Searcharxiv⌕ Search

arXiv subjects

W. Bednarek

Publications and source records attributed to W. Bednarek.

269 records · Page 15Linked to original sources
arXiv2000

Escape of VHE gamma-rays from close massive binary Cen X-3

We consider propagation of very high energy (VHE) gamma-rays in the radiation field of a massive star in the binary system Cen X-3, which has been reported as a source of gamma-ray photons in the GeV and TeV energies. VHE gamma-rays or electrons, injected by the compact object, should develop inverse Compton pair cascades. We predict the gamma-ray spectra and light curves for the parameters of Cen X-3 system. It is found that the gamma-ray spectra, observed at different directions, have different shape and intensity. The gamma-ray light curves, produced in the case of electron injection by the compact object in the Cen X-3 system, should have opposite tendencies for photons with energies above 100 MeV and above 300 GeV, i.e. the photon intensities increases with phase in the first case and decreases with phase in the second case. However the model with injection of primary electrons seems to be in contrary with the reported modulation of the GeV gamma-ray flux with the pulsar's period. The model with injection of primary photons allows such modulation with the pulsar's period, but predicts strong modulation of the TeV flux with the orbital period of the binary. Modulation of TeV emission with the orbital period has been reported by the early Cherenkov observations, but was not confirmed by the recent, more sensitive observations by the Durham Mark 6 telescope.

astro-ph↗
arXiv1999

Cascades initiated by EHE photons in the magnetic field of the Earth and the Sun

The content of extremely high energy (EHE) photons in the highest energy cosmic rays can be investigated by analyzing showers arriving from directions where the perpendicular component of the Earth's magnetic field is high or showers arriving from the region surrounding the Sun. We perform Monte Carlo simulations of cascades initiated by photons with parameters of the highest energy showers observed by the past and present detectors (AGASA, Fly's Eye, Yakutsk, Haverah Park). The purpose is to find out which events should cascade with high probability if initiated by photons. It is shown that EHE photons arriving from directions towards the magnetic poles cascade with higher probability. Alternatively, the lowest probabilities of cascading are expected for photons arriving from directions of the equator at the zenith angles equal to the angle between location of the specific observatory and the magnetic pole. We show that very unusual showers should arrive from direction of the Sun due to cascading of photons in the Sun's magnetosphere if EHE photons are numerous at the highest energies. The rate of such showers is estimated on about one per ten years. However extraordinary lateral distribution of secondary particles of such showers in the Earth's atmosphere may help to distinguish them from the ordinary showers arriving isotropically from the sky.

astro-ph↗
arXiv1999

Gamma-rays from Collisions of Compact Objects with AGN Jets?

We consider different scenarios of collisions of compact objects (clouds, massive stars, supernova shock waves, or young pulsars) with jets in active galactic nuclei. The purpose is to find out if such collisions can become plausible explanations for the gamma-ray production in blazars. We conclude that the relativistic proton beam - cloud collision scenario has problems with explanation of the $γ$-ray spectrum in the observed energy range from blazars. As a result of collisions of massive stars, supernova shock waves, or young pulsars with the jet plasma, a highly oblique shocks should be formed. These shocks can accelerate electrons, protons to energies high enough for production of $γ$-ray photons observed in blazars. However, in order to produce observable $γ$-ray fluxies, the particles should be accelerated strongly anisotropicly, which may be the case of highly oblique relativistic shocks in jets of AGNs.

astro-ph↗
arXiv1999

SN 1998bw--GRB 980425: A supernova explosion with a close massive companion?

