Paul J. Boyle

The TeV gamma-ray spectrum of markarian 421 during an intense flare

We present the gamma-ray spectrum of the BL Lacertae object, Markarian 421, above 500 GeV during its most intense recorded TeV flare, on 1996 May 7. The spectrum is well fitted by a power law with an exponent of 22.5650.0750.1 (statistical and systematic errors). The spectrum extends above 5 TeV with no evidence for a cutoff, favoring determinations of the extragalactic infrared energy density that do not produce a sharp cutoff at these energies.

A search for TeV counterparts to batse gamma-ray bursts

Intense effort has gone into the observation of optical, radio, and X-ray gamma-ray burst (GRB) counterparts, either simultaneous to the burst or as quasi-steady lingering remnants. Here we report on a similar study at higher energies of 250 GeV and above using ground-based telescopes. The recent technical advances represented by the atmospheric Cherenkov imaging technique (Cawley & Weekes 1995) have opened up the field of gamma-ray astronomy above 250 GeV and raised the possibility that these techniques can be used with excellent fluence sensitivity in exploring the GRB phenomenon. Observations by the Whipple collaboration of nine BATSE positions, one acquired within 2 minutes of the reported BATSE burst time, using coordinates distributed through the BATSE Coordinates Distribution Network (BACODINE) are reported. No evidence of TeV emission is found, and upper limits to the high-energy delayed or extended emission of observed candidates are calculated.

Detection of gamma rays with E> 100 MeV from BL Lacertae

We present evidence for the first detection of gamma rays from the extragalactic object BL Lacertae. Observations taken with EGRET on the Compton Gamma Ray Observatory between 1995 January 24 and 1995 February 14 indicate a 4.4 σ excess from the direction of BL Lacertae. The corresponding flux is (40 ± 12) × 10-8 photons cm-2 s-1 above 100 MeV. The combination of all previous observations where BL Lacertae was in EGRETs field of view result in a 2.4 σ excess and a corresponding 95% confidence upper limit of 14 × 10-8 photons cm-2 s-1, indicating that its gamma-ray emission is variable, at least on timescales of several months. Observations of BL Lacertae between 22 and 375 GHz were also taken between 1995 January 24 and 1995 February 14, and the flux levels for those measurements are similar to the historical average values for this object. A deep exposure on BL Lacertae with the Whipple Observatory 10 m gamma-ray telescope shows no evidence of emission above 350 GeV during a period 9 months after the EGRET observations. The 99.9% confidence flux upper limit derived from these observations is 0.53 × 10-11 photons cm-2 s-1, which implies a large reduction in the gamma-ray emission of BL Lacertae between EGRET and Whipple Observatory energies. This reduction should result from processes intrinsic to BL Lacertae because it is near enough to Earth that intergalactic background IR fields should not significantly reduce the flux of gamma rays to which the Whipple Observatory telescope is sensitive.

Very High-Energy Observations of PSR B1951+32

PSR B1951+32 is a γ-ray pulsar detected by the Energetic Gamma Ray Experiment Telescope (EGRET) and identified with the 39.5 ms radio pulsar in the supernova remnant CTB 80. The pulsar energy spectrum is consistent with an unbroken power law with differential photon index of -1.8 ± 0.1 between 50 MeV and 10 GeV. The EGRET data shows no evidence for a spectral turnover. Here we report on the first observations of PSR B1951+32 beyond 30 GeV. The observations were carried out with the 10 m γ-ray telescope at the Whipple Observatory on Mount Hopkins, Arizona. In 8.1 hours of observation we find no evidence for steady or periodic emission from PSR B1951+32 above ~260 GeV. Flux upper limits are derived and compared with model extrapolations from lower energies and the predictions of emission models.

