Future Cherenkov Telescope Array Research Articles

Constraints on redshifts of blazars from extragalactic background light attenuation using Fermi-LAT data

ABSTRACT The extragalactic high-energy γ-ray sky is dominated by blazars, which are active galactic nuclei with their jets pointing towards us. Distance measurements are of fundamental importance yet for some of these sources are challenging because any spectral signature from the host galaxy may be outshone by the non-thermal emission from the jet. In this paper, we present a method to constrain redshifts for these sources that relies only on data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. This method takes advantage of the signatures that the pair-production interaction between photons with energies larger than approximately 10 GeV and the extragalactic background light leaves on γ-ray spectra. We find upper limits for the distances of 303 γ-ray blazars, classified as 157 BL Lacertae objects, 145 of uncertain class, and 1 flat-spectrum-radio quasar, whose redshifts are otherwise unknown. These derivations can be useful for planning observations with imaging atmospheric Cherenkov telescopes and also for testing theories of supermassive black hole evolution. Our results are applied to estimate the detectability of these blazars with the future Cherenkov Telescope Array, finding that at least 21 of them could be studied in a reasonable exposure of 20 h.

A new derivation of the Hubble constant from γ-ray attenuation using improved optical depths for the Fermi and CTA era

ABSTRACT We present γ-ray optical-depth calculations from a recently published extragalactic background light (EBL) model built from multiwavelength galaxy data from the Hubble Space Telescope Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (HST/CANDELS). CANDELS gathers one of the deepest and most complete observations of stellar and dust emissions in galaxies. This model resulted in a robust derivation of the evolving EBL spectral energy distribution up to z ∼ 6, including the far-infrared peak. Therefore, the optical depths derived from this model will be useful for determining the attenuation of γ-ray photons coming from high-redshift sources, such as those detected by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope, and for multi-TeV photons that will be detected from nearby sources by the future Cherenkov Telescope Array. From these newly calculated optical depths, we derive the cosmic γ-ray horizon and also measure the expansion rate and matter content of the Universe including an assessment of the impact of the EBL uncertainties. We find H0 = 62.4 $^{+4.1}_{-3.9}$ km s−1 Mpc−1 when fixing Ωm = 0.32, and H0 = 65.1 $^{+6.0}_{-4.9}$ km s−1 Mpc−1 and Ωm = 0.19 ± 0.08, when exploring these two parameters simultaneously.

Impact of satellite trails on H.E.S.S. astronomical observations

Context. The number of satellites launched into Earth’s orbit has almost tripled in the last three years (to over 4000) due to the increasing commercialisation of space. Multiple satellite constellations, consisting of over 400 000 individual satellites, have either been partially launched or are proposed for launch in the near future. Many of these satellites are highly reflective, resulting in a high optical brightness that affects ground-based astronomical observations. Despite this caveat, the potential effect of these satellites on gamma-ray-observing Imaging Atmospheric Cherenkov Telescopes (IACTs) has largely been assumed to be negligible due to their nanosecond-scale integration times. However, this assumption has not been verified to date. Aims. As IACTs are sensitive to optical wavelength light, we aim to identify satellite trails in data taken by the High Energy Stereoscopic System (H.E.S.S.) IACT array. In particular, this study is aimed at quantifying the potential effects on data quality and extensive air shower event classification and reconstruction. Methods. Using night sky background measurements from H.E.S.S., we determined which observation times and pointing directions are affected most by these satellite trails. We then evaluated their impact on the standard Hillas parameter variables used for event analysis. Results. Due to the brightest trails, false trigger events can occur, however, for most modern analyses, the effect on astronomical results will be minimal. We observe a mild increase in the rate of trail detections over time (approximately doubling in three years), which is partially correlated with the number of satellite launches. Overall, the fraction of H.E.S.S. data affected (~0.2% of dark time observations) is currently minimal. We note that these trails could still have a non-negligible effect on future Cherenkov Telescope Array observations if advanced analysis techniques designed to lower the energy threshold of the instrument are applied.

