first results on the high energy cosmic ray electron spectrum from fermi lat

Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 11

FIRST RESULTS ON THE FIRST RESULTS ON THE HIGH ENERGY COSMIC HIGH ENERGY COSMIC

RAY ELECTRON RAY ELECTRON SPECTRUM FROM SPECTRUM FROM

FERMI LAT FERMI LAT

Alexander Moiseev

CRESST/NASA GSFC and University of Maryland

for the Fermi LAT Collaboration


2008: New results on high energy 2008: New results on high energy cosmic ray electrons and positronscosmic ray electrons and positrons

Astrophysicists are excited: •Spectral feature at ~ 620 GeV reported by ATICATIC and PPB-BETSPPB-BETS suggests a nearby source (astrophysical or exotic)• Pamela :Pamela : increase of positron fraction above 10 GeV also suggests new source or production process at high energy• H.E.S.S.H.E.S.S. detects spectrum steepening above ~1 TeV : local source? Weaker re-acceleration?• More than 100 papersMore than 100 papers mentioning these results within a few months

PAMELA

Electron + positron results above 100 GeV

Positron fraction

model


Fermi LAT Fermi LAT CollaborationCollaboration

United States (NASA and DOE)United States (NASA and DOE)• California State University at SonomaCalifornia State University at Sonoma• Goddard Space Flight CenterGoddard Space Flight Center• Naval Research LaboratoryNaval Research Laboratory• Ohio State UniversityOhio State University• Stanford University (HEPL, KIPAC and Stanford University (HEPL, KIPAC and

SLAC)SLAC)• University of California at Santa Cruz – University of California at Santa Cruz –

SCIPPSCIPP• University of DenverUniversity of Denver• University of WashingtonUniversity of Washington

FranceFrance• CEA/SaclayCEA/Saclay• IN2P3IN2P3

ItalyItaly• ASIASI• INFN (Bari, Padova, Perugia, Pisa, INFN (Bari, Padova, Perugia, Pisa,

Roma2, Trieste, Udine)Roma2, Trieste, Udine)• INAFINAF

JapanJapan• Hiroshima UniversityHiroshima University• Institute for Space and Institute for Space and Astronautical Science / JAXAAstronautical Science / JAXA• RIKEN RIKEN • Tokyo Institute of TechnologyTokyo Institute of Technology

SwedenSweden• Royal Institute of Technology Royal Institute of Technology (KTH)(KTH)• Stockholm UniversityStockholm University

122 full members

95 affiliated scientists

38 management, engineering and technical members

68 post-doctoral members

105 graduate students


Two instruments onboard Fermi:Two instruments onboard Fermi: Large Area Telescope LATLarge Area Telescope LAT

– main instrumentmain instrument, gamma-ray telescope, , gamma-ray telescope, 20 MeV - >300 GeV energy range 20 MeV - >300 GeV energy range

– scanning (main) mode - 20% of the sky all scanning (main) mode - 20% of the sky all the time; all parts of sky for ~30 min. the time; all parts of sky for ~30 min. every 3 hoursevery 3 hours

– ~ 2.4 sr field of view, 8000 cm~ 2.4 sr field of view, 8000 cm22 effective effective area above 1 GeVarea above 1 GeV

– high energy (5-10%) and spatial (~3high energy (5-10%) and spatial (~300 at at 100 MeV and <0.1100 MeV and <0.100 at 1 GeV) resolution at 1 GeV) resolution

– 1 1 μμs timing, <30 s timing, <30 μμs dead times dead time

GLAST Burst Monitor GBMGLAST Burst Monitor GBM

5-year mission (10-year goal), 565 km circular orbit, 25.60 inclination

Fermi Gamma-ray ObservatoryFermi Gamma-ray Observatory


The LAT Instrument The LAT Instrument OverviewOverview

e+

e–

Pair-conversion gamma-ray telescope: 16 identical “towers” providing conversion of γ into e+e- pair and determination of its arrival direction (Tracker) and energy (Calorimeter). Covered by segmented AntiCoincidence Detector which rejects the charged particles backgroundSilicon-stripped tracker:Silicon-stripped tracker: 18 double-plane single-side (x and y) interleaved with 3.5% X0 thick (first 12) and 18% X0 thick (next 4) tungsten converters. Strips pitch is 228 μm; total 8.8×105 readout channels

Hodoscopic CsI CalorimeterHodoscopic CsI Calorimeter Array of 1536 CsI(Tl) crystals in 8 layers.

Segmented Anticoincidence Detector:Segmented Anticoincidence Detector: 89 plastic scintillator tiles and 8 flexible scintillator ribbons. Segmentation reduces self-veto effect at high energy.

Electronics SystemElectronics System Includes flexible, robust hardware trigger and software filters.

