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The Galaxy contains a small but quite significant population of highly energetic denizens: supernova remnants with fast shocks, pulsars with powerful winds, intensely-interacting binary systems built from a compact object and a massive star. All of these environments conspire to generate non-thermal populations of particles, and radiation produced by these particles is gradually revealing the methods by which Nature accelerates cosmic rays, as well as the ways in which those cosmic rays escape and diffuse into the interstellar medium. In this talk, we discuss advances in our understanding of these environments and processes provided by recent results from VERITAS, an array of ground-based imaging air-Cherenkov telescopes located at the Whipple Observatory in southern Arizona and sensitive to gamma rays in the energy range from 85 GeV to > 30 TeV. These results include studies of cosmic-ray acceleration in the supernova remnants Cassiopeia A and IC 443, as well as the remarkable Fall 2017 periastron passage of VER J2032+4127, the binary system containing PSR J2032+4127 and the Be star MT91 213 with a 50-year period.

Our universe carries a small but important population of highly energetic denizens: supernova remnants with fast shocks, pulsars with powerful winds, intensely-interacting binary systems built from a compact object and a massive star, relativistic jets launched by supermassive black holes. All of these environments conspire to generate populations of nonthermal particles, and observations of the very high energy (VHE; E > 100 GeV) gamma rays produced by these particles are gradually revealing the methods by which Nature accelerates cosmic rays, as well as the ways in which those cosmic rays escape and diffuse into the interstellar medium. These observations include studies of cosmic-ray acceleration in the supernova remnants Cassiopeia A and IC 443, follow-up of unidentified HAWC sources, and the remarkable Fall 2017 periastron passage of VER J2032+4127, the 50-year-period binary system containing PSR J2032+4127 and a Be star. Fast TeV gamma-ray flares coincidental with the emergence of superluminal radio knots from the blazar BL Lac can be interpreted in terms of a coherent scenario of jet particle flow and radiation. The recent TeV gamma-ray discovery of the radio galaxy 3C 264 adds a new member to the small population of off-axis jets available for study. Meanwhile, the direct detection for the first time of gravitational wave (GW) transients by Advanced LIGO has motivated searches for their electromagnetic counterparts at all wavelengths. Neutrino astronomy is an emerging area of study in high-energy astrophysics, and astrophysical neutrinos are natural cousins of VHE gamma rays. The VERITAS gamma-ray observatory has an active program of follow-up observations in the directions of potentially astrophysical high-energy neutrinos detected by IceCube, as well as in the direction of GW transients. In this talk, we discuss recent results from the VERITAS galactic, extragalactic, and multi-messenger Follow-up programs.

Recent debate (Clack v. Jacobson, 2017) argues the feasibility of 100% terrestrial renewables (wind, water, solar) by 2050, on the premise of restricting world consumption to 2012 levels (∼12 TW-years). Given the expected population rise by 3 billion over the same time frame, and the correlation between prosperity and energy availability – are we to impose energy equality, requiring some to reduce consumption by 87%, or are we to condemn the majority to relentless poverty? Choosing neither implies ever-increasing carbon emissions and the risk of catastrophic climate change. Nuclear fission is one energy technology which could be expanded to provide sufficient carbon-free power, but faces widespread opposition from public fear and distrust. Future terrestrial fusion is another, but first pilot operations are not expected until 2050 – which may be too late. We could make much better use of one existing fusion power source, our Sun. The fundamentals of Space Solar Power (SSP) are well understood and could lead to a world of energy abundance; the deliverable energy from just a 10 km geostationary (GEO) band exceeds 570 TW-years – enough to supply ten billion people at six-times current US per-capita levels. Despite this, SSP has languished for fifty years. GEO is one of few candidates for baseload power, but physics dictates a kilometre-scale microwave transmitter irrespective of the power delivered – hence economics favours the multi-gigawatt (per-satellite) engineering limit. Given the complexity of the differentially rotating solar collector, sub-gigawatt SSP suffers both economically and technically, with different solutions required at different scales – which has led to exorbitant (hence prohibitive) start-up costs. CASSIOPeiA breaks this non-scaling paradigm by eliminating the rotating interfaces; all SPS subsystems are able to share one lightweight modular structure, with near-invariant areal power density from sub-megawatt to gigawatt systems. With additional fixed mirror concentrators, CASSIOPeiA can also be expanded into the multi-gigawatt regime. CASSIOPeiA's unique beam-steering capability facilitates baseload delivery from alternative, closer orbits, with the possibility of single payload deployment requiring no on-orbit assembly. By starting with sub-megawatt, near-term stratospheric station-keeping platforms – with retrieval, servicing and transfer of gained knowledge – the era of SSP may commence at much lower risk and expense.

