X-ray Spectra from Magnetar Candidates

Magnetic Fields and Neutron Star SMagnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007urface - Cocoyoc 14 February 2007

X-ray Spectra from X-ray Spectra from Magnetar CandidatesMagnetar Candidates

A Twist in the FieldA Twist in the Field

R TurollaDepartment of PhysicsUniversity of Padova, ItalyCreditsGL Israel, S. Mereghetti, L Nobili, N Rea, N Sartore,L Stella, A Tiengo, S Zane


Galactic NS Galactic NS PopulationPopulation

Present supernova rate in the Galaxy Present supernova rate in the Galaxy ≈ 0.01 yr ≈ 0.01 yr -1-1

The Galaxy is ≈ 10 Gyr old 10The Galaxy is ≈ 10 Gyr old 1088 – –101099 neutron stars neutron stars

Most neutron stars are known through Most neutron stars are known through their pulsed radio-emissiontheir pulsed radio-emission

Galactic pulsar population ≈ 10Galactic pulsar population ≈ 1055 ( (> > 1500 detected)1500 detected)


Pulsars and…Pulsars and… The majority of neutron stars are old, The majority of neutron stars are old,

dead objects dead objects Observations in the X- and Observations in the X- and γγ-rays -rays

revealed the existence of populations of revealed the existence of populations of radio-quiet neutron starsradio-quiet neutron stars X-ray binariesX-ray binaries X-ray dim isolated neutron starsX-ray dim isolated neutron stars Soft Soft γγ-repeaters-repeaters Anomalous X-ray pulsarsAnomalous X-ray pulsars

ISOLATED


Soft Gamma Repeaters - ISoft Gamma Repeaters - I Rare class of sources, 4 confirmed (+ 1): Rare class of sources, 4 confirmed (+ 1):

SGR 1900+14, SGR 1806-20, SGR 1627-41 SGR 1900+14, SGR 1806-20, SGR 1627-41 in the Galaxy and SGR 0526-66 in the LMC in the Galaxy and SGR 0526-66 in the LMC

Strong bursts of soft Strong bursts of soft γγ-/hard X-rays: L -/hard X-rays: L ~ ~ 10104141 erg/s, duration < 1 s erg/s, duration < 1 s

Bursts from SGR 1806-20 (INTEGRAL/IBIS,,Gőtz et al 2004)


Soft Gamma Repeaters - IISoft Gamma Repeaters - II Much more energetic “Giant Flares” (GFs, Much more energetic “Giant Flares” (GFs, L L

≈ 10≈ 104545-10-104747 erg/s) detected from 3 sources erg/s) detected from 3 sources No evidence for a binary companion, No evidence for a binary companion,

association with a SNR in one caseassociation with a SNR in one case Persistent X-ray emitters, Persistent X-ray emitters, L L ≈ 10≈ 103535 erg/s erg/s Pulsations discovered both in GFs tails and Pulsations discovered both in GFs tails and

persistent emission, P ≈ 5 -10 spersistent emission, P ≈ 5 -10 s Huge spindown rates, ṖHuge spindown rates, Ṗ/P /P ≈≈ 10 10-10 -10 ssss-1-1

(Kouveliotou et al. 1998; 1999)(Kouveliotou et al. 1998; 1999)


Anomalous X-ray Pulsars - IAnomalous X-ray Pulsars - I Seven sources known (+ 1 transient): Seven sources known (+ 1 transient):

1E 1048.1-5937, 1E 2259+586, 4U 1E 1048.1-5937, 1E 2259+586, 4U 0142+614, 1 RXS J170849-4009, 1E 1841-0142+614, 1 RXS J170849-4009, 1E 1841-045, CXOU 010043-721134, AX J1845-0258 045, CXOU 010043-721134, AX J1845-0258 (+ XTE J1810-197) (+ XTE J1810-197)

Persistent X-ray emitters, Persistent X-ray emitters, L L ≈ 10≈ 103434 -10 -103535 erg/s erg/s Pulsations with P ≈ 5 -10 sPulsations with P ≈ 5 -10 s Large spindown rates, ṖLarge spindown rates, Ṗ/P /P ≈≈ 10 10-11 -11 ssss-1-1

No evidence for a binary companion, No evidence for a binary companion, association with a SNR in three casesassociation with a SNR in three cases


