Cyclotron lines in accreting X-ray pulsars. - models and observations -

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1 Cyclotron lines in accreting X-ray pulsars - models and observations - Gabriele Schönherr - AIP, Germany on behalf of the MAGNET collaboration: J. Wilms (FAU, Germany), P. Kretschmar (ESAC,Spain), I. Kreykenbohm (FAU, Germany), R.E. Rothschild, S. Suchy (UCSD, USA), V. McBride (Univ. Southampton, UK), C. Ferrigno (IAAT & ISDC, Switzerland), I. Caballero, D. Klochkov, A. Santangelo, R. Staubert (IAAT, Germany), et al.

2 BASIC PHYSICS 1 Accreting neutron stars in X-ray binaries (Becker & Wolff 2007) Magnetospheric accretion Strong field: Flow couples to magnetic field Channeling of material along field lines ( I. Negeruela) Formation of accretion columns above polar caps

3 BASIC PHYSICS 1 Accreting neutron stars in X-ray binaries (Becker & Wolff 2007) Magnetospheric accretion Strong field: Flow couples to magnetic field Channeling of material along field lines ( I. Negeruela) Formation of accretion columns above polar caps

4 BASIC PHYSICS 2 Accreting neutron stars in X-ray binaries (Becker & Wolff 2007) free-fall stopping of matter bremsstrahlung dominated continuum lines? ( I. Negeruela)

5 BASIC PHYSICS 3 Cyclotron line formation X-ray photons scatter with quantized plasma electrons Classical: Spiraling motion of an electron in magnetic field If: Quantization of electron energy into Landau levels ( * ) ( * ) Photons induce electron transitions. Absorption features are observed.

6 Cyclotron lines directly probe the magnetic fields of neutron stars! more than 16 sources nearly harmonic spacing up to 5 lines observed counts/sec/kev COMPARISON TO DATA 4 Her X-1, Balloon-HEXE (Trümper et al. 1978) observationally resolved line profiles physical conditions of the plasma (T, τ, geo, ΔB) variation of line position with phase, luminosity, etc. MPR MPF Off1 SPR SPF Off2 Log Energy V , INTEGRAL (Kreykenbohm et al. 2005) line ratios ΔB E cyc 10 kev 4U , RXTE (Heindl et al. 1999) Currently: fit with Gaussians Wanted: physical model GX 301-2, RXTE (Heindl et al. 2004)

7 COMPARISON TO DATA 5 cyclomc : a new XSPEC model Expectations / Model features predict line positions for given B ( Gaussian) predict line profiles depending on physical setting and given continuum (>Gaussian) Model several lines simultanously (>Gaussian) Model development Based on Monte Carlo code by Araya & Harding 1999, 2000 calculate the response function of the plasma in the line forming region for an arbitrary continuum Implement in XSPEC spectral fitting software

8 Monte Carlo Simulations Ingredients: geometry, photons and electrons MODEL SETUP 6 θ isotropic injection internally or bottom irradiated plasma relativistic Maxwellian (parallel temperature) low-density plasma ground Landau state cylinder (based on Araya & Harding 1999,2000)

9 Monte Carlo Simulations Ingredients: geometry, photons and electrons MODEL SETUP 6 θ isotropic injection internally or bottom irradiated plasma relativistic Maxwellian (parallel temperature) low-density plasma ground Landau state slab (based on Araya & Harding 1999,2000)

10 MODEL SETUP 7 Resonant scattering (Schönherr et al after Araya & Harding 1999 based on calculations by Sina 1996)

11 MODEL SETUP 8 The response function of the line forming region : Energy redistribution

12 MODEL SETUP 9 Spectrum: Green s functions approach Convolve with Green s functions Arbitrary continuum Final spectrum with lines

13 SIMULATED LINE SHAPES 10 Variation with the magnetic field B (Schönherr et al. 2007)

14 SIMULATED LINE SHAPES 11 Temperature dependence Trümper et al. 1978: (Schönherr et al. 2007)

15 SIMULATED LINE SHAPES 12 Variation with μ and τ τ 1 τ 2 μ (Schönherr et al. 2007)

16 Variation with the geometry SIMULATED LINE SHAPES Energy [kev] Energy [kev] Energy [kev] Energy [kev] (Schönherr et al. 2007)

17 FITTED LINE SHAPES 14 Does cyclomc fit nature? normalized counts/sec/kev Δχ Δχ U (Suzaku) Energy [kev] Δχ normalized counts/sec/kev Δχ Δχ Δχ Δχ Δχ Δχ V (INTEGRAL) Energy [kev] (a) (b) (c) (c) (d) (d) (e) (e) Easy: one line Complicated: several lines

18 Beyond Gaussian modelling COMPARISON TO DATA 15 Expectations / Model features predict line positions for given B ( Gaussian) predict line profiles depending on physical setting and given continuum (>Gaussian) Model several lines simultanously (>Gaussian) Outcome Magnetic field + Indications for field structure B + temp., geometry, angle Line ratios + impossible line ratios-> physics!

19 Beyond Gaussian modelling Physical parameter correlations? COMPARISON TO DATA 16 4U (Suzaku) V (INTEGRAL) Cen X-3 (RXTE) normalized counts/sec/kev Δχ Δχ Energy [kev] V Cen X 3 normalized counts/sec/kev Δχ Δχ Energy [kev] 6 4U [Lamb 1990]

20 SUMMARY AND OUTLOOK 17 Conclusions and a quick outlook Cyclotron line analysis provides an elegant way to probe the magnetic field and the physics of accretion of neutron stars cyclomc Variation of position with pulse phase NOT predicted by an angular variation bulk velocity? Geometry change? No emission wings bottom illuminated slab geometry, possibly gradients to smear out lines! Modelled lines deeper than observed, fitting multiple line (positions) problematic B-field structure Still many d.o.f. where do we go? More fitting / sources / statistics Parallel cross-checks, e.g., depth&cross sections with other models (Isenberg, Nishimura, etc.) B-field structure implementation