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Arnon Dar

Publications and source records attributed to Arnon Dar.

At least 109 records · Page 6Linked to original sources
arXiv2001

The Cannonball Model of Gamma Ray Bursts: high-energy neutrinos and $γ$-rays

Recent observations suggest that $γ$-ray bursts (GRBs) and their afterglows are produced by jets of highly relativistic cannonballs (CBs), emitted in supernova (SN) explosions. The CBs, reheated by their collision with the shell, emit radiation that is collimated along their direction of motion and Doppler-boosted to the typical few-hundred keV energy of the GRB. Accompanying the GRB, there should be an intense burst of neutrinos of a few hundreds of GeV energy, made by the decay of charged pions produced in the collisions of the CBs with the SN shell . The neutrino beam carries almost all of the emitted energy, but is much narrower than the GRB beam and should only be detected in coincidence with the small fraction of GRBs whose CBs are moving very close to the line of sight. The neutral pions made in the transparent outskirts of the SN shell decay into energetic $γ$-rays (EGRs) of energy of ${\cal{O}}$(100) GeV. The EGR beam, whose energy fluence is comparable to that of the companion GRB, is as wide as the GRB beam and should be observable, in coincidence with GRBs, with existing or planned detectors. We derive in detail these predictions of the CB model.

astro-ph↗
arXiv2001

The Cannonball Model of Gamma Ray Bursts: Lines in the X-Ray Afterglow

Recent observations suggest that gamma-ray bursts (GRBs) and their afterglows are produced by jets of highly relativistic cannonballs (CBs), emitted in supernova explosions. The fully ionized CBs cool to a temperature below 4500 K within a day or two, at which point electron-proton recombination produces an intense Lyman-$α$ emission. The line energy is Doppler-shifted by the CBs' motion to X-ray energies in the observer's frame. The measured line energies, corrected for their cosmological redshift, imply Doppler factors in the range 600 to 1000, consistent with those estimated -in the CB model- from the characteristics of the $γ$-ray bursts. All other observed properties of the lines are also well described by the CB model. Scattering and self-absorption of the recombination lines within the CB also produce a wide-band flare-up in the GRB afterglow, as the observations indicate. A very specific prediction of the CB model is that the X-ray lines ought to be narrow and move towards lower line energies as they are observed: their current apparently large widths would be the effect of time integration, and/or of the blending of lines from CBs with different Doppler factors.

astro-ph↗
arXiv2001

Supernova, Hypernova and Gamma Ray Bursts

Recent observations suggest that gamma ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in core collapse supernova explosions (SNe). The result of the event, probably, is not just a compact object plus a spherical ejecta: within days, a fraction of the parent star falls back to produce a thick accretion disk around the compact object. Instabilities in the disk induce sudden collapses with ejection of highly relativistic ``cannonballs'' of plasma, similar to those ejected by microquasars. The jet of cannonballs exit the supernova shell/ejecta reheated by their collision with it, emitting highly forward-collimated radiation which is Doppler shifted to $γ$-ray energy. Each cannonball corresponds to an individual pulse in a GRB. They decelerate by sweeping up the ionised interstellar matter in front of them, part of which is accelerated to cosmic-ray energies and emits synchrotron radiation: the afterglow. The Cannonball Model cannot predict the timing sequence of these pulses, but it fares very well in describing the total energy, energy spectrum, and time-dependence of the individual $γ$-ray pulses and afterglows. It also predicts that GRB pulses are accompanied by detectable short pulses of TeV neutrinos and sub TeV $γ$-rays, that are much more energetic and begin and peak a little earlier.

astro-ph↗
arXiv2000

The Cannonball Model of Gamma Ray Bursts: Spectral and Temporal Properties of the Gamma Rays

Recent observations suggest that gamma ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in supernova explosions. We have proposed that the result of the event is not just a compact object plus the ejecta: within days, a fraction of the parent star falls back to produce a thick accretion disk. The subsequent accretion generates jets and constitutes the GRB ``engine'', as in the observed ejection of relativistic ``cannonballs'' of plasma by microquasars and active galactic nuclei. Here we investigate the production of a GRB as the jetted cannonballs exit the supernova shell reheated by their collision with it, emitting highly forward-collimated radiation. Each cannonball corresponds to an individual pulse in a GRB. We cannot predict the timing sequence of these pulses, but the Cannonball Model fares very well in describing the total energy, energy spectrum, and time-dependence of the individual pulses.

