arXiv subjects

Arnon Dar

Publications and source records attributed to Arnon Dar.

At least 91 records · Page 5Linked to original sources

arXiv2004

On the origin of X-ray Flashes

We use the cannonball (CB) model of gamma ray bursts (GRBs) and their afterglows (AGs) to analyze the observational data on X-ray flashes (XRFs) and their AGs. We show that the observations support the CB-model interpretation that XRFs, like GRBs, are produced by the explosions of core-collapse supernovae (SNe) akin to SN1998bw, by jets of highly-relativistic CBs. The XRFs and GRBs are intrinsically identical objects, but the XRFs are viewed from angles (relative to the jet direction) which are typically a few times larger than the typical viewing angles of ``classical'', long-duration GRBs. There should be XRFs, not observed so far, with durations similar to those of short GRBs.

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arXiv2004

The superluminal motion of Gamma-Ray-Burst sources and the complex afterglow of GRB 030329

The source of the very bright Gamma-Ray Burst GRB 030329 is close enough to us for there to be a hope to measure or significantly constrain its putative superluminal motion. Such a phenomenon is expected in the ``Cannonball'' (CB) model of GRBs. Recent precise data on the optical and radio afterglow of this GRB --which demonstrated its very complex structure-- allow us to pin down the CB-model's prediction for the afterglow-source position as a function of time. It has been stated that (the unpublished part of) the new radio data ``unequivocably disprove'' the CB model. We show how greatly exaggerated that obituary announcement was, and how precise a refined analysis of the data would have to be, to be still of interest.

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arXiv2003

Towards a complete theory of Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are notorious for their diversity. Yet, they have a series of common features. The typical energy of their $γ$ rays is a fraction of an MeV. The energy distributions are well described by a ``Band spectrum'', with ``peak energies'' spanning a surprisingly narrow range. The time structure of a GRB consists of pulses, superimposed or not, rising and decreasing fast. The number of photons in a pulse, the pulses' widths and their total energy vary within broad but given ranges. Within a pulse, the energy spectrum softens with increasing time. The duration of a pulse decreases at higher energies and its peak intensity shifts to earlier time. Many other correlations between pairs of GRB observables have been identified. Last (and based on one measured event!) the $γ$-ray polarization is very large. A satisfactory theory of GRBs should naturally and very simply explain, among others, all these facts. We show that the "cannonball" (CB) model does it. In the CB model the process leading to the ejection of highly relativistic jetted CBs in core-collapse supernova (SN) explosions is akin to the one observed in quasars and microquasars. The prompt $γ$-ray emission --the GRB-- is explained extremely well by inverse Compton scattering of light in the near environment of the SN by the electrons in the CBs' plasma. We have previously shown that the CB-model's description of GRB afterglows as synchrotron radiation from ambient electrons --swept in and accelerated within the CBs-- is also simple, universal and very successful. The only obstacle still separating the CB model from a complete theory of GRBs is the theoretical understanding of the CBs' ejection mechanism in SN explosions.

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arXiv2003

Cooling flows or warming rays?

The radiative cooling time of the X-ray-emitting plasma near the center in many clusters of galaxies is shorter than the age of the cluster, but neither the expected large drop in central temperature --nor the expected mass flow towards the pressure-depleted cluster centers-- are observed. We propose a solution to these ``cooling flow'' problems wherein energy is supplied to the plasma by hadronic cosmic rays (CRs). The solution requires an energy deposition more intense and more distributed than in conventional CR models, but this alternative model is also successful in describing the origin of CRs, as well as the properties of gamma ray bursts and of the diffuse gamma background radiation. The X-ray energy emitted by clusters is supplied, in a quasi-steady state, by the hadronic CRs, which act as ``warming rays''. The temperature distribution in the intracluster space is successfully predicted from the measured plasma-density distribution. Four other puzzling features of clusters can also be explained in simple terms: the discrepancy between their ``virial'' and ``lensing'' masses, their large magnetic fields, the correlation between their optical and X-ray luminosities, and the non-thermal tail of their X-ray spectrum.

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arXiv2003

The Supernova associated with GRB 030329

The relative proximity of the recent gamma ray burst (GRB) 030329 resulted in a large gamma-ray fluence and in the brightest-ever afterglow (AG), hours after the burst, in the radio, optical and X-ray bands, permitting precise AG measurements, sensitive tests of models and an excellent occasion to investigate the association of GRBs with supernovae (SNe). The Cannonball (CB) model provides a good, simple and universal description of all AGs of GRBs of known redshift, so that it is straightforward to use it to predict what the expected SN signatures are. In the case of GRB 030329, 10 days after burst the AG should begin to reveal the lightcurve, spectrum and polarization of an underlying SN --akin to SN1998bw-- which will peak in the NIR/optical band around day 15. These effects will be easily observable if indeed SN1998bw is a good ``standard candle'' for GRB-associated SNe and if the so far unknown extinction in the host galaxy is not too large.

