The Cosmic Ray electron spectrum below 20 GeV Melissa - PowerPoint PPT Presentation

The Cosmic Ray electron spectrum below 20 GeV Melissa Pesce-Rollins INFN–Pisa melissa.pesce.rollins@pi.infn.it on behalf of the Fermi LAT collaboration RICAP May 13, 2009

Outline ◮ Monte Carlo simulations ◮ Instrument simulation ◮ International Geomagnetic Reference Field (IGRF) ◮ Analysis strategy ◮ Event selection ◮ Estimation of the background ◮ Preliminary results ◮ Comparison between Fermi-LAT, PAMELA and AMS ◮ Searching for east-west asymmetries ◮ Count rates of secondaries particles. M. Pesce-Rollins (INFN) RICAP May 13, 2009 2 / 25

Cosmic ray electrons and the Fermi LAT Two different set of analyses, two different sources Fermi-LAT does not distinguish between e − and e + , we use the term electrons to refer to the sum of the two. Diagnostic (DGN) on-board filter ◮ Unbiased sample of all events that trigger Gamma on-board filter the LAT. ◮ Main on-board filter for γ s ◮ Prescaled 1:250 on-board(due to bandwidth limitations). ◮ High pass condition: ◮ Downlinks all events with onboard ◮ Designed for diagnostic purposes energy greater than 20 GeV ◮ And study of filter efficiencies ◮ Excellent data source for cosmic ray ◮ ∼ 20 Hz rate electrons above 20 GeV. ◮ Excellent data source of cosmic ray electrons! ◮ Overlap region (20 GeV - 100 GeV) also used for comparison of two analysis approaches. ⇒ Large fluxes for CR electrons below few GeV compensates for DGN 1:250 prescale! M. Pesce-Rollins (INFN) RICAP May 13, 2009 3 / 25

Monte-Carlo simulations ◮ Accurate detector model ◮ over 45000 volumes ◮ Physical interactions with GEANT4 ◮ Uses real LAT calibrations ◮ Monte Carlo is crucial for ◮ Event selection and LAT performance ◮ Instrument Response Functions ◮ Estimation of residual hadron contamination M. Pesce-Rollins (INFN) RICAP May 13, 2009 4 / 25

International Geomagnetic Reference Field (IGRF) ◮ We use the 10 th generation version of IGRF. ◮ http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html ◮ Fermi orbits at 565 km altitude with 25.6 ◦ inclination ◮ Fixes the geomagnetic latitude range and ◮ fixes the minimum energy at which we can measure the primary CR electrons ∼ 5 GeV. M. Pesce-Rollins (INFN) RICAP May 13, 2009 5 / 25

Selection criteria ◮ Event selection based on LAT’s Fraction of electrons after minimal quality cuts capability to discriminate between EM and hadronic showers in the subdetectors ◮ Similar approach to photon analysis ◮ Make use of Classification Trees for final boost in particle selection. ◮ Selection cuts need to be a smooth function of energy ◮ Make sure not to introduce ◮ Electron fraction has large variations over the any artificial features in the energy range of interest. ◮ The LAT does not distinguish electrons from measured spectrum positrons, we take the sum of the two ◮ Could also be a function of contributions. ◮ Therefore the term electrons refers to the sum geomagnetic latitude. of electrons and positrons. M. Pesce-Rollins (INFN) RICAP May 13, 2009 6 / 25

Geometry factor and rejection power Geometry Factor Hadron Rejection Power Preliminary Results Preliminary Results 5 10 Geometry Factor (m^2 sr) Hadron rejection power quality quality + acd quality + acd + HEEProb 0.015 quality + acd + HEEProb + cal 10 4 quality + acd + HEEProb + cal + CT 10 3 0.01 10 2 0.005 10 1 3 4 5 10 10 10 3 4 10 10 Energy (MeV) Energy (MeV) ◮ Geometry Factor: GF ( E ) = FOV · A eff ( E ) ◮ The geometry factor is small due to the 1:250 prescale of the DGN filter. ◮ Hadron rejection power at various stages of the event selection. ◮ Final boost in rejection power from the CT analysis is clear. M. Pesce-Rollins (INFN) RICAP May 13, 2009 7 / 25

Orbital averaged hadron contamination Preliminary Results Orbital Ave contamination 0.15 0.1 0.05 0 3 10 10 4 Energy (MeV) ◮ Average contamination below 20% throughout the energy range. ◮ Varies with energy. ◮ Varies for different orbital positions. ◮ Optimization of the selection cuts as a function of geomagnetic latitude still needs to be performed. M. Pesce-Rollins (INFN) RICAP May 13, 2009 8 / 25

Monte Carlo validation with flight data ◮ An ACD variable at an intermediate stage of the analysis. ◮ Total energy deposited in the ACD tiles per event. ◮ Analysis variables carefully checked over the energy range with flight data. Preliminary results M. Pesce-Rollins (INFN) RICAP May 13, 2009 9 / 25

