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Observations of γ-rays from the Crab pulsar suggest that the energy of the pulsar wind changes from electromagnetic to kinetic over a relatively short distance close to the light cylinder of the pulsar. Crab pulsar catches cold wind Recent observations of pulsed high-energy γ-ray emission from the Crab pulsar are explained here by the presence of a cold wind originating near the site of emission. Pulsars are neutron stars thought to eject electron–positron winds, dominated initially by electromagnetic energy. A new estimate of the position of the region in which the kinetic energy of bulk motion begins to dominate over electromagnetic energy suggests that the acceleration of electrons and positrons occurs abruptly in a small zone near the neutron star. Pulsars are thought to eject electron–positron winds that energize the surrounding environment, with the formation of a pulsar wind nebula 1 . The pulsar wind originates close to the light cylinder, the surface at which the pulsar co-rotation velocity equals the speed of light, and carries away much of the rotational energy lost by the pulsar. Initially the wind is dominated by electromagnetic energy (Poynting flux) but later this is converted to the kinetic energy of bulk motion 2 . It is unclear exactly where this takes place and to what speed the wind is accelerated. Although some preferred models imply a gradual acceleration over the entire distance from the magnetosphere to the point at which the wind terminates 3 , 4 , a rapid acceleration close to the light cylinder cannot be excluded 5 , 6 . Here we report that the recent observations of pulsed, very high-energy γ-ray emission from the Crab pulsar 7 , 8 , 9 are explained by the presence of a cold (in the sense of the low energy of the electrons in the frame of the moving plasma) ultrarelativistic wind dominated by kinetic energy. The conversion of the Poynting flux to kinetic energy should take place abruptly in the narrow cylindrical zone of radius between 20 and 50 light-cylinder radii centred on the axis of rotation of the pulsar, and should accelerate the wind to a Lorentz factor of (0.5–1.0) × 10 6 . Although the ultrarelativistic nature of the wind does support the general model of pulsars, the requirement of the very high acceleration of the wind in a narrow zone not far from the light cylinder challenges current models.

Gamma-ray bursts (GRBs) are brief flashes of [gamma]-rays and are considered to be the most energetic explosive phenomena in the Universe.sup.1. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow--produced by the interaction between the ejected matter and the circumburst medium--slows down, and a gradual decrease in brightness is observed.sup.2. GRBs typically emit most of their energy via [gamma]-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments.sup.3. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elusive.sup.4. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow--ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and [gamma]-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.

The recent detection of TeV γ-rays from the microquasar LS 5039 by HESS is one of the most exciting discoveries of observational gamma-ray astronomy in the very high energy regime. This result clearly demonstrates that X-ray binaries with relativistic jets (microquasars) are sites of effective acceleration of particles (electrons and/or protons) to multi-TeV energies. Whether the γ-rays are of hadronic or leptonic origin is a key issue related to the origin of Galactic Cosmic Rays. We discuss different possible scenarios for the production of γ-rays, and argue in favor of hadronic origin of TeV photons, especially if they are produced within the binary system. If so, the detected γ-rays should be accompanied by a flux of high energy neutrinos emerging from the decays of π± mesons produced at pp and/or pγ interactions. The flux of TeV neutrinos, which can be estimated on the basis of the detected TeV γ-ray flux, taking into account the internal γγ → e+ e− absorption, depends significantly on the location of γ-ray production region(s). The minimum neutrino flux above 1 TeV is expected to be at the level of 10−12 cm−2s−1; however, it could be up to a factor of 100 larger. The detectability of the signal of multi-TeV neutrinos significantly depends on the high energy cutoff in the spectrum of parent protons; if the spectrum of accelerated protons continues to 1 PeV and beyond, the predicted neutrino fluxes can be probed by the planned km3-scale neutrino detector.

High energy γ-rays from individual giant molecular clouds contain unique information about the hidden sites of acceleration of galactic cosmic rays, and provide a feasible method for study of propagation of cosmic rays in the galactic disk on scales ≤100 pc. I discuss the spectral features of π^sup 0^-decay γ-radiation from clouds/targets located in proximity of relatively young proton accelerators, and speculate that such `accelerator+target' systems in our Galaxy can be responsible for a subset of unidentified EGRET sources. Also, I argue that the recent observations of high energy γ-rays from the Orion complex contain evidence that the level of the `sea' of galactic cosmic rays may differ significantly from the flux and the spectrum of local (directly detected) particles.[PUBLICATION ABSTRACT]

The potential of a next-generation ground based gamma-ray telescope array has been investigated. In addition to the ideal Gaussian shaped PSF, more realistic non-Gaussian PSFs with tails have been considered and their impact on the detector performance has been studied. The capability of the instrument to resolve multiple sources has been analyzed and the corresponding detector sensitivity estimated. These scenarios are particularly interesting in the framework of Galactic objects, where the observation of more than one source in the same field of view (FoV) is very likely to happen.

