Review on the multiwavelength emission of the gamma-ray binary LS I +61 303
aa r X i v : . [ a s t r o - ph . H E ] A p r Review on the multiwavelength emission of thegamma-ray binary LS I +61 303
Benito Marcote ∗ Joint Institute for VLBI ERIC (JIVE)E-mail: [email protected]
Gamma-ray binaries are systems composed of a massive star and a compact object that produceemission from radio to very high energy γ -rays. LS I +61 303 is one of the only six gamma-ray binaries discovered so far. It is thought that gamma-ray binaries contain a young highlyrotating neutron star as compact object, and the emission is produced by the interaction betweenits relativistic pulsar wind and the stellar wind, However, in the case of LS I +61 303 a microquasarscenario is still considered and results pointing to oppose directions have been published duringthe last decades. Here we provide a review about the state of the art of LS I +61 303, summarizingthe observed emission from radio to very high energy γ -rays along all these years, and we discussthe proposed scenarios that can explain such emission. XII Multifrequency Behaviour of High Energy Cosmic Sources Workshop12-17 June, 2017Palermo, Italy ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ eview on the multiwavelength emission of the gamma-ray binary LS I +61 303
Benito Marcote
1. Introduction
Only a reduced number of binary systems has shown high-energy (HE; 0 . & . γ -ray emission so far. This requires the presence of powerfulmechanisms that accelerate particles up to relativistic energies, making these systems unique tostudy in detail particle acceleration given the short timescales involved and their relative proximity.During the last decades different types of binaries have been discovered as non-pulsed HEemitters: three high-mass X-ray binaries (Cyg X-1, Cyg X-3, SS 433), several neutron-star/low-mass star binaries, one colliding wind binary ( η -Car), and six gamma-ray binaries. Only the lattergroup displays significant VHE emission (see [1, 2] and references therein).All the known gamma-ray binaries are composed of a high-mass star (of either O or B spectraltype) and a compact object which only in one case (PSR B1259 −
63) has been confirmed to be aneutron star. In all the other systems the nature of this compact object remains unclear and couldbe either a black hole or a neutron star. In addition to the presence of VHE emission, gamma-raybinaries are characterized by displaying a Spectral Energy Distribution (SED) dominated by the γ -ray photons. This behavior makes gamma-ray binaries unique among all binaries displaying γ -rayemission, as for all the other kinds of binaries the SED is clearly dominated by the X-ray photons.A different origin for the emission is thought to be underlying this different behavior.In this work we present a review on the gamma-ray binary LS I +61 303. We present thesource in Sect. 2. We describe the multiwavelength emission in Sect. 3 and the proposed scenariosto explain the collected data in Sect. 4. Finally, we present the conclusions in Sect. 5.
2. The gamma-ray binary system LS I +61 303
The gamma-ray binary LS I +61 303 is composed of a young, rapidly rotating, 10–15 M ⊙ B0 Ve star [3] and a compact object orbiting it every P orb = . ± .
003 d [4] in an eccentric orbit( e = . ± . φ orb = . ± .
03 [5]. The whole system is located at a distante of 2 . ± . & ◦ ), whereas the latter one is favored otherwise. We note that inclinations above 60 ◦ arealready discarded according to the orbital parameters [5, 7]. Whereas pulse searches have beenconducted, no pulsed emission have been detected so far [8, 9]. This however does not reject thepresence of a pulsar as we would expect the pulses to be absorbed due to the compactness of thesystem, as it occurs in the much wider orbit of PSR B1259 −
63 during periastron.On the one hand, the mass of the compact object has been estimated to be 1 . ⊙ < M c < . ⊙ (implying the existence of a a neutron star) assuming that the orbit and the Be decretion diskare in the same plane, and the inclination of the orbit with respect to the observer is ∼ ◦ < i < ◦ [10]. Given the uncertainties in the measurement of the inclination, larger values of the mass areactually possible, allowing the existence of a black hole. On the other hand, the existence of a blackhole has been suggested by indirect studies on the X-ray luminosity and photon index [11].The companion star has a circumstellar disk that extends over periastron, causing a directinteraction with the compact object during periastron, where the disk could be disrupted. Although1 eview on the multiwavelength emission of the gamma-ray binary LS I +61 303 Benito Marcote the shape of the disk remains unclear, an elliptical shape [12] or one-armed spiral density wave[13, 14] have been suggested. In any case, it is clear that the size of the disk changes along theso-called superorbital modulation (see next section) [15, 16].
3. Multiwavelength emission
LS I +61 303 exhibits persistent and orbitally modulated emission from radio to VHE γ -rays.Whereas the first HE counterpart was already detected by COS B [17], its association could not beconfirmed until the discovery of a variable VHE source, almost 30 yr later [18].A long-term modulation have also been found at all wavelengths. The so-called superorbitalmodulation exhibits a period of P so = ± ∼ . γ -rays to radio frequencies. γ -ray emission LS I +61 303 has been detected at VHE (TeV energies) with the MAGIC Cherenkov Tele-scopes [18] and with VERITAS [21]. This emission show a modulation coincident with the orbitalperiod [22] with outbursts peaking at orbital phases φ orb ∼ . .
