The local energy production rates of GRB photons and of UHECRs

In a recent analysis it was found that the local (z=0) rate at which gamma-ray bursts (GRBs) produce energy in 1 MeV photons, Q_GRB(z=0), is 300 times lower than the local energy production rate in ultra-high energy cosmic-rays. This may appear to be in contradiction with earlier results, according to which Q_GRB(z=0) is similar to the local energy production rate in >10^{19} eV cosmic-rays, Q_{10EeV}(z=0). This short (1 page) note identifies the origin of the apparent discrepancy and shows that Q_GRB(z=0) \sim Q_{10EeV}(z=0) holds.Comment: 1 pag

High energy particles from gamma-ray bursts

A review is presented of the fireball model of gamma-ray bursts (GRBs), and of the production in GRB fireballs of high energy protons and neutrinos. Constraints imposed on the model by recent afterglow observations, which support the association of GRB and ultra-high energy cosmic-ray (UHECR) sources, are discussed. Predictions of the GRB model for UHECR production, which can be tested with planned large area UHECR detectors and with planned high energy neutrino telescopes, are reviewed.Comment: 33 pages. Based on lectures given at the ICTP Summer School (ICTP, Italy, June 2000), and at the VI Gleb Wataghin School (UNICAMP, Brazil, July 2000

On the origin of fast radio bursts (FRBs)

We derive stringent constraints on the persistent source associated with FRB 121102: Size $10^{17}$ cm $<R<10^{18}$ cm, age $<300$ yr, characteristic electron energy $\varepsilon_e\sim0.3$ GeV, total energy $\sim10^{49}$ erg. The hot radiating plasma is confined by a cold plasma of mass $M_c<0.03 (R/10^{17.5}{\rm cm})^4 M_\odot$. The source is nearly resolved, and may be resolved by 10 GHz observations. The fact that $\varepsilon_e\sim m_p c^2$ suggests that the hot plasma was created by the ejection of a mildly relativistic, $M\sim10^{-5} M_\odot$ shell, which propagated into an extended ambient medium or collided with a pre-ejected shell of mass $M_c$. The inferred plasma properties are inconsistent with typical "magnetar wind nebulae" model predictions. We suggest a physical mechanism for the generation of FRBs (independent of the persistent source model): Ejection from an underlying compact object, $R_s\sim10^{6}$ cm, of highly relativistic shells, with energy $E_s=10^{41}$ erg and Lorentz factor $\gamma_s$~$10^3$, into a surrounding e-p plasma with density $n\sim0.1/cm^3$ (consistent with that inferred for the plasma producing the persistent emission associated with FRB 121102). Such shell ejections with energy typical for FRB events lead to plasma conditions appropriate for strong synchrotron maser emission at the GHz range, $\nu_{ coh.}\sim0.5(E/10^{41}erg)^{1/4}$ GHz. In this model, a significant fraction of the deposited energy is converted to an FRB with duration $R_s/c$, accompanied by ~10 MeV photons carrying less energy than the FRB. The inferred energy and mass associated with the source are low compared to those of typical supernova ejecta. This may suggest some type of a "weak stellar explosion", where a neutron star is formed with relatively low mass and energy ejection. However, the current upper limit on R does not allow one to rule out $M_c\sim1M_\odot$.Comment: Accepted to ApJ. 7 pages, 1 figure. Some explanations expanded. Discussion of beaming added. Some references update

The high energy tail of gamma-ray burst 941017: Comptonization of synchrotron self absorbed photons

The recent detection of an unusually hard spectral component in GRB941017 extending to $\ge200$ MeV is hard to explain as a synchrotron emission from shock-accelerated electrons. It was argued to imply acceleration of protons to ultra-high energy. We show here that the "high energy tail" can be explained as emission from shock-accelerated electrons in the early afterglow epoch, taking into account the effect of synchrotron self-absorption. High energy observations set in this case stringent constraints on model parameters: A lower limit to the total explosion energy E\gsim5 \times 10^{53} erg (assuming spherical symmetry); An upper limit to the density of gas surrounding the explosion, n\lsim10^{-2}(E/10^{54}{\rm erg}){\rm cm}^{-3}; A lower limit to the expansion Lorentz factor \Gamma_i\gsim 200; and An upper limit to the fraction of thermal energy carried by the magnetic field behind the shock driven into the surrounding medium, $\epsilon_{B,f} <= 10^{-4}$. Such constraints can not be inferred from keV--MeV data alone. The unusually low value of $\epsilon_{B,f}$ and the unusually high ratio $E/n$ may account for the rareness of GRB941017-type high energy tails. Tighter constraints on model parameters may be obtained in the future from optical and sub-TeV observations.Comment: Revised version: Minor changes, 2 graphs combined into 1. Accepted to ApJ

Properties of the Radio-Emitting Gas Around SgrA*

We show that the radial profiles of the temperature and density of the electrons as well as the magnetic field strength around the massive black hole at the Galactic center, SgrA*, may be constrained directly from existing radio data without any need to make prior assumptions about the dynamics of the emitting gas. The observed spectrum and wavelength-dependent angular size of SgrA* indicate that the synchrotron emission originates from an optically-thick plasma of quasi-thermal electrons. We find that the electron temperature rises above the virial temperature within tens of Schwarzschild radii from the black hole, suggesting that the emitting plasma may be outflowing. Constraints on the electron density profile are derived from polarization measurements. Our best-fit results differ from expectations based on existing theoretical models. However, these models cannot be ruled out as of yet due to uncertainties in the source size measurements. Our constraints could tighten considerably with future improvements in the size determination and simultaneous polarization measurements at multiple wavelengths.Comment: 6 pages, 1 figure, accepted for publication in JCAP (Journal of Cosmology & Astroparticle Physics

