Imaging air Cherenkov telescopes (IACTs) detect the Cherenkov light from extensive air showers (EAS) initiated by very high energy (VHE)
γ
-rays impinging on the Earth's atmosphere. Due to the ...overwhelming background from hadron-induced EAS, the discrimination of the rare
γ
-like events is vital. The influence of the geomagnetic field (GF) on the development of EAS can further complicate the imaging air Cherenkov technique. The amount and the angular distribution of Cherenkov light from EAS can be obtained by means of Monte Carlo (MC) simulations. Here we present the results from dedicated MC studies of GF effects on images from
γ
-ray initiated EAS for the MAGIC telescope site, where the GF strength is
∼
40
μ
T
. The results from the MC studies suggest that GF effects degrade not only measurements of very low energy
γ
-rays below
∼
100
GeV
but also those at TeV-energies.
The blazar QSO B0218+357 is the first gravitationally lensed blazar detected in the very high energy (VHE, E > 100 GeV) gamma-ray spectral range (Ahnen et al. 2016). It is gravitationally lensed by ...the intervening galaxy B0218+357G (z
l
= 0.68466 ± 0.00004, Carilli et al. 1993), which splits the blazar emission into two components, spatially indistinguishable by gamma-ray instruments, but separated by a 10-12 days delay. In July 2014 a flare from QSO B0218+357 was observed by the Fermi-LAT (Large Area Telescope, Atwood et al. 2009, Ackermann et al. 2012), and followed-up by the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes, a stereoscopic system of two 17m Imaging Atmospheric Cherenkov Telescopes located on La Palma, Canary Islands (Aleksić et al. 2016a, 2016b), during the expected time of arrival of the delayed component of the emission. MAGIC could not observe the leading image due to the Full Moon. The MAGIC and Fermi-LAT observations were accompanied by optical data from KVA telescope at La Palma, and X-ray observations by Swift-XRT (Fig. 1 left). Variability in gamma-rays was of the order of one day, while no variability correlated with gamma-rays was observed at lower energies. The flux ratio of the leading to trailing image in HE gamma-rays was larger than in the flare of QSO B0218+357 observed by Fermi-LAT in 2012 (Cheung et al. 2014). Changes in the observed flux ratio can be caused by gravitational microlensing on individual stars in the host galaxy (Neronov et al. 2015), or by other compact objects like for ex. clumps in giant molecular clouds (Sitarek & Bednarek 2016).
ABSTRACT
We present a measurement of the extragalactic background light (EBL) based on a joint likelihood analysis of 32 gamma-ray spectra for 12 blazars in the redshift range z = 0.03–0.944, ...obtained by the MAGIC telescopes and Fermi-LAT. The EBL is the part of the diffuse extragalactic radiation spanning the ultraviolet, visible, and infrared bands. Major contributors to the EBL are the light emitted by stars through the history of the Universe, and the fraction of it that was absorbed by dust in galaxies and re-emitted at longer wavelengths.
The EBL can be studied indirectly through its effect on very high energy photons that are emitted by cosmic sources and absorbed via γγ interactions during their propagation across cosmological distances. We obtain estimates of the EBL density in good agreement with state-of-the-art models of the EBL production and evolution. The 1σ upper bounds, including systematic uncertainties, are between 13 per cent and 23 per cent above the nominal EBL density in the models. No anomaly in the expected transparency of the Universe to gamma-rays is observed in any range of optical depth. We also perform a wavelength-resolved EBL determination, which results in a hint of an excess of EBL in the 0.18–0.62 $\mu\mathrm{ m}$ range relative to the studied models, yet compatible with them within systematics.
