The Cherenkov Telescope Array (CTA) consortium aims to build the next-generation ground-based very-high-energy gamma-ray observatory. The array will feature different sizes of telescopes allowing it ...to cover a wide gamma-ray energy band from about 20GeV to above 100TeV. The highest energies, above 5TeV, will be covered by a large number of Small-Sized Telescopes (SSTs) with a field-of-view of around 9°. The Gamma-ray Cherenkov Telescope (GCT), based on Schwarzschild–Couder dual-mirror optics, is one of the three proposed SST designs. The GCT is described in this contribution and the first images of Cherenkov showers obtained using the telescope and its camera are presented. These were obtained in November 2015 in Meudon, 2http://www.cta-observatory.orgFrance.
Aims. Molecular clouds act as primary targets for cosmic-ray interactions and are expected to shine in γ-rays as a by-product of these interactions. Indeed, several detected γ-ray sources both in HE ...and VHE γ-rays (HE: 100 MeV < E < 100 GeV; VHE: E > 100 GeV) have been directly or indirectly associated with molecular clouds. Information on the local diffusion coefficient and the cosmic-ray population can be inferred from the observed γ-ray signals. In this work we explore the capability of the forthcoming Cherenkov Telescope Array observatory (CTA) to provide such measurements. Methods. We investigate the expected emission from clouds hosting an accelerator, surveying the parameter space for different modes of acceleration, age of the source, cloud density profile, and cosmic-ray diffusion coefficient. Results. We present some of the most interesting cases for CTA regarding this science topic. The simulated γ-ray fluxes depend strongly on the input parameters. In several cases, we find that it will be possible to constrain both the properties of the accelerator and the propagation mode of cosmic rays in the cloud from CTA data alone.
We present an analysis technique that uses the timing information of Cherenkov images from extensive air showers (EAS). Our emphasis is on distant, or large core distance
γ-ray induced showers at ...multi-TeV energies. Specifically, combining pixel timing information with an improved direction reconstruction algorithm, leads to improvements in angular and core resolution as large as ∼40% and ∼30%, respectively, when compared with the same algorithm without the use of timing. Above 10
TeV, this results in an angular resolution approaching 0.05°, together with a core resolution better than ∼15
m. The off-axis post-cut
γ-ray acceptance is energy dependent and its full width at half maximum ranges from 4° to 8°. For shower directions that are up to ∼6° off-axis, the angular resolution achieved by using timing information is comparable, around 100
TeV, to the on-axis angular resolution. The telescope specifications and layout we describe here are geared towards energies above 10
TeV. However, the methods can in principle be applied to other energies, given suitable telescope parameters. The 5-telescope cell investigated in this study could initially pave the way for a larger array of sparsely spaced telescopes in an effort to push the collection area to >10
km
2. These results highlight the potential of a ‘sparse array’ approach in effectively opening up the energy range above 10
TeV.
Massive star-forming regions assemble a large number of young stars with remnants of stellar evolution and a very dense environment. Therefore, particles accelerated in supernova remnants and pulsar ...wind nebulae encounter optimal conditions for interacting with target material and photon fields, and thus produce gamma-ray emission. However, observations are challenging because multiple phenomena may appear entangled within the resolution of current gamma-ray telescopes. We report on MAGIC observations aimed to understand the nature of the emission from the star-forming region W51 and the unidentified source HESS J1857+026. While gamma-ray emission from W51 is dominated by the interaction of the supernova remnant W51C with dense molecular clouds, HESS J1857+026 is associated to the pulsar wind nebula from PSR J1856+0245. However, an additional source is resolved north of HESSJ1857+026, with sufficient separation to determine that it cannot be powered by the same pulsar. We search for multiwavelength data to determine the origin of the new source.
Sub-TeV gamma-ray emission from the northwest rim of the supernova remnant RX J0852.0-4622 was detected with the CANGAROO II telescope and recently confirmed by the HESS group. In addition, the HESS ...data revealed a very wide (up to 2 in diameter), shell-like profile of the gamma-ray emission. We carried out CANGAROO III observations in 2005 January and February with three telescopes and show here the results of threefold coincidence data. We confirm the HESS results about the morphology and the energy spectrum and find that the energy spectrum in the NW rim is consistent with that of the whole remnant.
We report the detection, with the CANGAROO-III imaging atmospheric Cherenkov telescope array, of a very high energy gamma-ray signal from the unidentified gamma-ray source HESS J1614--518, which was ...discovered in the H.E.S.S. Galactic plane survey. Diffuse gamma-ray emission was detected above 760 GeV at the 8.9 Delta *s level during an effective exposure of 54 hr from 2008 May to August. The spectrum can be represented by a power law: (8.2 ? 2.2stat ? 2.5sys) X 10--12 X (E/1 TeV)-- Delta *g cm--2 s--1 TeV--1 with a photon index Delta *g of 2.4 ? 0.3stat ? 0.2sys, which is compatible with that of the H.E.S.S. observations. By combining our result with multiwavelength data, we discuss the possible counterparts for HESS J1614--518 and consider radiation mechanisms based on hadronic and leptonic processes for a supernova remnant (SNR), stellar winds from massive stars, and a pulsar wind nebula (PWN). Although a leptonic origin from a PWN driven by an unknown pulsar remains possible, hadronic-origin emission from an unknown SNR is preferred.
