arXiv subjects

W. Bednarek

Publications and source records attributed to W. Bednarek.

At least 235 records · Page 13Linked to original sources

arXiv2009

Upper limits for pulsars with MAGIC (2005/2006 observations)

During the last 4 years the MAGIC collaboration has searched for high-energy gamma-ray emission of some of the most promising pulsar candidates. The low energy threshold of MAGIC offered the opportunity for a high sensitivity search just above 50-100 GeV (since 2008 a new trigger system has decreased the energy threshold to 25 GeV), an energy region up to now not reachable to ground based instruments and past satellite borne detectors. No pulsed gamma-ray emission has been detected from any pulsar observed during the observation campaigns of 2005-2006. Here we present the upper limits obtained for tow canonical pulsars (PSR J0205+6449 and PSR J2229+6114) and their host nebulae (3C 58 and Boomerang, respectively) and the millisecond pulsar PSR J0218+4232. Physics implications will be discussed.

astro-ph.HE↗

arXiv2009

MAGIC observation of Globular Cluster M13 and its millisecond pulsars

Based on MAGIC observations from June and July 2007, we present upper limits to the E>140 GeV emission from the globular cluster M13. Those limits allow us to constrain the population of millisecond pulsars within M13 and to test models for acceleration of leptons inside their magnetospheres and/or surrounding. We conclude that in M13 either millisecond pulsars are fewer than expected or they accelerate leptons less efficiently than predicted.

astro-ph.HE↗

arXiv2009

TeV Gamma-rays from accreting magnetars in massive binaries

We consider the neutron star (NS) of the magnetar type inside the massive binary system. We determine the conditions under which the matter from the stellar wind can penetrate the inner magnetosphere of the magnetar. At some distance from the NS surface, the magnetic pressure can balance the gravitational pressure of the accreting matter creating very turbulent, magnetized transition region. It is suggested that this region provides good conditions for acceleration of electrons to relativistic energies. These electrons lose energy on the synchrotron process and the Inverse Compton (IC) scattering of the radiation from the nearby massive stellar companion, producing high energy radiation from the X-rays up to $\sim$TeV $γ$-rays. The primary $γ$-rays can be farther absorbed in the stellar radiation developing the IC $e^\pm$ pair cascade. We calculate the synchrotron X-ray emission from primary electrons and secondary $e^\pm$ pairs and the IC $γ$-ray emission from the cascade process. It is shown that the quasi-simultaneous observations of the TeV $γ$-ray binary system LSI +61 303 in the X-ray and the TeV $γ$-ray energy ranges can be explained in such an accreting magnetar model.

astro-ph.HE↗

arXiv2008

Gamma-rays from the vicinity of accreting neutron stars inside compact high-mass X-ray binaries

Dense wind of a massive star can be partially captured by a neutron star (NS) inside a compact binary system. Depending on the parameters of NS and the wind, the matter can penetrate the inner NS magnetosphere. At some distance from the NS a very turbulent and magnetized transition region is formed due to the balance between the magnetic pressure and the pressure inserted by accreting matter. This region provides good conditions for acceleration of particles to relativistic energies. The matter at the transition region can farther accrete onto the NS surface (the accretor phase) or is expelled from the NS vicinity (the propeller phase). We consider the consequences of acceleration of electrons at the transition region concentrating on the situation in which at least part of the matter falls onto the NS surface. This matter creates a hot spot on the NS surface which emits thermal radiation. Relativistic electrons lose energy on the synchrotron process and the inverse Compton (IC) scattering of this thermal radiation. We calculate the synchrotron spectra (from X-rays to soft $γ$-rays) and IC spectra (above a few tens MeV) expected in such a scenario. It is argued that a population of recently discovered massive binaries by the INTEGRAL observatory, which contain neutron stars hidden inside dense stellar winds of massive stars, can be detectable by the recently launched {\it Fermi} LAT telescope at GeV energy range. As an example, we predict the expected $γ$-ray flux from recently discovered source IGR J19140+0951.

astro-ph↗

arXiv2008

A model of the TeV flare of Cygnus X-1: electron acceleration and extended pair cascades

We consider theoretical models of emission of TeV photons by Cyg X-1 during a flare discovered by the MAGIC detector. We study acceleration of electrons to energies sufficient for TeV emission, and find the emission site is allowed to be close to the black hole. We then consider pair absorption in the photon field of the central X-ray source and a surrounding accretion disc, and find its optical depth is <1, allowing emission close to the black hole. On the other hand, the optical depth in the stellar field is about 10 at 1 TeV. However, the optical depth drops with increasing energy, allowing a model with the initial energy of >3 TeV, in which photons travel far away from the star, initiating a spatially extended pair cascade. This qualitatively explains the observed TeV spectrum, though still not its exact shape.

