arXiv subjects

E. Waxman

Publications and source records attributed to E. Waxman.

At least 37 records · Page 2Linked to original sources

arXiv2009

Long duration radio transients lacking optical counterparts are possibly Galactic Neutron Stars

(abridged) Recently, a new class of radio transients in the 5-GHz band was detected by Bower et al. We present new deep near-Infrared (IR) observations of the field containing these transients, and find no counterparts down to a limiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radio transients recently reported by Kida et al. are consistent with being additional examples of the Bower et al. transients. We refer to these groups of events as "long-duration radio transients". The main characteristics of this population are: time scales longer than 30 minute but shorter than several days; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2 (95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_ν~ ν^alpha, with alpha>0; and most notably the lack of any counterparts in quiescence in any wavelength. We rule out an association with many types of objects. Galactic brown-dwarfs or some sort of exotic explosions remain plausible options. We argue that an attractive progenitor candidate for these radio transients is the class of Galactic isolated old neutron stars (NS). We confront this hypothesis with Monte-Carlo simulations of the space distribution of old NSs, and find satisfactory agreement for the large areal density. Furthermore, the lack of quiescent counterparts is explained quite naturally. In this framework we find: the mean distance to events in the Bower et al. sample is of order kpc; the typical distance to the Kida et al. transients are constrained to be between 30 pc and 900 pc (95% C.L.); these events should repeat with a time scale of order several months; and sub-mJy level bursts should exhibit Galactic latitude dependence. We discuss possible mechanisms giving rise to the observed radio emission.

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arXiv2008

An extremely luminous X-ray outburst at the birth of a supernova

Massive stars end their short lives in spectacular explosions, supernovae, that synthesize new elements and drive galaxy evolution. Throughout history supernovae were discovered chiefly through their delayed optical light, preventing observations in the first moments (hours to days) following the explosion. As a result, the progenitors of some supernovae and the events leading up to their violent demise remain intensely debated. Here we report the serendipitous discovery of a supernova at the time of explosion, marked by an extremely luminous X-ray outburst. We attribute the outburst to the break-out of the supernova shock-wave from the progenitor, and show that the inferred rate of such events agrees with that of all core-collapse supernovae. We forecast that future wide-field X-ray surveys will catch hundreds of supernovae each year in the act of explosion, and thereby enable crucial neutrino and gravitational wave detections that may ultimately unravel the explosion mechanism.

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arXiv2007

Cosmic rays from trans-relativistic supernovae

We derive constraints that must be satisfied by the sources of ~10^{15} to ~10^{18} eV cosmic rays, under the assumption that the sources are Galactic. We show that while these constraints are not satisfied by ordinary supernovae (SNe), which are believed to be the sources of <10^{15} eV cosmic rays, they may be satisfied by the recently discovered class of trans-relativistic supernovae (TRSNe), which were observed in association with gamma-ray bursts. We define TRSNe as SNe that deposit a large fraction, f_R>10^{-2}, of their kinetic energy in mildly relativistic, γβ>1, ejecta. The high velocity ejecta enable particle acceleration to ~10^{18} eV, and the large value of f_R (compared to f_R~10^{-7} for ordinary SNe) ensures that if TRSNe produce the observed ~10^{18} eV cosmic ray flux, they do not overproduce the flux at lower energies. This, combined with the estimated rate and energy production of TRSNe, imply that Galactic TRSNe may be the sources of cosmic rays with energies up to ~10^{18}eV .

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arXiv2007

GRB060218: A Relativistic Supernova Shock Breakout

We show that the prompt and afterglow X-ray emission of GRB060218, as well as its early (t<=1 d) optical-UV emission, can be explained by a model in which a radiation- mediated shock propagates through a compact progenitor star into a dense wind. The prompt thermal X-ray emission is produced in this model as the mildly relativistic shock, v/c=0.85 carrying few x 10^49 erg, reaches the wind (Thomson) photosphere, where the post-shock thermal radiation is released and the shock becomes collisionless. Adopting this interpretation of the thermal X-ray emission, a subsequent X-ray afterglow is predicted, due to synchrotron emission and inverse-Compton scattering of SN UV photons by electrons accelerated in the collisionless shock. Early optical-UV emission is also predicted, due to the cooling of the outer δM ~10^{-3} M_sun envelope of the star, which was heated to high temperature during shock passage. The observed X-ray afterglow and the early optical-UV emission are both consistent with those expected in this model. Detailed analysis of the early optical-UV emission may provide detailed constraints on the density distribution near the stellar surface.