Some gamma ray bursts may be produced by supernovae exploding in close massive binary systems (type Ib/c supernovae) as suggested by the recent observation of SN 1998bw/GRB 980425. We propose that high energy radiation observed in such gamma ray bursts may be produced by synchrotron radiation of electrons accelerated by 1st order Fermi acceleration at a quasi-stationary shock in the high velocity SN ejecta colliding with the companion star or some other nearby massive object. Nuclei would also be accelerated, and could give rise to an observable fluence of high energy neutrinos at Earth. This paper has been withdrawn by the authors following evidence that SN 1998bw and GRB 980425 are two distinct sources.

astro-ph↗
arXiv1999

The physical parameters of Markarian 501 during flaring activity

We determine the physical parameters (magnetic field and Doppler factor) of the homogeneous synchrotron self-Compton model allowed by the observed X-ray to gamma-ray spectra and variability of Markarian~501 during the 15-16 April 1997 flaring activity. We find that magnetic fields between 0.07 G and 0.6 G and Doppler factors between 12 and 36 could fit (depending on observed variability time scale) these observations. We take account of photon-photon pair production interactions of gamma-ray photons occurring both inside the emission region and during propagation to Earth and find these to be extremely important in correctly determining the allowed model parameters. Previous estimates of the allowed parameter space have neglected this effect. Future multi-wavelength campaigns during strong flaring activity, including observations from optical to TeV gamma-rays, should enable the physical parameters to be further constrained.

astro-ph↗
arXiv1998

On the application of the mirror model for gamma-ray flare in 3C 27

Multiwavelength observations of high energy flare in 1996 from 3C 279 seems to favour the so called mirror model between different inverse Compton scattering models proposed as a possible explanation of gamma-ray emission in blazars. We performed kinematic analysis of the relativistic blob - mirror system and found that only part of the mirror located very close to the jet axis (very likely inside the jet cone) can re-emit soft photons which serve as a target for production of $γ$-rays by relativistic electrons in the blob. Since the presence of well localized scattering mirror inside the jet is problematic, this makes problems for the mirror model. The time scale and the shape of the $γ$-ray flare should reflect, in terms of the mirror model, the blob dimensions and the longitudinal distribution of relativistic electrons inside the blob. For the $γ$-ray light curve of the type observed in 1996 from 3C 279, i.e. the rising time of the flare during a few days with a sharp cut-off towards the end of the flare, the density of electrons inside the blob should increase exponentially starting from the front of the blob and reach maximum towards the end of the blob. Such distribution of electrons is difficult to explain in a model of a relativistic shock moving along the jet, which would rather inject electrons more efficiently at the front of the blob with a trail of particles on its downstream side.

astro-ph↗
arXiv1998

Gamma-ray and neutrino flares produced by protons accelerated on an accretion disk surface in AGN

We discuss the consequences of almost rectilinear acceleration of protons to extremely high energies in a reconnection region on the surface of an accretion disk which surrounds a central black hole in an active galaxy. The protons produce $γ$-rays and neutrinos in interactions with the disk radiation as considered in several previous papers. However, in this model the secondary $γ$-rays can initiate cascades in the magnetic and radiation fields above the disk. We compute the spectra of $γ$-rays and neutrinos emerging from regions close to the disk surface. Depending on the parameters of the reconnection regions, this model predicts the appearance of $γ$-ray and neutrino flares if protons takes most of the energy from the reconnection region. In contrast, if leptons take most of the energy, they produce pure $γ$-ray flares. The $γ$-ray spectrum expected in the case of hadronic cascading is compared with the spectrum observed during the flare in June 1991 from 3C 279. The neutrino flares which should accompany these gamma-ray flares may be detected by future large scale neutrino telescopes sensitive at $\sim 10^{5}$ TeV.

