Ground-based Cherenkov Telescopes Research Articles

Very High-energy (>50 GeV) Gamma-Ray Flux Variability of Bright Fermi Blazars

Understanding the high-energy emission processes and variability patterns are two of the most challenging research problems associated with relativistic jets. In particular, the long-term (months to years) flux variability at very high energies (VHE >50 GeV) has remained an unexplored domain so far. This is possibly due to the decreased sensitivity of the Fermi Large Area Telescope (LAT) above a few GeV, hence low photon statistics, and observing constraints associated with the ground-based Cherenkov telescopes. This paper reports the results obtained from the 0.05−2 TeV Fermi-LAT data analysis of a sample of 29 blazars with the primary objective to explore their months-to-year-long very high-energy (VHE) flux variability behavior. This systematic search has led to, for the first time, the detection of significant flux variations in five blazars at the >99% confidence level, whereas eight of them exhibit variability, albeit at a lower confidence level (∼95%–99%). A comparison of the 0.05–2 TeV flux variations with that observed at 0.1–50 GeV band has revealed similar variability behavior for most of the sources. However, complex variability patterns that are not reflected contemporaneously in both energy bands were also detected, thereby providing tantalizing clues about the underlying radiative mechanisms. These results open up a new dimension to unravel the VHE emission processes operating in relativistic jets, hence sowing the seeds for their future observations with the upcoming Cherenkov Telescope Array.

Interpretation of fluxes of cosmic rays’ nuclei and electrons in the nonclassical diffusion model

We discuss new scenario of the formation of the observed spectra of electrons/positrons and nuclei of ultrahigh-energy cosmic rays in the framework of the nonclassical (superdiffusion) model of particles propagation in a highly inhomogeneous interstellar and intergalactic media. It is shown that the proposed scenario provides a consistent description of the experimental data of precision satellite measurements, ground-based hybrid EAS arrays and Cherenkov telescopes on the spectra of leptons and nuclei, as well as the mass composition of cosmic rays in the range of super-high and ultrahigh energies.

Search for Gamma-Ray Spectral Lines from Dark Matter Annihilation up to 100 TeV toward the Galactic Center with MAGIC

Linelike features in TeV γ rays constitute a "smoking gun" for TeV-scale particle dark matter and new physics. Probing the Galactic Center region with ground-based Cherenkov telescopes enables the search for TeV spectral features in immediate association with a dense dark matter reservoir at a sensitivity out of reach for satellite γ-ray detectors, and direct detection and collider experiments. We report on 223hours of observations of the Galactic Center region with the MAGIC stereoscopic telescope system reaching γ-ray energies up to 100TeV. We improved the sensitivity to spectral lines at high energies using large-zenith-angle observations and a novel background modeling method within a maximum-likelihood analysis in the energy domain. No linelike spectral feature is found in our analysis. Therefore, we constrain the cross section for dark matter annihilation into two photons to ⟨σv⟩≲5×10^{-28} cm^{3} s^{-1} at 1TeV and ⟨σv⟩≲1×10^{-25} cm^{3} s^{-1} at 100TeV, achieving the best limits to date for a dark matter mass above 20TeV and a cuspy dark matter profile at the Galactic Center. Finally, we use the derived limits for both cuspy and cored dark matter profiles to constrain supersymmetric wino models.

Search for Dark Matter Annihilation Signals in the H.E.S.S. Inner Galaxy Survey.

The central region of the MilkyWay is one of the foremost locations to look for dark matter (DM) signatures. We report the first results on a search for DM particle annihilation signals using new observations from an unprecedented γ-ray survey of the Galactic Center (GC) region, i.e., the Inner Galaxy Survey, at very high energies (≳100 GeV) performed with the H.E.S.S. array of five ground-based Cherenkov telescopes. No significant γ-ray excess is found in the search region of the 2014-2020 dataset and a profile likelihood ratio analysis is carried out to set exclusion limits on the annihilation cross section ⟨σv⟩. Assuming Einasto and Navarro-Frenk-White (NFW) DM density profiles at the GC, these constraints are the strongest obtained so far in the TeV DM mass range. For the Einasto profile, the constraints reach⟨σv⟩ values of 3.7×10^{-26} cm^{3} s^{-1} for 1.5TeV DM mass in the W^{+}W^{-} annihilation channel, and 1.2×10^{-26} cm^{3} s^{-1} for 0.7TeV DM mass in the τ^{+}τ^{-} annihilation channel. With the H.E.S.S. Inner Galaxy Survey, ground-based γ-ray observations thus probe ⟨σv⟩ values expected from thermal-relic annihilating TeV DM particles.

