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What can QED tell us aboutneutron stars and vice versa?
Jeremy Heyl, TheoreticalAstrophysics, Harvard-Smithsonian
Center for Astrophysics
Introduction
• The structure, cooling and observations ofneutron stars probes all four forces inregimes inaccessible to Earthboundexperiments. Neutron stars uniquely probe:– Strong-field general relativity
– Nuclear and neutrino physics at supernucleardensities
– QED in ultrastrong magnetic fields
The Nuclear Equation of State
• Softer equations ofstate result in morecompact stars.– Relativistic effects
– Higher surfacegravity
• Heat capacity andemissivity dependsthe composition ofthe core.
Lattimer & Prakash ‘01
Young Cooling Neutron Stars
Yakovlev et al. ‘02
• The luminosity ofyoung cooling neutronstars is a direct probeof the physics ofultradense matter.
• Is there a quark-gluonphase transition athigh chemicalpotential?
Chakrabarty et al. ‘01
ATMOSPHERE: Magnetic Atoms
ENVELOPE: Anisotropic Heat Conduction
MAGNETOSPHERE: Magnetic Lensing
MAGNETOSPHERE: Gravitational Lensing
JSH, Hernquist; Shaviv, JSH, Lithwick ‘99
Polarized Light near NeutronStars
• The thermal radiationfrom the surface ofneutron stars is slightlyto fully polarized.
• How strongly polarizedis the radiation that wecould observe andwhat can it tell us?
• c.f. Pavlov & Zavlin
PikachuTM is a trademark of the Nintendo corporation. Copyright © 1999 with Nir Shaviv (CITA) andDon Lloyd (CfA)
Contents
• Neutron Stars:– What happens to photons as they pass through
neutron star magnetospheres?
– How does this affect what we see?• Ionized hydrogen atmosphere
– How can we verify that this happens (withtoday’s instruments)?
Neutron Star Atmospheres
• In the atmosphere of a magnetized neutronstar, the opacity of photons in the twopropagating modes may vary by severalorders of magnitude.– κ || ∼ (Eγ/Ecyl) κ ⊥
• The emergent radiation is polarizedperpendicular to the local magnetic field.
• How does it all add up?
The Photon Trajectory
Strong-field QED
• In the magnetic fieldnear a neutron star,many process maybecome important thatwe cannot otherwiseprobe.
• Tracers of theseprocesses aregenerally polarized.
What happens to the polarizationof photons near neutron stars?
• The polarization of light travelling througha transparent medium evolves as:
– s is the normalized Stokes polarization vector
– Ω is the birefringent vector which for QEDpoints in the direction of the projection of themagnetic field onto the Poincaré sphere.
dlds~ = Ω~ â s~
Polarization-Limiting Radii• The polarization modes
of high-energy photonscouple further from thestellar surface thanthose of low-energyphotons.
• Also, if the direction ofthe magnetic fieldchanges appreciablyduring coupling we geta circular component.
1021Hz
1013Hz
1015Hz
1017Hz
Why Is This Important?
• Cheng and Ruderman exploited the plasma-induced low-energy PL radius to explain thehigh net polarization of pulsars in the radio.
• We can do the same here! Except:– We understand the emission in detail.
– The emission comes from the surface.
– Plasma birefringence is familiar; vacuumbirefringence is unprobed.
Low-Energy Polarized Images
Zero Hz - QED neglected 1015 Hz - Optical/UV
High-Energy Polarized Images
1021 Hz - Gamma-Ray1017 Hz - Soft X-Ray
Polarized Spectra from Neutron Stars
101012 12 GG 101014 14 GG
||-mode||-modeQEDQED
||-mode||-modeQEDQED
Total FluxTotal Flux
⊥⊥ -mode QED-mode QED
⊥⊥ -mode no QED-mode no QED
||-mode no QED||-mode no QEDTotal FluxTotal Flux
⊥⊥ -mode QED-mode QED
⊥⊥ -mode no QED-mode no QED
||-mode no QED||-mode no QED
Polarized Fractions
101012 12 GG
101014 14 GG
With QED
Without QED
Implications
• The best diagnostics for understanding NSstructure are in optical/UV not X-ray!
• A detection of the polarized thermalradiation at any energy from a NS willverify the birefringence of the magneticvacuum!
• The polarized fraction depends strongly ongeometry and field strength.
X-ray Prospects
• XPE SMEXfor thecurrentNASA AO
Sensitivitiy
Let’s Do It!
• Optical/UV emission originating from thestellar surface has been detected from:
• Performing the observations as far into theUV as possible reduces the contaminationfrom synchrotron emission plus the objectsare brighter and more polarized in the UV.
RXJ 1856.5-3754RXJ 0720-3125PSR B0656+14PSR B0950+08Geminga
U=24.4B=26.6B=24.98F(130LP)=27.1V=25.3
36ks120ks210ks900ks90ks
Polarizer forACS is only15% efficient at2500Å
Optical Prospects
• Ground-based polarimeters achieve higherthroughput. Prism analyzers can achievenearly fully transmission.
• 24ks on Keck in B would detect 20%polarization from RXJ 0720-3125.
• Polarimetry of these objects is possible(although one would need a generous TAC)using HST or 8-meter telescopes!
What can QED tell us about neutron stars and vice versa?IntroductionThe Nuclear Equation of StateYoung Cooling Neutron StarsPolarized Light near Neutron StarsContentsNeutron Star AtmospheresThe Photon TrajectoryStrong-field QEDWhat happens to the polarization of photons near neutron stars?Polarization-Limiting RadiiWhy Is This Important?Low-Energy Polarized ImagesHigh-Energy Polarized ImagesPolarized Spectra from Neutron StarsPolarized FractionsImplicationsX-ray ProspectsSensitivitiyLet’s Do It!Optical Prospects