The prompt emission of gamma-ray bursts (GRBs) is widely thought to be radiation from accelerated electrons, but an appreciably larger amount of energy could be carried by accelerated protons, particularly if GRBs are the sources of ultra-high-energy cosmic rays (UHECRs). We model the expected photon spectra for such "proton-dominated" GRBs in the internal shock scenario through Monte Carlo simulations, accounting for various processes related to high-energy electrons and protons. Besides proton and muon synchrotron components, emission from photomeson-induced secondary pair cascades becomes crucial, generally enhancing the GeV-TeV and/or eV-keV photons and offering a signature of UHE protons. In some cases, it can overwhelm the primary electron component and result in GRBs peaking in the 10 MeV - 1 GeV range, which may be relevant to some bursts discussed in a recent re-analysis of EGRET TASC data. The dependence of the spectra on key quantities such as the bulk Lorentz factor, magnetic field and proton-to-electron ratio is nontrivial due to the nonlinear nature of cascading and the interplay of electron- and proton-induced components. Observations by {\it Fermi}, ground-based telescopes and other facilities should test these expectations and provide critical constraints on the proton acceleration efficiency.
The hydrogen-rich accreted envelopes of neutron stars in binary systems are the site for the rapid proton capture nucleosynthethic process (rp-process), which involves nuclei close to the proton dripline. An overview of the relevant reactions and nuclear properties for rp-process studies is given, along with motivations for further experimental nuclear physics studies.
Since 1996, a hybrid experiment consisting of the emulsion chamber and burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level, 606 g/cm^2) in Tibet. This experiment can detect air-shower cores, called as burst events, accompanied by air showers in excess of about 100 TeV. We observed about 4300 burst events accompanied by air showers during 690 days of operation and selected 820 proton-induced events with its primary energy above 200 TeV using a neural network method. Using this data set, we obtained the energy spectrum of primary protons…
We first refine the fixed concept in the literature that the usage of the Newtonian potential in studies of black hole accretion is invalid and the general relativistic effect must be considered. As our main results, we then show that the energy released by neutrino annihilation in neutrino-dominated accretion flows is sufficient for gamma-ray bursts when the contribution from the optically thick region of the flow is included, and that in the optically thick region advection does not necessarily dominate over neutrino cooling because the advection factor is relevant to the geometrical depth r…
In the framework of the internal shock scenario, we model the broadband prompt emission of gamma-ray bursts (GRBs) with emphasis on the GeV-TeV bands, utilizing Monte Carlo simulations that include various processes associated with electrons and protons accelerated to high energies. While inverse Compton emission from primary electrons is often dominant, different proton-induced mechanisms can also give rise to distinct high-energy components, such as synchrotron emission from protons, muons or secondary electrons/positrons injected via photomeson interactions. In some cases, they give rise to…
Using Monte Carlo simulations, we demonstrate photopion production from Fermi-accelerated protons and the resulting neutrino production in gamma-ray bursts. Unless internal shocks occur at quite large distance from the center, ultra high-energy protons are depleted by photopion production and synchrotron radiation. Internal shocks at fiducial distance cause neutrino bursts, which accompany gamma-ray bursts originating from electromagnetic cascades.
