J. E. Carson

Observations of the BL Lacertae Object 3C 66A with STACEE

We present the analysis and results of recent high-energy gamma-ray observations of the BL Lac object 3C 66A conducted with the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE). During the 2003-2004 observing season, STACEE extensively observed 3C 66A as part of a multiwavelength campaign on the source. A total of 33.7 hours of data was taken on the source, plus an equivalent-duration background observation. After cleaning the data set a total of 16.3 hours of live time remained, and a net on-source excess of 1134 events was seen against a background of 231742 events. At a significance of 2.2 standard deviations this excess is insufficient to claim a detection of 3C 66A, but is used to establish flux upper limits for the source.

HIGH-ENERGY GAMMA-RAY OBSERVATIONS OF W COMAE WITH THE SOLAR TOWER ATMOSPHERIC CERENKOV EFFECT EXPERIMENT (STACEE)

We report on observations of the blazar W Com (ON+231) with the Solar Tower Atmospheric Cerenkov Effect Experiment (STACEE), a wave front-sampling atmospheric Cerenkov telescope, in the spring of 2003. In a data set comprising 10.5 hr of on-source observing time, we detect no significant emission from W Com. We discuss the implications of our results in the context of the composition of the relativistic jet in W Com, examining both leptonic and hadronic models for the jet. We derive 95% confidence level upper limits on the flux at the level of (1.5-3.5) × 10-10 cm-2 s-1 above 100 GeV for the leptonic models, or (0.5-1.1) × 10-10 cm-2 s-1 above 150 GeV for the hadronic models.We report on observations of the blazar W Comae (ON+231) with the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE), a wavefront-sampling atmospheric Cherenkov telescope, in the spring of 2003. In a data set comprising 10.5 hours of ON-source observing time, we detect no significant emission from W Comae. We discuss the implications of our results in the context of the composition of the relativistic jet in W Comae, examining both leptonic and hadronic models for the jet. We derive 95% confidence level upper limits on the flux at the level of 1.5--3.5 x 10^{-10} cm^{-2} s^{-1} above 100 GeV for the leptonic models, or 0.5--1.1 x 10^{-10} cm^{-2} s^{-1} above 150 GeV for the hadronic models.

The STACEE ground-based gamma-ray detector

We describe the design and performance of the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) in its complete configuration. STACEE uses the heliostats of a solar energy research facility to collect and focus the Cherenkov photons produced in gamma-ray induced air showers. The light is concentrated onto an array of photomultiplier tubes located near the top of a tower. The large Cherenkov photon collection area of STACEE results in a gamma-ray energy threshold below that of previous ground-based detectors. STACEE is being used to observe pulsars, supernova remnants, active galactic nuclei, and gamma-ray bursts

VERY HIGH ENERGY OBSERVATIONS OF GAMMA-RAY BURSTS WITH STACEE

Gamma-ray bursts (GRBs) are the most powerful explosions known in the universe. Sensitive measurements of the high-energy spectra of GRBs can place important constraints on the burst environments and radiation processes. Until recently, there were no observations during the first few minutes of GRB afterglows in the energy range between 30 GeV and ~1 TeV. With the launch of the Swift GRB Explorer in late 2004, GRB alerts and localizations within seconds of the bursts became available. The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) was a ground-based, gamma-ray telescope with an energy threshold of ~150 GeV for sources at zenith. At the time of Swifts launch, STACEE was in a rare position to provide >150 GeV follow-up observations of GRBs as fast as three minutes after the burst alert. In addition, STACEE performed follow-up observations of several GRBs that were localized by the HETE-2 and INTEGRAL satellites. Between 2002 June and 2007 July, STACEE made follow-up observations of 23 GRBs. Upper limits are placed on the high-energy gamma-ray fluxes from 21 of these bursts.

PULSED VERY HIGH ENERGY γ-RAY EMISSION CONSTRAINTS FOR PSR B1951+32 FROM STACEE OBSERVATIONS

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a ground-based telescope that uses the wave-front-sampling technique to detect very high energy (VHE) gamma rays. STACEEs sensitivity in the energy range near 100 GeV permits useful observations of pulsars with the potential to discriminate between various proposed mechanisms for pulsed gamma-ray emission. Based on the 11.3 hr of data taken during the 2005 and 2006 observing seasons, we derive an upper limit on the pulsed gamma-ray emission from PSR B1951+32 of <6.53 × 10–11 photons cm–2 s–1 above an energy threshold of 117 GeV.

Follow-up Observations of Gamma-ray Bursts with STACEE

STACEE is an atmospheric Cherenkov telescope using the large mirror area of a solar research facility to obtain a low energy threshold. The telescope has a peak in the detected signal of a power‐law gamma‐ray spectrum near 100 GeV. The low energy threshold of STACEE allows detection of gamma rays from higher redshifts than most other ground‐based experiments. The STACEE instrument can be re‐targeted to the position of a GRB within a few minutes of an alert to search for emission above 50 GeV. So far, data have been acquired within a few hours of the burst for five GRB. We discuss the STACEE sensitivity to high energy gamma‐ray emission from GRB and preliminary results of the observations.

Recent AGN Observations by the Solar Tower Atmospheric Cherenkov Effect Experiment

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a ground‐based atmospheric Cherenkov telescope for the detection of very high energy gamma rays from Galactic and extra‐galactic sources. By utilizing the large collection area provided by the solar mirrors of the National Solar Thermal Test Facility in Albuquerque, New Mexico, STACEE achieves a low energy threshold, around 100 GeV, for the detection of gamma rays. We briefly describe the STACEE detector and detail recent observations of Active Galactic Nuclei.