In a simple model it is demonstrated that the neutron star surface temperature evolution is sensitive to the phase state of the triplet superfluid condensate. A multicomponent triplet pairing of superfluid neutrons in the core of a neutron star with participation of several magnetic quantum numbers leads to neutrino energy losses exceeding the losses from the unicomponent pairing. A phase transition of the neutron condensate into the multicomponent state triggers more rapid cooling of superfluid core in neutron stars. This makes it possible to simulate an anomalously rapid cooling of neutron stars within the minimal cooling paradigm without employing any exotic scenarios suggested earlier for rapid cooling of isolated neutron star in Cassiopeia A., Comment: 15 pages, 1 figure, Phys. Lett. B (2014)

We have searched for the three decay lines of the 44Ti in Cassiopeia A with the spectrometer SPI on-board INTEGRAL. The lines at 78.4 and 1157.0 keV have been detected with a flux of (2.1 ± 0.7) × 10−5 ph cm−2 s−1, consistent with all previous measurements combined. The spectral capabilities of SPI indicate that the 44Ti ejecta expands at about 430 km s−1 and moves away from the observer at about 500 km s−1. Such a small expansion velocity together with the 44Ti yield of the event are consistent with a spherical explosion but an asymmetric explosion due for instance to prolate neutrino emission could not be definitely excluded.

The development of observational techniques has enriched our knowledge of supernova remnants. In this paper, we review the main progresses in the last decade, including new discoveries of supernova remnants and the associated (rare type of) pulsars, nucleosynthesis, the interaction between supernova remnants and molecular clouds, dust in the supernova remnants, shock physics, and cosmic ray accelerations. (C) 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved.

Radioactive 44 Ti is of great interest as a diagnostic of the core collapse supernova explosion mechanism and as the source of stable 44 Ca in our Galaxy. Its decay gamma-ray lines have been measured several times in the supernova remnant Cassiopeia A, with some inconsistencies among the measurements. This isotope’s decay ( 44 Ti → 44 Sc) also produces X-ray fluorescence emission following the electron capture, which might provide an alternate method of direct measurement of the mass of 44 Ti ejected, and the location of the 44 Ti within the remnant. Here we search for the Sc K-α emission in Chandra ACIS observations of Cassiopeia A. This instrument has sufficient effective area to detect the expected line flux, but because of the large continuum flux, we thus far have obtained only upper limits on the line emission. In two Chandra observations the 2 σ upper limit for the 4.09 keV line flux from the entire remnant is ∼3.8 × 10 −5 γ /cm 2 s. We also study more limited parts of the remnant, including apparently iron-rich regions of CasA, and discuss possible complications and limitations of this method.

The presence of photon induced ionization from supernovae at detectable levels in the polar ice sheets has been an interesting issue for some time. Evidence for such a signal is reported in 1200-year time sequences from the Antarctic continent at South Pole and Vostok, respectively. The supernova candidate events all appear as nitrate concentration spikes apparently caused by ionization from high energy radiation interacting with the upper atmosphere of the Earth. Further support for this interpretation has been obtained from an ice core drilled in Greenland covering 430 years. These ultrahigh resolution measurements (averaging 18 data points/year) have revealed nitrate anomalies at the times of the Tycho and Kepler Supernovae. In addition, the possibility is being suggested that the supernova Cassiopeia A appears in the record, thus pinpointing its year of occurrence. Other, less understood phenomena such as γ-ray bursts also occasionally may produce anomalies in the nitrate record.

We present an analysis of the BeppoSAX high X-ray energy spectrum of the supernova remnant Cassiopeia A with an observation time of 83 103 s. We measure a flux upper limit of 4.1 10−5 ph cm−2 s−1 (99.7% confidence) of the nuclear decay lines of44Ti at 68 keV and 78 keV that is lower and inconsistent with the flux of an accompanying line at 1157 keV measured by CGRO's Comptel. However, if the underlying X-ray continuum is lower, because the spectrum is steepening, the actual44Ti may be higher and consistent with the Comptel result, although the measured flux of (2.9±1.0) 10−5 ph cm−2 s−1 under this assumption is still lower than the flux measured by Comptel.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the COMPTON Observatory was designed to undertake a broad range of high energy astronomical observations. Many of these have been realized during the first three years of the mission. In this paper we present several of the recent results from OSSE. We highlight recent OSSE results on the galactic center region and on transient galactic X-ray sources. In addition, we present OSSE results on the average spectrum of Seyfert galaxies, the search for Ti-44 emission from Cas A, and hard X-ray emission from SN1993j.

