arXiv subjects

Eli Waxman

Publications and source records attributed to Eli Waxman.

At least 145 records · Page 8Linked to original sources

arXiv2000

Gamma-Ray Background from Structure Formation in the Intergalactic Medium

The universe is filled with a diffuse and isotropic extragalactic background of gamma-ray radiation, containing roughly equal energy flux per decade in photon energy between 3 MeV-100 GeV. The origin of this background is one of the unsolved puzzles in cosmology. Less than a quarter of the gamma-ray flux can be attributed to unresolved discrete sources, but the remainder appears to constitute a truly diffuse background whose origin has hitherto been mysterious. Here we show that the shock waves induced by gravity during the formation of large-scale structure in the intergalactic medium, produce a population of highly-relativistic electrons with a maximum Lorentz factor above 10^7. These electrons scatter a small fraction of the microwave background photons in the present-day universe up to gamma-ray energies, thereby providing the gamma-ray background. The predicted diffuse flux agrees with the observed background over more than four decades in photon energy, and implies a mean cosmological density of baryons which is consistent with Big-Bang nucleosynthesis.

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arXiv1998

High Energy Neutrinos from Astrophysical Sources: An Upper Bound

We show that cosmic-ray observations set a model-independent upper bound to the flux of high-energy, > 10^14 eV, neutrinos produced by photo-meson (or p-p) interactions in sources of size not much larger than the proton photo-meson (or pp) mean-free-path. The bound applies, in particular, to neutrino production by either AGN jets or GRBs. This upper limit is two orders of magnitude below the flux predicted in some popular AGN jet models, but is consistent with our predictions from GRB models. We discuss the implications of these results for future km^2 high-energy neutrino detectors.

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arXiv1998

A Sub-Relativistic Shock Model for the Radio Emission of SN1998bw

SN1998bw is the most luminous radio supernova ever observed. Previous discussions argued that its exceptional radio luminosity, 4e38 erg/s, must originate from a highly relativistic shock which is fully decoupled from the supernova ejecta. Here we present an alternative model in which the radio emission originates from a sub-relativistic shock with a velocity of 0.3c, generated in the surrounding gas by the expanding ejecta. In this model, thermal electrons heated by the shock to a relativistic temperature of 60 MeV emit synchrotron self-absorbed radiation in the post-shock magnetic field. This model provides an excellent fit to the observed spectra provided that the thermal electrons are in equipartition with the ions behind the shock. The required magnetic field is much weaker than its equipartition value and could have been carried out by the progenitor's wind prior to the supernova explosion. According to this model, the radio emission from SN1998bw is unrelated to the highly relativistic blast wave that produced the gamma-ray burst GRB980425.

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arXiv1998

Gamma-Ray Burst Afterglow: Polarization and Analytic Light Curves

GRB afterglow polarization is discussed. We find an observable, up to 10%, polarization, if the magnetic field coherence length grows at about the speed of light after the field is generated at the shock front. Detection of a polarized afterglow would show that collisionless ultrarelativistic shocks can generate strong large scale magnetic fields and confirm the synchrotron afterglow model. Non-detection, at a 1% level, would imply that either the synchrotron emission model is incorrect, or that strong magnetic fields, after they are generated in the shock, somehow manage to stay un-dissipated at ``microscopic'', skin depth, scales. Analytic lightcurves of synchrotron emission from an ultrarelativistic self-similar blast wave are obtained for an arbitrary electron distribution function, taking into account the effects of synchrotron cooling. The peak synchrotron flux and the flux at frequencies much smaller than the peak frequency are insensitive to the details of the electron distribution function; hence their observational determination would provide strong constraints on blast wave parameters.

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arXiv1998

Can high energy neutrino annihilation on relic neutrinos generate the observed highest energy cosmic-rays?

Annihilation of high energy, $\sim 10^{21}$eV, neutrinos on big bang relic neutrinos of $\sim 1$eV mass, clustered in the Galactic halo or in a nearby galaxy cluster halo, has been suggested to generate, through hadronic Z decay, high energy nucleons and photons which may account for the detected flux of >10^{20}eV cosmic-rays. We show that the flux of high energy nucleons and photons produced by this process is dominated by annihilation on the uniform, non-clustered, neutrino background, and that the energy generation rate of 10^{21}eV neutrinos required to account for the detected flux of >10^{20}eV particles is >10^{48} erg/Mpc^3 yr. This energy generation rate, comparable to the total luminosity of the universe, is 4 orders of magnitude larger than the rate of production of high energy nucleons required to account for the flux of >10^{19}eV cosmic-rays. Thus, in order for neutrino annihilation to contribute significantly to the detected flux of >10^{20}eV cosmic-rays, the existence of a new class of high-energy neutrino sources, likely unrelated to the sources of >10^{19}eV cosmic-rays, must be invoked.