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.

Search for dark matter annihilation in the Wolf-Lundmark-Melotte dwarf irregular galaxy with H.E.S.S.

We search for an indirect signal of dark matter through very high-energy gamma rays from the Wolf-Lundmark-Melotte (WLM) dwarf irregular galaxy. The pair annihilation of dark matter particles would produce Standard Model particles in the final state such as gamma rays, which might be detected by ground-based Cherenkov telescopes. Dwarf irregular galaxies represent promising targets as they are dark matter dominated objects with well measured kinematics and small uncertainties on their dark matter distribution profiles. In 2018, the H.E.S.S. five-telescope array observed the dwarf irregular galaxy WLM for 18 hours. We present the first analysis based on data obtained from an imaging atmospheric Cherenkov telescope for this subclass of dwarf galaxy. As we do not observe any significant excess in the direction of WLM, we interpret the result in terms of constraints on the velocity-weighted cross section for dark matter pair annihilation as a function of the dark matter particle mass for various continuum channels as well as the prompt gamma-gamma emission. For the $\tau^+\tau^-$ channel the limits reach a $\langle \sigma v \rangle$ value of about $4\times 10^{-22}$ cm3s-1 for a dark matter particle mass of 1 TeV. For the prompt gamma-gamma channel, the upper limit reaches a $\langle \sigma v \rangle$ value of about $5 \times10^{-24}$ cm3s-1 for a mass of 370 GeV. These limits represent an improvement of up to a factor 200 with respect to previous results for the dwarf irregular galaxies for TeV dark matter search.

Multi-wavelength observations of the BL Lac object Fermi J1544-0649: One year after its awakening

We report observations of a transient source FermiJ1544-0649 from radio to γ-rays. FermiJ1544-0649 was discovered by the Fermi-LAT in May 2017. Follow-up Swift-XRT observations revealed three flaring episodes through March 2018, and the peak X-ray flux is about 103 higher than the ROSAT all-sky survey (RASS) flux upper limit. Optical spectral measurements taken by the Magellan 6.5-m telescope and the Lick-Shane telescope both show a largely featureless spectrum, strengthening the BL Lac interpretation first proposed by Bruni et al. (2018). The optical and mid-infrared (MIR) emission goes to a higher state in 2018, when the flux in high energies goes down to a lower level. Our RATAN-600 m measurements at 4.8 GHz and 8.2 GHz do not indicate any significant radio flux variation over the monitoring seasons in 2017 and 2018, nor deviate from the archival NVSS flux level. During GeV flaring times, the spectrum is very hard (Γγ∼1.7) in the GeV band and at times also very hard ((ΓX≲2) in the X-rays, similar to a high-synchrotron-peak (or even an extreme) BL Lac object, making FermiJ1544-0649 a good target for ground-based Cherenkov telescopes.

Fast Sampling and Read-Out for Ground-Based Imaging Cherenkov Telescopes

The SCT telescope has been proposed as a medium-sized telescope for the Cherenkov Telescope Array. One of the major upgrades is the design of the new Front End Electronics Module (FEEM). New FEEMs aim to read-out and digitize the SiPM pre-amplified signals down to the single photoelectron (phe). This phe signal is assumed equivalent to a signal with 2 mV peak amplitude and 500 MHz maximum bandwidth. FEEM should have a linear response up to 2 V for a required dynamic range of 1000 phe. FEEM noise equivalent of 0.5 phe - 1 mV of RMS noise - is an acceptable value. Due to the severe mechanical constraints, and to have a very compact electronics and low noise performance, FEEM consists of two stacked-up submodules, one dedicated to the power supplies and the other to house the FPGA, which reads out and sends digitized data to the main backplane. An FEEM is capable of digitizing 64 analog channels with a sampling frequency of 1 GSamples/s.

