Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

We consider implications of the IceCube signal for hadronuclear (pp) scenarios of neutrino sources such as galaxy clusters/groups and star-forming galaxies. Since the observed neutrino flux is comparable to the diffuse -ray background flux obtained by Fermi, we place new, strong upper limits on the source spectral index, À & 2:1-2:2. In addition, the new IceCube data imply that these sources contribute at least 30%-40% of the diffuse -ray background in the 100 GeV range and even $100% for softer spectra. Our results, which are insensitive to details of the pp source models, are one of the first strong examples of the multimessenger approach combining the measured neutrino and -ray fluxes. The pp origin of the IceCube signal can further be tested by constraining À with sub-PeV neutrino observations, by unveiling the sub-TeV diffuse -ray background and by observing such pp sources with TeV -ray detectors. We also discuss specific pp source models with a multi-PeV neutrino break/cutoff, which are consistent with the current IceCube data.

We study high-energy neutrino production in inner jets of radio-loud active galactic nuclei (AGN), taking into account effects of external photon fields and the blazar sequence. We show that the resulting diffuse neutrino intensity is dominated by quasar-hosted blazars, in particular, flat spectrum radio quasars, and that PeV-EeV neutrino production due to photohadronic interactions with broadline and dust radiation is unavoidable if the AGN inner jets are ultrahigh-energy cosmic-ray (UHECR) sources. Their neutrino spectrum has a cutoff feature around PeV energies since target photons are due to Lyα emission. Because of infrared photons provided by the dust torus, neutrino spectra above PeV energies are too hard to be consistent with the IceCube data unless the proton spectral index is steeper than 2.5, or the maximum proton energy is 100 PeV. Thus, the simple model has difficulty in explaining the IceCube data. For the cumulative neutrino intensity from blazars to exceed ∼ 10 −8 GeV cm −2 s −1 sr −1 , their local cosmic-ray energy generation rate would be ∼ 10-100 times larger than the local UHECR emissivity, but is comparable to the averaged γ-ray blazar emissivity. Interestingly, future detectors such as the Askaryan Radio Array can detect ∼ 0.1-1 EeV neutrinos even in more conservative cases, allowing us to indirectly test the hypothesis that UHECRs are produced in the inner jets. We find that the diffuse neutrino intensity from radio-loud AGN is dominated by blazars with γ-ray luminosity of 10 48 erg s −1 , and the arrival directions of their ∼ 1-100 PeV neutrinos correlate with the luminous blazars detected by Fermi.PACS numbers: 95.85. Ry, 98.54.Cm, 98.70.Rz, 98.70.Vc

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. ?? 2013 Elsevier B.V. All rights reserved

We consider limits on the local (z = 0) density (n0) of extragalactic neutrino sources set by the nondetection of steady high-energy neutrino sources producing 50 TeV muon multiplets in the present IceCube data, taking into account the redshift evolution, luminosity function and neutrino spectrum of the sources. We show that the lower limit depends weakly on source spectra and strongly on redshift evolution. We find n0 10 −8 −10 −7 Mpc −3 for standard candle sources evolving rapidly, Applying these results to a wide range of classes of potential sources, we show that powerful "blazar" jets associated with active galactic nuclei are unlikely to be the dominant sources. For almost all other steady candidate source classes (including starbursts, radio galaxies, and galaxy clusters and groups), an order of magnitude increase in the detector sensitivity at ∼ 0.1 − 1 PeV will enable a detection (as point sources) of the few brightest objects. Such an increase, which may be provided by next-generation detectors like IceCube-Gen2 and an upgraded KM3NET, can improve the limit on n0 by more than two orders of magnitude. Future gamma-ray observations (by Fermi, HAWC and CTA) will play a key role in confirming the association of the neutrinos with their sources.PACS numbers: 95.85. Ry, 98.70.Sa, 98.70.Vc

Detection of the IceCube-170922A neutrino coincident with the flaring blazar TXS 0506+056, the first and only ∼3σ high-energy neutrino source association to date, offers a potential breakthrough in our understanding of highenergy cosmic particles and blazar physics. We present a comprehensive analysis of TXS 0506+056 during its flaring state, using newly collected Swift, NuSTAR, and X-shooter data with Fermi observations and numerical models to constrain the blazar's particle acceleration processes and multimessenger (electromagnetic and high-energy neutrino) emissions. Accounting properly for electromagnetic cascades in the emission region, we find a physically-consistent picture only within a hybrid leptonic scenario, with γ-rays produced by external inverse-Compton processes and highenergy neutrinos via a radiatively-subdominant hadronic component. We derive robust constraints on the blazar's neutrino and cosmic-ray emissions and demonstrate that, because of cascade effects, the 0.1-100 keV emissions of TXS 0506+056 serve as a better probe of its hadronic acceleration and high-energy neutrino production processes than its GeV-TeV emissions. If the IceCube neutrino association holds, physical conditions in the TXS 0506+056 jet must be close to optimal for high-energy neutrino production, and are not favorable for ultra-high-energy cosmic-ray acceleration. Alternatively, the challenges we identify in generating a significant rate of IceCube neutrino detections from TXS 0506+056 may disfavor single-zone models, in which γ-rays and high-energy neutrinos are produced in a single emission region. In concert with continued operations of the high-energy neutrino observatories, we advocate regular X-ray monitoring of TXS 0506+056 and other blazars in order to test single-zone blazar emission models, clarify the nature and extent of their hadronic acceleration processes, and carry out the most sensitive possible search for additional multimessenger sources.

