arXiv subjects

Eli Waxman

Publications and source records attributed to Eli Waxman.

At least 91 records · Page 5Linked to original sources

arXiv2008

Searching for a Correlation Between Cosmic-Ray Sources Above 10^{19} eV and Large-Scale Structure

We study the anisotropy signature which is expected if the sources of ultra high energy, >10^{19} eV, cosmic-rays (UHECRs) are extragalactic and trace the large scale distribution of luminous matter. Using the PSCz galaxy catalog as a tracer of the large scale structure (LSS), we derive the expected all sky angular distribution of the UHECR intensity. We define a statistic, that measures the correlation between the predicted and observed UHECR arrival direction distributions, and show that it is more sensitive to the expected anisotropy signature than the power spectrum and the two point correlation function. The distribution of the correlation statistic is not sensitive to the unknown redshift evolution of UHECR source density and to the unknown strength and structure of inter-galactic magnetic fields. We show, using this statistic, that recently published >5.7x10^{19} eV Auger data are inconsistent with isotropy at ~98% CL, and consistent with a source distribution that traces LSS, with some preference to a source distribution that is biased with respect to the galaxy distribution. The anisotropy signature should be detectable also at lower energy, >4x10^{19} eV. A few fold increase of the Auger exposure is likely to increase the significance to >99% CL, but not to >99.9% CL (unless the UHECR source density is comparable or larger than that of galaxies). In order to distinguish between different bias models, the systematic uncertainty in the absolute energy calibration of the experiments should be reduced to well below the current ~25%.

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arXiv2008

X-ray Hotspot Flares and Implications for Cosmic Ray Acceleration and Magnetic Field amplification in Supernova Remnants

For more than fifty years, it has been believed that cosmic ray (CR) nuclei are accelerated to high energies in the rapidly expanding shockwaves created by powerful supernova explosions. Yet observational proof of this conjecture is still lacking. Recently, Uchiyama and collaborators reported the detection of small-scale X-ray flares in one such supernova remnant, dubbed 'RX J1713-3946' (a.k.a. G347.3-0.5), which also emits very energetic, TeV (10^12 eV) range, gamma-rays. They contend that the variability of these X-ray 'hotspots' implies that the magnetic field in the remnant is about a hundred times larger than normally assumed; and this, they say, means that the detected TeV range photons were produced in energetic nuclear interactions, providing 'a strong argument for acceleration of protons and nuclei to energies of 1 PeV (10^15 eV) and beyond in young supernova remnants.' We point out here that the existing multiwavelength data on this object certainly do not support such conclusions. Though intriguing, the small-scale X-ray flares are not the long sought-after 'smoking gun' of nucleonic CR acceleration in SNRs.

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arXiv2008

Prompt optical emission from residual collisions in GRB outflows

The prompt gamma-ray emission in gamma-ray bursts is believed to be produced by internal shocks within a relativistic unsteady outflow. The recent detection of prompt optical emission accompanying the prompt gamma-ray emission appears to be inconsistent with this model since the out flowing plasma is expected to be highly optically thick to optical photons. We show here that fluctuations in flow properties on short, ~ 1 ms, time scale, which drive the gamma-ray producing collisions at small radii, are expected to lead to "residual" collisions at much larger radii, where the optical depth to optical photons is low. The late residual collisions naturally account for the relatively bright optical emission. The apparent simultaneity of gamma-ray and optical emission is due to the highly relativistic speed with which the plasma expands. Residual collisions may also account for the X-ray emission during the early "steep decline" phase, where the radius is inferred to be larger than the gamma-ray emission radius. Finally, we point out that inverse-Compton emission from residual collisions at large radii is expected to contribute significantly to the emission at high energy, and may therefore "smear" the pair production spectral cut-off.

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arXiv2007

In which shell-type SNRs should we look for gamma-rays and neutrinos from p-p collisions?