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arXiv2007

Non-photonic emission from gamma-ray bursts

gamma-ray bursts (GRBs) are likely sources of ultra-high energy, >10^{19} eV, protons and high energy, >1 TeV, neutrinos. Large volume detectors of ultra high energy cosmic rays (UHECRs) and high energy neutrinos, which are already operating and are being expanded, may allow to test in the coming few years the predictions of the GRB model for high energy proton and neutrino production. Detection of the predicted signals will allow to identify the sources of UHECRs and will provide a unique probe, which may allow to resolve open questions related to the underlying physics of GRB models. Moreover, detection of GRB neutrinos will allow to test for neutrino properties (e.g., flavor oscillations for which tau'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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arXiv2007

Neutrino astronomy and gamma-ray bursts

The construction of large volume detectors of high energy, >1 TeV, neutrinos is mainly driven by the search for extra-Galactic neutrino sources. The existence of such sources is implied by observations of ultra-high energy, >10^{19} eV, cosmic-rays, the origin of which is a mystery. In this lecture I briefly discuss the expected extra-Galactic neutrino signal and the current state of the experimental efforts. Neutrino emission from gamma-ray bursts (GRBs), which are likely sources of both high energy protons and neutrinos, is discussed in some detail. The detection of the predicted GRB neutrino signal, which may become possible in the coming few years, will allow one to identify the sources of ultra-high energy cosmic-rays and to resolve open questions related to the underlying physics of GRB models. Moreover, detection of GRB neutrinos will allow one to test for neutrino properties (e.g. flavor oscillations and coupling to gravity) with an accuracy many orders of magnitude better than is currently possible.

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arXiv2007

Neutrino astrophysics: A new tool for exploring the universe

In the past four decades a new type of astronomy has emerged, where instead of looking up into the sky "telescopes" are buried miles underground or deep under water or ice and search not for photons (that is, light), but rather for particles called neutrinos. Neutrinos are nearly massless particles that interact very weakly with matter. The detection of neutrinos emitted by the Sun and by a nearby supernova provided direct tests of the theory of stellar evolution and led to modifications of the standard model describing the properties of elementary particles. At present, several very large neutrino detectors are being constructed, aiming at the detection of the most powerful sources of energy and particles in the universe. The hope is that the detection of neutrinos from these sources, which are extra-Galactic and are most likely powered by mass accretion onto black-holes, will not only allow study of the sources, but, much like solar neutrinos, will also provide new information about fundamental properties of matter.

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arXiv2006

Gamma-ray bursts and collisionless shocks

Particle acceleration in collisionless shocks is believed to be responsible for the production of cosmic-rays over a wide range of energies, from few GeV to >10^{20}eV, as well as for the non-thermal emission of radiation from a wide variety of high energy astrophysical sources. A theory of collisionless shocks based on first principles does not, however, exist. Observations of gamma-ray burst (GRB) "afterglows" provide a unique opportunity for diagnosing the physics of relativistic collisionless shocks. Most GRBs are believed to be associated with explosions of massive stars. Their "afterglows," delayed low energy emission following the prompt burst of gamma-rays, are well accounted for by a model in which afterglow radiation is due to synchrotron emission of electrons accelerated in relativistic collisionless shock waves driven by the explosion into the surrounding plasma. Within the framework of this model, some striking characteristics of collisionless relativistic shocks are implied. These include the generation of downstream magnetic fields with energy density exceeding that of the upstream field by ~8 orders of magnitude, the survival of this strong field at distances ~10^{10} skin-depths downstream of the shock, and the acceleration of particles to a power-law energy spectrum, d\log n/d\log E ~ -2, possibly extending to 10^{20}eV. I review in this talk the phenomenological considerations, based on which these characteristics are inferred, and the challenges posed to our current models of particle acceleration and magnetic field generation in collisionless shocks. Some recent theoretical results derived based on the assumption of a self-similar shock structure are briefly discussed.