We have used the CANGAROO-III imaging atmospheric Cerenkov telescopes to observe the high-frequency-peaked BL Lacertae (HBL) object PKS 2155-304 between 2006 July 28 (MJD 53,944) and August 2, ...triggered by the H.E.S.S. report that the source was in a high state of TeV gamma-ray emission. A signal was detected at the 4.8 capital sigma level in an effective live time of 25.1 hr during the outburst period. The flux of very high energy gamma rays from the CANGAROO-III observations shows variability on the timescale of less than a few hours. The averaged integral flux above 660 GeV is image cm super(-2) s super(- 1), which corresponds to image45% of the flux observed from the Crab Nebula. Follow-up observations between August 17 (MJD 53,964) and 25 indicate that the source activity had decreased.
Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but ...still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole. We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet.
► A partly new analysis was applied to CANGAROO-III data. ► No evidence of TeV gamma-ray emissions were found from four blazars. ► Non-simultaneous multiwavelength SEDs of blazars were modeled with a ...leptonic scenario successfully. ► The magnetic field of H 2356-309 might be lower than so far reported values. ► We find a consistency with the blazar sequence in terms of the magnetic field and the size of blob.
We have searched for very high energy (VHE) gamma rays from four blazars using the CANGAROO-III imaging atmospheric Cherenkov telescope. We report the results of the observations of H 2356-309, PKS 2155-304, PKS 0537-441, and 3C 279, performed from 2005 to 2009, applying a new analysis to suppress the effects of the position dependence of Cherenkov images in the field of view. No significant VHE gamma ray emission was detected from any of the four blazars. The GeV gamma-ray spectra of these objects were obtained by analyzing Fermi/LAT archival data. Wide range (radio to VHE gamma-ray bands) spectral energy distributions (SEDs) including CANGAROO-III upper limits, GeV gamma-ray spectra, and archival data, even though they are non-simultaneous, are discussed using a one-zone synchrotron self-Compton (SSC) model in combination with a external Compton (EC) radiation. The HBLs (H 2356-309 and PKS 2155-304) can be explained by a simple SSC model, and PKS 0537-441 and 3C 279 are well modeled by a combination of SSC and EC model. We find a consistency with the blazar sequence in terms of strength of magnetic field and component size.
Context. Because of its proximity, Mrk 421 is one of the best sources on which to study the nature of BL Lac objects. Its proximity allows us to characterize its broadband spectral energy ...distribution (SED). Aims. The goal is to better understand the mechanisms responsible for the broadband emission and the temporal evolution of Mrk 421. These mechanisms may also apply to more distant blazars that cannot be studied with the same level of detail. Methods. A flare occurring in March 2010 was observed for 13 consecutive days (from MJD 55 265 to MJD 55 277) with unprecedented wavelength coverage from radio to very high energy (VHE; E> 100 GeV) γ-rays with MAGIC, VERITAS, Whipple, Fermi-LAT, MAXI, RXTE, Swift, GASP-WEBT, and several optical and radio telescopes. We modeled the day-scale SEDs with one-zone and two-zone synchrotron self-Compton (SSC) models, investigated the physical parameters, and evaluated whether the observed broadband SED variability can be associated with variations in the relativistic particle population. Results. The activity of Mrk 421 initially was high and then slowly decreased during the 13-day period. The flux variability was remarkable at the X-ray and VHE bands, but it was minor or not significant at the other bands. The variability in optical polarization was also minor. These observations revealed an almost linear correlation between the X-ray flux at the 2–10 keV band and the VHE γ-ray flux above 200 GeV, consistent with the γ-rays being produced by inverse-Compton scattering in the Klein-Nishina regime in the framework of SSC models. The one-zone SSC model can describe the SED of each day for the 13 consecutive days reasonably well, which once more shows the success of this standard theoretical scenario to describe the SEDs of VHE BL Lacs such as Mrk 421. This flaring activity is also very well described by a two-zone SSC model, where one zone is responsible for the quiescent emission, while the other smaller zone, which is spatially separated from the first, contributes to the daily variable emission occurring at X-rays and VHE γ-rays. The second blob is assumed to have a smaller volume and a narrow electron energy distribution with 3 × 104<γ< 6 × 105, where γ is the Lorentz factor of the electrons. Such a two-zone scenario would naturally lead to the correlated variability at the X-ray and VHE bands without variability at the optical/UV band, as well as to shorter timescales for the variability at the X-ray and VHE bands with respect to the variability at the other bands. Conclusions. Both the one-zone and the two-zone SSC models can describe the daily SEDs via the variation of only four or five model parameters, under the hypothesis that the variability is associated mostly with the underlying particle population. This shows that the particle acceleration and cooling mechanism that produces the radiating particles might be the main mechanism responsible for the broadband SED variations during the flaring episodes in blazars. The two-zone SSC model provides a better agreement with the observed SED at the narrow peaks of the low- and high-energy bumps during the highest activity, although the reported one-zone SSC model could be further improved by varying the parameters related to the emitting region itself (δ, B and R), in addition to the parameters related to the particle population.
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