Exploiting morphological data from Pulsar Wind Nebulae via a spatiotemporal leptonic transport code

ABSTRACT The next era of ground-based Cherenkov telescope development will see a great increase in both quantity and quality of γ-ray morphological data. This initiated the development of a spatiotemporal leptonic transport code to model pulsar wind nebulae. We present results from this code that predicts the evolution of the leptonic particle spectrum and radiation at different radii in a spherically symmetric source. We simultaneously fit the overall broad-band spectral energy distribution, the surface brightness profile, and the X-ray photon index versus radius for PWN 3C 58, PWN G21.5 − 0.9, and PWN G0.9 + 0.1. Such concurrent fitting of disparate data sets is non-trivial and we thus investigate the utility of different goodness-of-fit statistics, specifically the traditional χ2 test statistic and a newly developed scaled-flux-normalized test statistic to obtain best-fitting parameters. We find reasonable fits to the spatial and spectral data of all three sources, but note some remaining degeneracies that will have to be broken by future observations.

Hunting for heavy winos in the Galactic Center

Observing gamma rays using ground-based atmospheric Cherenkov telescopes provides one of the only probes of heavy weakly interacting dark matter. A canonical target is the thermal wino, for which the strongest limits come from searches for photon lines from annihilations in the Galactic Center. Irreducible finite energy resolution effects motivate refining the prediction for a wino signal beyond the photon line approximation; recently, modern effective field theory techniques have been utilized to obtain a precise calculation of the full photon energy spectrum from wino annihilation. In this paper, we investigate the implications for a realistic mock H.E.S.S.-like line search. We emphasize the impact of including the non-trivial spectral shape, and we carefully treat the region of interest, presenting results for choices between $1^{\circ}$ and $4^{\circ}$ from the Galactic Center. Projected limits for wino masses from $1$-$70$ TeV are interpreted as a constraint on the wino annihilation rate, or alternatively as the minimum core size required such that the wino is not excluded. If there is a thermal wino, H.E.S.S. will be able to probe cores of several kpc, which would begin to cause tension between this dark matter candidate and astrophysical observations/simulations.

AGN at very high energies: Cosmic accelerators and probes of space-time

During the last decade, very high energy astrophysics emerged as a new branch of astronomy with major discoveries achieved by the present ground-based gamma-ray Cherenkov telescopes. The sample of cosmic sources firmly detected at very high energy (VHE) now exceeds two hundred objects, including active galactic nuclei (AGN), pulsar wind nebulae, and several other types of sources of which a significant number are unidentified ones. The scientific return from recent VHE data is particularly interesting for AGN science, shedding new light on particle acceleration and emission processes around supermassive black holes, and probing the intergalactic space by the analysis of VHE photons propagating from bright remote sources to the Earth. The perspectives of this research field are promising with new generation VHE instruments such as CTA, a project of open observatory at extreme energies at the horizon 2023, allowing a deep analysis of the sky in the highest part of the electromagnetic spectrum, from 20 GeV to 300 TeV.

The origin of HE and VHE gamma-ray flares from FSRQs

The discovery of very-high-energy gamma-ray emission from Flat Spectrum Radio Quasars (FSRQs) by ground-based Cherenkov telescopes (HESS, MAGIC, VERITAS) provides a new view of blazar emission processes. The available data from multiwavelength observations of FSRQs, allow us to constrain the size (possibly also location) of the emitting region, magnetic field, electron energy distribution, etc., which are crucial for the understanding of the jet properties. We investigate the origin of emission from FSRQs (PKS 1510-089, PKS 1222+216 and 3C 279) by modeling the broadband spectral energy distribution in their quiescent and flaring states, using estimation of the parameter space that describes the underlying particle distribution responsible for the emission through the Markov Chain Monte Carlo (MCMC) technique.

Search for γ-Ray Line Signals from Dark Matter Annihilations in the Inner Galactic Halo from 10 Years of Observations with H.E.S.S.

Spectral lines are among the most powerful signatures for dark matter (DM) annihilation searches in very-high-energy γ rays. The central region of the MilkyWay halo is one of the most promising targets given its large amount of DM and proximity to Earth. We report on a search for a monoenergetic spectral line from self-annihilations of DM particles in the energy range from 300GeV to 70TeV using a two-dimensional maximum likelihood method taking advantage of both the spectral and spatial features of the signal versus background. The analysis makes use of Galactic center observations accumulated over ten years (2004-2014) with the H.E.S.S. array of ground-based Cherenkov telescopes. No significant γ-ray excess above the background is found. We derive upper limits on the annihilation cross section ⟨σv⟩ for monoenergetic DM lines at the level of 4×10^{-28} cm^{3} s^{-1} at 1TeV, assuming an Einasto DM profile for the MilkyWay halo. For a DM mass of 1TeV, they improve over the previous ones by a factor of 6. The present constraints are the strongest obtained so far for DM particles in the mass range 300GeV-70TeV. Ground-based γ-ray observations have reached sufficient sensitivity to explore relevant velocity-averaged cross sections for DM annihilation into two γ-ray photons at the level expected from the thermal relic density for TeV DM particles.

