arXiv subjects

Eli Waxman

Publications and source records attributed to Eli Waxman.

At least 109 records · Page 6Linked to original sources

arXiv2005

High energy neutrinos from a slow jet model of core collapse supernovae

It has been hypothesized recently that core collapse supernovae are triggered by mildly relativistic jets following observations of radio properties of these explosions. Association of a jet, similar to a gamma-ray burst jet but only slower, allows shock acceleration of particles to high energy and non-thermal neutrino emission from a supernova. Detection of these high energy neutrinos in upcoming kilometer scale Cherenkov detectors may be the only direct way to probe inside these astrophysical phenomena as electromagnetic radiation is thermal and contains little information. Calculation of high energy neutrino signal from a simple and slow jet model buried inside the pre-supernova star is reviewed here. The detection prospect of these neutrinos in water or ice detector is also discussed in this brief review. Jetted core collapse supernovae in nearby galaxies may provide the strongest high energy neutrino signal from point sources.

astro-ph↗

arXiv2005

A Complete Survey of the Transient Radio Sky and Implications for Gamma-Ray Bursts, Supernovae, and other Relativistic Explosions

We had previously reported on a survey for radio transients, used to set an upper limit on the number of orphan gamma-ray burst (GRB) radio afterglows, and thus a lower limit on the typical GRB beaming factor. Here we report radio and optical follow-up observations of these possible transients, achieving the first full characterization of the transient radio sky. We find that only two source are likely to be real radio transients, an optically obscured radio supernova (SN) in the nearby galaxy NGC 4216, and a source not associated with a bright host galaxy, which is too radio luminous to be a GRB afterglow. We speculate that this may be a flare from a peculiar active galactic nucleus, or a burst from an unusual Galactic compact object. We place an upper limit of 65 radio transients above 6 mJy over the entire sky at the 95% confidence level. The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find f_b^{-1} >~ 60, ~5 times higher than our earlier results. Second, we impose an upper limit on the rate of events that eject >~ 10^{51} erg in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate, <=1000/y/Gpc, is about two orders of magnitude smaller than the rate of core-collapse SNe (and type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that future wider and/or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows. Our survey also illustrates the great potential of sensitive surveys with new instruments to revolutionize the study of nearby SNe (abridged).

astro-ph↗

arXiv2005

The stability of decelerating shocks revisited

We present a new method for analyzing the global stability of the Sedov-von Neumann-Taylor self-similar solutions, describing the asymptotic behavior of spherical decelerating shock waves, expanding into ideal gas with density \propto r^{-ω}. Our method allows to overcome the difficulties associated with the non-physical divergences of the solutions at the origin. We show that while the growth rates of global modes derived by previous analyses are accurate in the large wave number (small wavelength) limit, they do not correctly describe the small wave number behavior for small values of the adiabatic index γ. Our method furthermore allows to analyze the stability properties of the flow at early times, when the flow deviates significantly from the asymptotic self-similar behavior. We find that at this stage the perturbation growth rates are larger than those obtained for unstable asymptotic solutions at similar [γ,ω]. Our results reduce the discrepancy that exists between theoretical predictions and experimental results.

astro-ph↗

arXiv2005

High energy photon emission in the early afterglow of GRB's

We consider the emission within the fireball model framework of very high energy, ~1 GeV to >1 TeV photons, on a minute time scale, during the onset of fireball deceleration due to interaction with surrounding medium. Our time dependent numerical model includes exact treatment of electron synchrotron emission, inverse-Compton scattering, pair production, and evolution of electromagnetic cascades (initiated by pair production or photo-production of pions). We find that (i) The 1 GeV--10 GeV flux is not sensitive to model parameters and is ~10^{-7} erg cm^-2 s^-1 for z=1 bursts, well within the detection capabilities of GLAST; (ii) The sub-TeV flux depends on the surrounding medium density and on the fraction of thermal energy carried by the magnetic field, epsilon_B: It ranges from ~10^{-7} erg cm^-2 s^-1 in the case of typical ISM density and epsilon_B <~ 10^{-4} to 10^{-10} erg cm^-2 s^-1 in the case of a source surrounded by a wind and epsilon_B ~ 10^{-0.5}; (iii) The sub-TeV flux is detectable by high energy gamma-ray experiments such as HESS, MAGIC, MILAGRO, and VERITAS; (iv) Combined ~1 keV, ~1 GeV and sub-TeV observations will allow to determine both epsilon_B and the ambient medium density; (v) The spectra depend only weakly on the spectral index of the energy distribution of the accelerated electrons. Pion production energy loss of high energy protons may contribute significantly in the wind case to the luminosity of high energy photons. However, it is difficult to distinguish in this case between the electron and proton contributions since the spectral shape is determined primarily by the energy dependence of the pair production optical depth.