Anomalous X-ray Pulsars - Anomalous X-ray Pulsars - IIII

Bursts of soft Bursts of soft γγ-/hard X--/hard X-rays quite similar to those rays quite similar to those of SGRs (AXPs much less of SGRs (AXPs much less active though, bursts active though, bursts from two sources only)from two sources only)

Time (sec)

Woods & Thompson (2005)


R < ctrise ≈ 300 km: a compact objectPulsed X-ray emission: a neutron star

A Tale of Two Populations ?A Tale of Two Populations ?SGRs: bursting X/γ-ray sources

Single class ofobjects

AXPs: peculiar class of steady X-ray sources

A Magnetar


MagnetarsMagnetars Strong convection in a rapidly rotating (P Strong convection in a rapidly rotating (P

~ 1 ms) newborn neutron star generates ~ 1 ms) newborn neutron star generates a very strong magnetic field via dynamo a very strong magnetic field via dynamo actionaction

Magnetars: neutron stars with surface Magnetars: neutron stars with surface field B > 10 Bfield B > 10 BQEDQED ~ 4 x10 ~ 4 x101414 G (Duncan & G (Duncan & Thomson 1992; Thomson & Duncan 1993)Thomson 1992; Thomson & Duncan 1993)

Rapid spin-down due to magneto-dipolar Rapid spin-down due to magneto-dipolar losses, losses,

1121411 ss )G 10/(10 PBP


Why magnetars ?Why magnetars ? . . No evidence for a companion starNo evidence for a companion star Spin down to present periods in ≈ Spin down to present periods in ≈

10104 4 yrs requires B > 10yrs requires B > 101414 G G Large measured spin-down rates Large measured spin-down rates Quite natural explanation for the Quite natural explanation for the

burstsbursts

IEL rotX

SPIN - DOWN ENERGY LOSSX

-RA

Y L

UM

INO

SITY

SGRs+AXPsL X = Ė rot

Photon splitting threshold

SGRs + AXPs

PSRs

High-field PSRs


SGRs and AXPs X-ray SGRs and AXPs X-ray Spectra - ISpectra - I

0.5 – 10 keV emission well 0.5 – 10 keV emission well represented by a blackbody plus a represented by a blackbody plus a power lawpower law

SGR 1806-20 (Mereghetti et al 2005)

AXP 1048-5937 (Lyutikov & Gavriil 2005)


SGRs and AXPs X-ray SGRs and AXPs X-ray Spectra - IISpectra - II

kTkTBB BB ~~ 0.5 keV, does not change 0.5 keV, does not change much in different sourcesmuch in different sources

Photon index Photon index ГГ ≈≈ 1 – 4, 1 – 4, AXPs tend to AXPs tend to be softerbe softer

SGRs and AXPs persistent emission is SGRs and AXPs persistent emission is variable (months/years)variable (months/years)

Variability mostly associated with the Variability mostly associated with the non-thermal componentnon-thermal component


Hard X-ray EmissionHard X-ray EmissionMereghetti et al 2006INTEGRAL revealed

substantial emission in the 20 -100 keV band from SGRs and APXs

Hard power law tails with Г ≈ 1-3, hardeningwrt soft X-ray emissionrequired in AXPs Hard emission pulsed


Hardness vs Spin-down RateHardness vs Spin-down Rate

Harder X-ray spectrum

Larger Spin-down rate

Correlation Correlation between spectral between spectral hardness and hardness and spin-down rate in spin-down rate in SGRs and AXPs SGRs and AXPs (Marsden & White (Marsden & White 2001)2001)Correlation holds Correlation holds also for different also for different states within a states within a single source single source (SGR (SGR 1806-20, Mereghetti et 1806-20, Mereghetti et al 2005; al 2005; 1 RXS 1 RXS J170849-4009, Rea et al J170849-4009, Rea et al 20052005))


SGR 1806-20 - I SGR 1806-20 - I SGR 1806-20 displayed a gradual increase in the level of activity during 2003-2004 (Woods et al 2004; Mereghetti et al 2005)

enhanced burst rate increased persistent luminosity

The 2004 December 27 EventSpring2003

Spring2004

Autumn2003 Autumn

2004

Bursts / day (IPN)

20-60 keV flux (INTEGRAL IBIS)

Mereghetti et al 2005


SGR 1806-20 - IISGR 1806-20 - II Four XMM-Newton observations (last on Four XMM-Newton observations (last on