Luminous and high-frequency peaked blazars: the origin of the γ-ray emission from PKS 1424+240

Context. The current generation of ground-based Cherenkov telescopes, together with the LAT instrument on-board the Fermi satellite, have greatly increased our knowledge of γ-ray blazars. Among them, the high-frequency-peaked BL Lacertae object (HBL) PKS 1424+240 (z ≃ 0.6) is the farthest persistent emitter of very-high-energy (VHE; E ≥ 100 GeV) γ-ray photons. Current emission models can satisfactorily reproduce typical blazar emission assuming that the dominant emission process is synchrotron-self-Compton (SSC) in HBLs; and external-inverse-Compton (EIC) in low-frequency-peaked BL Lacertae objects and flat-spectrum-radio-quasars. Alternatively, hadronic models are also able to correctly reproduce the γ-ray emission from blazars, although they are in general disfavored for bright quasars and rapid flares. Aims. The blazar PKS 1424+240 is a rare example of a luminous HBL, and we aim to determine which is the emission process most likely responsible for its γ-ray emission. This will impact more generally our comprehension of blazar emission models, and how they are related to the luminosity of the source and the peak frequency of the spectral energy distribution. Methods. We have investigated different blazar emission models applied to the spectral energy distribution of PKS 1424+240. Among leptonic models, we study a one-zone SSC model (including a systematic study of the parameter space), a two-zone SSC model, and an EIC model. We then investigated a blazar hadronic model, and finally a scenario in which the γ-ray emission is associated with cascades in the line-of-sight produced by cosmic rays from the source. Results. After a systematic study of the parameter space of the one-zone SSC model, we conclude that this scenario is not compatible with γ-ray observations of PKS 1424+240. A two-zone SSC scenario can alleviate this issue, as well as an EIC solution. For the latter, the external photon field is assumed to be the infra-red radiation from the dusty torus, otherwise the VHE γ-ray emission would have been significantly absorbed. Alternatively, hadronic models can satisfactorily reproduce the γ-ray emission from PKS 1424+240, both as in-source emission and as cascade emission.

Rare Events searches with Cherenkov Telescopes

Ground-based Imaging Cherenkov Telescope Arrays observe the Cherenkov radiation emitted in extended atmospheric showers generated by cosmic gamma rays in the TeV regime. The rate of these events is normally overwhelmed by 2-3 orders of magnitude more abundant cosmic rays induced showers. A large fraction of these 'background' events is vetoed at the online trigger level, but a substantial fraction still goes through data acquisition system and is saved for the offline reconstruction. What kind of information those events carry, normally rejected in the analysis? Is there the possibility that an exotic signature is hidden in those data? In the contribution, some science cases, and the problems related to the event reconstruction for the current and future generation of these telescopes will be discussed.

The inner 300 parsecs of the Milky Way seen by H.E.S.S.: a Pevatron in the Galactic Centre

The Galactic Centre region has been observed by the High Energy Stereoscopic System (H.E.S.S.) array of ground-based Cherenkov telescopes since 2004 leading to the detection of the very-high-energy γ-ray source HESS J1745-290 spatially coincident with the supermassive black hole Sagittarius A*. Diffuse TeV gamma-ray emission has been detected along the Galactic plane, most likely due to hadronic cosmic-ray interactions with the dense gas of the Central Molecular Zone. The rich 2004-2013 dataset permits detailed spectral and morphological studies of the diffuse emission in the inner 300 pc of the Galactic Centre region. The new results provide an important statement regarding the location and origin of the accelerator of PeV protons. The H.E.S.S. observations of the Pevatron are discussed in the context of the origin of Galactic cosmic rays.

Cut-off characterisation of energy spectra of bright fermi sources: Current instrument limits and future possibilities

In this paper some of the brightest GeV sources observed by the Fermi-LAT were analysed, focusing on their spectral cut-off region. The sources chosen for this investigation were the brightest blazar flares of 3C 454.3 and 3C 279 and the Vela pulsar with a reanalysis with the latest Fermi-LAT software. For the study of the spectral cut-off we first explored the Vela pulsar spectrum, whose statistics in the time interval of the 3FGL catalog allowed strong constraints to be obtained on the parameters. We subsequently performed a new analysis of the flaring blazar SEDs. For these sources we obtained constraints on the cut-off parameters under the assumption that their underlying spectral distribution is described by a power-law with a stretched exponential cut-off. We then highlighted the significant potential improvements on such constraints by observations with next generation ground based Cherenkov telescopes, represented in our study by the Cherenkov Telescope Array (CTA). Adopting currently available simulations for this future observatory, we demonstrate the considerable improvement in cut-off constraints achievable by observations with this new instrument when compared with that achievable by satellite observations.

