Search

Milagrito, a detector sensitive to very high energy gamma rays, monitored the northern sky from 1997 February through 1998 May. With a large field of view and a high duty cycle, this instrument was well suited to perform a search for TeV gamma-ray bursts (GRBs). We report on a search made for TeV counterparts to GRBs observed by BATSE. BATSE detected 54 GRBs within the field of view of Milagrito during this period. An excess of events coincident in time and space with one of these bursts, GRB 970417a, was observed by Milagrito. The excess has a chance probability of 2.8 × 10-5 of being a fluctuation of the background. The probability for observing an excess at least this large from any of the 54 bursts is 1.5 × 10-3. No significant correlations were detected from the other bursts.

The Milagrito water Cherenkov detector near Los Alamos, New Mexico, operated as a sky monitor at energies of a few TeV between February 1997 and May 1998 including the period of the strong, long-lasting 1997 flare of Markarian 501. Milagrito served as a test run for the full Milagro detector. An event excess with a significance of 3.7 sigma from Markarian 501 was observed, in agreement with expectations.

Milagro will be the first water-Cerenkov detector specifically built to study extensive air showers. It is being built in an existing man-made pond located near Los Alamos, NM. Milagro will be the only air shower detector to have an energy threshold as low as a few hundred GeV and an angular resolution as good as 0.3°. Milagro will observe the entire overhead sky at all times. We describe here the Milagro detector, it capabilities and its physics goals.

Data taken with the CYGNUS detector between 1989 September and 1993 January have been used to search for 1 s bursts of ultrahigh-energy gamma rays from point sources at arbitrary locations in the northern sky. We find no evidence for such bursts. We set a theory-dependent upper limit on the rate-density of evaporating black holes of 8.5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{pc}}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{yr}}^{\mathrm{\ensuremath{-}}1}$ at the 99% C.L. When the same emission spectrum is used to recalculate previous upper limits based on direct searches, this limit is the most restrictive by nearly 2 orders of magnitude.

The Tokamak Physics Experiment is designed to develop the scientific basis for a compact and continuously operating tokamak fusion reactor. It is based on an emerging class of tokamak operating modes, characterized by beta limits well in excess of the Troyon limit, confinement scaling well in excess of H-mode, and bootstrap current fractions approaching unity. Such modes are attainable through the use of advanced, steady state plasma controls including strong shaping, current profile control, and active particle recycling control. Key design features of the TPX are superconducting toroidal and poloidal field coils; actively-cooled plasma-facing components; a flexible heating and current drive system; and a spacious divertor for flexibility. Substantial deuterium plasma operation is made possible with an in-vessel remote maintenance system, a lowactivation titanium vacuum vessel, and shielding of ex-vessel components. The facility will be constructed as a national project with substantial participation by U.S. industry. Operation will begin with first plasma in the year 2000.

Searches for point astrophysical sources of ultra high energy (UHE) gamma rays are plagued by large numbers of background events from isotropic cosmic rays. Some of the methods that have been used to estimate the expected number of background events coming from the direction of a possible source are found to contain biases. Search techniques that avoid this problem are described. There is also a discussion of how to optimize the sensitivity of a search to emission from a point source.

The CYGNUS extensive air-shower experiment is described. The design criteria, construction and operation details, and performance characteristics are presented. A discussion of the data analysis techniques is given. Finally, several enhancements and improvements in the apparatus are described.

Data collected with the Cerenkov telescope at the Haleakala Gamma Observatory have been analyzed in an effort to establish evidence for emission of VHE γ-rays for a number of candidate sources. Brief episodes of emission from Her X-1 have been detected. The results from searches for emission from other sources, which has not been detected by our group, are discussed. Continued observations with an upgraded telescope facility are currently underway.

Milagro is a large field of view (∼ 2 sr), high duty cycle (∼90%), ground‐based observatory sensitive to gamma‐rays above ∼100 GeV. This unique detector is ideal for observing the highest energy gamma‐rays from gamma‐ray bursts. The highest energy gamma rays supply very strong constraints on the nature of gamma‐ray burst sources as well as fundamental physics. Because the highest energy gamma‐rays are attenuated by pair production with the extragalactic infrared background light, Milagro’s sensitivity decreases rapidly for bursts with redshift > 0.5. While only 10 % of bursts have been measured to be within z=0.5, these bursts are very well studied at all wavelengths resulting in the most complete understanding of GRB phenomena. Milagro has sufficient sensitivity in units of E2 dN/dE to detect VHE luminosities lower than the observed luminosities at ∼ 100 keV for these nearby bursts. Therefore, the launch of SWIFT and its ability to localize and measure redshifts of many bursts points to great future poss...