October 5 2004, October 5 2004, Mereghetti et al 2005)) Pulsations clearly detected in all observationsPulsations clearly detected in all observations ṖṖ ~ 5.5x10 ~ 5.5x10-10-10 s/s, higher than the “historical” s/s, higher than the “historical”

valuevalue Blackbody component in addition to an Blackbody component in addition to an

absorbed power law (kT ~ 0.79 keV)absorbed power law (kT ~ 0.79 keV) Harder spectra: Harder spectra: ΓΓ ~ 1.5 vs. ~ 1.5 vs. ΓΓ ~ 2~ 2 The 2-10 keV luminosity almost doubled (LThe 2-10 keV luminosity almost doubled (LXX ~ ~

10103636 erg/s) erg/s)


Twisted Magnetospheres – ITwisted Magnetospheres – I The magnetic field inside a magnetar The magnetic field inside a magnetar

is “wound up”is “wound up” The presence of a toroidal component The presence of a toroidal component

induces a rotation of the surface layersinduces a rotation of the surface layers The crust tensile strength resists The crust tensile strength resists A gradual (quasi-plastic ?) deformation A gradual (quasi-plastic ?) deformation

of the crustof the crust The external field twists up The external field twists up (Thompson, (Thompson,

Lyutikov & Kulkarni 2002)Lyutikov & Kulkarni 2002) Thompson & Duncan 2001


Twisted Magnetospheres - IITwisted Magnetospheres - II TLK02 investigated TLK02 investigated

force-free force-free magnetic equilibria magnetic equilibria

A sequence of A sequence of

models labeled by models labeled by the twist anglethe twist angle

)0( BJ

BRB

),(

sin2 2

0

dBB

SN


Twisted Magnetospheres - IIITwisted Magnetospheres - III Twisted magnetospheres are threaded by Twisted magnetospheres are threaded by

currentscurrents Charged particles provide large optical Charged particles provide large optical

depth to resonant cyclotron scattering depth to resonant cyclotron scattering Because and , a power-Because and , a power-

law tail expected instead of an absorption law tail expected instead of an absorption line line

, and , and Both and increase with the twist Both and increase with the twist

angleangle

),( Rcc NScurrent RR

rs

)2( ptwist RB 3 RBdip diptwist PP

rs twistP


A Growing Twist in SGR 1806-A Growing Twist in SGR 1806-20 ?20 ?

Evidence for spectral Evidence for spectral hardening AND hardening AND enhanced spin-downenhanced spin-down

and and correlationscorrelations

Growth of bursting Growth of bursting activityactivity

Possible presence of Possible presence of proton cyclotron line proton cyclotron line only during bursts only during bursts

P All these features are consistent with an increasingly twisted magnetosphere

L


A Monte Carlo ApproachA Monte Carlo Approach Follow individually a large sample of Follow individually a large sample of

photons, treating probabilistically their photons, treating probabilistically their interactions with charged particles interactions with charged particles

Can handle very general (3D) geometriesCan handle very general (3D) geometries Quite easy to code, fastQuite easy to code, fast Ideal for purely scattering mediaIdeal for purely scattering media Monte Carlo techniques work well when Monte Carlo techniques work well when

NNscatscat ≈ 1 ≈ 1

Basic ingredients: Space and energy distribution of the scattering particles Same for the seed (primary) photons Scattering cross sections

Preliminary investigation (1D) by Lyutikov& Gavriil (2005)More detailed modeling by Fernandez & Thompson (2006)New, up-to-dated code (Nobili, Turolla, Zane & Sartore 2007)


Select seed photon (energy and direction)

Generate a uniform deviate 0<R<1

Advance photon,compute depth R ?ln

Compute scattering

No

Yes

Escape ?

Store data

Yes

NoNo

Select particle from distributionTransform photon energy and direction to ERFCompute photon energy after scatteringCompute new photon direction

Transform back to LAB

)]cos1(1/[' 2

4

'cos

1

2

0' '/)',,('/'/)',,('' dkkdddkkdddR


Magnetospheric CurrentsMagnetospheric Currents Charges move along the field lines Charges move along the field lines Spatial distributionSpatial distribution Particle motion characterized by a Particle motion characterized by a

bulk velocity, vbulk velocity, vbulkbulk, and by a velocity , and by a velocity spread spread ΔΔv v (Beloborodov & Thompson 2006)(Beloborodov & Thompson 2006)

