A. D. Kerrick

Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare

A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.

Outburst of TeV photons from Markarian 421

Markarian 421, an active galactic nucleus (AGN) of the BL Lacertae type, is the closest Energetic Gamma Ray Experiment Telescope (EGRET)-detected AGN. It has been monitored by the Whipple Observatory very high energy gamma-ray telescope since its discovery at TeV energies, for approximately 90 nights, totaling 130 hours of observation. Observations from 1993 December 23 to 1994 May 10 showed an average sourve flux only half that of its 1992 discovery level. However, observations on 1994 May 14 and 15 show an increase in flux above this quiescent level by a factor of approximately 10. The timescale of this increase provides the best geometric constraint on the extent of TeV photon emission regions within AGNs. The observation of the high TeV flux occurred 1 day prior to the observation by ASCA of a very high 2-10 keV X-ray flux. This strong TeV outburst is reminiscent of the behavior seen for the stronger 100 MeV-GeV EGRET sources (e.g. 3C 279) but was not anticipated in view of the fact that the six EGRET observations of Mrk 421 from 1991 June to 1993 July showed no evidence for variability.

Upper Limits on TeV Gamma-Ray Emission from Active Galactic Nuclei

The results of a search for TeV gamma-ray emission from 35 active galactic nuclei (AGNs) using the Whipple Observatory High Resolution Atmospheric Cerenkov Camera are reported. Fifteen of these objects have been detected at GeV energies by the EGRET experiment on the {ital Compton} {ital Gamma} {ital Ray} {ital Observatory}. None of the 35 objects gave a signal at the 3 {sigma} level; Mrk 421 remains the only AGN detected at TeV energies. The absence of a TeV signal may imply a change in the primary source spectrum and/or the effect of absorption by pair production on intergalactic infrared photons. {copyright} {ital 1995 The American Astronomical Society.}

The use of an ultra-violet camera in the atmospheric Cherenkov technique

Abstract The atmospheric Cherenkov technique for gamma ray astronomy is shown to be feasible using an ultraviolet (UV) camera on moon-lit-nights. The Whipple telescope has been used as the reflector and the Crab Nebula as the standard gamma ray beam. Our energy threshold is 1.2 TeV which is compared to 0.4 TeV in the visible.

Detection of TeV gamma rays from the AGN Markarian 421

We present here the detection of TeV gamma rays from the extragalactic source Markarian 421. Observations obtained with the Whipple 10 m imaging Cherenkov telescope in March--June 1992 give a gamma ray signal with statistical significance of 6.3[sigma] above background. The excess corresponds to an average flux of 1.5[times]10[sup [minus]11] cm[sup [minus]2] s[sup [minus]1] above 0.5 TeV, equivalent to 0.3 of that from the Crab Nebula. The estimated location of the source agrees with the position of Mrk 421 to the angular uncertainty of the Whipple instrument (6 arc minutes).

Searches for bursts of TEV gamma rays on time‐scales of seconds

The Whipple Observatory gamma‐ray telescope has a high sensitivity to sources of gamma rays in the 0.4 to 4 TeV energy range. Although this sensitivity is used primarily to search for discrete sources of gamma‐rays the instrument also has sensitivity to gamma‐ray bursts on time‐scales from milliseconds to seconds. The field of view is limited but the source location capability is good. Such bursts could radiate with peak luminosity at TeV energies and could originate from(a) primordial black holes or (b) cosmic strings; they could also be the high energy counterparts of BATSE‐type bursts and hence of unknown origin. The search of the Whipple data‐base for statistically unlikely consecutive events on time‐scales of second will be described and compared with the theoretical predictions.

Detection of TeV photons from Markarian 421

We report on the detection by the Whipple Observatory’s γ‐ray telescope of TeV γ‐rays from the Markarian 421 at the 6.3σ level. The flux above 0.5 TeV is 0.3 that of the Crab Nebula. This is the most distant source detection at TeV γ‐ray energies.

Search for TeV gamma‐ray emission from AGN’s using the Whipple imaging telescope

MeV--GeV emission has been reported by the EGRET team for a number of AGNs. We report the results of observations taken with the Whipple Gamma Ray imaging telescope during the spring of 1992, on QSO0836+710, PKS0528+134, 4C38.1, 3C273, 3C279, Mrk421, Mrk501, IZW187, and VRO42.22.01. The EGRET team also sees a short time-scale variation in luminosity for some of these objects. The results of a search for variability at TeV energies in these objects is reported.

A method for searching the Whipple Observatory gamma ray data base for evidence of GRB’s

In light of recent Gamma Ray Burst (GRB) data from GRO, it is desirable to have a method for searching data from the Fred Lawrence Whipple Observatory 10 meter Cherenkov telescope for evidence of GRB’s at TeV energies. In particular, this instrument’s narrow field of view and high angular resolution make it well suited for observations of M31 as a test of the galactic halo model of GRB origin. We present here such a method, and give the results of its application to data taken on M31. Also reported here is an upper limit for the detection of a DC signal from M31.