Search

Growing observational evidence suggests that enhanced mass loss from the progenitors of core-collapse supernovae (SNe) is common during $\sim1$ yr preceding the explosion, creating an optically thick circum-stellar medium (CSM) shell at $\sim10^{14.5}$ cm radii. We show that if such mass loss is indeed common, then the breakout of the SN shock through the dense CSM shell produces a neutrino flux that may account for a significant fraction of the observed $\gtrsim10$ TeV neutrino background. The neutrinos are created within a few days from the explosion, during and shortly after the shock breakout, which produces also large UV (and later X-ray) luminosity. The compact size and large UV luminosity imply a pair production optical depth of $\sim10^4$ for $>100$ GeV photons, naturally accounting for the lack of a high-energy gamma-ray background accompanying the neutrino background. SNe producing $>1$ neutrino event in a 1 km$^2$ detector are expected at a rate of $\lesssim0.1$/yr. A quantitative theory describing the evolution of the electromagnetic spectrum during a breakout, as the radiation-mediated shock is transformed into a collisionless one, is required to enable (i) using data from upcoming surveys that will systematically detect large numbers of young, $<1$ d old SNe, to determine the pre-explosion mass loss history of the SN progenitor population, and (ii) a quantitative determination of the neutrino luminosity and spectrum., Comment: Accepted to ApJ. Added discussion of current IceCube limits and non-type II SNe contribution

Light emission in the first hours and days following core-collapse supernovae is dominated by the escape of photons from the expanding shock-heated envelope. In preceding papers, we provided a simple analytic description of the time-dependent luminosity, $L$, and color temperature, $T_{\rm col}$, as well as of the small ($\simeq10\%$) deviations of the spectrum from blackbody at low frequencies, $h\nu< 3T_{\rm col}$, and of `line dampening' at $h\nu> 3T_{\rm col}$, for explosions of red supergiants (RSGs) with convective polytropic envelopes (without significant circum-stellar medium). Here, we extend our work to provide similar analytic formulae for explosions of blue supergiants with radiative polytropic envelopes. The analytic formulae are calibrated against a large set of spherically symmetric multi-group (frequency-dependent) calculations for a wide range of progenitor parameters (mass, radius, core/envelope mass ratios) and explosion energies. In these calculations we use the opacity tables we constructed (and made publicly available), that include the contributions of bound-bound and bound-free transitions. They reproduce the numeric $L$ and $T_{\rm col}$ to within 10\% and 5\% accuracy, and the spectral energy distribution to within $\sim20-40\%$. The accuracy is similar to that achieved for RSG explosions., Comment: 12 pages, 10 figures

The impact of the Double Asteroid Redirection Test (DART) spacecraft with Dimorphos allows us to study asteroid collision physics, including momentum transfer, the ejecta properties, and the visibility of such events in the Solar System. We report observations of the DART impact in the ultraviolet (UV), visible light, and near-infrared (IR) wavelengths. The observations support the existence of at least two separate components of the ejecta: a fast and a slow component. The fast-ejecta component is composed of a gaseous phase, moving at about 1.6 km/s with a mass of <10^4 kg. The fast ejecta is detected in the UV and visible light, but not in the near-IR $z$-band observations. Fitting a simplified optical thickness model to these observations allows us to constrain some of the properties of the fast ejecta, including its scattering efficiency and the opacity of the gas. The slow ejecta component is moving at typical velocities of up to about 10 m/s. It is composed of micrometer-size particles, that have a scattering efficiency, at the direction of the observer, of the order of 10^-3 and a total mass of about 10^6 kg. The larger particles in the slow ejecta, whose size is bound to be in the range between ~1 mm to ~1 m, likely have a scattering efficiency larger than that of the pre-impact Didymos system., Comment: Submitted to MNRAS

