On the X-ray/TeV connection in Galactic jet sources
aa r X i v : . [ a s t r o - ph ] M a y October 30, 2018 14:53 WSPC - Proceedings Trim Size: 11in x 8.5in vbosch On the X-ray/TeV connection in Galactic jet sources
V. Bosch-Ramon
Max Planck Institut f¨ur KernphysikSaupfercheckweg 1, Heidelberg 69117, GermanyE-mail: [email protected]
D. Khangulyan
Max Planck Institut f¨ur KernphysikSaupfercheckweg 1, Heidelberg 69117, GermanyE-mail: [email protected]
F. A. Aharonian
Max Planck Institut f¨ur KernphysikSaupfercheckweg 1, Heidelberg 69117, GermanyE-mail: [email protected]; Dublin Institute for Advanced Studies, Dublin, Ireland
There are three Galactic jet sources, from which TeV emission has been detected: LS 5039, LS I +61 303 and Cygnus X-1. These three sources show power-law tails at X-rays and soft gamma-rays that could indicate a non-thermal originfor this radiation. In addition, all three sources apparently show correlated and complex behavior at X-ray and TeVenergies. In some cases, this complex behavior is related to the orbital motion (e.g. LS 5039, LS I +61 303), and in someothers it is related to some transient event occurring in the system (e.g. Cygnus X-1, and likely also LS I +61 303 andLS 5039). Based on modeling or energetic grounds, it seems difficult to explain the emission in the X-/soft gamma-rayand the TeV bands as coming from the same region (i.e. one-zone). We also point out the importance of the paircreation phenomena in these systems, which harbor a massive and hot star, for the radio and the X-ray emission, sincea secondary pair radiation component may be significant in these energy ranges. Finally, we discuss that in fact thepresence of the star can indeed have strong impact on, beside the non-thermal radiation production, the jet dynamics.
Keywords : X-rays: binaries – stars: individual: LS 5039 – Radiation mechanisms: non-thermal
1. Introduction
Three X-ray binary systems presenting extended ra-dio emission have been detected in the TeV range:LS 5039 [1]; LS I +61 303 [2]; and Cygnus X-1 [3].These three sources seem to show similarities be-tween the X-ray and the TeV lightcurves (LS 5039:see fig. 3 and fig. 5 in [4] and [5], respectively;LS I +61 303: see fig. 3 in [6]; Cygnus X-1: see fig. 4in [3]). In addition, LS 5039 show apparently similarphoton index/flux changes in both energy ranges [5].In the case of LS 5039 and LS I +61 303, the ra-diation variability seems to be associated with theorbital motion [2, 5]. Otherwise, Cygnus X-1 was de-tected during a transient event, and a possible or-bital link cannot be neither stated nor discarded;short TeV flares have been also detected in LS 5039and LS I +61 303 (see [7] and [8], respectively). Theshort ( ∼ hours) TeV flares observed in these three sources could have X-ray counterparts, since activeX-ray states (quasi) simultaneous with TeV activ-ity have been reported: possibly in LS 5039 [9], inLS I +61 303 [ ? , 10], and in Cygnus X-1 [3].
2. On the X-ray/TeV connection andthe importance of the primary star2.1.
The X-ray and the TeV emission
The link between the X-ray and the TeV emissionin all these three sources is certainly far from beingclear. Despite showing similar behavior, the regionproducing the radiation in these two energy bands islikely different, as explained in this section.In the case of LS 5039, strong theoretical ar-guments (basically: the expected low magnetic fieldin the emitter; see [12]) show that the X-ray andthe TeV emission cannot come from the same elec-tron population a , although some sort of physical link a Even if the TeV radiation has a hadronic origin, a leptonic population different from the secondary pairs produced in hadronicinteractions would be required to explain the X-ray fluxes. ctober 30, 2018 14:53 WSPC - Proceedings Trim Size: 11in x 8.5in vbosch ε [eV] ) l og ( ε L ε [ e r g / s ] ) X−ray levels/high BTeV levels/low B
MAGIC high statesynchrotron ICXMM high state
Fig. 1. Computed spectral energy distribution of the synchrotron and IC emission produced in LS I +61 303. We show twocases the only difference between them is the magnetic field ( B ) value: a case in which the X-ray emission is explained in thecontext of synchrotron radiation ( B = 2 . B = 0 . × erg s − , and the accelerationefficiency is ∼ . qBc , enough to explain the observed maximum photon energy. For a description of the used one-zone modeland the system parameters, see [21]. From the curves, it is seen that to explain the highest energy band of the very high-energyspectrum [2] requires X-ray fluxes below the observed ones [22]. would be required to explain the similar lightcurveand photon index evolution along the orbit.Regarding LS I +61 303, it is also difficult to rec-oncile its X-ray and TeV radiation during the phaseswith a one-zone model if we want to explain the high-est energy band of the TeV