Xray Spectra from Magnetar Candidates A Twist in

X-ray Spectra from Magnetar Candidates A Twist in the Field R Turolla Department of Physics University of Padova, Italy Credits GL Israel, S. Mereghetti, L Nobili, N Rea, N Sartore, L Stella, A Tiengo, S Zane Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Galactic NS Population § Present supernova rate in the Galaxy ≈ 0. 01 yr -1 § The Galaxy is ≈ 10 Gyr old 108 – 109 neutron stars § Most neutron stars are known through their pulsed radio-emission § Galactic pulsar population ≈ 105 (> 1500 detected) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Pulsars and… § The majority of neutron stars are old, dead objects § Observations in the X- and γ-rays revealed the existence of populations of radio-quiet neutron stars § X-ray binaries § X-ray dim isolated neutron stars § Soft γ-repeaters ISOLATED § Anomalous X-ray pulsars Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Soft Gamma Repeaters - I Rare class of sources, 4 confirmed (+ 1): SGR 1900+14, SGR 1806 -20, SGR 1627 -41 in the Galaxy and SGR 0526 -66 in the LMC n Strong bursts of soft γ-/hard X-rays: L ~ 1041 erg/s, duration < 1 s n Bursts from SGR 1806 -20 (INTEGRAL/IBIS, , Gőtz et al 2004) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Soft Gamma Repeaters - II n n n Much more energetic “Giant Flares” (GFs, L ≈ 1045 -1047 erg/s) detected from 3 sources No evidence for a binary companion, association with a SNR in one case Persistent X-ray emitters, L ≈ 1035 erg/s Pulsations discovered both in GFs tails and persistent emission, P ≈ 5 -10 s Huge spindown rates, Ṗ/P ≈ 10 -10 ss-1 (Kouveliotou et al. 1998; 1999) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Anomalous X-ray Pulsars - I n n n Seven sources known (+ 1 transient): 1 E 1048. 1 -5937, 1 E 2259+586, 4 U 0142+614, 1 RXS J 170849 -4009, 1 E 1841 -045, CXOU 010043 -721134, AX J 1845 -0258 (+ XTE J 1810 -197) Persistent X-ray emitters, L ≈ 1034 -1035 erg/s Pulsations with P ≈ 5 -10 s Large spindown rates, Ṗ/P ≈ 10 -11 ss-1 No evidence for a binary companion, association with a SNR in three cases Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Anomalous X-ray Pulsars - II n Bursts of soft γ-/hard X-rays quite similar to those of SGRs (AXPs much less active though, bursts from two sources only) Woods & Thompson (2005) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007 Time (sec)

A Tale of Two Populations ? SGRs: bursting X/γ-ray sources AXPs: peculiar class A Magnetar of steady X-ray sources Single class of objects R < ctrise ≈ 300 km: a compact object Pulsed X-ray emission: a neutron star Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Magnetars n Strong convection in a rapidly rotating (P ~ 1 ms) newborn neutron star generates a very strong magnetic field via dynamo action n Magnetars: neutron stars with surface field B > 10 BQED ~ 4 x 1014 G (Duncan & Thomson 1992; Thomson & Duncan 1993) n Rapid spin-down due to magneto-dipolar losses, Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

ot n splitt ing evidence for a companion star n Spin down to present periods in ≈ 104 SPIN - DOWN ENERGY LOSS yrs requires B > 1014 G n Large measured spin-down rates n Quite natural explanation for the bursts Photo X-RAY LUMINOSITY Ėr = LX PSRs n No SGRs+AXPs High-field PSRs n. Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007 Why magnetars ? hold SGRs + AXPs thres

SGRs and AXPs X-ray Spectra - I n 0. 5 – 10 ke. V emission well represented by a blackbody plus a power law AXP 1048 -5937 (Lyutikov & Gavriil 2005) SGR 1806 -20 (Mereghetti et al 2005) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

SGRs and AXPs X-ray Spectra - II n k. TBB ~ 0. 5 ke. V, does not change much in different sources n Photon index Г ≈ 1 – 4, AXPs tend to be softer n SGRs and AXPs persistent emission is variable (months/years) n Variability mostly associated with the nonthermal component Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Hard X-ray Emission INTEGRAL revealed substantial emission in the 20 -100 ke. V band from SGRs and APXs Mereghetti et al 2006 Hard power law tails with Г ≈ 1 -3, hardening wrt soft X-ray emission required in AXPs Hard emission pulsed Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Hardness vs Spin-down Rate Correlation between spectral hardness and spin-down rate in SGRs and AXPs (Marsden & White 2001) Correlation holds also for different states within a single source (SGR 1806 -20, Mereghetti et al 2005; 1 RXS J 1708494009, Rea et al 2005) Harder X-ray spectrum Larger Spin-down rate Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

