Gamma-ray Binaries Research Articles

Monte Carlo Simulations of Radio Emitting Secondaries in-Ray Binaries

Several binary systems that contain a massive star have been detected in both the radio band and at very high energies. In the dense stellar photon field of these sources, gamma-ray absorption and pair creation are expected to occur, and the radiation from these pairs may contribute significantly to the observed radio emission. We aim at going deeper in studying the properties, and in particular the morphology, of the pair radio emission in gamma-ray binaries. We applied a Monte Carlo code that computes the creation location, the spatial trajectory and the energy evolution of the pairs produced in the binary system and its surroundings. The radio emission produced by these pairs, with its spectral, variability and spatial characteristics, was calculated as it would be seen from a certain direction. A generic case was studied first, and then the specific case of LS 5039 was also considered. We found that, confirming previous results, the secondary radio emission should appear as an extended radio structure of a few milliarcseconds size. This radiation would be relatively hard, with fluxes of up to $\sim$ 10 mJy. Modulation is expected, depending on the gamma-ray production luminosity, system eccentricity, and wind ionization fraction, and to a lesser extent on the magnetic-field structure. In gamma-ray binaries in general, the pairs created due to photon-photon interactions can contribute significantly to the core, and generate an extended structure. In the case of LS 5039, the secondary radio emission is likely to be a significant fraction of the detected core flux, with a marginal extension.

Revealing the extended radio emission from the gamma-ray binary HESS J0632+057

Context. After the detection of a 321-days periodicity in X-rays, HESS J0632+057 can be robustly considered a new member of the selected group of gamma-ray binaries. These sources are known to show extended radio structure at scales of milliarcseconds (mas). Aims. We present the expected extended radio emission on mas scales from HESS J0632+057. Methods. We observed HESS J0632+057 with the European VLBI Network (EVN) at 1.6 GHz in two epochs: during the January/February 2011 X-ray outburst and 30 days later. Results. The VLBI image obtained during the outburst shows a compact ~0.4 mJy radio source, whereas 30 days later the source has faded and appears extended, with a projected size of ~75 AU. The peak of the emission is displaced between runs 21+/-5 AU, which is bigger than the orbit size. The position of the radio source is compatible with the Be star MWC 148, which sets the proper motion of the binary system below 3 mas yr^-1 in each coordinate. The brightness temperature of the source is above 2 \times 10^6 K. We compare the multiwavelength properties of HESS J0632+057 with those of the previously known gamma-ray binaries. Conclusions. HESS J0632+057 displays extended and variable non-thermal radio emission. Its morphology, size, and displacement at AU scales are similar to those found in the other gamma-ray binaries, PSR B1259-63, LS 5039 and LS I +61 303, supporting a similar nature for HESS J0632+057.

A RADIO PULSAR SEARCH OF THE γ-RAY BINARIES LS I +61 303 AND LS 5039

LS I +61 303 and LS 5039 are exceptionally rare examples of HMXBs with MeV-TeV emission, making them two of only five known or proposed "gamma-ray binaries". There has been disagreement within the literature over whether these systems are microquasars, with stellar winds accreting onto a compact object to produce high energy emission and relativistic jets, or whether their emission properties might be better explained by a relativistic pulsar wind colliding with the stellar wind. Here we present an attempt to detect radio pulsars in both systems with the Green Bank Telescope. The upper limits of flux density are between 4.1-14.5 uJy, and we discuss the null results of the search. Our spherically symmetric model of the wind of LS 5039 demonstrates that any pulsar emission will be strongly absorbed by the dense wind unless there is an evacuated region formed by a relativistic colliding wind shock. LS I +61 303 contains a rapidly rotating Be star whose wind is concentrated near the stellar equator. As long as the pulsar is not eclipsed by the circumstellar disk or viewed through the densest wind regions, detecting pulsed emission may be possible during part of the orbit.

Implication of the striped pulsar wind model for gamma-ray binaries

(abridged) Gamma-ray binaries are massive stars with compact object companions that are observed to emit most of their energy in the gamma-ray range. One of these binaries is known to contain a radio pulsar, PSR B1259-63. Synchrotron and inverse Compton emission from particles accelerated beyond the light cylinder in striped pulsar winds has been proposed to explain the X-ray to high energy (HE, $>$ 100 MeV) gamma-ray emission from isolated pulsars. This pulsar model extends naturally to binary environments, where seed photons for inverse Compton scattering are provided by the companion star. Here, we investigate the possibility of gamma-ray emission from PSR B1259-63 in the framework of the striped pulsar wind model. The orbital geometry of PSR B1259-63 is well constrained by observations and the double radio pulse suggests an almost orthogonal rotator so that the solid angle covered by the striped region is close to $4\pi$. We calculate the orbital and rotational phase-resolved spectral variability and light-curves to expect.