Rapid identification of the optical counterparts of Neutron Star (NS) merger events discovered by gravitational wave detectors may require observing a large error region and sifting through a large number of transients to identify the object of interest. Given the expense of spectroscopic observations, a question arises: How can we utilize photometric observations for candidate prioritization, and what kinds of photometric observations are needed to achieve this goal? NS merger kilonova exhibits low ejecta mass (~5x10^-2 solar mass) and a rapidly evolving photospheric radius (with a velocity ~0.2c). As a consequence, these sources display rapid optical-flux evolution. Indeed, selection based on fast flux variations is commonly used for young supernovae and NS mergers. In this study, we leverage the best currently available flux-limited transient survey - the Zwicky Transient Facility Bright Transient Survey - to extend and quantify this approach. We focus on selecting transients detected in a 3-day cadence survey and observed at a one-day cadence. We explore their distribution in the phase space defined by g-r, g-dot, and r-dot. Our analysis demonstrates that for a significant portion of the time during the first week, the kilonova AT 2017gfo stands out in this phase space. It is important to note that this investigation is subject to various biases and challenges; nevertheless, it suggests that certain photometric observations can be leveraged to identify transients with the highest probability of being fast-evolving events. We also find that a large fraction (~0.75) of the transient candidates with |g-dot|>0.7 mag/day, are cataclysmic variables or active galactic nuclei with radio counterparts., Comment: 9 pages, 4 figures, MNRAS in press

The Ultraviolet Transient Astronomy Satellite (ULTRASAT) is scheduled to be launched to geostationary orbit in 2026. It will carry a telescope with an unprecedentedly large field of view (204 deg$^2$) and NUV (230-290nm) sensitivity (22.5 mag, 5$\sigma$, at 900s). ULTRASAT will conduct the first wide-field survey of transient and variable NUV sources and will revolutionize our ability to study the hot transient universe: It will explore a new parameter space in energy and time-scale (months long light-curves with minutes cadence), with an extra-Galactic volume accessible for the discovery of transient sources that is $>$300 times larger than that of GALEX and comparable to that of LSST. ULTRASAT data will be transmitted to the ground in real-time, and transient alerts will be distributed to the community in $<$15 min, enabling a vigorous ground-based follow-up of ULTRASAT sources. ULTRASAT will also provide an all-sky NUV image to $>$23.5 AB mag, over 10 times deeper than the GALEX map. Two key science goals of ULTRASAT are the study of mergers of binaries involving neutron stars, and supernovae: With a large fraction ($>$50%) of the sky instantaneously accessible, fast (minutes) slewing capability and a field-of-view that covers the error ellipses expected from GW detectors beyond 2025, ULTRASAT will rapidly detect the electromagnetic emission following BNS/NS-BH mergers identified by GW detectors, and will provide continuous NUV light-curves of the events; ULTRASAT will provide early (hour) detection and continuous high (minutes) cadence NUV light curves for hundreds of core-collapse supernovae, including for rarer supernova progenitor types., Comment: 40 pages, 16 figures, 3 tables. Submitted to the AAS journals

The Large Array Survey Telescope (LAST) is a wide-field visible-light telescope array designed to explore the variable and transient sky with a high cadence. LAST will be composed of 48, 28-cm f/2.2 telescopes (32 already installed) equipped with full-frame backside-illuminated cooled CMOS detectors. Each telescope provides a field of view (FoV) of 7.4 deg^2 with 1.25 arcsec/pix, while the system FoV is 355 deg^2 in 2.9 Gpix. The total collecting area of LAST, with 48 telescopes, is equivalent to a 1.9-m telescope. The cost-effectiveness of the system (i.e., probed volume of space per unit time per unit cost) is about an order of magnitude higher than most existing and under-construction sky surveys. The telescopes are mounted on 12 separate mounts, each carrying four telescopes. This provides significant flexibility in operating the system. The first LAST system is under construction in the Israeli Negev Desert, with 32 telescopes already deployed. We present the system overview and performances based on the system commissioning data. The Bp 5-sigma limiting magnitude of a single 28-cm telescope is about 19.6 (21.0), in 20 s (20x20 s). Astrometric two-axes precision (rms) at the bright-end is about 60 (30)\,mas in 20\,s (20x20 s), while absolute photometric calibration, relative to GAIA, provides ~10 millimag accuracy. Relative photometric precision, in a single 20 s (320 s) image, at the bright-end measured over a time scale of about 60 min is about 3 (1) millimag. We discuss the system science goals, data pipelines, and the observatory control system in companion publications., Comment: Submitted to PASP, 15pp

