Session B16 - Neutrino and Gamma Ray Astrophysics.
MIXED session, Saturday morning, April 29
202C, Convention Center

[B16.001] STATUS OF THE RICE EXPERIMENT - HARDWARE AND DEPLOYMENT

The Radio Ice Cerenkov Experiment (RICE) is an astrophysics neutrino experiment, co-deployed with AMANDA, and based at the South Pole. The experiment uses an array of 16 radio receivers, deployed at depths up to 200 m., to detect showers in the ice initiated by electron neutrinos. We summarize the deployments to date, over the last 3 austral summers, and testing and calibration of the transmitters and receivers used in the experiment, both on-site and at the home institutions.

[B16.002] STATUS OF THE RICE EXPERIMENT - ANALYSIS AND RESULTS

The Radio Ice Cerenkov Experiment (RICE) is an astrophysics neutrino experiment, co-deployed with AMANDA, and based at the South Pole. The experiment uses an array of 16 radio receivers, deployed at depths up to 200 m., to detect showers in the ice initiated by electron neutrinos. We discuss analysis of the 1999 data, including an estimated upper limit on the neutrino flux above 1 PeV from that dataset.

[B16.003] New Population of Unidentified High Energy Gamma Ray Sources

One of the great mysteries of the high energy gamma-ray sky is the existence of a large number of unidentified point sources. They account for more than half of the sources detected by the EGRET instrument on CGRO, numbering 171 out of 270 total sources in the recent 3rd EGRET catalog. No clear counterparts have been found at other wavelengths despite 20 years of effort. In this presentation, we will show evidence for a new population of these sources in the 3rd EGRET catalog. A cluster of ~20 faint sources appears concentrated near the galactic center, extending to ~30š galactic latitude and with more sources toward positive latitudes than negative. They are part of a broader class of faint sources sprinkled around the sky at mid-latitudes. Several tests show that they are real point sources and that they are distinct from the population of bright unidentified sources that lie along the galactic plane. Candidates for the counterparts include massive stars (gamma rays generated in the stellar wind), radio quiet gamma-ray pulsars and jets from accreting black holes.

[B16.004] Rapid Variability of TeV Blazars

Observations of blazars have revealed hour-scale or shorter flux variations on several occasions, from radio to gamma-ray wavelengths. These very fast flares provide a means of investigating the flaring mechanisms and acceleration processes in these objects and, when observed simultaneously at several wavelengths, provide a means of investigating the environment of the blazar jet and the mechanism which produces the gamma-ray emission. In this talk, I will review recent observations of hour-scale variability at X-ray and TeV energies from the TeV-emitting blazars, Markarian 421 and Markarian 501, and discuss what we have learned about these objects from these observations.

[B16.005] Extragalactic Background Light and \gamma-ray observations of AGN.

The spectral density of the Extragalactic Background Light (EBL) (0.1-100 \mum) contains extensive information on the early history of the Universe: cosmology, galaxy and star formation, metal and dust production, and the rate of re-processing of starlight to infrared wavelengths by the dust. In spite of great interest, EBL has so far escaped direct detection due to strong backgrounds from interplanetary dust scattering and emission, stars and galactic foregrounds, and interstellar dust emission. In the late sixties Gould and Schréder pointed out that observations of extragalactic photons in the region 0.1 to 100 TeV can be used for indirect detection of EBL because the two photons are coupled via pair production. Recent detections of AGN which indeed emit such energetic photons provide the data necessary to utilize this technique. We will review recent experimental results and uncertainties associated with theoretical predictions of EBL as well as the contribution and potential of the ground based \gamma-ray observatories, such as Whipple and VERITAS, to measure the EBL field.

[B16.006] Fast Timing Bi-Directional CCD for Use in Gamma-Ray Astronomy

A CCD coupled with a pixellated inorganic scintillator (such as segmented CsI) can provide high position resolution (\sim300 \mum). However, standard CCDs are integrating devices typically operating no faster than video rates. For a balloon-borne \gamma-ray telescope capable of measuring the energy of individual photons, the CCD must have a time resolution better than the average time interval between cosmic ray hits on the veto shield (\sim10 kHz). A Fast Timing Bi-Directional CCD has better than 10 \mus time resolution and 50 \mum position resolution. We describe the CCD readout architecture, the ASIC readout design, the present status of the development, and the application to a \gamma-ray astronomy telescope suitable for a 100-day Ultra Long Duration Balloon mission.

[B16.007] High x-ray yields in CO2 laser driven relativistic Thomson scattering

We report results of the first stage proof-of-principle demonstration of the ultra-high intensity laser synchrotron source (LSS) that combines a photocathode RF linac and a picosecond CO2 laser. Selection of such components is based on a systematic approach to optimize the LSS towards the maximum photon yield. Simultaneously, this approach opens a prospect to ultra-high peak intensity femtosecond x-ray and gamma sources. The 600 MW CO2 laser interacted in a head-on collision with the 60 MeV, 140 A, 3.5 ps electron beam. Both beams were focused to the r(rms)=32 um spot. 6.4*10^6 of the 1.8-2.3 Å x-ray photons per 3.5 ps pulse are detected in agreement with simulations that model the experimental conditions. Our next plan is to demonstrate 10^10 x-ray photons per pulse using a CO2 laser of the ~1 TW peak power. The next stage of the ATF LSS experiment will also open an opportunity for detailed study of Thomson harmonics generated on relativistic electron beams at the normalized laser strength of a~1.

