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Session 56 - BL Lacs & Blazars.
Oral session, Thursday, January 08

[56.02] TeV Gamma-ray Observations of BL Lacertae Galaxies

T. C. Weekes (Harvard-Smithsonian CfA), Whipple Gamma Ray Collaboration

The Whipple Observatory imaging atmospheric Cherenkov telescope has been used in an extensive survey of nearby BL Lacertae objects (out to a redshift of 0.2). The three closest objects, Markarian 421, Markarian 501 and 1E2344+514, have been detected with high statistical significance. Only the first has been seen by EGRET on CGRO. In 1997 Markarian 501 exhibited extraordinary activity and on average was the brightest object in the known TeV sky. The observed gamma-ray energies extend from 200 GeV to >12 TeV. Time variations on scales of years, months, days and hours are reported; the shortest doubling-time seen is 15 minutes. The energy spectra do not reveal a sharp cutoff that would be expected if intergalactic space was permeated by a high density of infrared photons. The daily variability in TeV gamma-rays is found to correlate with variations in optical, UV, and X-ray emissions.

Program listing for Thursday