We present a simple analytic model for the various contributions to the non-thermal emission from shell type SNRs, and show that this model's results reproduce well the results of previous detailed calculations. We show that the \geq 1 TeV gamma ray emission from the shell type SNRs RX J1713.7-3946 and RX J0852.0-4622 is dominated by inverse-Compton scattering of CMB photons (and possibly infra-red ambient photons) by accelerated electrons. Pion decay (due to proton-proton collisions) is shown to account for only a small fraction, \lesssim10^-2, of the observed flux, as assuming a larger fractional contribution would imply nonthermal radio and X-ray synchrotron emission and thermal X-ray Bremsstrahlung emission that far exceed the observed radio and X-ray fluxes. Models where pion decay dominates the \geq 1 TeV flux avoid the implied excessive synchrotron emission (but not the implied excessive thermal X-ray Bremsstrahlung emission) by assuming an extremely low efficiency of electron acceleration, K_ep \lesssim 10^-4 (K_ep is the ratio of the number of accelerated electrons and the number of accelerated protons at a given energy). We argue that observations of SNRs in nearby galaxies imply a lower limit of K_ep \gtrsim 10^-3, and thus rule out K_ep values \lesssim 10^-4 (assuming that SNRs share a common typical value of K_ep). It is suggested that SNRs with strong thermal X-ray emission, rather than strong non-thermal X-ray emission, are more suitable candidates for searches of gamma rays and neutrinos resulting from proton-proton collisions. In particular, it is shown that the neutrino flux from the SNRs above is probably too low to be detected by current and planned neutrino observatories (Abridged).

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arXiv2007

EeV neutrinos associated with UHECR sources

Electromagnetic energy losses of charged pions and muons suppress the expected high energy, >1E18 eV, neutrino emission from sources of ultrahigh energy, >1E19 eV, cosmic-rays. We show here that >1E19 eV photons produced in such sources by neutral pion decay may escape the sources, thanks to the Klein-Nishina suppression of the pair production cross section, and produce muon pairs in interactions with the cosmic microwave background. The flux of muon decay neutrinos, which are expected to be associated in time and direction with the electromagnetic emission from the sources, may reach a few percent of the Waxman-Bahcall bound. Their detection may allow one to directly identify the sources of >1E19 eV cosmic-rays, and will provide the most stringent constraints on quantum-gravity-induced Lorentz violation.

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arXiv2007

Probing CP violation in neutrino oscillations with neutrino telescopes

Measurements of flavor ratios of astrophysical neutrino fluxes are sensitive to the two yet unknown mixing parameters $θ_{13}$ and $δ$ through the combination $\sinθ_{13}\cosδ$. We extend previous studies by considering the possibility that neutrino fluxes from more than a single type of sources will be measured. We point out that, if reactor experiments establish a lower bound on $θ_{13}$, then neutrino telescopes might establish an upper bound on $|\cosδ|$ that is smaller than one, and by that prove that CP is violated in neutrino oscillations. Such a measurement requires several favorable ingredients to occur: (i) $θ_{13}$ is not far below the present upper bound; (ii) The uncertainties in $θ_{12}$ and $θ_{23}$ are reduced by a factor of about two; (iii) Neutrino fluxes from muon-damped sources are identified, and their flavor ratios measured with accuracy of order 10% or better. For the last condition to be achieved with the planned km^3 detectors, the neutrino flux should be close to the Waxman-Bahcall bound. It motivates neutrino telescopes that are effectively about 10 times larger than IceCube for energies of O(100 TeV), even at the expense of a higher energy threshold.

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arXiv2007

Photon Acceleration at Shock Breakout of Trans-Relativistic Supernova

The predicted thermal flash from SN shock breakout might have been detected for the first time by Swift in GRB 060218/SN 2006aj. The detected thermal X-ray emission in this event implies emergence of a trans-relativistic (TR) SN shock with kinetic energy of E_k>1E49 erg. During TRSN shock breakout, the thermal photons could be "accelerated" by the shock through repeated bulk Compton scattering, forming a nonthermal gamma/X-ray component with dominant energy over thermal one. This mechanism of "photon acceleration" at TRSN shock breakout might also account for gamma-rays in the other similar low-luminosity GRBs, implying that they are atypical GRBs with only TR outflows. TRSNe form a peculiar type of SNe with large kinetic energy, >1E49 erg, in TR ejecta, Γβ~2.