[B16.008] Inherently Bursting Accretion

Magnetized accretion disks are shown to be subject to collective processes, that can induce the needed outward transport of angular momentum, whose growth rates, as in the case of laboratory plasmas depend on the finiteness of the plasma temperature(B. Coppi and P.S. Coppi, \itPhys. Lett. A)239\rm, 261 (1998) and on the gradient of the rotation frequency(E. Velikhov, \itSoviet Phys.) JETP36\rm, 1938 (1959); S.A. Balbus and J.F. Hawley, \itAp. J. \bf376\rm, 214 (1991) combined. Therefore accretion is envisioned to occur as a sequence of two stages: i) a preheating phase of accumulated plasmas at the outer edge of the disk and ii) a rapid infalling phase following the excitation of the relevant modes. The proposed scenario is similar to that of the so called ``monster sawtooth oscillations'' occuring in magnetically confined laboratory plasmas where the central temperature is observed to rise, as a result of the injection of a high energy particle population and then to crash periodically. The crash leads to a spatial redistribution of the thermal energy and is caused by the excitation of internal modes driven by the plasma pressure gradient when this is raised above a characteristic threshold by the energy transferred from the injected particles to the thermal plasma.

Sponsored in part by the U.S. Department of Energy

[B16.009] Flash Heating of Circumstellar Clouds by Gamma-Ray Bursts

The identification of flaring and fading X-ray, optical and radio counterparts to gamma-ray burst (GRB) sources, and the large energy releases implied by redshift measurements, find a consistent explanation in an expanding relativistic blast-wave model. This model has been called into question by observations of spectra from GRBs that are harder than can be produced through the synchrotron process. If GRBs originate from the collapse of massive stars, then circumstellar clouds near burst sources will be illuminated by intense gamma radiation, and the electrons in these clouds will be rapidly scattered to energies as large as several hundred keV. Low-energy photons that subsequently pass through the hot plasma will be scattered to higher energies, and this effect resolves the line-of-death objection to the synchrotron shock model. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electron-positron pairs that can be detected within ~ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy would reveal past GRB explosions.

[B16.010] The Rapid Gamma-ray Burst Response Campaign with ROTSE

The main goal of the Robotic Optical Transient Search Experiment (ROTSE) is to detect optical emission from Gamma-ray Bursts (GRBs) during and immediately following GRBs. The instruments comprising ROTSE consist of wide-field optics on rapidly slewing mounts optimized for quick response to GRBs localized to several degree regions. Thus far, only one prompt optical counterpart to a GRB has been discovered although simple scaling arguments suggest more should be easily detected. This talk will cover ROTSE responses to GRBs detected by the Burst and Transient Source Experiment and the soon-to-be-launched High Energy Transient Experiment. The expansion of ROTSE to a global network of identical telescopes will also be briefly discussed.

[B16.011] Search for a TeV Component of GRBs Using the Milagrito Detector

Observing gamma-ray bursts (GRBs) in the TeV energy range can be extremely valuable in providing insight into GRB radiation mechanisms and in constraining source distances. The Milagrito detector was an air shower array which used the water Cherenkov technique to search for TeV sources. Unlike other detectors which have attempted to search for GRBs at TeV energies, Milagrito combined the characteristics of a low energy threshold, a large field of view, and a high duty cycle, making it a practical instrument for detecting gammay ray events which lasted but fleetingly and which were unpredictable in both time and position of occurrence. Data from this detector was analyzed to look for TeV emission from 54 BATSE GRBs which were in the field of view of the Milagrito detector during its lifetime (January 1997 to May 1998). Results from this analysis, including evidence for TeV emission from GRB 970417a, will be presented.

[B16.012] Considerations for a Next Generation GRB Observatory

A framework will be described for the planning of a Next Generation Burst Observatory. This Observatory, using Swift as a pathfinder mission, would study early star formation and early galaxy formation at very high redshifts through observations of thousands of GRBs, their afterglows and environments. It is suggested that the international GRB community should begin some initial studies for such an observatory.

[B16.013] Gamma-Ray Bursts Cannot Produce the Observed Cosmic Rays above ~10 EeV

It has been suggested that cosmological \gamma-ray bursts can produce the observed flux of cosmic rays at the highest energies. However, recent observations indicate that the redshift distribution of \gamma-ray bursts most likely follows the redshift distribution of the average star formation rate in the universe, a rate which was much higher at redshifts between 1.5 and 2 than it is today. We show that as a consequence, energy losses suffered by ultrahigh energy cosmic rays caused by meson-producing interactions with photons of the big-bang relic background radiation would have a profound effect on both the flux and energy spectrum of these cosmic rays which would be observed at Earth during the present epoch. The cosmic rays with energies above 10 EeV from \gamma-ray bursts which we predict would have a much lower flux than that observed and would have a sharp cutoff in their spectrum at an energies above \sim 30 EeV, an order of magnitude lower in energy than the highest energy cosmic ray event reported by the Fly's Eye detector array.

[B16.014]

This abstract not available.

Part B of program listing