GeV Signatures of Short Gamma-Ray Bursts in Active Galactic Nuclei

The joint detection of gravitational waves and the gamma-ray counterpart of a binary neutron star merger event, GW170817, unambiguously validates the connection between short gamma-ray bursts and compact binary object (CBO) mergers. We focus on a special scenario where short gamma-ray bursts produced by CBO mergers are embedded in disks of active galactic nuclei (AGNs), and we investigate the γ-ray emission produced in the internal dissipation region via synchrotron, synchrotron self-Compton, and external inverse Compton (EIC) processes. In this scenario, isotropic thermal photons from the AGN disks contribute to the EIC component. We show that a low-density cavity can be formed in the migration traps, leading to the embedded mergers producing successful GRB jets. We find that the EIC component would dominate the GeV emission for typical CBO mergers with an isotropic-equivalent luminosity of L j,iso = 1048.5 erg s−1 that are located close to the central supermassive black hole. Considering a long-lasting jet of duration T dur ∼ 102–103 s, we find that the future Cherenkov Telescope Array (CTA) will be able to detect its 25–100 GeV emission out to a redshift z = 1.0. In the optimistic case, it is possible to detect the on-axis extended emission simultaneously with GWs within one decade using MAGIC, H.E.S.S., VERITAS, CTA, and LHAASO-WCDA. Early diagnosis of prompt emissions with Fermi-GBM and HAWC can provide valuable directional information for the follow-up observations.

The RTApipe framework for the gamma-ray real-time analysis software development

In the multi-messenger era, coordinating observations between astronomical facilities is mandatory to study transient phenomena (e.g. Gamma-ray bursts) and is achieved by sharing information with the scientific community through networks such as the Gamma-ray Coordinates Network. The facilities usually develop real-time scientific analysis pipelines to detect transient events, alert the astrophysical community, and speed up the reaction time of science alerts received from other observatories. We present in this work the RTApipe framework, designed to facilitate the development of real-time scientific analysis pipelines for present and future gamma-ray observatories. This framework provides pipeline architecture and automatisms, allowing the researchers to focus on the scientific aspects and integrate existing science tools developed with different technologies. The pipelines automatically execute all the configured analyses during the data acquisition. This framework can be interfaced with science alerts networks to perform follow-up analysis of transient events shared by other facilities. The analyses are performed in parallel and can be prioritised. The workload is highly scalable on a cluster of machines. The framework provides the required services using containerisation technology for easy deployment. We present the RTA pipelines developed for the AGILE space mission and the prototype of the SAG system for the ground-based future Cherenkov Telescope Array observatory confirming that the RTApipe framework can be used to successfully develop pipelines for the gamma-ray observatories, both space and ground-based.

Inter-calibration of atmospheric Cherenkov telescopes with UAV-based airborne calibration system

The recent advances in the flight capability of remotely piloted aerial vehicles (here after referred to as UAVs) have afforded the astronomical community the possibility of a new telescope calibration technique: UAV-based calibration. Building upon a feasibility study which characterised the potential that a UAV-based calibration system has for the future Cherenkov Telescope Array, we created a first-generation UAV-calibration prototype and undertook a field-campaign of inter-calibrating the sensitivity of the H.E.S.S. telescope array with two successful calibration flights. In this paper we report the key results of our first test campaign: firstly, by comparing the intensity of the UAV-calibration events, as recorded by the individual HESS-I cameras, we find that a UAV-based inter-calibration is consistent with the standard muon inter-calibration technique at the level of 5.4% and 5.8% for the two individual UAV-calibration runs. Secondly, by comparing the position of the UAV-calibration signal on the camera focal plane, for a variety of telescope pointing models, we were able to constrain the pointing accuracy of the HESS-I telescopes at the tens of arc-second accuracy level. This is consistent with the pointing accuracy derived from other pointing calibration methods. Importantly both the inter-calibration and pointing accuracy results were achieved with a first-generation UAV-calibration prototype, which eludes to the potential of the technique and highlights that a UAV-based system is a viable calibration technique for current and future ground-based γ-ray telescope arrays.

Absorption effects in the blazar’s γ-ray spectra due to luminous stars crossing the jet

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.

Constraining models of the pulsar wind nebula in SNR G0.9+0.1 via simulation of its detection properties using the Cherenkov Telescope Array

ABSTRACT SNR G0.9+0.1 is a well-known source in the direction of the Galactic Centre composed by a Supernova Remnant (SNR) and a Pulsar Wind Nebula (PWN) in the core. We investigate the potential of the future Cherenkov Telescope Array (CTA), simulating observations of SNR G0.9 + 0.1. We studied the spatial and spectral properties of this source and estimated the systematic errors of these measurements. The source will be resolved if the very high-energy emission region is bigger than ∼0.65′. It will also be possible to distinguish between different spectral models and calculate the cutoff energy. The systematic errors are dominated by the Instrument Response Function instrumental uncertainties, especially at low energies. We computed the evolution of a young PWN inside an SNR using a one-zone time-dependent leptonic model. We applied the model to the simulated CTA data and found that it will be possible to accurately measure the cutoff energy of the γ-ray spectrum. Fitting of the multiwavelength spectrum will allow us to constrain also the magnetization of the PWN. Conversely, a pure power-law spectrum would rule out this model. Finally, we checked the impact of the spectral shape and the energy density of the Inter-Stellar Radiation Fields on the estimate of the parameters of the PWN, finding that they are not significantly affected.

