chris thompson- magnetars
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MAGNETARS
Chris Thompson
CITA, University of Toronto
CMSO, Chicago2 August 2006
Collaborators:
Andrei BeloborodovRobert DuncanMaxim LyutikovPhil Arras, Andrew Cumming
OUTLINE
MAGNETAR FORMATION
GIANT SOFT GAMMA REPEATER FLARESformation of e+ e- fireballs in B>BQED,
confinement [relativistic e+e- winds]
TWISTED MAGNETOSPHERES,PERSISTENT NON-THERMAL EMISSION,AND LONG-TERM TORQUE VARIATIONS
bright > 100 keV hard X-ray emissionoptical-IR emission
Known Galactic Population of Magnetars
Soft Gamma Repeaters
Anomalous X-ray Pulsars
Two basic classes, discovered by independent methods:
Recent review: Woods & Thompson (astro-ph/0406133)
Spinning-down Neutron Stars (non-accreting)
(radio data includes recentParkes survey results)
spin period (s)
perio
d de
rivat
ive
(s/s
)
magnetars
LX ~ 1035-1036 erg/s(persistent sources)
27 Dec 2004Giant Flare
SGR 1806-20
0 100 200 300 400
Tim e, s
10
100
1000
10000
Co
un
ts/0.5
s
2 3 4 5 6
Tim e, s
0
40
80
120
160
200
Co
un
ts/7.8
ms
1
10
100
kT
,keV
a
b
E ~ 4x1046 erg
(no beaming)
⇒ B > 5x1015 G (N/100)1/2
Hurley et al. 2005, Nature, 434, 1098
(N comparable flaresover SGR active lifetime)
Magnetars from Supernova Collapse
Buras et al. 2005(astro-ph/0507135)
§ Violent convection extends close to ν-sphere:
ms Helical dynamo when ms
§ Strong magnetic buoyancy
Rν
Rgain
( rapid thermalization by νe / νe absorption; T & Murray ‘01 )_
Need to make1016 G r.m.s.!
Dynamo vs. Flux Conservation
(Braithwaite & Spruit 2004)
1. Magnetic helicity needed to stabilize B-field inconvectively stable star
3. Massive progenitor:
Bdipole ~ 1015 G + magnetic flux conservation
⇒ in H-burning core
2. Free Energy for B-field amplification much larger post-
collapse: 10-3 proto-neutron star10-8 core H-burning
4. White dwarf collapse: Bdipole < 1014 G from obs. WD fields
? Minimum Seed Field to Reach Equipartition B-field ?
5. Neutron star convection only persists for ~103 overturnsBUTMagnetic flux transport across stellar boundary
(neutrinosphere) is facilitated by intense νe fluxWHEREASBoundary of He core in the progenitor star
is stabilized to magnetic flux transport by He/H gradient
Quasi-Periodic Oscillationsin SGR Giant Flares
Detected in 2004 and (subsequently) 1998 giant flares byIsrael et al.; Watts & Strohmayer
ν= 90 Hz, 630 Hz seen in both giant flares(consistent with l = 7 and n = 0, 1 crustal elastic modes)
Seen only during parts of outburst that have i) Harder X-ray spectra andii) Diminished pulsed amplitude
Seen only at some rotational phases
⇒ Continued crustal yielding & localized current dissipation
QPOs due to: Torsional modes of solid crust (Duncan 1998)(or possibly: Torsional mode in liquid core magnetic field)
Relaxation Behavior inBursting Soft Gamma Repeaters
(earthquakes; solar flares)
SGR 1806-20 (continuous coverage 40 days 1983)
Palmer 1999, ApJ, 512, L113
↑cumulative
fluence
Something’s Creeping:
manybursts
crustal yield strain > 0.03
(untwisting motion // magnetic flux surfaces
⇒ volumetric breakdown of solid structure)
Formation of Magnetically Trapped Fireballs
Common 0.1-s Bursts;Giant Flare `Corona’
Intermediate-EnergyBursts & Flare Tails
tcool < tcurrentrelaxation
tcool > tcurrentrelaxation
Crustal yielding ⇒ Current Relaxation in Magnetosphere
⇒ Heating and thermalization of e+-e-, γ
Critical Heating Rate: 1042 erg s-1
npair ∝ exp[-mec2/kT] T↑ -- cooling rate ↓Thompson &Duncan 2001
Magnetic Confinement in SGR Flares
e-/e+ plasma, kBT ~ 1 MeV,
B ~ 1015 G ~ (10-30) BQED
Trapped plasma energy < 1044 ergs
at ⇒ Bdipole > 1014 G
e-/e+ gyroradius(indeed only lowest Landau level populated at stellar surface)
Microturbulence?