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arXiv2007

Nonthermal gamma-ray and X-ray flashes from shock breakout in gamma-ray bursts/supernovae

Thermal X-ray emission which is simultaneous with the prompt gamma-rays has been detected for the first time from a supernova connected with a gamma-ray burst (GRB), namely GRB060218/SN2006aj. It has been interpreted as arising from the breakout of a mildly relativistic, radiation-dominated shock from a dense stellar wind surrounding the progenitor star. There is also evidence for the presence of a mildly relativistic ejecta in GRB980425/SN1998bw, based on its X-ray and radio afterglow. Here we study the process of repeated bulk Compton scatterings of shock breakout thermal photons by the mildly relativistic ejecta. During the shock breakout process, a fraction of the thermal photons would be repeatedly scattered between the pre-shock material and the shocked material as well as the mildly relativistic ejecta and, as a result, the thermal photons get boosted to increasingly higher energies. This bulk motion Comptonization mechanism will produce nonthermal gamma-ray and X-ray flashes, which could account for the prompt gamma-ray burst emission in low-luminosity supernova-connected GRBs, such as GRB060218. A Monte Carlo code has been developed to simulate this repeated scattering process, which confirms that a significant fraction of the thermal photons get "accelerated" to form a nonthermal component, with a dominant luminosity. This interpretation for the prompt nonthermal emission of GRB060218 may imply that either the usual internal shock emission from highly relativistic jets in these low-luminosity GRBs is weak, or alternatively, that there are no highly relativistic jets in this peculiar class of bursts.

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arXiv2007

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.

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arXiv2006

Self-Similar Collisionless Shocks

Observations of gamma-ray burst afterglows suggest that the correlation length of magnetic field fluctuations downstream of relativistic non-magnetized collisionless shocks grows with distance from the shock to scales much larger than the plasma skin depth. We argue that this indicates that the plasma properties are described by a self-similar solution, and derive constraints on the scaling properties of the solution. For example, we find that the scaling of the characteristic magnetic field amplitude with distance from the shock is B \propto D^{s_B} with -1 \propto x^{2s_B} (for x>>D). We show that the plasma may be approximated as a combination of two self-similar components: a kinetic component of energetic particles and an MHD-like component representing "thermal" particles. We argue that the latter may be considered as infinitely conducting, in which case s_B=0 and the scalings are completely determined (e.g. dn/dE \propto E^{-2} and B \propto D^0). Similar claims apply to non- relativistic shocks such as in supernova remnants, if the upstream magnetic field can be neglected. Self-similarity has important implications for any model of particle acceleration and/or field generation. For example, we show that the diffusion function in the angle μof momentum p in diffusive shock acceleration models must satisfy D_{μμ}(p,D) = D^{-1}D'_{μμ}(p/D), and that a previously suggested model for the generation of large scale magnetic fields through a hierarchical merger of current-filaments should be generalized. A numerical experiment testing our analysis is outlined (Abridged).

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arXiv2006

Assessing The Starburst Contribution to the Gamma-Ray and Neutrino Backgrounds

If cosmic ray protons interact with gas at roughly the mean density of the interstellar medium in starburst galaxies, then pion decay in starbursts is likely to contribute significantly to the diffuse extra-galactic background in both gamma-rays and high energy neutrinos. We describe the assumptions that lead to this conclusion and clarify the difference between our estimates and those of Stecker (2006). Detection of a single starburst by GLAST would confirm the significant contribution of starburst galaxies to the extra-galactic neutrino and gamma-ray backgrounds.

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arXiv2006

The Starburst Contribution to the Extra-Galactic Gamma-Ray Background

Cosmic ray protons interacting with gas at the mean density of the interstellar medium in starburst galaxies lose energy rapidly via inelastic collisions with ambient nuclei. The resulting pions produce secondary electrons and positrons, high-energy neutrinos, and gamma-ray photons. We estimate the cumulative gamma-ray emission from starburst galaxies. We find a total integrated background above 100 MeV of F_gamma ~ 10^{-6} GeV/cm^2/s/sr and a corresponding specific intensity at GeV energies of nuI_nu ~ 10^{-7} GeV/cm^2/s/sr. Starbursts may thus account for a significant fraction of the extra-galactic $γ$-ray background. We show that the FIR-radio correlation provides a strong constraint on the gamma-ray emission from starburst galaxies because pions decay into both gamma-rays and radio-emitting electron/positron pairs. We identify several nearby systems where the potential for observing gamma-ray emission is the most favorable (M82, NGC 253, and IC 342), predict their fluxes, and predict a linear FIR-gamma-ray correlation for the densest starbursts. If established, the FIR-gamma-ray correlation would provide strong evidence for the ``calorimeter'' theory of the FIR-radio correlation and would imply that cosmic rays in starburst galaxies interact with gas at approximately the mean density of the interstellar medium (ISM), thereby providing an important constraint on the physics of the ISM in starbursts.

