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Measurement of the cosmic ray electron spectrum with FERMI in the energy region 20 GeV ­ 1 TeV

Alexandre Chekhtman
George Mason University / Naval Research Laboratory

for the Fermi LAT Collaboration
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Fermi LAT Collaboration
United States (NASA and DOE)
California State University at Sonoma Goddard Space Flight Center Naval Research Laboratory Ohio State University Stanford University (HEPL, KIPAC and SLAC) University of California at Santa Cruz ­ SCIPP University of Denver University of Washington

Japan · Hiroshima University
· Institute for Space and

Astronautical Science / JAXA · RIKEN · Tokyo Institute of Technology

Sweden
· Royal Institute of Technology (KTH) · Stockholm University

France

CEA/Saclay IN2P3

122 full members 95 affiliated scientists 38 management, engineering and technical members 68 post-doctoral members 105 graduate students
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Italy

ASI INFN (Bari, Padova, Perugia, Pisa, Roma2, Trieste, Udine) Triest INAF

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Fermi Gamma-ray Space Telescope
Two instruments onboard Fermi:
Large Area Telescope LAT
· main instrument, gamma-ray telescope, 20 MeV - >300 GeV energy range · scanning (main) mode - 20% of the sky all the time; all parts of sky for ~30 min. every 3 hours · ~ 2.4 sr field of view, 8000 cm2 effective area above 1 GeV · good energy (5-10%) and spatial (~30 at 100 MeV and <0.10 at 1 GeV) resolution

GLAST Burst Monitor GBM 5-year mission (10-year goal), 565 km circular orbit, 25.60 inclination
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The LAT as an electron spectrometer
Pair-conversion gamma-ray telescope: 16 identical "towers" providing conversion of into e+e- pair and determination of its arrival direction (Tracker) and energy (Calorimeter). Covered by segmented AntiCoincidence Detector which rejects the charged particles background
Silicon-strip tracker: 18 double-plane singleside (x and y) interleaved with 3.5% X0 thick (first 12) and 18% X0 thick (next 4) tungsten converters. Strips pitch is 228 m Segmented Anticoincidence Detector: 89 plastic scintillator tiles and 8 flexible scintillator ribbons. Hodoscopic CsI Calorimeter Array of 1536 CsI(Tl) crystals in 8 layers. Electronics System Includes flexible, robust hardware trigger and software filters.

~1 m

e
+

e­

~1.7 m

· LAT intrinsically is an electron spectrometer. We only needed to teach
it how to distinguish electrons from hadrons
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· expected statistics ~ 10M electrons per year for E>20 GeV
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FERMI FLIGHT DATA ANALYSIS FOR ELECTRONS Our strong points: Main challenges:
Energy reconstruction:
­ optimized for photon energy < 300 GeV; we extended it up to 1 TeV

Extensive MC simulations High precision 1.5 X0 thick tracker:
­ ­ powerful in event topology recognition serves as a pre-shower detector

Electron-hadron separation
achieved needed 103 ­ 104 rejection against hadrons

Segmented calorimeter with imaging capability Segmented ACD:
­ removes gammas and contributes to event pattern recognition

Validation of Monte Carlo with the beam tests and flight data Assessment of systematic errors

Extensive beam tests:
­ SLAC, DESY, GSI, CERN, GANIL

High flight statistics:
­
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Expecting ~10 M electrons above 20 GeV a year
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Energy reconstruction
Energy

Reconstruction of the most probable value for the event energy: - based on calibration of the response of each of 1536 calorimeter crystals

Beam test shower profiles for on-axis incidence: theta=0 (worst case ­ smallest calorimeter thickness) Beam Energy = 20 GeV

Red ­ beam test Black ­ Monte Carlo CAL Layer

- calorimeter imaging capability is heavily used for fitting shower profile - tested at CERN beams up to 280 GeV with the LAT Calibration Unit Very good agreement between shower profile in beam test data (red) and Monte Carlo (black)

Energy Energy

- energy reconstruction is optimized for each event

Beam Energy = 100 GeV

Beam Energy = 280 GeV CAL Layer

CAL Layer 6 Alexandre Chekhtman 14th Lomonosov conference, Moscow August 21, 2009


Energy resolution
Agreement between MC and beam test within a few percent up to 280 GeV we can be confident in MC

we have reasonable grounds to extend the energy range to 1 TeV relying on Monte Carlo simulations

· Energy resolution defined as full width containing 68% (95%) of events · high energy tail is exponential and drops much faster than E-3 spectrum

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Achieved electron-hadron separation and effective geometric factor
Candidate electrons pass on average 12.5 X0 ( Tracker and Calorimeter added together) Simulated residual hadron contamination (5-21% increasing with energy) is deducted from resulting flux of electron candidates Effective geometric factor exceeds 2.5 m2sr for 30 GeV to 200 GeV, and decreases to ~1 m2sr at 1 TeV Full power of all LAT subsystems is in use: tracker, calorimeter and ACD act together

Key issue: good
knowledge and confidence in Instrument Response Function
Geometric Factor Residual hadron contamination
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Validation of the flight data
Example for the variable (shower transverse size)

Task: compare the efficiency of all
"cuts" for flight data and MC events

Approach:
- Plot from the flight data the histogram of each variable involved in the electron selections, one at a time, after applying all other cuts - check if the flight histograms match the simulated ones, and account for the differences in systematic errors for the reconstructed spectrum

