Isolated Neutron Stars XDINSs, AXPs, SGRs, RRATs R Turolla Department of Physics University of Padova, Italy Artist impression of a magnetar Proper motion of RX J1856.5-3754 with HST (F. Walter) A detected RRAT Cagliari 25-26 maggio 2007 Basics Compact objects are born in the core collapse following a type II supernova explosion Present rate of SN events in the Galaxy: ≈ 0.01/yr (possibly higher in the past) Galactic population of compact objects: ≈ 108 – 109 (≈ 1% of stars) Cagliari 25-26 maggio 2007 Nature of compact remnant depends on progenitor mass 8 < M/M < 20-25 M/M > 20-25 If N(>M) ~ M-1.3 only ~ 20% of stars with M > 8 M are more massive than 25 M Very massive stars may form “magnetars” (Muno et al 2006), black holes about 10% of the total Cagliari 25-26 maggio 2007 Pulsars and… Most neutron stars are known through their pulsed radio-emission Galactic pulsar population ≈ 105 (≈ 1800 detected) The majority of neutron stars are old, dead objects Observations in the X- and γ-rays revealed the existence of different populations of neutron stars X-ray binaries Geminga, CCOs in SNR X-ray dim isolated neutron stars (XDINSs) Soft γ-repeaters (SGRs) ISOLATED Anomalous X-ray pulsars (AXPs) Rotating Radio Transients (RRATs) Cagliari 25-26 maggio 2007 Neutron Stars in a Nutshell Vast majority (~ 80-90%) of Galactic compact objects are compact stars (no horizon) Electron capture e-+p → n+ν energetically favorable at ρ > 107 g/cm3, or R < 104 km for M ~ 1 M Typical radius ≈ 10 km, masses in the range 0.1 < M/M < 3 (theoretical estimates) Highly relativistic objects M/R ~ 0.15 (M/M)(R/10 km)-1 J/M ~ 0.25 (1 ms/P) (M/M)(R/10 km)2 (the Kerr solution does not describe spacetime outside a rotating star though) Cagliari 25-26 maggio 2007 ds 2 exp( 2)dt 2 exp( 2)dr 2 r 2 (d 2 sin 2 d 2 ) Spherically symmetric space-time d 1 dp dr p dr Einstein equation dp m p 4r 2 1 1 dr r m 3 dm 4r 2 dr exp g 00 p 2m 1 r Gravitational mass Cagliari 25-26 maggio 2007 1 2M 1 r Hydrostatic equilibrium (TOV) (0) c m(0) 0 R, p ( R ) 0 M m( R ) or R, p ( R ) 0 m(0) 0 M m( R ) Compute sequence of equilibrium models: the Mass-radius relation P = P(ρ) ? Cagliari 25-26 maggio 2007 Nuclear Matter EOS: The Holy Grail “Neutron” stars: a misnomer Inner core may contain – Hyperons (Σ, Ξ, Λ) – Meson (K-,π-) condensates – Deconfined quarks (u, d, s) – Hybrid Stars Strange quark stars Page & Reddy (2006) Cagliari 25-26 maggio 2007 R=2GM/c2 P=ρ R~3GM/c2 Page & Reddy (2006) R∞=R(1-2GM/Rc2)-1/2 Lattimer & Prakash (2004) Cagliari 25-26 maggio 2007 ω=ωK NS Masses Stellar masses directly measured only~ Initial in Mass binary systems Accurate NS mass determination for PSRs in relativistic systems by measuring PK > Initial Mass corrections Gravitational redshift may provide M/R in NSs by detecting a known spectral line, BHs ? E∞ = E(1-2GM/Rc2)1/2 Fe and O lines in EXO 0748-676, M/R ~ 0.22 (Cottam et al 2002) Page & Reddy (2006) Cagliari 25-26 maggio 2007 NS Cooling NSs are born very hot, T > 1010 K At early stages neutrino cooling dominates The core is isothermal dEth dT CV L L dt dt Photon luminosity Neutrino luminosity L 4 R 2 Ts4 , Ts T 1/ 2 ( 1) Cagliari 25-26 maggio 2007 Fast Cooling (URCA cycle) n p e e p e n e Slow Cooling (modified URCA cycle) n n n p e e n p e n n e p n p p e e p p e p n e Fast cooling possible only if np > nn/8 Nucleon Cooper pairing important Minimal cooling scenario (Page et al 2004): no exotica no fast processes pairing included Cagliari 25-26 maggio 2007 “Minimal” Cooling Curves