Several proposed source models for Ultra-High Energy Cosmic Rays (UHECRs) consist of dipole distributions oriented towards major astrophysical landmarks such as the galactic center, M87, or Centaurus A. We use a comparison between real data and simulated data to show that the HiRes-I monocular data for energies above 10^{18.5} eV is, in fact, consistent with an isotropic source model. We then explore methods to quantify our sensitivity to dipole source models oriented towards the Galactic Center, M87, and Centaurus A., Comment: 17 pages, 31 figures

Evidence of TeV emission from GRB970417a has been previously reported using data from the Milagrito detector. Constraints on the TeV fluence and the energy spectrum are now derived using additional data from a scaler system that recorded the rate of signals from the Milagrito photomultipliers. This analysis shows that if emission from GRB970417a has been observed, it must contain photons with energies above 650 GeV. Some consequences of this observation are discussed., Comment: Submitted to the Astrophysical Journal

Milagrito, a detector sensitive to gamma-rays at TeV energies, monitored the northern sky during the period February 1997 through May 1998. With a large field of view and high duty cycle, this instrument was used to perform a search for TeV counterparts to gamma-ray bursts. Within the Milagrito field of view 54 gamma-ray bursts at keV energies were observed by the Burst And Transient Satellite Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. This paper describes the results of a preliminary analysis to search for TeV emission correlated with BATSE detected bursts. Milagrito detected an excess of events coincident both spatially and temporally with GRB 970417a, with chance probability $2.8 \times 10^{-5}$ within the BATSE error radius. No other significant correlations were detected. Since 54 bursts were examined the chance probability of observing an excess with this significance in any of these bursts is $1.5 \times 10^{-3}$. The statistical aspects and physical implications of this result are discussed., Comment: 6 pages, 5 figures, to appear in "GeV-TeV Astrophysics: Toward a Major Atmospheric Cherenkov Telescope VI," Snowbird, Utah (August, 1999)

A new type of very high energy (> a few 100 GeV) gamma-ray observatory, Milagro, has been built with a large field of view of >1 steradian and nearly 24 hours/day operation. Milagrito, a prototype for Milagro, was operated from February 1997 to May 1998. During the summer of 1998, Milagrito was dismantled and Milagro was built. Both detectors use a 80 m×60 m×8 m pond of water in which a 3 m×3 m grid of photomultiplier tubes detects the Cherenkov light produced in the water by the relativistic particles in extensive air showers. Milagrito was smaller and had only one layer of photomultipliers, but allowed the technique to be tested. Milagrito observations of the Moon’s shadow and Mrk 501 are consistent with the Monte Carlo prediction of the telescopes parameters, such as effective area and angular resolution. Milagro is larger and consists of two layers of photomultiplier tubes. The bottom layer detects penetrating particles that are used to reject the background of cosmic-ray initiated showers.

Progress in high-energy gamma-ray astronomy has depended upon the development of sophisticated detectors and analysis techniques. Observations in this decade using space-based and ground-based detectors have observed gamma-ray emission from a variety of sources. For the first time a consistent picture of the {gamma}-ray sky has emerged. This article describes the detection techniques in {gamma}-ray astronomy, the nature of the astrophysical objects studied, and the present state of the observations. Several possible new directions in the field are also described. {copyright} {ital 1999} {ital The American Physical Society}

This paper describes how data from a new type of air shower detector, MILAGRO, can shed light on a variety of interesting problems in astrophysics. MILAGRO has the capability to make observations of VHE/UHE emission from the recently discovered TeV gamma‐ray source Markarian 421, an Active Galactic Nucleus (AGN). An observation of the attenuation of this signal in the range of 1–20 TeV can be used to make the first measurement of the intergalactic infrared radiation. We will also describe how MILAGRO can improve the existing limits on the density of Primordial Black Holes (PBH) by three orders of magnitude. In addition, MILAGRO will have a large aperture and continuous operation which gives it the capability to search for VHE gamma ray bursts.