⇒
Magnetizationparameter:
Decaying Torsional Wave Turbulenceand Electrostatic heating of e+ e-
Alfven modes (quasi-transverse waves):
Alfven wave slows down when
Strong longitudinal excitationof charges:
>> 1
Alfven waves Landau dampedon parallel motion of e+-e-
Critically balanced cascade: (ISM: Goldreich & Sridhar;large σ: T+Blaes; Cho)
Anisotropic cascade:
Strong collisional couplingbetween Alfven wavepackets:
Inner Scale:
Critical Wavenumber:
Thompson 2006
(other applications to GRBs, black hole coronae)
Post-Burst Afterglow Aftermath of 1998giant flare:
Seen following severalintermediate-energySGR bursts and 1998giant flare of 1900+14
LX ~ t-0.7
Strong b.b. component
Radiative area ~ 10-2 ( 4πRNS2 )
Lyubarsky, Eichler &Thompson 2002
data
Volumetric heating of crust:
(const)
+ passive cooling
Helicity Injection into the Magnetosphere
Actively bursting magnetars show:§ Strong non-thermal X-ray emission when not bursting
§ Long term (up to years) stable variations in X-raypulse profile and spindown torque after outbursts
Helicity
decays slowly, on aresistive timescale,
in a confinedmagnetized plasma
Helical Magnetosphere
(relative twist of N/S poles)
(rad)
self-similar:
⇒
(stronger open-fieldcurrent and persistent,accelerated spindown)
Thompson, Lyutikov, & Kulkarni 2002, ApJ, 574 332
Magnetar Spindown
↓Giant 2004 Dec 27 Flare
P. Woods et al. 2005
Long TermSpindown
Variations ofBursting SGRs
> factor 4 increase in torque;stable over > 4 yrs
`Radio pulsars’ vs. `Magnetars’:
decay of magnetospheric twist:
decay of internal twist:
DUTY CYCLE OF EXTERNAL TWIST ∝ B2
(hall drift)
Multiple Resonant Cyclotron Scattering
⇒ optical depth
twisted dipole:
(in rest frame of e- / e+)
Rodrigo Fernandez & C. Thompson 2006
Thermal photons emitted from N.S. surface and multiply scattered in magnetosphere.
Monte Carlo treats mode exchange self-consistently
TLK ‘02
Pulse ProfilesX-ray Spectra
Pulsed fraction <~ 50% for uniform surface temperature
Relativistic Double Layer with e+/e- creationBeloborodov & Thompson 2006
1D circuit
Infinitely conducting boundaries, ϕ = 0
Thermal particles injected at boundaries
scale height
Pair creation regulated by threshold energyfor resonant absorption of soft X-ray photon
Landau level 0 → 1
B > 1014 Gimmediate conversion ofde-excitation photon to e+-e-
VOLTAGE ~ γresmec2/e
increasing twist
voltage
no paircreation
time
Net dissipation rate:
erg / s
Observed:
Thermal (keV) black body emission from N.S. surface:
1x1035 erg/s (⇒ regulated by core neutrino cooling)
Hard (20 - 200 keV) output:
up to 1036 erg/s (⇒ magnetospheric emission)
Hard X-ray Emission
Kuiper et al. 2006
4U 0142+61L100 keV ~ (1-10) x L1 keV
⇒ active plasma corona
Coronal transition layer:
if
NB: magnetospheric beam does appear to be stopped in transition layerthe observed thermal keV X-ray emissionis tightly regulated near 1035 erg/s (by core neutrino emission)
T + Beloborodov ‘05Beloborodov & T ‘06
Bremsstrahlungemission fromhydrostatic layerheated from above
thermal resistivity:Coulomb collisions
T & Beloborodov ‘05
Mild suppression of thermal conductivity κ ⇒
runaway heating of transition layer & copious e+-e- creation
Optical-IR Emission
M. Durant (’06)
Beloborodov &Thompson ‘06
Alternative: direct conversion of beam-drivenLangmuir mode to propagating O-mode (Eichler et al. 2002)
Charges in twisted magnetosphere emit curvature radiation in
optical-IR band
Also: absorption by ions at higher altitude andre-radiation at ion cyclotron cooling frequency (~ 0.5 eV)
4U 0142+61
;
LIR ~ IΩΩ !.
Conclusions
Magnetars differ from radio pulsars in several ways:
i) magnetospheric structure is globally non-potential
ii) persistent non-thermal optical/IR and X-ray emissionfrom closed magnetosphere
iii) presence/absence of core superfluidity (?)
Giant flares and persistent magnetospheric activityare driven by magnetic helicity loss from the deep interior
Persistent emission appears to be truly persistent inspite of some short-timescale variability
Strongest B-fields in N.S. population are probablygenerated post-collapse
Regulation of Photon Temperature inSuper-QED B-fields
Vacuum is birefringent
Photons are linearly polarized
E-mode:
O-mode:
⇒ thermal X-ray photonscan split and merge
Short bursts: T ~ 11 keV component (magnetospheric fireball)T ~ 4 keV component (re-radiation from surface)
(Hurley et al. ‘04)
Arras, Cumming, & Thompson 2004, ApJ, 608, L49
Internal Heating by Hall Drift andAmbipolar Diffusion
chemical potentialimbalance:
delayed pairingtransition of
core superfluidneutrons
(Tcn
< 6x10^8 K)
normal coreneutrons:
⇒ erg s-1
Lbb(AXP) ~102 Lbb(Vela)!
LX = 1x1035 erg s-1
OBSERVED
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