Analysis of the H.E.S.S. public data release with ctools

The ctools open-source software package was developed for the scientific analysis of astronomical data from Imaging Air Cherenkov Telescopes (IACTs), such as H.E.S.S., VERITAS, MAGIC, and the future Cherenkov Telescope Array (CTA). To date, the software has been mainly tested using simulated CTA data; however, upon the public release of a small set of H.E.S.S. observations of the Crab nebula, MSH 15–52, RX J1713.7–3946, and PKS 2155–304 validation using real data is now possible. We analysed the data of the H.E.S.S. public data release using ctools version 1.6 and compared our results to those published by the H.E.S.S. Collaboration for the respective sources. We developed a parametric background model that satisfactorily describes the expected background rate as a function of reconstructed energy and direction for each observation. We used that model, and tested all analysis methods that are supported by ctools, including novel unbinned and joint or stacked binned analyses of the measured event energies and reconstructed directions, and classical On-Off analysis methods that are comparable to those used by the H.E.S.S. Collaboration. For all analysis methods, we found a good agreement between the ctools results and the H.E.S.S. Collaboration publications considering that they are not always directly comparable due to differences in the datatsets and event processing software. We also performed a joint analysis of H.E.S.S. and Fermi-LAT data of the Crab nebula, illustrating the multi-wavelength capacity of ctools. The joint Crab nebula spectrum is compatible with published literature values within the systematic uncertainties. We conclude that the ctools software is mature for the analysis of data from existing IACTs, as well as from the upcoming CTA.

Book Review: "Cherenkov Reflections: Gamma-Ray Imaging and the Evolution of TeV Astronomy by David Fegan"

Journal of Astronomical InstrumentationVol. 08, No. 04, 1980003 (2019) Book ReviewFree AccessBook Review: "Cherenkov Reflections: Gamma-Ray Imaging and the Evolution of TeV Astronomy by David Fegan"Stefan WagnerStefan WagnerHeidelberg-Königstuhl State Observatory, Center for Astronomy, Heidelberg University, Germanyhttps://doi.org/10.1142/S2251171719800035Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Librarian ShareShare onFacebookTwitterLinked InRedditEmail (World Scientific, 2019)Astronomical research is conducted in an amazingly broad part of the electromagnetic spectrum. The exploration of the universe at the very high energy end is a relatively recent chapter in this story — as far as actual discoveries and physics interpretation is concerned. The experimental techniques have been in development for more than two generations, however, and the early days have not been recorded in a comprehensive fashion as often and as detailed as in other branches of astronomy. David Fegan’s reflections on the history of Cherenkov astronomy provide a detailed and comprehensive insider’s account on the meandering path that led to the recognition that this field enjoys today.Cherenkov astronomy is different from other branches in many ways. The primary gamma-photons are reconstructed after having launched an avalanche of relativistic particles, which, in turn, generate a nano-second-lasting flash of Cherenkov light. The atmosphere — generally a troublesome, annoying or forbidding screen in observational astronomy — provides the detector volume, the light recorded by the cameras is emitted several kilometers above the telescopes rather than from kiloparsec or gigaparsec distances, and the original photons have to be reconstructed one by one.With photon energies 10 to 18 orders of magnitude higher than in the radio to X-ray regime, photon rates are typically more than 20 to 36 orders of magnitude lower, and there is the overwhelming isotropic background of cosmic rays that also initiate showers and Cherenkov flashes that are difficult to discriminate against. These obstacles provided many challenges to the pioneers and many generations of PhD students and it is amazing to follow their persistence.David Fegan takes the reader on a tour that covers this difficult period between the early ideas and the breakthrough that was triggered by the first indisputable discoveries. Following a brief introduction on the motivations provided by the discovery of cosmic rays and a concise chapter of the early work in the 1950s, 12 of the 15 chapters describe the progress from the early 1960s, when the author started his active involvement, to the early 1990s when the first real source discoveries started the path that took the field to mainstream astrophysics.The text has a rich store of personal recollections and insider information that can hardly be distilled from published work. The main focus lies on detailed descriptions of the different experimental set-ups, cameras, telescopes and experimental methods developed by many small and widely distributed groups. The text reiterates the motivation and enables the reader to understand why many paths have been taken that ultimately proved to be dead ends. While it is difficult in several cases to see why certain choices were made, the reader cannot fail to appreciate the persistence of the small community to overcome obstacles and disappointments. The need for rigor in eliminating plausible but ultimately false detections and the necessity of open-mindedness are made very obvious throughout the book.The detailed story ends with the first discoveries made with the Whipple telescope around 1990 and reflections on whether the path could have been straighter. The subsequent decade of building the current generation of observatories, the many discoveries, and the development of VHE gamma-ray astrophysics are only sketched in a final chapter that takes the reader to the current planning of the future Cherenkov Telescope Array (CTA). The book is not about the history of Cherenkov astronomy but a book on the evolution of experimental concepts in a branch of what is now known as particle astrophysics. It will be an interesting read for any interested in the history of science in general and high-energy astrophysics (or particle astrophysics) in particular. The admirable persistence of the founding fathers of TeV astronomy in what appears to have been a more daring endeavor in retrospect is well captured. FiguresReferencesRelatedDetails Recommended Vol. 08, No. 04 Metrics History Received 5 November 2019 Published: 13 November 2019 PDF download

