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 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 (~10 6 K) warm ionised gas (~8000 K) warm atomic gas ( 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 4U (Yao & Wang 2006): Crab has 2x N H, but % of O VIII/ O VII  N H (hot) ≤ 1% N H (cold)  Hot gas can be ignored 4U , Chandra LETGS

10 Limits on molecular gas H 2 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  N H2 <0.001N HI  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 N H ~2x10 21 cm -2 (Seward et al.)  Scattering column ~2/3 of total absorption column (3x10 21 cm -2 ) Our fit also shows this ratio directly in absorption

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

14 Fine structure near Fe-L edge Possible to distinguish ferrous (Fe 2+ ) from ferric (Fe 3+ ) iron Van Aken & Liebscher 2002

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

16 Fine structure near edges: Ne & Fe Ne-KFe-L

17 Composition of the ISM CompoundO I 1s-2p or main line Fe 2p-3d main Fe 2p-3d 2nd Ferrous, Fe 2+ (e.g., olivine) Ferric, Fe 3+ (e.g., Fe 2 O 3 ) Atomic, O I or Fe I Crab23.466± ± ±0.016 Wavelengths in Ångstrom  Mixture half atomic, half ferric?

18 Abundances (gas & dust) Neutral hydrogen column: 3.21±0.02 x cm -2 (compare to 3.0±0.5 x from Lyα absorption, Sollerman et al. 2000) N I1.04±0.10N II-1.01±0.09 O I1.017±0.011O II0.013± ±0.016 Ne I1.55±0.07Ne II0.17± ±0.11 Mg I0.85±0.20Mg II0.00± ±0.21 Fe I0.66±0.03Fe II0.12± ±0.05 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 N H ~2x10 21 cm -2, from modeling of halo images  Scattering column ~2/3 of absorption column (3x10 21 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 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)