Gravitational light-bending preventsγγabsorption in gravitational lenses

The magnification effect due to gravitational lensing enhances the chances of detecting moderate-redshift ($z \sim 1$) sources in very-high-energy (VHE; $E > 100$ GeV) $\gamma$-rays by ground-based Atmospheric Cherenkov Telescope facilities. It has been shown in previous work that this prospect is not hampered by potential $\gamma-\gamma$ absorption effects by the intervening (lensing) galaxy, nor by any individual star within the intervening galaxy. In this paper, we expand this study to simulate the light-bending effect of a realistic ensemble of stars. We first demonstrate that, for realistic parameters of the galaxy's star field, it is extremely unlikely (probability $\lesssim 10^{-6}$) that the direct line of sight between the $\gamma$-ray source and the observer passes by any star in the field close enough to be subject to significant $\gamma\gamma$ absorption. Our simulations then focus on the rare cases where $\gamma\gamma$ absorption by (at least) one individual star might be non-negligible. We show that gravitational light bending will have the effect of avoiding the $\gamma-\gamma$ absorption spheres around massive stars in the intervening galaxy. This confirms previous results by Barnacka et al. and re-inforces arguments in favour of VHE $\gamma$-ray observations of lensed moderate-redshift blazars to extend the redshift range of objects detected in VHE $\gamma$-rays, and to probe the location of the $\gamma$-ray emission region in those blazars.

Search for Dark Matter Annihilations towards the Inner Galactic Halo from 10 Years of Observations with H.E.S.S.

The inner region of the MilkyWay halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the annihilations of DM particles using γ-ray observations towards the inner 300pc of the MilkyWay, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic Center (GC) data accumulated by H.E.S.S. over the last 10years (2004-2014), and does not show any significant γ-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section ⟨σv⟩. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach ⟨σv⟩ values of 6×10^{-26} cm^{3} s^{-1} in the W^{+}W^{-} channel for a DM particle mass of 1.5TeV, and 2×10^{-26} cm^{3} s^{-1} in the τ^{+}τ^{-} channel for a 1TeV mass. For the first time, ground-based γ-ray observations have reached sufficient sensitivity to probe ⟨σv⟩ values expected from the thermal relic density for TeV DM particles.

A significant hardening and rising shape detected in the MeV/GeV νFν spectrum from the recently discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February

Abstract We report on Fermi Large Area Telescope (LAT) and multi-wavelength results on the recently discovered very-high-energy (VHE, E > 100 GeV) blazar S4 0954+65 (z = 0.368) during an exceptionally bright optical flare in 2015 February. During the time period (2015 February 13/14, or MJD 57067) when the MAGIC telescope detected VHE γ-ray emission from the source, the Fermi-LAT data indicated a significant spectral hardening at GeV energies, with a power-law photon index of 1.8 ± 0.1—compared with the 3FGL (The Fermi LAT 4-Year Point Source Catalog) value (averaged over four years of observation) of 2.34 ± 0.04. In contrast, Swift X-Ray Telescope data showed a softening of the X-ray spectrum, with a photon index of 1.72 ± 0.08 (compared with 1.38 ± 0.03 averaged during the flare from MJD 57066 to 57077), possibly indicating a modest contribution of synchrotron photons by the highest-energy electrons superposed on the inverse Compton component. Fitting of the quasi-simultaneous (<1 d) broad-band spectrum with a one-zone synchrotron plus inverse-Compton model revealed that GeV/TeV emission could be produced by inverse-Compton scattering of external photons from the dust torus. We emphasize that a flaring blazar showing high flux of ≳1.0 × 10−6 photons cm−2 s−1 (E > 100 MeV) and a hard spectral index of ΓGeV < 2.0 detected by Fermi-LAT on daily timescales is a promising target for TeV follow-up by ground-based Cherenkov telescopes to discover high-redshift blazars, investigate their temporal variability and spectral features in the VHE band, and also constrain the intensity of the extragalactic background light.