Luca Latronico
INFN-Pisa (luca.latronico@pi.infn.it)
on behalf of the Fermi-LAT Collaboration

Fermi Symposium
2-5 November 2009
Washington DC Measurement 20 GeV – 1 TeV– hard (~ E-3)
– flat (no spectral features)– cutoff above ~1 TeV (HESS) Observational consequences– Pure diffusive models– pre-Fermi too soft– proper choice ofmodel params fit data
– source stochasticity
can explain hardness– Models with additional local
electron source– Many fit data well
– Local component
nature is astrophysicalor Dark MatterAbdo, A. A. et al, PRL 102 181101 (2009)
 > 1 ref/day (>60% related to DM scenarios) Huge electron statistics (~8M/yr)– Large area, high duty cycle
 3 powerful detectors (ACD, TKR, CAL)
– All contribute to electron ID bysampling EM vs hadron shower
development
 Very accurate MonteCarlo
instrument simulation– Performance metrics, event
selection, residual
contamination
 Validations with flight andground data
– Energy reconstruction– MC simulation3 ACD: large energy deposit per tile
 TKR: small number of extra clusters around main track, large number ofclusters away from the track CAL: large shower size, low probability of good energy reconstruction4

 ACD: few hits in conjunction with track
 TKR: single clean track, extra clusters around main track clusters(preshower)
 CAL: clean EM shower not fully contained in CAL

5 Critical for high energies– Shower leakage from CAL
 Select subsample of eventswith long path-length (HI-X0)– X0>1315.9 X0 avg
1TeV shower peaks
at 10.9X0– 12 in CAL + minimum
track length in TKR +events contained in asingle CAL module Energy resolution X ~ 2 – 4– Down to 5% at 1 TeV (68%containment half-width)
 Instrument acceptance to ~5% of standard and limited toa specific portion of
instrument phase space– Much higher systematics
6 already demonstrated bysimulation of LAT response tospectral features withartificially worsened resolution
E 3! J(E) (m-2 s -1 sr -1 GeV 2)

 Consistent within their own
systematics

Input spectrum
4% #E/E (gaussian ")12% #E/E (gaussian ")25% #E/E (gaussian ")40% #E/E (gaussian ")

102

102

103

Energy (GeV) the LAT energy resolution is adequate todetect prominent spectral features
 the Fermi spectrum is NOT dependent on theenergy resolution of the bulk of the events7 Determine geomagnetic
cutoff energy as afunction of geomagnetic
orbital coordinates– Higher McIlwainL,
lower cutoff energy
 Measure spectrum forprimary component
above cutoff
 Recombine spectra into
global spectrum

see M. Pesce-Rollins poster P4 – 1248Extended Energy Range (7 GeV – 1 TeV) – One year statistics (8M evts)

 Two-component scenario– Modified background(primary electron index at
source γ0 = -2.70)– Additional local component(index = -1.5; Ecut = 1.0 TeV)
– Fits Pamela data too Revised diffusion model– compliant with gamma-ray
data (Fermi) and other CR
measurements– Modifications to standard
diffusion and propagation
processes– Modifications to solar
modulation effects

see D. Grasso poster P4 - 237

 Fermi CRE measurement extended down to 7 GeV and to 1year statistics
 Event selection checks with long path-length requests indicateno dependence of the measured spectrum on energy
resolution
 Spectrum adds valuable information below 10 GeV wherestrong constraints to propagation models can be imposed
 Several possible interpretations– Revised diffusion model– Extra component, astrophysical or Dark Matter Further work
– extend energy above 1 TeV to find TeV spectral cut-off– Reduce systematics to constrain different components inthe overall spectrum– Search for anisotropies (see poster P4 – 121 Vasileiou &Mazziotta)