~1.7 m

~1 m


Launch from Cape Canaveral, June 11, 2008


Being a Being a γγ–ray telescope, LAT intrinsically is an –ray telescope, LAT intrinsically is an electron spectrometer. electron spectrometer. We onlyWe only needed to teach it needed to teach it how to distinguish how to distinguish electrons from hadronselectrons from hadrons

We found that:We found that:

• Photon reconstruction works perfectly for electronsPhoton reconstruction works perfectly for electrons

• All events above ~20 GeV are downlinkedAll events above ~20 GeV are downlinked

• Residual hadron contamination below 20% is confirmedResidual hadron contamination below 20% is confirmed

• Energy range can be extended to at least 1 TeV Energy range can be extended to at least 1 TeV

Advantages:Advantages:

• Large geometric factor (> 1 mLarge geometric factor (> 1 m22 sr) sr)

• Continuous (high duty cycle) and long observation Continuous (high duty cycle) and long observation

• Huge statistics expected: ~ ~10M electrons above 20 GeV a Huge statistics expected: ~ ~10M electrons above 20 GeV a yearyear

• No atmospheric correction is neededNo atmospheric correction is needed

But: Can not separate electrons from positronsBut: Can not separate electrons from positrons

History of Cosmic Ray electrons History of Cosmic Ray electrons topic for Fermi: topic for Fermi:


Energy reconstruction:– optimized for energy <

300 GeV; we extended it up to 1 TeV

Electron-hadron separation

- achieved needed 103 – 104 rejection power against hadrons, with hadron residual contamination < 20%Validation of Monte Carlo with the flight data:

– carefully compared MC and flight data

Assessment of systematic errors:

– uncertainty in the resulting spectrum is systematic dominated due to very large statistics

Extensive MC simulations:– different particles, all energies and

angles– comparison with beam test– accurate model of CR background

High flight statistics:– ~10 M electrons above 20 GeV a year

High precision 1.5 X0 thick tracker:– powerful in event topology

recognition– serves as a pre-shower detector

Segmented ACD:– removes gammas and contributes to

event pattern recognition

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

Main challenges:

Our strong points:

FERMI FLIGHT DATA ANALYSIS FOR FERMI FLIGHT DATA ANALYSIS FOR ELECTRONSELECTRONS

Segmented calorimeter with imaging capability:

– fraction of mm to a few mm accuracy position reconstruction depending on energy


Event energy Event energy reconstructionreconstruction

Beam Energy = 20 GeV



CAL Layer

CAL Layer

CAL Layer

En

erg

yEn

erg

yEn

erg

y

BT DataMC

1. Reconstruction of the most probable value for the event energy:

- based on calibration of the response of each of 1536 calorimeter crystals

- energy reconstruction is optimized for each event

- calorimeter imaging capability is heavily used for fitting shower profile

- tested at CERN beams with LAT Calibration Unit

Very good agreement between beam test and Monte Carlo


Energy resolutionEnergy resolution

Agreement between MC and beam test within a few percent up to 280 GeV we can be confident in MC we can be confident in MC we we have reasonable grounds to extend the energy have reasonable grounds to extend the energy range to 1 TeV relying on Monte Carlo simulations range to 1 TeV relying on Monte Carlo simulations


Electron-hadron Electron-hadron separationseparation

1.1. Goal 1Goal 1 – keep residual hadron contamination at less than 20% of the number of electrons which pass event classification, over the whole energy range

2.2. Goal 2Goal 2 – maximize the effective geometric factor for electrons

3.3. Approach Approach

- optimize and tune the selections to separate electrons from hadrons and photons (basic cuts + Classification Tree analysis ). It utilizes about 30 LAT ntuple variables from all the LAT subsystems

- use MC-generated “background” runs which contain the best known fluxes of HE charged particles on Fermi orbit in order to optimize electron selections and determine residual proton contamination

- use MC- generated uniform isotropic “all Electrons” runs in order to determine instrument response function (effective geometric factor)


Achieved electron-hadron Achieved electron-hadron separation and effective separation and effective

geometric factorgeometric factor• Candidate electrons pass on average 12.5 X0 ( Tracker and

Calorimeter added together)

• Simulated residual hadron contamination (5-21% increasing with the 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

Geometric Factor

Residual hadron

contamination

Key issue: good knowledge and confidence in Instrument Response Function


Flight event Flight event display display

Electron candidate, Electron candidate, 844 GeV844 GeV

Background Background event, 765 GeVevent, 765 GeV


Validation of the flight dataValidation of the flight data

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

Approach: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

Example for the variable (shower transverse size)

Analysis variables demonstrate good agreement between the flight data and MC


Assessment of systematic errorsAssessment of systematic errors

Contributors:Contributors: 1. 1. Uncertainty in 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 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 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.


Fermi-LAT electron spectrum from 20 Fermi-LAT electron spectrum from 20 GeV to 1 TeVGeV to 1 TeV

Submitted to PRL on March 19, 2009

Accepted April 21

Measurement of the Cosmic Ray e++e- Spectrum from 20 GeV to 1 TeV with the Fermi Large Area TelescopeA. A. Abdo et al. (Fermi LAT Collaboration)

Published 4 May 2009 See accompanying Viewpoint Physics 2, 37 (2009) Designed as a high-sensitivity gamma-ray observatory, the Fermi Large Area Telescope is also an electron detector with a large acceptance exceeding 2 m2 sr at 300 GeV. Building on the gamma-ray analysis, we have developed an efficient electron detection strategy which provides sufficient background rejection for measurement of the steeply falling electron spectrum up to 1 TeV. Our high precision data show that the electron spectrum falls with energy as E-3.0 and does not exhibit prominent spectral features. Interpretations in terms of a conventional diffusive model as well as a potential local extra component are briefly discussed.

