MarieHlne Grondin

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Fermi-LAT Observations of the Crab Pulsar and
Nebula
Marie-Hélène Grondin Marianne
Lemoine-Goumard CEN Bordeaux-Gradignan on behalf
of the Fermi-LAT Collaboration and the Pulsar
Timing Consortium
TeV Particle Astrophysics 13th - 17th
July 2009
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The Crab Nebula and Pulsar

• Pulsar Wind Nebula G184.6-5.8 aka Crab Nebula,
  powered by the Crab Pulsar (B053121, dE/dt
  4.6x1038 erg/s)?
• No SNR shell detected
• Remnant of a supernova explosion, occurred in
  1054 A.D.
• Distance (2.0 0.2) kpc, characteristic pulsar
  age 1240 yr
• Spectrum of the nebula spanning 21 decades in
  frequency, from radio to 80 TeV, emission
  predominantly by non-thermal processes
• Crab Pulsar detected and studied in all
  wavelength bands from the radio to high energy
  gamma-rays

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Earlier gamma-ray observations of the Crab region

• Crab Nebula
• EGRET observations of the synchrotron/Inverse
  Compton spectrum in the 70 MeV 30 GeV energy
  band (De Jager et al., ApJ 457253, 1996)
• Large uncertainties on the spectrum shapes for
  both components

• Variability observed (on year time scales) at
  3.1? level below 150 MeV, no variability for the
  Inverse Compton component
• no cut-off detected for the Inverse-Compton
  component
• Tcherenkov observations
• Inverse Compton peak energy estimated at (77
  35) GeV ( MAGIC observations above 60 GeV (Albert
  et al, ApJ 6741037, 2008))?

Spectral energy distribution of the Crab Nebula
(Horns Aharonian, ESAPS 552439, 2004)?
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Earlier gamma-ray observations of the Crab region

• Crab Pulsar
• EGRET observations (Nolan et al., ApJ 409697,
  1993 Fierro et al., ApJ 494734, 1998 Kuiper et
  al., AA 378918,2001)
• Large uncertainties on the spectrum at high
  energy
• No cut-off detected in the EGRET energy band
• Study of the pulsar spectroscopy

• MAGIC observations above 25 GeV (MAGIC
  collaboration, Science 3221221, 2008)
• Pulsations observed above 25 GeV
• Estimation of the cut-off energy, assuming the
  EGRET spectrum (spectral index of 2.022) and a
  simple exponential cut-off (b1) scenario Ec
  (17.7 2.8 5.0) GeV
• Emitting region located well above the neutron
  star (NS) surface

High energy spectral energy distribution of the
Crab Pulsar (MAGIC collaboration, Science
3221221, 2008)?
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Analysis of the Fermi-LAT data

• Event selection
• 20circular region around the Crab Pulsar radio
  position
• 8 months of data in survey mode
• Photons above 100 MeV
•  diffuse  class events (highest quality photon
  data)?
• Temporal analysis
• Timing solution built with the TEMPO2 timing
  package (Hobbs et al., MNRAS 369655, 2006),
  using observations made with the Jodrell Bank
  (England) and Nançay (France) radio telescopes
• Spectral analysis
• Sources with a statistical significance larger
  than 5? (after 6 months of survey) are taken into
  account
• Galactic diffuse background modelled using
  GALPROP (Strong et al ApJ 613962, 2004)?
• Extragalactic and residual backgrounds assumed as
  isotropic and described with a power-law spectrum

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Fermi-LAT light curve above 100 MeV
P1
P2

• Events in an energy-dependent circular region are
  selected (optimized for the Crab case)?
• 14563 240 pulsed photons above the background
  level
• The radio main pulse lags the first gamma-ray
  peak by 0.014 in phase ?280 µs
• ? implications on the emitting region

Light curve above 100 MeV (LAT, black curve,
binned to 0.01 of pulsar phase) and at 1.4 GHz
(Nançay radio-telescope, red curve). Two cycles
are shown.
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Fermi-LAT light curves above 100 MeV

• The peaks are asymmetric
• The positions of the peak are stable with energy
• The peak half-widths and the P1/P2 ratio decrease
  with energy
• The off-pulse window is defined as the 0.52-0.87
  phase interval
• A 2.3? significant enhancement is observed above
  10 GeV at phase 0.74, coincident with a radio
  feature (HFC2) reported by Moffett and Hankins
  (ApJ 46877, 1996)?

Light curve (binned to 0.01 of pulsar phase
except above 10 GeV) in different energy bands.
Two cycle are shown.
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Spectral analysis of the Crab Nebula

• Analysis in the off-pulse window
• Synchrotron and inverse Compton (IC) components
  are resolved
• Crab Nebula spectral parameters (renormalized to
  the total phase)

Spectral energy distribution of the Crab Nebula

• Spectral index (synchrotron) (3.99 0.12
  0.08)?
• Spectral index (IC) (1.64 0.05 0.07)?
• Flux above 100 MeV (9.8 0.7 1.0)x10-7 cm-2
  s-1
• No significant cut-off and variability can be
  observed, neither for the synchrotron nor for the
  IC component
• The spectral parameters for the IC component are
  consistent with EGRET (De Jager et al, ApJ
  457253, 1996)?

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Study of the synchrotron component

• Synchrotron component
• Using COMPTEL results (without systematic
  errors), the spectrum below 400 MeV can be
  modeled with an exponential cut-off power-law (as
  done by de Jager, 0.C.. et al., ApJ 457253,
  1996).
• The cut-off energy is estimated at 100 MeV.

Spectral energy distribution of the Crab Nebula.
The blue dashed line represents the fit of the
synchrotron component
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The GeV-TeV connection

• Inverse Compton component
• No cut-off seen with LAT only
• The LAT spectrum links up satisfactorily to the
  Cherenkov results.
• A joint fit (using Cherenkov and LAT results)
  could be performed
• Estimation of the cut-off or break energy
• Cross-calibration of the ground-based telescopes
  (Bastieri et al., 2005)?
• The IC spectrum (LAT and Cherenkov) is consistent
  with a mean magnetic field 100 µG lt B lt 200 µG

SED of the Crab Nebula
Predictions of Atoyan Aharonian, MNRAS 1996 100
µG 200 µG 300 µG
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Spectral analysis of the Crab Pulsar

• Analysis in the whole phase interval
• Best fit above 100 MeV obtained with a power-law
  with an exponential cut-off
• An hyper exponential cut-off (b2) spectrum is
  excluded at 5?
• Crab Pulsar spectral parameters

Spectral energy distribution of the Crab Pulsar

• Spectral Index (1.97 0.02 0.06)
• Cut-off energy (5.8 0.5 1.2) GeV
• Flux above 100 MeV (2.09 0.03 0.18)x10-6
  cm-2 s-1
• Pulsed photons observed up to 20 GeV
• ? Consistent with an emitting region well above
  the NS surface, as reported by the MAGIC
  collaboration

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Summary

• Large photon counts number obtained in 8 months
  of survey
• Analysis of the gamma-ray pulse profile with
  unprecedented precision
• Detailed analysis of the nebular spectrum between
  100 MeV and 300 GeV, covering the falling edge of
  the synchrotron and the rising edge of the IC
  components
• implications on physical parameters such as the
  mean magnetic field strength
• First measurement of the cut-off energy of the
  Crab Pulsar
• implications on the models of pulsed emission
• Results are reported in an upcoming paper (to be
  submitted), including a detailed phase-resolved
  spectroscopic study of the pulsed emission