the fermi pulsar revolution - istituto nazionale di fisica...
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The Fermi pulsar revolution
Patrizia Caraveo
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Fermi LAT FIRST Fermi LAT FIRST resultresult
• 2008The Fermi Large Area Telescope discovers the pulsar in the young galactic supernova remnant CTA 1Abdo, A. A. et al. 2008, Science, 322,
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Up Up toto date date γγ--rayray pulsar pulsar censuscensus
64 64 gammagamma--rayray emittingemitting neutronneutron starsstars25 gamma-selected NSs (24 new + Geminga)25 radio pulsars14 msec radio pulsar
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Neutron stars are efficient particleaccelerators
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Accelerators and global modelsAccelerators and global modelsAccelerators and global models
Radio/Polar cap
beam
Slotgap
Outergap
Magneticaxis
Rotationaxis
Openmagneticfield lines
Gammarays
αζInclination angle
Observer angle
Accelerator Accelerator gapsgaps
Charges Charges ((ee++ee--))
Global Global currentscurrents
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Gamma-ray astronomy tells us that the magnetosphere of a NS (of sufficient Edot) is filledwith high energy e+ e-
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out
in
Magnetic field lines are 2-way streetsOUT γ−rays, non thermal X-rays- towards PWNeIN thermal X-rays
in
out
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Observational Constraints
• γ-ray light curves# of pulses and how they change with energy
• γ-ray spectra (averaged and space resolved)• comparison with radio light curves
-alignment/misalignment- delay
• lack of radio emission• comparison with X-ray behaviour• energetics Lγ vs. Edot
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Radio Radio PSRsPSRs’’ light light curvescurves
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Gamma-ray intensity of Vela pulsar versus energy and phase
Fermi Large Area Telescope Observations of the Vela PulsarAbdo, A. A. et al. 2009, ApJ, 696, 1084 arXiv: 0812.2960
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PhasePhase--averagedaveraged Vela Vela spectrumspectrum
b
Exponential cutoff
b<1
Γ = 1.38 ± 0.02 ± 0.07Eo= 1.36 ± 0.15 ± 1.0GeVb = 0.69 ± 0.02 ± 0.18
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P1
P2
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More radio PSR light More radio PSR light curvescurves
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More light More light curvescurves
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Spectral fitting
b=1 high altitude emission.(strong magnetic fields near surface "absorb" gammas.)
Spectral index Γ~1.5Cutoff E0 ~2.5 GeVEnergy flux > 100 MeV~ 4.3e-10 ergs/cm²/s
b
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Millisecond pulsarsAbdo et al 2009 Science 325 848
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Blind search pulsarsAbdo et al 2009 Science 325 840
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Geminga pulsar with Fermi LAT
P0 = 237 ms 300
Phase-resolved
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GammaGamma--RayRay pulsarspulsars light light curvescurves
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First Fermi LAT Pulsar CatalogueFirst Fermi LAT Pulsar Catalogue
A global viewDistances
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F is the integral energy flux.
fΩ – beam geometry
The distance D is a big source of uncertainty:• For radio pulsars, DM (dispersion measure, the electron column density along the
line of sight) gives a distance estimation, via e.g. Cordes & Lazio NE2001 electron model.• Can be off by +/- 50%. Or by a factor of 3.• For nearby pulsars, parallax.• For X-ray sources, absorption, plus a model.• If associated with a known object… e.g. HII doppler shift of the co-located cloud…)
Energetics
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MODELSMODELS
M. Pierbattista, I. Grenier , A.Harding but also Romani, Watters
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GammaGamma--ray ray luminosityluminosity versus versus spindownspindown powerpower
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PulsarsPulsars’’ facts facts
• Of the 24 pulsars discovered by Fermi thus far, only 3 have been detected in radio, despite deep searches on all 24.
• Including Geminga, about 50% of known younggamma-ray pulsars are radio quiet (despite the significantly reduced flux sensitivity of the blind search compared to folding gamma-ray data with known radio ephemerides)
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About the LAT pulsars
• Generally (but not always), two peaks separated by ½ rotation.
• Generally (but not always), gamma peak offset from radio.
• Exponential cut-offs at ~1 to ~3 GeV. (not super-exponential)
• Favors outer magnetospheric emission.
• Edot's as low as 3E33 erg/s
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