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 Cerenkov 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 hr of data was taken on the source, plus an equivalent-duration background observation. After cleaning the data set a total of 16.3 hr of live time remained, and a net on-source excess of 1134 events was seen against a background of 231,742 events. At a significance of 2.2 this excess is insufficient to claim a detection of 3C 66A but is used to establish flux upper limits for the source.

The BL Lac object 3C 66A was the target of an extensive multiwavelength monitoring campaign from , including 3 epochs contemporaneous with the core campaign.A gradual brightening of the source over the course of the campaign was observed at all optical frequencies, culminating in a very bright maximum around Feb. 18, 2004. The WEBT campaign revealed microvariability with flux changes of ∼ 5 % on time scales as short as ∼ 2 hr. The source was in a relatively bright state, with several bright flares on time scales of several days. The spectral energy distribution (SED) indicates a νF ν peak in the optical regime. A weak trend of optical spectral hysteresis with a trend of spectral softening throughout both the rising and decaying phases, has been found. On longer time scales, there appears to be a weak indication of a positive hardness-intensity correlation for low optical fluxes, which does not persist at higher flux levels.The 3 -10 keV X-ray flux of 3C 66A during the core campaign was historically high and its spectrum very soft, indicating that the low-frequency component of the broadband SED extends beyond ∼ 10 keV. No significant X-ray flux and/or spectral variability was detected. STACEE and Whipple observations provided upper flux limits at > 150 GeV and > 390 GeV, respectively.The 22 and 43 GHz data from the 3 VLBA epochs made between September 2003 and January 2004 indicate a rather smooth jet with only very moderate internal structure. Evidence for superluminal motion (8.5 ± 5.6 h −1 c) was found in only one out of 6 components, while the apparent velocities of all other components are consistent with 0. The radial radio brightness profile suggests a magnetic field decay ∝ r −1 and, thus, a predominantly perpendicular magnetic field orientation.

We present here new results on the space density of rich, optically-selected, clusters of galaxies at low redshift (z < 0.15). These results are based on the application of the matched filter clusterfinding algorithm (as outlined by Postman et al. 1996 andKawasaki et al. 1998) to 1067deg 2 of the Edinburgh/Durham Southern Galaxy Catalogue (EDSGC). This is the first major application of this methodology at low redshift and in total, we have detected 2109 clusters above a richness cutoff of R m ≥ 50 (or Λ cl ≥ 10; Postman et al. 1996). This new catalogue of clusters is known as the Edinburgh/Durham Cluster Catalogue II (or EDCCII). We have used extensive Monte Carlo simulations to define the detection thresholds for our algorithm, to measure the effective area of the EDCCII and to determine our spurious detection rate. These simulations have shown that our detection efficiency is strongly correlated with the presence of large-scale structure in the EDSGC data. We believe this is due to the assumption of a flat, uniform background in the matched filter algorithm. Using these simulations, we are able to compute the space density of clusters in this new survey. We find 83.These three richness bands roughly correspond to Abell Richness Classes 0, 1 and ≥ 2 respectfully. These new measurements of the local space density of clusters are in agreement with those found at higher redshift (0.2 < z est < 0.6) in the Palomar Distant Cluster Survey (PDCS; Postman et al. 1996 and therefore, removes one of the major uncertainties associated with the PDCS as it had previously detected a factor of 5 ± 2 more clusters at high redshift than expected compared to the space density of low redshift Abell clusters. This discrepancy is now lessened and, at worst, is only a factor of 4 +10 −4 . This result illustrates the need to use the same cluster-finding algorithm at both high and low redshift to avoid such apparent discrepancies. We also confirm that the space density of clusters remains nearly constant out to z ∼ 0.6 in agreement with previous optical and X-ray measurements of the space density of clusters (Couch et al. 1991;Postman et al. 1996;Ebeling et al. 1997;Nichol et al. 1999). Finally, we have compared the EDCCII with the Abell catalogue. We detect nearly 60% of all Abell clusters in the EDCCII area regardless of their Abell Richness and Distance Classes. For clusters in common between the two surveys, we find no strong correlation between the two richness estimates in agreement with the work of Lumsden et al. (1992). In comparison, ∼ 90% of the EDCCII systems are new, although a majority of them have a richness lower than an Abell Richness Class of 0 and therefore, would be below Abell's original selection criteria. However, we do detect 143 new clusters with R m ≥ 100 (which corresponds to a Richness Class of greater than, or equal to, 0) that are not in the Abell catalogue i.e. 63% of the rich EDCCII systems. These numbers lend credence to the idea that the Abell catalogue may be incomplete, especially at lower richnesses.

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.