Geppert & Weber (2006) Page &Page, Reddy (2006) Cagliari 25-26 maggio 2007 NS Radii A NS with homogeneous surface temperature and local blackbody emission L 4 R T L 2 4 F R / D T 2 4 D 2 4 From X-ray spectroscopy Cagliari 25-26 maggio 2007 From dispersion measure NS Radii - II Real life is a trifle more complicated… Because of the strong B field Photon propagation different Surface temperature is not homogeneous Local emission may be not exactly planckian Gravity effects are important Cagliari 25-26 maggio 2007 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” The extraordinary (X) and ordinary (O) modes Cagliari 25-26 maggio 2007 NS Thermal Maps Electrons move much more easily along B than across B Thermal conduction is highly anisotropic inside a NS: Kpar >> Kperp until EF >> hνB or ρ >> 104(B/1012 G)3/2 g/cm3 Envelope scaleheight L ≈ 10 m << R, B ~ const and heat transport locally 1D Greenstein & Hartke (1983) Cagliari 25-26 maggio 2007 TS cos K perp / K par sin 2 2 1/ 4 Tpole K perp / K par 1 TS cos 1/ 2 Tpole Core centered dipole Core centered quadrupole Cagliari 25-26 maggio 2007 Local Surface Emission Much like normal stars NSs are covered by an atmosphere Because of enormous surface gravity, g ≈ 1014 cm/s2, Hatm ≈ 1-10 cm Spectra depend on g, chemical composition and magnetic field Plane-parallel approximation (locally) Cagliari 25-26 maggio 2007 Free-free absorption dominates 3 , h kT High energy photons decouple deeper in the atmosphere where T is higher Zavlin & Pavlov (2002) Cagliari 25-26 maggio 2007 Gravity Effects Redshift Ray bending L 4 R T 2 4 2 2 1 0 0 0 4 T d d du 4 2 E , 2 E ,1 dE I ( E, B, cos , Ts , ) Cagliari 25-26 maggio 2007 STEP 1 Specify viewing geometry and B-field topology; compute the surface temperature distribution STEP 2 Compute emission from every surface patch STEP 4 Predict lightcurve and phase-resolved spectrum Compare with observations Cagliari 25-26 maggio 2007 STEP 3 GR ray-tracing to obtain the spectrum at infinity Further Readings Page, D., Reddy, S. 2006, Ann. Rev. Nucl. Part. Sci., 56, 327 (astro-ph/0608360) Page, D. Geppert, U., Weber, F. 2006, Nucl. Phys. A, 777, 497 (astro-ph/0508056) Weber, F. 2005, Progr. Part. Nucl. Phys., 54, 193 (astroph/0407155) Lattimer, J.M., Prakash, M. 2004, Science, 304, 536 (astroph/0405262) Yakovlev, D.G., Pethick, C.J. 2004, Ann. Rev. Astron. Astrophys., 42, 169 (astro-ph/0402143) Zavlin, V.E., Pavlov, G.G. 2002, Proceedings of the 270 WEHeraeus Seminar on Neutron Stars, Pulsars, and Supernova Remnants. MPE Report 278, p.26 (astro-ph/0206025) Cagliari 25-26 maggio 2007 The Seven X-ray Dim Isolated Neutron stars (XDINSs) Soft thermal spectrum (kT 50-100 eV) No hard, non-thermal tail Radio-quiet, no association with SNRs Low column density (NH 1020 cm-2) X-ray pulsations in 6 sources (P 3-10 s) Very faint optical counterparts Cagliari 25-26 maggio 2007 The Magnificent Seven Source kT (eV) P (s) Amplitude/2 Optical RX J1856.5-3754 60 7.06 1.5% V = 25.6 RX J0720.4-3125 (*) 85 8.39 11% B = 26.6 RX J0806.4-4123 96 11.37 6% - RX J0420.0-5022 45 3.45 13% B = 26.6 ? RX J1308.6+2127 (RBS 1223) 86 10.31 18% m50CCD = 28.6 RX J1605.3+3249 (RBS 1556) 96 - - m50CCD = 26.8 1RXS J214303.7+065419 (RBS 1774) 104 9.43 4% - (*) variable source Cagliari 25-26 maggio 2007 Featureless ? No Thanks ! RX J1865.5-3754 is convincingly featureless RX J0720.4-3125 (Haberl et al 2004) (Chandra 500 ks DDT; Drake et al 2002; Burwitz et al 2003) A broad absorption feature detected in all other XDINSs (Haberl