We report on the discovery of AT2018lqh (ZTF18abfzgpl) -- a rapidly-evolving extra-galactic transient in a star-forming host at 242 Mpc. The transient g-band light curve's duration above half-maximum light is about 2.1 days, where 0.4/1.7 days are spent on the rise/decay, respectively. The estimated bolometric light curve of this object peaked at about 7x10^42 erg/s -- roughly seven times brighter than AT2017gfo. We show that this event can be explained by an explosion with a fast (v~0.08 c) low-mass (~0.07 Msun) ejecta, composed mostly of radioactive elements. For example, ejecta dominated by Ni-56 with a time scale of t_0=1.6 days for the ejecta to become optically thin for gamma-rays fits the data well. Such a scenario requires burning at densities that are typically found in the envelopes of neutron stars or the cores of white dwarfs. A combination of circumstellar material (CSM) interaction power at early times and shock cooling at late times is consistent with the photometric observations, but the observed spectrum of the event may pose some challenges for this scenario. The observations are not consistent with a shock breakout from a stellar envelope, while a model involving a low-mass ejecta ramming into low-mass CSM cannot explain both the early- and late-time observations., Comment: 10 pages, ApJ in press

Non-thermal particles and high-energy radiation can play a role in the dynamical processes in star-forming regions and provide an important piece of the multiwavelength observational picture of their structure and components. Powerful stellar winds and supernovae in compact clusters of massive stars and OB associations are known to be favourable sites of high-energy particle acceleration and sources of non-thermal radiation and neutrinos. Namely, young massive stellar clusters are likely sources of the PeV (petaelectronvolt) regime cosmic rays (CRs). They can also be responsible for the cosmic ray composition, e.g., 22Ne/20Ne anomalous isotopic ratio in CRs. Efficient particle acceleration can be accompanied by super-adiabatic amplification of the fluctuating magnetic fields in the systems converting a part of kinetic power of the winds and supernovae into the magnetic energy through the CR-driven instabilities. The escape and CR propagation in the vicinity of the sources are affected by the non-linear CR feedback. These effects are expected to be important in starburst galaxies, which produce high-energy neutrinos and gamma-rays. We give a brief review of the theoretical models and observational data on high-energy particle acceleration and their radiation in star-forming regions with young stellar population., Comment: 41 pages, Space Science Reviews v.216, id 42, topical collection Star Formation

The IceCube collaboration reports a detection of extra-terrestrial neutrinos. The isotropy and flavor content of the signal, and the coincidence, within current uncertainties, of the 50 TeV to 2 PeV flux and the spectrum with the Waxman-Bahcall bound, suggest a cosmological origin of the neutrinos, related to the sources of ultra-high energy, $>10^{10}$ GeV, cosmic-rays (UHECR). The most natural explanation of the UHECR and neutrino signals is that both are produced by the same population of cosmological sources, producing CRs (likely protons) at a similar rate, $E^2d\dot{n}/dE\propto E^{0}$, over the [$1$ PeV,$10^{11}$ GeV] energy range, and residing in "calorimetric" environments, like galaxies with high star formation rate, in which $E/Z<100$ PeV CRs lose much of their energy to pion production. A tenfold increase in the effective mass of the detector at $\gtrsim100$ TeV is required in order to significantly improve the accuracy of current measurements, to enable the detection of a few bright nearby starburst "calorimeters", and to open the possibility of identifying the CR sources embedded within the calorimeters, by associating neutrinos with photons accompanying transient events responsible for their generation. Source identification and a large neutrino sample may enable one to use astrophysical neutrinos to constrain new physics models., Comment: To appear in "Neutrino Astronomy- Current status, future prospects", Eds. T. Gaisser & A. Karle (World Scientific)

