ABSTRACT
Shocked winds of massive stars in young stellar clusters have been proposed as possible sites in which relativistic particles are accelerated. Electrons, accelerated in such an environment, ...are expected to efficiently Comptonize optical radiation (from massive stars) and the infrared radiation (rescattered by the dust within the cluster) producing GeV–TeV γ-rays. We investigate the time-dependent process of acceleration, propagation, and radiation of electrons in the stellar wind of the massive star Θ1 Ori C within the Trapezium Cluster. This cluster is located within the nearby Orion Nebula (M42). We show that the γ-ray emission expected from the Trapezium Cluster is consistent with the present observations of the Orion molecular cloud by the Fermi-Large Area Telescope provided that the efficiency of energy conversion from the stellar wind to relativistic electrons is very low, i.e. χ < 10−4. For such low efficiencies, the γ-ray emission from electrons accelerated in the stellar wind of Θ1 Ori C can be only barely observed by the future Cherenkov telescopes, e.g. the Cherenkov Telescope Array.
ABSTRACT
Galactic novae are at present a well established class of γ-ray sources. We wonder for how long the mechanism of acceleration of electrons operates in the shells of novae. In order to put ...constraints on the time-scale of the electron acceleration, we consider a specific model for the injection and propagation of electrons within the shell of the recurrent nova RS Ophiuchi. We calculate the equilibrium spectra of electrons within the nova shell and the γ-ray fluxes produced by these electrons in the Comptonization of the soft radiation from the red giant within a nova binary system and also radiation from the nova photosphere. We investigate a two-component time-dependent model in which a spherically ejected nova shell propagates freely in the polar region of a nova binary system. However, the shell is significantly decelerated in the dense equatorial region of the binary system. We discuss the conditions under which electrons can produce γ-rays that might be detectable by present and/or future γ-ray observatories. It is concluded that freely expanding shells of novae in the optimal case (strongly magnetized shell and efficiency of acceleration of electrons of the order of 10 per cent) can produce TeV γ-rays within the sensitivity of the Cherenkov Telescope Array within 1–2 yr of explosion only. On the other hand, decelerated shells of novae have a chance to be detected during the whole recurrence period of RS Ophiuchi, i.e. ∼15 yr.
ABSTRACT
A part of early-type stars is characterized by strong dipole magnetic field that is modified by the outflow of dense wind from the stellar surface. At some distance from the surface (above ...the Alfvén radius), the wind drives the magnetic field into the reconnection in the equatorial region of the dipole magnetic field. We propose that electrons accelerated in these reconnection regions can be responsible for efficient Comptonization of stellar radiation producing gamma-ray emission. We investigate the propagation of electrons in the equatorial region of the magnetosphere by including their advection with the equatorial wind. The synchrotron and Inverse Compton (IC) spectra are calculated assuming that a significant part of the wind energy is transferred to relativistic electrons. As an example, the parameters of luminous, strongly magnetized star HD 37022 (Θ1 Ori C) are considered. The IC gamma-ray emission is predicted to be detected either in the GeV energy range by the Fermi-LAT telescope or in the sub-TeV energies by the Cherenkov Telescope Array. However, since the stellar winds are often time variable and the magnetic axis can be inclined to the rotational axis of the star, the gamma-ray emission is expected to show variability with the rotational period of the star and, on a longer time-scale, with the stellar circle of the magnetic activity. Those features might serve as tests of the proposed scenario for gamma-ray emission from single, luminous stars.
Abstract AR Scorpii is a close binary system containing a rotation powered white dwarf and a low-mass M type companion star. This system shows non-thermal emission extending up to the X-ray energy ...range. We consider hybrid (lepto-hadronic) and pure hadronic models for the high energy non-thermal processes in this binary system. Relativistic electrons and hadrons are assumed to be accelerated in a strongly magnetised, turbulent region formed in collision of a rotating white dwarf magnetosphere and a magnetosphere/dense atmosphere of the M-dwarf star. We propose that the non-thermal X-ray emission is produced either by the primary electrons or the secondary e± pairs from decay of charged pions created in collisions of hadrons with the companion star atmosphere. We show that the accompanying γ-ray emission from decay of neutral pions, which are produced by these same protons, is expected to be on the detectability level of the present and/or the future satellite and Cherenkov telescopes. The γ-ray observations of the binary system AR Sco should allow us to constrain the efficiency of hadron and electron acceleration and also the details of the radiation processes.
TeV γ rays are observed from a few nearby radio galaxies whose jets are viewed at relatively large angles toward the observer. This emission can be produced in kiloparsec-scale jets whose Lorentz ...factors are decelerated from values of the order of several at parsec-scale distances. We consider in detail the model in which TeV γ-ray emission is produced by the relativistic electrons in kiloparsec-scale jets that comptonize strongly beamed radiation from inner (parsec-scale) jets. As an example, we study the morphology of the TeV γ-ray emission from the decelerated kiloparsec-scale jet in the nearby radio galaxy Cen A. We show that TeV γ-ray emission can extend throughout kiloparsec-scale distances, as it is relatively smoothly distributed along jets for some parameters of the model we consider. Investigation of the morphological structure of such specific γ-ray emission by the future Cerenkov Telescope Array should provide important constraints on the content and dynamics of the kiloparsec-scale jet in Cen A.