et al 2003, 2004, 2004a; Van Kerkwijk et al 2004; Zane et al 2005) Eline ~ 300-700 eV; evidence for two lines with E1 ~ 2E2 in RBS 1223 (Schwope et al 2006) Proton cyclotron lines ? H/He transitions at high B ? Cagliari 25-26 maggio 2007 Period Evolution . RX J0720.4-3125: bounds on P derived by Zane et al. (2002) and Kaplan et al (2002) Timing solution by Cropper et al (2004), further improved by Kaplan & Van Kerkwijk (2005): . P = 7x10-14B s/s, B =13 2x1013 14 G ~ 10 -10 G RX J1308.6+2127: timing . solution by Kaplan & Van Kerkwijk (2005a), P = 10-13 s/s, B = 3x1013 G Spin-down values of B in agreement with absorption features being proton cyclotron lines Cagliari 25-26 maggio 2007 Source Energy (eV) EW (eV) Bline (Bsd) (1013 G) Notes RX J1856.5-3754 no no ? - RX J0720.4-3125 270 40 5 (2) Variable line RX J0806.4-4123 460 33 9 - RX J0420.0-5022 330 43 7 - RX J1308.6+2127 300 150 6 (3) - RX J1605.3+3249 450 36 9 - 1RXS J214303.7+065419 700 50 14 - Cagliari 25-26 maggio 2007 XDINSs: The Perfect Neutron Stars XDINSs are key in neutron star astrophysics: these are the only sources for which we have a “clean view” of the star surface Information on the thermal and magnetic surface distributions Estimate of the star radius (and mass ?) Direct constraints on the EOS Cagliari 25-26 maggio 2007 XDINSs: What Are They ? XDINSs are neutron stars Powered by ISM accretion, ṀBondi ~ nISM/v3 if v < 40 km/s and D < 500 pc (e.g. Treves et al 2000) Measured proper motions imply v > 100 km/s Just cooling NSs Cagliari 25-26 maggio 2007 Simple Thermal Emitters ? Recent detailed observations of XDINSs allow direct testing of surface emission models “STANDARD MODEL” thermal emission from the surface of a neutron star with a dipolar magnetic field and covered by an atmosphere The optical excess XDINS lightcurves The puzzle of RX J1856.5-3754 Spectral evolution of RX J0720.4-3125 Cagliari 25-26 maggio 2007 The Optical Excess In the four sources with a confirmed optical counterpart Fopt 5-10 x B(TBB,X) Fopt 2 ? Deviations from a RayleighJeans continuum in RX J0720 (Kaplan et al 2003) and RX J1605 (Motch et al 2005). A non-thermal power law ? RX J1605 multiwavelength SED (Motch et al 2005) Cagliari 25-26 maggio 2007 Pulsating XDINSs - I Quite large pulsed fractions Skewed lightcurves Harder spectrum at pulse minimum Phase-dependent absorption features RX J0420.0-5022 (Haberl et al 2004) Cagliari 25-26 maggio 2007 Pulsating XDINSs - II Too Too small small pulsed pulsed fractions fractions Core-centred Core-centred Atmosphere Blackbody == ++ Symmetrical Symmetrical dipole dipole field field emission emission pulse pulse profiles profiles (Page (Zane1995) & Turolla 2006) + = Cagliari 25-26 maggio 2007 Pulsating XDINSs - III Pure dipole-induced thermal maps do not match XDINS pulse profiles Observed Synthetic Principal component representation of dipolar lightcurves More complex thermal and/or magnetic surface distributions Cagliari 25-26 maggio 2007 Beyond the Dipole Addition of quadrupolar components (even assuming BB emission) results in larger pulsed fractions and non-symmetric pulse shapes (Page & Sarmiento 1996) Star-centred dipolar+quadrupolar fields can reproduce observed lightcurves (Zane & Turolla 2006) Cagliari 25-26 maggio 2007 Crustal Magnetic Fields Star centred dipole + poloidal/toroidal field in the envelope (Geppert, Küker & Page 2005; 2006) Purely poloidal crustal fields produce a steeper meridional temperature gradient Addition of a toroidal component introduces a