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arXiv2000

A cannonball model of gamma-ray bursts: superluminal signatures

Recent observations suggest that the long-duration gamma ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in supernova explosions. We propose that the result of the event is not just a compact object plus the ejecta: within a day, a fraction of the parent star falls back to produce a thick accretion disk. The subsequent accretion generates jets and constitutes the GRB ``engine'', as in the observed ejection of relativistic ``cannonballs'' of plasma by microquasars and active galactic nuclei. The GRB is produced as the jetted cannonballs exit the supernova shell reheated by the collision, re-emitting their own radiation and boosting the light of the shell. They decelerate by sweeping up interstellar matter, which is accelerated to cosmic-ray energies and emits synchrotron radiation: the afterglow. We emphasize here a smoking-gun signature of this model of GRBs: the superluminal motion of the afterglow, that can be searched for ---the sooner the better--- in the particular case of GRB 980425.

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arXiv2000

Comment on ``Violation of the Greisen-Zatsepin-Kuzmin Cutoff: A Tempest in a (Magnetic) Teapot? Why Cosmic Ray Energies above $10^{20}$ eV May Not Require New Physics''

In a recent letter [1] with the same title, Farrar and Piran offered an explanation for the near isotropy of the arrival directions [2] of ultrahigh energy cosmic rays (UHECRs) and the apparent absence [3] of the so called `GZK cutoff' in their spectrum around $10^{20}$ eV due to pion photoproduction on the cosmic background radiation (CMB) that was predicted independently by Greisen [4] and by Zatsepin and Kuz'min [5]. They suggested that the extragalactic magnetic fields near the Milky Way are strong enough to deflect and isotropise the arrival directions of the UHECRs from a few nearby sources for which their travel time to Earth is shorter than their attenuation time by pion photoproduction on the CMB. They also estimated that this allows active galactic nuclei (AGNs) or gamma ray bursts (GRBs) to be the source of the UHECRs. However, these suggestions are inconsistent with various observations

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arXiv2000

Is the diffuse gamma background radiation generated by galactic cosmic rays?

We explore the possibility that the diffuse gamma-ray background radiation (GBR) at high galactic latitudes could be dominated by inverse Compton scattering of cosmic ray (CR) electrons on the cosmic microwave background radiation and on starlight from our own galaxy. Assuming that the mechanisms accelerating galactic CR hadrons and electrons are the same, we derive simple and successful relations between the spectral indices of the GBR above a few MeV, and of the CR electrons and CR nuclei above a few GeV. We reproduce the observed intensity and angular dependence of the GBR, in directions away from the galactic disk and centre, without recourse to hypothetical extragalactic sources.

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arXiv2000

The late afterglow and host galaxy of GRB 990712

We present deep HST imaging, as well as ground-based imaging and spectroscopy, of the optical afterglow associated with the long-duration gamma-ray burst GRB 990712 and its host galaxy. The data were obtained 48--123 days after the burst occurred. The magnitudes of the host (R = 21.9, V = 22.5) and optical afterglow (R = 25.4, V = 25.8, 47.7 days after the burst) favor a scenario where the optical light follows a pure power-law decay with an index of alpha ~ -1.0. We find no evidence for a contribution from a supernova like SN1998bw. This suggests that either there are multiple classes of long-duration gamma-ray bursts, or that the peak luminosity of the supernova was > 1.5 mag fainter than SN1998bw. The HST images and EFOSC2 spectra indicate that the gamma-ray burst was located in a bright, extended feature (possibly a star-forming region) 1.4 kpc from the nucleus of a 0.2 L^*_B galaxy at z = 0.434, possibly a Seyfert 2 galaxy. The late-time afterglow and host galaxy of GRB 990712 bear some resemblance to those of GRB 970508.

