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.
Strong gravitational lensing leads to an occurrence of multiple images, with different magnifications, of a lensed source. Those magnifications can in turn be modified by microlensing on smaller mass ...scales within the lens. Recently, measurements of the changes in the magnification ratio of the individual images have been proposed as a powerful tool for estimation of the size and velocity of the emission region in the lensed source. The changes of the magnification ratios in blazars PKS1830-211 and QSO B0218+357, if interpreted as caused by a microlensing on individual stars, put strong constraints on those two variables. These constraints are difficult to accommodate with the current models of gamma-ray emission in blazars. In this paper we study if similar changes in the magnification ratio can be caused by microlensing on intermediate size structures in the lensing galaxy. We investigate in details three classes of possible lenses: globular clusters (GCs), open clusters (OCs) and giant molecular clouds (GMCs). We apply this scenario to the case of QSO B0218+357. Our numerical simulations show that changes in magnifications with similar time-scales can be obtained for relativistically moving emission regions with sizes up to 0.01 pc in the case of microlensing on the cores of GCs or clumps in GMCs. From the density of such structures in spiral galaxies we estimate however that lensing in GMCs would be more common.
Two energetic millisecond pulsars (MSPs) within globular clusters (GCs), J1823-3021A in NGC 6624 and PSR B1821-24 in M28, have been recently discovered to emit pulsed GeV γ-rays. These MSPs are ...expected to eject energetic leptons. Therefore, GCs have been proposed to produce GeV-TeV γ-rays as a result of the Comptonization process of the background radiation within a GC. We develop this general scenario by taking into account not only the diffusion process of leptons within a GC but also their advection with the wind from the GC. Moreover, we consider distribution of MSP within a GC and the effects related to the non-central location of the dominating, energetic MSP. Such more complete scenario is considered for the modelling of the GeV-TeV γ-ray emission from the core-collapsed GC M15 and also for GCs which contain recently discovered energetic MSPs within NGC 6624 and M28. The confrontation of the modelling of the γ-ray emission with the observations with the present Cherenkov telescopes and the future Cherenkov Telescope Array (CTA) allows us to constrain more reliably the efficiency of lepton production within the inner magnetosphere of the MSPs and re-accelerated in their vicinity. We discuss the expected limits on this parameter in the context of expectations from the pulsar models. We conclude that deep observations of GCs, even with the present sensitivity of Cherenkov telescopes (the High Energy Stereoscopic System, the Major Atmospheric Gamma-Ray Imaging Cherenkov, the Very Energetic Radiation Imaging Telescope Array System), should start to constrain the models for the acceleration and radiation processes of leptons within the inner pulsar magnetosphere and its surrounding.
The current generation of ground-based imaging atmospheric Cherenkov telescopes (IACTs) operate in the very-high-energy (VHE) domain from ~100 GeV to ~100 TeV. They use electronic digital trigger ...systems to discern the Cherenkov light flashes emitted by extensive air showers (EASs), from the overwhelming light of the night sky (LoNS) background. Near the telescope energy threshold, the number of emitted Cherenkov photons by gamma-ray-induced EASs is comparable to the fluctuations of the LoNS and the photon distribution at the Cherenkov-imaging camera plane becomes patchy. This results in a severe loss of effectiveness of the digital triggers based on combinatorial logic of thresholded signals. A stereoscopic analog trigger system has been developed for improving the detection capabilities of the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes at the lowest energies. It is based on the analog sum of the photosensor electrical signals. In this article, the architectural design, technical performances, and configuration of this stereoscopic analog trigger, dubbed " Sum-Trigger-II ," are described.
High-energy γ-rays from globular clusters Bednarek, W.; Sitarek, J.
Monthly Notices of the Royal Astronomical Society,
05/2007, Letnik:
377, Številka:
2
Journal Article
Recenzirano
Odprti dostop
It is expected that specific globular clusters (GCs) 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 GC 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 GCs assuming reasonable, of the order of 1 per cent, efficiency of energy conversion from the pulsar winds into the relativistic leptons. It is concluded that leptons accelerated in the GC cores should produce well localized γ-ray sources which are concentric with these GCs. The results are shown for four specific GCs (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.
Context
. We are operating an elastic light detecting and ranging system (LIDAR) for the monitoring of atmospheric conditions during regular observations of the MAGIC telescopes.
Aims
. We present ...and evaluate methods for converting aerosol extinction profiles, obtained with the LIDAR, into corrections of the reconstructed gamma-ray event energy and instrument response functions of Imaging Atmospheric Cherenkov Telescopes.
Methods
. We assess the performance of these correction schemes with almost seven years of Crab Nebula data obtained with the MAGIC telescopes under various zenith angles and different aerosol extinction scenarios of Cherenkov light.
Results
. The methods enable the reconstruction of data taken under nonoptimal atmospheric conditions with aerosol transmissions down to ~0.65 with systematic uncertainties comparable to those for data taken under optimal conditions. For the first time, the correction of data affected by clouds has been included in the assessment. The data can also be corrected when the transmission is lower than 0.65, but the results are less accurate and suffer from larger systematics.