Many models of gamma-ray bursts suggest a common central engine: a black hole of several solar masses accreting matter from a disk at an accretion rate from 0.01 to 10 M_\sun / s. The inner region of the disk is cooled by neutrino emission and large amounts of its binding energy were liberated, which could trigger the fireball. We improve the neutrino-dominated accreting flows by considering the effects of the magnetic fields, and find that more than half of the liberating energy can be extracted directly by the large-scale magnetic fields on the disk. And it turns out that the temperature of …
We consider in this paper the effect of synchrotron self-Compton process on X-ray afterglows of gamma-ray bursts. We find that for a wide range of parameter values, especially for the standard values which imply the energy in the electrons behind the afterglow shock is tens times as that in the magnetic field, the electron cooling is dominated by Compton cooling rather than synchrotron one. This leads to a different evolution of cooling frequency in the synchrotron emission component, and hence a different (flatter) light curve slope in the X-ray range. This effect should be taken into account…
Relativistic neutron-loaded outflows in gamma-ray bursts are studied at their early stages, before deceleration by a surrounding medium. The outflow has four components: radiation, electrons, protons and neutrons. The components interact with each other and exchange energy as the outflow expands. The presence of neutrons significantly changes the outflow evolution. Before neutrons decouple from protons, friction between the two components increases their temperatures by many orders of magnitude. After the decoupling, the gradual neutron decay inside the outflow has a drag effect on the protons…
Protons accelerated to high energies in the relativistic shocks that generate gamma ray bursts photoproduce pions, and then neutrinos in situ. I show that ultra high energy neutrinos (> 10^19 eV) are produced during the burst and the afterglow. A larger flux, also from bursts, is generated via photoproduction off CMBR photons in flight but is not correlated with currently observable bursts, appearing as a bright background. Adiabatic/synchrotron losses from protons/pions/muons are negligible. Temporal and directional coincidences with bursts detected by satellites can separate correlated neutr…
Gamma-ray bursts are generally considered to be the result of internal shocks generated in an inhomogeneous relativistic outflow that arises from a fireball. In such shocks, the Fermi acceleration of protons is naturally expected to be at least as efficient as that of electrons. We investigate the consequences of proton acceleration in the residual thermal photon field of a fireball, especially those on the emission spectra of photons. In contrast to most other studies, we do not invoke a direct electron acceleration in shock waves. We show that the residual radiation field of the fireball can…
The late-time optical/radio afterglows of $γ$-ray bursts (GRBs) are believed to be synchrotron emission of electrons accelerated in relativistic collisionless shocks propagating in the ambient medium of the sources. However, the fraction $f$ of electrons that are coupled to protons and accelerated remains unclear and a large number of thermal electrons that are not coupled to protons may be left behind. If $f<1$, the true explosion energies of GRBs are $f^{-1}$ times larger than those commonly estimated with $f=1$. Thus the value of $f$ gives an important constraint on the nature of the centra…
I construct a simple model of the convective burning layer during a type I X-ray burst to investigate the effects convection has on the stability of the layer to nonradial oscillations. A linear perturbation analysis demonstrates that the region is stable to nonradial oscillations when energy transport is convection-dominated, but it is unstable when energy transport is radiation-dominated. Thus, efficient convection always dampens oscillations. These results may explain the nondetection of oscillations during the peak of some X-ray bursts.
The high energy neutrino signature from proton-proton and photo-meson interactions in a supernova remnant shell ejected prior to a gamma-ray burst provides a test for the precursor supernova, or supranova, model of gamma-ray bursts. Protons in the supernova remnant shell, and photons entrapped from a supernova explosion or a pulsar wind from a fast-rotating neutron star remnant provide ample targets for protons escaping the internal shocks of the gamma-ray burst to interact and produce high energy neutrinos. We calculate the expected neutrino fluxes, which can be detected by current and future…
It is widely believed that multiwavelength afterglows of gamma-ray bursts (GRBs) originate from relativistic blast waves. We here show that in such blast waves, a significant fraction of the energy of shock-accelerated protons would be lost due to pion production by interactions with afterglow photons. This could lead to long-term production of $10^{16}$--$10^{18}$ eV neutrinos and sub-TeV $γ$-rays that accompany with usual afterglows, provided that the protons are accelerated to $10^{19}$ eV in the blast waves.