N-S asymmetry Geppert, Küker & Page 2006 Gepper, Küker & Page 2006 Cagliari 25-26 maggio 2007 Schwope et al. 2005 RBS 1223 (Zane & Turolla 2006) Indications for non-antipodal caps (Schwope et al 2005) Need for a non-axsymmetric treatment of heat transport Cagliari 25-26 maggio 2007 RX J1856.5-3754 - I Blackbody featureless spectrum in the 0.1-2 keV band (Chandra 500 ks DDT, Drake et al 2002); possible broadband deviations in the XMM 60 ks observation (Burwitz et al 2003) RX J1856 multiwavelength SED (Braje & Romani 2002) Thermal emission from NSs is not expected to be a featureless BB ! H, He spectra are featureless but only blackbody-like (harder). Heavy elements spectra are closer to BB but with a variety of features Cagliari 25-26 maggio 2007 RX J1856.5-3754 - II A quark star (Drake et al 2002; Xu 2002; 2003) What spectrum ? A The optical excess ? and cooler NS with hotter caps equatorial region (Pons et al 2002; Braje & Romani 2002; Trűmper et al 2005) A bare NS (Burwitz A perfect BB ? et al 2003; Turolla, Zane & Drake 2004; Van Adelsberg et al 2005; Perez-Azorin, Miralles & Pons 2005) Cagliari 25-26 maggio 2007 Bare Neutron Stars At B >> B0 ~ 2.35 x 109 G atoms attain a cylindrical shape Turolla, Zane & Drake 2004 Formation of molecular chains by covalent bonding along the field direction RX J0720.4-3125 Interactions between molecular chains can RX lead to the formation J1856.5-3754 of a 3D condensate Fe condensation H temperature Critical depends on B and chemical composition (Lai & Salpeter 1997; Lai 2001) Cagliari 25-26 maggio 2007 Spectra from Bare NSs - I The cold electron gas approximation. Reduced emissivity expected below p (Lenzen & Trümper 1978; Brinkmann 1980) Spectra are very close to BB in shape in the 0.1 - 2 keV range, but depressed wrt the BB at Teff. Reduction factor ~ 2 - 3. Turolla, Zane & Drake (2004) Cagliari 25-26 maggio 2007 Spectra from Bare NS - II Proper account for damping of free electrons by lattice interactions (e-phonon scattering; Yakovlev & Urpin 1980; Potekhin 1999) Spectra deviate more from BB. Fit in the 0.1 – 2 keV band still acceptable. Features may be present. Reduction factors higher. Cagliari 25-26 maggio 2007 Turolla, Zane & Drake (2004) Is RX J1856.5-3754 Bare ? Fit of X-ray data in the 0.152 keV band acceptable Radiation radius problem eased Optical excess may be produced by reprocessing of surface radiation in a very rarefied atmosphere (Motch, Does the atmosphere keep the star surface temperature ? Zavlin & Haberl 2003; Zane, Turolla & Drake 2004; Ho et al. 2006) Details of spectral shape (features, low-energy behaviour) still uncertain Cagliari 25-26 maggio 2007 What is the ion contribution to the dielectric tensor ? (Van Adelsberg et al. 2005; Perez-Azorin, Miralles & Pons 2005) Long Term Variations in RX J0720.4-3125 A gradual, long term change in the shape of the X-ray spectrum AND the pulse profile (De Vries et al 2004; Vink et al 2004) Steady increase of TBB and of the absorption feature EW (faster during 2003) Evidence for a reversal of the evolution in 2005 De Vries et al. 2004 Obs. Date kTBB (eV) EW (eV) 13-05-2000 86.6±0.4 -5.0 21/22-11-2001 86.5±0.5 +8.7 06/09-11-2002 88.3±0.3 -21.5 27/28-10-2003 91.3±0.6 -73.7 22/23-05-2004 93.8±0.4 -72.4 28-04-2005 93.5±0.4 -68.3 23-09-2005 93.2±0.4 -67.4 12/13-11-2005 92.6±0.4 -67.5 (Vink et al 2005) Cagliari 25-26 maggio 2007 Cagliari 25-26 maggio 2007 A Precessing Neutron Star ? Evidence for a periodic modulation in the spectral parameters (Tbb, Rbb) but no