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X-ray Spectra from Magnetar Candidates

X-ray Spectra from Magnetar Candidates. R Turolla Department of Physics University of Padova, Italy. A Twist in the Field. Credits GL Israel, S . Mereghetti, L Nobili, N Rea, N Sartore, L Stella, A Tiengo, S Zane. Galactic NS Population.

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X-ray Spectra from Magnetar Candidates

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  1. X-ray Spectra from Magnetar Candidates R Turolla Department of Physics University of Padova, Italy A Twist in the Field 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

  2. 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

  3. ISOLATED 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 • Anomalous X-ray pulsars Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

  4. 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 • Strong bursts of soft γ-/hard X-rays: L ~ 1041 erg/s, duration < 1 s Bursts from SGR 1806-20 (INTEGRAL/IBIS,,Gőtz et al 2004) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

  5. Soft Gamma Repeaters - II • 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

  6. Anomalous X-ray Pulsars - I • Seven sources known (+ 1 transient): 1E 1048.1-5937, 1E 2259+586, 4U 0142+614, 1 RXS J170849-4009, 1E 1841-045, CXOU 010043-721134, AX J1845-0258 (+ XTE J1810-197) • 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

  7. Time (sec) Anomalous X-ray Pulsars - II • 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

  8. A Tale of Two Populations ? AXPs: peculiar class of steady X-ray sources SGRs: bursting X/γ-ray sources A Magnetar 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

  9. Magnetars • Strong convection in a rapidly rotating (P ~ 1 ms) newborn neutron star generates a very strong magnetic field via dynamo action • Magnetars: neutron stars with surface field B > 10 BQED ~ 4 x1014 G (Duncan & Thomson 1992; Thomson & Duncan 1993) • Rapid spin-down due to magneto-dipolar losses, Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

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

  11. SGR 1806-20 (Mereghetti et al 2005) SGRs and AXPs X-ray Spectra - I • 0.5 – 10 keV emission well represented by a blackbody plus a power law AXP 1048-5937 (Lyutikov & Gavriil 2005) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

  12. SGRs and AXPs X-ray Spectra - II • kTBB ~ 0.5 keV, does not change much in different sources • Photon index Г≈ 1 – 4, AXPs tend to be softer • SGRs and AXPs persistent emission is variable (months/years) • Variability mostly associated with the non-thermal component Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

  13. Mereghetti et al 2006 Hard X-ray Emission INTEGRAL revealed substantial emission in the 20 -100 keV band from SGRs and APXs 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

  14. Hardness vs Spin-down Rate Correlation between spectral hardness and spin-down rate in SGRs and AXPs (Marsden & White 2001) Harder X-ray spectrum Correlation holds also for different states within a single source (SGR 1806-20, Mereghetti et al 2005; 1 RXS J170849-4009, Rea et al 2005) Larger Spin-down rate Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

  15. Autumn 2003 Spring 2004 Autumn 2004 Spring 2003 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 Bursts / day (IPN) 20-60 keV flux (INTEGRAL IBIS) The2004 December 27 Event Mereghetti et al 2005 Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

  16. SGR 1806-20 - II • Four XMM-Newton observations (last on October 5 2004, Mereghetti et al 2005) • Pulsations clearly detected in all observations • Ṗ ~ 5.5x10-10 s/s, higher than the “historical” value • Blackbody component in addition to an absorbed power law (kT ~ 0.79 keV) • Harder spectra: Γ~ 1.5 vs. Γ~ 2 • The 2-10 keV luminosity almost doubled (LX ~ 1036 erg/s) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

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

  18. Twisted Magnetospheres - II • TLK02 investigated force-free magnetic equilibria • A sequence of models labeled by the twist angle Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

  19. Twisted Magnetospheres - III • Twisted magnetospheres are threaded by currents • Charged particles provide large optical depth to resonant cyclotron scattering • Because and , a power-law 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

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

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

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

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

  24. 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

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

  26. Scattering Cross Sections - I • QED cross section available (Herold 1979, Harding & Daugherty 1991) but unwieldy • Non-relativistic (Thompson) cross section (ε<mc2/γ≈50 keV, B/BQED < 1) Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

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

  28. 1014 G hardness increases 1015 G twist increases Model Spectra - I Model parameters: ΔΦN-S, Bpole, Te, vbulk Surface emission geometry, viewing angle Emission from entire star surface at Tγ=0.5 keV Magnetic Fields and Neutron Star Surface - Cocoyoc 14 February 2007

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

  30. Conclusions & Future Developments • Twisted magnetosphere model, within magnetar scenario, in general agreement with observations • Resonant scattering of thermal, surface photons produces spectra with right properties • Many issues need to be investigated further • 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 keV 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

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

  32. 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

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