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)