Extragalactic background light inhomogeneities and Lorentz-Invariance Violation in gamma-gamma absorption and Compton scattering

AbstractAt energies approaching the Planck energy scale 1019GeV, several quantum-gravity theories predict that familiar concepts such as Lorentz (LIV) symmetry can be broken. Such extreme energies are currently unreachable by experiments on Earth, but for photons traveling over cosmological distances the accumulated deviations from the Lorentz symmetry may be measurable using the Cherenkov Telescope Array (CTA). To study the spectral hardening feature observed in some VHE gamma-ray blazars, we calculate the reduction of the EBL gamma-gamma opacity due to the existence of underdense regions along the line of sight to VHE -gamma ray sources and we compared with the possibility of a LIV signature. Considering the LIV effect, we found that the cosmic opacity for VHE-gamma rays with energy more than 10 TeV can be strongly reduced. I will further discuss the impact of LIV on the Compton scattering process, and how future CTA observations may open an exciting window on studies of the fundamental physics.

Using Muon Rings for the Calibration of the Cherenkov Telescope Array: A Systematic Review of the Method and Its Potential Accuracy

The analysis of ring images produced by muons in an Imaging Atmospheric Cherenkov Telescope (IACT) provides a powerful and precise method to calibrate the IACT optical throughput and monitor its optical point-spread function (PSF). First proposed by the Whipple collaboration in the early 90's, this method has been refined by the so-called second generation of IACT experiments: H.E.S.S., MAGIC and VERITAS. We review here the progress made with these instruments and investigate the applicability of the method as the primary throughput calibration method for the different telescope types forming the future Cherenkov Telescope Array (CTA). We find several additional systematic effects not yet taken into account by previous authors and propose several new analytical methods to include these in the analysis. Slight modifications in hardware and analysis need to be made to ensure that such a calibration works as accurately as required for the CTA. We derive analytic estimates for the expected muon data rates for optical throughput calibration, camera pixel flat-fielding and monitoring of the optical PSF. The achievable statistical and systematic uncertainties of the method are also assessed.

Very-high-energy Emission from Magnetic Reconnection in the Radiative-inefficient Accretion Flow of SgrA*

Abstract The cosmic-ray (CR) accelerator at the Galactic center (GC) is not yet established by current observations. Here we investigate the radiative-inefficient accretion flow (RIAF) of Sagittarius A* (SgrA*) as a CR accelerator assuming acceleration by turbulent magnetic reconnection, and derive possible emission fluxes of CRs interacting within the RIAF (the central ∼1013 cm). The target environment of the RIAF is modeled with numerical, general relativistic magnetohydrodynamics together with leptonic radiative transfer simulations. The acceleration of the CRs is not computed here. Instead, we inject CRs constrained by the magnetic reconnection power of the accretion flow and compute the emission/absorption of γ-rays due to these CRs interacting with the RIAF, through Monte Carlo simulations employing the CRPropa 3 code. The resulting very-high-energy (VHE) fluxes are not expected to reproduce the point source HESS J1745-290 as the emission of this source is most likely produced at parsec scales. The emission profiles derived here intend to trace the VHE signatures of the RIAF as a CR accelerator and provide predictions for observations of the GC with improved angular resolution and differential flux sensitivity as those of the forthcoming Cerenkov Telescope Array (CTA). Within the scenario presented here, we find that for mass accretion rates ≳10−7 M ⊙yr−1, the RIAF of SgrA* produces VHE fluxes that are consistent with the High Energy Stereoscopic System (H.E.S.S.) upper limits for the GC and potentially observable by the future CTA. The associated neutrino fluxes are negligible compared with the diffuse neutrino emission measured by the IceCube.