We consider gamma-ray burst outflows with a substantial neutron component that are either dominated by thermal energy (fireballs) or by magnetic energy. In the latter case, we focus on the recently introduced `AC' model which relies on magnetic reconnection to accelerate the flow and power the prompt emission. For both the fireball and the AC model, we investigate the dynamical importance of neutrons on the outflow. We study particle creation in inelastic neutron - proton collisions and find that in both models the resulting neutrino emission is too weak to be detectable. The inelastic collisi…
We demonstrate that ``hot'' MHD outflows from neutron-rich black-hole debris disks can significantly alleviate the baryon-loading problem in gamma-ray burst (GRB) sources. We argue that the neutron-to-proton ratio in disk-fed outflows might be as high as ~30 and show, with the help of an exact semianalytic relativistic-MHD solution, that the neutrons can decouple at a Lorentz factor gamma_d ~ 15 even as the protons continue to accelerate to Lorentz factor ~200 and end up acquiring ~30% of the injected energy. We clarify the crucial role that the magnetic field plays in this process and prove t…
We have observed an unusually strong X-ray burst as a part of our regular eclipse timing observations of the low mass binary system EXO0748-676. The burst peak flux was 5.2x10^-8 ergs cm^-2 s^-1, approximately five times the normal peak X-ray burst flux observed from this source by RXTE. Spectral fits to the data strongly suggest that photospheric radius expansion occurred during the burst. In this Letter we examine the properties of this X-ray burst, which is the first example of a radius expansion burst from EXO0748-676 observed by RXTE. We find no evidence for coherent burst oscillations. A…
We extend and reconcile recent work on turbulence and particle heating in advection-dominated accretion flows. For approximately equipartition magnetic fields, the turbulence primarily heats the electrons. For weaker magnetic fields, the protons are primarily heated. The division between electron and proton heating occurs between $β\sim 5$ and $β\sim 100$ (where $β$ is the ratio of the gas to the magnetic pressure), depending on unknown details of how Alfvén waves are converted into whistlers on scales of the proton Larmor radius. We also discuss the possibility that magnetic reconnection coul…
We compute the luminosity of K_alpha emission lines produced by astrophysically abundant elements in the soft X-ray spectra of the early afterglow of gamma-ray bursts. We find that the detection of these lines can be a diagnostic for the geometrical set-up of the reprocessing material. In particular we can distinguish between a ``geometry dominated'' model, in which the line emission is coming from an extended region and its duration arises from light-travel-time effects and an ``engine dominated'' model, where the line emitting gas is in a smaller region, irradiated for a longer period. These…
Roughly half of the observed gamma-ray bursts (GRBs) may arise from the shocking of an ultra-relativistic shell of protons with the interstellar medium (ISM). Any neutrons originally present in the GRB fireball may, depending on the characteristics of the central engine, dynamically decouple as the fireball accelerates. This leads to outflow consisting of separate fast proton and slow neutron components. We derive detailed implications of neutron decoupling for the observed lightcurves of short bursts. We show that the collision of a neutron decayed shell with a decelerating outer shell is exp…
Core collapse of massive stars and binary neutron stars or black hole-neutron star binary mergers are likely progenitors of long and short duration gamma-ray bursts respectively. Neutronized material in the former and neutron star material in the latter are ejected by the central engine implying a neutron-rich jet outflow. A free neutron, however, beta decays to a proton, an electron (beta) and an anti-neutrino in about fifteen minutes in its rest frame. Sudden creation of a relativistic electron is accompanied by radiation with unique temporal and spectral signature. We calculate here this ra…
We demonstrate a qualitatively new aspect of the dynamics of Gamma-Ray Burst (GRB) fireballs: the development of a substantial dispersion in the proton component in fireballs in which neutron decoupling occurs and is sufficiently pronounced. This effect depends sensitively on the neutron to proton ratio in the fireball, becoming more dramatic with increasing neutron excess. Simple physical arguments and transport calculations indicate that the dispersion in Lorentz factor of the protons can be of order the final mean Lorentz factor of the fireball. We show how plasma instabilities could play a…
A population of X-ray dominated gamma-ray bursts (GRBs) observed by Ginga, BeppoSax and Hete-2 should be represented in the BATSE data as presumably soft bursts. We have performed a search for soft GRBs in the BATSE records in the 25--100 keV energy band. A softness of a burst spectrum can be a reason why it has been missed by the on-board procedure and previous searches for untriggered GRBs tuned to 50--300 keV range. We have found a surprisingly small number (~20/yr down to 0.1 ph cm$^{-2}$ s$^{-1}$) of soft GRBs where the count rate is dominated by 25--50 keV energy channel. This fact as we…
