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
2
Intrinsic continuum nebula:
Power law from 1-100 keV
(Kuiper et al. 2001)
3
Interstellar absorption
4
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
5
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)
6
Best fit Crab spectrum
• Rebinning factor 5
• Fit only 7-30 Å range
• Exclude regions near
O-K and Fe-L edges
7
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
8
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
9
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
10
Dust
• Two main effects dust:
• Scattering (no photons lost, but halo’s)
• Modifies absorption fine structure near
edges
11
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
12
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
13
Fine structure near Fe-L edge
• Possible to
distinguish ferrous
(Fe2+) from ferric
(Fe3+) iron
Van Aken & Liebscher 2002
14
Fine structure near edges: O & N
O-K
N-K
15
Fine structure near edges: Ne & Fe
Ne-K
Fe-L
16
Composition of the ISM
Wavelengths in Ångstrom
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
 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)
Abundances:
NI
1.04±0.10
N II
OI
1.017±0.011 O II
Total:
-
1.01±0.09
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
Fe II 0.12±0.03
0.78±0.05
0.66±0.03
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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
19
Is Crab a straight power law?
Spatial/spectral variations Crab
(Mori et al. 2004, Chandra imaging)
Circle has r=50”
20
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
21
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
22
Spectral broadening in dispersion
direction due to spatial extent
(taken into account in spectral fitting)
FWHM = 0.2 Å
23
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
24
Dust absorption
• Absorption cross
section per atom for
dust grains differs
from free atoms
• Due to self-shielding
•  dust has less
opacity
25
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)
26