ABSTRACT We investigate the consequences of acceleration of nuclei in jets of active galaxies not far from the surface of an accretion disk. The nuclei can be accelerated in the re-connection regions ...in the jet and/or at the jet boundary, between the relativistic jet and its cocoon. It is shown that the relativistic nuclei can efficiently fragment onto specific nucleons in collisions with the disk radiation. Neutrons, directed toward the accretion disk, take a significant part of energy from the relativistic nuclei. These neutrons develop a cascade in the dense accretion disk. We calculate the neutrino spectra produced in such a hadronic cascade within the accretion disk. We propose that the neutrinos produced in such a scenario, from the whole population of super-massive black holes in active galaxies, can explain the extragalactic neutrino background recently measured by the IceCube neutrino detector, provided that a 5% fraction of galaxies have an active galactic nucleus and a few percent of neutrons reach the accretion disk. We predict that the neutrino signals in the present neutrino detectors, produced in terms of such a model, will not be detectable even from the nearby radio galaxies similar to M87.
Context. A significant number of the millisecond pulsars (MSPs) have been discovered within binary systems. Tens of these MSPs emit γ-rays that are modulated with the pulsar period since this ...emission is produced in the inner pulsar magnetosphere. In several such binary systems, the masses of the companion stars have been derived allowing two classes of objects to be distinguished, which are called the black widow and the redback binaries. Pulsars in these binary systems are expected to produce winds that create conditions for acceleration of electrons, when colliding with stellar winds. These electrons should interact with the anisotropic radiation from the companion stars producing γ-ray emission modulated with the orbital period of the binary system, similar to what is observed in the massive TeV γ-ray binary systems. Aims. We consider the interaction of a MSP wind with a very inhomogeneous stellar wind from the companion star within binary systems of the black widow and redback types. Our aim is to determine the features of γ-ray emission produced in the collision region of the winds from a few typical MSP binary systems. Methods. It is expected that the pulsar wind should mix efficiently with the inhomogeneous stellar wind. The mixed winds move outside the binary with relatively low velocity. Electrons accelerated in such mixed, turbulent winds can interact with the magnetic field and strong radiation from the companion star, producing not only synchrotron radiation but also γ-rays in the inverse Compton process, fluxes of which are expected to be modulated on the periods of the binary systems. Applying numerical methods, we calculated the GeV-TeV gamma-ray spectra and the light curves expected from some MSP binary systems. Results. Gamma-ray emission, produced within the binary systems, is compared with the sensitivities of the present and future gamma-ray telescopes. It is concluded that energetic MSP binary systems create a new class of TeV γ-ray sources that could be detectable by the future Cherenkov arrays (e.g., CTA) and possibly also by the extensive campains with the present arrays (HESS, MAGIC, VERITAS). However, γ-ray emission from the MSP binary systems is predicted to have different features than those observed in the case of massive TeV gamma-ray binaries such as LS I 303 61 or LS 5039. The maximum in the TeV γ-ray orbital light curve should appear when the MSP is behind the companion star. This is in contrast to the observations of the orbital light curves from the massive TeV γ-ray binaries (LS I 303 61 or LS 5039). Moreover, the GeV and orbital TeV γ-ray light curves should be positively correlated unlike the case of massive TeV γ-ray binaries. Conclusions. We conclude that TeV γ-ray emission, modulated on the orbital period of MSP binary systems, should be detected by the future CTA. Moreover, some MSP binary systems of the Redback type might also show GeV γ-ray emission modulated on the binary periods on the level detectable by Fermi-LAT.
Long expected transition states between the rotation powered and accretion powered non-thermal emission in the millisecond pulsar binary systems have been recently observed in the case of three ...objects PSR J1023+0038, PSR J1824−2452, and PSR J1227−4859. Surprisingly, the transition is related to the significant change in the γ-ray flux being a factor of a few higher with the presence of an accretion disc. The origin of this enhanced emission seems to be related to the penetration of the inner pulsar magnetosphere by the accretion disc. We propose that the radiation processes, characteristic for the rotation powered pulsar, can co-exist with the presence of an accretion disc in the inner pulsar magnetosphere. In our scenario additional γ-ray emission is produced by secondary leptons, originated close to the acceleration gap, which Compton up-scatter thermal radiation from the accretion disc to GeV energies. The accretion disc penetrates deep into the pulsar magnetosphere allowing the matter to fall on to the neutron star surface producing pulsed X-ray emission. We show that the sum of the rotation powered pulsar γ-ray emission, produced by the primary electrons in the curvature process, and the γ-ray emission, produced by secondary leptons, can explain the observed high-energy radiation from the redback binary pulsar PSR J1227−4853 in the state with evidences of the accretion disc.
ABSTRACT
γ-ray emission in active galaxies is likely produced within the inner jet, or in the close vicinity of the supermassive black hole (SMBH) at sub-parsec distances. γ-rays have to pass through ...the surrounding massive stellar cluster in which luminous stars can accidentally appear close to the observer’s line of sight. In such a case, soft radiation of massive stars can create enough target for transient absorption of the γ-rays in multi-GeV to TeV energy range. We consider the effect of such stellar encounters on the γ-ray spectrum produced within the massive stellar cluster surrounding a central SMBH. We predict characteristic, time-dependent effects on the γ-ray spectra due to the encounter with the single luminous star and also stellar binary system. We conclude that during the encounter, the γ-ray spectrum of an active galaxy should steepen at tens of GeV and harden in the range of hundreds of GeV. As an example, we consider such effects on the spectra observed from a typical blazar, 1ES 1959+650 (in an active state) and also in the case of a radio galaxy M87 (in a low state). It is shown that observation of such transient characteristic features in the γ-ray spectra, observed from blazars and radio galaxies, lays within the sensitivity of the future Cherenkov Telescope Array.
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