The flux, spectrum and angular distribution of the excess neutrino signal detected by IceCube between 50TeV and 2PeV are inconsistent with those expected for Galactic sources. The coincidence of the excess, $E_\nu^2\Phi_\nu=3.6\pm1.2\times10^{-8}(GeV/ cm^2 sr s)$, with the Waxman-Bahcall (WB) bound, $E_\nu^2\Phi_{WB}=3.4\times10^{-8}(GeV/cm^2 sr s)$, is probably a clue to the origin of IceCube's neutrinos. The most natural explanation of this coincidence is that both the neutrino excess and the ultra-high energy, $>10^{19}$ eV, cosmic-ray (UHECR) flux are produced by the same population of cosmologically distributed sources, producing CRs, likely protons, at a similar rate, $E^2 dQ/dE=0.5\times10^{44}(erg/Mpc^3yr)$ (at z=0), across a wide range of energies, from $10^{15}$ eV to $>10^{20}$ eV, and residing in environments (such as starburst galaxies) in which CRs of rigidity $E/Z< 10^{17}$ eV lose much of their energy to pion production. Identification of the neutrino sources will allow one to identify the UHECR accelerators, to resolve open questions related to the accelerator models, and to study neutrino properties (related e.g. to flavor oscillations and coupling to gravity) with an accuracy many orders of magnitude better than is currently possible. The most promising method for identifying the sources is by association of a neutrino with an electromagnetic signal accompanying a transient event responsible for its generation. The neutrino flux that is produced within the sources, and that may thus be directly associated with transient events, may be significantly lower than the total observed neutrino flux, which may be dominated by neutrino production at the environment in which the sources reside., Comment: 8 pages, 2 figures, to appear in the Proc. of the 9th Rencontres du Vietnam : Windows on the universe

The time-variable electromagnetic sky has been well-explored at a wide range of wavelengths. Numerous high-energy space missions take advantage of the dark Gamma-ray and X-ray sky and utilize very wide field detectors to provide almost continuous monitoring of the entire celestial sphere. In visible light, new wide-field ground-based surveys cover wide patches of sky with ever decreasing cadence, progressing from monthly-weekly time scale surveys to sub-night sampling. In the radio, new powerful instrumentation offers unprecedented sensitivity over wide fields of view, with pathfinder experiments for even more ambitious programs underway. In contrast, the ultra-violet (UV) variable sky is relatively poorly explored, even though it offers exciting scientific prospects. Here, we review the potential scientific impact of a wide-field UV survey on the study of explosive and other transient events, as well as known classes of variable objects, such as active galactic nuclei and variable stars. We quantify our predictions using a fiducial set of observational parameters which are similar to those envisaged for the proposed ULTRASAT mission. We show that such a mission would be able to revolutionize our knowledge about massive star explosions by measuring the early UV emission from hundreds of events, revealing key physical parameters of the exploding progenitor stars. Such a mission would also detect the UV emission from many tens of tidal-disruption events of stars by super massive black holes at galactic nuclei and enable a measurement of the rate of such events. The overlap of such a wide-field UV mission with existing and planned gravitational-wave and high-energy neutrino telescopes makes it especially timely.

Many of the astrophysical sources and violent phenomena observed in our Universe are potential emitters of gravitational waves (GW) and high-energy neutrinos (HEN). Both GWs and HENs may escape very dense media and travel unaffected over cosmological distances, carrying information from the innermost regions of the astrophysical engines. Such messengers could also reveal new, hidden sources that have not been observed by conventional photon-based astronomy. Coincident observation of GWs and HENs may thus play a critical role in multimessenger astronomy. This is particularly true at the present time owing to the advent of a new generation of dedicated detectors: IceCube, ANTARES, VIRGO and LIGO. Given the complexity of the instruments, a successful joint analysis of this data set will be possible only if the expertise and knowledge of the data is shared between the two communities. This review aims at providing an overview of both theoretical and experimental state-of-the-art and perspectives for such a GW+HEN multimessenger astronomy., Comment: Revised version, to appear in Reviews of Modern Physics (Colloquium Series). Section II on Potential GW+HEN emitters thoroughly updated. Section IV expanded with recent experimental results from joint GW+HEN searches with ANTARES, IceCube, LIGO and VIRGO, and with an extended discussion of the reach and implications of future searches