complete cycle yet Phase residuals (coherent timing solution by Kaplan & Van Kerkwijk 2005) show periodic behavior over a much longer Haberl et al. 2006 timescale (> 10 yrs) Periods consistent within the errors, Pprec ~ 7.1-7.7 yr (Haberl et al. 2006) Cagliari 25-26 maggio 2007 A Simple Model Precessing neutron star Blackbody emission from two “hot spots” of different size and temperature A nearly aligned rotator seen almost equator-on Non-antipodal spots, rather large precession angle (~ 10o) Haberl et al. 2006 A bare NS with a crustal field ? Cagliari 25-26 maggio 2007 (Perez-Azorin et al 2006) RRATs - I 11 sources detected in the Parkes Multibeam survey (McLaughlin et al 2006) Burst duration 2-30 ms, interval 4 min-3 hr Periods in the range 0.4-7 s Period derivative measured in 3 sources: B ~ 1012-1014 G, age ~ 0.1-3 Myr RRAT J1819-1458 detected in the X-rays, spectrum soft and thermal, kT ~ 120 eV (Reynolds et al 2006) Cagliari 25-26 maggio 2007 RRATs - II P, B, ages and X-ray properties of RRATs very similar to those of XDINSs Estimated number of RRATs ~ 3-5 times that of PSRs If τRRAT ≈ τPSR, βRRAT ≈ 3-5 βPSR βXDINS > 3 βPSR (Popov et al 2006) Are RRATs far away XDINSs ? Cagliari 25-26 maggio 2007 Conclusions Rather complex thermal maps required to explain XDINS observations Progresses on the theoretical side but no self-consistent model yet – crustal fields (outside axial symmetry) – phase transition and emission properties of condensed surface – radiative transfer in the B > BQED regime Find more sources – is RX J1856.5-3754 unique ? – Relationship with other NS populations: Pulsating XDINSs are quite likely strongly magnetized objects, B > 1013 G. A XDINS-magnetar connection ? XDINSs = RRATs ? Search for RRAT-like radio-emission from XDINSs under way Cagliari 25-26 maggio 2007 X-ray Spectra from Magnetar Candidates A Twist in the Field Cagliari 25-26 maggio 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 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) Cagliari 25-26 maggio 2007 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) Cagliari 25-26 maggio 2007 Anomalous X-ray Pulsars - I Eight 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, CXOU J164710-455216 (+ 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 Cagliari 25-26 maggio 2007 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) Time (sec) Cagliari 25-26 maggio 2007 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 Cagliari 25-26 maggio 2007 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, P 1011 ( B / 1014 G) 2 P 1 ss 1 Cagliari 25-26 maggio 2007 Why magnetars ? .LX E rot I SGRs+AXPs SGRs + AXPs No evidence for a companion star High-field PSRs Spin down to present periods in ≈ 104 yrs requires B > 1014 G PSRs Large measured spin-down rates SPIN - DOWN ENERGY LOSS Quite natural explanation for the bursts X-RAY LUMINOSITY Cagliari 25-26 maggio 2007 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) SGR 1806-20 (Mereghetti et al 2005) Cagliari 25-26 maggio 2007 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 nonthermal component Cagliari 25-26 maggio 2007 Hard X-ray Emission INTEGRAL revealed substantial emission in the 20 -100 keV 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 Cagliari 25-26 maggio 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 J1708494009, Rea et al 2005) Harder X-ray spectrum Larger Spin-down rate Cagliari 25-26 maggio 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 keV flux (INTEGRAL IBIS) The 2004 December 