Pulsar High Energy Emission Models: What Works and What Doesn't
Pulsar High Energy Emission Models: What Works and What Doesn't
• “Standard” outer magnetosphere models - successes
• Shortcomings of the models
• Next steps?
Alice K. HardingNASA Goddard Space Flight Center
“Standard” outer magnetosphere models - successes
“Standard” outer magnetosphere models - successes
• Outer gap, slot gap and separatrix layer models can roughly produce observed gamma-ray light curves – but require thin emission layers
• Predicted cutoff energies for curvature radiation roughly match observed cutoff energies
• Variation of curvature radius similar to phase resolved cutoff energies
Pulsar accelerator geometriesPulsar accelerator geometriesPulsar accelerator geometriesPulsar accelerator geometries
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γγ→ e±
€
γ→ e±
pair-starvedpolar cap
slotgap / TPC
Accelerator GeometriesAccelerator GeometriesAccelerator GeometriesAccelerator Geometries
Narrow accelerators require screening of E field by pairs
Narrow accelerators require screening of E field by pairs
Caustic emissionCaustic emission• Photons emitted parallel to Photons emitted parallel to
dipole magnetic fielddipole magnetic field• Aberration and time-of-flight Aberration and time-of-flight
delaysdelays• Trailing outer edge of open Trailing outer edge of open
volumevolume
Caustic emissionCaustic emission• Photons emitted parallel to Photons emitted parallel to
dipole magnetic fielddipole magnetic field• Aberration and time-of-flight Aberration and time-of-flight
delaysdelays• Trailing outer edge of open Trailing outer edge of open
volumevolume
Emission on trailing field Emission on trailing field lineslines
• Bunches in phaseBunches in phase• Arrives at inertial Arrives at inertial
observer observer simultaneouslysimultaneously
Emission on trailing field Emission on trailing field lineslines
• Bunches in phaseBunches in phase• Arrives at inertial Arrives at inertial
observer observer simultaneouslysimultaneously
Emission on leading field Emission on leading field lineslines
• Spreads out in phaseSpreads out in phase• Arrives at inertial Arrives at inertial
observer at different observer at different timestimes
Emission on leading field Emission on leading field lineslines
• Spreads out in phaseSpreads out in phase• Arrives at inertial Arrives at inertial
observer at different observer at different timestimes
Formation of causticsFormation of causticsFormation of causticsFormation of caustics
Slot gap
Outer gap
= 300 = 600 = 900
Sky distribution of intensitySky distribution of intensitySky distribution of intensitySky distribution of intensity
Phase
Observer angle
Phase
Light curves in force-free geometryLight curves in force-free geometryLight curves in force-free geometryLight curves in force-free geometry
Bai & Spitkovsky 2010
Young Radio-loud PulsarsYoung Radio-loud Pulsars
9
PSR J1028-5819 (Abdo et al., ApJL 695, 72, 2009)
PSR J2021+3651 (Abdo et al., ApJ 700, 1059, 2009)
PSR J0205+6449 (Abdo et al., ApJL 699, 102, 2009)PSR J1048-5832 & J2229+6114 (Abdo et al., ApJ 2009 accepted)
PSR J1420-6048 (Weltevrede et al., ApJ 2009 submitted)
16 Pulsars Found in Blind 16 Pulsars Found in Blind SearchesSearches
After 4 months of data taking, 16 pulsars were found with blind search technique!(Abdo et al., Science 325, 840, 2009).
13 were unidentified sources for EGRET
Radio-loud millisecond pulsarsRadio-loud millisecond pulsars
Fermi LAT detected pulsed gamma-Fermi LAT detected pulsed gamma-ray emission from J0030+0451, ray emission from J0030+0451, making it the first firm detection of an making it the first firm detection of an MSP in gamma rays (Abdo et al., ApJ MSP in gamma rays (Abdo et al., ApJ 699, 1171, 2009).699, 1171, 2009).
After 9 months of data taking, the LAT After 9 months of data taking, the LAT had detected 8 gamma-ray MSPs had detected 8 gamma-ray MSPs (Abdo et al. Science 325, 848, 2009).(Abdo et al. Science 325, 848, 2009).
For the first time, a population of For the first time, a population of gamma-ray MSPs has been gamma-ray MSPs has been observed.observed.
10
PSR J0034-0534
Modeling young pulsar light curvesModeling young pulsar light curvesModeling young pulsar light curvesModeling young pulsar light curves
(, , w) = (73, 53, 0.07)
Decesar et al., in prep.