Analysis variables demonstrate good agreement between the flight data and MC 9
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Assessment of systematic errors
Contributors:
1. Uncertainty in effective geometric factor ­ comes from the residual discrepancy between Monte Carlo and the data. Carefully estimated for each variable used in the analysis 2. Uncertainty in determination of residual hadron contamination - comes mostly from the uncertainty of the primary proton flux (~ 20%) - we validated the hadronic interaction model with the beam test data Contributors 1 and 2 result in total systematic error in the spectrum ranging from 10% at low energy end to 25-30% at high energy end (full width)

3. Possible bias in absolute energy determination
- Included separately in the resulting spectrum as (+5, -10)% estimated from MC simulations, calorimeter calibration and CERN beam test.
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Fermi-LAT electron spectrum from 20 GeV to 1 TeV
Phys. Rev. Letters 102, 181101 (2009) Cited 38 times within a month APS Viewpoint

Total statistics collected for 6 months of Fermi LAT observations: > 4 million electrons above 20 GeV > 400 electrons in last energy bin (770-1000 GeV)
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Final check ­ could we miss "ATIC-like" spectral feature?
We validated the spectrum reconstruction by several ways including simulation of the LAT response to a spectrum with an "ATIC-like" feature:

Blue line shows how Fermi-LAT would detect this feature if its energy resolution was worse by a factor of 2

This demonstrates that the Fermi LAT would have been able to reveal "ATIC-like" spectral feature with high confidence if it were there. Energy resolution is not an issue with such a wide feature 12
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Some interpretations...
Can our result be fitted with pre-Fermi model?
Based on our interpretation paper: D.Grasso et al., astro-ph 0905. 0636 (May 4, 2009); submitted to Astroparticle Physics

Pre-Fermi Diffuse Galactic Cosmic-Ray Source Model: electrons accelerated by continuously distributed astrophysical sources, likely SNR
Pre-Fermi model

Spectrum can be fitted by model with harder injection spectral index (-2.42) than in pre-Fermi model (-2.54). All that within our current uncertainties, both statistical and systematic Remark: if we subtract the e+ fraction of the flux, using Pamela data, the e- spectrum becomes softer by ~0.1 and consequently requires softer injection spectrum
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Now include recent Pamela result on positron fraction:
Qualitative approach: the harder primary CRE spectrum is, the steeper secondary-to-primary e+/e- ratio should be. Pamela shows the opposite

New "conventional" CRE models Old "conventional" CRE Model

Precise Fermi measurement of the hard e+e- spectrum increases the discrepancy between a purely secondary origin for positrons and the positron fraction measured by Pamela
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It is becoming clear that we are dealing with at least three distinct origins of high energy electrons and positrons
· One is uniformly distributed "distant" sources, likely SNR · Another is unavoidable e+e- production in CR interactions with ISM

What creates positron excess at high energy? Nearby (d < 1 kpc) and
Mature (104 < T/yr < 106) pulsars ?

Example of fit to both Fermi and Pamela data with Monogem and Geminga pulsars and with a single, nominal choice for the e+/einjection parameter ­ works better
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What if we randomly vary the pulsar parameters, relevant for e+e- production (injection
spectrum, e+e- production efficiency, PWN "trapping" time), and include more

contributing pulsars stochastically?
These "wings" are our next target

Under reasonable assumptions, electron/positron emission from pulsars offers a viable interpretation of Fermi CRE data which is also consistent with the HESS and Pamela results. Many degrees of freedom, but the assumption is plausible and realistic
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Dark matter: the impact of the new Fermi CRE data
1. Everything said on previous slides about pulsars as sources of e+e- is applicable to DM. Dark matter origin of e+e- is not ruled out
2. If the Pamela positron excess comes from DM annihilation or decay, the Fermi CRE data set puts constraints on such interpretation (e.g. pair annihilation or decay rate for a given DM mass and diffusion setup) the DM constraints, favoring pure e , lepto-philic, or super-heavy DM models 4. Need precise spectral shape! Irregularities on the falling slope of the spectrum above ~ 1 TeV, if found, may help to determine the origin of high energy electrons, favoring nearby pulsars scenario
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3. Pamela and Fermi-LAT data tighten

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preferred

Likely excluded

Instrumental energy smearing is not included

Dark matter origin of CRE is not ruled out. Origin of the local source 18 is still unclear ­ astrophysical or "exotic"
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Future plans:
Search for anisotropy in the electron flux ­ contributes to the understanding of the "extra" source origin Study systematic errors in energy and instrument response to determine whether or not the observed spectral structure is significant ­ also critical for understanding of the source origin, as well as models constrains Work on understanding of calorimeter calibration to decrease systematic uncertainty in the absolute energy scale Expand energy range down to ~ 5 GeV (lowest possible for Fermi orbit) and up to ~ 2 TeV, in order to reveal the spectral shape above 1 TeV Measurement at higher energies require the use of bigger incident angles (currently theta < 70 degrees) Thicker CsI calorimeter ­ up to 1.5 m of CsI at 90 degrees Requires modified reconstruction algorithms for calorimeter and tracker Increase the statistics at high energy end. Each year Fermi-LAT will collect ~ 400 electrons above 1 TeV with the current selections if the spectral index stays unchanged

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SUMMARY
· Real breakthrough during last 1-1.5 years in cosmic ray electrons: ATIC, HESS, Pamela, and finally Fermi-LAT. New quality data are available · With the new data more puzzles than was before; need "multiwavelength" campaign: electrons, positrons, gammas, X-ray, radio, neutrino... · We may be coming close to the first direct detection of cosmic ray source · Source nature: astrophysical or exotic ­ unclear. Possible that other models will be suggested · More results from Fermi-LAT are coming: high energy electrons anisotropy at a level of ~ 1%, extended energy range to 5 GeV ­ 2 TeV
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