Transcript Rolling Search For a GRB Cascade Signal

Rolling Search For a GRB
Cascade Signal
Brennan Hughey
March 19, 2005
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Our Story so far……
The Rolling cascade search scans the entire data sample (year 2001) for a
significant clustering of events above background
The signal MC spectrum is νe-produced cascades from a Waxman-Bahcall
(Band function) spectrum with break energies at 7.7 X 104 and 1.0 X 107 GeV
Background rejection is accomplished in 3 steps:
1. The high energy filter is applied – cuts on nhits and OMs with > 2 hits
2. Ndird(muon fit)/nhits – a loose cut is applied on this parameter
3. 6 variable svm cut – a support vector machine (which functions in a manner
similar to a neural network) is used for further reduction
Total background rejection ~106
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Two Searches
BATSE experiment finds two classes of
bursts, so we have two searches:
1 second and 100 seconds
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Support Vector Machine Selection
SVMs were chosen such that the total chance of getting a false detection was
less than 1% for both searches in the whole analysis – p value of .01 in the
event of a detection – 1 second search svm retains 80% of signal and
requires 3 events for significance. 100 second search svm retains 53% of
signal and requires 4 events for significance.
___ - signal Monte Carlo _____ - real data _ _ _ _ - background MC (dCorsika)
keep
keep
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Background rates
Number of surviving events are shown as a
function of run number
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Poissonian Distribution of Events
Chi2 test yields a result of 76% for both plots.
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Poissonian Distribution of Events
Chi2 test yields a result of 75% for left plot and 86% for right plot.
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Check - Bins With 2 Events
Occurrence of bins with 2 events is not
significant, but provides a useful check.
Real data is compared to distribution produced
by a Toy Monte Carlo
18 or more bins with 2 events will occur > 5% of
the time, so this is a little on the
high side, but not a statistically significant
fluctuation
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Cascade Effective Volume
Use of uncontained events allows effective volume greater than detector size
(but means no directional reconstruction)
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Neutrino Effective Area
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Expected Distribution of Detected
Signal Energies
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Comparison Between my Search
and Satellite-Coincident Searches
Disadvantage
Requires multiple events for a detection.
This means far lower likelihood of getting lucky from a
faint source: 1X10-4 events fluctuates to 1 way more
often than 3, so requires a particularly bright event even
more than other analyses
Limit on Waxman-Bahcall style flux from 667 equivalent
sources only ~7X10-6 Gev/cm2 s sr
(This will look better if a more realistic distribution of
fluences is used)
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Comparison Between my Search
and Satellite-Coincident Searches
Advantages:
- Can pick up GRBs missed by satellites -particularly useful
given that CGRO went down in early 2000
-
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Can detect γ-ray dark choked bursts as described in
2001 Meszaros and Waxman paper: astro-ph/0103275.
Choked bursts would produce ν as normal GRBs but
photons would fail to escape. Frequency unknown but
could be as high as ~100 times conventional GRB rate.
Utilizes cascade channel and therefore searches all sky
rather than 2π (okay, Ignacio’s analysis can do this too)
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Sphere of Sensitivity
Starting with a GRB at redshift of z=1
Bring closer: 2 effects happen
-
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Geometric effect: Fluence at Earth proportional to 1/r2
This makes a huge difference in the expected event
rate.
Relativistic effect: Spectrum changes shape as it moves
closer: Ebν increases as 1/(1+z)2.
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Sphere of Sensitivity: long bursts
Closest measured GRB redshift z=.009 (anomalous burst)
Center of Virgo Cluster
Fluence for “typical” burst used by Waxman-Bahcall
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Sphere of Sensitivity: short bursts
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Coincidences With Other
Experiments: IPN3
With framework already in place, a check for temporal
coincidence with satellite detections is fairly
straightforward
81 bursts during livetime from IPN3 catalogs time estimates
obtained mostly from Konus-Wind lightcurves
For 100 second search cuts:
Probability of 2 background events during burst times 0.01.
Probability of 3 background events during burst times 5X10-4.
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Coincidences With Other
Experiments: MILAGRO
I will also be collaborating with the MILAGRO air shower array on
AMANDA-MILAGRO coincidence studies
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Backup Slide: cuts
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Backup Slide
50 TeV
43.43%
100 TeV
48.99%
300 TeV
56.24%
700 TeV
61.01%
1100 TeV
64.08%
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Backup Slide: cuts
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Backup Slide: cuts
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Backup Slide: ndird cut
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