Monte Carlo validation with flight data ◮ The low energy CT variable at an intermediate stage of the analysis ◮ Continuous probability variable ◮ Overall agreement is fair but we plan to optimize the CT variable for better agreement. ◮ Analysis variables carefully checked over the energy range with flight data Preliminary results M. Pesce-Rollins (INFN) RICAP May 13, 2009 10 / 25

Comparison with PAMELA and AMS ◮ Geomagnetic latitude interval: 0.0 ≤ λ ≤ 0.3 ◮ Good overall agreement with PAMELA and AMS. ◮ Some differences in the energy range ∼ 3 GeV and 10 GeV. ◮ Systematics not included! Estimated to be around 20% M. Pesce-Rollins (INFN) RICAP May 13, 2009 11 / 25

Comparison with PAMELA and AMS ◮ Geomagnetic latitude interval: 0.3 ≤ λ ≤ 0.6 ◮ Cutoff position in rough agreement. ◮ Systematics not included! Estimated to be around 20% ◮ Large differences at lower energies. ◮ Different fraction of time spent in each lambda interval. ◮ Different acceptance. M. Pesce-Rollins (INFN) RICAP May 13, 2009 12 / 25

Fraction of time spent in each latitude band Fraction of time Fermi and PAMELA spend in each latitude band ◮ Comparison at this stage intended to be a simple sanity check. ◮ Cutoff position is quite similar between the three experiments in both intervals. ◮ Fluxes below the cutoff are not isotropic: ◮ The three experiments have different acceptances (Fermi is accepting particles from ∼ 60 ◦ ). ◮ Larger fraction of time spent in lower geomagnetic latitude could explain the higher fluxes seen by Fermi: Fermi in 0.3 ≤ λ ≤ 0.6 interval (larger concentration of ◮ 0.0 ≤ λ ≤ 0.3, 49% of total orbital time. secondaries located near the ◮ 0.3 ≤ λ ≤ 0.6 spend 46% of total orbital time magnetic equator). ◮ Roughly half of which is spent in 0.3 ◮ PAMELA data correspond to ≤ λ ≤ 0.4 interval. August-September 2006; solar PAMELA: modulation? ◮ 0.0 ≤ λ ≤ 0.1, 23% of total orbital time. ◮ 0.3 ≤ λ ≤ 0.6 roughly equal fraction of time spent in each lambda interval. M. Pesce-Rollins (INFN) RICAP May 13, 2009 13 / 25

Searching for East West asymmetries East west ratio Flux Geomagnetic latitude interval 0.1 < λ < 0.2 ◮ Magnetic rigidity is also a function of the particle’s charge, expect to see an east/west effect on the magnetic cutoff. ◮ Clear separation between east and west intensities. ◮ Larger intensities coming from the east. M. Pesce-Rollins (INFN) RICAP May 13, 2009 14 / 25

Searching for East West asymmetries East west ratio Flux Geomagnetic latitude interval 0.4 < λ < 0.5 ◮ Cutoff energy decreases with increasing lambda as expected. ◮ Gradual decrease in the east west ratio at higher geomagnetic latitude. M. Pesce-Rollins (INFN) RICAP May 13, 2009 15 / 25

Searching for East West asymmetries ◮ Asymmetry peak value in energy decreases with increaing lambda values, this is expected due to the rigidity cutoff. ◮ The amount of asymmetry decreases with increasing lambda (maximum in the geomagnetic equator region). M. Pesce-Rollins (INFN) RICAP May 13, 2009 16 / 25

AMS secondary electrons and positrons ◮ Geographical origin of electrons (a) and positrons (b) measured by AMS 01. ◮ Secondary electrons and positrons are concentrated around the geomagnetic equator. M. Pesce-Rollins (INFN) RICAP May 13, 2009 17 / 25

Distribution of secondary electrons by Fermi Preliminary Results! ◮ Count rate of secondary electrons and positrons as measured by Fermi. ◮ Flux distribution is still work in progress. M. Pesce-Rollins (INFN) RICAP May 13, 2009 18 / 25

Conclusions ◮ Event selection in an advanced stage ◮ Orbital averaged hadron contamination well below 20% ◮ Preliminary cosmic ray electron fluxes are roughly consistent with past experiments. ◮ Still some discrepancy at high geomagnetic latitude ◮ Working to pin down the cause. ◮ Preliminary signs of east west asymmetries ◮ Trapped count rates show a concentration in the geomagnetic equitorial region (as also observed by M.Aguillar et al. , Phys. Rep., 366 , 331 (2002)) M. Pesce-Rollins (INFN) RICAP May 13, 2009 19 / 25

Future work ◮ Study the systematic uncertainties ◮ Data/Monta Carlo simulations ◮ Background model ◮ Spectrum unfolding ◮ Geomagnetic latitude dependent event selections ◮ Update the background models with Fermi electron data. M. Pesce-Rollins (INFN) RICAP May 13, 2009 20 / 25

Extra Slides M. Pesce-Rollins (INFN) RICAP May 13, 2009 21 / 25

Energy resolution ◮ Energy resolution integrated over all angles. ◮ Compatible with energy resolution of the LAT for photons. M. Pesce-Rollins (INFN) RICAP May 13, 2009 22 / 25