Soebur Razzaque

Modeling the Extragalactic Background Light from Stars and Dust

The extragalactic background light (EBL) from the far-infrared through the visible and extending into the ultraviolet is thought to be dominated by starlight, either through direct emission or through absorption and reradiation by dust. This is the most important energy range for absorbing γ-rays from distant sources such as blazars and gamma-ray bursts and producing electron-positron pairs. In previous work, we presented EBL models in the optical through ultraviolet by consistently taking into account the star formation rate (SFR), initial mass function (IMF), and dust extinction, and treating stars on the main sequence as blackbodies. This technique is extended to include post-main-sequence stars and reprocessing of starlight by dust. In our simple model, the total energy absorbed by dust is assumed to be re-emitted as three blackbodies in the infrared, one at 40 K representing warm, large dust grains, one at 70 K representing hot, small dust grains, and one at 450 K representing polycyclic aromatic hydrocarbons. We find that our best-fit model combining the Hopkins and Beacom SFR using the Cole et al. parameterization with the Baldry and Glazebrook IMF agrees with available luminosity density data at a variety of redshifts. Our resulting EBL energy density is quite close to the lower limits from galaxy counts, though in two cases below the lower limits, and agrees fairly well with other recent EBL models shortward of about 5 μm. Deabsorbing TeV γ-ray spectra of various blazars with our EBL model gives results consistent with simple shock acceleration theory. We also find that the universe should be optically thin to γ-rays with energies less than 20 GeV.

GeV and higher energy photon interactions in gamma-ray burst fireballs and surroundings

We have calculated the opacities and secondary production mechanisms of high-energy photons arising in gamma-ray burst internal shocks, using exact cross sections for the relevant processes. We find that for reasonable choices of parameters, photons in the range of tens to hundreds of GeV may be emitted in the prompt phase. Photons above this range are subject to electron-positron pair production with fireball photons and would be absent from the spectrum escaping the gamma-ray burst. We find that, in such cases, the fireball becomes optically thin again at ultrahigh energies (PeV). On the other hand, for sufficiently large fireball bulk Lorentz factors, the fireball is optically thin at all energies. Both for γγ self-absorbed and optically thin cases, the escaping high-energy photons can interact with infrared and microwave background photons to produce delayed secondary photons in the GeV-TeV range. These may be observable with GLAST or at low redshifts with ground-based air Cerenkov telescopes. Detection of the primary prompt spectrum constrains the bulk Lorentz factor, while detection of delayed secondary gamma rays would provide a consistency check for the primary spectrum and the bulk Lorentz factor, as well as constraints on the intergalactic magnetic field strength.

Neutrino Tomography of Gamma Ray Bursts and Massive Stellar Collapses

The gamma-ray bursts (GRB) which have so far been accurately localized are associated with regions of active star formation, and their progenitors are thought to be massive stars. The leading model for such bursts involves a relativistic jet, produced following the collapse of the core of the massive stellar progenitor [1]. In this model the -rays are produced by synchrotron or inverse Compton radiation from Fermi accelerated electrons in optically thin shocks (see [2] for a review), after the jet has emerged from the stellar envelope. The same optically thin shocks should accelerate relativistic protons [3], and lead to ∼ 100 TeV neutrinos via interactions with the observed MeV -rays [4]. However, while the jets are still inside the star, shock-accelerated protons can produce ∼ TeV neutrinos through photomeson interactions with thermal X-rays in the sub-stellar jet cavity [5]. In this paper we discuss a more general class of massive stellar collapses, in which jet formation may be ubiquitous, but not all of which emerge to be associated with detectable GRBs. Before their successful or failed emergence from the star, the jets can accelerate protons which undergo a more complex sequence of high energy interactions than previously realized. These depend not only on the jet and central engine characteristics but also on the location of the shocks and on the outer dimensions of the stellar progenitor, thus providing potentially useful diagnostics for the type of progenitor as well as the jet and shock parameters. Protons accelerated in substellar jet shocks first undergo photomeson interactions with thermalized shock photons, as well as pp, pn interactions with thermal nucleons in the jet frame. This modifies the relativistic proton spectrum reaching the end of the jet cavity, where the protons undergo a second set of photomeson interactions with stellar X-ray photons and pp, pn interactions with cold nucleons in the stellar frame. The fraction of collapses producing jets which subsequently emerge from the star to produce electromagnetically detectable GRBs are expected to be preceded by a precursor neutrino signal at energies & TeV, which is significantly different from the previously calculated & 100 TeV neutrino signals coincident with the -rays [4]. The fraction of stellar collapses leading to jets which do not emerge would have similar neutrino signals, but they could be more numerous and hence their diffuse flux could be more important. We discuss our jet models in Sec. II, proton and electron acceleration in the internal shocks in Sec. III and proton interactions in Sec. IV. We discuss neutrino production mechanisms in Sec. V and calculate observed neutrino flux in Sec. VI. We summarize and discuss implications of our results in Sec. VII.

Synchrotron Radiation from Ultra-High Energy Protons and the Fermi Observations of GRB 080916C

Fermi gamma-ray telescope data of GRB 080916C with ~1e55 erg in apparent isotropic gamma-ray energy, show a several second delay between the rise of 100 MeV - GeV radiation compared with keV - MeV radiation. Here we show that synchrotron radiation from cosmic ray protons accelerated in GRBs, delayed by the proton synchrotron cooling timescale in a jet of magnetically-dominated shocked plasma moving at highly relativistic speeds with bulk Lorentz factor Gamma ~ 500, could explain this result. A second generation electron synchrotron component from attenuated proton synchrotron radiation makes enhanced soft X-ray to MeV gamma-ray emission. Long GRBs with narrow, energetic jets accelerating particles to ultra-high energies could explain the Auger observations of UHE cosmic rays from sources within 100 Mpc for nano-Gauss intergalactic magnetic fields. The total energy requirements in a proton synchrotron model are proportional to Gamma^(16/3). This model for GRB 080916C is only plausible if Gamma ~< 500 and the jet opening angle is ~ 1 degree.

High-energy cosmic rays and neutrinos from semirelativistic hypernovae

The origin of the ultrahigh-energy (UHE) cosmic rays (CRs) from the second knee (

Limits on the Ultra-High Energy Electron Neutrino Flux from the RICE Experiment

\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{eV}

On the Origin and Survival of Ultra-High-Energy Cosmic-Ray Nuclei in Gamma-Ray Bursts and Hypernovae

) above in the CR spectrum is still unknown. Recently, there has been growing evidence that a peculiar type of supernovae, called hypernovae, are associated with subenergetic gamma-ray bursts, such as SN1998bw/GRB980425 and SN2003lw/GRB031203. Such hypernovae appear to have high (up to mildly relativistic) velocity ejecta, which may be linked to the subenergetic gamma-ray bursts. Assuming a continuous distribution of the kinetic energy of the hypernova ejecta as a function of its velocity

TeV neutrinos from core collapse supernovae and hypernovae

{E}_{k}\ensuremath{\propto}(\ensuremath{\Gamma}\ensuremath{\beta}{)}^{\ensuremath{-}\ensuremath{\alpha}}

Searching for sterile neutrinos in ice

with

Neutrino mass hierarchy extraction using atmospheric neutrinos in ice

\ensuremath{\alpha}\ensuremath{\sim}2