We present numerical solutions of the 2D relativistic hydrodynamics equations describing the deceleration and expansion of highly relativistic conical jets, of opening angles 0.05R/c the emission of radiation from the jet blast wave is similar to that of a spherical blast wave carrying the same energy. Thus, the total (calorimetric) energy of GRB blast waves may be estimated with only a small fractional error based on t>R/c observations., Comment: 5 pages, 5 figures

Cosmic-rays with energies exceeding 10^{19} eV are referred to as Ultra High Energy Cosmic Rays (UHECRs). The sources of these particles and their acceleration mechanism are unknown, and for many years have been the issue of much debate. The first part of this review describes the main constraints, that are implied by UHECR observations on the properties of candidate UHECR sources, the candidate sources, and the related main open questions. In order to address the challenges of identifying the UHECR sources and of probing the physical mechanisms driving them, a "multi-messenger" approach will most likely be required, combining electromagnetic, cosmic-ray and neutrino observations. The second part of the review is devoted to a discussion of high energy neutrino astronomy. It is shown that detectors, which are currently under construction, are expected to reach the effective mass required for the detection of high energy extra-Galactic neutrino sources, and may therefore play a key role in the near future in resolving the main open questions. The detection of high energy neutrinos from extra-Galactic sources will not only provide constraints on the identity and underlying physics of UHECR sources, but may furthermore provide information on fundamental neutrino properties., Comment: 16 pages, 10 figures; To be published in {\guillemotleft} Astronomy at the Frontiers of Science {\guillemotright}, ed. J.-P. Lasota, Springer 2011. The original publication will be available at http://www.springerlink.com

Type-IIn supernovae (SNe), which are characterized by strong interaction of their ejecta with the surrounding circumstellar matter (CSM), provide a unique opportunity to study the mass-loss history of massive stars shortly before their explosive death. We present the discovery and follow-up observations of a Type IIn SN, PTF 09uj, detected by the Palomar Transient Factory (PTF). Serendipitous observations by GALEX at ultraviolet (UV) wavelengths detected the rise of the SN light curve prior to the PTF discovery. The UV light curve of the SN rose fast, with a time scale of a few days, to a UV absolute AB magnitude of about -19.5. Modeling our observations, we suggest that the fast rise of the UV light curve is due to the breakout of the SN shock through the dense CSM (n~10^10 cm^-3). Furthermore, we find that prior to the explosion the progenitor went through a phase of high mass-loss rate (~0.1 solar mass per year) that lasted for a few years. The decay rate of this SN was fast relative to that of other SNe IIn., Comment: Accepted to Apj, 6 pages, 4 figures

(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_\nu ~ \nu^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., Comment: Submitted to ApJ, 17 pages, 10 figures

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., Comment: Article to appear in the May 22 issue of Nature. Final version: 21 pages, 5 figures. High resolution figures and Supplementary Information available at http://www.astro.princeton.edu/~alicia/SN2008D/ Note: the results presented in this paper are under embargo by Nature until the publication date

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, \gamma\beta>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 ., Comment: Accepted to ApJ. Expanded abstract, introduction, discussion

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., Comment: contribution to the proc. of the Sixteenth Maryland Astrophysics Conference (was not posted earlier by mistake)

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 \delta M ~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., Comment: Discussion of several points expanded, 1 citation corrected; accepted to ApJ

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., Comment: Review, R. Soc. Discussion Meeting on GRBs (To appear in Phil. Trans. R. Soc. Lond. A)

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., Comment: Perspective article, Science Special Issue on "particle astrophysics" (Jan 2007)

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., Comment: Expanded discussion of the derivation of collisionless shock characteristics from afterglow observations. Accepted to Plasma Phys. and Controlled Fusion (Invited review, 33rd annual European Physical Society Conference on Plasma Physics, Rome 2006)

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., Comment: 12 pages, 3 color figures (final version). Accepted for publication in Nature

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., Comment: 8 pages; Invited talk, XIII International Symposium on Very High Energy Cosmic Ray Interactions (Pylos, Greece 2004)

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, $L\propto\theta^{-2}$. 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 $L\propto\theta^{-2}$ 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 $\propto \theta$ 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 $\sim 20 \pm 10$., Comment: 11 pages, 5 figures, accepted for publication in ApJ