spectrum at the sametime as the hard X-ray fluxes (see Fig. 1).Finally, concerning Cygnus X-1, the emission inthe TeV range cannot be produced by the same elec-trons that emit hard X-rays because the emittingprocesses are expected to be different; the hard X-rays would come from the base of the jet or from acorona-like region (see, e.g. [13, 14]); the TeV couldbe produced either by hadronic processes or via in-verse Compton (IC) where non-thermal particles in-teract with the targets likely provided by the star(i.e. wind ions [15] or UV photons [16]). Soft gamma-ray radiation showing a power-law-like steep spec-trum has been found in Cygnus X-1 [17]. Unfortu- nately, there are no soft gamma-ray data simulta-neous with the TeV observations, preventing from ameaningful comparison. Nevertheless, we note thatif this soft gamma-ray emission were produced inthe jet, an injection luminosity of relativistic elec-trons equivalent to the kinetic luminosity of the jet[18] would be needed, unless the injected electrondistribution had a extremely high low-energy cutoff( ∼ TeV).In short, although there should be a physical linkbetween the X-ray and the TeV emission in LS 5039and LS I +61 303, the emitter modeling must go be-yond the one-zone approximation. Even in the caseof Cygnus X-1, to explain the soft gamma-rays andthe TeV emission requires a very peculiar particledistribution with an unrealistic low-energy cutoff of ∼ TeV, incompatible with TeV observations [3]. Oth-erwise, the required relativistic particle luminosity ishardly tenable. ctober 30, 2018 14:53 WSPC - Proceedings Trim Size: 11in x 8.5in vbosch The role of the star
To understand the X-ray/TeV connection in thesesystems, there are additional complexities that areto be taken into account before looking for a phys-ical explanation. The presence of the massive hotstar, with its strong photon and magnetic field, andstrong stellar wind, could have a significant impactin the emission and absorption processes, and on thephysics of the jet or outflow, which is commonly as-sumed to be the accelerator/emitter in the sourcesdiscussed here. For instance, in Fig. 2 it is shown theemission generated by electron-positron pairs pro-duced via pair creation in the photon field of the pri-mary star under reasonable assumptions for the sys-tem environment. Such a secondary radiation couldbe a significant fraction, if not a dominant one, ofthe non-thermal radiation produced in TeV close bi-nary systems [19]. In addition, the stellar wind rampressure could distort the jet dynamics significantly,producing also non-thermal emission via the genera-tion of shocks [20].
3. Discussion
The aim of this work is to remark the complexityof the phenomena that can lead to the productionof non-thermal emission in microquasars, and galac-tic compact sources in general, which harbor a mas-sive hot star. This complexity arises either based: onmodeling grounds (i.e. observations cannot be easilyexplained); or on an exploration of the importanceof several elements, which are unavoidably playingsome role in the considered scenario (i.e. the stellarradiation field and wind).In some cases the TeV radiation and its evolu-tion could be explained via geometrical effects re-lated to the emission and absorption processes (e.g.LS 5039 [12]). However, the similar X-ray and TeVbehaviors could be additionally pointing to under-lying variability of the non-thermal emitter intrin-sic properties (e.g. acceleration efficiency, magneticfield, relativistic particle energy budget, etc.). In ad-dition, the spectra and fluxes of the radiation in theX-ray and the TeV band seem to be incompatiblewith a simple approach considering only one elec-tron population (or even one hadronic-secondary lep-tonic population). Finally, the fact that the emitteris embedded in a powerful material outflow comingfrom the primary star likely implies that isolated jet models can hardly work to explain the radio-to-TeVemission. This could already happen at the first or-der approximation, or even at the zero one. All thisshould be taken into account when trying to general-ize the behavior of these sources, generalization thatseems far from trivial.
4. Summary
The X-ray and the TeV emission from the three jetGalactic sources detected up to now at very highenergies shows a very complex behavior and cannotbe explained in the simple context of one-zone mod-els. The presence of the primary star cannot be ne-glected regarding the non-thermal processes occur-ring in (and the dynamics of) the jet and its sur-roundings.
Acknowledgments
V.B-R. gratefully acknowledges support from theAlexander von Humboldt Foundation. V.B-R. ac-knowledges support by DGI of MEC under grantAYA2007-68034-C03-01, as well as partial sup-port by the European Regional Development Fund(ERDF/FEDER).
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