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arXiv2006

The shock break-out of GRB 060218/SN 2006aj

Although the link between long Gamma Ray Bursts (GRBs) and supernovae (SNe) has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know however how a GRB jet emerges from the star surface nor how a GRB progenitor explodes. Here we report on observations of the close GRB060218 and its connection to SN2006aj. In addition to the classical non-thermal emission, GRB060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/UV band as time passes. We interpret these features as arising from the break out of a shock driven by a mildly relativistic shell into the dense wind surrounding the progenitor. Our observations allow us for the first time to catch a SN in the act of exploding, to directly observe the shock break-out and to provide strong evidence that the GRB progenitor was a Wolf-Rayet star.

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arXiv2004

Gamma-ray bursts: Potential sources of ultra high energy cosmic-rays

The arguments suggesting an association between the sources of cosmological gamma-ray bursts (GRBs) and the sources of ultra-high energy cosmic rays (UHECRs) are presented. Recent GRB and UHECR observations are shown to strengthen these arguments. Predictions of the GRB model for UHECR production, that may be tested with large area high energy cosmic-ray detectors which are either operating or under construction, are outlined.

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arXiv2004

The Luminosity and Angular Distributions of Long GRBs

The realization that the total energy of GRBs is correlated with their jet break angles motivates the search for a similar relation between the peak luminosity, $L$, and the jet break angles, $L\proptoθ^{-2}$. Such a relation implies that the GRB luminosity function determines the angular distribution. We re-derive the GRB luminosity function using the BATSE peak flux distribution and compare the predicted distribution with the observed redshift distribution. The luminosity function can be approximated by a broken power law with a break peak luminosity of $4.4 \times 10^{51}$ erg/sec, a typical jet angle of 0.12 rad and a local GRB rate of $0.44 h_{65}^{3}$ Gpc$^{-3}$yr$^{-1}$. The angular distribution implied by $L\proptoθ^{-2}$ agrees well with the observed one, and implies a correction factor to the local rate due to beaming of $75\pm25$ (instead of 500 as commonly used). The inferred overall local GRB rate is $33\pm11 h_{65}^{3}$ Gpc$^{-3}$yr$^{-1}$. The luminosity function and angle distribution obtained within the universal structured jet model, where the angular distribution is essentially $\propto θ$ and hence the luminosity function must be $\propto L^{-2}$, deviate from the observations at low peak fluxes and, correspondingly, at large angles. The corresponding correction factor for the universal structure jet is $\sim 20 \pm 10$.

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arXiv2004

High energy cosmic-rays from gamma-ray burst sources: A stronger case

The suggested association between the sources of gamma-ray bursts (GRB's) and the sources of ultra-high energy cosmic rays (UHECR's) is based on two arguments: (i) The average energy generation rate of UHECR's is similar to the gamma-ray generation rate of GRB's, and (ii) The constraints that UHECR sources must satisfy to allow proton acceleration to >10^{20} eV are similar to those inferred for GRB sources from gamma-ray observations. We show that recent GRB and UHECR observations strengthen both arguments, and hence strengthen the suggested association.