27 Event Spring 2003 Autumn 2003 Spring 2004 Autumn 2004 Mereghetti et al 2005 Cagliari 25-26 maggio 2007 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) Cagliari 25-26 maggio 2007 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) Cagliari 25-26 maggio 2007 Thompson & Duncan 2001 Twisted Magnetospheres - II TLK02 investigated force-free magnetic equilibria ( J B 0) B ( R, ) B A sequence of models labeled by the twist angle N S 2 2 0 B d B sin Cagliari 25-26 maggio 2007 Twisted Magnetospheres - III Twisted magnetospheres are threaded by currents Charged particles provide large optical depth rs to resonant cyclotron scattering Because c c ( R, ) and Rcurrent RNS , a powerlaw tail expected instead of an absorption line Btwist R ( 2 p ,) Bdip R 3 and Ptwist Pdip Both rs and Ptwist increase with the twist angle Cagliari 25-26 maggio 2007 A Growing Twist in SGR 1806-20 ? Evidence for spectral hardening AND enhanced spin-down and L P correlations Growth of bursting activity Possible presence of proton cyclotron line only during bursts All these features are consistent with an increasingly twisted magnetosphere Cagliari 25-26 maggio 2007 A Monte Carlo Approach Follow individually a large sample of photons, treating probabilistically their Preliminary investigation (1D) by Lyutikov interactions with charged particles Basic ingredients: & Gavriil (2005) Can handle general (3D) geometries Space andvery energy distribution of the More detailed modeling by Fernandez & scattering Quite easy toparticles code, fast Thompson (2006) Same for the seed (primary) photons New, Ideal up-to-dated for purely scattering media code (Nobili, Turolla, Scattering cross sections Zane Monte Carlo techniques work well when 2007) Nscat ≈ 1 Cagliari 25-26 maggio 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 ln R ? 2 Escape Compute ? photoncompute energy after depthscattering ' /[1 (1 cos )] Compute new photon direction Yes 2 cos ' R ' d ' 0 1 d ' d ( , k , k ' ) / d' / d' d ( , k , k ' ) / d' Transform back to LAB Compute scattering Yes Store data Cagliari 25-26 maggio 2007 4 Magnetospheric Currents Charges move along the field lines v B Spatial distribution 1 p 1 B n 4e B -3 B p RNS 14 only cm 10 contribution Electron G 10 km 10 Mawellian 1D relativistic at Particle motion characterized Te centred at vbulk 16 cB r vbulk 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 Cagliari 25-26 maggio 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 Cagliari 25-26 maggio 2007 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) Completely differenti al cross sections at resonance (ERF) 3r0 c d ( c ) cos 2 cos 2 ' d' O O 8 3r0 c d ( c ) d' X X 8 3r0 c d ( c ) cos 2 d' O X 8 3r0c d ( c ) cos 2 ' d' X O 8 r0 e 2 / mc2 , c eB / mc, , ' angles between photon direction and particle velocity before and after scattering Cagliari 25-26 maggio 2007 Scattering Cross Sections - II Because of charge motion resonance at res c (1 cos ) For a given photon (energy ω, direction k) (c / ) (c / ) 1 res 1, 2 (c / ) 2 2 2 (1 i ) 1 ( res ) ( i ) c i 1, 2 i i 2 Cagliari 25-26 maggio 2007 2 Model Spectra Model parameters: ΔΦN-S, Bpole, Te, vbulk Surface emission geometry, viewing angle hardness increases Emission from entire star surface at Tγ=0.5 keV 1014 G B 1014 G B 10 B G 10 G N -S 0.7 14 14 B 100.G N -S N -S7 N -S 0.7 14 0.7 1015 G twist B 1015 G increases N -S 1B.210 15 G N -S 1.2 Cagliari 25-26 maggio 2007 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) Cagliari 25-26 maggio 2007