Microquasars are expected to emit high-energy gamma-rays owing to their general similarities to gamma-ray-emitting blazars (evidence of relativistic jets, non-thermal radio to X-ray emission). In ...fact, the first source of this type, Cyg X-3, has recently been unambiguously discovered by satellite telescopes. We study the features of the gamma-ray radiation produced in these sources by relativistic electrons accelerated in the inner part of the jet. The electrons initiate an inverse Compton e± pair cascade in the radiation field of the accretion disc. Owing to the anisotropy of the accretion disc radiation field, the spectra of gamma-rays show a strong dependence on the observation angle, the location of the emission region within the jet, and the details of the acceleration process. As an example, we test our model with observations of the microquasar Cyg X-3, which has recently been reported as a transient GeV gamma-ray source by the Agile and Fermi observatories. Satisfactory descriptions of the gamma-ray spectra observed from Cyg X-3 are obtained in the case of injection of electrons in the inner part of the jet (located within 300 inner disc radius from the jet base), provided that the observer is located at a relatively small angle to the jet axis. PUBLICATION ABSTRACT
Context. QSO B0218+357 is a gravitationally lensed blazar located at a redshift of 0.944. The gravitational lensing splits the emitted radiation into two components that are spatially ...indistinguishable by gamma-ray instruments, but separated by a 10–12 day delay. In July 2014, QSO B0218+357 experienced a violent flare observed by the Fermi-LAT and followed by the MAGIC telescopes. Aims. The spectral energy distribution of QSO B0218+357 can give information on the energetics of z ~ 1 very high energy gamma-ray sources. Moreover the gamma-ray emission can also be used as a probe of the extragalactic background light at z ~ 1. Methods. MAGIC performed observations of QSO B0218+357 during the expected arrival time of the delayed component of the emission. The MAGIC and Fermi-LAT observations were accompanied by quasi-simultaneous optical data from the KVA telescope and X-ray observations by Swift-XRT. We construct a multiwavelength spectral energy distribution of QSO B0218+357 and use it to model the source. The GeV and sub-TeV data obtained by Fermi-LAT and MAGIC are used to set constraints on the extragalactic background light. Results. Very high energy gamma-ray emission was detected from the direction of QSO B0218+357 by the MAGIC telescopes during the expected time of arrival of the trailing component of the flare, making it the farthest very high energy gamma-ray source detected to date. The observed emission spans the energy range from 65 to 175 GeV. The combined MAGIC and Fermi-LAT spectral energy distribution of QSO B0218+357 is consistent with current extragalactic background light models. The broadband emission can be modeled in the framework of a two-zone external Compton scenario, where the GeV emission comes from an emission region in the jet, located outside the broad line region.
ABSTRACTNew high-energy emission features from active galaxies have recently been discovered using the Cherenkov telescopes; for example, a time-scale variability of a few minutes for TeV emission ...from Mrk 501 and PKS 2155-304, sub-TeV gamma -ray emission from the GeV peaked blazar 3C 279, and TeV emission from two nearby active galaxies, M87 and Cen A, whose jets are inclined at a relatively large angle to the line of sight. These results have shed new light on the high-energy processes occurring in the central parts of active galaxies and have stimulated more detailed studies of gamma -ray emission models. In this paper, we report on the results of a detailed analysis concerning the most general version of the model for gamma -ray production by leptons injected in the jet, which interact with the thermal radiation from an accretion disc: the so-called external inverse Compton (IC) model. We investigate the gamma -ray spectra produced in an anisotropic IC e plus or minus pair cascade in the whole volume above the accretion disc. The cascade gamma -ray spectra are obtained for different locations of the observer with respect to the direction of the jet. We also study the time evolution of this gamma -ray emission caused by the propagation of the relativistic leptons along the jet and the delays resulting from different places of the origin of gamma -rays above the accretion disc. We discuss the main features of such a cascade model, assuming a constant injection rate of electrons along the jet. We are investigating two models for maximum energies of injected electrons: with a constant value independent of the distance along the jet or limited by the synchrotron energy losses considered locally in the jet. The model is discussed in the context of blazars observed at small and large inclination angles, taking as an example the parameters of the two famous sources, Cen A and 3C 279.
The MAGIC telescopes discovered very high energy (VHE, E>100 GeV) gamma-ray emission coming from the distant Flat Spectrum Radio Quasar (FSRQ) PKS 1222+21 (4C +21.35, z 0.432). It is the second most ...distant VHE gamma-ray source, with well measured redshift, detected until now. The observation was performed on 2010 June 17 (MJD 55364.9) using the two 17 m diameter imaging Cherenkov telescopes on La Palma (Canary Islands, Spain). The MAGIC detection coincides with high energy MeV/GeV gamma-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The averaged integral flux above 100 GeV is equivalent to 1 Crab Nebula flux. The VHE flux measured by MAGIC varies significantly within the 30 minutes of exposure implying a flux doubling time of about 10 minutes. The VHE and MeV/GeV spectra, corrected for the absorption by the extragalactic background light, can be described by a single power law with photon index 2.72±0.34 between 3 GeV and 400 GeV, consistent with gamma-ray emission belonging to a single component in the jet. The absence of a spectral cut-off constrains the gamma-ray emission region to lie outside the broad line region, which would otherwise absorb the VHE gamma-rays. Together with the detected fast variability, this challenges present emission models from jets in FSRQs.
PDF
