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