astro-ph↗
arXiv1998

On the Hadronic Beam Model for Gamma-ray Production in Blazars

We consider, herein, a model for gamma-ray production in blazars in which a relativistic, highly-collimated electron-proton beam interacts with a dense, compact cloud as the jet propagates through the broad and perhaps narrow line regions (BLR and NLR) of active galactic nuclei (AGN). During the propagation of the beam through the cloud, the process of excitation of plasma waves becomes an important energy loss mechanism, especially for mildly relativistic proton beams. We compute the expected spectra of gamma-rays from the decay of neutral pions produced in hadronic collisions of the beam with the cloud, taking into account collisionless losses of the electron-proton beam. This model may explain the X-ray and TeV gamma-ray (both low and high emission states) of Mrk 421 as a result of synchrotron emission of secondary pairs from the decay of charged pions and gamma-ray emission from the decay of neutral pions for the plausible cloud parameters. However clouds can not be too hot and too dense. Otherwise the TeV gamma-rays can be attenuated by the bremsstrahlung radiation in the cloud and the secondary pairs are not able to efficiently produce synchrotron flares because of the dominant role of inverse Compton scattering. The non-variable $γ$-ray emission observed from Mrk 421 in the EGRET energy range cannot be described by the $γ$-rays from decay of neutral pions provided that the spectrum of protons in the beam is well described by a simple power law. These $γ$-rays might only be produced by secondary pairs scattering the soft non-variable X-rays which might originate in the inner part of the accretion disk.

astro-ph↗
arXiv1997

Inverse Compton scattering model for gamma-ray production in Mev blazars

It is proposed that the spectra of so called 'MeV blazars' can be explained in terms of previously developed models of the external comptonization of accretion disk radiation provided that the structure of the inner and outer parts of the accretion disk is different. The electron acceleration is saturated by the inverse Compton losses in the inner geometrically thick disk and the outer geometrically thin disk at different maximum energies which causes the appearance of two spectral components, one strongly peaked in the MeV energy range and the second one of a power law type extending through the GeV energy range. The spectra, computed in terms of such a simple geometrical model, are in a good agreement with observations of the MeV blazar PKS0208-512. They are consistent with the transient appearance of a strong MeV peak, the power law spectrum in the EGRET energy range, and a possible cut-off at high energies.

astro-ph↗
arXiv1997

Gamma-Rays and Neutrinos from Very Young Supernova Remnants

We consider the result of acceleration of heavy ions in the slot gap potential of a very young pulsar with a hot polar cap. Photodisintegration of the heavy ions in the radiation field of the polar cap and pulsar surface gives rise to a flux of energetic neutrons. Some fraction of these neutrons interact with target nuclei in the supernova shell to produce neutrino and gamma-ray signals which should be observable from very young supernova remnants in our galaxy for a range of pulsar parameters.

astro-ph↗
arXiv1997

Testing the homogeneous synchrotron self Compton model for gamma ray production in Mrk 421

Based on the detected variability time scales of X-ray and TeV gamma-ray emission, and the observed multiwavelength photon spectrum, of Mrk 421 we place constraints on the allowed parameter space (magnetic field and Doppler factor of the emission region) for the homogeneous synchrotron self-Compton model. The spectra calculated for the allowed parameters are marginally consistent with the available spectral information above $\sim 1$ TeV reported by the Whipple Observatory in the case of a 1 day flare time scale. However, for the recently reported very short duration flares varying on a time scale of 15 min, the calculated spectra are significantly steeper, suggesting that the homogeneous synchrotron self Compton model has problems in describing the relatively flat observed spectra extending above a few TeV. We determine the maximum ratio of TeV gamma-ray luminosity to X-ray luminosity during flaring which is allowed by the homogeneous synchrotron self-Compton model for the case of no significant photon-photon absorption in the source.

astro-ph↗
arXiv1997

Gamma rays and neutrinos from the Crab Nebula produced by pulsar accelerated nuclei

We investigate the consequences of the acceleration of heavy nuclei (e.g. iron nuclei) by the Crab pulsar. Accelerated nuclei can photodisintegrate in collisions with soft photons produced in the pulsar's outer gap, injecting energetic neutrons which decay either inside or outside the Crab Nebula. The protons from neutron decay inside the nebula are trapped by the Crab Nebula magnetic field, and accumulate inside the nebula producing gamma-rays and neutrinos in collisions with the matter in the nebula. Neutrons decaying outside the Crab Nebula contribute to the Galactic cosmic rays. We compute the expected fluxes of gamma-rays and neutrinos, and find that our model could account for the observed emission at high energies and may be tested by searching for high energy neutrinos with future neutrino telescopes currently in the design stage.