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., Comment: 13 pages, 1 figure; 2 references added; ApJ, in press

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., Comment: 12 pages, 7 figures; Summary of review talk, PASCOS 03 (Mumbai, India)

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., Comment: Nature "News & Views", RE Coburn & Boggs (astro-ph/0305377)

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., Comment: In "Supernovae and Gamma Ray Bursters", ed. K. W. Weiler, Lecture Notes in Physics, Springer-Verlag (in press); 26 pages, 2 figures

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., Comment: Invited talk, Neutrino 2002 (Munich)

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., Comment: Arguments strengthened by the inclusion of updated constraints on GRB rate and model parameters, based on new afterglow data and analysis. Accepted to ApJ

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., Comment: emulateapj style, 6 pages, 1 figure, ApJ, v595, in press. Important contributions from Dr. Harding added. Major revisions made. More conservative and realistic estimates about the neutrino threshold condition and emission efficiency performed. More realistic typical beaming angle and magnetar birth rate adopted

Several high-frequency peaked BL Lac objects such as Mrk 501 are strong TeV emitters. However, a significant fraction of the TeV gamma rays emitted are likely to be absorbed in interactions with the diffuse IR background, yielding electron-positron pairs. Hence, the observed TeV spectrum must be steeper than the intrinsic one. Using the recently derived intrinsic $\gamma$-ray spectrum of Mrk 501 during its 1997 high state, we study the inverse-Compton scattering of cosmic microwave photons by the resulting electron-positron pairs, which implies the existence of a hitherto undiscovered GeV emission. The typical duration of the GeV emission is determined by the flaring activity time and the energy-dependent magnetic deflection time. We numerically calculate the scattered photon spectrum for different intergalactic magnetic field (IGMF) strengths, and find a spectral turnover and flare duration at GeV energies which are dependent on the field strength. We also estimate the scattered photon flux in the quiescent state of Mrk 501. The GeV flux levels predicted are consistent with existing EGRET upper limits, and should be detectable above the synchrotron -- self Compton (SSC) component with the {\em Gamma-Ray Large Area Space Telescope} ({\em GLAST}) for IGMFs $\lesssim 10^{-16}$ G, as expected in voids. Such detections would provide constraints on the strength of weak IGMFs., Comment: ApJL in press, expanded version with one extra figure

We discuss the possibility that microquasar jets may be powerful emitters of TeV neutrinos. We estimate the neutrino fluxes produced by photopion production in the jets of a sample of identified microquasars and microquasar candidates, for which available data enables rough determination of the jet parameters. We demonstrate that in several of the sources considered, the neutrino flux at Earth, produced in events similar to those observed, can exceed the detection threshold of a km^2 neutrino detector. Sources with bulk Lorentz factors larger than those characteristic of the sample considered here, directed along our line of sight may be very difficult to resolve at radio wavelengths and hence may be difficult to identify as microqusar candidates. However these sources can be identified through their neutrino and gamma-ray emission., Comment: To appear in the Proceedings of the 4th Microquasar Workshop, 2002, eds. Durouchoux, Fuchs & Rodriguez, published by the Center for Space Physics: Kolkata

We show that the collapsar model of gamma-ray bursts results in a series of successive shocks and rarefaction waves propagating in the "cork" of stellar material being pushed ahead of the jet, as it emerges from the massive stellar progenitor. These shocks result in a series of characteristic, increasingly shorter and harder thermal X-ray pulses, as well as a non-thermal gamma-ray pulse, which precede the usual non-thermal MeV gamma-rays. We consider jets escaping from both compact (CO or He dwarf) and blue supergiant stellar progenitors. The period, fluence and hardness of the pulses serves as a diagnostic for the size and density of the outer envelope of the progenitor star., Comment: Illustrative figures and expanded discussion of derivations added. Accepted for publication in ApJ