(Abridged) We discuss a Gamma-ray burst (GRB) fireball with an additional energy injection, either in the form of a Poynting-flux-dominated outflow or a kinetic-energy-dominated matter shell injected after the burst. A total injection energy comparable to that of the impulsive energy in the initial fireball is required to make a detectable signature in the afterglow lightcurves. For the case of a Poynting-flux-dominated injection, this leads to a gradual achromatic bump feature. For the case when the injection is kinetic-energy-dominated, the collision between the rear (injected) and the forwa…
We investigate two high energy radiation mechanisms, the proton synchrotron and the electron inverse Compton emission, and explore their possible signatures in the broad-band spectra and in the keV to GeV light curves of gamma-ray burst afterglows. We develop a simple analytical approach, allowing also for the effects of photon-photon pair production, and explore the conditions under which one or the other of these components dominates. We identify three parameter space regions where different spectral components dominate: (I) a region where the proton synchrotron and other hadron-related emis…
High energy neutrino emission from GRBs is discussed. In this paper, by using the simulation kit GEANT4, we calculate proton cooling efficiency including pion-multiplicity and proton-inelasticity in photomeson production. First, we estimate the maximum energy of accelerated protons in GRBs. Using the obtained results, neutrino flux from one burst and a diffuse neutrino background are evaluated quantitatively. We also take account of cooling processes of pion and muon, which are crucial for resulting neutrino spectra. We confirm the validity of analytic approximate treatments on GRB fiducial pa…
Using a sample of 2408 time-resolved spectra for 91 BATSE gamma-ray bursts (GRBs) presented by Preece et al., we show that the relation between the isotropic-equivalent luminosity (L_iso) and the spectral peak energy (E_p) in the cosmological rest frame, L_iso \propto E_p^2, not only holds within these bursts, but also holds among these GRBs, assuming that the burst rate as a function of redshift is proportional to the star formation rate. The possible implications of this relation for the emission models of GRBs are discussed. We suggest that both the kinetic-energy-dominated internal shock m…
Here we propose that the excess flux of particle events of energy near 1 EeV from the direction of the Galactic Center region is due to the production of cosmic rays by the last few Gamma Ray Bursts in our Galaxy. The basic idea is that protons get accelerated inside Gamma Ray Bursts, then get ejected as neutrons, decay and so turn back into protons, meander around the inner Galaxy for some time, and then interact again, turning back to neutrons to be observed at our distance from the Galactic Center region, where most star formation is happening in our Galaxy. We demonstrate that this suggest…
Simple arguments concerning power and acceleration efficiency show that ultra-high energy cosmic rays (UHECRS) with energies >~ 10^{19} eV could originate from GRBs. Neutrons formed through photo-pion production processes in GRB blast waves leave the acceleration site and travel through intergalactic space, where they decay and inject a very energetic proton and electron component into intergalactic space. The neutron-decay protons form a component of the UHECRs, whereas the neutron-decay electrons produce optical/X-ray synchrotron and gamma radiation from Compton-scattered background radiatio…
Gamma-Ray Bursts (GRBs) are expected to efficiently accelerate protons up to relativistic energies. High-energy photons can originate from decay of neutral pions produced by the interaction of these protons with the medium surrounding the source. In the same hadronic chain, high energy neutrinos are expected to be produced from decay of charged pions, as well as from decay of generated muons. Neutrinos can travel cosmological distances being much less absorbed than photons, thus providing a powerful tool for the investigation of cosmic sources. In the frame of a recently proposed hadronic mode…
We calculate the flux and spectrum of γ-rays emitted by a two-temperature advection-dominated accretion flow (ADAF) around a black hole. The γ-rays are from the decay of neutral pions produced through proton-proton collisions. We discuss both thermal and power-law distributions of proton energies and show that the γ-ray spectra in the two cases are very different. We apply the calculations to the γ-ray source, 2EG J1746-2852, detected by EGRET from the direction of the Galactic Center. We show that the flux and spectrum of this source are consistent with emission from an ADAF around the superm…
We present Monte Carlo simulations of radiative transfer in magnetar atmospheres. We include the effects of vacuum polarization, electron and proton scattering, and free-free absorption. Simulations are performed for the atmosphere model with the magnetic field perpendicular and also tilted with respect to the neutron star surface, and we show that the average spectrum does not strongly depend on the orientation of the magnetic field. We investigate the region of the parameter space where the vacuum absorption-like feature appears in the spectrum and we analyze the shape of the proton cyclotro…
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. Moreove…
Neutrinos produced in gamma-ray bursters (GRBers) may provide a unique probe for the physics of these extreme astrophysical systems. Here we discuss neutrino production in inelastic neutron-proton collisions within the relativistic outflows associated with GRBers. We consider both the widely used fireball model and a recently proposed magneto-hydrodynamic (MHD) model for the GRB outflow.
