Constantine Sinnis

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.

A search of the northern sky for ultra-high-energy point sources

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

Search for emission of ultra-high-energy radiation from active Galactic nuclei

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

Daily search for emission of ultra-high-energy radiation from point sources

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

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.

Astrophysics with MILAGRO

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.