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The diffuse X-ray Galactic Centre with XMM-Newton

Anne Decourchelle Service d'Astrophysique, CEA Saclay. 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.

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The diffuse X-ray Galactic Centre with XMM-Newton

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  1. Anne Decourchelle Service d'Astrophysique, CEA Saclay 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

  2. - 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 Before XMM-Newton and Chandra

  3. 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)

  4. 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)

  5. 0.0° 0.3-9 keV XMM-Newton EPIC MOS 0.0° +0.5° 0.0° Radio VLA 90 cm 0.0° -0.5° 1.0° 359.0°

  6. S K Si K Fe 6.4 keV Ca K Ar K Fe Ka GC6 GC4 GC7 GC1 Multi-component spectra of the diffuse emission Counts/s/keV Counts/sec/keV Energy (keV) Energie en keV

  7. Continuum subtracted line emission Morphology of the different components: XMM-Newton Sulphur K 4-6 keV +0.5° Fe K 6.4 keV Radio VLA Fe K 6.7 keV

  8. Sulphur K Continuum 4-6 keV Fe K 6.4 keV Fe K 6.7 keV

  9. Fe K 6.4 keV S K Soft thermal equilibrium Low Energy CR Electrons Fe K 6.7 keV Hot thermal equilibrium plasma

  10. 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 • c2 = 3731 / 3332 ~ 1.1 • 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.)

  11. 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

  12. 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-a with • a~ -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)

  13. Spectral characteristics of the highly ionised iron component • 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 Equivalent width • 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

  14. CONCLUSIONS • 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

  15. Discrete sources ? Results from Chandra • - 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

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