Gamma-ray bursts (GRBs) have been an enigma since their discovery forty years ago. However, considerable progress unraveling their mysteries has been made in recent years. Developments in ...observations, theory, and instrumentation have prepared the way so that the next decade can be the one in which we finally answer the question, "What are gamma-ray bursts?" This question encompasses not only what the progenitors are that produce the GRBs, but also how the enormous luminosity of the GRBs, concentrated in gamma rays, is achieved. Observations across the electromagnetic spectrum, from both the ground and space, will be required to fully tackle this important question. This white paper, mostly distilled from a recent study commissioned by the Division of Astrophysics of the American Physical Society, focuses on what very high energy (~100 GeV and above) gamma-ray observations can contribute. Very high energy gamma rays probe the most extreme high energy particle populations in the burst environment, testing models of lepton and proton acceleration in GRBs and constraining the bulk Lorentz factor and opacity of the outflow. Sensitivity improvements of more than an order of magnitude in the very high energy gamma-ray band can be achieved early in the next decade, in order to contribute to this science.
This is a short report on the preliminary findings of the gamma ray burst (GRB) working group for the white paper on the status and future of very high energy (VHE; >50 GeV) gamma-ray astronomy. The ...white paper discusses the status of past and current attempts to observe GRBs at GeV-TeV energies, including a handful of low-significance, possible detections. The white paper concentrates on the potential of future ground-based gamma-ray experiments to observe the highest energy emission ever recorded for GRBs, particularly for those that are nearby and have high Lorentz factors in the GRB jet. It is clear that the detection of VHE emission would have strong implications for GRB models, as well as cosmic ray origin. In particular, the extended emission phase (including both afterglow emission and possible flaring) of nearby long GRBs could provide the best possibility for detection. The difficult-to-obtain observations during the prompt phase of nearby long GRBs and short GRBs could also provide particularly strong constraints on the opacity and bulk Lorentz factors surrounding the acceleration site. The synergy with upcoming and existing observatories will, of course, be critical for both identification of GRBs and for multiwavelength/multimessenger studies.
This is a report on the findings of the gamma ray burst working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper is an APS commissioned document, and the ...overall version has also been released and can be found on astro-ph. This detailed section of the white paper discusses the status of past and current attempts to observe gamma ray bursts at GeV-TeV energies. We concentrate on the potential of future ground-based gamma-ray experiments to observe the highest energy emission ever recorded for GRBs, particularly for those that are nearby and have high Lorentz factors in the GRB jet. It is clear that major advances are possible and that the detection of very high energy emission would have strong implications for GRB models, as well as cosmic ray origin.
Recent accurate measurements of cosmic-ray (CR) species by ATIC-2, CREAM, and PAMELA reveal an unexpected hardening in the proton and He spectra above a few hundred GeV, a gradual softening of the ...spectra just below a few hundred GeV, and a harder spectrum of He compared to that of protons. These newly-discovered features may offer a clue to the origin of high-energy CRs. We use the \({\it Fermi}\) Large Area Telescope observations of the \(\gamma\)-ray emission from the Earth's limb for an indirect measurement of the local spectrum of CR protons in the energy range \(\sim 90~\)GeV-\(6~\)TeV (derived from a photon energy range \(15~\)GeV-\(1~\)TeV). Our analysis shows that single power law and broken power law spectra fit the data equally well and yield a proton spectrum with index \(2.68 \pm 0.04\) and \(2.61 \pm 0.08\) above \(\sim 200~\)GeV, respectively.
Connective tissue remodeling of the extracellular matrix (ECM) is an essential and dynamic process associated with both physiological responses, such as wound healing, and pathological conditions, ...such as renal tubulointerstitial fibrosis (TIF). Data from the published literature indicate that collagens and several non-collagenous ECM glycoproteins actively contribute to TIF. The early phase of TIF is usually associated with an inflammatory process mediated by soluble factors released by activated resident cells and by infiltrating cells. Fibrogenic cytokines and growth factors secreted by inflammatory cells and fibroblasts are actively involved in connective tissue remodeling, possibly by regulating the rate of synthesis and degradation of the ECM. An uncontrolled balance of this process usually results in TIF. We review the physiology of wound healing and the pathology of fibrosis, emphasizing TIF.
We reevaluate our published calculations of electromagnetic showers generated by GEANT 3.21 and the radio frequency pulses they produce in ice. We are prompted by a recent report showing that GEANT ...3.21-modeled showers are sensitive to internal settings in the electron tracking subroutine. We report the shower and pulse characteristics obtained with different settings of GEANT 3.21 and with GEANT 4. The default setting of electron tracking in GEANT 3.21 we used in previous work speeds up the shower simulation at the cost of information near the end of the tracks. We find that settings tracking electron and positron to lower energy yield a more accurate calculation, a more intense shower, and proportionately stronger radio pulses at low frequencies. At high frequencies the relation between shower tracking algorithm and pulse spectrum is more complex. We obtain radial distributions of shower particles and phase distributions of pulses from 100 GeV showers that are consistent with our published results.
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