astro-ph↗

arXiv2005

The Remnants of Intergalactic Supernovae

Intergalactic type-Ia supernovae (SNe-Ia) have been discovered recently in rich galaxy clusters, likely the descendants of an intergalactic stellar population found in recent years through a variety of tracers. We estimate the observational signatures of the associated SN remnants (SNRs) in the unusual intracluster medium (ICM) environment. If SNe-Ia still have a circumstellar medium (CSM) at the time of explosion, then their remnants are visible in the optical for ~100-1000 years, with properties similar to young galactic SNRs. In contrast with galactic SNRs, in which the ejecta from the explosion interacts with the ISM, intracluster SNRs become undetectable in the optical once their ejecta passes beyond the CSM and enters the hot and tenuous ICM. If SNe-Ia have a CSM, there should be ~150 young SNRs in the Virgo cluster, with L(H-alpha)~10^{35} erg/s and angular size ~0.1''. We investigate the possibility that members of this SNR population may have recently been detected, but incorrectly identified as intergalactic HII regions. Alternatively, if optical intergalactic SNRs do not exist in Virgo, this will be evidence that SNe-Ia are devoid of a CSM, with implications for progenitor scenarios. Regardless of the presence of a CSM, about 10 older radio SNRs per square degree should be detectable in Virgo, with fluxes of ~0.1 mJy at 1 GHz. Their angular sizes, morphologies, and lack of optical association with distant galaxies can distinguish them from the much more numerous background population. Their detection would provide a measurement of the intracluster SN rate. Observations toward the site of SN1980I, a possibly intergalactic Virgo SN-Ia, can test the existence of a CSM by comparison to our early-time predictions for intergalactic SNR development.

astro-ph↗

arXiv2005

Time dependent numerical model for the emission of radiation from relativistic plasma

We describe a numerical model constructed for the study of the emission of radiation from relativistic plasma under conditions characteristic, e.g., to gamma-ray bursts (GRB's) and active galactic nuclei (AGN's). The model solves self consistently the kinetic equations for e^\pm and photons, describing cyclo-synchrotron emission, direct Compton and inverse Compton scattering, pair production and annihilation, including the evolution of high energy electromagnetic cascades. The code allows calculations over a wide range of particle energies, spanning more than 15 orders of magnitude in energy and time scales. Our unique algorithm, which enables to follow the particle distributions over a wide energy range, allows to accurately derive spectra at high energies, >100 \TeV. We present the kinetic equations that are being solved, detailed description of the equations describing the various physical processes, the solution method, and several examples of numerical results. Excellent agreement with analytical results of the synchrotron-SSC model is found for parameter space regions in which this approximation is valid, and several examples are presented of calculations for parameter space regions where analytic results are not available.

astro-ph↗

arXiv2005

The efficiency of electron acceleration in collisionless shocks and GRB energetics

Afterglow observations are commonly used to determine the parameters of GRB explosions, the energy E, surrounding density n, post-shock magnetic field equipartition fraction ε_B and electron equipartition fraction ε_e, under the frequently made assumption that the efficiency of electron "injection" into relativistic shock acceleration is high, i.e. that the fraction f of electrons which undergo acceleration is f~1. We show that the value of f can not be determined by current observations, since currently testable model predictions for a parameter choice {E'=E/f,n'=n/f,ε'_B=fε_B,ε'_e=fε_e} are independent of the value of f for m_e/m_p< f< 1. Current observations imply that the efficiency f is similar for highly relativistic and for sub relativistic shocks, and plausibly suggest that f~1, quite unlike the situation in the Crab Nebula. However, values m_e/m_p< f<< 1 can not be ruled out, implying a factor m_e/m_p uncertainty in determination of model parameters. We show that early, <10 hr, radio afterglow observations, which will be far more accessible in the SWIFT era, may provide constraints on f. Such observations will therefore provide a powerful diagnostic of GRB explosions and of the physics of particle acceleration in collisionless shocks.

