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.
New 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 γ-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 γ-ray emission models. In this paper, we report on the results of a detailed analysis concerning the most general version of the model for γ-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 γ-ray spectra produced in an anisotropic IC e± pair cascade in the whole volume above the accretion disc. The cascade γ-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 γ-ray emission caused by the propagation of the relativistic leptons along the jet and the delays resulting from different places of the origin of γ-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.
Microquasars are expected to emit high-energy γ-rays owing to their general similarities to γ-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 γ-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 γ-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 γ-ray source by the Agile and Fermi observatories. Satisfactory descriptions of the γ-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.
The very short time-scale variability of TeV γ-ray emission from active galaxies suggests that the acceleration process of particles and the production of primary γ-rays likely occur relatively close ...to the accretion disc. We calculate the γ-ray spectra produced in an inverse Compton e± pair cascade initiated by primary γ-rays which are injected close to the surface of the accretion disc. Possible synchrotron energy losses of secondary cascade e± pairs are also taken into account. Since the soft radiation field is anisotropic, the resulting γ-ray spectra strongly depend on the observation angle. We investigate their basic properties for different parameters describing such a model. The model is applied to the misaligned blazar Cen A recently detected in the TeV γ-rays. We conclude on the site of the γ-ray emission region in Cen A based on the comparison of the model with the observations of this source in the GeV–TeV energy range.
Abstract
The Cherenkov Telescope Array (CTA) is the next-generation ground-based very-high-energy gamma-ray observatory. By using three types of telescopes CTA can cover a wide energy range (20 ...GeV–300 TeV) with an order of magnitude higher sensitivity than the current telescopes. The Large-Sized Telescope (LST) is designed to detect 20 GeV–1 TeV gamma rays thanks to the large light collection area, sensitive photosensors, a fast trigger system, and readout electronics. The camera readout system must have a high signal-to-noise ratio and a linear signal sampling with a large dynamic range in order to efficiently detect dim and low-energy atmospheric showers. To meet this requirement we use the Domino Ring Sampler version 4 (DRS4), which also enables ultra-fast sampling with low power consumption. Some of the intrinsic characteristics of DRS4 chips require software corrections. These procedures lower the effect of non-Gaussian noise contribution and improve the timing resolution of the system. In this contribution we discuss the calibration algorithms and the resulting performance.
We investigate the influence of the geomagnetic field (GF) on the Imaging Air Cherenkov Telescope technique for two northern (Tenerife and San Pedro Martir) and three southern (Salta, Leoncito and ...Namibia (the H.E.S.S.-site)) site candidates for Cherenkov Telescope Array (CTA) observatories. We use the CORSIKA and sim_telarray programs for Monte Carlo simulations of gamma ray showers, hadronic background and the telescope response. We focus here on gamma ray measurements in the low energy, sub-100GeV, range. Therefore, we only consider the performance of arrays of several large telescopes. Neglecting the GF effect, we find (in agreement with previous studies) that such arrays have lower energy thresholds, and larger collection areas below 30GeV, when located at higher altitudes. We point out, however, that in the considered ranges of altitudes and magnetic field intensities, 1800–3600m a.s.l. and 0–40μT, respectively, the GF effect has a similar magnitude to this altitude effect. We provide the trigger-level performance parameters of the observatory affected by the GF effect, in particular the collection areas, detection rates and the energy thresholds for all five locations, which information may be useful in the selection of sites for CTA. We also find simple scaling of these parameters with the magnetic field strength, which can be used to assess the magnitude of the GF effect for other sites; in this work we use them to estimate the performance parameters for five sites: South Africa-Beaufort West, USA-Yavapai Ranch, Namibia-Calapanzi, Chile-La Silla and India-Hanle. We roughly investigate the impact of the geophysical conditions on gamma/hadron separation procedures involving image shape and direction cuts. We note that the change of altitude has an opposite effect at the trigger and analysis levels, i.e. gains in triggering efficiency at higher altitudes are partially balanced by losses in the separation efficiency. In turn, a stronger GF spoils both the shape and the direction discrimination of gamma rays, thus its effects at the trigger and analysis levels add up resulting in a significant reduction of the observatory performance. Overall, our results indicate that the local GF strength at a site can be equally important as its altitude for the low-energy performance of CTA.
Novae were discovered to emit transient γ rays during the period of several days to a few weeks after initial explosion, indicating presence of acceleration processes of particles in their expanding ...shells. In the case of recurrent novae, electrons can be in principle accelerated in the nova shells for the whole recurrence period of nova producing delayed γ ray emission as considered in Bednarek (2022). Here we extend the ideas presented in this article by considering the fate of electrons which diffuse out of the shells of novae supplying fresh relativistic electrons to the recurrent nova super-remnants during the whole active period of nova (≥104 yrs). We develop a model for the acceleration of electrons and their escape from the nova shells. The electrons within the recurrent nova super-remnants produce γ rays in the comptonization process of the radiation from the red giant companion and the Cosmic Microwave Background Radiation. As an example, the case of a symbiotic nova RS Oph (with the recurrence period estimated on ∼10-50 yrs) is considered in more detail. Predicted γ-ray emission from the nova super-remnant around RS Oph is discussed in the context of its observability by satellite experiments (i.e. Fermi-LAT) as well as current and future Cherenkov telescopes.
The search for detection of γ-rays from distant AGNs by Imaging Atmospheric Cherenkov Telescopes (IACTs) is challenging at high redshifts, not only because of lower flux due to the distance of the ...source, but also due to the consequent absorption of γ-rays by the extragalactic background light (EBL). Before the MAGIC discoveries reported in this work, the farthest source ever detected in the VHE domain was the blazar PKS 1424+240, at z > 0.6. MAGIC, a system of two 17 m of diameter IACTs located in the Canary island of La Palma, has been able to go beyond that limit and push the boundaries for VHE detection to redshifts z ~ 1. The two sources detected and analyzed, the blazar QSO B0218+357 and the FSRQ PKS 1441+25 are located at redshift z = 0.944 and z = 0.939 respectively. QSO B0218+357 is also the first gravitational lensed blazar ever detected in VHE. The activity, triggered by Fermi-LAT in high energy γ-rays, was followed up by other instruments, such as the KVA telescope in the optical band and the Swift-XRT in X-rays. In the present work we show results on MAGIC analysis on QSO B0218+357 and PKS 1441+25 together with multiwavelength lightcurves. The collected dataset allowed us to test for the first time the present generation of EBL models at such distances.
Context.
Modelling the broadband emission of jetted active galactic nuclei (AGN) constitutes one of the main research topics of extragalactic astrophysics in the multi-wavelength and multi-messenger ...domain.
Aims.
We present
agnpy
, an open-source python package modelling the radiative processes of relativistic particles accelerated in the jets of AGN. The package includes classes describing the galaxy components responsible for line and thermal emission and it calculates the absorption due to
γ
γ
pair production on several photon fields.
agnpy
aims to extend the effort of modelling and interpreting the emission of extragalactic sources to a wide number of astrophysicists.
Methods.
We present the package content and illustrate a few examples of applications of its functionalities. We validate the software by comparing its results against the literature and against other open-source software.
Results.
We illustrate the utility of
agnpy
in addressing the most common questions encountered while modelling the emission of jetted active galaxies. When comparing its results against the literature and other modelling tools adopting the same physical assumptions, we achieve an agreement within 10 − 30%.
Conclusions.
agnpy
represents one of the first systematic and validated collection of established radiative processes for jetted active galaxies in an open-source python package. We hope it will also stand among the first endeavours providing reproducible and transparent astrophysical software not only for data reduction and analysis, but also for modelling and interpretation.
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