Tomography of Interacting Binary Systems: Algols to Gamma-Ray Binaries

AbstractThree-dimensional Doppler tomography of interacting binaries has now provided some interesting perspectives of the gas flows beyond the central plane corresponding to the orbital plane. These images suggest that the magnetic field of the mass losing star influences the gas flows in some cases. Earlier 2D tomograms displayed evidence of gas flows associated with the gas stream, accretion disks, accretion annuli, and hot spots as well as evidence of magnetic flows associated with the mass loser. These indirect images have revealed the active environments that exist in the slow-mass-transfer Algols, cataclysmic variables, polars, x-ray binaries, and gamma ray binaries.

A SEARCH FOR VERY HIGH ENERGY GAMMA-RAY EMISSION FROM SCORPIUS X-1 WITH THE MAGIC TELESCOPES

The acceleration of particles up to GeV or higher energies in microquasars has been the subject of considerable theoretical and observational efforts in the past few years. Sco X-1 is a microquasar from which evidence of highly energetic particles in the jet has been found when it is in the so-called Horizontal Branch (HB), a state when the radio and hard X-ray fluxes are higher and a powerful relativistic jet is present. Here we present the first very high energy gamma-ray observations of Sco X-1 obtained with the MAGIC telescopes. An analysis of the whole dataset does not yield a significant signal, with 95% CL flux upper limits above 300 GeV at the level of 2.4x10^{-12} ph/cm^2/s. Simultaneous RXTE observations were conducted to search for TeV emission during particular X-ray states of the source. A selection of the gamma-ray data obtained during the HB based on the X-ray colors did not yield a signal either, with an upper limit of 3.4x10^{-12} ph/cm^2/s. These upper limits place a constraint on the maximum TeV luminosity to non-thermal X-ray luminosity of L_{VHE}/L_{ntX}<0.02, that can be related to a maximum TeV luminosity to jet power ratio of L_{VHE}/L_{j}<10^{-3}. Our upper limits indicate that the underlying high-energy emission physics in Sco X-1 must be inherently different from that of the hitherto detected gamma-ray binaries.

ASTROPHYSICAL PARAMETERS OF LS 2883 AND IMPLICATIONS FOR THE PSR B1259-63 GAMMA-RAY BINARY

Only a few binary systems with compact objects display TeV emission. The physical properties of the companion stars represent basic input to understand the physical mechanisms behind the particle acceleration, emission, and absorption processes in these so-called gamma-ray binaries. Here we present high-resolution and high signal-to-noise optical spectra of LS2883, the Be star forming a gamma-ray binary with the young non-accreting pulsar PSR B1259-63, showing it to rotate faster and be significantly earlier and more luminous than previously thought. Analysis of the interstellar lines suggest that the system is located at the same distance as (and thus is likely a member of) CenOB1. Taking the distance to the association, d=2.3kpc, and a color excess of E(B-V)=0.85 for LS2883, results in Mv=-4.4. Because of fast rotation, LS2883 is oblate (R_eq=9.7R_sun and R_pole=8.1R_sun) and presents a temperature gradient (T_eq=27500K, log g_eq=3.7; T_pole=34000K, log g_pole=4.1). If the star did not rotate, it would have parameters corresponding to a late O-type star. We estimate its luminosity at log(L*/L_sun)=4.79, and its mass at M=30M_sun. The mass function then implies an inclination of the binary system i_orb=23deg,slightly smaller than previous estimates. We discuss the implications of these new astrophysical parameters of LS2883 for the production of high energy and very high energy gamma rays in the PSR B1259-63/LS2883 gamma-ray binary system. In particular, the stellar properties are very important for prediction of the line-like bulk Comptonization component from the unshocked ultra-relativistic pulsar wind.

