225 research outputs found
The TeV Energy Spectrum of Mrk 421 Measured in A High Flaring State
The BL Lac object (blazar) Mrk 421 was observed during its outburst in April
2004 with the Whipple 10 m telescope for a total of about 24.5 hours. The
measured gamma-ray rate varied substantially over the range from 4 to 10
gamma's/min and eventually exceeded the steady gamma-ray rate of the Crab
Nebula (standard candle) by a factor of 3. The overall significance of the
gamma-ray signal exceeded 70 sigma and the total number of excess events was
more than 10,000. The signal light curve does not show any particular
variability pattern. This unique Mrk 421 outburst enabled the measurement of a
high quality spectrum of very high-energy gamma rays in a high state of
emission. This spectrum is a power-law and it extends beyond 10 TeV.Comment: 4 pages, 5 figures, Proc. 30th International Cosmic Ray Conference,
Merida, Mexico, 200
OSETI with STACEE: A Search for Nanosecond Optical Transients from Nearby Stars
We have used the 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 STACEE's sensitivity to be
approximately 10 photons per square meter 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.Comment: 38 pages, 12 figures. Accepted for publication in Astrobiolog
Very high-energy observations of the two high-frequency peaked BL Lac objects 1ES 1218+304 and H 1426+428
We present results of very-high-energy gamma-ray observations (E > 160 GeV)
of two high-frequency-peaked BL Lac (HBL) objects, 1ES 1218+304 and H 1426+428,
with the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE). Both
sources are very-high-energy gamma-ray emitters above 100 GeV, detected using
ground-based Cherenkov telescopes. STACEE observations of 1ES 1218+304 and H
1426+428 did not produce detections; we present 99% CL flux upper limits for
both sources, assuming spectral indices measured mostly at higher energies
Observations of the BL Lac 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.Comment: Accepted for publication in the Astrophysical Journa
Multiwavelength Observations of Markarian 421 in March 2001: an Unprecedented View on the X-ray/TeV Correlated Variability
(Abridged) We present a detailed analysis of week-long simultaneous
observations of the blazar Mrk421 at 2-60 keV X-rays (RXTE) and TeV gamma-rays
(Whipple and HEGRA) in 2001. The unprecedented quality of this dataset enables
us to establish firmly the existence of the correlation between the TeV and
X-ray luminosities, and to start unveiling some of its more detailed
characteristics, in particular its energy dependence, and time variability. The
source shows strong, highly correlated variations in X-ray and gamma-ray. No
evidence of X-ray/gamma-ray interband lag is found on the full week dataset (<3
ks). However, a detailed analysis of the March 19 flare reveals that data are
not consistent with the peak of the outburst in the 2-4 keV X-ray and TeV band
being simultaneous. We estimate a 2.1+/-0.7 ks TeV lag. The amplitudes of the
X-ray and gamma-ray variations are also highly correlated, and the TeV
luminosity increases more than linearly w.r.t. the X-ray one. The strong
correlation supports the standard model in which a unique electrons population
produces the X-rays by synchrotron radiation and the gamma-ray component by
inverse Compton scattering. However, for the individual best observed flares
the gamma-ray flux scales approximately quadratically w.r.t. the X-ray flux,
posing a serious challenge to emission models for TeV blazars. Rather special
conditions and/or fine tuning of the temporal evolution of the physical
parameters of the emission region are required in order to reproduce the
quadratic correlation.Comment: Correction to authorship. Minor editorial changes to text, figures,
references. 22 pages (emulateapj), 12 figures (47 postscript files) Published
in ApJ, 2008 April 20 (ADS: 2008ApJ...677..906F
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