The Astrophysical Journal, Volume 621, Issue 1, pp. 181-187.

astro-ph/0503085 

Abstract:  The blazar 1ES 1959+650 was observed in a flaring state with the Whipple 10 m Imaging Atmospheric Cerenkov Telescope in 2002 May. A spectral analysis has been carried out on the data from that time period, and the resulting very high energy gamma-ray spectrum (E〉=316 GeV) can be well fitted by a power law of differential spectral index α=2.78+/-0.12_stat+/-0.21_sys. On 2002 June 4, the source flared dramatically in the gamma-ray range without any coincident increase in the X-ray emission, providing the first unambiguous example of an ``orphan'' gamma-ray flare from a blazar. The gamma-ray spectrum for these data can also be described by a simple power-law fit with α=2.82+/-0.15_stat+/-0.30_sys. There is no compelling evidence for spectral variability or for any cutoff to the spectrum.

Astroparticle Physics, Volume 23, Issue 2, p. 235-248.

arXiv:astro-ph/0501199

Abstract:  The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) is operated at the Whipple Observatory utilizing the Whipple 10 m γ-ray telescope. SGARFACE is sensitive to γ-ray bursts of more than 100 MeV with durations from 100 ns to 35 μs and provides a fluence sensitivity as low as 0.8 γ-rays/m2 above 200 MeV (0.05 γ-rays/m2 above 2 GeV) and allows to record the burst time structure.

The Astrophysical Journal, Volume 618, Issue 2, pp. 657-674.

arXiv:astro-ph/0406565

Abstract:  Very high energy (~TeV) γ-rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity for determining the EBL intensity if their intrinsic spectrum is known. Conversely, knowledge of the EBL intensity can be used to determine the intrinsic blazar spectrum. Unfortunately, neither the EBL intensity nor the intrinsic blazar spectrum is known with high enough precision to accurately derive one quantity from the other. In this paper we use the most recent data on the EBL to construct 12 different realizations representing all possible permutations between EBL limits and the detections in the different wavelength regions. These realizations explore a significantly larger range of allowable EBL spectra than any previous studies. We show that these realizations can be used to explore the effects of the EBL on the inferred spectra of blazars. Concentrating on the two relatively nearby (z~0.03) blazars Mrk 421 and 501, we derive their intrinsic spectra and peak γ-ray energies for the different EBL realizations. Some EBL spectra give rise to ``unphysical'' intrinsic blazar spectra, characterized by an exponential rise at high TeV energies. We use the F-test to show that some of these exponential rises are statistically insignificant. Consequently, statements regarding the existence of an ``IR background-TeV γ-ray crisis'' are unfounded on the basis of our present knowledge of the EBL. EBL spectra that do give rise to unphysical blazar spectra are regarded as invalid realizations of the EBL. Those that do not thus define new constraints on the EBL spectrum and are used to derive new limits on the intensity and the peak γ-ray energy of these two blazars. In particular, we derive an upper limit of ~15 nW m^-2 sr^-1 on the 60 μm EBL intensity and find the peak energies of the Mrk 421 and Mrk 501 sources to be very similar, between 0.5-1.2 and 0.8-2.5 TeV, respectively. We also show that the intrinsic spectrum of Mrk 421 during a period of intense flaring activity has a peak energy that seems to shift to higher energies at higher flux states. For the most distant blazar H1426+428 (z=0.129), most EBL realizations give rise to spectra that peak between ~1 and 5 TeV, with some suggesting peaks below 400 GeV and above ~10 TeV. These results provide important constraints on the different particle acceleration mechanisms and the generation of the γ-ray emission. Finally, we also show that uncertainties in the absolute calibration of the γ-ray energies with atmospheric Cerenkov telescopes have an important impact on the intrinsic blazar spectra. These systematic uncertainties will be improved with the next generations of telescopes that will also cover a wider range of γ-ray energies, providing further insights into the intrinsic spectrum of TeV blazars.

Whipple 10-m Telescope Observations of Potential TeV Gamma-Ray Sources Found by the Tibet Air Shower Array

Gary Walker 

Department of Physics

University of Utah

December 2004