Hα EMISSION VARIABILITY IN THE γ-RAY BINARY LS I +61 303

LS I +61 303 is an exceptionally rare example of a high mass X-ray binary (HMXB) that also exhibits MeV-TeV emission, making it one of only a handful of "gamma-ray binaries". Here we present H-alpha spectra that show strong variability during the 26.5 day orbital period and over decadal time scales. We detect evidence of a spiral density wave in the Be circumstellar disk over part of the orbit. The H-alpha line profile also exhibits a dramatic emission burst shortly before apastron, observed as a redshifted shoulder in the line profile, as the compact source moves almost directly away from the observer. We investigate several possible origins for this red shoulder, including an accretion disk, mass transfer stream, and a compact pulsar wind nebula that forms via a shock between the Be star's wind and the relativistic pulsar wind.

X-ray absorption and occultation in LS 5039

Gamma-ray binaries are systems containing a massive star and a compact object that have been detected up to TeV energies. The high-energy emission could result from particle acceleration in the region where the stellar wind from the massive star interacts with the relativistic wind from a young pulsar. LS 5039 has the most compact orbit amongst gamma-ray binaries and its X-ray light curve shows a stable modulation synchronized with the orbital period. Photoelectric absorption of X-rays in the O star wind and occultation of the X-ray emitting region by the massive star can alter the X-ray light curve and spectrum along the orbit. Yet, the X-ray spectrum and light curve of LS 5039 do not show intrinsic absorption or X-ray eclipses. We study these effects in the framework of the pulsar wind scenario as a function of the binary inclination angle, the stellar wind mass-loss rate and the size of the X-ray emitter. An extended X-ray emission region ≳3R★ appears necessary to reconcile the pulsar wind scenario with observations.

Relativistic Doppler-boosted emission in gamma-ray binaries

<i>Context. <i/>Gamma-ray binaries could be compact pulsar wind nebulae formed when a young pulsar orbits a massive star. The pulsar wind is contained by the stellar wind of the O or Be companion, creating a relativistic comet-like structure accompanying the pulsar along its orbit.<i>Aims. <i/>The X-ray and the very high energy (<i>><i/>100 GeV, VHE) gamma-ray emission from the binary LS 5039 are modulated on the orbital period of the system. Maximum and minimum flux occur at the conjunctions of the orbit, suggesting that the explanation is linked to the orbital geometry. The VHE modulation has been proposed to be due to the combined effect of Compton scattering and pair production on stellar photons, both of which depend on orbital phase. The X-ray modulation could be due to relativistic Doppler boosting in the comet tail where both the X-ray and VHE photons would be emitted.<i>Methods. <i/>Relativistic aberrations change the seed stellar photon flux in the comoving frame so Doppler boosting affects synchrotron and inverse Compton emission differently. The dependence with orbital phase of relativistic Doppler-boosted (isotropic) synchrotron and (anisotropic) inverse Compton emission is calculated, assuming that the flow is oriented radially away from the star (LS 5039) or tangentially to the orbit (LS I +61303, PSR B1259-63).<i>Results. <i/>Doppler boosting of the synchrotron emission in LS 5039 produces a lightcurve whose shape corresponds to the X-ray modulation. The observations imply an outflow velocity of 0.15–0.33<i>c<i/> consistent with the expected flow speed at the pulsar wind termination shock. In LS I +61303, the calculated Doppler boosted emission peaks in phase with the observed VHE and X-ray maximum.<i>Conclusions. <i/>Doppler boosting is not negligible in gamma-ray binaries, even for mildly relativistic speeds. The boosted modulation reproduces the X-ray modulation in LS 5039 and could also provide an explanation for the puzzling phasing of the VHE peak in LS I +61303.

SECONDARY EMISSION BEHIND THE RADIO OUTFLOWS IN GAMMA-RAY BINARIES

Several binary systems consisting of a massive star and a compact object have been detected above 100 GeV in the Galaxy. In most of these sources, gamma-rays show a modulation associated to the orbital motion, which means that the emitter should not be too far from the bright primary star. This implies that gamma-ray absorption will be non-negligible, and large amounts of secondary electron–positron pairs will be created in the stellar surroundings. In this work, we show that the radio emission from these pairs should be accounted for when interpreting the radio spectrum, variability, and morphology found in gamma-ray binaries. Relevant features of the secondary radio emission are the relatively hard spectrum, the orbital motion of the radio peak center and the radio structure extension following a spiral-like trajectory. The impact of the stellar wind free–free absorption should not be neglected.

