X-ray Absorption and Scattering by Interstellar Dust:

the XMM viewElisa Costantini

Max Planck Institute for extraterrestrial Physics (MPE)

P. Predehl, J. Trümper, G. Hasinger, M.Freyberg, N. Schulz (MIT)

XMM-EPIC consortium meeting Palermo 14-16 Sept 2003

Extinction from IR to Far-UV

Reddened stars Extinction = Absorption + Scattering

• scattering in 4• scattering dominant in Vis.

Both components of extinction CANNOT be separated

Extinction in X-rays

40°

• small angle scattering• X-ray sources surrounded by haloes

Both components of extinction CAN be separated

sca

x

source

dust grain

oberserver

Scattering and Absorption

Scattering grain size distributiondistribution along line of sightgrain number densitydistance determination

Absorption total column density (metals: C,O,Fe,Si...)

Scattering + Absorption physical state of grains chemistry of grains

ID Scattering theory

a = grain size

= wavelenght of incident radiation

Rayleigh-Gans approximation

Differential Scattering cross section:

Often violated in X-ray band !

Exact Mie Theory

X= 2a /

XMM & Chandra•high resolution imaging (0.5'') + broad band spectroscopy Chandra-ACIS-S:

energy dependence of the halo shape constraints on the grain models distance determination

* high sensitivity imaging + spectroscopy XMM-Newton

fainter haloes, lower energies, lower NH

* high resolution spectroscopy Chandra-HETGS, (LETGS) ID composition and abundances

Technical Limitations

Bright fluxes imply strong PILE UP:

• Distortion of surface brightness profiles hampering in flight calibration of PSF & loss of information on the scattering halo • Incorrect determination of the flux additional errors in the comparison halo-PSF

XMM Observation of Cyg X-2Imaging technical details:- vignetting corrected- exposure corrected - annuli of ~0.6' ranging from 0.8' to 4' - central region avoided because of pileup- RAWY > 180 to use only Y9 response matrix- trailed events cut out

Spectral analysis:- cut on the Out of Time events to avoid pile up in the central column

- Information on the Flux lost- Recovered Cyg X-2 spectrum

Cyg X-2 Spectral Analysis (1)0.5-2 keV

6-8 keV

Cyg X-2 Spectral Analysis (2)

Comparison with an halo free source: PKS 0558-504

PKS 0558-504 Cyg X-2

The Scattered RadiationComparison between Rayleigh Gans Approximation and exact Mie Theory

(Smith & Dwek 1998)

Halo ModellingFor each compound / element:(Mg, Fe, Si, C, SiO4, SiO2, Mg2SiO4, Fe2SiO4)

- Density- Energy dependent refractive index- Calculated scattered intensity for each Energy and grain size using the complete Mie theory

- Distributed according to MRN model and WD model - Best fit through minimization

Costantini et al 2003 in prep

(Weingartner & Draine 2001)

Extinction by ID

- There is no evidence of Mg and Si in the absorbed spectrum of Cyg X-2

(NH~2 1021 cm-2)

(Draine et al 2003)

Summary- XMM spectral resolution is suitable for the study of scattering features- Successful detection in Cyg X-2!

- Oxigen, Magnesium, Silicon clearly detected- Standard grain size distribution models satisfactorily fit the data, in first approximation.

Future:- Identification of the scattering compounds- Constraints on grain size distribution- Chemistry of ID towards different line of sights in the Milky Way