FRAM telescopes and their measurements of aerosol content at the Pierre Auger Observatory and at future sites of the Cherenkov Telescope Array

A FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope is a system of a robotic mount, a large-format CCD camera and a fast telephoto lens that can be used for atmospheric monitoring at any site when information about the atmospheric transparency is required with high spatial or temporal resolution and where continuous use of laser-based methods for this purpose would interfere with other observations. The original FRAM has been operated at the Pierre Auger Observatory in Argentina for more than a decade, while three more FRAMs are foreseen to be used by the Cherenkov Telescope Array (CTA). The CTA FRAMs are being deployed ahead of time to characterize the properties of the sites prior to the operation of the CTA telescopes; one FRAM has been running on the planned future CTA site in Chile for a year while two others are expected to become operational before the end of 2018. We report on the hardware and current status of operation and/or deployment of all the FRAM instruments in question as well as on some of the preliminary results of integral aerosol measurements by the FRAMs in Argentina and Chile.

Spatial likelihood analysis for MAGIC telescope data

Context. The increase in sensitivity of Imaging Atmospheric Cherenkov Telescopes (IACTs) has lead to numerous detections of extended γ-ray sources at TeV energies, sometimes of sizes comparable to the instrument’s field of view. This creates a demand for advanced and flexible data analysis methods that are able to extract source information using the photon counts in the entire field of view. Aims. We present a new software package, “SkyPrism”, aimed at performing 2D (3D if energy is considered) fits of IACT data that possibly contain multiple and extended sources. The fits are based on sky images binned in energy. Although the development of this package was focused on the analysis of data collected with the MAGIC telescopes, it can further be adapted to other instruments, such as the future Cherenkov Telescope Array. Methods. We have developed a set of tools that in addition to sky images (count maps) compute the instrument response functions of MAGIC (effective exposure throughout the field of view, point spread function, energy resolution, and background shape) based on the input data, Monte Carlo simulations, and the pointing track of the telescopes. With this information, the package can perform a simultaneous maximum likelihood fit of source models of arbitrary morphology to the sky images providing energy spectra, detection significances, and upper limits. Results. We demonstrate that the SkyPrism tool accurately reconstructs the MAGIC point spread function, on- and off-axis performance as well as the underlying background. We further show that for a point source analysis with the MAGIC default observational settings, SkyPrism gives results compatible with those of the standard tools while being more flexible and widely applicable.

Gamma rays from dark matter annihilation in three-loop radiative neutrino mass generation models

We present the Sommerfeld enhanced Dark Matter (DM) annihilation into gamma ray for a class of three-loop radiative neutrino mass models with large electroweak multiplets where the DM mass is in O(TeV) range. We show that in this model, the DM annihilation rate becomes more prominent for larger multiplets and it is already within the reach of currently operating Imaging Atmospheric Cherenkov telescopes (IACTs), High Energy Stereoscopic System (H.E.S.S.). Furthermore, Cherenkov Telescope Array (CTA), which will begin operating in 2030, will improve this sensitivity by a factor of O(10) and may exclude a large portion of parameter space of this radiative neutrino mass model with larger electroweak multiplet. This implies that the only viable option is the model with lowest electroweak multiplets i.e. singlets of SU(2)L where the DM annihilation rate is not Sommerfeld enhanced and hence it is not yet constrained by the indirect detection limits from H.E.S.S. or future CTA.

Gamma-ray emission in radio galaxies, from MeV to TeV

AbstractThanks to the Fermi λ-ray satellite and the current Imaging Atmospheric Cherenkov Telescopes, radio galaxies have arisen as a new class of high- and very-high energy emitters. The favourable orientation of their jets makes radio galaxies extremely relevant in addressing important issues such as: (i) revealing the jet structure complexity; (ii) localising the emitting region(s) of high- and very-high energy radiation; (iii) understanding the physical processes producing these photons. In this review the main results on the λ-ray emission studies of radio galaxies from the MeV to TeV regimes will be presented, and the impact of future Cherenkov Telescope Array observations will be discussed.