astro-ph↗

arXiv2005

TeV neutrinos from core collapse supernovae and hypernovae

A fraction of core collapse supernovae of type Ib/c are associated with Gamma-ray bursts, which are thought to produce highly relativistic jets. Recently, it has been hypothesized that a larger fraction of core collapse supernovae produce slower jets, which may contribute to the disruption and ejection of the supernova envelope, and explain the unusually energetic hypernovae. We explore the TeV neutrino signatures expected from such slower jets, and calculate the expected detection rates with upcoming Gigaton Cherenkov experiments. We conclude that individual jetted SNe may be detectable from nearby galaxies.

astro-ph↗

arXiv2005

Extra galactic sources of high energy neutrinos

The main goal of the construction of large volume, high energy neutrino telescopes is the detection of extra-Galactic neutrino sources. The existence of such sources is implied by observations of ultra-high energy, >10^{19} eV, cosmic-rays (UHECRs), the origin of which is a mystery. The observed UHECR flux sets an upper bound to the extra-Galactic high energy neutrino intensity, which implies that the detector size required to detect the signal in the energy range of 1 TeV to 1 PeV is >=1 giga-ton, and much larger at higher energy. Optical Cerenkov neutrino detectors, currently being constructed under ice and water, are expected to achieve 1 giga-ton effective volume for 1 TeV to 1 PeV neutrinos. Coherent radio Cerenkov detectors (and possibly large air-shower detectors) will provide the >> 1 giga-ton effective volume required for detection at ~10^{19} eV. Detection of high energy neutrinos associated with electromagnetically identified sources will allow to identify the sources of UHECRs, will provide a unique probe of the sources, which may allow to resolve open questions related to the underlying physics of models describing these powerful accelerators, and will provide information on fundamental neutrino properties.

astro-ph↗

arXiv2004

The spectrum of particles accelerated in relativistic, collisionless shocks

We analytically study diffusive particle acceleration in relativistic, collisionless shocks. We find a simple relation between the spectral index s and the anisotropy of the momentum distribution along the shock front. Based on this relation, we obtain s = (3beta_u - 2beta_u*beta_d^2 + beta_d^3) / (beta_u - beta_d) for isotropic diffusion, where beta_u (beta_d) is the upstream (downstream) fluid velocity normalized to the speed of light. This result is in agreement with previous numerical determinations of s for all (beta_u,beta_d), and yields s=38/9 in the ultra-relativistic limit. The spectrum-anisotropy connection is useful for testing numerical studies and for constraining non-isotropic diffusion results. It implies that the spectrum is highly sensitive to the form of the diffusion function for particles travelling along the shock front.

astro-ph↗

arXiv2004

Imprint of Intergalactic Shocks on the Radio Sky

Strong intergalactic shocks are a natural consequence of structure formation in the universe. They are expected to deposit large fractions of their energy in relativistic electrons (xi_e~0.05 according to SNR observations) and magnetic fields (xi_B~0.01 according to cluster halo observations). We calculate the synchrotron emission from such shocks using an analytical model, calibrated with a hydrodynamical LCDM simulation. The resulting signal composes a large fraction of the extragalactic radio background (ERB) below 500 MHz. The associated angular fluctuations dominate the sky for frequencies nu<10 GHz and angular scales arcmin-deg (after a modest removal of point sources), provided that xi_e*xi_B>3*10^-4. The fluctuating signal is most pronounced for nu<500 MHz, dominating the sky even for xi_e*xi_B=5*10^-5. The signal will be easily observable by next generation radio telescopes such as LOFAR and SKA, and is marginally observable with present telescopes. It may be identified using cross-correlations with tracers of large scale structure, possibly even in existing <10 GHz CMB anisotropy maps and high resolution ~1 GHz radio surveys. Detection of the signal will provide the first identification of intergalactic shocks and of the WHIM, and gauge the unknown intergalactic magnetic field. We show that existing observations of the diffuse <500 MHz radio background are well fit by a simple, double-disk Galactic model, precluding a direct identification of the diffuse ERB. Modelling the frequency-dependent anisotropy pattern observed at very low (1-10 MHz) frequencies can disentangle the distributions of Galactic cosmic-rays, ionized gas and magnetic fields. Space missions such as ALFA will thus provide important insight into the structure and composition of our Galaxy (abridged).