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) Bursts / day (IPN) § enhanced burst rate § increased persistent luminosity 20 -60 ke. V flux (INTEGRAL IBIS) The 2004 December 27 Event Spring 2003 Autumn 2003 Spring 2004 Autumn 2004 Mereghetti et al 2005 Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

SGR 1806 -20 - II n n n Four XMM-Newton observations (last on October 5 2004, Mereghetti et al 2005) Pulsations clearly detected in all observations Ṗ ~ 5. 5 x 10 -10 s/s, higher than the “historical” value Blackbody component in addition to an absorbed power law (k. T ~ 0. 79 ke. V) Harder spectra: Γ ~ 1. 5 vs. Γ ~ 2 The 2 -10 ke. V luminosity almost doubled (LX ~ 1036 erg/s) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Twisted Magnetospheres – I n The magnetic field inside a magnetar is “wound up” n The presence of a toroidal component induces a rotation of the surface layers n The crust tensile strength resists n A gradual (quasi-plastic ? ) deformation of the crust n The external field twists up (Thompson, Lyutikov & Kulkarni 2002) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007 Thompson & Duncan 2001

Twisted Magnetospheres - II n TLK 02 investigated force-free magnetic equilibria n n A sequence of models labeled by the twist angle Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Twisted Magnetospheres - III n n n Twisted magnetospheres are threaded by currents Charged particles provide large optical depth to resonant cyclotron scattering Because and , a powerlaw tail expected instead of an absorption line , and Both and increase with the twist angle Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

A Growing Twist in SGR 1806 -20 ? Evidence for spectral hardening AND enhanced spin-down n and correlations n Growth of bursting activity n Possible presence of proton cyclotron line only during bursts n All these features are consistent with an increasingly twisted magnetosphere Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

A Monte Carlo Approach n Follow individually a large sample of photons, treating probabilistically their Preliminary investigation (1 D) by Lyutikov interactions with charged particles Basic ingredients: & Gavriil (2005) n Can handle general (3 D) geometries § Space andvery energy distribution of the More detailed modeling by Fernandez & scattering n Quite easy toparticles code, fast Thompson (2006) § Same for the seed (primary) photons n. New, Ideal up-to-dated for purely scattering media code (Nobili, Turolla, § Scattering cross sections n. Zane Monte techniques & Carlo Sartore 2007) work well when Nscat ≈ 1 Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Generate a uniform deviate 0<R<1 No Select seed photon (energy and direction) No Select particle from distribution Transform photon energyphoton, and direction to ERF Advance Escape Compute ? photoncompute energy after depthscattering Compute new photon direction Yes Transform back to LAB Compute scattering Yes Store data Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007 R?

Magnetospheric Currents n Charges move along the field lines n Spatial distribution Electron contribution only 1 D relativistic Mawellian at n Particle motion characterized Te centred at vbulk by a bulk velocity, vbulk, and by a velocity spread Δv (Beloborodov & Thompson 2006) n There may be e± in addition to e-p, but no detailed model as yet Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Surface Emission The star surface is divided into patches by a cos θ – φ grid Each patch has its own temperature to reproduce different thermal maps Blackbody (isotropic) emission Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Photons in a Magnetized Medium n Magnetized plasma is anisotropic and birefringent, radiative processes sensitive to polarization state n Two normal, elliptically polarized modes in the magnetized “vacuum+cold plasma” n At the modes are almost linearly polarized The extraordinary (X) and ordinary (O) modes Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Scattering Cross Sections - I QED cross section available (Herold 1979, Harding & Daugherty 1991) but unwieldy n Non-relativistic (Thompson) cross section (ε<mc 2/γ≈50 ke. V, B/BQED < 1) n Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Scattering Cross Sections - II n Because of charge motion resonance at n For a given photon (energy ω, direction k) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Model Spectra - I Model parameters: ΔΦN-S, Bpole, Te, vbulk Surface emission geometry, viewing angle 1014 G 1015 G twist increases Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007 hardness increases Emission from entire star surface at Tγ=0. 5 ke. V

Model Spectra - II Line of sight effects Emission from a single patch at the equator LOS same LOSat atthe opposite longitudeof the patch Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Conclusions & Future Developments Twisted magnetosphere model, within magnetar scenario, in general agreement with observations n Resonant scattering of thermal, surface photons produces spectra with right properties n Many issues need to be investigated further n – Twist of more general external fields – Detailed models for magnetospheric currents – More accurate treatment of cross section including QED effects and electron recoil (in progress) – 10 -100 ke. V tails: up-scattering by (ultra)relativistic (e±) particles ? – Create an archive to fit model spectra to observations (in progress) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Post-Flare Evolution n After the GF SGR 1806 -20 persistent X-ray emission is softer and spindown rate smaller n Evidence for an untwisting of the magnetosphere Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

Part I: Observational Facts (mainly) Part II: Theoretical Implications (and Speculations…) Soft Gamma Repeaters are ULTRA-MAGNETIZED NEUTRON STARS, i. e. MAGNETARS Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007