A. Jarvis

CONSTRAINTS ON THE VERY HIGH ENERGY EMISSION FROM BL LACERTAE OBJECTS

We present results from observations of 29 BL Lacertae objects, taken with the Whipple Observatory 10 m gamma-ray telescope between 1995 and 2000. The observed objects are mostly at low redshift (z < 0:2), but observations of objects of up to z ¼ 0:444 are also reported. Five of the objects are EGRET sources and two are unconfirmed TeV sources. Three of the confirmed sources of extragalactic TeV gamma rays were originally observed as part of this survey and have been reported elsewhere. No significant excesses are detected from any of the other objects observed, on timescales of days, months, or years. We report 99.9% confidence level flux upper limits for the objects for each observing season. The flux upper limits are typically 20% of the Crab flux, although for some sources, limits as sensitive as 6% of the Crab flux were derived. The results are consistent with the synchrotron self-Compton model predictions considered in this work. Subject headings: BL Lacertae objects: general — galaxies: jets — gamma rays: observations

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.

A Search for TeV Gamma-Ray Emission from High-peaked Flat-Spectrum Radio Quasars Using the Whipple Air Cerenkov Telescope

Blazars have traditionally been separated into two broad categories based on their optical emission characteristics. Blazars with faint or no emission lines are referred to as BL Lacertae objects (BL Lacs), and blazars with prominent, broad emission lines are commonly referred to as flat-spectrum radio quasars (FSRQs). The spectral energy distribution of FSRQs has generally been thought of as being more akin to the low-peaked BL Lacs, which exhibit a peak in the infrared region of the spectrum, as opposed to high-peaked BL Lacs (HBLs), which exhibit a peak in UV/X-ray region of the spectrum. All blazars that are currently confirmed as sources of TeV emission fall into the HBL category. Recent surveys have found several FSRQs that exhibit spectral properties, particularly the synchrotron peak frequency, similar to HBLs. These objects are potential sources of TeV emission according to several models of blazar jet emission and the evolution of blazars. Measurements of TeV flux or flux upper limits could impact existing theories explaining the links between different blazar types and could have a significant impact on our understanding of the nature of objects that are capable of TeV emission. In particular, the presence (or absence) of TeV emission from FSRQs could confirm (or cast doubt on) recent evolutionary models that expect intermediate objects in a transitional state between FSRQ and BL Lac. The Whipple 10 m imaging air Cerenkov gamma-ray telescope is well suited for TeV gamma-ray observations. Using the Whipple telescope, we have taken data on a small selection of nearby (z < 0.1 in most cases) high-peaked FSRQs. Although one of the objects, B2 0321+33, showed marginal evidence of flaring, no significant emission was detected. The implications of this paucity of emission and the derived upper limits are discussed.

A survey of unidentified egret sources at very high energies

The Whipple Observatory 10 m ?-ray telescope has been used to survey the error boxes of EGRET unidentified sources in an attempt to find counterparts at energies of 350 GeV and above. Twenty-one unidentified sources detected by EGRET (more than 10% of the total number) have been included in this survey. In no case is a statistically significant signal found in the EGRET error box, which implies that, at least for this sample, the ?-ray spectra of these sources steepen between 100 MeV and 350 GeV. For each EGRET source location, we list candidate associations and derive upper limits on the integral ?-ray flux above 350 GeV.

OBSERVATION OF M87 AT 400 GeV WITH THE WHIPPLE 10 METER TELESCOPE

We present results from observations taken with the Whipple 10 m very high energy γ-ray telescope with maximal sensitivity at 400 GeV during 39 hr between 2000 and 2003 in the direction of the giant radio galaxy M87. Using the entire data set, we derive a 99% confidence level upper limit on the flux of γ-ray emission above 400 GeV from M87 to be ≤6.9 × 10-12 cm-2 s-1. This suggests variability at the 90% confidence level when compared to the flux measured by the HEGRA collaboration in 1999 if the differential spectrum is steeper than a power law of index 3.75. Our search for a correlation between the Rossi X-Ray Timing Explorer all-sky monitor observation and a potential γ-ray signal is inconclusive.

OSETI with STACEE: a search for nanosecond optical transients from nearby stars.

We have used the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) high-energy gamma-ray detector to look for fast blue-green laser pulses from the vicinity of 187 stars. The STACEE detector offers unprecedented light-collecting capability for the detection of nanosecond pulses from such lasers. We estimate STACEEs sensitivity to be approximately 10 photons/m(2) at a wavelength of 420 nm. The stars have been chosen because their characteristics are such that they may harbor habitable planets, and they are relatively close to Earth. Each star was observed for 10 minutes, and we found no evidence for laser pulses in any of the data sets. Key Words: Search for extraterrestrial intelligence-Optical search for extraterrestrial intelligence-Interstellar communication-Laser.

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.

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.

The Energy Spectrum of the Blazar Markarian 421 above 130 GeV

According to leptonic models for the high-energy emission from blazars, relativistic electrons in the inner jets inverse-Compton scatter photons from a variety of sources. Seed photons are certainly introduced via the synchrotron process from the electrons themselves, but external sources of seed photons may also be present. In this paper, we present detailed derivations of the equations describing external inverse-Compton scattering from two sources of seed photons: direct emission from the accretion disk, and accretion disk photons that have scattered off the broad line region. For each source, we derive the seed photon spectrum incident on the jet, the single electron energy loss rate, and the emitted photon spectrum.Markarian 421 (Mrk 421) was the first blazar detected at gamma-ray energies above 300 GeV, and it remains one of only twelve TeV blazars detected to date. TeV gamma-ray measurements of its flaring activity and spectral variability have placed constraints on models of the high-energy emission from blazars. However, observations between 50 and 300 GeV are rare, and the high-energy peak of the spectral energy distribution (SED), predicted to be in this range, has never been directly detected. We present a detection of Mrk 421 above 100 GeV as made by the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) during a multiwavelength campaign in early 2004. STACEE is a ground-based atmospheric Cherenkov telescope using the wave-front sampling technique to detect gamma rays at lower energies than achieved by most imaging Cherenkov telescopes. We also outline a method for reconstructing gamma-ray energies using a solar heliostat telescope. This technique was applied to the 2004 data, and we present the differential energy spectrum of Mrk 421 above 130 GeV. Assuming a differential photon flux dN/dE ∝ E-α, we measure a spectral index α = 2.1 ± 0.2stat . Finally, we discuss the STACEE spectrum in the context of the multiwavelength results from the same epoch.

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.