astro-ph↗
arXiv1996

Modulation of AGN gamma rays by interaction with X-rays from an accretion disk hot spot

We have developed a model for the variability of gamma ray emission in jets of active galactic nuclei in which the variability arises as a result of photon-photon pair production interactions with X-rays emitted by a hot spot in the inner part of the accretion disk. As the hot spot orbits around the central engine, the amount of absorption varies periodically. Our model may account for the observed variability of TeV emission from Markarian 421 and other blazars detected by the EGRET instrument on the Compton Gamma Ray Observatory, as well as correlated variations of X-ray and TeV gamma flux from Markarian 421. Quasi-periodic variations observed in Markarian 421 enable us to place an upper limit on the black hole mass of (2-30) times 10^8 solar masses.

astro-ph↗
arXiv1996

Gamma rays from interactions of stars with AGN jets

We have developed a model for gamma ray emission in jets of active galactic nuclei in which particle acceleration takes place at a shock in the relativistic jet plasma due to a massive star in the central region of the host galaxy moving through the jet. The gamma rays are produced in a pair-Compton cascade in the radiation field of the star initiated by accelerated electrons. Our model may account for the observed GeV to TeV gamma ray spectrum and variability of Markarian 421 and other blazars detected by the EGRET instrument on the Compton Gamma Ray Observatory.

astro-ph↗
arXiv1996

On the production of very high energy beamed gamma-rays in blazars

The variable flux of TeV gamma-rays detected from Mkn 421 and Mkn 501 requires the presence of high energy electrons, which could in principle produce large numbers of electron/positron pairs, leading to an electromagnetic cascade. We point out that this scenario can be avoided if electrons are accelerated to high energy rectilinearly, rather than being injected isotropically into a blob, as in most of the models of the GeV gamma-ray emission. By balancing linear acceleration by an electric field against inverse Compton losses in the radiation field of the accretion disk we calculate the emitted spectra and find the conditions which must be fulfilled in order to exclude the development of electromagnetic cascades during acceleration. Assuming these to be fulfilled, we show that the maximum possible photon energy is approximately $10M_8^{2/5}\,$TeV, where $M_8$ is the mass of the central black hole in units of $10^8\,\msolar$. In addition we compute the optical depth to absorption of TeV photons on a possible isotropic scattered component and on the observed nonthermal radiation (in the case of Mkn 421) and find that TeV photons can escape provided the nonthermal X-rays originate in a jet moving with a Lorentz factor of around 8.

astro-ph↗
arXiv1994

Gamma-Rays from the Funnels of Thick Accretion Disks Around Active Galactic Nuclei

We analyse the propagation of particles in the narrow funnel of a thick accretion disk. It is assumed that: (1) the funnel walls emit black body radiation with temperature decreasing outwards; (2) the magnetic and electric fields are longitudinal in the funnel. Such a scenario is consistent with models in which a large potential drop is induced by a rotating massive black hole threaded by a magnetic field (Blandford & Znajek 1977, Macdonald & Thorne 1982). The interaction of relativistic protons with thermal photons from the funnel results in direct production of pairs and/or pions. We discuss the relative importance of these processes for different conditions in the funnel (temperature and electric field profiles). Injected pairs interact with thermal photons in the Thomson or Klein-Nishina regime. Under some conditions, a pair avalanche results, which we assume saturates in the production of stable bunches, containing almost equal numbers of electrons and positrons. As a consequence, highly collimated gamma-ray photons are produced. We obtain gamma-ray spectra from our model in order to test its applicability to the gamma-ray emitting AGNs recently reported by EGRET.

astro-ph↗