We consider nebulae which are created around binary systems containing rotation-powered pulsars and companion stars with strong stellar winds. It is proposed that the stellar and pulsar winds have to ...mix at some distance from the binary system, defined by the orbital period of the companion stars and the velocity of the stellar wind. The mixed pulsar-stellar wind expands with a specific velocity determined by the pulsar power and the mass loss rate of the companion star. Relativistic particles, either from the inner pulsar magnetosphere and/or accelerated at the shocks between stellar and pulsar winds, are expected to be captured and isotropized in the reference frame of the mixed wind. Therefore, they can efficiently Comptonize stellar radiation producing GeV-TeV γ-rays in the inverse Compton process. We calculate the γ-ray spectra expected in such scenario for the two example binary systems: J1816+4510 which is the Redback-type millisecond binary and LS 5039 which is supposed to contain energetic pulsar. It is concluded that the steady TeV γ-ray emission from J1816+4510 should be on the 100 h sensitivity limit of the planned Cherenkov Telescope Array, provided that ∼ 10 per cent of the rotational energy lost by the pulsar is transferred to TeV electrons. On the other hand, the comparison of the predicted steady TeV γ-ray emission, expected from γ-ray binary LS 5039, with the observations of the TeV emission in a low state, reported by the High Energy Stereoscopic System Collaboration, allows us to put stringent upper limit on the product of the part of the hemisphere in which the mixed pulsar-stellar wind is confined, Δmix, and the energy conversion efficiency, , from the supposed pulsar to the TeV electrons injected in this system, Δmix < 1 per cent. This lower limit can be understood provided that either the acceleration efficiency of electrons is rather low ( ∼ 1 per cent), or the parameters of the stellar wind from the companion star are less extreme than expected, or the injection/acceleration process of electrons occurs highly anisotropically with the predominance towards the companion star.
Recent observations with the low-threshold Cherenkov telescopes proved that sub-TeV γ-rays are able to arrive from active galaxies at relatively large distances in spite of the expected severe ...absorption in the extragalactic background light (EBL). We calculate the γ-ray spectra at TeV energies from two example optically violently variable quasars, 3C 273 and 3C 279, assuming that γ-rays are injected in the inner parts of the jets launched by the accretion discs. It is assumed that γ-rays in the broad energy range (from MeV up to TeV) are produced in these blazars with a 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 γ-rays by considering a number of models for the radiation field surrounding the jet. The classical picture of a relativistic blob in a jet for the injection of primary γ-rays is considered, with the injection rate of γ-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 γ-ray fluxes from these two sources. It is concluded that, even with the Stecker EBL model, the level of γ-ray emission from 3C 279 is close to the recent measurements in the sub-TeV γ-ray energies, provided that the injected γ-ray spectrum extends from the GeV energies over the next two decades with this same spectral index. We also suggest that a flare with a time-scale of a few days from 3C 273 could be detected by the MAGIC II stereo telescopes.
Context. The features of pulsed γ-ray emission from classical and millisecond pulsars indicate that the high energy radiation processes in their inner magnetospheres occur in a similar way. In the ...past decade several TeV γ-ray nebulae have been discovered around classical pulsars. The above facts suggest that γ-rays should be produced also in the surroundings of millisecond pulsars. Aims. We discuss a model for the bow shock nebula around the well known Black Widow binary system containing the millisecond pulsar B1957+20. This model predicts the existence of a synchrotron X-ray and inverse Compton γ-ray nebula around this system. We want to find out whether γ-ray emission from the nebula around B1957+20 could be detected by the future and present Cherenkov telescopes. Methods. Using the Monte Carlo method we followed the propagation of relativistic electrons in the vicinity of the pulsar. We calculated the very high energy radiation produced by them in the synchrotron process and the inverse Compton scattering of the microwave background radiation and of the infrared radiation from the galactic disk. We also computed the X-ray emission produced by the electrons in the synchrotron process. Results. We show that the hard X-ray tail emission observed from the vicinity of B1957+20 can be explained by our model. Moreover, we predict that the TeV γ-ray emission produced by the electrons in the inverse Compton process should be detectable by the future Cherenkov Telescope Array and possibly by the long term observations with the present Cherenkov arrays such as MAGIC and VERITAS. The γ-ray emission from B1957+20 is expected to be extended, inhomogeneous, and shifted from the present location of the binary system by a distance comparable to the radius of the nebula.
Imaging Atmospheric Cherenkov Telescopes (IACT) are designed to detect γ-ray photons in the very-high energy (VHE) band from 50 GeV to more than 10 TeV. The few degree-scale field of view and the ...good angular resolution < 0.1 deg, favor morphological studies on extended sources and on diffuse emission. The description of the PSF (Point Spread Function) of IACT is a prerequisite to assess, for example, the possibility to reveal diffuse emission around point like sources. We study the PSF on γ-ray photons of IACTs using Crab Nebula data collected with the MAGIC stereoscopic system. Through a careful description of the background and response of the instrument we can build the γ-ray PSF corrected for the angular acceptance. We find a good analytical model describing the γ-ray PSF for IACTs and assess its stability.
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