astro-ph↗

arXiv2004

Searching for Intergalactic Shocks with the SKA

Strong intergalactic shocks are a natural consequence of structure formation in the universe. These shocks are expected to deposit large fractions of their energy in relativistic electrons (xi_e~0.05 of the thermal energy according to supernova remnant observations) and magnetic fields (xi_B~0.01 according to cluster halo observations). We discuss the expected synchrotron emission from such shocks, and the observational consequences for next generation radio telescopes such as the Square Kilometer Array. We present an analytical model, calibrated and verified based on a hydrodynamical LCDM simulation. The resulting signal composes a large fraction (up to a few 10%) of the extragalactic radio background below 500 MHz. The associated angular fluctuations, e.g. delta T_l>260(xi_e*xi_B/5*10^-4)(nu/100 MHz)^-3 K for multipoles 400 3*10^-4. The fluctuating signal is most pronounced for nu<500 MHz, dominating the sky there even for xi_e*xi_B=5*10^-5. We find that the signal is easily observable by next generation telescopes such as the SKA, and marginally observable with present telescopes. The signal may be identified using cross-correlations with tracers of large-scale structure (e.g. gamma-ray emission from intergalactic shocks). Detection of the signal will provide the first identification of intergalactic shocks and of the warm-hot intergalactic medium (believed to contain most of the baryons in the low redshift universe), and gauge the unknown intergalactic magnetic field.

astro-ph↗

arXiv2004

Probing the Magnetic Field Structure in Gamma-Ray Bursts through Dispersive Plasma Effects on the Afterglow Polarization

(Abr) The origin and structure of magnetic fields in Gamma-Ray Burst (GRB) fireball plasmas are two of the most important open questions in all GRB models. We show that the structure and strength of the magnetic field may be constrained by radio and IR observations of the early afterglow, where plasma effects on the polarization of propagating radiation are significant. We calculate these propagation effects for cold and relativistic plasmas, and find that in the presence of a uniform equipartition field the degree of linear polarization is suppressed, and circular polarization prevails at low frequencies, nu < 1-3 GHz, (2x10^11 Hz < nu < few x 10^14 Hz) in the forward (reverse) shock. At higher frequencies linear polarization dominates. At the frequency of the transition between circular and linear polarization, the net level of polarization is minimal, ~10-20%. These features are nearly independent of the circumburst density. The transition frequency is smaller by a factor of ~10 when the uniform field is much weaker than equipartition. The dependence of these results on viewing geometry, outflow collimation and magnetic field orientation is discussed. When the configuration of the field is entangled over length scales much smaller than the extent of the emitting plasma, the aforementioned effects should not be observed and a linear polarization at the few % level is expected. Polarimetric observations during the early afterglow, and particularly of the reverse shock emission, may therefore place strong constraints on the structure and strength of the magnetic field within the fireball plasma.

astro-ph↗

arXiv2004

Prompt GRB spectra: detailed calculations and the effect of pair production

We present detailed calculations of the prompt spectrum of gamma-ray bursts (GRBs) predicted within the fireball model framework, where emission is due to internal shocks in an expanding relativistic wind. Our time dependent numerical model describes cyclo-synchrotron emission and absorption, inverse and direct Compton scattering, and e^\pm pair production and annihilation (including the evolution of high energy electro-magnetic cascades). It allows, in particular, a self-consistent calculation of the energy distribution of e^\pm pairs produced by photon annihilation, and hence a calculation of the spectra resulting when the scattering optical depth due to pairs, τ_\pm, is high. We show that emission peaks at ~1MeV for moderate to large τ_\pm, reaching τ_\pm ~ 10^2. In this regime of large compactness we find that (i) A large fraction of shock energy can escape as radiation even for large τ_\pm; (ii) The spectrum depends only weakly on the magnetic field energy fraction; (iii) The spectrum is hard, epsilon^2 dN/depsilon \propto epsilon^αwith 0.5<α<1, between the self absorption (epsilon_{ssa}= 10^{0.5\pm0.5} keV) and peak (epsilon_{peak}= 10^{0.5\pm0.5} MeV) photon energy, (iv) and shows a sharp cutoff at ~10 MeV; (v) Thermal Comptonization leads to emission peaking at epsilon_peak>=30 MeV, and can not therefore account for observed GRB spectra. For small compactness, spectra extend to >10 GeV with flux detectable by GLAST, and the spectrum at low energy depends on the magnetic field energy fraction. Comparison of the flux at ~1 GeV and ~ 100 keV may therefore allow to determine the magnetic field strength. For both small and large compactness, the spectra depend only weakly on the spectral index of the energy distribution of accelerated electrons.