We recalculate the diffuse flux of high energy neutrinos produced by Gamma Ray Bursts (GRB) in the relativistic fireball model. Although we confirm that the average single burst produces only ~10^{-2} high energy neutrino events in a detector with 1 km^2 effective area, i.e. about 10 events per year, we show that the observed rate is dominated by burst-to-burst fluctuations which are very large. We find event rates that are expected to be larger by one order of magnitude, likely more, which are dominated by a few very bright bursts. This greatly simplifies their detection.
We report on observations of GRB 080503, a short gamma-ray burst with very bright extended emission (about 30 times the gamma-ray fluence of the initial spike) in conjunction with a thorough comparison to other short Swift events. In spite of the prompt-emission brightness, however, the optical counterpart is extraordinarily faint, never exceeding 25 mag in deep observations starting at ~1 hr after the BAT trigger. The optical brightness peaks at ~1 day and then falls sharply in a manner similar to the predictions of Li & Paczynski (1998) for supernova-like emission following compact-binary me…
It is shown that fusion of neutrons and protons to He-4 nuclei occurs in gamma-ray burst outflows in a process similar to big-bang nucleosynthesis in the early Universe. Only the surviving free neutrons can then decouple kinematically from the charged fluid so that the multi-GeV neutrino signal predicted from inelastic nuclear n-p collisions is significantly reduced. It is also argued that a sizeable fraction of ultra-high energy cosmic rays accelerated in gamma-ray bursts should be He-4 nuclei.
We suggest that coherent radiation may occur in relativistic collisionless shocks via two-stream Weibel instabilities. The coherence amplifies the radiation power by many orders [$\sim 10^{12}$ in Gamma-Ray Bursts (GRBs)] and particles cool very fast before being randomized. We imply (1) GRBs accompany strong infrared emission, (2) protons efficiently transfer energy to electrons and (3) prompt GRBs might be the upscattered coherent radiation.
We explore several models which might be proposed to explain recent possible detections of high-energy (TeV) gamma rays in association with low-energy gamma-ray bursts (GRBs). Likely values (and/or upper limits) for the source energies in low- and high-energy gamma rays and hadrons are deduced for the burst sources associated with possible TeV gamma-ray detections by the Project GRAND array. Possible spectra for energetic gammas are deduced for three models: 1) inverse-Compton scattering of ambient photons from relativistic electrons; 2) proton-synchrotron emission; and 3) inelastic scattering…
The rate of terrestrial irradiation events by galactic gamma-ray bursts (GRBs) is estimated using recent standard-energy results. We assume that GRBs accelerate high-energy cosmic rays, and present results of three-dimensional simulations of cosmic rays moving in the Galactic magnetic field and diffusing through pitch-angle scattering. An on-axis GRB extinction event begins with a powerful prompt gamma-ray and neutron pulse, followed by a longer-lived phase from cosmic-ray protons and neutron-decay protons that diffuse towards Earth. Our results force a reinterpretation of reported ~ 10^{18} e…
We study the spin-down of a neutron star atmosphere during the Type I X-ray burst in low mass X-ray binaries. Using polar cap acceleration models, we show that the resulting stellar ``wind'' torque on the burning shell due to the flowing charged particles (electrons, protons and ions) from the star's polar caps may change the shell's angular momentum during the burst. We conclude that the net change in the angular momentum of the star's atmosphere can account for rather large frequency drifts observed during Type I X-ray burst.