8. The emission outside the out-bursts (e.g. at φ orb ∼ .
0) is faint and thus difficult to detect, though it has been reported with moresensitive observations [23]. Therefore it is thought that there is persistent VHE emission at allorbital phases. This emission is explained by a power-law with a photon index of Γ ≈ . . . . × − TeV − cm − s − [24].A long-term modulation has recently been reported at VHE [25] with a period of 1 610 ±
58 d,thus consistent with the superorbital modulation observed at other wavelengths. However, stillmore data are required to study in detail the behavior of LS I +61 303 at these energies. γ -ray emission LS I +61 303 is the 12 th brightest source in the HE sky, and was detected as a variable sourcein the Fermi /LAT data [26] with a modulation consistent with the orbital period [27]. The HE light-curve is roughly anti-correlated with respect to the VHE, X-ray, and radio ones, with the maximataking place right after periastron. The HE spectrum is described by a power-law with a photonindex of Γ ≈ . . × − cm − s − [27]. An exponential cutoff isobserved at 3.9 GeV, though at higher energies (above 30 GeV) a new component emerges.The HE emission also exhibits superorbital modulation [28, 12], and a significant dip aroundthe periastron has been reported at particular superorbital phases [12]. LS I +61 303 exhibits a non-thermal X-ray emission, which is also orbitally modulated andexhibits outbursts between orbital phases 0.4 and 0.8 [29, 30, 31]. A correlation between the X-ray light-curve and the TeV one has been suggested but not confirmed [32, 33]. Rapid activity,2 eview on the multiwavelength emission of the gamma-ray binary LS I +61 303
Benito Marcote with flares lasting for tens to hundreds of seconds, has been reported [34, 35], although quasi-periodic oscillations have been discarded. The superorbital modulation is also clearly observed atX-rays with a sinusoidal evolution [36, 31, 37], but a shift of 280 ±
44 d with respect to the radiomodulation is reported (we note the period of 1 667 d).The X-ray emission is well-described by an absorbed power-law with an index of Γ = . . . × − erg s − cm − [38]. No significant intrinsic absorption is howeverreported, and the spectrum does not reveal signatures of accretion, absorption and/or emission lines[39]. It has been reported the existence of a correlation between the flux and photon index, with aharder spectrum at higher fluxes [34, 35].Finally, hints of X-ray extended emission at scales of ∼ −
63 that are ejected after periastronpassage and move away from the system at ∼ . c [40]. The optical light-curve of LS I +61 303 is also orbitally [3, 41] and superorbitally [14] mod-ulated. Measurements of the equivalent width of the H α ( EW H α ) emission line, which is a goodtracer of the conditions in the outer circumstellar disk, also show the orbital and superorbital vari-ability [42, 43, 16, 14]. The photometric and EW H α light-curves show the same behavior but with ashift with respect to the radio and X-ray light-curves, suggesting a coupling between these differentwavelengths, and thus between the thermal and non-thermal emission. The spectrum of LS I +61 303 has been widely studied at gigahertz frequencies along theorbit [20, 44, 45, 46] and along the superorbital modulation [4]. Whereas an outburst is observedevery orbital cycle, their shapes and peak times slightly change from cycle to cycle [44]. Thebursts are known to peak roughly at the same times ( φ orb ≈ . .
9) for frequencies 1–25 GHz,exhibiting an average spectral index of α ≈ − . α ≈ . ∆φ orb ≈ . ∼
100 – 700 MHz) have also been conducted [47, 48].A significant delay in the peak of the outbursts and their shapes has been reported [48], probablydue to absorption. Additionally, these observations show evidences of a low-frequency turnover, totake place between 0.6 and 2 GHz.Very long baseline interferometric (VLBI) radio observations show that the radio emission isresolved on milliarcsecond scales (5–10 AU) [49]. The structure shows periodic morphologicalchanges along the orbit that have been interpreted as a signature of a cometary tail produced as aresult of colliding winds [50, 51, 52, 53]. Although other interpretations of the data suggest thepresence of a precessing jet as in a microblazar [54].
