Post on 16-Dec-2015
RGS spectroscopy of the Crab nebula
Jelle S. KaastraCor de Vries, Elisa Costantini,
Jan-Willem den HerderSRON
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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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Intrinsic continuum nebula: Power law from 1-100 keV
(Kuiper et al. 2001)
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Interstellar absorption
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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
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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)
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Best fit Crab spectrum
• Rebinning factor 5• Fit only 7-30 Å range• Exclude regions near
O-K and Fe-L edges
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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
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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 ignored4U 1820-303, Chandra LETGS
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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
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Dust
• Two main effects dust:
• Scattering (no photons lost, but halo’s)
• Modifies absorption fine structure near edges
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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
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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
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Fine structure near Fe-L edge
• Possible to distinguish ferrous (Fe2+) from ferric (Fe3+) iron
Van Aken & Liebscher 2002
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Fine structure near edges: O & N
O-K N-K
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Fine structure near edges: Ne & Fe
Ne-K Fe-L
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Composition of the ISM
Compound O I 1s-2p or
main line
Fe 2p-3d
main
Fe 2p-3d
2nd
Ferrous, Fe2+ (e.g., olivine)
23.09 17.498 17.196
Ferric,
Fe3+ (e.g., Fe2O3)
23.42 17.456 17.130
Atomic,
O I or Fe I
23.508 17.453 17.142
Crab 23.466±0.009 17.396±0.009 17.120±0.016
Wavelengths in Ångstrom
Mixture half atomic, half ferric?
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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)
N I 1.04±0.10 N II - 1.01±0.09
O I 1.017±0.011 O II 0.013±0.008 1.030±0.016
Ne I 1.55±0.07 Ne II 0.17±0.08 1.72±0.11
Mg I 0.85±0.20 Mg II 0.00±0.07 0.85±0.21
Fe I 0.66±0.03 Fe II 0.12±0.03 0.78±0.05
Abundances: Total:
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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
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Is Crab a straight power law?Spatial/spectral variations Crab
(Mori et al. 2004, Chandra imaging)
Circle has r=50”
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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
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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
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Spectral broadening in dispersion direction due to spatial extent
(taken into account in spectral fitting)
FWHM = 0.2 Å
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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
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Dust absorption
• Absorption cross section per atom for dust grains differs from free atoms
• Due to self-shielding dust has less
opacity
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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)