The CYGNUS air-shower array has been used to search for emission of ultra-high-energy gamma radiation from 13 active galactic nuclei (AGNs) that were detected by EGRET. The data set spans the period 1986 April 2 to 1993 January 1. The data set has been searched for continuous emission, for emission on the time scale of 1 week, and for emission on the time scale of 1 day. No evidence for emission from any of the AGNs on any of the time scales examined was found. The 90% Confidence Level (CL) upper limit to the continuous flux from Mrk 421 above 50 TeV is 7.5×10 -14 cm -1 s -1

A daily search for emission of ultra-high-energy radiation from astrophysical point sources using the CYGNUS extensive air shower array is described. The data set spans the period from 1986 April 4 to 1992 June 22. Fifty-one astrophysical objects have been examined, including Cyg X-3, Her X-1, the Crab, a number of gamma-ray and X-ray sources from the COS B and the fourth Uhuru catalogs, and several cataclysmic variables, nearby galaxies, and radio pulsars. The observed daily number of events from the source directions are consistent with expected statistical fluctuations of the number of events from backgrounds cosmic rays

A search has been made for steady emission or ultra-high-energy radiation from individual point sources over most or the northern sky. Over 2×10 8 air showers induced by primary particles or energy greater than about 10 TeV were recorded from 1986 April until 1991 May by the CYGNUS air shower array. No statistically significant excess above the background from the isotropic flux or cosmic rays was found for any direction in the sky from 0° to 80° declination. In addition, 49 specific potential sources were examined, and none showed a statistically significant excess or events

A 10 m2 multi-mirror telescope for observing Cherenkov light signals from atmospheric cascades is now operating at Mount Haleakala, Maui, Hawaii. It differs from other atmospheric Cherenkov detectors in accepting pulses that originate from single photoelectrons, employing two sets of 18 optically independent phototubes in a logic system with nanosecond time resolution to reject ambient light from the night sky. With an angular aperture of 1.3 × 10−4 sr, cosmic ray showers are observed at a rate of ∼ 0.5 hz at the zenith, with nearly complete rejection of ambient light. This rate for hadronic showers implies an effective threshold near 100 GeV for electromagnetic showers. Two regions of the sky, one centered on the source and the other separated by from it by 3.6° are simultaneously monitored. Examples of observations of episodic and periodic (pulsar) sources are given.

Neutral-beam heating of plasmas in the Tokamak Fusion Test Reactor at low preinjection densities (n/sub e/(0)approx. =10/sup 19/ m/sup -3/) were characterized by T/sub e/(0) = 6.5 keV, T/sub i/(0) = 20 keV, n/sub e/(0) = 7 x 10/sup 19/ m/sup -3/, tau/sub E/ = 170 msec, ..beta../sub t//sub h//sub e//sub t//sub a/ = 2, and a d(d,n)/sup 3/He neutron emission rate of 10/sup 16/ sec/sup -1/. The ion temperature and the deuterium-fusion neutron yields were significantly higher than for previous tokamak experiments. The low initial densities were achieved by operation of the Tokamak Fusion Test Reactor with low plasma currents (less than or equal to1 MA) and by extensive limiter conditioning.

It is shown that the constant D of transverse diffusion of drifting electrons in gases depends on the magnetic field B or the cyclotron frequency ω = (em)B in such a way that the reduction factor is D(0)D(B) = 1 + ω2τ12 for small B and D(0)D(B) = C + ω2τ22 for large B. The measurements were done between B = 0 and 7 kG, using a time projection chamber. The gas was 91% argon and 9% methane at 1 atm, the drift field 115 V/cm.

Milagrito, a detector sensitive to γ-rays at TeV energies, monitored the northern sky during the period February 1997 through May 1998. With a large field of view and high duty cycle, this instrument was used to perform a search for TeV counterparts to γ-ray bursts. BATSE detected 54 GRBs within the field of view of Milagrito during this period. This paper describes the results of an analysis to search for TeV emission correlated with BATSE detected bursts. Milagrito detected an excess of events coincident both spatially and temporally with GRB 970417a, with chance probability 2.8×10−5 within the BATSE error radius. No other significant correlations were detected. Since 54 bursts were examined, the chance probability of observing an excess with at least this significance in any of these bursts is 1.5×10−3.