4. Microquasar or young non-accreting pulsar wind scenario?
Different scenarios have been proposed to explain the aforementioned emission of LS I +61 303and we still do not have a clear, full, picture. While a young highly rotating pulsar producing a3 eview on the multiwavelength emission of the gamma-ray binary LS I +61 303
Benito Marcote shock between its relativistic wind and the stellar wind is thought to be present in all gamma-raybinaries (which would explain the significant differences observed with respect to the other γ -rayemitting X-ray binaries), in the case of LS I +61 303 a microquasar scenario is still considered.In the following we summarize the proposed scenarios that has been proposed in the literatureto explain the available data. In the microquasar scenario [55] an accretion onto the compact object is required. This accre-tion would lead the formation of jets where the particles are accelerated and would emit up to VHE γ -rays due to upscattering of stellar photons [56]. This is the scenario observed in X-ray binaries,either including black holes or accreting powered neutron stars.The reported anti-correlation between the X-ray flux and the photon index is similar to the oneobserved in other black-hole X-ray binaries, but not in PSR B1259 −
63, which has leaded to thesuggestion of the existence of a black hole in LS I +61 303 [11]. Furthermore, through frequencyradio studies it has been suggested that there are actually two close periods of ≈ . ≈ . To explain the vast differences observed in the SED between the gamma-ray binaries andthe γ -ray emitting X-ray binaries a different underlying scenario was proposed, which has beenconfirmed at least for the only gamma-ray binary with a confirmed pulsar, PSR B1259 −
63. Thisscenario requires the presence of a neutron star, which must be young and must be highly rotatingto guarantee to be rotation powered, and not accretion powered. The neutron star thus producesrelativistic winds that collide with the stellar wind originating a strong shock due to the proximityof both components [62]. In this case particles are accelerated at the interaction region betweenboth winds but also in the Coriolis turnover behind the compact object [63].LS I +61 303 hosts a B spectral-type star with a circumstellar disk which is perturbed by thecompact object during periastron. In this sense we would expect a system qualitatively similar toPSR B1259 −
63, but in a much closer orbit. Indeed, we observe an outburst per orbital cycle thatis produced after the compact object crosses the circumstellar disk. Given the lack of evidences ofaccretion in the system, a wind interaction is favored in LS I +61 303.It has been suggested that the superorbital variability is related to periodic changes in themass-loss rate of the Be star and/or variations in the circumstellar disk [43, 14]. Therefore thisvariability could not be linked with only the compact object (e.g. a precessing jet). This connectionis supported by the evidences of periodic changes in the circumstellar disk, with the presence ofa spiral density wave [43] which has been interpreted as an non-uniform, elongated, disk [12] orwith one-armed spiral shape [14]. In any of these scenarios, the rotation of the disk with a longperiod would naturally produce the observed superorbital modulation. When the putative neutronstar approaches periastron it would face a region of the disk with different density, producing achange in the strength of the interaction and emission each time.4 eview on the multiwavelength emission of the gamma-ray binary LS I +61 303
Benito Marcote
Independently to this, we note the X-ray extended emission that has been reported in LS I+61 303 [38]. This phenomena could be related to the similar emission observed in PSR B1259 − A third scenario, the so-called flip-flop model, has been proposed to explain the emissionobserved in LS I +61 303 [65, 66]. This scenario also considers the presence of a neutron staras compact object. However, in this case the neutron star would change state from a propellerregime (during periastron) to an ejector region (at apastron). These changes of state would bedriven by the interaction with the circumstellar disk and the stellar wind. At periastron the neutronstar magnetosphere would be compressed and disrupted due to the much denser surrounding ofthe disk, suppressing the synchrotron emission and favoring the propeller regime. At apastron, farfrom the circumstellar disk, the neutron star would change again to the ejector regime favoring thestrong VHE emission that is observed.
5. Conclusions
LS I +61 303 is one of the six known gamma-ray binaries. Whereas it is thought that allgamma-ray binaries share a common scenario, different from the microquasar scenario observed inX-ray binaries, in the case of LS I +61 303 this scenario is still considered.LS I +61 303 exhibits a SED dominated by the HE photons, as all the other gamma-ray bina-ries, and in contrast to the ones observed in γ -ray emitting X-ray binaries, where the X-ray photonsdominate due to the presence of accretion and a strong cutoff is observed above those energies.The stablished connection between changes in the thermal emission (arising from the circumstellardisk) and the non-thermal emission (from the interaction of the compact object and the disk/star),among other phenomena, points to an emission driven by wind collisions instead of jet-related.The nature of the compact object would allow us to unveil easily the kind of interaction thattakes place in LS I +61 303. However, we note the existence of indirect evidences pointing toopposed directions. Therefore only a direct measurement of the mass of the compact object wouldfirmly unveil its nature. Acknowledgements
The author acknowledges support from the Spanish Ministerio de Economía y Competitividad(MINECO) under grants AYA2016-76012-C3-1-P and MDM-2014-0369 of ICCUB (Unidad deExcelencia “María de Maeztu”).
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Among the six γ -ray binaries, what source would be the best candidateto contain a black hole instead of a neutron star? What would be the scenario for γ -ray productionin case of a such BH/HMXB system? 9 eview on the multiwavelength emission of the gamma-ray binary LS I +61 303 Benito Marcote
BENITO MARCOTE:
We only know the presence of a neutron star in one system, PSR B1259 − − γγ