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arXiv2003

What we learn from the afterglow of GRB 021211

The behaviour of the afterglow (AG) of gamma-ray bursts (GRBs) directly provides, in the cannonball (CB) model, information about the environment of their progenitor stars. The well observed early temporal decline of the AG of GRB 021211 is precisely the one predicted in the presence of a progenitor's ``wind'' which resulted in a density profile $\propto 1/r^2$ around the star. The subsequent fast fading --which makes this GRB ``quasi-dark''-- is the one anticipated if, further away, the interstellar density is roughly constant and relatively high. The CB-model fit to the AG clearly shows the presence of an associated supernova akin to SN1998bw, and allows even for the determination of the broad-band spectrum of the host galaxy. GRB 990123 and GRB 021004, whose AGs were also measured very early, are also discussed.

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arXiv2003

The cannonball model of gamma ray bursts

The cannonball model (CB) of gamma ray bursts (GRBs) is incredibly more successful than the standard blast-wave models (SM) of GRBs, which suffer from profound inadequacies and limited predictive power. The CB model is falsifiable in its hypothesis and results. Its predictions are summarized in simple analytical expressions, derived, in fair approximations, from first principles. It provides a good description on a universal basis of the properties of long-duration GRBs and of their afterglows (AGs).

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arXiv2002

Fireballs and cannonballs confront the afterglow of GRB 991208

Galama et al. have recently reported their follow-up measurements of the radio afterglow (AG) of the Gamma Ray Burst (GRB) 991208, up to 293 days after burst, and their reanalysis of the broad-band AG, in the framework of standard fireball models. They advocate a serious revision of their prior analysis and conclusions, based on optical data and on their earlier observations during the first two weeks of the AG. We comment on their work and fill a lacuna: these authors have overlooked the possibility of comparing their new data to the available predictions of the cannonball (CB) model, based --like their incorrect predictions-- on the first round of data. The new data are in good agreement with these CB-model predictions. This is in spite of the fact that, in comparison to the fireball models, the CB model is much simpler, much more predictive, has many fewer parameters, practically no free choices... and it describes well --on a universal basis-- all the measured AGs of GRBs of known redshift.

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arXiv2002

On the radio afterglow of gamma ray bursts

We use the cannonball (CB) model of gamma ray bursts (GRBs) to predict the spectral and temporal behaviour of their radio afterglows (AGs). A single simple expression describes the AGs at all times and frequencies; its high-frequency limit reproduces the successful CB model predictions for optical and X-ray AGs. We analyze all of the observed radio AGs of GRBs with known redshifts, including those of the exceptionally close-by GRB 980425. We also study in detail the time-evolution of the AGs' spectral index. The agreement between theory and observations is excellent, even though the CB model is extremely frugal in the number of parameters required to explain the radio observations. We propose to use the scintillations in the radio AGs of GRBs to verify and measure the hyperluminal speed of their jetted CBs, whose apparent angular velocity is of the same order of magnitude as that of galactic pulsars, consistently measured directly, or via scintillations.

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arXiv2002

On the X-ray lines in the afterglows of GRBs

The observation of X-ray lines in the afterglow of GRB 011211 has been reported, and challenged. The lines were interpreted as blue-shifted X-rays characteristic of a set of photoionized ``metals'', located in a section of a supernova shell illuminated by a GRB emitted a couple of days after the supernova explosion. We show that the most prominent reported lines coincide with the ones predicted in the ``cannonball'' model of GRBs. In this model, the putative signatures are Hydrogen lines, boosted by the (highly relativistic) motion of the cannonballs (CBs). The corresponding Doppler boost can be extracted from the fit to the observed I-, R- and V-band light-curves of the optical afterglow of GRB 011211, so that, since the redshift is also known, the line energies are --in the CB model-- predicted. We also discuss other GRBs of known redshift which show spectral features generally interpreted as Fe lines, or Fe recombination edges. The ensemble of results is very encouraging from the CB-model's point of view, but the data on each individual GRB are not good enough to draw (any) objectively decisive conclusions. We outline a strategy for X-ray observers to search for lines which, in the CB model, move predictably from higher to lower energies.

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arXiv2002

Is the afterglow of Gamma Ray Burst 021004 unusual?

The bumpy light-curve of the bright optical afterglow (AG) of gamma ray burst (GRB) 021004, its spectral evolution and its radio emission have been claimed to be unusual. In the Cannonball Model of GRBs that is not the case. The very early AG's shape is, as for GRB 990123, a direct tracer of the expected circumburst density profile. The unprecedented precision of the data allows for the ``resolution'' of two cannonballs (CBs) in the AG. These two CBs correspond to the two pulses in the GRB and to the two wide humps in the AG light curve. The smaller wiggles in the AG are, as for GRBs 000301c and 970508, to be expected: they trace moderate deviations from a constant density interstellar medium. The observed evolution of the optical spectrum is that predicted in the CB model. The X-ray and radio emissions of GRB 021004 are also normal.