Signals of HE atmospheric μ decay in flight around the Sun’s albedo versus astrophysical νμ and ντ traces in the Moon shadow

The Sun albedo of Cosmic Rays (CRs) at GeVs energy has been discovered recently by the FERMI satellite. They are traces of atmospheric CRs hitting solar atmosphere and reflecting skimming gamma photons. Even if relevant for astrophysics, as being a trace of atmospheric solar CR noises they cannot offer any signal of neutrino astronomy. On the contrary, the Moon with no atmosphere, may become soon a novel filtering calorimeter and an amplifier of energetic muon astronomical neutrinos (at TeV up to hundred TeVs energy); these lepton tracks leave an imprint in their beta decay while in flight to Earth. Their TeV electron air-shower are among the main signals. Also, a more energetic, but more rare, PeV up to EeV tau lunar neutrino events may be escaping as a tau lepton from the Moon: [Formula: see text] PeV secondaries, then, may be shining on Earth’s atmosphere in lunar shadows in a surprising way. One or a few gamma air-shower events inside the Moon shadows may occur each year in near future Cherenkov telescope array (CTA) or large high altitude air shower observatory (LHAASO) TeV gamma array detector, assuming a nonnegligible astrophysical TeV up to hundred TeV neutrino component (with respect to our terrestrial ruling atmospheric ones); these signals will open a new wonderful passe-partout keyhole for neutrino, been seen along the Moon. The lunar solid angle is small and the muon or tau expected rate is rare, but with the future largest tau radio array as the giant radio array for neutrino detection (GRAND), one might well discover such neutrino imprint.

Expected signatures from hadronic emission processes in the TeV spectra of BL Lacertae objects

Context. The wealth of recent data from Imaging Air Cherenkov telescopes (IACTs), ultra-high energy cosmic-ray experiments and neutrino telescopes have fuelled a renewed interest in hadronic emission models for γ-loud blazars. Aims. We explore physically plausible solutions for a lepto-hadronic interpretation of the stationary emission from high-frequency peaked BL Lac objects (HBLs). The modelled spectral energy distributions are then searched for specific signatures at very high energies that could help to distinguish the hadronic origin of the emission from a standard leptonic scenario. Methods. By introducing a few basic constraints on parameters of the model, such as assuming the co-acceleration of electrons and protons, we significantly reduced the number of free parameters. We then systematically explored the parameter space of the size of the emission region and its magnetic field for two bright γ-loud HBLs, PKS 2155-304 and Mrk 421. For all solutions close to equipartition between the energy densities of protons and of the magnetic field, and with acceptable jet power and light-crossing timescales, we inspected the spectral hardening in the multi-TeV domain from proton-photon induced cascades and muon-synchrotron emission inside the source. Very-high-energy spectra simulated with the available instrument functions from the future Cherenkov Telescope Array (CTA) were evaluated for detectable features as a function of exposure time, source redshift, and flux level. Results. A range of hadronic scenarios are found to provide satisfactory solutions for the broad band emission of the sources under study. The TeV spectrum can be dominated either by proton-synchrotron emission or by muon-synchrotron emission. The solutions for HBLs cover a parameter space that is distinct from the one found for the most extreme BL Lac objects in an earlier study. Over a large range of model parameters, the spectral hardening due to internal synchrotron-pair cascades, the “cascade bump”, should be detectable for acceptable exposure times with the future CTA for a few nearby and bright HBLs.

Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7−3946

Abstract We perform simulations for future Cherenkov Telescope Array (CTA) observations of RX J1713.7−3946, a young supernova remnant (SNR) and one of the brightest sources ever discovered in very high energy (VHE) gamma rays. Special attention is paid to exploring possible spatial (anti)correlations of gamma rays with emission at other wavelengths, in particular X-rays and CO/H i emission. We present a series of simulated images of RX J1713.7−3946 for CTA based on a set of observationally motivated models for the gamma-ray emission. In these models, VHE gamma rays produced by high-energy electrons are assumed to trace the nonthermal X-ray emission observed by XMM-Newton, whereas those originating from relativistic protons delineate the local gas distributions. The local atomic and molecular gas distributions are deduced by the NANTEN team from CO and H i observations. Our primary goal is to show how one can distinguish the emission mechanism(s) of the gamma rays (i.e., hadronic versus leptonic, or a mixture of the two) through information provided by their spatial distribution, spectra, and time variation. This work is the first attempt to quantitatively evaluate the capabilities of CTA to achieve various proposed scientific goals by observing this important cosmic particle accelerator.