Arnon Dar

Gamma-ray bursts from mini - jets

Striking similarities exist between high energy gamma ray emission from active galactic nuclei (AGN) and gamma ray bursts (GRBs). They suggest that GRBs are generated by inverse Compton scattering from highly relativistic electrons in transient jets. Such jets may be produced along the axis of an accretion disk formed around stellar black holes (BH) or neutron stars (NS) in BH-NS and NS-NS mergers and in accretion induced collapse of magnetized white dwarfs (WD) or neutron stars in close binary systems. Such events may produce the cosmological GRBs. Transient jets formed by single old magnetized neutron stars in an extended Galactic halo may produce a local population of GRBs. Here we show that jet production of GRBs by inverse Compton scattering can explain quite simply the striking correlations that exist between various temporal features of GRBs, their duration histogram, the power spectrum of their complex multipeak light curves, their power-law high energy spectra and other features of GRBs. Some additional predictions are made including the expected polarization of gamma-rays in the bursts.

Towards a complete theory of gamma-ray bursts

Abstract 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 specific 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 may be 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-models 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.

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. Although our emphasis is on optical afterglows, we also provide an excellent description of X-ray afterglows.

DIFFRACTION MECHANISM FOR ELASTIC SCATTERING AND DIRECT NUCLEAR REACTIONS

Abstract A phenomenological model is presented which is suitable for the description of direct nuclear reactions with unbound particles strongly interacting with complex nuclei. Expressions in closed form are given for elastic and reaction scattering amplitudes. Good agreement with experiment is found.

Can the Soft X-Ray Opacity Toward High-redshift Sources Probe the Missing Baryons?

Observations with the Swift satellite of X-ray afterglows of more than a hundred gamma-ray bursts (GRBs) with known redshift reveal ubiquitous soft X-ray absorption. The directly measured optical depth ? at a given observed energy is found to be constant on average at redshift z > 2, i.e., ?(0.5 keV) z > 2 = 0.40 ? 0.02. Such an asymptotic optical depth is expected if the foreground diffuse intergalactic medium (IGM) dominates the absorption effect and if the metallicity of the diffuse IGM reaches 0.2-0.4 solar at z = 0. To further test the IGM absorption hypothesis, we analyze the 12 highest signal-to-noise ratio (S/N) (>5000 photons) z > 2 quasar spectra from the XMM-Newton archive, which are all extremely radio loud. The quasar optical depths are found to be consistent with the mean GRB value. The four lowest-z quasars (2 2 provide only upper limits to the absorption, which are still consistent with the radio-loud quasers (RLQs), albeit with much lower S/N ( 1000 photons at z 4). Lack of quasar absorption poses a challenge to the smooth IGM interpretation and could allude to the opacity being rather due to the jets in RLQs and GRBs. However, the jet absorbing column would need to appear in RLQs only at z 2.5 and in GRBs to strongly increase with z in order to produce the observed tendency to a constant mean ?. High X-ray spectral resolution can differentiate between an absorber intrinsic to the source that produces discernible spectral lines, and the diffuse IGM that produces significant absorption, but no discrete features.

Can Fireball Models Explain Gamma-Ray Bursts?

The observed afterglows of gamma-ray bursts (GRBs), in particular the afterglow of GRB 970228 after 6 months, seem to rule out, as the origin of GRBs, relativistic fireballs driven by the mergers or accretion-induced collapse of compact stellar objects in galaxies. GRBs can be produced by superluminal jets from such events.

Discovery of the optical counterpart and early optical observations of GRB 990712

We present the discovery observations of the optical counterpart of the gamma-ray burst GRB 990712 taken 4.16 hr after the outburst and discuss its light curve observed in the V, R, and I bands during the first ~35 days after the outburst. The observed light curves were fitted with a power-law decay for the optical transient (OT), plus an additional component that was treated in two different ways. First, the additional component was assumed to be an underlying galaxy of constant brightness. The resulting slope of the decay is 0.97, and the magnitudes of the underlying galaxy are V = 22.3 ± 0.05, R = 21.75 ± 0.05, and I = 21.35 ± 0.05. Second, the additional component was assumed to be a galaxy plus an underlying supernova with a time-variable brightness identical to that of GRB 980425, appropriately scaled to the redshift of GRB 990712. The resulting slope of the decay is similar, but the goodness of fit is worse, which would imply that either this GRB is not associated with an underlying supernova or the underlying supernova is much fainter than the supernova associated with GRB 980425. The galaxy in this case is fainter: V = 22.7 ± 0.05, R = 22.25 ± 0.05, and I = 22.15 ± 0.05, and the OT plus the underlying supernova at a given time is brighter. Measurements of the brightnesses of the OT and the galaxy by late-time Hubble Space Telescope observation and ground-based observations can thus assess the presence of an underlying supernova.

Gamma-ray bursts and cosmic rays from accretion-induced collapse

It is shown that the birth of naked or nearly naked neutron stars in accretion-induced collapse or in the bare collapse of white dwarfs can produce cosmological gamma-ray bursts and can provide the required injection rate of cosmic rays into the interstellar space. It is estimated that most of the e(+)e(-) pairs annihilate in flight on a short time scale in the vicinity of the neutron star. It is shown that the gamma-ray bursts, the 0.511 MeV Galactic annihilation radiation, and the cosmic rays exclude the possibility that the large uncertainties in the Galactic pulsar birthrate and the Galactic SN II explosion rate would allow a significant contribution to the pulsar birthrate from naked or nearly naked neutron star formation. The upper bound on the Galactic birthrate of naked or nearly naked neutron stars of less than 1 in 1000 yr makes it very unlikely that a neutrino burst unaccompanied by optical emission from the birth of a naked or nearly naked neutron star will be detected in the near future by underground neutrino telescopes.

Origin of the high energy extragalactic diffuse gamma ray background.

We show that cosmic rays in external galaxies, groups and clusters rich in gas, with an average flux similar to that observed in the Milky Way, could have produced the observed extragalactic diffuse gamma radiation.

The host galaxy and optical light curve of the gamma-ray burst GRB 980703

We present deep HST /STIS and ground-based photometry of the host galaxy of the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the burst. We find that the host is a blue, slightly over-luminous galaxy with