It has been proposed recently that Galactic microquasars may be prodigious emitters of TeV neutrinos that can be detected by upcoming km^2 neutrino telescopes. In this paper we consider a sample of identified microquasars and microquasar candiates, for which available data enables rough determination of the jet parameters. By employing the parameters inferred from radio observations of various jet ejection events, we determine the neutrino fluxes that should have been produced during these events by photopion production in the jet. Despite the large uncertainties in our analysis, we demonstrate that in several of the sources considered, the neutrino flux at Earth, produced in events similar to those observed, would exceed the detection threshold of a km^2 neutrino detector. The class of microquasars may contain also sources with bulk Lorentz factors larger than those characteristic of the sample considered here, directed along our line of sight. Such sources, which may be very difficult to resolve at radio wavelengths and hence may be difficult to identify as microqusar candidates, may emit neutrinos with fluxes significantly larger than typically obtained in the present analysis. These sources may eventually be identified through their neutrino and gamma-ray emission., Comment: 17 pages. Submitted to ApJ

We present X-ray observations of the afterglow of GRB000926 performed around and after the break observed in the optical light curve two days after the burst. The steep X-ray light curve observed around the break confirms the presence of this feature in X-rays. However, the spectral and temporal properties are not consistent with a standard jet scenario based on synchrotron emission, requiring a more complicated model. We find that X-ray and optical data are compatible with a moderately collimated fireball (with opening angle $\theta\approx25 \deg$) expanding in a dense medium ($n\approx 4\times 10^4$ cm$^{-3}$). This produces two breaks in the light curve. The first, at $t\approx$ 2 days, is due to jet behavior. The second, around 5 days, is attributed to the transition of the fireball to a non-relativistic expansion. This transition predicts a{\it flattening} of the light curve, that explains the late X-ray measurement in excess above the extrapolation of the simple jet scenario, and is also consistent with optical data., Comment: 19 pages, 3 fig.s, minor revisions, accepted for publication in ApJ

Core collapse of massive stars resulting in a relativistic fireball jet which breaks through the stellar envelope is a widely discussed scenario for gamma-ray burst production. For very extended or slow rotating stars, the fireball may be unable to break through the envelope. Both penetrating and choked jets will produce, by photo-meson interactions of accelerated protons, a burst of neutrinos with energies in excess of 5 TeV while propagating in the envelope. The predicted flux, from both penetrating and chocked fireballs, should be easily detectable by planned cubic kilometer neutrino telescopes., Comment: Phys.Rev.Letters, in press, final version accepted 8/31/01 (orig. 3/17/01)

Observations imply that gamma-ray bursts (GRBs) are produced by the dissipation of the kinetic energy of a highly relativistic fireball. Photo-meson interactions of protons with gamma-rays within the fireball dissipation region are expected to convert a significant fraction of fireball energy to >10^14 eV neutrinos. We present an analysis of the internal shock model of GRBs, where production of synchrotron photons and photo-meson neutrinos are self-consistently calculated, and show that the fraction of fireball energy converted to high energy neutrinos is not sensitive to uncertainties in fireball model parameters, such as the expansion Lorentz factor and characteristic variability time. This is due in part to the constraints imposed on fireball parameters by observed GRB characteristics, and in part to the fact that for parameter values for which the photo-meson optical depth is high (implying high proton energy loss to pion production) neutrino production is suppressed by pion and muon synchrotron losses. The neutrino flux is therefore expected to be correlated mainly with the observed gamma-ray flux. The time averaged neutrino intensity predicted by the model, ~10^-8.5 GeV/cm^2 s sr, is consistent with the flux predicted by the assumption that GRBs are the sources of >10^19 eV cosmic-rays., Comment: 17 pages, 7 ps-figures. Submitted to ApJ

We present an analysis of the Internal Shock Model of GRBs, where gamma-rays are produced by internal shocks within a relativistic wind. We show that observed GRB characteristics impose stringent constraints on wind and source parameters. We find that a significant fraction, of order 20 %, of the wind kinetic energy can be converted to radiation, provided the distribution of Lorentz factors within the wind has a large variance and provided the minimum Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time R/c < 1 ms, suggesting a possible explanation for the observed clustering of spectral break energies in this range. The lower limit to wind Lorenz factor and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies are set by Thomson optical depth due to electron positron pairs produced by synchrotron photons. Natural consequences of the model are absence of bursts with peak emission energy significantly exceeding 1 MeV, and existence of low luminosity bursts with low, 1 keV to 10 keV, break energies., Comment: 10 pages, 5 ps-figures. Expanded discussion of magnetic field and electron energy fraction. Accepted for publication in Astrophysical Journal