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arXiv2000

SGRs and AXPs - Magnetars or Young Quark Stars ?

Recent measurements of the spin-down rates of soft gamma ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) have been interpreted as evidence that these objects are ``magnetars'': neutron stars spinning down by magnetic dipole radiation, but with a magnetic field two orders of magnitude larger than that of ordinary neutron stars. We discuss the evidence disfavouring this interpretation. We argue that, instead, the observations support the hypothesis that SGRs and AXPs are neutron stars that have suffered a transition into a denser form of nuclear matter to become, presumably, strange stars or quark stars. Their gravitational energy release while they are cooling and contracting can power both their quiescent X-ray emission and their star quakes which produce `soft' gamma ray bursts.

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arXiv1999

Where are the young pulsars?

We show that young pulsars with normal magnetic fields, which are born as fast rotating neutron stars (NSs) in Type II/Ib supernova explosions, can slow down quickly by relativistic particles emission along their magnetic axis. When they slow down sufficiently, they can undergo phase transitions and collapse to strange stars, quark stars or black holes in explosions that remove a substantial fraction of their initial angular momentum. The newly born strange or quark stars continue to cool and spin down very quickly as soft gamma ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) to become slowly rotating, radio-quiet, dim X-ray pulsars or stellar black holes, depending on their initial mass. It can explain the apparent dearth of young radio pulsars in SNRs and in the Galaxy.

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arXiv1999

Will relativistic heavy-ion colliders destroy our planet?

Experiments at the Brookhaven National Laboratory will study collisions between gold nuclei at unprecedented energies. The concern has been voiced that ``strangelets''-hypothetical products of these collisions - may trigger the destruction of our planet. We show how naturally occurring heavy-ion collisions can be used to derive a safe and stringent upper bound on the risk incurred in running these experiments.

hep-ph↗
arXiv1999

A common origin of all the species of high-energy cosmic rays?

Cosmic ray nuclei, cosmic ray electrons with energy above a few GeV, and the diffuse gamma-ray background radiation (GBR) above a few MeV, presumed to be extragalactic, could all have their origin or residence in our galaxy and its halo. The mechanism accelerating hadrons and electrons is the same, the electron spectrum is modulated by inverse Compton scattering on starlight and on the microwave background radiation; the $γ$-rays are the resulting recoiling photons. The spectral indices of the cosmic-ray electrons and of the GBR, calculated on this simple basis, agree with observations. The angular dependence and the approximate magnitude of the GBR are also explained.

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arXiv1999

High Energy Cosmic Rays, Gamma Rays And Neutrinos From Jetted GRBs

Recent observations suggest that gamma ray bursts (GRBs) and their afterglows are produced in star formation regions in distant galaxies by highly relativistic jets that happen to point in our direction. Relativistic beaming collimates the emission from the highly relativistic jets into small solid angles along the jet direction. It implies that we are seeing only a small fraction of the events that produce GRBs. The observed GRB rate then requires an event rate which is comparable to the birth rate of neutron stars (NS). The highly relativistic jets sweep up ambient matter along their trajectories, accelerate it to cosmic ray (CR) energies and disperse it in hot spots which they form when they stop in the galactic halo. With an event rate comparable to the NS birth rate, such events in our Galaxy may be the main source of Galactic cosmic rays at all energies. Internal interactions and/or external interactions of these jets with high column density matter and/or radiation at their production sites or along their trajectories can produce high energy gamma rays and neutrinos that are highly beamed along the jet direction. Jetted GRBs, like blazars, may be much more fluent in high energy gamma rays and neutrinos than in MeV gamma rays. But, TeV gamma rays from large cosmological distances are unobservable because of their attenuation by electron-positron pair creation on the intergalactic infrared background radiation. However, high energy neutrinos from distant GRBs may be observed with large surface/volume telescopes which are under construction. TeV gamma rays and high energy neutrinos may also be detected from relatively nearby GRBs by the existing moderate size detectors, but with a much smaller rate.