Recently, it was suggested that a photospheric component that results from the internal dissipation occurring in the optically thick inner parts of relativistic outflows may be present in the prompt $γ$/X-ray emission of gamma-ray bursts or X-ray flashes. We explore high-energy neutrino emission in this dissipative photosphere model, assuming that the composition of the outflow is baryon-dominated. We find that neutrino emission from proton-proton collision process forms an interesting signature in the neutrino spectra. Under favorable conditions for the shock dissipation site, these low-energ…
A gamma-ray burst fireball is likely to contain an admixture of neutrons, in addition to protons, in essentially all progenitor scenarios. Inelastic collisions between differentially streaming protons and neutrons in the fireball produce muon neutrinos (antineutrinos) of ~ 10 GeV as well as electron neutrinos (antineutrinos) of ~ 5 GeV, which could produce ~ 7 events/year in kilometer cube detectors, if the neutron abundance is comparable to that of protons. Photons of ~ 10 GeV from pi-zero decay and ~ 100 MeV electron antineutrinos from neutron decay are also produced, but will be difficult t…
Magnetars are modelled as sources that derive their output from magnetic energy that substantially exceeds their rotational energy. An implication of the recent polarization measurement of GRB 021206 is that the emission mechanism may be dominated by a magnetic field that originates in the central engine. Similarities in the temporal properties of SGRs and GRBs are considered in light of the fact that the central engine in GRBs may be magnetically dominated. The results show that 1) the time intervals between outbursts in SRG 1806-20 and pulses in GRBs are consistent with lognormal distributio…
The extreme energetics of the gamma-ray burst (GRB) 990123 has revealed that some of GRBs emit quite a large amount of energy, and total energy release from GRBs seems to change from burst to burst by a factor of 10^{2-3} as E_{gamma, iso} \sim 10^{52-55} erg, where E_{gamma, iso} is the observed GRB energy when radiation is isotropic. If all GRBs are triggered by similar events, such a wide dispersion in energy release seems odd. Here we propose a unified picture for these various energetics of GRBs, in which all GRB events release roughly the same amount of energy of E_{iso} \sim 10^{55-56} …
The 3 seismic sources S1, S2 and S3 detected from MDI dopplergrams using the time-distance diagram technique are presented with the locations, areas and vertical and horizontal velocities of the visible wave displacements. Within the datacube of 120 Mm the horizontal velocities and the wave propagation times slightly vary from source to source. The momenta and start times measured from the TD diagrams in the sources S1-S3 are compared with those delivered to the photosphere by different kinds of high energy particles with the parameters deduced from hard X-ray and $γ$-ray emission as well as b…
Bohdan Paczy'nski was one of the pioneers of the cosmological GRB model. His ideas on how GRBs operate and what are their progenitors have dominated the field of GRBs in the hectic nineties during which the distances and the origin of GRBs were revealed. I discuss here Bohdan's contributions in some historical perspective.
A high-speed halo-type coronal mass ejection (CME), associated with a GOES M4.6 soft X-ray flare in NOAA AR 0180 at S12W29 and an EIT wave and dimming, occurred on 9 November 2002. A complex radio event was observed during the same period. It included narrow-band fluctuations and frequency-drifting features in the metric wavelength range, type III burst groups at metric--hectometric wavelengths, and an interplanetary type II radio burst, which was visible in the dynamic radio spectrum below 14 MHz. To study the association of the recorded solar energetic particle (SEP) populations with the pro…
We present a unified description of all the three known classes of optically thick accretion disks around black holes, namely Shakura-Sunyaev disks, slim disks, and neutrino-dominated accretion flows (NDAFs). It is found that NDAFs have both a maximal and a minimal possible mass accretion rate at their each radius. This may be suggestive of an interpretation for the origin of X-ray flares observed in gamma-ray bursts.
We investigate the form of the momentum distribution function for protons and electrons in an advection-dominated accretion flow (ADAF). We show that for all accretion rates, Coulomb collisions are too inefficient to thermalize the protons. The proton distribution function is therefore determined by the viscous heating mechanism, which is unknown. The electrons, however, can exchange energy quite efficiently through Coulomb collisions and the emission and absorption of synchrotron photons. We find that for accretion rates greater than \sim 10^{-3} of the Eddington accretion rate, the electrons…
For Gamma-Ray Bursts occurring in dense star-forming regions, the X-ray afterglow behavior minutes to days after the trigger may be dominated by the small-angle scattering of the prompt X-ray emission off dust grains. We give a simple illustrative model for the X-ray light curves at different X-ray energies, and discuss possible implications. A bump followed by a steeper decay in soft X-rays is predicted for bursts which are heavily obscured in the optical.
