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Arnon Dar

Publications and source records attributed to Arnon Dar.

At least 73 records · Page 4Linked to original sources
arXiv2007

Long gamma-ray bursts without visible supernovae: a case study of redshift estimators and alleged novel objects

It has been argued that the observational limits on a supernova (SN) associated with GRB060614 convincingly exclude a SN akin to SN1998bw as its originator, and provide evidence for a new class of long-duration GRBs. We discuss this issue in the contexts of indirect 'redshift estimators' and of the fireball and cannonball models of GRBs. The latter explains the unusual properties of GRB060614: at its debated but favoured low redshift (0.125) they are predicted, as opposed to exceptional, if the associated core-collapse SN is of a recently discovered, very faint type. We take the occasion to discuss the 'association' between GRBs and SNe.

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arXiv2007

Gamma ray bursts, supernovae and metallicity in the intergalactic medium

The mean iron abundance observed in the intergalactic medium (IGM) within galaxy clusters and without galaxy clusters is consistent with the mean amount of iron per unit volume in the Universe which has been produced by standard supernova (SN) explosions with a rate proportional to the cosmic star-formation rate. If most SNe took place inside galaxies, then the IGM could have been enriched with their metals by galactic winds and jets that swept most of the galactic gas with the SNe ejecta into the IGM. A significant fraction of the early SNe, however, could have taken place outside galaxies or within dwarf galaxies, which were later disrupted by tidal interactions, and/or mass loss through fast winds, SN ejecta and jets. Little is known about such intergalactic SNe at high red-shifts. They could have occurred primarily in highly obscured environments, avoiding detection. Supporting evidence for intergalactic SNe is provided by SNe associated with gamma ray bursts (GRBs) without a host galaxy and from the ratio of well localized GRBs with and without a host galaxy. A direct test of whether a significant contribution to the iron abundance in the IGM came from intergalactic SNe would require the measurement of their rate per comoving unit volume as function of red-shift. This may be feasible with IR telescopes, such as the Spitzer Space Telescope.

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arXiv2007

On the X-Ray emission of Gamma Ray Bursts

Recent data gathered and triggered by the SWIFT satellite have greatly improved our knowledge of long-duration gamma ray bursts (GRBs) and X-ray flashes (XRFs). This is particularly the case for the X-ray data at all times. We show that the entire X-ray observations are in excellent agreement with the predictions of the `cannonball' model of GRBs and XRFs, which are based on simple physics and were published long before the launch of SWIFT. Two mechanisms underlie these predictions: inverse Compton scattering and synchrotron radiation, generally dominant at early and late times, respectively. The former mechanism provides a unified description of the gamma-ray peaks, X-ray flares and even the optical `humps' seen in some favourable cases; i.e. their very different durations, fluxes and peak-times are related precisely as predicted. The observed smooth or bumpy fast decay of the X-ray light curve is correctly described case-by-case, in minute detail. The `canonical' X-ray plateau, as well as the subsequent gradual steepening of the afterglow to an asymptotic power-law decay, are as foretold. So are the chromatic and achromatic properties of the light-curves.

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arXiv2007

On the origin of the diffuse gamma-ray background radiation

We show that inverse Compton scattering of cosmic-microwave-background and starlight photons by cosmic-ray electrons in the interstellar and intergalactic space explains well the spectrum and intensity of the diffuse gamma-ray background radiation (GBR), which was measured by EGRET aboard the Compton Gamma Ray Observatory (CGRO) in directions away from the Galactic disk and centre. The Gamma Ray Large Area Space Telescope (GLAST) will be able to separate the Galactic foreground from the extragalactic gamma-rays, and to provide stringent tests of the theory.

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arXiv2007

The time ending the shallow decay of the X-ray light curves of long GRBs

We show that the mean values and distributions of the time ending the shallow decay of the light curve of the X-ray afterglow of long gamma ray bursts (GRBs), the equivalent isotropic energy in the X-ray afterglow up to that time and the equivalent isotropic GRB energy, as well as the correlations between them, are precisely those predicted by the cannonball (CB) model of GRBs. Correlations between prompt and afterglow observables are important in that they test the overall consistency of a GRB model. In the CB model, the prompt and afterglow spectra, the endtime, the complex canonical shape of the X-ray afterglows and the correlations between GRB observables are not surprises, but predictions.