Ultra-high-energy, >10^19 eV, cosmic-ray and high energy, ~10^14 eV, neutrino production in GRBs is discussed in the light of recent GRB and cosmic-ray observations. Emphasis is put on model predictions that can be tested with operating and planned cosmic-ray and neutrino detectors, and on the prospects of testing for neutrino properties., Comment: 7 pages; Invited talk presented at the 7th International Symposium on Particles, Strings and Cosmology (Dec. 1999, Lake Tahoe, California)

The gamma-ray burst (GRB) model for production of ultra-high-energy cosmic-rays (UHECRs) is based on the hypothesis that GRBs arise from the dissipation of the kinetic energy of relativistic fireballs at cosmological distances. Recent GRB afterglow observations support the validity of this hypothesis and provide quantitative support for the model. The inferred physical fireball parameters imply that protons may be accelerated to >10^20 eV, and the inferred GRB energy generation rate is similar to that required to account for the observed flux of UHECRs. Strong suppression of UHECR flux is expected in this model above 10^19.7 eV, due to proton interaction with CMB photons. Strong deviations from model flux derived under the assumption of uniform source distribution is expected above 10^20 eV, due to source clustering. The flux above 10^20.5 eV is expected to be dominated by few, narrow spectrum sources. While model predictions can not be tested (with high confidence level) at present, the predicted signatures should be observed with the planned Auger and Telescope-Array UHECR detectors. A natural consequence of the GRB model of UHECR production is the conversion of a large fraction, ~10%, of the fireball energy to accompanying burst of \~10^14 eV and ~10^18 eV neutrinos. A km^2 neutrino detector would observe several tens of events per year correlated with GRBs, and test for neutrino properties (e.g. flavor oscillations, for which upward moving tau's would be a unique signature, and coupling to gravity) with an accuracy many orders of magnitude better than is currently possible., Comment: 10 pages; in press, Proc. TAUP99 [Nucl. Phys. B (Proc. Suppl.)], ed. M. Froissart, J. Dumarchez and D. Vignaud

We show that gamma-ray burst (GRB) afterglow observations strongly suggest, within the fireball model framework, that radiating electrons are shock accelerated to a power-law energy distribution, with universal index p \approx 2.2, and that the fraction of shock energy carried by electrons, \xi_e, is universal and close to equipartition, \xi_e ~ 1/3. For universal p and \xi_e, a single measurement of the X-ray afterglow flux on the time scale of a day provides a robust estimate of the fireball energy per unit solid angle, \epsilon, averaged over a conical section of the fireball of opening angle \theta ~ 0.1. Applying our analysis to BeppoSAX afterglow data we find that: (i) Fireball energies are in the range of 4\pi\epsilon=10^{51.5} to 10^{53.5} erg; (ii) The ratio of observed $\gamma$-ray to total fireball energy per unit solid angle, \epsilon_\gamma / \epsilon, is of order unity, satisfying abs[log10(\epsilon_\gamma/\epsilon)]<0.5; (iii) If fireballs are jet like, their opening angle should satisfy \theta>=0.1. Our results imply that if typical opening angles are \theta ~ 0.1, a value consistent with our analysis, the total energy associated with a GRB event is in the range of 10^{50} erg to 10^{51.5} erg., Comment: 16 pages; Submitted to ApJ

The recent detection of delayed, low energy emission from Gamma-Ray Burst (GRB) sources confirmed the cosmological origin of the bursts and provided support for models where GRBs are produced by the dissipation of the kinetic energy of relativistic fireballs. In this review, ultra high energy, >10^{19} eV, cosmic-ray and high energy, 100 TeV, neutrino production in GRBs is discussed in the light of recent GRB and cosmic-ray observations. Emphasis is put on model predictions that can be tested with operating and planned cosmic-ray and neutrino detectors. The predicted neutrino intensity, E^2 dN/dE=3\times 10^{-9} GeV/(cm^2 s sr) for 10^{14} eV