FermiLAT Observations of the Crab Pulsar and Nebula

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 Te. V Particle Astrophysics 13 th - 17 th July 2009 1

The Crab Nebula and Pulsar • • • Pulsar Wind Nebula G 184. 6 -5. 8 aka Crab Nebula, powered by the Crab Pulsar (B 0531+21, d. E/dt = 4. 6 x 1038 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 Te. V, emission predominantly by nonthermal processes Crab Pulsar detected and studied in all wavelength bands from the radio to high energy gamma-rays 2

Earlier gamma-ray observations of the Crab region • Crab Nebula: – EGRET observations of the synchrotron/Inverse Compton spectrum in the 70 Me. V – 30 Ge. V energy band (De Jager et al. , Ap. J 457: 253, 1996): Large uncertainties on the spectrum shapes for both components Variability observed (on year time scales) at 3. 1 level below 150 Me. V, 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) Ge. V ( MAGIC observations above 60 Ge. V (Albert et al, Ap. J 674: 1037, 2008)) Spectral energy distribution of the Crab Nebula (Horns & Aharonian, ESAPS 552: 439, 2004) 3

Earlier gamma-ray observations of the Crab region • Crab Pulsar: – EGRET observations (Nolan et al. , Ap. J 409: 697, 1993; Fierro et al. , Ap. J 494: 734, 1998; Kuiper et al. , A&A 378: 918, 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 Ge. V (MAGIC collaboration, Science 322: 1221, 2008): • Pulsations observed above 25 Ge. V • Estimation of the cut-off energy, assuming the EGRET spectrum (spectral index of 2. 022) and a simple exponential cut-off (b=1) scenario: Ec = (17. 7 ± 2. 8 ± 5. 0) Ge. V • Emitting region located well above the neutron star (NS) High energy spectral energy distribution of surface the Crab Pulsar (MAGIC collaboration, 4 2008) Science 322: 1221,

Analysis of the Fermi-LAT data • Event selection: – – 20°circular region around the Crab Pulsar radio position 8 months of data in survey mode Photons above 100 Me. V « diffuse » class events (highest quality photon data) • Temporal analysis: – Timing solution built with the TEMPO 2 timing package (Hobbs et al. , MNRAS 369: 655, 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 Ap. J 613: 962, 2004) – Extragalactic and residual backgrounds assumed as isotropic and described with a power-law spectrum 5

Fermi-LAT light curve above 100 Me. V • Events in an energydependent circular region are selected (optimized for the Crab case) • 14563 ± 240 pulsed photons above the background level The radio main pulse lags the first gammaray peak by ~0. 014 in phase �~280 µs implications on the emitting region P 1 P 2 • Light curve above 100 Me. V (LAT, black curve, binned to 0. 01 of pulsar phase) and at 1. 4 GHz (Nançay radio-telescope, red curve). Two cycles are shown. 6

Fermi-LAT light curves above 100 Me. V • The peaks are asymmetric • The positions of the peak are stable with energy • The peak half-widths and the P 1/P 2 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 Ge. V at phase ~0. 74, coincident with a radio feature (HFC 2) reported by Moffett and Hankins (Ap. J 468: 77, 1996) Light curve (binned to 0. 01 of pulsar phase except above 10 Ge. V) in different energy bands. Two cycle are shown. 7

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 Me. V ~ (9. 8 ± 0. 7 ± 1. 0)x 10 -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, Ap. J 457: 253, 1996) 8

Study of the synchrotron component • Synchrotron component: – Using COMPTEL results (without systematic errors), the spectrum below 400 Me. V can be modeled with an exponential cut-off powerlaw (as done by de Jager, 0. C. . et al. , Ap. J 457: 253, 1996). – The cut-off energy is estimated at ~100 Me. V. Spectral energy distribution of the Crab Nebula. The blue dashed line represents the fit of the synchrotron component 9

The Ge. V-Te. V 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 < B < 200 µG SED of the Crab Nebula Predictions of Atoyan & Aharonian, MNRAS 1996 100 µG 200 µG 300 µG 10

Spectral analysis of the Crab Pulsar • Analysis in the whole phase interval • Best fit above 100 Me. V obtained with a power-law with an exponential cut-off • An hyper exponential cut-off (b=2) 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) Ge. V – Flux above 100 Me. V ~ (2. 09 ± 0. 03 ± 0. 18)x 10 -6 cm-2 s-1 • Pulsed photons observed up to ~20 Ge. V Consistent with an emitting region well above the NS surface, as reported by the MAGIC collaboration 11

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 Me. V and 300 Ge. V, 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 12