Hard X-ray emission from accretion shocks around galaxy clusters

We show that the hard X-ray (HXR) emission observed from several galaxy clusters is naturally explained by a simple model, in which the nonthermal emission is produced by inverse Compton scattering of cosmic microwave background photons by electrons accelerated in cluster accretion shocks: The dependence of HXR surface brightness on cluster temperature is consistent with that predicted by the model, and the observed HXR luminosity is consistent with the fraction of shock thermal energy deposited in relativistic electrons being \lesssim 0.1. Alternative models, where the HXR emission is predicted to be correlated with the cluster thermal emission, are disfavored by the data. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) gamma-ray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed.Comment: 7 pages, 3 figures, somewhat revised, published in JCA

Nonthermal emission from clusters of galaxies

We show that the spectral and radial distribution of the nonthermal emission of massive, M>10^{14.5}M_sun, galaxy clusters (GCs) may be approximately described by simple analytic expressions, which depend on the GC thermal X-ray properties and on two model parameter, beta_{core} and eta_e. beta_{core} is the ratio of CR energy density (within a logarithmic CR energy interval) and the thermal energy density at the GC core, and eta_{e(p)} is the fraction of the thermal energy generated in strong collisionless shocks, which is deposited in CR electrons (protons). Using a simple analytic model for the evolution of ICM CRs, which are produced by accretion shocks (primary CRs), we find that beta_{core} ~ eta_{p}/200, nearly independent of GC mass and with a scatter Delta ln(beta_{core}) ~ 1 between GCs of given mass. We show that the HXR and gamma-ray luminosities produced by IC scattering of CMB photons by primary electrons exceed the luminosities produced by secondary particles (generated in hadronic interactions within the GC) by factors ~500(eta_e/eta_p)(T/10 keV)^{-1/2} and ~150(eta_e/eta_p)(T/10 keV)^{-1/2} respectively, where T is the GC temperature. Secondary particle emission may dominate at the radio and VHE (> 1 TeV) gamma-ray bands. Our model predicts, in contrast with some earlier work, that the HXR and gamma-ray emission from GCs are extended, since the emission is dominated at these energies by primary electrons. Our predictions are consistent with the observed nonthermal emission of the Coma cluster for eta_peta_e ~ 0.1. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) gamma-ray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed. Finally, we show that our model's results agree with results of detailed numerical calculations.Comment: 22 pages, 16 figures, somewhat revised, published in JCA

Maser and other instabilities in a weakly magnetized relativistic plasma: Theory and the astrophysical relevance of the maser

A sufficient condition for maser instability in a weakly magnetized relativistic plasma with an isotropic particle distribution function is given. The maser growth rates and polarizations are computed starting from the exact dielectric permittivity tensor of a magnetized plasma. For very weak magnetic fields, our results confirm the approximate validity of the 'standard maser theory', which is based on the Einstein coefficients method, with one significant exception. For inclined propagation and realistic (small but finite) field, the growth rates of the two (nearly circular) polarizations differ significantly, while the standard theory predicts two (nearly circular) polarizations with similar growth rates. We show that this deviation is due to circularly polarized synchrotron emission, which is neglected in the standard theory. The maser is shown to grow slower than Langmuir waves. Nevertheless, significant generation of EM waves is seen in (highly simplified) direct numerical simulations. We study the nonlinear saturation of the maser instability and find that it offers a mechanism for the conversion of a significant fraction of the plasma energy into radio waves. We briefly discuss the conditions under which the maser instability may operate in astrophysical sources, and provide rough estimates that may be used as a guidance when studying particular astrophysical sources/phenomena

The Cumulative Bakground of High-Energy Neutrinos from Starburst Galaxies

We show that starburst galaxies convert efficiently cosmic-rays into pions, which in turn decay into high-energy neutrinos and photons. The cumulative background of GeV neutrinos is 10^{-7}GeV/cm^2/s/sr. Its extrapolation to higher neutrino energies depends on the energy spectrum of the injected cosmic-rays and is proportional to E^{-0.15+-0.1} up to E~0.3PeV and possibly higher neutrino energies. This flux, which constitutes a lower limit to the high energy extra-Galactic neutrino flux, is likely to be detectable by forthcoming km-scale neutrino telescopes.Comment: Accepted for publication in JCA

Flavoring Astrophysical Neutrinos: Flavor Ratios Depend on Energy

Electromagnetic (and adiabatic) energy losses of pions and muons modify the flavor ratio (measured at Earth) of neutrinos produced by pion decay in astrophysical sources, $\Phi_{\nu_e}:\Phi_{\nu_\mu}:\Phi_{\nu_\tau}$, from 1:1:1 at low energy to 1:1.8:1.8 at high energy. The transition occurs over 1-2 decades of nuetrino energy, and is correlated with a modification of the neutrino spectrum. For gamma-ray bursts, e.g., the transition is expected at \~100 TeV, and may be detected by km-scale neutrino telescopes. Measurements of the transition energy and energy-width will provide unique probes of the physics of the sources. pion and muon energy losses also affect the ratio of $\bar\nu_e$ flux to total neutrino flux, which may be measured at the W-resonance (6.3 PeV): It is modified from 1/6 (1/15) at low energy to 1/9 (practically 0) at high energy for neutrinos produced in pp ($p\gamma$) interactions.Comment: v1: 4 pages, 1 figure; v2: added reference; v3: improved introduction, accepted to PRL; v4: added note about matter oscillation