During the first three years of operation, the imaging Compton telescope COMPTEL on board of the Compton Gamma-Ray Observatory has observed MeV-gamma radiation from a variety of objects including pulsars, stellar black-hole candidates, supernova remnants, extended regions of interstellar space, nuclei of active galaxies, gamma-ray bursters and solar flares. In the field of gamma-ray line spectroscopy, COMPTEL has achieved a major break-through with the establishment of the first map of the galactic plane in the light of the 1.8 MeV line from radioactive 26Al, with the discovery of the 1.15 MeV 44Ti-line from Cas A, and with the first detection of gamma-ray emission from the Orion complex in the 3 to 7 MeV range that can be identified with the 4.44 MeV and 6.13 MeV nuclear deexcitation lines of 12C∗ and 16O∗, respectively. An overview about the COMPTEL highlight results is given.

Thanks to mid-infrared observations, we provide new clues to the element mixing during a supernova explosion by probing the mixing between three adjacent layers: the oxygen burning products layer (sulfur, argon,...), the silicate layer and the neon layer. The silicate and neon layers are both contaminated by sulfur and argon in a macroscopic way, but appear segregated, so that the mixing is heterogeneous. This finding complements the microscopic mixing information deduced from presolar grains found in meteorites and implies that, at present time, supernovae are probably not the main dust factory in the Galaxy. The mixing is often interpreted in terms of hydrodynamical instabilities driven by the outward shock following the implosion of the supernova core. Testing whether such instabilities can lead to the injection of material from a layer into upper layers without complete mixing, as suggested by the observations presented in this paper, should be possible with the intense lasers which are starting to be used to simulate astrophysical plasmas.

A multiwire proportional counter capable of imaging soft X-rays has been constructed. The position of an incident photon is obtained by measuring the induced charge signals on two orthogonal sets of cathode wire grids. The anode signal is used as trigger and energy signal. At a photon energy of 0.94 keV the achieved position resolution is 300 μm (FWHM), and the energy resolution is 45% (FWHM). With a five sided anticoincidence the background rejection efficiency is 90–95%. A small version of the counter has successfully been flown twice on sounding rocket payloads. X-ray images of the supernova remnants Puppis A and Cassiopeia A have been taken. As the prime focal plane detector for the ROSAT project we are building a version with an aperture of 80 mm.

The young supernova remnant Cas A has been observed with the imaging proportional counter (PSD) of the EXOSAT observatory. High quality spatially resolved, spectral data allow for the first time the determination of the temperature structure of the remnant. Preliminary results on the distribution of temperature and emission measureover the remnant are presented.

Records of scintillations of the source Cassiopeia A made on 38 Mc/s using three spaced interferometers have been analysed to determine whether Fourier components of different frequencies give different values of drift velocity. Such an effect is found to occur, and in most cases the speed of drift increases with frequency, while the direction of drift does not change. This effect is related to a systematic ‘skewness’ of the cross-correlation functions. The dispersion could be produced by the propagation of some type of dispersive wave through the F-region of the ionosphere, but it is considered more likely that it is produced by a variation of drift velocity along the line of sight.

A statistical comparison of the depth of scintillation of the 54 MHz beacon of satellite, Transit 4A, to the depth consistent with 63 and 113 MHz observations of radio star, Cassiopeia A, indicates that (a) a thin screen of irregularities of height about 450 km and size somewhat greater than 1 km may represent weak scintillation during quiet geomagnetic periods but that (b) for scintillation indices above about 20 per cent or during periods with local magnetic K-index above 2, the analysis should be based on a thick layer of irregularities.

Recordings of signal power from satellites Transit 4A and Cosmos I and from radiostar sources Cassiopeia A and Cygnus A were made during April 1962. The percentage of signal variation was analyzed at frequencies from 20 Mc/s to 200 Mc/s. Satellite records show that the elevation angle dependence of scintillation index is partially obscured by the latitude distribution of the ionosphere irregularities. A region of dense irregularities is found near the auroral zone. During periods of magnetic activity this region moves away from the auroral zone. From a study of the dependence of radio-star scintillation index on wavelength, sizes of individual irregularities are derived. A model for large scale irregularity regions is proposed to account for the latitude variations found on satellite records.

Some observations of the phenomenon known as ‘inversion’ in records of scintillation of signals from the sources Cassiopeia A and Cygnus A are reviewed briefly. It is shown that the phenomenon is more likely to be explained by the finite angular diameters of these sources than, as suggested by previous authors, by the aperture size of the antenna used in making the observations. An experiment is suggested which should distinguish between these and other criteria which can bring about the observation of inversion.