On the possibility of sub-TeV -ray emission from Cyg X-3

The compact X-ray binary system Cyg X-3 has been recently discovered as a source of GeV gamma-rays by the AGILE and the {\it Fermi} satellites. It shows emission features in the GeV gamma-rays similar to other gamma-ray binaries which were also observed in the TeV gamma-rays (LS 5039 and LSI +61 303). The question appears whether Cyg X-3 can be also detected in the TeV gamma-rays by the Cherenkov telescopes. Here we discuss this problem in detail based on the anisotropic inverse Compton (IC) e-p pair cascade model successfully applied to TeV gamma-ray binaries. We calculate the gamma-ray light curves and gamma-ray spectra expected from the cascade process occurring inside the Cyg X-3 binary system. It is found that the gamma-ray light curves at GeV energies can be consistent with the gamma-ray light curve observed by the Fermi for reasonable parameters of the orbit of the injection source of relativistic electrons. Moreover, we show that in such a model the sub-TeV gamma-ray emission (above 100 GeV) is expected to be below sensitivities of the present Cherenkov telescopes assuming that electrons are accelerated in Cyg X-3 to TeV energies. The next stage Cherenkov telescopes (MAGIC II, HESS II) should have the energy threshold in the range 20-30 GeV, in order to have a chance to detect the signal from Cyg X-3. Otherwise, the positive detection of gamma-rays at energies above a few tens of GeV requires a telescope with the sensitivity of ~0.1% of Crab Units. We conclude that detection of sub-TeV gamma-rays from Cyg X-3 by on-ground telescopes has to probably wait for construction of the Cherenkov Telescope Array (CTA).

HESS upper limits on very high energy gamma-ray emission from the microquasar GRS 1915+105

Context. High energy particles reside in the relativistic jets of microquasars, making them possible sources of very high energy radiation (VHE, >100 GeV). Detecting this emission would provide a new handle on jet physics. Aims. Observations of the microquasar GRS 1915+105 with the HESS telescope array were undertaken in 2004-2008 to search for VHE emission. Methods. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. Results. There is no evidence for a VHE gamma-ray signal either from the direction of the microquasar or its vicinity. An upper limit of $6.1\times 10^$ ph cm-2 s-1 (99.9% confidence level) is set on the photon flux above 410 GeV, equivalent to a VHE luminosity of ${\sim} 10^$ erg s-1 at 11 kpc. Conclusions. The VHE to X-ray luminosity ratio in GRS 1915+105 is at least four orders of magnitude lower than the ratio observed in gamma-ray binaries. The VHE radiative efficiency of the compact jet is less than 0.01% based on its estimated total power of 1038 erg s-1. Particle acceleration in GRS 1915+105 is not efficient at high energies and/or the magnetic field is too strong. It is also possible that VHE gamma-rays are produced by GRS 1915+105, but the emission is highly time-dependent.

Gamma-ray emission from pulsar/massive-star binaries

AbstractI present a review of the main phenomenological properties at high energies related to massive gamma-ray binaries and I discuss some aspects of pulsar models for these objects.

One-dimensional pair cascade emission in gamma-ray binaries

In gamma-ray binaries such as LS 5039 a large number of electron-positron pairs are created by the annihilation of primary very high energy (VHE) gamma-rays with photons from the massive star. The radiation from these particles contributes to the total high energy gamma-ray flux and can initiate a cascade, decreasing the effective gamma-ray opacity in the system. The aim of this paper is to model the cascade emission and investigate if it can account for the VHE gamma-ray flux detected by HESS from LS 5039 at superior conjunction, where the primary gamma-rays are expected to be fully absorbed. A one-dimensional cascade develops along the line-of-sight if the deflections of pairs induced by the surrounding magnetic field can be neglected. A semi-analytical approach can then be adopted, including the effects of the anisotropic seed radiation field from the companion star. Cascade equations are numerically solved, yielding the density of pairs and photons. In LS 5039, the cascade contribution to the total flux is large and anti-correlated with the orbital modulation of the primary VHE gamma-rays. The cascade emission dominates close to superior conjunction but is too strong to be compatible with HESS measurements. Positron annihilation does not produce detectable 511 keV emission. This study provides an upper limit to cascade emission in gamma-ray binaries at orbital phases where absorption is strong. The pairs are likely to be deflected or isotropized by the ambient magnetic field, which will reduce the resulting emission seen by the observer. Cascade emission remains a viable explanation for the detected gamma-rays at superior conjunction in LS 5039.