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arXiv2006

The upstream magnetic field of GRB shocks

Gamma-ray burst (GRB) afterglow emission is believed to be produced by synchrotron emission of electrons accelerated to high energy by a relativistic collisionless shock propagating into a weakly magnetized plasma. Afterglow observations have been used to constrain the post-shock magnetic field and structure, as well as the accelerated electron energy distribution. Here we show that X-ray afterglow observations on day time scale constrain the pre-shock magnetic field to satisfy B>0.2[n/(1/cc)]^{5/8} mG, where n is the pre-shock density. This suggests that either the shock propagates into a highly magnetized fast, v~10^3 km/s, wind, or that the pre-shock magnetic field is strongly amplified, most likely by the streaming of high energy shock accelerated particles. More stringent constraints may be obtained by afterglow observations at high photon energy at late, >1 d, times.

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arXiv2006

The redshift distribution of short gamma-ray bursts from dynamically formed neutron star binaries

Short-hard gamma-ray bursts (SHBs) may arise from gravitational wave (GW) driven mergers of double neutron star (DNS) systems. DNSs may be "primordial" or can form dynamically by binary exchange interactions in globular clusters during core-collapse. For primordial binaries, the time delay between formation and merger is expected to be short, tau~0.1 Gyr, implying that the redshift distribution of merger events should follow that of star-formation. We point out here that for dynamically formed DNSs, the time delay between star-formation and merger is dominated by the cluster core-collapse time, rather than by the GW inspiral time, yielding delays comparable to the Hubble time. We derive the redshift distribution of merger events of dynamically formed DNSs, and find it to differ significantly from that typically expected for primordial binaries. The observed redshift distribution of SHBs favors dynamical formation, although a primordial origin cannot be ruled out due to possible detection biases. Future red-shift observations of SHBs may allow to determine whether they are dominated by primordial or dynamically formed DNSs.

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arXiv2006

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.

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arXiv2006

Enhancement of the $\barν_e$ flux from astrophysical sources by two photon annihilation interactions

The ratio of anti-electron to total neutrino flux, $Φ_{\barν_e}:Φ_ν$, expected from $pγ$ interactions in astrophysical sources is $\le1:15$. We point out that this ratio is enhanced by the decay of $μ^+μ^-$ pairs, created by the annihilation of secondary high energy photons from the decay of the neutral pions produced in $pγ$ interactions. We show that, under certain conditions, the $Φ_{\barν_e}:Φ_ν$ ratio may be significantly enhanced in gamma-ray burst (GRB) fireballs, and that detection at the Glashow resonance of $\barν_e$ in kilometer scale neutrino detectors may constrain GRB fireball model parameters, such as the magnetic field and energy dissipation radius.

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arXiv2006

Magnetic Fields in Starburst Galaxies and The Origin of the FIR-Radio Correlation

We estimate minimum energy magnetic fields (B_min) for a sample of galaxies with measured gas surface densities, spanning from normal spirals to starbursts. We show that the ratio of the minimum energy magnetic pressure to the total pressure in the ISM decreases substantially with increasing surface density; for Arp 220 this ratio is ~10^-4. Therefore, if the minimum energy estimate is applicable, magnetic fields in starbursts are dynamically weak compared to gravity, in contrast to normal spiral galaxies. We argue, however, that rapid cooling of relativistic electrons in starbursts invalidates the minimum energy estimate. We critically assess a number of independent constraints on the magnetic field strength in starbursts. In particular, we argue that the existence of the FIR-radio correlation implies that the synchrotron cooling timescale for cosmic ray electrons is much shorter than their escape time from the galactic disk; this in turn implies that the true magnetic field in starbursts is significantly larger than B_min. The strongest argument against such large fields is that one might expect starbursts to have steep radio spectra indicative of strong synchrotron cooling, which is not observed. We show, however, that ionization and bremsstrahlung losses can flatten the nonthermal spectra of starburst galaxies even in the presence of rapid cooling, providing much better agreement with observed spectra. We further demonstrate that ionization and bremsstrahlung losses are likely to be important in shaping the radio spectra of most starbursts at GHz frequencies, thereby preserving the linearity of the FIR-radio correlation. We thus conclude that magnetic fields in starbursts are significantly larger than B_min. We highlight several observations that can test this conclusion.

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arXiv2005

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, $Φ_{ν_e}:Φ_{ν_μ}:Φ_{ν_τ}$, 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 $\barν_e$ 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 ($pγ$) interactions.

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