We study high-energy neutrino production in collimated jets inside progenitors of gamma-ray bursts (GRBs) and supernovae, considering both collimation and internal shocks. We obtain simple, useful constraints, using the often overlooked point that shock acceleration of particles is ineffective at radiation-mediated shocks. Classical GRBs may be too powerful to produce high-energy neutrinos inside stars, which is consistent with IceCube nondetections. We find that ultralong GRBs avoid such constraints and detecting the TeV signal will support giant progenitors. Predictions for lowpower GRB classes including low-luminosity GRBs can be consistent with the astrophysical neutrino background that IceCube may detect, with a spectral steepening around PeV. The models can be tested with future GRB monitors.PACS numbers: 95.85. Ry, 97.60.Bw, 98.70.Rz Long gamma-ray bursts (GRBs) are believed to originate from relativistic jets launched at the death of massive stars. Associations with core-collapse supernovae (CCSNe) have provided strong evidence for the GRB-CCSN relationship [1]. But, there remain many important questions. What makes the GRB-CCSN connection? How universal is it? What is the central engine and progenitor of GRBs? How are jets launched and accelerated? Observationally, it is not easy to probe physics inside a star with photons until the jet breaks out and the photons leave the system. This is always the case if the jet is "chocked" rather than "successful" [2]; that is, the jet stalls inside the star, where the electromagnetic signal is unobservable. Such failed GRBs may be much more common than GRBs (whose true rate is ∼ 10 −3 of that of all CCSNe), and CCSNe driven by mildly relativistic jets may make up a few present of all CCSNe [3][4][5].Recent observations suggest interesting diversity in the GRB population. "Low-power GRBs" such as lowluminosity (LL) GRBs [3,6,7] and ultralong (UL) GRBs [8,9] have longer durations (∼ 10 3 -10 4 s) compared to that of classical long GRBs, suggesting different GRB classes and larger progenitors. While they were largely missed in previous observations, they are important for the total energy budget and the GRB-CCSN connection.Neutrinos and gravitational waves (GWs) can present special opportunities to address the above issues. In particular, IceCube is powerful enough to see high-energy (HE) neutrinos at 1 TeV [10] and has reported the first detections of cosmic PeV neutrinos [11]. Efficient HE neutrino production inside a star has been proposed assuming shock acceleration of cosmic rays (CRs) [12][13][14], and investigated by a lot of authors, since their detection allows us to study the GRB-CCSN connection [13,15], joint searches with GWs [16], neutrino mixing including the matter effect [17], the nature of GRB progenitors [18] and so on. However, IceCube has not detected neutrinos from GRBs, putting limits on this scenario as well as the classical prompt emission scenario [19,20]. It also constrains orphan neutrinos from a CCSN [21].In this work, we consider HE ne...

The spectra of BL Lac objects and Fanaroff-Riley I radio galaxies are commonly explained by the one-zone leptonic synchrotron self-Compton (SSC) model. Spectral modeling of correlated multiwavelength data gives the comoving magnetic field strength, the bulk outflow Lorentz factor and the emission region size. Assuming the validity of the SSC model, the Hillas condition shows that only in rare cases can such sources accelerate protons to much above 10 19 eV, so 10 20 eV ultra-high-energy cosmic rays (UHECRs) are likely to be heavy ions if powered by this type of radio-loud active galactic nuclei (AGN). Survival of nuclei is shown to be possible in TeV BL Lacs and misaligned counterparts with weak photohadronic emissions. Another signature of hadronic production is intergalactic UHECR-induced cascade emission, which is an alternative explanation of the TeV spectra of some extreme non-variable blazars such as 1ES 0229+200 or 1ES 1101-232. We study this kind of cascade signal, taking into account effects of the structured extragalactic magnetic fields in which the sources should be embedded. We demonstrate the importance of cosmic-ray deflections on the γ-ray flux, and show that required absolute cosmic-ray luminosities are larger than the average UHECR luminosity inferred from UHECR observations and can even be comparable to the Eddington luminosity of supermassive black holes. Future TeV γ-ray observations using the Cherenkov Telescope Array and the High Altitude Water Cherenkov detector array can test for UHECR acceleration by observing > 25 TeV photons from relatively low-redshift sources such as 1ES 0229+200, and TeV photons from more distant radio-loud AGN.