion disk (e.g. Blandford & Znajek 1977; Blandford & Payne 1982), and part of this power is even- tually converted into relativistic particles and radiation. For several decades, microquasars were considered strong candidates to gamma-ray sources (e.g. Chadwick et al. 1985; see also Chardin & Gerbier 1989; Levinson & Blandford 1996; Paredes et al. 2000), but they Send offprint requests to: V. Bosch-Ramon have not become fully recognized as power- ful gamma-ray emitters until recent years, af- ter the most recent generation of ground-based Cherenkov (HESS, MAGIC, VERITAS) and satellite-borne instruments (Fermi, AGILE) ar- rived. The most relevant cases are the micro- quasars Cygnus X-11 and Cygnus X-3 (Albert et al. 2007; Sabatini et al. 2010; Tavani et al. 2009; Abdo et al. 2009a; Sabatini 2011). Other sources, like for instance SS 433, Scorpius X- 1 or GRS 1915−105, have been also observed in GeV and TeV energies but only upper limits have been obtained (Saito et al. 2009; Acero et al. 2011; Aleksic et al. 2010; Bordas et al. 2010). It is noteworthy that there are four other binary systems that may be also micro- quasars: LS I+61+303 (e.g. Albert et al. 2006; Abdo et al. 2009b; Pittori et al. 2009), LS 5039 1 This source has been detected in GeV and TeV energies with significances close, but slightly below, 5 σ, and thus these detections are still to be firmly established. Bosch-Ramon: Microquasars in the GeV–TeV era 283 (e.g. Aharonian et al. 2005; Abdo et al. 2009c; Pittori et al. 2009), HESS J0632+057 (e.g. Hinton et al. 2009; Falcone et al. 2011; Mold´on et al. 2011), and 1FGL J1018.6−5856 (e.g. Corbet et al. 2011), although they could as well host a non-accreting pulsar. Regarding the mi- croquasar and pulsar scenarios, LS I+61+303 and LS 5039 have been extensively discussed in the literature (see, e.g., Bosch-Ramon & Khangulyan 2009 and references therein). Although the detected radio emission is already evidence of particle acceleration in microquasar jets, the finding of microquasar gamma-ray emission proves that these sources can very efficiently channel accretion or black- hole rotational energy into radiation. In addi- tion, together with this high efficiency, the tem- poral characteristics of the detected radiation may favor leptonic models, although hadronic mechanisms cannot be discarded. Also, the extreme conditions under which gamma-rays are produced can put restrictions in the emit- ter structure. Morphological studies can be also of help, since non-thermal processes can take place not only at the binary scales, but also far away (e.g. the jet termination region). Although the complexity of microquasar phe- nomenology can make the characterization of the ongoing processes difficult, high quality data together with semi-analytical modeling can provide sensible information on the non- thermal physics of the sources. Numerical cal- culations are also important, since they can in- form about the conditions of the background plasma in which emission takes place. In this paper, we briefly review relevant aspects of the non-thermal emission in mi- croquasars. We will focus mainly in the GeV and TeV energy bands, for which photon pro- duction requires extreme conditions in these sources. In Figure 1, a sketch of the micro- quasar scenario is presented.
2. Non-thermal emission in microquasars Microquasar jets can produce non-thermal populations of relativistic particles via diffu- sive shock acceleration or other mechanisms at different spatial scales (e.g. Rieger et al. 2007). These particles, electro

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