The diffuse X-ray Galactic Centre with XMM-Newton

1- Problematic of the hot component

2- Morphology and spectral modeling of the different components at high

energy

Anne DecourchelleService d'Astrophysique, CEA Saclay

Before XMM-Newton and Chandra

-- Hard X-ray emission from the Galactic Ridge and particularly from the Galactic Centre region (Worrall et al. 1982)

-- ASCA: emission lines from highly ionised elements (Si, S, Ar, Ca, Fe)

--> ionization equilibrium multi-temperature diffuse gas with a component at 10 keV -(Koyama et al. 1996)

--> Observed spectra identical in shape from place to place except for the 6.4 keV iron K line

Enigma of the hot diffuse X-ray emission

Hot thermal component at ~10 keV

But confinement and production of a 10 keV plasma problematic !

- What is the heating source ? Young supernova remnants: kT ~ few keV

- Plasma not bound by the Galactic gravitational potential

- Constant replenishement required: What is the energy source ?

-Alternative explanations for the origin of the hard component ?

-Discrete sources (see talks by Muno and Bykov)

-Diffuse non-thermal emission:

-- bremsstrahlung from CRs interacting with neutral material (Valinia et al. 2000)

-- charge-exchange interactions between CR ions and ISM (Tanaka 2002)

-- quasi-thermal emission from continuously accel. particles (Dogiel et al. 2002, Masai et al. 2002)

-Hot helium plasma (see talk by Belmont)

XMM-Newton observations

(Saclay, Leicester, MPE)

Total exp. time: 250 ks

Goals:- diffuse emission - Sgr A*, X-ray binaries- supernova remnants:

G0.9+0.1, Sgr A East,…

Collaborators :

- R. Warwick, M. Sakano (Leicester, UK)

- A. Goldwurm (Saclay, F)- D. Porquet , P. Predehl

(MPE, Germany)

XMM-Newton EPIC MOS 0.3-9 keV

Radio VLA 90 cm

0.0°

0.0°

0.0°

0.0°

359.0°1.0°

+0.5°

-0.5°

Energie en keV

Cou

nts

/s/k

eV

GC1

GC4GC6

GC7

Si K S KCa KAr K

Fe 6.4 keV Fe K

Multi-component spectra of the diffuse emission

Energy (keV)

Cou

nts/

sec/

keV

+0.5°

Morphology of the different components:

XMM-Newton

Continuum subtracted line emission

Sulphur K

Fe K 6.4 keV

Fe K 6.7 keV

4-6 keV

Radio VLA

Sulphur K Continuum 4-6 keV

Fe K 6.4 keV Fe K 6.7 keV

Soft thermal

equilibrium

S K

Fe K 6.7 keV

Hot thermal equilibrium

plasma

Fe K 6.4 keV

Low EnergyCR Electrons

We impose over all the set of considered regions:- same spectral index for the electron distribution function - same temperature for the hard thermal component ass. with iron (high ionis.)- same abundance for the two components ass. with iron (low and high ionis.)

Spectral characteristics of the hard components derived from combined fitting of a set of individual homogeneous regions

Model:

- soft thermal equilibrium plasma

- low energy cosmic ray electron excitation

- hot thermal equilibrium plasma

2 = 3731 / 3332 ~ 1.1

Spectral characteristics of the component associated to the sulphur K line emission

- Structured morphology

- Variation of the interstellar absorption in the line of sight:

NH ~ 3- 7 10 22 cm-2

- Variation of the temperature from place to place:

kT ~ 0.36 -1.0 keV

=> supernova remnants population

Spectral characteristic of the weakly ionised iron

- Structured morphology

(molecular clouds)

- Strong interstellar absorption:

NH ~ 8- 15 10 22 cm-2

- Compatible with excitation by low energy (~ MeV) cosmic ray electrons of spectrum E- with

~ -0.3 (Valinia et al. 2000)

Abundance ~ 1.3 [1.2-1.4] solar

See talk by Warwick

Alternative to accelerated electrons: excitation by photons of E > 7.1 keV Bright past activity of the Galactic centre (see talk by Revnitsev) Internal source of irradiation (see talk by Law)

Spectral characteristics of the highly ionised iron component

Large scale mechanism required to explain the 6.7 keV component not related to Sgr A* and not related directly to the soft component

not consistent with a quasi-thermal emission model

Equivalent width

- Density effect towards the centre

- Strong interstellar absorption:

NH ~ 8- 15 1022 cm-2

- Uniform large scale morphology

- Uniform spectral characteristics compatible with a thermal equilibrium plasma:

kT ~ 7.7 [7.2-8.2] keV

Abundance ~ 1.0 [0.8-1.1] solar

Morphological and spectral identification of three different components

- Soft X-ray emission (sulphur K line): various supernova remnants

- structured morphology and different emitting conditions of a soft plasma at equilibrium

- 6.4 keV line emission and associated continuum

- structured morphology (molecular clouds) and spectra compatible with low energy cosmic ray electrons impact

- Hard X-ray component (ionised iron K line at 6.7 keV)

- uniform distribution and uniform spectral characteristics

=> Incompatible with a quasi-thermal plasma of continuously accelerated particles

CONCLUSIONS

-- Not enough discrete sources with Lx > 1031 erg/s to account for more than 10 % of

the diffuse emission (Ebisawa et al. 2001).

-- Less than 10 % of the flux from point sources detected (Muno et al. 2003)

-However source spectrum at high energy similar to diffuse emission (Muno et al. 2004)

- see talk by A. Bykov

-Muno et al. 2003,2004: ~ 2350 sources subtracted

Discrete sources ? Results from Chandra