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RGS spectroscopy of the Crab nebula

RGS spectroscopy of the Crab nebula. Jelle S. Kaastra Cor de Vries, Elisa Costantini, Jan-Willem den Herder SRON. Introduction. RGS Crab spectrum used for calibration purposes Here focus on astrophysics : ISM absorption. XMM-Newton OM (231, 291 , 344 nm) Courtesy A. Talavera, ESA.

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RGS spectroscopy of the Crab nebula

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  1. RGS spectroscopy of the Crab nebula Jelle S. Kaastra Cor de Vries, Elisa Costantini, Jan-Willem den Herder SRON

  2. Introduction • RGS Crab spectrum used for calibration purposes • Here focus on astrophysics: ISM absorption XMM-Newton OM (231, 291,344 nm) Courtesy A. Talavera, ESA

  3. Intrinsic continuum nebula: Power law from 1-100 keV(Kuiper et al. 2001)

  4. Interstellar absorption

  5. Fit to RGS Crab spectrum Basic idea: • use fixed intrinsic continuum shape Crab from Kuiper model (with Crab Curvature Correction) • Determine foreground absorption from spectral curvature & edges measured with RGS

  6. Absorption model • Absorption model hot of SPEX (transmission of plasma in Collisional Ionisation Equilibrium) • Take kT low (quasi-neutral) • Free parameters: columns of H, N, O, Ne, Mg and Fe (plus singly ionised ions) • Other elements coupled to H using protosolar (Lodders) abundances • Correction for dust (cf. Wilms et al. 2000)

  7. Best fit Crab spectrum • Rebinning factor 5 • Fit only 7-30 Å range • Exclude regions near O-K and Fe-L edges

  8. Composition of the ISM(after Ferrière 2001) • hot ionised gas (~106 K) • warm ionised gas (~8000 K) • warm atomic gas (6000-10000 K) • cold atomic gas (20-50 K) • molecular gas (10-20 K) • dust

  9. Limits on hot gas • Little O VIII / O VII (from weak lines) • Comparison with 4U1820-303 (Yao & Wang 2006): Crab has 2x NH, but 10-30 % of O VIII/ O VII • NH(hot) ≤ 1% NH(cold) • Hot gas can be ignored 4U 1820-303, Chandra LETGS

  10. Limits on molecular gas • H2 has 1.42 x X-ray opacity per atom as compared to H I • Typically, Galaxy has 20 % molecules •  opacity ~8 % higher if molecules present abundances affected • CO map NH2<0.001NHI •  molecules can be ignored CO map (Dame et al. 2001) of 10°x10° around Crab

  11. Dust • Two main effects dust: • Scattering (no photons lost, but halo’s) • Modifies absorption fine structure near edges

  12. Dust scattering • Chandra modeling halo: scattering column NH~2x1021 cm-2 (Seward et al.) • Scattering column ~2/3 of total absorption column (3x1021 cm-2) • Our fit also shows this ratio directly in absorption

  13. Fine structure near O-K edge • Laboratory measurements Van Aken et al. 1998 • Different line position 1s-2p transition of atomic O I and bound oxygen

  14. Fine structure near Fe-L edge • Possible to distinguish ferrous (Fe2+) from ferric (Fe3+) iron Van Aken & Liebscher 2002

  15. Fine structure near edges: O & N O-K N-K

  16. Fine structure near edges: Ne & Fe Ne-K Fe-L

  17. Composition of the ISM Wavelengths in Ångstrom  Mixture half atomic, half ferric?

  18. Abundances (gas & dust) • Neutral hydrogen column: 3.21±0.02 x 1021 cm-2 (compare to 3.0±0.5 x 1021 from Lyα absorption, Sollerman et al. 2000) Abundances: Total:

  19. Conclusions • Excellent RGS spectra Crab nebula provide: • Accurate ISM abundances (Ne 1.7 times overabundant, O & N solar) • Spectral evidence for ~half gas, half dust mixture

  20. Is Crab a straight power law?Spatial/spectral variations Crab(Mori et al. 2004, Chandra imaging) Circle has r=50”

  21. Crab Curvature Correction • Addition of softer and harder parts of remnant, each with power law spectrum, leads to curvature (softening at low E) • Apply this Crab Curvature Correction to Kuiper et al. continuum

  22. Dust scattering I • Dust scattering along line of sight gives halo’s • Crab has ~ 10 % of flux in halo • Scattering is energy dependent, but no photons destroyed • Example: Chandra, Seward et al. 2006

  23. Spectral broadening in dispersion direction due to spatial extent(taken into account in spectral fitting) FWHM = 0.2 Å

  24. Dust scattering II • Seward et al. find scattering column NH~2x1021 cm-2, from modeling of halo images • Scattering column ~2/3 of absorption column (3x1021 cm-2) • Our fit also shows this ratio directly in absorption

  25. Dust absorption • Absorption cross section per atom for dust grains differs from free atoms • Due to self-shielding •  dust has less opacity

  26. Comparison with other results • Similar columns for O I, Mg I+II, Fe II from opt/UV obs. Crab, but they have 0.3-0.6 dex errors (Sollerman et al. 2000) • Also solar O/H found in absorption towards 11 clusters (Baumgartner & Mushotzky 2006) • Overabundance Ne is 1.8, not as high as factor 2.6 by Drake & Testa (2005)

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