We propose a gamma-ray burst scenario involving relativistic jets dominated by Poynting flux with alternating toroidal magnetic fields. Such a structure may arise naturally if the jet is formed and powered by the accretion flow in the core of a collapsar. We conjecture that the polarity of the toroidal magnetic field changes randomly due to hydromagnetic turbulence driven by the magneto-rotational instability (MRI), with the typical reversal time determined by the time scale for amplifying magnetic fields up to a dynamically important level. Poynting flux-dominated jets with reversing B-fields…
We present numerical simulations of Gamma-Ray Bursts arising from external shocks in the impulsive and wind models, including a weak or a strong coupling between electrons and protons plus magnetic fields, and analyze the burst features in each scenario. The dynamics of the ejecta and external medium are followed into the late stages of deceleration, in order to study the hydrodynamics of the remnant and the temporal and spectral evolution of the afterglow. A brief comparison with the optical and radio afterglows of GRB 970228 and GRB 970508 is made.
We calculate the neutrino emission from individual gamma-ray bursts observed by the BATSE detector on the Compton Gamma-Ray Observatory. Neutrinos are produced by photoproduction of pions when protons interact with photons in the region where the kinetic energy of the relativistic fireball is dissipated allowing the acceleration of electrons and protons. We also consider models where neutrinos are predominantly produced on the radiation surrounding the newly formed black hole. From the observed redshift and photon flux of each individual burst, we compute the neutrino flux in a variety of mode…
I discuss the interpretation of the prompt phase in gamma-ray bursts as being dominated by quasi-thermal emission, rather than by synchrotron emission. Such an interpretation gives a more natural explanation of (i) the observed variety of spectral shape and spectral evolution, (ii) the observed narrowness of the distribution of peak energies, as well as (iii) the observed correlations between peak energy and luminosity. However, the physical setting that could produce such a scenario is not yet clear.
Low frequency electromagnetic turbulence is generated in relativistically outflowing plasma that sweeps up particles from the surrounding environment. Electrons are energized by stochastic gyroresonant acceleration with the turbulence produced by the isotropization of captured protons and charged dust. Protons are accelerated stochastically by the dust-induced turbulence. Analytical solutions for the proton and electron energy distributions are obtained and used to calculate broadband synchrotron emission. The solutions are compared with generic spectral behavior of blazars and gamma-ray burst…
Many astrophysical sources of high energy emission, such as black hole magnetospheres, superstrongly magnetized neutron stars (magnetars), and probably relativistic jets in Active Galactic Nuclei and Gamma Ray Bursts involve relativistically magnetically dominated plasma. In such plasma the energy density of magnetic field greatly exceeds the thermal and the rest mass energy density of particles. Therefore the magnetic field is the main reservoir of energy and its dissipation may power the bursting emission from these sources, in close analogy to Solar flares. One of the principal dissipative …
The supranova model for gamma-ray bursts (GRBs) has recently gained popularity. In this scenario the GRB occurs weeks to years after a supernova explosion, and is located inside a pulsar wind bubble (PWB). High energy protons from the PWB can interact with photons from the rich radiation field inside the PWB or collide with cold protons from the supernova remnant, producing pions which decay into \sim 10-10^3 TeV neutrinos. The predicted neutrino flux from the PWBs that host the GRBs should be easily detectable by planned 1 km^2 detectors.
Short-hard gamma-ray bursts (SHBs) may arise from gravitational wave (GW) driven mergers of double neutron star (DNS) systems. DNSs may be "primordial" or can form dynamically by binary exchange interactions in globular clusters during core-collapse. For primordial binaries, the time delay between formation and merger is expected to be short, tau~0.1 Gyr, implying that the redshift distribution of merger events should follow that of star-formation. We point out here that for dynamically formed DNSs, the time delay between star-formation and merger is dominated by the cluster core-collapse time…