A search for TeV gamma-ray bursts on a 1-second time-scale

Abstract Although atmospheric Cherenkov telescopes have restricted fields of view, their fluence sensitivity warrants a search for gamma-ray burst phenomena. A search for 400 GeV gamma-ray bursts on a 1 s time-scale using archival data taken between 1988 and 1992 with the Whipple Collaboration 10 m reflector is presented. No evidence of such bursts is found. Bursts of TeV gamma rays have been predicted from exotic astrophysical objects such as Primordial Black Holes and Cosmic Strings. An upper limit to the number density of exploding PBH of 3.0±1.0×10 6 pc −3 yr −1 is calculated.

Search for shadowing of primary cosmic radiation by the moon at TeV energies

Abstract The antimatter research through the earth moon ion spectrometer (ARTEMIS) experiment is described in terms of searching for the moons shadow in the primary cosmic radiation at TeV energies. Observations were made using the Whipple 10 m Imaging Atmospheric Cerenkov Telescope, in conjunction with a special optical filter to suppress unwanted moonlight. Monte Carlo simulations were used to predict that the magnitude of shadowing based on a pure proton beam would be 1% of the cosmic ray background. Observations and event classification are discussed, and results of a search for shadowing based on both protons and antiprotons are presented. Atmospheric fluctuations giving rise to non-statistical event rates constituted a limiting factor to the sensitivity of the shadow search as they rivaled or exceeded statistical errors. It is demonstrated that the experiment does not have sufficient sensitivity to moon shadowing which would allow a search for an antimatter component in the primary cosmic ray flux at the percent level. Possible future improvements of the technique are discussed.

Constraints on cosmic-ray origin from TeV gamma-ray observations of supernova remnants

Ifsupernovaremnants(SNRs)arethesiteofcosmic- ray acceleration, the associated nuclear interactions should re- sult in an observable flux of -rays for the nearest SNRs. Mea- surements of the TeV -ray flux from six nearby, radio-bright SNRs have been made with the Whipple Observatory imag- ing air Cerenkov telescope over the period September 1993 to June 1996. No signicant emission has been detected and up- per limits on the >300 GeV flux are reported. Three of these SNRs (IC443, -Cygni and W44) are spatially coincident with low-latitude otherwise unidentied sources detected with the Energetic Gamma-Ray Experiment Telescope (EGRET). If the EGRET -ray fluxes result from cosmic-ray interactions then theEGRETandWhippledataarefoundtobecollectivelyincon- sistent with a cosmic-ray source spectrum flatter than E 2:4 . The Whipple upper limits for IC443 and -Cygni are also in- consistent with a priori predictions if these remnants are indeed expanding into regions where the average density of the inter- stellarmediumisenhancedbythepresenceofmolecularclouds. These data weaken the case for the simplest models of shock acceleration and energy dependent propagation of cosmic rays.

Gamma-ray variability of BL lacs at E>300 GeV

Abstract We report on observations of Mrk 421 and Mrk 501 at E γ >300 GeV during the 1995 and 1996 seasons. A multiwavelength observing campaign on Mrk 421 in the period 1995 April 20 to May 5 indicates correlations in the γ-ray, X-ray, extreme ultraviolet, and optical wavelengths. Dramatic outbursts of TeV γ-rays were detected on two occasions in May of 1996 where the γ-ray flux reached the highest levels ever recorded (a factor of ∼20 greater than typical levels) with variability timescales ≲1hr. Together with the multiwavelength correlations these provide serious constraints on the Doppler factor ( δ ≳ 9.9), the size and location of the emission region ( R ≲ 10 −4 pc) and the mechanism responsible for the γ-ray emission.

Very energy observations of PSR B1951+32

PSR B1951+32 is a γ-ray pulsar detected by the Energetic Gamma Ray Experiment Telescope (EGRET) and identified with the 39.5 ms radio pulsar in the supernova remnant CTB 80. The EGRET data shows no evidence for a spectral turnover. Here we report on the first observations of PSR B1951+32 beyond 30 GeV. The observations were carried out with the 10 m γ-ray telescope at the Whipple Observatory on Mt. Hopkins, Arizona. In 8.1 hours of observation we find no evidence for steady or periodic emission from PSR B1951+32 above ∼260 GeV. Flux upper limits are derived and compared with model extrapolations from lower energies and the predictions of emission models.