TPC
Off-peak emission too high
(, , w) = (68, 26, 0.05)
TPC
Modeling MSP Light CurvesModeling MSP Light CurvesModeling MSP Light CurvesModeling MSP Light Curves
OGTPC~ 0.45~ 0.45
~ 0.16~ 0.16
OG1 (80,70)
TPC1 (70,80)
= 70o, P = 5 ms
γ-ray
Radio
PSR J0030+0451PSR J0030+0451
Venter, Harding & Guillemot 2009
PSR J0613-0200PSR J0613-0200PSR J0613-0200PSR J0613-0200
OGTPC
OG1 (30,60)
TPC2 (30,60)
Radio
= 30= 3000, , = 60 = 6000
Radio 1400 MHz
Radio 1400 MHz
γ-ray
Predicted CR cutoff energies roughly match observed cutoff energies
Balance CR losses with acceleration gain
Steady-state Lorentz factor
Curvature radiation peak energy:
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εCR =2
3
λ cγCRR3
ρ c
=3
2
⎛
⎝ ⎜
⎞
⎠ ⎟7 / 4
E ||
e
⎛
⎝ ⎜
⎞
⎠ ⎟3 / 4
λ cρ c1/ 2 ≈ 3 GeV
1/ 42|| 73
2 102
cCRR
E
e
γ
€
eE|| = ˙ γ CR =2e2γ 4
3ρ c2
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E || ~ w2BLC
Observed cutoff energies show remarkably small spreadObserved cutoff energies show remarkably small spread
Phase resolved cutoff energiesVela
Geminga
J1836+5925CTA1
Emission radius
Curvature radius
Light curve
= 700
Model variation of emission, curvature Model variation of emission, curvature radiusradiusDeCesar et al., in prep.
Shortcomings of the modelsShortcomings of the models• Energetics
too small for both slot gap and outer gap models by a factor of 5 – 10 (for narrow gap widths)
• Lack of sufficient pairs –
for either PWNe: Crab nebula requires ~1040 s-1 particles pair multiplicity ~ 106
or MSPs with narrow gaps emission
• Dipole field geometry and lack of current closure
€
Lγ = ˙ n γmc 2
near the surfacenot efficient screening
Ne2/Ne1 = 10-3-102
Sturner & Dermer ‘94Hibschmann & Arons ’01
higher above the surfaceE// screening
Ne2/Ne1 = 10-105
Daugherty & Harding ’82
Zhang & Harding ‘00
e (1-10 TeV)
CRCR< 50 GeV< 50 GeVCRCR< 50 GeV< 50 GeV
SYNSYN
ICSICS
e±
X(surface)X(surface)
X(surfacesurface)
ICSICS
SYNSYNe±
e±
e±
e±
e±
e (0.05-500 GeV)
γ + B eγ + B e
Polar cap pair cascadesPolar cap pair cascadesPolar cap pair cascadesPolar cap pair cascades
Pulsar pair cascadesPulsar pair cascadesPulsar pair cascadesPulsar pair cascadesPolar cap pair death linesPolar cap pair death lines
Harding & Muslimov 2002, Harding et al. 2005
PC cascade pair spectrumPC cascade pair spectrum
Pair energies can reach 100 GeV Pair energies can reach 100 GeV
Cascade pair multiplicityCascade pair multiplicity
Above/below CR pair death line – high/low pair multiplicity (for dipole field)Above/below CR pair death line – high/low pair multiplicity (for dipole field)
High pair multiplicityHigh pair multiplicity
Low pair multiplicityLow pair multiplicity
What about millisecond pulsars?What about millisecond pulsars?• Many detected by Fermi• Pair cascades in dipole fields not expected to have high
multiplicity ‘pair starved’• But gamma-ray light curves show narrow peaks, not in phase
with radio narrow acceleration gaps
Field geometry and current closureField geometry and current closure
• Emission models assume retarded vacuum dipole field
• Emission models ignore current closure
• Force-free current is not the same as GJc assumed by models
Force-free magnetosphereSpitkovsky 2008
Bai & Spitkovsky 2010
NS SURFACE
e+
i+
e-
e-
Next steps• Understand acceleration in self-consistent global geometry?
(not likely anytime soon!)• Accelerate both signs of charge above polar cap?
e-
e+
• Space-charge limited ion (as well as electron)acceleration
• Lorentz-boosted pair production(Cheng & Ruderman 1977)
• Positrons initiate pair cascade
QuestionsQuestions
• How do MSPs (and maybe all pulsars) produce higher pair multiplicities?
• How do polar cap pair cascades link to global magnetosphere currents (and what happens when pulsars are pair-starved)?
• How do thin gaps produce observed gamma-ray luminosity?