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arXiv1999

Jetted GRBs, Afterglows and SGRs from the Birth of Quark Stars

Recent studies suggest that when cold nuclear matter is compressed to high nuclear densities, diquarks with spin zero and antisymmetric color wave function Bose condensate into a superfluid/superconducting state that is several times as dense. Various astrophysical phenomena may be explained by gravitational collapse of neutron stars (NSs) to (di)quark stars (QSs) as a result of a first order phase transition in NSs within $\sim 10^{4}$ years after their birth in supernova explosions, when they cooled and spun down sufficiently (by magnetic braking ?). The gravitational energy release drives an explosion which may eject both highly relativistic narrowly collimated jets and a mildly relativistic ``spherical'' shell. The slow contraction/cooling of the remnant QSs can power soft gamma ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs), without invoking a huge magnetic energy storage. The jets can produce the observed gamma ray bursts (GRBs) in distant galaxies when they happen to point in our direction and the GRBs afterglows. Also the expanding shell from the explosion may produce a GRB afterglow. The jets distort the original SNR, sweep up ambient matter along their trajectories, accelerate it to cosmic ray (CR) energies and disperse it in hot spots which they form when they stop in the galactic halo. Such events in our Galaxy may be the main source of Galactic cosmic rays at all energies.

astro-ph↗
arXiv1998

Comment on ``Cosmological Gamma Ray Bursts and the Highest Energy Cosmic Rays''

In a letter with the above title, published some time ago in PRL, Waxman made the interesting suggestion that cosmological gamma ray bursts (GRBs) are the source of the ultra high energy cosmic rays (UHECR). This has also been proposed independently by Milgrom and Usov and by Vietri. However, recent observations of GRBs and their afterglows and in particular recent data from the Akeno Great Air Shwoer Array (AGASA) on UHECR rule out extragalactic GRBs as the source of UHECR.

astro-ph↗
arXiv1998

Cosmic Rays and Gamma Ray Bursts From Microblazars

Highly relativistic jets from merger and accretion induced collapse of compact stellar objects, which may produce the cosmological gamma ray bursts (GRBs), are also very efficient and powerful cosmic ray accelerators. The expected luminosity, energy spectrum and chemical composition of cosmic rays from Galactic GRBs, most of which do not point in our direction, can explain the observed properties of Galactic cosmic rays.

astro-ph↗
arXiv1998

The Solar Neutrino Problem - An Update

The $^8$B solar neutrino flux as measured by Super-Kamiokande is consistent with the $^{37}$Ar production rate in $^{37}$Cl at Homestake. GALLEX and SAGE, continue to observe $^{71}$Ge production rates in $^{71}$Ga that are consistent with the minimal signal expected from the solar luminosity. The observed $^8$B solar neutrino flux is in good agreement with that predicted by the standard solar model of Dar and Shaviv with nuclear reaction rates that are supported by recent measurements of nuclear fusion cross sections at low energies. The measurements of Super-Kamiokande, SAGE and GALLEX suggest that the expected the pep, $^7$Be and NO solar neutrino fluxes are strongly suppressed. This can be explained by neutrino oscillations and the Mikheyev-Smirnov-Wolfenstein effect. Since neither a flavor change, nor a terrestrial variation, nor a spectral distortion of the $^8$B solar neutrino flux has been observed yet, the solar neutrino problem does not provide conclusive evidence for neutrino properties beyond the standard electroweak model. The deviations of the experimental results from those predicted by the standard solar models may reflect the approximate nature of of solar models and of our knowledge of nuclear reaction rates, radiation transport and particle diffusion in dense stellar plasmas. Only future observations of spectral distortions, or terrestrial modulation or flavor change of solar neutrinos in solar neutrino experiments, such as Super-Kamiokande, SNO, Borexino and HELLAZ will be able to establish that neutrino properties beyond the minimal standard electroweak model are responsible for the solar neutrino problem.

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