Observing gamma ray bursts (GRBs) in the TeV energy range can be extremely valuable in providing insight to 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. Data from this detector was analyzed to look for a TeV component of GRBs coincident with low energy -rays detected by the BATSE instrument on the Compton Gamma Ray Observatory. A sample of 54 BATSE GRBs which were in the field of view of the Milagrito detector during its lifetime (February 1997 to May 1998) was used.
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.
The Milagrito water Cherenkov telescope operated for over a year. The most probable gamma-ray energy was ~1 TeV and the trigger rate was as high as 400 Hz. We have developed an efficient technique for searching the entire sky for short duration bursts of TeV photons. Such bursts may result from traditional gamma-ray bursts that were not in the field-of-view of any other instruments, the evaporation of primordial black holes, or some as yet undiscovered phenomenon. We have begun to search the Milagrito data set for bursts of duration 10 seconds. Here we will present the technique and the expected results. Final results will be presented at the conference.
Milagrito, a large, covered water-Cherenkov detector, was the worlds 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.
The Milagro water Cherenkov detector near Los Alamos, New Mexico, has been operated as a sky monitor at energies of a few TeV between February 1997 and April 1998. Serving as a test run for the full Milagro detector, Milagrito has taken data during the strong and long-lasting 1997 flare of Mrk 501. We present results from the analysis of Mrk 501 and compare the excess and background rates with expectations from the detector simulations.
H.E.S.S. is an array of five Imaging Atmospheric Cherenkov Telescopes (IACTs) located 1800 m above sea level in the Khomas Highland of Namibia and is sensitive to very-high-energy (VHE) gamma rays between tens of GeV to tens of TeV. The very-high background rejection capabilities of IACTs provide excellent sensitivity of H.E.S.S. to GRBs. In this contribution the status of the H.E.S.S. GRB programme, already started in 2003, is reviewed. A highlight is the recent addition of the fifth telescope, which is the worlds largest IACT. Its 600 square metre mirror lowers the energy threshold to tens of GeV and provides an effective area that is ten thousands of times larger than Fermi-LAT at these energies. The higher performance drive system will reduce the response time to a GRB alert, which will significantly enhance the chances of a H.E.S.S. GRB detection. Recent results on selected GRBs will be shown.