There may be eThere may be e±± in addition to e-p, in addition to e-p,

but no detailed model as yetbut no detailed model as yet

bulkvrcB

BB

epn

41

Bv

3-1

1416 cm

km 10G 1010

NSp RB

Electron contribution only1D relativistic Mawellian atTe centred at vbulk


Surface EmissionSurface EmissionThe star surface is divided into patches by a cos θ – φ grid

Each patch has its own temperature toreproduce differentthermal maps

Blackbody (isotropic) emission


Photons in a Magnetized Photons in a Magnetized MediumMedium

Magnetized plasma is anisotropic and Magnetized plasma is anisotropic and birefringent, radiative processes birefringent, radiative processes sensitive to polarization statesensitive to polarization state

Two normal, elliptically polarized Two normal, elliptically polarized modes in the magnetized modes in the magnetized “vacuum+cold plasma”“vacuum+cold plasma”

At the At the modes are almost linearly polarizedmodes are almost linearly polarized

32214 gcm )keV 1/()G 10/( BV

The extraordinary (X) and ordinary (O) modes


Scattering Cross Sections - IScattering Cross Sections - I QED cross section available QED cross section available (Herold 1979, (Herold 1979,

Harding & Daugherty 1991) Harding & Daugherty 1991) but unwieldybut unwieldy Non-relativistic (Thompson) cross section Non-relativistic (Thompson) cross section

((εε<mc<mc22//γ≈γ≈50 keV, B/B50 keV, B/BQED QED < 1)< 1)

scatteringafter and beforevelocity particle anddirection photon between angles ', ,/ ,/

'cos)(8

3'

)(8

3'

cos)(8

3'

'coscos)(8

3'

(ERF) resonanceat sections cross aldifferenti Completely

220

200

20220

mceBmcer

crddcr

dd

crddcr

dd

c

cOX

cXX

cXO

cOO


Scattering Cross Sections - IIScattering Cross Sections - II Because of charge motion resonance atBecause of charge motion resonance at

For a given photon (energy For a given photon (energy ωω, direction k), direction k) )cos1(

cres

2,1

2

)()1(1)(i

iii

i

cres

22

22

2,1 )/(1)/()/(

c

ccres


7.0G 10

S-N

14

B 7.0

G 10

S-N

14

B

Model Spectra - IModel Spectra - I

2.1G 10

S-N

15

B

Model parameters: ΔΦN-S, Bpole, Te, vbulkSurface emission geometry, viewing angle

2.1G 10

S-N

15

B

Emission from entire star surface at Tγ=0.5 keV

twist increases

hard

ness

incr

ease

s

1015 G

1014 G

7.0G 10

S-N

14

B 7.0

G 10

S-N

14

B


Model Spectra - IIModel Spectra - IILine of sight effectsEmission from a single patch at the equator

LOS at the samelongitude of the patchLOS at oppositelongitude


Conclusions & Future Conclusions & Future DevelopmentsDevelopments

Twisted magnetosphere model, within Twisted magnetosphere model, within magnetar scenario, in general agreement with magnetar scenario, in general agreement with observationsobservations

Resonant scattering of thermal, surface Resonant scattering of thermal, surface photons produces spectra with right propertiesphotons produces spectra with right properties

Many issues need to be investigated furtherMany issues need to be investigated further– Twist of more general external fieldsTwist of more general external fields– Detailed models for magnetospheric currentsDetailed models for magnetospheric currents– More accurate treatment of cross section including More accurate treatment of cross section including

QED effects and electron recoil (in progress)QED effects and electron recoil (in progress)– 10-100 keV tails: up-scattering by (ultra)relativistic 10-100 keV tails: up-scattering by (ultra)relativistic

(e(e±±) particles ?) particles ?– Create an archive to fit model spectra to Create an archive to fit model spectra to

observations (in progress)observations (in progress)


Post-Flare EvolutionPost-Flare Evolution After the GF SGR After the GF SGR

1806-20 1806-20 persistent X-ray persistent X-ray emission is softer emission is softer and spin-down and spin-down rate smallerrate smaller

Evidence for an Evidence for an untwisting of the untwisting of the magnetospheremagnetosphere


Part I: Observational Facts (mainly)

Part II: Theoretical Implications (andSpeculations…)

Soft Gamma Repeaters are ULTRA-MAGNETIZED NEUTRON STARS, i.e. MAGNETARS

X-ray Spectra from Magnetar Candidates

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