astro-ph↗

arXiv2004

The Case for a Low Extragalactic Gamma-ray Background

Measurements of the diffuse extragalactic gamma-ray background (EGRB) are complicated by a strong Galactic foreground. Estimates of the EGRB flux and spectrum, obtained by modeling the Galactic emission, have produced a variety of (sometimes conflicting) results. The latest analysis of the EGRET data found an isotropic flux I_x=1.45+-0.05 above 100 MeV, in units of 10^-5 s^-1 cm^-2 sr^-1. We analyze the EGRET data in search for robust constraints on the EGRB flux, finding the gamma-ray sky strongly dominated by Galactic foreground even at high latitudes, with no conclusive evidence for an additional isotropic component. The gamma-ray intensity measured towards the Galactic poles is similar to or lower than previous estimates of I_x. The high latitude profile of the gamma-ray data is disk-like for 40<|b[deg]|<70, and even steeper for |b|>70; overall it exhibits strong Galactic features and is well fit by a simple Galactic model. Based on the |b|>40 data we find that I_x<0.5 at a 99% confidence level, with evidence for a much lower flux. We show that correlations with Galactic tracers, previously used to identify the Galactic foreground and estimate I_x, are not satisfactory; the results depend on the tracers used and on the part of the sky examined, because the Galactic emission is not linear in the Galactic tracers and exhibits spectral variations across the sky. The low EGRB flux favored by our analysis places stringent limits on extragalactic scenarios involving gamma-ray emission, such as radiation from blazars, intergalactic shocks and production of ultra-high energy cosmic rays and neutrinos. We suggest methods by which future gamma-ray missions such as GLAST and AGILE could indirectly identify the EGRB.

astro-ph↗

arXiv2004

Does the detection of X-ray emission from SN1998bw support its association with GRB980425?

We show that the recent identification of X-ray emission from SN1998bw is naturally explained as synchrotron emission from a shock driven into the wind surrounding the progenitor by a mildly relativistic shell ejected by the supernova, the existence of which was inferred earlier from radio observations. X-ray observations imply a shell energy E~10^{49.7}erg, and constrain the initial shell velocity β*c and normalized wind mass loss rate, \dot{m}=(\dot{M}/10^{-5}M_sun/yr)/(v_w/10^3 km/s), to satisfy β^3*\dot{m}~10^{-1.5}. The inferred energy is consistent with energy estimates based on radio observations provided \dot{m}~0.04, in which case radio observations imply β~0.8, consistent with the X-ray constraint β^3*\dot{m}~10^{-1.5}. While X-ray observations allow to determine the parameters characterizing the pre-explosion wind and the mildly relativistic shell ejected by SN1998bw, they do not provide evidence for existence of an off-axis "standard" GRB jet associated with SN1998bw, that may have produced GRB980425. However, as recently pointed out in (astro-ph/0310320), the lack of observational signatures typically expected to be produced by such an off-axis jet on a 1yr time scale, may be due to a low \dot{m}<0.1, which implies that an off-axis jet will become observable only on >10yr time scale.

astro-ph↗

arXiv2004

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 $Γ_i\gsim 200$; and An upper limit to the fraction of thermal energy carried by the magnetic field behind the shock driven into the surrounding medium, $ε_{B,f} <= 10^{-4}$. Such constraints can not be inferred from keV--MeV data alone. The unusually low value of $ε_{B,f}$ 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.

astro-ph↗

arXiv2003

Neutrino Tomography of Gamma Ray Bursts and Massive Stellar Collapses

Neutrinos at energies above TeV can serve as probes of the stellar progenitor and jet dynamics of gamma ray bursts arising from stellar core collapses. They can also probe collapses which do not lead to gamma-rays, which may be much more numerous. We calculate detailed neutrino spectra from shock accelerated protons in jets just below the outer stellar envelope, before their emergence. We present neutrino flux estimates from such pre-burst jets for two different massive stellar progenitor models. These should be distinguishable by IceCube, and we discuss the implications.

astro-ph↗