astro-ph↗

arXiv2008

Modification of the gamma-ray spectra by internal absorption in OVV blazars: The example cases of 3C 273 and 3C 279

Recent observations with the low threshold Cherenkov telescopes proved that sub-TeV gamma-rays are able to arrive from active galaxies at relatively large distances in spite of expected severe absorption in the extragalactic background light (EBL). We calculate the gamma-ray spectra at TeV energies from two example OVV quasars, 3C 273 and 3C 279, assuming that gamma-rays are injected in the inner parts of the jets launched by the accretion disks. It is assumed that gamma-rays in the broad energy range (from MeV up to TeV) are produced in these blazars with the power law spectrum with the spectral index as observed from these objects by the EGRET telescope at GeV energies. We take into account the internal absorption of these gamma-rays by considering a few different models for the radiation field surrounding the jet. The classical picture of the relativistic blob in jet model for the injection of primary gamma-rays is considered with the injection rate of gamma-rays as observed by the EGRET telescope in the GeV energy range. The results of calculations are compared with positive detection and the upper limits on the sub-TeV gamma-ray fluxes from these two sources. It is concluded that even with the Stecker EBL model, the level of the gamma-ray emission from 3C~279 is close to the recent measurements in the sub-TeV gamma-ray energies provided that the injected gamma-ray spectrum extends from the GeV energies over the next two decades with this same spectral index. We also suggest that a few day time scale flare from 3C 273 can be detected by the MAGIC II stereo telescopes.

astro-ph↗

arXiv2008

High energy gamma-rays from massive binary systems

During last years a few massive binary systems have been detected in the TeV gamma-rays. This gamma-ray emission is clearly modulated with the orbital periods of these binaries suggesting its origin inside the binary system. In this paper we summarize the anisotropic IC e-p pair cascade model as likely explanation of these observations. We consider scenarios in which particles are accelerated to relativistic energies, either due to the presence of an energetic pulsar inside the binary, or as a result of accretion process onto the compact object during which the jet is launched from the inner part of the accretion disk, or in collisions of stellar winds from the massive companions.

astro-ph↗

arXiv2008

A model for delayed emission in a very-high energy gamma-ray flare in Markarian 501

Recently, the MAGIC collaboration reported evidence for a delay in the arrival times of photons of different energies during a gamma-ray flare from the blazar Markarian 501 on 2005 July 9. We apply a homogeneous synchrotron self-Compton (SSC) model under the assumption that the blob containing relativistic electrons was observed in its acceleration phase. This modified SSC model predicts the appearance of a gamma-ray flare first at lower energies and subsequently at higher energies. Based on the reported time delay of approx. 240 s between the flare observed at 190 GeV and 2.7 TeV, we predict a delay on the order of 1 h if observed between 10 GeV and 100 GeV. Such delay timescales can be tested in the future by simultaneous flare observations with the Gamma Ray Large Area Space Telescope (GLAST) and Cherenkov telescopes.

astro-ph↗

arXiv2007

Gamma-Ray Emission from PWNe Interacting with Molecular Clouds

We consider a situation in which a pulsar is formed inside or close to a high density region of a molecular cloud. Right after birth, the pulsar was very active and accelerated hadrons and leptons to very high energies. Hadrons diffuse through the supernova remnant (SNR) and some of them are trapped in the nearby cloud interacting with the matter. We extend a recent time-dependent model for the gamma-radiation of pulsar wind nebulae (PWNe) to describe this more complicated astrophysical scenario. The example calculations have been performed for two objects, IC443 and W41, which have recently been discovered as sources of TeV gamma-rays. In this model the low energy TeV emission should be correlated with the birth place of the pulsar and the region of dense soft radiation rather than with its present position, provided that the injection rate of relativistic particles into the PWNa has been much more efficient at early times. The high energy TeV emission should be correlated with the location of dense clouds which were able to capture high energy hadrons due to their strong magnetic fields.

astro-ph↗

arXiv2007

The MAGIC Project: Contributions to ICRC 2007

The MAGIC Project: Contributions to ICRC 2007, Merida, Mexico. Contents pages for the Contribution on behalf of the MAGIC Collaboration to the 30th ICRC that took place in July 2007 in Merida, Mexico. The contents are in html form with clickable links to the papers that exist on the Astrophysics archive. We hope that this will make it easier to access the output of the conference in a systematic way. Comments on how useful this is/ how it could be improved should be sent to michela.demaria@iuav.it.