Quick-fire cosmic rays New data from the orbiting Chandra X-ray observatory reveal that the X-ray hot spots in the shell of supernova remnant SNR RX J1713.7-3946 undergo brightening and decay on a one-year timescale. This supernova remnant is unusual in that its X-ray emissions are largely non-thermal in origin. The rapid variability points to ultrarelativistic electrons acting via a synchrotron process as the probable X-ray source. All this adds up to unexpectedly rapid cosmic-ray acceleration in a magnetic field amplified by a factor of more than 100. A discovery of the brightening and decay of X-ray hot spots in the shell of the SNR RX J1713.˜73946 on a one-year timescale is reported. This rapid variability shows that the X-rays are produced by ultrarelativistic electrons through a synchrotron process. Galactic cosmic rays (CRs) are widely believed to be accelerated by shock waves associated with the expansion of supernova ejecta into the interstellar medium 1 . A key issue in this long-standing conjecture is a theoretical prediction that the interstellar magnetic field can be substantially amplified at the shock of a young supernova remnant (SNR) through magnetohydrodynamic waves generated by cosmic rays 2 , 3 . Here we report a discovery of the brightening and decay of X-ray hot spots in the shell of the SNR RX J1713.7-3946 on a one-year timescale. This rapid variability shows that the X-rays are produced by ultrarelativistic electrons through a synchrotron process and that electron acceleration does indeed take place in a strongly magnetized environment, indicating amplification of the magnetic field by a factor of more than 100. The X-ray variability also implies that we have witnessed the ongoing shock-acceleration of electrons in real time. Independently, broadband X-ray spectrometric measurements 4 of RX J1713.7-3946 indicate that electron acceleration proceeds in the most effective (‘Bohm-diffusion’) regime. Taken together, these two results provide a strong argument for acceleration of protons and nuclei to energies of 1 PeV (10 15  eV) and beyond in young supernova remnants.

The Enrico Fermi Schools, a cultural initiative promoted by the Italian Physical Society (SIF), were initiated in 1953 in a period that marked the beginnings of what is now called cosmic ray astrophysics.

Gamma-ray bursts (GRBs) are brief flashes of γ-rays and are considered to be the most energetic explosive phenomena in the Universe1. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow—produced by the interaction between the ejected matter and the circumburst medium—slows down, and a gradual decrease in brightness is observed2. GRBs typically emit most of their energy via γ-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments3. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elusive4. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow—ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and γ-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies

Galactic cosmic rays reach energies of at least a few petaelectronvolts1 (of the order of 1015 electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators (‘PeVatrons’), but all proposed models of Galactic cosmic-ray accelerators encounter difficulties at exactly these energies2. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of teraelectronvolts (of the order of 1013 electronvolts) were inferred from recent γ-ray observations3. However, none of the currently known accelerators—not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays—has shown the characteristic tracers of petaelectronvolt particles, namely, power-law spectra of γ-rays extending without a cut-off or a spectral break to tens of teraelectronvolts4. Here we report deep γ-ray observations with arcminute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of petaelectronvolt protons within the central 10 parsecs of the Galaxy. We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outbursts5and an outflow from the Galactic Centre6. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 106–107 years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays.

The nearby radio galaxy Centaurus A belongs to a class of active galaxies that are luminous at radio wavelengths. Most show collimated relativistic outflows known as jets, which extend over hundreds of thousands of parsecs for the most powerful sources. Accretion of matter onto the central supermassive black hole is believed to fuel these jets and power their emission 1 . Synchrotron radiation from relativistic electrons causes the radio emission, and it has been suggested that the X-ray emission from Centaurus A also originates in electron synchrotron processes 2 – 4 . Another possible explanation is inverse Compton scattering with cosmic microwave background (CMB) soft photons 5 – 7 . Synchrotron radiation needs ultrarelativistic electrons (about 50 teraelectronvolts) and, given their short cooling times, requires some continuous re-acceleration mechanism 8 . Inverse Compton scattering, on the other hand, does not require very energetic electrons, but the jets must stay highly relativistic on large scales (exceeding 1 megaparsec). Some recent evidence disfavours inverse Compton-CMB models 9 – 12 , although other work seems to be compatible with them 13 , 14 . In principle, the detection of extended γ-ray emission, which directly probes the presence of ultrarelativistic electrons, could distinguish between these options. At gigaelectronvolt energies there is also an unusual spectral hardening 15 , 16 in Centaurus A that has not yet been explained. Here we report observations of Centaurus A at teraelectronvolt energies that resolve its large-scale jet. We interpret the data as evidence for the acceleration of ultrarelativistic electrons in the jet, and favour the synchrotron explanation for the X-rays. Given that this jet is not exceptional in terms of power, length or speed, it is possible that ultrarelativistic electrons are commonplace in the large-scale jets of radio-loud active galaxies. Observations of the radio galaxy Centaurus A at teraelectronvolt energies resolve its large-scale jet and favour electron synchrotron processes as the source of its X-ray emission.