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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.

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 x 10**9 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 degrees and 71.7 degrees. Upper limits are derived for the photon flux above 1 TeV from any steady point source in the northern sky., 2 Figures

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.

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...

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

The Milagro water Cherenkov detector began full operation in January 2000. This detector is capable of monitoring the Northern sky at energies above 500 GeV for sources of equivalent strength to the Crab Nebula over one year of integration. We report on the current performance and sensitivity of Milagro.

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)

The Milagro water Cherenkov detector near Los Alamos, New Mexico is the first air shower detector capable of continuously monitoring the sky at energies between 500 GeV and 20 TeV. Preliminary results of the Milagro experiment are presented. A predecessor of the Milagro detector, Milagrito, was operational from February 1997 to May 1998. Milagrito consisted of 228 8″ photomultiplier tubes (PMTs) arranged in a grid with a 2.8 meter spacing and submerged in 1–2 meters of water. During its operation, Milagrito collected in excess of 9 billion events with a median energy of about 3 TeV. The detector’s sensitivity extends below 1 TeV for showers from near zenith. The results of an all sky search for the Milagrito data for both transient and DC sources will be presented, including the Crab Nebula and active galaxies Markarian 501 and 421, which are known sources of TeV gamma-rays. Also presented will be a study of the TeV emission from gamma ray bursts (GRBs) in Milagrito’s field of view detected by the BATSE e...

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.

Milagrito, a large, covered water-Cherenkov detector, was the world's first air-shower-particle detector sensitive to cosmic gamma rays below 1 TeV. It served as a prototype for the Milagro detector and operated from February 1997 to May 1998. This paper gives a description of Milagrito, a summary of the operating experience, and early results that demonstrate the capabilities of this technique., Comment: 38 pages including 24 figures

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.