The Active Galaxy Mrk501

Mrk501 is an AGN (active galactic nucleus) that lies about 300 million light-years from the Earth. This object was first discovered in the TeV energy regime by the Whipple telescope (an air Cherenkov telescope operating in Arizona). During 1997 this AGN entered into a "high" state, where its luminosity became highly variable, and for periods of time it became very bright. During 1997 it was the brightest known source of TeV gamma rays. Below is a plot of the region around Mrk501 as seen by the Milagrito prototype telescope. The center of the plot is the direction of Mrk501. The circle drawn around the center is the size of the region used to integrate the signal (radius of 1 degree). This is determined by the angular resolution of Milagrito. The color axis is in standard deviations, which is related to the statistical significance of the observed excess. Neighboring bins are highly correlated.

Unlike air Cherenkov telescopes Milagro could observe Mrk501 during the time of year that it was overhead during the day. Thus, we could try and determine when the high-state ended. (When Mrk501 again went overhead during the night in 1998 it was no longer in a high state.) Below is a graph of our measured event rate excess from Mrk501 as a function of the Julian Day. The shaded region indicates the time period during which Mrk501 was overhead during the day. Unfortunately, Milagrito did not have sufficient sensitivity to answer this question. The full detector, Milagro, will have the sensitivity required to perform these types of studies.