We present a detailed investigation of the gamma -ray emission in the vicinity of the supernova remnant (SNR) W28 (G6.4-0.1) observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray ...Space Telescope. We detected significant gamma -ray emission spatially coincident with TeV sources HESS J1800-240A, B, and C, located outside the radio boundary of the SNR. Their spectra in the 2-100 GeV band are consistent with the extrapolation of the power-law spectra of the TeV sources. We also identified a new source of GeV emission, dubbed Source W, which lies outside the boundary of TeV sources and coincides with radio emission from the western part of W28. All of the GeV gamma -ray sources overlap with molecular clouds in the velocity range from 0 to 20 km s super(-1). Under the assumption that the gamma -ray emission toward HESS J1800-240A, B, and C comes from pi super(0) decay due to the interaction between the molecular clouds and cosmic rays (CRs) escaping from W28, they can be naturally explained by a single model in which the CR diffusion coefficient is smaller than the theoretical expectation in the interstellar space. The total energy of the CRs escaping from W28 is constrained through the same modeling to be larger than ~2 x 10 super(49) erg. The emission from Source W can also be explained with the same CR escape scenario.
Two possible planetary nebulae (PN G035.5-00.4 and IRAS 18551+0159), one newly re-identified supernova remnant (SNR G35.6-0.4), and one H II region (G35.6-0.5) form a line-of-sight-overlapping ...complex known as G35.6-0.5. We analyze 21 cm H I absorption spectra toward the complex to constrain the kinematic distances of these objects. PN G035.5-00.4 has a distance from 3.8 + or - 0.4 kpc to 5.4 + or - 0.7 kpc. IRAS 18551+0159 is at 4.3 + or - 0.5 kpc. We discuss the distance for SNR 35.6-0.4, for which the previous estimate was 10.5 kpc, and find a plausible distance of 3.6 + or - 0.4 kpc. The new distance of SNR G35.6-0.4 and the derived mass for the ~55 km s super(-1) CO molecular cloud can accommodate an association with HESS J1858+020. We also conclude that SNR G35.6-0.4 is unlikely to be associated with PSR J1857+0210 or PSR J1857+0212, which are projected onto the SNR area.
Aims. The association of very-high energy sources with regions of the sky rich in dust and gas has been noticed in the study of individual very-high energy (VHE; E ≳ 100 GeV) sources. However, the ...statistical significance of this correlation for the whole population of TeV detections has not been assessed yet. Here, we present a study of the association of VHE sources in the central Galactic region with positions of enhanced material content. Methods. We obtain estimates of the material content through two classical tracers: dust emission and intensity of the 12CO(1 → 0) line. We make use of the recently released all-sky maps of astrophysical foregrounds of the Planck Collaboration, and of an extensive existing CO mapping of the Galactic sky. To test the correlation, we construct randomized samples of VHE source positions starting from the inner Galactic plane survey sources detected by the High Energy Stereoscopic System (H.E.S.S.) array. Results. We find hints of a positive correlation between positions of VHE sources and regions rich in molecular material, which reaches the 3.9σ level in the best of cases. The latter confidence is however decreased if variations in the selection criteria are considered, which lead us to conclude that a positive correlation cannot be firmly established yet. Forthcoming VHE facilities will be needed to firmly establish the correlation.
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 describe the impact that CTA is expected to have in cosmic ray studies. ► Population studies of supernova remnants will be possible. ► CTA will study the diffuse emission from our Galaxy and ...other galactic systems.
Galactic cosmic rays are commonly believed to be accelerated at supernova remnants via diffusive shock acceleration. Despite the popularity of this idea, a conclusive proof for its validity is still missing. Gamma-ray astronomy provides us with a powerful tool to tackle this problem, because gamma rays are produced during cosmic ray interactions with the ambient gas. The detection of gamma rays from several supernova remnants is encouraging, but still does not constitute a proof of the scenario, the main problem being the difficulty in disentangling the hadronic and leptonic contributions to the emission. Once released by their sources, cosmic rays diffuse in the interstellar medium, and finally escape from the Galaxy. The diffuse gamma-ray emission from the Galactic disk, as well as the gamma-ray emission detected from a few galaxies is largely due to the interactions of cosmic rays in the interstellar medium. On much larger scales, cosmic rays are also expected to permeate the intracluster medium, since they can be confined and accumulated within clusters of galaxies for cosmological times. Thus, the detection of gamma rays from clusters of galaxies, or even upper limits on their emission, will allow us to constrain the cosmic ray output of the sources they contain, such as normal galaxies, AGNs, and cosmological shocks. In this paper, we describe the impact that the Cherenkov Telescope Array, a future ground-based facility for very-high energy gamma-ray astronomy, is expected to have in this field of research.
