S. Hugenberger

Evidence for T[CLC]e[/CLC]V Emission from GRB 970417[CLC]a[/CLC]

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 High-Energy Gamma-Ray Fluence and Energy Spectrum of GRB 970417a from Observations with Milagrito

Evidence of TeV emission from GRB 970417a 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 GRB 970417a has been observed, it must contain photons with energies above 650 GeV. Some consequences of this observation are discussed.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.

Observation of GeV Solar Energetic Particles from the 1997 November 6 Event Using Milagrito

Milagrito was an extensive air-shower observatory that served as a prototype for the larger Milagro instrument. Milagrito operated from 1997 February to 1998 May. Although it was designed as a very high energy (few hundred GeV threshold) water-Cerenkov gamma-ray observatory, it could also be used to study solar energetic particles (SEPs). By recording scaler data, which correspond to photomultiplier tube singles rates, it was sensitive to muons and small showers from hadronic primary particles with rigidities above ~4 GV. Milagrito simultaneously recorded air-shower trigger data of primary particles with energies greater than ~100 GeV that provide the data to help reconstruct event directions. The Milagrito scalers registered a ground-level enhancement associated with the 1997 November 6 SEP event and X9 solar flare. At its peak, the enhancement was 22 times the background rms fluctuations. Based on comparisons to neutron monitor and satellite data, we find evidence that the rigidity power-law spectrum for the differential flux of energetic protons steepened above ~4 GV and that the acceleration site was high in the corona (at ~3 R☉ above the photosphere), assuming that a CME-driven shock was responsible for the ground-level enhancement.

Evidence for TeV Emission from GRB 970417a

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 2.8x10-5 of being a fluctuation of the background. The probability for observing an excess at least this large from any of the 54 bursts is 1.5x10-3. No significant correlations were detected from the other bursts.

A SURVEY OF THE NORTHERN SKY FOR TeV POINT SOURCES

A search for steady TeV point sources anywhere in the northern sky has been made with data from the Milagrito air-shower particle detector. Over 3 × 109 events, collected from 1997 February to 1998 May, have been used in this study. No statistically significant excess above the background from the isotropic flux of cosmic rays was found for any direction of the sky with declination between -5° and 717. Upper limits are derived for the photon flux above 1 TeV from any steady point source in the northern sky.

TeV OBSERVATIONS OF MARKARIAN 501 WITH THE MILAGRITO WATER CERENKOV DETECTOR

The Milagrito water Cerenkov detector near Los Alamos, New Mexico, was operated as a sky monitor at energies of a few TeV between 1997 February and 1998 May, 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.

High energy gamma ray astroparticle physics with milagro

Abstract 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.

First results of a study of TeV emission from GRBs in Milagrito

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-rays bursts. Within the Milagrito field of view 54 γ-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 2.8×10−5 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 1.5×10−3. The statistical aspects and physical implications of this result are discussed.

Milagro: A TeV gamma-ray monitor of the Northern Hemisphere Sky

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.

The Milagro gamma-ray observatory

Milagro is a unique water Cherenkov detector designed for very high energy (VHE) gamma-ray astronomy. The water Cherenkov technique gives Milagro an unprecedented sensitivity and energy threshold. A deep detector layer is used to identify penetrating particles (muons and hadrons) present in hadron induced showers allowing for the rejection of backgrounds which substantially increases sensitivity. This paper describes the Milagro detector, its mode of operation and sensitivity to gamma-ray sources. Recent results of Milagro are presented including the recent detection of diffuses gamma-ray emission from the Galactic plane and the Cygnus region, search for VHE radiation from gamma-ray bursts and a survey of the northern sky for gamma-ray sources.