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Results listed here at the CWRU STACEE web site are early results (through June, 2000). For a complete summary of current results and publications (including the latest STACEE observations of extra-galactic sources and the detection of Markarian 421 in outburst, see the main STACEE Collaboration Home Page at UCLA.
6/22/00: The official analysis of the STACEE 1998-99 Crab data has been completed. We detect the Crab Nebula at high significance (6.75 sigma) at an energy threshold of 190 +/- 60 GeV, and place a limit of 5.5% at the 90% confidence level for the fraction of pulsed emission from the Crab pulsar above this energy threshold. The results are written up in Scott Oser's completed Ph.D. thesis and have also been submitted to the Astrophysical Journal (see our preprint in astro-ph/0006304).
Excerpts from Scott Oser's Ph.D. thesis defense talk. Click on any thumbnail image to enlarge.
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Detailed ray-trace simulations of heliostat optics were made and compared with
CCD images of sunlight reflected from real heliostats, called "sunspots". Each
of four figures shown is a plot of the percentage of light contained within a
circular contour centered on the centroid of the sunspot versus the radius of
that contour. The red curves were measured from the CCD data, and the black curves
represent multiple Monte Carlo trials attempting to reproduce the observed sunspot
intensity profile. The data and the simulations agree quite well on a statistical
basis. Such plots assisted us in understanding the performance of our heliostats
(pointing accuracy, alignment and focusing of individual heliostat facets) during
the past observing season, and also in fine-tuning the optics for optimal performance
in the upcoming observing season. |
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Monte Carlo simulations of the overall response of our detector to showers
initiated by gamma-rays and by hadrons produce this effective area curve. |
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Multiplying our effective area curve by an appropriate expected energy spectrum
for the objects STACEE will view (in this case, the Crab Nebula) produces a distribution
of energies of gamma-ray photons successfully detected by STACEE. This distribution
peaks at about 190 GeV, our quoted energy threshold. |
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The Cherenkov wavefront of the air shower produced by a gamma-ray in the range
of energies which STACEE is designed to study (50-250 GeV) are spherical to a
good approximation, as opposed to wavefronts of air showers produced by protons,
which are irregular in shape. Fitting the wavefront to a spherical shape and removing
events with large chi-square is therefore expected to increase our signal. The
top plot shows the cumulative distribution of chi-squares for events taken with
the Crab in the field of view (small dots) and without it (solid line). The bottom
plot shows the distribution for the excess events, which peaks at chi-square less
than 1, as we expect for gamma-rays according to our simulations. |
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We see a strong excess (5.27 sigma) in the raw data. After cutting events,
first based on the re-imposition of the trigger conditions in software, and then
on the chi-square of a fit to a spherical Cherenkov wavefront -- cuts which should
select for gamma-rays -- the significance of the signal increases to 6.75 sigma. |
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The distribution of pairwise significances in the final data set has a positive
mean and unit width, as would be expected for a gamma-ray signal. |
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Plotting the observed excess vs. the phase of the Crab Pulsar (as determined
from radio observations), we see a distribution which, according to the H-test
(h = 0.12), is consistent with being flat. This allows us to put an upper limit
on the pulsed fraction of the total gamma-ray emission at about 5.5% at the 90%
confidence level. |
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Using our effective area curve, we can calculate the total flux of gamma-rays
from the Crab Nebula observed by STACEE above its energy threshold of 190 GeV.
The results are consistent with observations made by other groups (CAT and Whipple
are shown on this plot). |
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Conclusion: Just as we suspected, the Crab Nebula really is there... |
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