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arXiv1997

Gamma-ray burst after-glow: Confirming the cosmological fireball model

The recent detection of delayed X-ray and optical emission, ``afterglow,'' associated with gamma-ray bursts (GRBs) supports models, where the bursts are produced by relativistic expanding blastwaves, ``fireballs,'' at cosmological distances. The detection of absorption lines in the optical afterglow of the GRB of 8 May 1997 confirms that the sources lie at cosmological distance. We show here that the new features detected in GRB970508 afterglow, radio emission one week following the burst and a 2 day increase in optical flux, are consistent with the blastwave model. The fireball optical depth at radio frequencies is much smaller than previously estimated, which accounts for the observed radio emission. The initial suppression of optical flux is consistent with that predicted due to electron cooling. The combined radio and optical data imply that the fireball energy is $\sim10^{52}$erg, and that the density of the medium into which the blastwave expands is $\sim 1cm^{-3}$, a value typical for gas within galaxies. We predict the time dependence of the radio flux and the absorption frequency, which constitute tests of the fireball model as described in this paper.

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arXiv1997

Implications of the radio afterglow from the gamma-ray burst of May 8, 1997

Radio observations of the afterglow of the gamma-ray burst GRB970508 provide unique new constraints on afterglow models. The quenching of diffractive scintillation at 4 week delay provides the first direct estimate of source size and expansion rate. It implies an apparent size $R\sim10^{17}$cm and expansion at a speed comparable to that of light at 4 weeks delay, in agreement with the fireball model prediction $R=10^{17}(t/week)^{5/8}$cm. The radio flux and its dependence on time and frequency at 1--5 week delay are in agreement with the model and imply a fireball energy (assuming spherical symmetry) of order 10^{52}erg, consistent with the value inferred from observations at shorter delay. The observed radio behavior deviates from model predictions at delays larger than 5 weeks. This is expected, since at this delay the fireball is in transition from highly-relativistic to sub-relativistic expansion, with Lorentz factor γ<2. Deviation may be due to a change in the physical processes associated with the shock wave as it becomes sub-relativistic (e.g. a decrease in the fraction of energy carried by magnetic field), or to the fireball being a cone of opening angle $1/γ\sim0.5$. We predict the future behavior of the radio flux assuming that the latter interpretation is valid. These predictions may be tested by radio observations in the frequency range 0.1--10GHz on time scale of months.

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arXiv1997

Angular size and emission time scales of relativistic fireballs

The detection of delayed X-ray, optical and radio emission, ``afterglow,'' associated with gamma-ray bursts (GRBs) is consistent with models, where the bursts are produced by relativistic expanding blast waves, driven by expanding fireballs at cosmological distances. In particular, the time scales over which radiation is observed at different wave bands agree with model predictions. It had recently been claimed that the commonly used relation between observation time t and blast wave radius r, t=r/2γ^{2}c where γ(r) is the fluid Lorentz factor, should be replaced with t=r/16γ^{2}c due to blast wave deceleration. Applying the suggested deceleration modification would make it difficult to reconcile observed time scales with model predictions. It would also imply an apparent source size which is too large to allow attributing observed radio variability to diffractive scintillation. We present a detailed analysis of the implications of the relativistic hydrodynamics of expanding blast waves to the observed afterglow. We find that modifications due to shock deceleration are small, therefore allowing for both the observed afterglow time scales and for diffractive scintillation. We show that at time t the fireball appears on the sky as a narrow ring of radius h=r/γand width 0.1h, where r and t are related by t=r/2γ^{2}c.

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arXiv1997

GRB after-glow: Supporting the cosmological fireball model, constraining parameters, and making predictions

Cosmological fireball models of gamma-ray bursts (GRBs) predict delayed emission, ``after-glow,'' at longer wavelengths. We present several new results regarding the model predictions, and show that X-ray to optical observations of GRB970228 and GRB970402 are naturally explained by the model: The scaling of flux with time and frequency agrees with model predictions and requires a power law distribution of shock accelerated electrons, dlog N/dlog E=2.3+-0.1 (implying, and consistent with the observed, 1/t decline of flux observed at a given frequency); The absolute flux value agrees with that inferred through the model from observed gamma-ray fluence. The future after-glow emission of these bursts is predicted. The observations indicate that the ratio of magnetic field to equipartition value and the fraction F of dissipated kinetic energy carried by electrons are not much smaller than 1. More frequent observations at a fixed wavelength, or a wide spectrum at a fixed time, would put strong constraints on these parameters. We show that inverse-Compton scattering suppresses X-ray/ optical emission at delays t 1GeV photons and providing a natural explanation to the delayed GeV emission observed in several strong bursts.

