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Register a new userHigh energy emission from transients
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Added by
Rhaana L. C. Starling
Publication date
2013
fields
Physics
and research's language is
English
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Cosmic explosions dissipate energy into their surroundings on a very wide range of time-scales: producing shock waves and associated particle acceleration. The historical culprits for the acceleration of the bulk of Galactic cosmic rays are supernova remnants: explosions on ~10000 year time-scales. Increasingly however, time-variable emission points to rapid and efficient particle acceleration in a range of different astrophysical systems. Gamma-ray bursts have the shortest time-scales, with inferred bulk Lorentz factors of ~1000 and photons emitted beyond 100 GeV, but active galaxies, pulsar wind nebulae and colliding stellar winds are all now associated with time-variable emission at ~TeV energies. Cosmic photons and neutrinos at these energies offer a powerful probe of the underlying physical mechanisms of cosmic explosions, and a tool for exploring fundamental physics with these systems. Here we discuss the motivations for high-energy observations of transients, the current experimental situation, and the prospects for the next decade, with particular reference to the major next-generation high-energy observatory CTA.
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We present an overview of high energy transients in astrophysics, highlighting important advances over the past 50 years. We begin with early discoveries of gamma-ray transients, and then delve into physical details associated with a variety of phenomena. We discuss some of the unexpected transients found by Fermi and Swift, many of which are not easily classifiable or in some way challenge conventional wisdom. These objects are important insofar as they underscore the necessity of future, more detailed studies.
Search for high energy transients in the millisecond domain has come to the focus in recent times due to the detection of Gravitational Wave events and the identification of Fast Radio Bursts as cosmological sources. I will highlight the sensitivity limitations in the currently operating hard X-ray telescopes and give some details of the search for millisecond events in the AstroSat CZT Imager data.
In this chapter we review some aspects of X-ray binaries, particularly those presenting steady jets, i.e. microquasars. Because of their proximity and similarities with active galactic nuclei (AGN), galactic jet sources are unique laboratories to test astrophysical theories of a universal scope. Due to recent observational progress made with the new generation of gamma-ray imaging atmospheric Cherenkov telescopes and in view of the upcoming km3-size neutrino detectors, we focus especially on the possible high-energy gamma radiation and neutrino emission. In connection with this, we also comment about astrophysical jets present in young stellar objects, and we briefly discuss similarities and differences with extragalactic AGN and gamma-ray bursters.
The vast majority of pulsars detected by the Fermi Large Area Telescope (LAT) display exponentially cutoff spectra with cutoffs falling in a narrow band around a few GeV. Early spectral modelling predicted spectral cutoffs at energies of up to 100 GeV, assuming curvature radiation. It was therefore not expected that pulsars would be visible in the very-high energy (VHE) regime (>100 GeV). The VERITAS announcement of the detection of pulsed emission from the Crab pulsar at energies up to 400 GeV (and now up to 1.5 TeV as detected by MAGIC) therefore raised important questions about our understanding of the electrodynamics and local environment of pulsars. H.E.S.S. has now detected pulsed emission from the Vela pulsar down to tens of GeV, making this the second pulsar detected by a ground-based Cherenkov telescope. Deep upper limits have also been obtained by VERITAS and MAGIC for the Geminga pulsar. We will review the latest developments in VHE pulsar science, including an overview of the latest observations, refinements, and extensions to radiation models and magnetic field structures, and the implementation of new radiation mechanisms. This will assist us in understanding the VHE emission detected from the Crab pulsar, and predicting the level of VHE emission expected from other pulsars, which is very important for the upcoming CTA.
The number of Gamma-Ray Bursts (GRBs) detected at high energies ($sim,0.1-100$ GeV) has seen a rapid increase over the last decade, thanks to observations from the Fermi-Large Area Telescope. The improved statistics and quality of data resulted in a better characterisation of the high-energy emission properties and in stronger constraints on theoretical models. In spite of the many achievements and progresses, several observational properties still represent a challenge for theoretical models, revealing how our understanding is far from being complete. This paper reviews the main spectral and temporal properties of $sim,0.1-100$ GeV emission from GRBs and summarises the most promising theoretical models proposed to interpret the observations. Since a boost for the understanding of GeV radiation might come from observations at even higher energies, the present status and future prospects for observations at very-high energies (above $sim$ 100 GeV) are also discussed. The improved sensitivity of upcoming facilities, coupled to theoretical predictions, supports the concrete possibility for future ground GRB detections in the high/very-high energy domain.
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