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arXiv2007

On the origin of the correlations between Gamma-Ray Burst observables

Several pairs of observable properties of Gamma Ray Bursts (GRBs) are known to be correlated. Many such correlations are straightforward predictions of the 'cannonball' model of GRBs. We extend our previous discussions of the subject to a wealth of new data, and to correlations between 'lag-time', 'variability' and 'minimum rise-time', with other observables. Schaefer's recent systematic analysis of the observations of many GRBs of known red-shift gives us a good and updated data-basis for our study.

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arXiv2007

Implications of the Large Polarization Measured in Gamma Ray Bursts

The polarization of the prompt $γ$-ray emission has been measured in four bright gamma ray bursts (GRBs). It was nearly maximal in all cases, as predicted by the Cannonball (CB) model of GRBs long before the observations. These results are inconsistent with standard models of GRBs wherein the prompt emission is due to synchrotron radiation. A much smaller linear polarization is predicted by the CB model for the prompt emission in X-ray flashes (XRFs) and in extremely luminous GRBs. These measurements would provide yet another stringent test of the CB model and its unification of GRBs and XRFs.

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arXiv2006

The origin of cosmic rays at all energies

There is mounting evidence from observations of long duration gamma ray bursts (GRBs), supernova remnants (SNR) and the supernova (SN) explosion 1987A, that SN explosions eject highly relativistic bipolar jets of plasmoids (cannonballs) of ordinary matter. Here we use the remarkably successful cannonball (CB) model of GRBs to show that bipolar jets from Galactic SN explosions can produce the bulk of the Galactic cosmic rays at energies below the ankle, while the CRs which escape into the intergalactic space or are deposited there directly by jets from SNe in external galaxies can produce the observed cosmic ray flux with energies above the ankle. The model predict well all the observed properties of cosmic rays: their intensity, their spectrum including their elemental knees and ankle, their composition and the distribution of their arrival directions. At energies above the CR ankle, the Galactic magnetic fields can no longer delay the free escape of such ultra high energy CRs (UHECRs) from the Galaxy. The UHECRs, which are injected into the intergalactic medium (IGM) by the SN jets from our Galaxy and all the other galaxies and are isotropized there by the IGM magnetic fields, dominate the flux of UHECRs. Almost all the extragalactic UHECRs heavier than helium photo-disintegrate in collisions with the far infrared (FIR), microwave and radio background radiations. The CR protons and He nuclei however suffer a Greisen-Zatsepin-Kuzmin (GZK) cutoff due to pion photo-production in collisions with the FIR, microwave and radio background photons.

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arXiv2005

Hyperstars - Main Origin of Short Gamma Ray Bursts?

The first well-localized short-duration gamma ray bursts (GRBs), GRB 050509b, GRB 050709 and GRB 050724, could have been the narrowly beamed initial spike of a burst/hyper flare of soft gamma ray repeaters (SGRs) in host galaxies at cosmological distances. Such bursts are expected if SGRs are young hyperstars, i.e. neutron stars where a considerable fraction of their neutrons have converted to hyperons and/or strange quark matter.

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arXiv2005

The Magnetic Field in Galaxies, Galaxy Clusters, and the InterGalactic Space

Magnetic fields of debated origin appear to permeate the Universe on all large scales. There is mounting evidence that supernovae produce not only roughly spherical ejecta and winds, but also highly relativistic jets of ordinary matter. These jets, which travel long distances, slow down by accelerating the matter encountered on their path to cosmic-ray energies. We show that, if the turbulent motions induced by the winds and the cosmic rays generate magnetic fields in rough energy equipartition, the predicted magnetic-field strengths coincide with the ones observed not only in galaxies (5 $μ$G in the Milky Way) but also in galaxy clusters (6 $μ$G in Coma). The prediction for the intergalactic (or inter-cluster) field is 50 nG.

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arXiv2004

Double-peak spectral energy density of GRBs and the true identity of GRB 031203

A double-peak spectral-energy-density of gamma-rays, similar to that observed in blazars, is expected in gamma-ray bursts (GRBs) produced in supernova (SN) explosions. The second peak, which is formed by inverse Compton scattering of ambient SN light by cosmic ray electrons accelerated by the jets from the SN explosion, has a much higher peak-energy than the first ordinary peak. However, in X-ray flashes (XRFs), which in the cannonball (CB) model are normal GRBs viewed farther off axis, the first peak-energy is shifted to the soft X-ray region while the second peak-energy moves to the MeV range. In far-off-axis GRBs, such as GRBs 980425 and 031203, the second peak may have been confused with the normal GRB peak. In most GRBs, which have been observed so far, the gamma-ray detectors ran out of statistics far below the second peak. However, in bright GRBs, the two peaks may be resolved by simultaneous measurements with SWIFT and GLAST.