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arXiv1997

High Energy Neutrinos from Cosmological Gamma-Ray Burst Fireballs

Observations suggest that $γ$-ray bursts (GRBs) are produced by the dissipation of the kinetic energy of a relativistic fireball. We show that a large fraction, $\ge 10%$, of the fireball energy is expected to be converted by photo-meson production to a burst of $\sim10^{14} eV$ neutrinos. A km^2 neutrino detector would observe at least several tens of events per year correlated with GRBs, and test for neutrino properties (e.g. flavor oscillations, for which upward moving $τ$'s would be a unique signature, and coupling to gravity) with an accuracy many orders of magnitude better than is currently possible.

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arXiv1996

Predictions of the Gamma-Ray Burst Model of Ultra High Energy Cosmic Rays

The cosmological gamma-ray burst (GRB) model for the production of ultra- high energy cosmic rays is described, and the current observational evidence which support it discussed. Several predictions of the model are presented, which would allow it to be tested by future high energy cosmic ray and gamma- ray experiments. If the predicted signatures of the GRB model are observed, they will not only corroborate the model, but will also provide information about the source population, and will allow to investigate the unknown structure of the inter-galactic magnetic field.

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arXiv1996

Images of Bursting Sources of High-Energy Cosmic Rays. I: Effects of Magnetic Fields

It has recently been shown that the highest energy cosmic rays (CRs) may originate in the same cosmological objects producing $γ$-ray bursts. This model requires the presence of intergalactic magnetic fields (IGMF) to delay the arrival times of $\sim 10^{20}$ eV CRs by 50 years or longer relative to the $γ$-rays, of an amplitude that is consistent with other observational constraints. Sources of CRs coming from individual bursts should be resolved with the planned ``Auger'' experiment, with as many as hundreds of CRs for the brightest sources. We analyze here the apparent angular and energy distribution of CRs from bright sources below the pion production threshold (in the energy range $10^{19}{\rm eV} < E < 4\times10^{19}{\rm eV}$) expected in this model. This observable distribution depends on the structure of the IGMF: the apparent spectral width $ΔE$ is small, $ΔE/E\lesssim1\%$, if the intergalactic field correlation length $λ$ is much larger than $1{\rm Mpc}$, and large, $ΔE/E=0.3$, in the opposite limit $λ\ll 1{\rm Mpc}$. The apparent angular size is also larger for smaller $λ$. If the sources of CRs we predict are found, they will corroborate the bursting model and they will provide us with a technique to investigate the structure of the IGMF.

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arXiv1996

The Signature of a Correlation between $>10^{19}{\rm eV}$ Cosmic Ray Sources and Large Scale Structure

We analyze the anisotropy signature expected if the high energy (above $10^{19}$eV) cosmic ray (CR) sources are extra-Galactic and trace the distribution of luminous matter on large scales. We investigate the dependence of the anisotropy on both the relative bias between the CR sources and the galaxy distribution and on the (unknown) intrinsic CR source density. We find that the expected anisotropy associated with the large scale structure (LSS) should be detected once the number of CR events observed above $10^{19}{\rm eV}$ is increased by a factor of $\sim10$. This would require $\sim30$ observation-years with existing experiments, but less then $1$ year with the proposed $\sim5000\ {\rm km}^2$ Auger detectors. We find that the recently reported concentration of the Haverah Park CR events towards the super-galactic plane is not consistent with the known LSS. If real, the Haverah Park result suggests that the CR sources are much more concentrated towards the super-galactic plane than the known LSS. Our results are not sensitive to the number density of CR sources. We show that once the number of detected events is increased by a factor of $\sim10$, the number density would be strongly constrained by considering the probability for having repeating sources.

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arXiv1996

Delayed GeV--TeV Photons from Gamma-Ray Bursts Producing High-Energy Cosmic Rays

A scenario in which cosmic rays (CRs) above $10^{20}{\rm eV}$ are produced by cosmological gamma-ray bursts (GRBs) is consistent with observations provided that deflections by the inter-galactic magnetic field (IGMF) delay and spread the arrival time of the CRs over $\geq50{\rm yr}$. The energy lost by the CRs as they propagate and interact with the microwave background is transformed by cascading into secondary GeV-TeV photons. We show that a significant fraction of these photons can arrive with delays much smaller than the CR delay if much of inter-galactic space is occupied by large-scale magnetic ``voids'', regions of size $\gtrsim5{\rm Mpc}$ and field weaker than $10^{-15}{\rm G}$. Such voids might be expected, for example, in models where a weak primordial field is amplified in shocked, turbulent regions of the intergalactic medium during the formation of large-scale structure. For a field strength $\sim4\times10^{-11}{\rm G}$ in the high field regions, the value required to account for observed galactic fields if the IGMF were frozen in the protogalactic plasma, the delay of CRs produced by a burst at a distance of $100{\rm Mpc}$ is $\sim100{\rm yr}$, and the fluence of secondary photons above $10{\rm GeV}$ on hour--day time scales is $I(>E)\sim10^{-6}E_{\rm TeV}^{-1}{\rm cm}^{-2}$. This fluence is close to the detection threshold of current high-energy $γ$-ray experiments. Detection of the delayed flux would support the GRB-CR association and would also provide information on the IGMF structure.