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arXiv2002

The supernova associated with GRB 020405

We use the very simple and successful Cannonball (CB) model of gamma ray bursts (GRBs) and their afterglows (AGs) to analyze the observations of the mildly extinct optical AG of the relatively nearby GRB 020405. We show that GRB 020405 was associated with a 1998bw-like supernova (SN) at the GRB's redshift that appeared dimmer and redder than SN1998bw because of extinction in the host and our Galaxy. The case for the SN/GRB association --advocated in the CB model-- is becoming indubitable. We discuss the extent to which the GRB/SN connection is model-dependent.

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arXiv2002

On the optical and X-ray afterglows of gamma ray bursts

We severely criticize the consuetudinary analysis of the afterglows of gamma-ray bursts (GRBs) in the conical-ejection fireball scenarios. We argue that, instead, recent observations imply that the long-duration GRBs and their afterglows are produced by highly relativistic jets of cannonballs (CBs) emitted in supernova explosions. The CBs are heated by their collision with the supernova shell. The GRB is the boosted surface radiation the CBs emit as they reach the transparent outskirts of the shell. The exiting CBs further decelerate by sweeping up interstellar matter (ISM). The early X-ray afterglow is dominated by thermal bremsstrahlung from the cooling CBs, the optical afterglow by synchrotron radiation from the ISM electrons swept up by the CBs. We show that this model fits simply and remarkably well all the measured optical afterglows of the 15 GRBs with known redshift, including that of GRB 990123, for which unusually prompt data are available. We demonstrate that GRB 980425 was a normal GRB produced by SN1998bw, with standard X-ray and optical afterglows. We find that the very peculiar afterglow of GRB 970508 can be explained if its CBs encountered a significant jump in density as they moved through the ISM. The afterglows of the nearest 8 of the known-redshift GRBs show various degrees of evidence for an association with a supernova akin to SN1998bw. In all other cases such an association, even if present, would have been undetectable with the best current photometric sensitivities. This gives strong support to the proposition that most, maybe all, of the long-duration GRBs are associated with supernovae. Though our emphasis is on optical afterglows, we also provide an excellent description of X-ray afterglows.

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arXiv2002

The threat to life from Eta Carinae and gamma ray bursts

Eta Carinae, the most massive and luminous star known in our galaxy, is rapidly boiling matter off its surface. At any time its core could collapse into a black hole, which may result in a gamma-ray burst (GRB) that can devastate life on Earth. Auspiciously, recent observations indicate that the GRBs are narrowly beamed in cones along the rotational axis of the progenitor star. In the case of Eta Carinae the GRBs will not point to us, but will be ravaging to life on planets in our galaxy that happen to lie within the two beaming cones. The mean rate of massive life extinctions by jets from GRBs, per life-supporting planet in galaxies like ours, is once in 100 million years, comparable to the rate of major extinctions observed in the geological records of our planet.

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arXiv2002

Is there a 1998bw-like supernova in the afterglow of gamma ray burst 011121?

We use the very simple and successful Cannonball Model (CB) of gamma ray bursts (GRBs) and their afterglows (AGs) to analyze the observations of the strongly extinct optical AG of the relatively nearby GRB 011121, which were made with ground-based telescopes at early times, and with the HST at later time. We show that GRB 011121 was indeed associated with a 1998bw-like supernova at the GRB's redshift, as we had specifically predicted for this GRB before the supernova could be observed.

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arXiv2002

Is there a 1998bw-like supernova in the afterglow of gamma ray burst 010921?

We use the very simple and successful Cannonball Model (CB) of gamma ray bursts (GRBs) and their afterglows (AGs) to analyze the observations of the strongly extinct optical AG of GRB 010921 with ground-based telescopes at early times, and with the HST at later time. We show that GRB 010921 was indeed associated with a 1998bw-like supernova at the GRB's redshift.

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arXiv2001

Supernova search in the afterglow of gamma ray burst 011121

In order to assist in planing the search of supernova (SN) in the late time afterglow (AG) of the gamma ray burst (GRB) 011121 we use the Cannonball Model of GRBs and the observations of its afterglow in the R band in the first two days to predict its late time decline and intrinsic spectrum.

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arXiv2001

Galactic origin of the alleged extragalactic gamma-ray background radiation

The diffuse gamma-ray background radiation (GBR) at high Galactic latitudes could be dominated by inverse Compton scattering (ICS) of cosmic ray (CR) electrons on the cosmic microwave background radiation (CBR) and on starlight (SL) in an extended CR halo of our galaxy. Assuming the locally observed CR electron spectrum beyond a few GeV, which follows from equilibrium between Fermi acceleration and radiative cooling, to be a universal spectrum throughout our galaxy and its CR halo, we reproduce the observed spectral index, the intensity and the angular dependence of the GBR, in directions away from the galactic disk and centre, without recourse to hypothetical extragalactic sources.

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