Using GLAST observations to calibrate MAGIC

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Using GLAST observationsto calibrate MAGIC

• Denis Bastieri
• Alessandro De AngelisFrancesco Longo

MAGIC Coll. February 22nd 2005 Berlin
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GLAST and MAGIC An Outline
B

• y using the GLAST observations of
  steady gamma sources, as the Crab Nebula, the
  energy calibration of IACT, and MAGIC in
  particular, becomes feasible. We show that at
  around 100 GeV, exploiting the features in the
  spectrum of the Crab Nebula, the absolute energy
  calibration uncertainty of Cherenkov telescopes
  can be reduced to lt10.

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Intro Satellite versus IACT

• Satellite experiments
• Primary detection
• Test beam
• Low background
• Effective area m2
• Duty-cycle 100
• Energy lt 200 GeV
• Field of view 1 sr
• High cost

• IACT experiments
• Secondary detection
• Strong MC dependence
• High background
• Effective area 1045m2
• Duty-cycle 1020
• Energy gt 50 GeV
• Field of view 0.01 sr
• Low cost

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Intro Energy rec. by GLAST

• Satellites as GLAST (or AGILE), contrary to
  IACTs, are calibrated in a well-controlled
  laboratory environment, using test beams of
  electrons and gammas,
• ? relative uncertainties of 10 or better is
  expected

Energy resolution vs. Energy GLAST and EGRET
compared
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Intro Energy rec. by MAGIC

• Measuring spectral features
• Intrinsic energy resolution lt 5
• Absolute energy scale quite elusiveenergy rec.
  in the 30300 GeV range
• dominated by uncertainties onMonte Carlo
  simulations
• dominated by uncertainties onatmospheric models
• dominated by atmospheric variations

?? 30
LIDAR
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Intro the Object (Crab Nebula)

• Why the Crab Nebula?
• Because the Crab Nebula is steady.
• Because it will be observed intensively by GLAST
  already in the first year.
• Because it will have been observed,in 2007,
  quite a lot by MAGIC.

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GLAST 1st year policy

• 1st year GLAST will observe the sky in survey
  (scanning) mode with a uniform exposure of 90.

GLAST FoV 2.4 sr ? 1/5 of the full sky ? Crab
obs 1/5 year
On average, Crab will be off-axis by 40º ?
effective area reduced by a factor of
0.8
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GLAST Effective area

• The performance of GLAST was studied by means
  of a full simulation based on Geant4.

The average value for the effective area of GLAST
Large Area Telescope (LAT) is1.3 m2 around 100
GeV
The actual on-axiseff. area of GLAST as a
function of energy is shown on the right.
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GLAST Crab Nebula

• The spectrum of the Crab Nebula in the overlap
  region is poorly known, but under different
  hypotheses on the magnetic field in a Inverse
  Compton scenario looks like the figure on the
  right.

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GLAST Tackling spectral features

• The spectrum can be parameterised as a two-slope
  spectrum
• Spectral index 2.0 for E lt Ebrk
• Spectral index 2.7 for E gt Ebrk
• Ebrk 100 GeV and depends on the model assumed

A bigger differencebetween indexes willmark
even more thespectral feature andmake the
determina-tion of Ebrk easier
The position of this spectral breakwell
determined by GLASTcan be used to calibrate
MAGIC.
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GLAST Crab observations

• Gammas from Crab Nebula between 30 and 300
  GeV detected in the first year by GLAST in survey
  modeas a function of Ebrk
• (90 data efficiency)
• taking into account
• South Atlantic Anomaly
• Data downlink failures
• Scheduled maintenances
• Ebrk fitted assuming the actual E resln. of
  GLAST.

• Gammas
• Ebrk seen by
• GLAST
• 50 3763 6.2
• 100 3249 8.2
• 150 2988 12.7
• 200 2818 17.2

dEbrk Ebrk
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MAGIC Simulating Crab

• Total flux of 50,000 gammas coming from the Crab
  Nebula in 50 hours of observation time.
• Energy resolution dE/E 30 (E/30 GeV)0.3
• Set the energy threshold Ethr 30 GeV
• Unfold with a migration matrix
• Minimise residual values, properly weighted,of
  data from template distribution with fixed
  spectral idxs and given Ebrk (50, 100, 150, 200)
• Linearly interpolate the value of Ebrk

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MAGIC Unfolding

• Divide the interval 30300 GeV into 10 equally
  spaced log bins
• Fix a Ebrk according with GLAST output
• Fill up the extended migration matrix (with
  under- and overflows)
• Normalisethe matrixrow by row
• Cut outunder andoverflows
• Apply thematrixobtainedin this way

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MAGIC Results
? 30

• The error on Ebrk is the spread of the estimate
  obtained from 100 indep. MC.
• The value of Ebrkas determined by MAGIC should
  be offset to match GLAST one.
• The absolute scale uncertainty between 30 and 200
  GeV will not exceed GLAST one it will go from
  about 6 to 17.

• Gammas
• Ebrk seen by
• GLAST GLAST MAGIC
• 50 3763 6.2 4.0
• 100 3249 8.2 3.5
• 150 2988 12.7 2.9
• 200 2818 17.2 5.2

dEbrk/Ebrk
?
?
?
?
or
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Conclusions

• GLAST observations of steady gamma sources,as
  the Crab Nebula, can be used to calibratethe
  absolute energy scale of IACT and MAGICin
  particular.
• The absolute energy uncertainty of Cherenkov
  telescopes, at around 100 GeV, can be reduced to
  lt10.
• A spectral break at higher energies will be
  harder to measure and of little help to MAGIC.
• Other features, as the exponential cut-off of AGN
  spectra, due to the interaction of AGN gammas
  with the MRF, can also be well-suited for IACT
  calibration.