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arXiv1996

Signatures of the Origin of High-Energy Cosmic Rays in Cosmological Gamma-Ray Bursts

We derive observational consequences of the hypothesis that cosmic rays (CR's) of energy $>10^{19}eV$ originate in the same cosmological objects producing gamma-ray bursts (GRB's). Inter-galactic magnetic fields $\gtrsim 10^{-12} G$ are required in this model to allow CR's to be observed continuously in time by producing energy dependent delays in the CR arrival times. This results in individual CR sources having very narrow observed spectra, since at any given time only those CR's having a fixed time delay are observed. Thus, the brightest CR sources should be different at different energies. The average number of sources contributing to the total CR flux decreases with energy much more rapidly than in a model of steady CR sources, dropping to one at $E_{crit} \simeq2\times10^{20}$~eV with very weak sensitivity to the inter-galactic magnetic field strength. Below $E_{crit}$, a very large number of sources is expected, consistent with observations. Above $E_{crit}$, a source may be observed with a flux considerably higher than the time-averaged CR flux from all sources, if a nearby GRB occurred recently. If such a source is present, its narrow spectrum may produce a ``gap'' in the total spectrum. These signatures should be detectable by the planned ``Auger'' CR experiment.

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arXiv1995

Cooling Flows and Metallicity Gradients in Clusters of Galaxies

The X-ray emission by hot gas at the centers of clusters of galaxies is commonly modeled assuming the existence of steady-state, inhomogeneous cooling flows. We derive the metallicity profiles of the intracluster medium expected from such models. The inflowing gas is chemically enriched by type Ia supernovae and stellar mass loss in the outer parts of the central galaxy, which may give rise to a substantial metallicity gradient. The amplitude of the expected metallicity enhancement towards the cluster center is proportional to the ratio of the central galaxy luminosity to the mass inflow rate. The metallicity of the hotter phases is expected to be higher than that of the colder, denser phases. The metallicity profile expected for the Centaurus cluster is in good agreement with the metallicity gradient recently inferred from ASCA measurements (Fukazawa et al. 1994). However, current data do not rule out alternative models where cooling is balanced by some heat source. The metallicity gradient does not need to be present in all clusters, depending on the recent merging history of the gas around the central cluster galaxy, and on the ratio of the stellar mass in the central galaxy to the gas mass in the cooling flow.

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arXiv1995

Cosmological Origin for Cosmic Rays Above $10^{19}$ eV

The cosmic ray spectrum at $10^{19}{\rm eV}-10^{20}{\rm eV}$, reported by the Fly's Eye and the AGASA experiments, is shown to be consistent with a cosmological distribution of sources of protons, with a power law generation spectrum ${\rm d}\ln N/{\rm d}\ln E=-2.3\pm0.5$ and energy production rate of $4.5\pm1.5\times10^{44}{\rm erg}\ {\rm Mpc}^{-3}\ {\rm yr}^{-1}$. The two events measured above $10^{20}{\rm eV}$ are not inconsistent with this model. Verifying the existence of a ``black-body cutoff'', currently observed with low significance, would require $\sim30$ observation-years with existing experiments, but only $\sim1$ year with the proposed $\sim5000\ {\rm km}^2$ detectors. For a cosmological source distribution, no anisotropy is expected in the angular distribution of events with energies up to $\sim5\times10^{19}{\rm eV}$.

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arXiv1995

COSMOLOGICAL GAMMA RAY BURSTS AND THE HIGHEST ENERGY COSMIC RAYS

We discuss a scenario in which the highest energy cosmic rays (CR's) and cosmological $γ$-ray bursts (GRB's) have a common origin. This scenario is consistent with the observed CR flux above $10^{20}\text{eV}$, provided that each burst produces similar energies in $γ$-rays and in CR's above $10^{20}\text{eV}$. Protons may be accelerated by Fermi's mechanism to energies $\sim10^{20}\text{eV}$ in a dissipative, ultra-relativistic wind, with luminosity and Lorentz factor high enough to produce a GRB. For a homogeneous GRB distribution, this scenario predicts an isotropic, time-independent CR flux.

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