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arXiv2003

The nature of GRB980425 and the search for off-axis GRB signatures in nearby type Ib/c supernovae emission

The identification of type Ib/c supernovae as GRB progenitors is motivated by the association of GRB980425 with SN1998bw and of GRB030329 with SN2003dh. While the gamma-ray luminosity of GRB030329 was typical to cosmological GRBs, the luminosity of the nearby (40 Mpc) GRB980425 was ~5 orders of magnitude lower. The large luminosity difference is commonly explained by hypothesizing that either SNe Ib/c produce two different classes of GRBs, or that GRB980425 was a typical cosmological GRB jet viewed off-axis. In the latter scenario, strong radio emission, ~10^{30} erg/s Hz, is expected at ~1 yr delay due to jet deceleration to sub-relativistic speed, as observed from GRB970508. The radio luminosity of SN1998bw was 3 orders of magnitude lower than this value. We show that the low radio flux may be consistent with the off-axis jet interpretation, if the density of the wind surrounding the progenitor is lower than typically expected, \dot{m}=(\dot{M}/10^{-5} M_sun/yr)/(v/10^3 km/s)~0.1 instead of \dot{m}>=1. The lower value of \dot{m} is consistent with the observed radio emission from the supernova shock driven into the wind. This interpretation predicts transition to sub-relativistic expansion at ~10 yr delay, with current \~1 mJy 10GHz flux and m_V~23 optical flux, and with ~10 mas angular source size. It also implies that in order to search for the signature of off-axis GRBs associated with nearby Ib/c supernovae, follow up observations should be carried on a multi-yr time scale.

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arXiv2003

High energy cosmic-rays: puzzles, models, and giga-ton neutrino telescopes

The existence of cosmic rays of energies exceeding 10^20 eV is one of the mysteries of high energy astrophysics. The spectrum and the high energy to which it extends rule out almost all suggested source models. The challenges posed by observations to models for the origin of high energy cosmic rays are reviewed, and the implications of recent new experimental results are discussed. Large area high energy cosmic ray detectors and large volume high energy neutrino detectors currently under construction may resolve the high energy cosmic ray puzzle, and shed light on the identity and physics of the most powerful accelerators in the universe.

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arXiv2003

High energy neutrinos from magnetars

Magnetars can accelerate cosmic rays to high energies through the unipolar effect, and are also copious soft photon emitters. We show that young, fast-rotating magnetars whose spin and magnetic moment point in opposite directions emit high energy neutrinos from their polar caps through photomeson interactions. We identify a neutrino cut-off band in the magnetar period-magnetic field strength phase diagram, corresponding to the photomeson interaction threshold. Within uncertainties, we point out four possible neutrino emission candidates among the currently known magnetars, the brightest of which may be detectable for a chance on-beam alignment. Young magnetars in the universe would also contribute to a weak diffuse neutrino background, whose detectability is marginal, depending on the typical neutrino energy.

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arXiv2003

New direction for gamma-rays

The origin of energetic gamma-ray bursts is still unknown. But the detection of polarization of gamma-rays provides fresh insight into the mechanism driving these powerful explosions.

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arXiv2003

Gamma-Ray Bursts: The Underlying Model

A pedagogical derivation is presented of the ``fireball'' model of gamma-ray bursts, according to which the observable effects are due to the dissipation of the kinetic energy of a relativistically expanding wind, a ``fireball.'' The main open questions are emphasized, and key afterglow observations, that provide support for this model, are briefly discussed. The relativistic outflow is, most likely, driven by the accretion of a fraction of a solar mass onto a newly born (few) solar mass black hole. The observed radiation is produced once the plasma has expanded to a scale much larger than that of the underlying ``engine,'' and is therefore largely independent of the details of the progenitor, whose gravitational collapse leads to fireball formation. Several progenitor scenarios, and the prospects for discrimination among them using future observations, are discussed. The production in gamma- ray burst fireballs of high energy protons and neutrinos, and the implications of burst neutrino detection by kilometer-scale telescopes under construction, are briefly discussed.

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arXiv2002

Astrophysical sources of high energy neutrinos

Several high energy, >100 GeV, neutrino telescopes are currently operating or under construction. Their main motivation is the extension of the horizon of neutrino astronomy to cosmological scales. We show that general, model independent, arguments imply that ~1 Gton detectors are required to detect cosmic high energy neutrino sources. Predictions of models of some of the leading candidate sources, gamma-ray bursts and micro-quasars, are discussed, and the question of what can be learned from neutrino observations is addressed.

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