Very high energy (VHE, >100 GeV) Delta *g-rays are expected to be emitted from the vicinity of supermassive black holes (SMBHs), irrespective of their activity state. In the magnetosphere of rotating ...SMBH, efficient acceleration of charged particles can take place through various processes. These particles could reach energies up to E ~ 1019 eV. VHE Delta *g-ray emission from these particles is then feasible via leptonic or hadronic processes. Therefore, passive systems, where the lack of a strong photon field allows the VHE Delta *g-rays to escape, are expected to be detected by Cherenkov telescopes. We present results from recent VHE experiments on the passive SMBH in the nearby elliptical galaxy NGC 1399. No Delta *g-ray signal has been found, neither by the H.E.S.S. experiment nor in the Fermi data analyzed here. We discuss possible implications for the physical characteristics of the system. We conclude that in a scenario where particles are accelerated in vacuum gaps in the magnetosphere, only a fraction ~0.3 of the gap is available for particle acceleration, indicating that the system is unlikely to be able to accelerate protons up to E ~ 1019 eV.
► Extensive simulations for the design and optimization of CTA were carried out. ► Different sets of simulation tools were checked against each other. ► Large-scale simulations with 275 telescopes ...for later layout selection.► Different analysis methods were applied and compared. ► Resulting sensitivity predictions confirmed the goals of CTA.
The Cherenkov Telescopes Array (CTA) is planned as the future instrument for very-high-energy (VHE) gamma-ray astronomy with a wide energy range of four orders of magnitude and an improvement in sensitivity compared to current instruments of about an order of magnitude. Monte Carlo simulations are a crucial tool in the design of CTA. The ultimate goal of these simulations is to find the most cost-effective solution for given physics goals and thus sensitivity goals or to find, for a given cost, the solution best suited for different types of targets with CTA. Apart from uncertain component cost estimates, the main problem in this procedure is the dependence on a huge number of configuration parameters, both in specifications of individual telescope types and in the array layout. This is addressed by simulation of a huge array intended as a superset of many different realistic array layouts, and also by simulation of array subsets for different telescope parameters. Different analysis methods – in use with current installations and extended (or developed specifically) for CTA – are applied to the simulated data sets for deriving the expected sensitivity of CTA. In this paper we describe the current status of this iterative approach to optimize the CTA design and layout.
Fast radio bursts (FRBs) are bright flashes observed typically at GHz frequencies with millisecond duration, whose origin is likely extragalactic. Their nature remains mysterious, motivating searches ...for counterparts at other wavelengths. FRB 121102 is so far the only source known to repeatedly emit FRBs and is associated with a host galaxy at redshift z ≃ 0.193.We conducted simultaneous observations of FRB 121102 with the Arecibo and MAGIC telescopes during several epochs in 2016-2017. This allowed searches for millisecond time-scale burst emission in very-high-energy (VHE) gamma-rays as well as the optical band. While a total of five FRBs were detected during these observations, no VHE emission was detected, neither of a persistent nature nor burst-like associated with the FRBs. The average integral flux upper limits above 100 GeV at 95 per cent confidence level are 6.6 × 10 -12 photons cm -2 s -1 (corresponding to luminosity LVHE ≲ 10 45 erg s -1 ) over the entire observation period, and 1.2 × 10 -7 photons cm -2 s -1 (LVHE ≳ 10 49 erg s -1 ) over the total duration of the five FRBs. We constrain the optical U-band flux to be below 8.6 mJy at 5σ level for 1-ms intervals around the FRB arrival times. A bright burst with U-band flux 29 mJy and duration ~12 ms was detected 4.3 s before the arrival of one FRB. However, the probability of spuriously detecting such a signal within the sampled time space is 1.5 per cent (2.2, post-trial), i.e. consistent with the expected background. We discuss the implications of the obtained upper limits for constraining FRB models.
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