Searching for GRBs with the Single Particle Technique

1
Searching for GRBs with the Single Particle
Technique
Tristano Di Girolamo
Dipartimento di Scienze Fisiche, Universita di
Napoli Federico II
ARGO-YBJ General Meeting, October 31, 2006
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Determination of the Effective Areas

• MC simulation for both protons and photons
• energy range 1 GeV 1 TeV
• zenith angles ? 0, 10, 20, 30, 40
• code CORSIKA/QGSJet 6.204
• Ethr 0.05 MeV for electrons and photons
• Ethr 50 MeV for muons and hadrons
• cluster size taken into account
• reciprocity technique exploited (AS5000?5000
  m2)

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Resulting Effective Areas
The curves refer to 154 Clusters ( 6700 m2) and
? 20
Scaling from these values is possible for any
carpet dimension
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Convolution with Primary Spectra
Multiplicity ltE?gt (GeV) ltEpgt (GeV)
n 1 41 104
n 2 43 138
n 3 45 172
n ? 4 50 248
The spectral index ? 2 is the mean value for
GRBs as measured by EGRET
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Search for GRBs

• Started with the first GRB detection by the
  Swift satellite on
• December 17, 2004
• Up to September 2006, 33 GRBs detected by
  satellites were
• within ? 40
• Data for some GRBs are lost because of detector
  installation
• and debugging operations

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Fluence Upper Limit

• When the statistical significance n?lt 3, the 3?
  fluence upper limit for
• n 1 in the 1-100 GeV energy range is
  calculated, using the spectral
• indices ? measured by satellites (in ?E15-150
  keV for Swift)

B ? Background counts during GRB duration L? ?
Width ( gt1) of the GRB normalized fluctuation
distribution

• For GRBs with known redshift, the Kneiske et al.
  model
• for ?? absorption by the Extragalactic
  Background Light
• was taken into account

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Results for 19 GRBs analysed
GRB Sat. T90/dur (sec) ?() Redshift z ? Carpet Area (m2) n? 3? U.L. (erg/cm2)
041228 Swift 62 28.1 1.56 693 -0.34 5.810-4
050408 HETE 15 20.4 1.24 1.98 1820 -1.2 1.110-4
050509A Swift 12 34.0 2.10 1820 0.44 1.810-4
050528 Swift 11 37.8 2.30 1820 -0.03 6.210-4
050802 Swift 13 22.5 1.71 1.55 1820 0.82 8.510-5
051105A Swift 0.03 28.5 1.33 3379 -1.5 1.310-5
051114 Swift 2 32.8 1.22 3379 1.2 2.510-5
051227 Swift 8 22.8 1.31 3379 -0.89 2.110-5
060105 Swift 55 16.3 1.11 3379 1.3 1.610-4
060111A Swift 13 10.8 1.63 3379 -0.54 3.410-5
060115 Swift 142 16.6 3.53 1.76 4505 0.17 1.210-3
060421 Swift 11 39.3 1.53 4505 -0.71 1.910-4
060424 Swift 37 6.7 1.72 4505 -0.05 7.610-5
060427 Swift 64 32.6 1.87 4505 -0.39 4.110-4
060510A Swift 21 37.4 1.55 4505 0.63 1.510-4
060526 Swift 14 31.7 3.21 1.66 4505 0.63 1.510-4
060717 Swift 3 7.4 1.72 5632 1.08 1.310-5
060801 Swift 0.5 16.8 0.47 5632 0.10 4.810-6
060807 Swift 34 12.4 1.57 5632 0.61 7.610-5
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Distribution of Statistical Significances
Even if the statistics is very poor, the
distribution is consistent with a normalized
Gaussian ? we are observing background signals
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Constraints to the Extension of GRB Spectra
? 1.31
? 1.22
The full red line is our 3? upper limit as a
function of Emax, while the green dashed line is
the extrapolation of the satellite spectrum ?
a constraint to Emax can be fixed at the crossing
energy
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Work in Progress

• Assumption of the Band function for the
  spectrum of GRBs
• smooth steepening to a second power
  law index
• New calculation of limits on fluence and Emax of
  the spectrum
• Search for possible signals delayed or
  anticipated with respect
• to the satellite detection, or with a different
  time duration T90

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Conclusions

• No significant excess has been found from 19
  GRBs exploded
• within ? ? 40? in the period December 2004
  September 2006
• Comparing our fluence upper limits with
  extrapolations of satellite
• measurements, constraints to the GRB maximum
  energy can be set
• The 0.5 cm thick lead converter will increase
  our sensitivity by a
• factor ?2