astro-ph↗

arXiv2007

Gamma-Ray Emission from PWNe Interacting with Molecular Clouds

We consider a situation in which a pulsar (and its nebula) is formed inside or close to a high density regions of a molecular cloud. We apply a recent model for the gamma radiation of pulsar wind nebulae (PWN), which includes not only radiation processes due to injected leptons but also processes due to injection of relativistic hadrons, in order to calculate the expected gamma-ray emission from such interacting PWNe. The example calculations have been performed for two objects of this type from which directions TeV gamma-ray sources have recently been observed (IC443 and W41). We show that the gamma-ray emission below a few TeV can be produced by leptons accelerated in the past in the vicinity of the pulsars. gamma-rays with energies above ~10 TeV can be produced by hadrons interacting with the matter inside the supernova remnant and surrounding dense clouds. In contrary to the low energy TeV emission, this high energy TeV emission should be correlated with the location of dense clouds able to capture hadrons due to their strong magnetic fields.

astro-ph↗

arXiv2007

Gamma-ray production in young open clusters: Berk 87, Cyg OB2 and Westerlund 2

Young open clusters are likely sites of cosmic ray acceleration as indicated by recent detections of the TeV gamma-ray sources in the directions of two open clusters (Cyg OB2 and Westerlund 2) and their directional proximity to some unidentified EGRET sources. In fact, up to now a few different scenarios for acceleration of particles inside open clusters have been considered, i.e. shocks in massive star winds, pulsars and their nebulae, supernova shocks, massive compact binaries. Here we consider in detail the radiation processes due to both electrons and hadrons accelerated inside the open cluster. As a specific scenario, we apply the acceleration process at the shocks arising in the winds of WR type stars. Particles diffuse through the medium of the open cluster during the activity time of the acceleration scenario defined by the age of the WR star. They interact with the matter and radiation, at first inside the open cluster and, later in the dense surrounding clouds. We calculate the broad band spectrum in different processes for three example open clusters (Berk 87, Cyg OB2, Westerlund 2) for which the best observational constraints on the spectra are at present available. It is assumed that the high energy phenomena, observed from the X-ray up to the GeV-TeV gamma-ray energies, are related to each other. We conclude that the most likely description of the radiation processes in these objects is achieved in the hybrid (leptonic-hadronic) model in which leptons are responsible for the observed X-ray and GeV gamma-ray emission and hadrons are responsible for the TeV gamma-ray emission, which is produced directly inside and in dense clouds surrounding the open cluster.

astro-ph↗

arXiv2007

MAGIC upper limits on the very high energy emission from GRBs

The fast repositioning system of the MAGIC Telescope has allowed during its first data cycle, between 2005 and the beginning of year 2006, observing nine different GRBs as possible sources of very high energy gammas. These observations were triggered by alerts from Swift, HETE-II, and Integral; they started as fast as possible after the alerts and lasted for several minutes, with an energy threshold varying between 80 and 200 GeV, depending upon the zenith angle of the burst. No evidence for gamma signals was found, and upper limits for the flux were derived for all events, using the standard analysis chain of MAGIC. For the bursts with measured redshift, the upper limits are compatible with a power law extrapolation, when the intrinsic fluxes are evaluated taking into account the attenuation due to the scattering in the Metagalactic Radiation Field (MRF).

astro-ph↗

arXiv2007

High Energy Gamma-rays from Globular Clusters

It is expected that specific globular clusters can contain up to a hundred of millisecond pulsars. These pulsars can accelerate leptons at the shock waves originated in collisions of the pulsar winds and/or inside the pulsar magnetospheres. Energetic leptons diffuse gradually through the globular cluster comptonizing stellar and microwave background radiation. We calculate the GeV-TeV $γ$-ray spectra for different models of injection of leptons and parameters of the globular clusters assuming reasonable, of the order of 1%, efficiency of energy conversion from the pulsar winds into the relativistic leptons. It is concluded that leptons accelerated in the globular cluster cores should produce well localized $γ$-ray sources which are concentric with these globular clusters. The results are shown for four specific globular clusters (47 Tuc, Ter 5, M13, and M15), in which significant population of millisecond pulsars have been already discovered. We argue that the best candidates, which might be potentially detected by the present Cherenkov telescopes and the planned satellite telescopes (AGILE, GLAST), are 47 Tuc on the southern hemisphere, and M13 on the northern hemisphere. We conclude that detection (or non-detection) of GeV-TeV $γ$-ray emission from GCs by these instruments put important constraints on the models of acceleration of leptons by millisecond pulsars.

astro-ph↗

arXiv2006

Very high energy gamma-ray production inside the massive binary system Cyg X-1/HDE 226868

We apply the IC e$^\pm$ pair cascade model in order to investigate the possible $γ$-ray emission features from another compact massive binary of the microquasar type, Cyg X-1. We conclude that the observational constraints at lower energies (from MeV to GeV) suggest that the spectrum of electrons injected in the jet is likely steeper than in other TeV $γ$-ray microquasars. The cascade $γ$-ray spectrum produced by electrons with such spectrum in Cyg X-1 should be below the sensitivities of the MAGIC and VERITAS class Cherenkov telescopes. However, if the electron spectrum is flatter, then the highest TeV $γ$-ray fluxes are predicted at $\sim$7 hrs before and after the phase when the compact object is in front of the massive star. We suggest that Cherenkov telescopes should concentrate on these range of phases since the TeV flux can vary by a factor of $\sim$20 with the period of the Cyg X-1 binary system. Moreover, the model predicts clear anticorrelation of the GeV and TeV $γ$-ray emission. This feature can be tested by the future multiwavelength observations with the AGILE and GLAST telescopes in the GeV energy range and the MAGIC and VERITAS telescopes in the TeV energy range.