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arXiv2004

The Origin of Cosmic Rays - A 96-Year-Old Puzzle Solved?

There is mounting evidence that long duration gamma ray bursts (GRBs) are produced by ultra-relativistic jets of ordinary matter which are ejected in core collapse supernova (SN) explosions. Such jets are extremely efficient cosmic ray (CR) accelerators which can accelerate the swept up ambient particles on their way to the highest observed CR energies. The bulk of the jet kinetic energy is used to accelerate CRs while only a tiny fraction is used to produce the GRB and its afterglow. Here we use the remarkably successful cannonball (CB) model of GRBs to show that the bipolar jets from SN explosions, which produce GRBs most of which are not beamed towards Earth, can be the main origin of cosmic rays at all energies. The model explains very simply the elemental composition of CRs and their observed spectra at all energies. In particular it explains the origin of the CR knees and ankle. Above the CR ankle, the Galactic magnetic fields can no longer delay the free escape of ultra-high energies CRs (UHECR) from the Galaxy, and the CRs from the intergalactic medium (IGM), which were injected there by SN jets from all the galaxies and isotropized there by the IGM magnetic fields, dominate the Galactic CR spectrum. A Greisen-Zatsepin-Kuzmin (GZK) effect due to the interaction of UHECRs with the microwave background radiation is expected. The CR nuclei which diffuse out of galaxies, or are directly deposited in the IGM by the relativistic SN jets, may be the origin of the IGM magnetic fields. Inverse Compton scattering of the cosmic microwave background radiation (MBR) by the CR electrons in the IGM produces the diffuse extragalactic gamma-ray background radiation (GBR).

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arXiv2004

The Discovery of a Hyperluminal Source in the Radio Afterglow of GRB 030329

Taylor, Frail, Berger and Kulkarni have made precise VLBI measurements of the size and position of the source of the radio afterglow of GRB 030329. They report a size evolution compatible with standard fireball models, proper motion limits inconsistent with the cannonball model, and a double source, i.e. "an additional compact component" on day 51 after the GRB, totally unexpected in the standard models. We outline a consistent interpretation of the ensemble of the data in the realm of the cannonball model. The observed double source is a radio image of the two cannonballs required in this model to explain the gamma-ray and optical light curves of this GRB; their separation agrees with the expectation. Thus interpreted, the observation of the two sources --separated by a "hyperluminal" distance-- is a major discovery in astrophysics: it pins down the origin of GRBs.

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arXiv2004

The GRB/XRF-SN Association

There is mounting evidence that long duration gamma ray bursts (GRBs) and X-ray flashes (XRFs) are produced by highly relativistic and narrowly collimated jets ejected in core collapse supernova (SN) explosions akin to SN 1998bw. We review the history of the GRB-SN association idea and its observational verification. We summarize the present evidence for a GRB/XRF-SN association. We comment on the possibility that most, perhaps all, SN explosions produce GRBs, including SNe of Type Ia which may produce short GRBs/XRFs. We list the major open questions that follow from a GRB/XRF-SN association. Possible uses of the GRB-SN association in cosmology are pointed out.

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arXiv2004

On the Polarization of Gamma Ray Bursts and their Optical Afterglows

The polarization of the optical afterglow (AG) of Gamma-Ray Bursts (GRBs) has only been measured in a few instances at various times after the GRB. In all cases except the best measured one (GRB 030329) the observed polarization and its evolution are simple and easy to explain in the most naive version of the "Cannonball'' model of GRBs: the "intrinsic" AG polarization is small and the observations reflect the "foreground" effects of the host galaxy and ours. The polarization observed in GRB 030329 behaves chaotically, its understanding requires reasonable but ad-hoc ingredients. The polarization of the gamma rays of a GRB has only been measured in the case of GRB 021206. The result is debated, but similar measurements would be crucial to the determination of the GRB-generating mechanism.

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