28 Bastieri

Cross Calibration between Satellite Detectors and IACT : 

Cross Calibration between Satellite Detectors and IACT – Denis Bastieri – INFN & U. Padova keV2TeV – October 19th 2006 – Rome

Satellite detectors and the IACT: 

Satellite detectors and the IACT y using the satellite observations of steady gamma sources, as the Crab Nebula, energy calibration of the IACT becomes feasible. We show that at around 100 GeV, exploiting spectral features of the Crab Nebula, the absolute energy calibration uncertainty of Cherenkov telescopes can be reduced to <10%. B

The Object (Crab Nebula): 

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

IACT & the Crab Nebula: 

IACT & the Crab Nebula Whipple (1998): dN 3.3×10–7×E[TeV]–2.60 dE TeV•m2•s HEGRA (2000): dN 2.8×10–7×E[TeV]–2.59 dE TeV•m2•s Mohanty et al, ApP 9 (1998), 15 Crab flux: dN/dE×E2.5 = Crab flux: dN/dE Aharonian et al, ApJ 539 (2000), 317 =

IACT & the Crab Nebula (2): 

IACT & the Crab Nebula (2) “Synchronization” of Montecarlo for atmospheric showers Common model for “standard” atmosphere 15% agreement “Whipple” TeV Aharonian et al, ApJ 539 (2000), 317 Crab flux: dN/dE×E2 1 “TeV”

Energy reconstruction by IACT: 

Energy reconstruction by IACT Measuring spectral features: Intrinsic energy resolution ~ 5% dominated by atmospheric variations Cosmic Rays flux & muon yields atmospheric monitoring with LIDARs Absolute energy scale quite elusive atmospheric models doubling 03 and aerosol conc.  –6% halving 03 and aerosol conc.  +4% uncertainties on MC (atmo showers) estimated to be around 30%

Satellite vs. IACT: 

Satellite vs. IACT Satellite experiments • Primary detection – Test beam – Low background • Effective area ~ m2 • Duty-cycle ~ 100% • Energy < 300 GeV • Field of view ~ 1 sr IACT experiments • Secondary detection – Strong MC dependence – High background • Effective area ~ 104÷5m2 • Duty-cycle ~ 10%÷20% • Energy > 50 GeV • Field of view ~ 0.01 sr

Energy reconstruction onboard satellites: 

Energy reconstruction onboard satellites Satellites as GLAST and AGILE are calibrated in a well-controlled laboratory environment, by using test beams.  relative uncertainties of ~ 10% or better are expected Energy resolution vs. Energy GLAST and EGRET compared

GLAST: 1st year policy: 

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 off-axis by 40º  effective area reduced by a factor of 0.8

GLAST: Effective area: 

GLAST: Effective area The performance of GLAST (LAT) was studied by means of a full simulation based on Geant4 Around 100 GeV, the average value for the effective area of the LAT is ~1.3 m2 The actual on-axis eff. area of the LAT increases with higher energies

GLAST: Crab Nebula: 

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 Hillas et al, ApJ 503 (1998), 744 dN/dE ×E2

GLAST: Tackling spectral features: 

GLAST: Tackling spectral features The spectrum can be parameterised as a broken power law: Spectral index = 2.0 for E < Ebrk Spectral index = 2.6 for E > Ebrk Ebrk ~ 100 GeV and depends on the model assumed

GLAST: Crab observations: 

GLAST: Crab observations Gammas from Crab Nebula between 30 and 300 GeV detected in the first year by GLAST in survey mode as 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% 100 3249 8% 150 2988 13% 200 2818 17% dEbrk Ebrk DB et al., ApP 23 (2005), 572

Simulating the Crab with MAGIC: 

Simulating the Crab with MAGIC 50,000 gammas coming from the Crab Nebula (~50 hours of observation time) E resolution dE/E = 30% × (E/30 GeV)–0.3 Set the energy threshold Ethr = 30 GeV Unfold with a “spectrum-independent” migration matrix  Compare spectrum with template distributions of fixed spectral indices and given Ebrk (50, 100, 150, 200)

The spectral feature seen by MAGIC: 

The spectral feature seen by MAGIC dEbrk: “rms” from 100 indep. MCs MAGIC’s Ebrk moved to match GLAST one Absolute scale uncertainty will not exceed GLAST one  From 6% to 17% Gammas Ebrk seen by GLAST GLAST MAGIC 50 3763 6% 4.0% 100 3249 8% 3.5% 150 2988 13% 2.9% 200 2818 17% 5.2% dEbrk/Ebrk  30 % … or …    

Conclusions: 

Conclusions GLAST observations of steady gamma sources, as the Crab Nebula, can be used to calibrate the absolute energy scale of IACT  less than 10% at around 100 GeV A spectral break at higher energies will be harder to measure and of little help to IACT 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