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)

First public First public release on release on May 2, 2009 May 2, 2009 at April’s APS at April’s APS meeting in meeting in DenverDenver

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000102000018181101000001&idtype=cvips&gifs=Yes







http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Abdo%2C+A.+A.&possible1zone=author&maxdisp=25&smode=strresults&&aqs=true

http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Abdo%2C+A.+A.&possible1zone=author&maxdisp=25&smode=strresults&&aqs=true

http://link.aps.org/doi/10.1103/Physics.2.37


And finally we want to check – could we And finally we want to check – could we miss “ATIC-like” spectral feature?miss “ATIC-like” spectral feature?

We validated the spectrum reconstruction by:– comparing the results for different path length subsets– varying the electron selections – simulating the LAT response to a spectrum with an “ATIC-like” feature:

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

Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1818with 2 per degree of freedom of 9.7 / 23

Profumo - 1/8

(χ2 =9.7, d.o.f 24)

astro-ph 0905. 0636 (May 4, 2009)

Some interpretation…Some interpretation…

Spectrum can be fit by Diffuse Galactic Cosmic-Ray Source Model (electrons accelerated by continuously distributed astrophysical sources, likely SNR), with harder injection spectral index (-2.42) than in previous CR models (-2.54). All that within our current uncertainties, both statistical and systematic


Fermi CRE data exacerbates the discrepancy between a

purely secondary diffuse cosmic-ray origin for positrons

and the positron fraction measured by Pamela

Now – let’s include recent Pamela result Now – let’s include recent Pamela result on positron fraction:on positron fraction:

Harder primary CRE spectrum steeper secondary-to-primary e+/e- ratio

New “conventional” CRE models

Old “conventional” CRE Model

Profumo - 2/8

S. Profumo, APS, 050409


Need other contributors of electrons:Need other contributors of electrons:

PulsarsPulsars:: Most significant contribution to high-energy CRE:

Nearby (d < 1 kpc) and Mature (104 < T/yr < 106) Pulsars

Example of fit to both Fermi and Pamela data with known

(ATNF catalogue) nearby, mature pulsars and with a single,

nominal choice for the e+/e- injection parameters

Profumo - 4/8



What if we randomly vary the pulsar parameters

relevant for e+e- production?

(injection spectrum, e+e- production efficiency, PWN “trapping” time)

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. Maybe too many degrees of freedom, but the assumption is plausible

Profumo - 5/8



Dark matter: the impact Dark matter: the impact

of the new Fermi CRE dataof the new Fermi CRE data

1. Much weaker rationale to postulate a DM mass in the 0.3-1 TeV range (“ATIC bump”) motivated by the CR electron+positron spectrum

2. If the Pamela positron excess is from DM annihilation or decay, Fermi CRE data set stringent constraints on such interpretation

3. Even neglecting Pamela, Fermi CRE data are useful to put limits on rates for particle DM annihilation or decay

4. We find that a DM interpretation to the Pamela positron fraction data consistent with the new Fermi-LAT CRE is a viable possibility.

Profumo - 6/8

Origin of the local source is still unclear – astrophysical or “exotic”


H.E.S.S. astro-ph 0905.0105, May 1, 2009

• The measured spectrum is compatible with a power law within our current systematic errors. The spectral index (-3.04) is harder than expected from previous experiments and simple theoretical considerations

• “Pre-Fermi” diffusive model requires a harder electron injection spectrum (by 0.12) to fit the Fermi data, but inconsistent with positron excess reported by Pamela if it extends to higher energy

• Additional component of electron flux from local source(s) may solve the problem; its origin, astrophysical or exotic, is still unclear

• Valuable contribution to the calculation of IC component of diffuse gamma radiation

SUMMARY


The last thing: Positron spectrum, derivedPositron spectrum, derived from FermiFermi electron + positron spectrum and PamelaPamela

positron ratio


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

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

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


BACK-UPS


And finally we want to check – could we And finally we want to check – could we miss “ATIC-like” spectral feature?miss “ATIC-like” spectral feature?

We validated the spectrum reconstruction by:– comparing the results for different path length subsets– varying the electron selections – simulating the LAT response to a spectrum with an “ATIC-like” feature:

6 month Fermi LAT spectrum with the added spectral feature reported by ATIC, scaled for the statistics to be obtained by LAT for the same observation time:

Black points – LAT electron spectrum

Blue points: LAT spectrum + “ATIC-like” bump. LAT ΔE/E (68%, FW) = 18%. The excess would be ~ 7,000 events on the top of “background” of ~ 14,000 events between 300 and 800 GeV

Red points – the same but if LAT had twice worse energy resolutionThis 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

first results on the high energy cosmic ray electron spectrum from fermi lat

Documents

energy calorimeter

gevhigh energy

fermi lat collaboration2008

new results

separate electrons

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new source

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