THE ORBITS OF THE γ-RAY BINARIES LS I +61 303 AND LS 5039

LS I +61 303 and LS 5039 are two of only a handful of known high mass X-ray binaries (HMXBs) that exhibit very high energy emission in the MeV-TeV range, and these "gamma-ray binaries" are of renewed interest due to the recent launch of the Fermi Gamma-ray Space Telescope. Here we present new radial velocities of both systems based on recent red and blue optical spectra. Both systems have somewhat discrepant orbital solutions available in the literature, and our new measurements result in improved orbital elements and resolve the disagreements. The improved geometry of each orbit will aid in studies of the high energy emission region near each source.

GAMMA-RAY VARIABILITY FROM WIND CLUMPING IN HIGH-MASS X-RAY BINARIES WITH JETS

In the subclass of high-mass X-ray binaries known as "microquasars", relativistic hadrons in the jets launched by the compact object can interact with cold protons from the star's radiatively driven wind, producing pions that then quickly decay into gamma rays. Since the resulting gamma-ray emissivity depends on the target density, the detection of rapid variability in microquasars with GLAST and the new generation of Cherenkov imaging arrays could be used to probe the clumped structure of the stellar wind. We show here that the fluctuation in gamma rays can be modeled using a "porosity length" formalism, usually applied to characterize clumping effects. In particular, for a porosity length defined by h=l/f, i.e. as the ratio of the characteristic size l of clumps to their volume filling factor f, we find that the relative fluctuation in gamma-ray emission in a binary with orbital separation a scales as sqrt(h/pi a) in the "thin-jet" limit, and is reduced by a factor 1/sqrt(1 + phi a/(2 l)) for a jet with a finite opening angle phi. For a thin jet and quite moderate porosity length h ~ 0.03 a, this implies a ca. 10 % variation in the gamma-ray emission. Moreover, the illumination of individual large clumps might result in isolated flares, as has been recently observed in some massive gamma-ray binaries.

Diffusive shock acceleration in radiation-dominated environments

<i>Context. <i/>Radio, X-ray, and gamma-ray observations provide us with strong evidence of particle acceleration to multi-TeV energies in various astrophysical sources. Diffusive shock acceleration is one of the most successful models explaining the presence of such high-energy particles.<i>Aims. <i/>We discuss the impact of inverse Compton losses on the shock acceleration of electrons that takes place in radiation dominated environments, i.e. in regions where the radiation energy density exceeds that of the magnetic field.<i>Methods. <i/>We perform a numerical calculation, including an energy-loss term in the transport equation of accelerated particles.<i>Results. <i/>We discuss the implications of this effect on the hard X-ray synchrotron and gamma-ray inverse Compton radiation, produced by shock-accelerated electrons in young supernova remnants in the presence of large radiation fields (e.g. in the Galactic centre). We also discuss possible implications of our results for clusters of galaxies and gamma-ray binaries.<i>Conclusions. <i/>We demonstrate that the inverse Compton losses of electrons, in the Klein-Nishina regime, lead to spectra of ultra-relativistic electrons that may significantly differ from classical diffusive shock acceleration solution. The most prominent feature is the appearance of a pile-up in the spectrum around the cut-off energy.

MODELS FOR GAMMA-RAY PRODUCTION IN LOW-MASS MICROQUASARS

Unlike high-mass gamma-ray binaries, low-mass microquasars lack external sources of radiation and matter that could produce high-energy emission through interactions with relativistic particles. In this work, we consider the synchrotron emission of protons and leptons that populate the jet of a low-mass microquasar. In our model photohadronic and inverse Compton (IC) interactions with synchrotron photons produced by both protons and leptons result in a high-energy tail of the spectrum. We also estimate the contribution from secondary pairs injected through photopair production. The high-energy emission is dominated by radiation of hadronic origin, so we can call these objects "proton microquasars".

High energy gamma-ray emission from binaries

The current Cherenkov telescopes together with GLAST are opening up a new window into the physics at work close to black holes and rapidly rotating neutron stars with great breakthrough potential. Very high energy gamma-ray emission up to 10 TeV is now established in several binaries. The radiative output of gamma-ray binaries is in fact dominated by emission above 1–10 MeV. Most are likely powered by the rotational spindown of a young neutron star that generates a highly relativistic wind. The interaction of this pulsar wind with the companion’s stellar wind is responsible for the high energy gamma-ray emission. There are hints that microquasars, accretion-powered binaries emitting relativistic jets, also emit gamma-ray flares that may be linked to the accretion–ejection process. Studying high energy gamma-ray emission from binaries offers good prospects for the study of pulsar winds physics and may bring new insights into the link between accretion and ejection close to black holes.