astro-ph↗

arXiv2006

GeV-TeV gamma-ray light curves expected in the IC electron-positron pair cascade model for massive binaries: Application to LS 5039

TeV gamma-ray emission from two massive binaries of the microquasar type, LS 5039 and LS I +61$^{\rm o}$ 303, show clear variability with their orbital periods. Our purpose is to calculate the GeV and TeV $γ$-ray light curves from the massive binary LS 5039 which are expected in the specific Inverse Compton $e^\pm$ pair cascade model. This model successfully predicted the basic features of the high energy $γ$-ray emission from LS 5039 and LS I +61 303. In the calculations we apply the Monte Carlo code which follows the IC $e^\pm$ pair cascade in the anisotropic radiation of the massive star. The $γ$-ray light curves and spectra are obtained for different parameters of the acceleration scenario and the inclination angles of the binary system. It is found that the GeV and TeV $γ$-ray light curves should be anti-correlated. This feature can be tested in the near future by the simultaneous observations of LS 5039 with the AGILE and GLAST telescopes in GeV energies and the Cherenkov telescopes in the TeV energies. Considered model also predicts a broad maximum in the TeV $γ$-ray light curve between the phases $\sim 0.4-0.8$ consistently with the observations of LS 5039 by the HESS telescopes. Moreover, we predict additional dip in the TeV light curve for large inclination angles $\sim 60^{\rm o}$. This feature could serve as a diagnostic for independent measuring of the inclination angle of this binary system indicating also on the presence of a neutron star in LS 5039.

astro-ph↗

arXiv2006

High Energy Processes in Pulsar Wind Nebulae

Young pulsars produce relativistic winds which interact with matter ejected during the supernova explosion and the surrounding interstellar gas. Particles are accelerated to very high energies somewhere in the pulsar winds or at the shocks produced in collisions of the winds with the surrounding medium. As a result of interactions of relativistic leptons with the magnetic field and low energy radiation (of synchrotron origin, thermal, or microwave background), the non-thermal radiation is produced with the lowest possible energies up to $\sim$100 TeV. The high energy (TeV) gamma-ray emission has been originally observed from the Crab Nebula and recently from several other objects. Recent observations by the HESS Cherenkov telescopes allow to study for the first time morphology of the sources of high energy emission, showing unexpected spectral features. They might be also interpreted as due to acceleration of hadrons. However, theory of particle acceleration in the PWNe and models for production of radiation are still at their early stage of development since it becomes clear that realistic modeling of these objects should include their time evolution and three-dimensional geometry. In this paper we concentrate on the attempts to create a model for the high energy processes inside the PWNe which includes existence not only relativistic leptons but also hadrons inside the nebula. Such model should also take into account evolution of the nebula in time. Possible high energy expectations based on such a model are discussed in the context of new observations.

astro-ph↗

arXiv2006

Inverse Compton e-p pair cascade model for the gamma-ray production in massive binary LSI +61^o 303

We apply an inverse Compton $e^\pm$ pair cascade model for $γ$-ray production in massive binary system LSI +61$^{\rm o}$ 303 assuming that electrons are accelerated already inside the inner part of the jet launched by the compact object. $γ$-ray spectra, affected by the cascade process, and lower energy spectra, from the synchrotron cooling of the highest energy electrons in the jet, are calculated as a function of the phase of this binary system. $γ$-ray spectra expected in such model have different shape than that ones produced by electrons in the jet directly to observer. Moreover, the model predicts clear anti-correlation between $γ$-ray fluxes in the GeV (1-10 GeV) and TeV ($>200$ GeV) energy ranges with the peak of the TeV emission at the phase $\sim$0.5 (the peak half width ranges between the phases $\sim$0.4-0.9 for the inclination of the binary system equal to $60^{\rm o}$, and $\sim$0.4-0.1 for $30^{\rm o}$). The fine features of TeV $γ$-ray emission (fluxes and spectral shapes) as a function of the phase of the binary system are consistent with recent observations reported by the MAGIC collaboration. Future simultaneous observations in the GeV energies (by the GLAST and AGILE telescopes) and in the TeV energies (by the MAGIC and VERITAS telescopes) should test other predictions of the considered model supporting or disproving the hypothersis of acceleration of electrons already in the inner part of the microquasar jets.

astro-ph↗