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Astrophysics from Space Lecture 12: The cosmic high-energy background

Astrophysics from Space Lecture 12: The cosmic high-energy background. Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year 2013-2014. Background radiation. CMB is very famous, but cosmic background radiation exists at (nearly) all wavelengths.

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Astrophysics from Space Lecture 12: The cosmic high-energy background

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  1. Astrophysics from Space Lecture 12: The cosmic high-energy background Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year 2013-2014

  2. Background radiation CMB is very famous, but cosmic background radiation exists at (nearly) all wavelengths. E.g. DIRBE onboard COBE: measurement of the absolute IR background radiation. Measuring the cosmic background is challenging: requires elimination of instrumental effects and foreground (MW) emission…

  3. CXB: HEAO-1

  4. CXB: ROSAT

  5. Cosmic X-ray background • CXB already discovered in 1962. • Two options • superposition of unresolved point sources (CIRB) • truly diffuse mechanism (CMB) • 1970s: first X-ray all-sky surveys (Uhuru, Ariel-V) • X-ray background is very isotropic (and hence extragalactic). • Most popular option: superposition of large numbers of AGNs.

  6. CXB: the spectral paradox HEAO-1 results puzzling: the spectral shape of the CXB (in the 3-50 keV range) was very well fit by a thermal Bremsstrahlung model of 5 x 108 K. Spectral paradox: reliable X-ray spectra of essentially all Seyfert-1 galaxies have a completely different shape. Strongly suggests that the CXB is due to a diffuse thermal plasma.

  7. CXB: Einstein and COBE Pointed Einstein observations of known X-ray sources revealed a class of very luminous AGN. Deep Einstein surveys resolved 20% of the soft CXB radiation into discrete sources. Many identified as AGN. Subtracting these AGN from CXB destroys the apparent thermal nature of the CXB. Final conclusive observation: exact blackbody nature of CMB leaves no room for hot thermal plasma (strong limit on so-called comptonization parameter).

  8. ROSAT Lockman Hole survey Lockman Hole: region in the sky with minimal contamination from galactic absorption/emission: ideal for extragalactic background radiation. 1991: ROSAT ultra-deep observation of Lockman Hole (1.4 Ms = 390 h). The X-ray analog to the HDF.

  9. ROSAT Lockman Hole survey • Important result: 70% to 80% of the soft X-ray CXB could be resolved into point sources. • Follow-up observations (radio, IR, optical…) • Extremely important: optical (multi-object) spectroscopy • type identification • redshift determination • Most objects turned out to be classical broad-line • AGN (often quasars).

  10. XRB completely resolved ? • One remaining problem: at the faintest levels, classical quasars contribute at most 50% to the total number of X-ray point sources. • Solution: extremely deep surveys with Chandra and XMM. • Chandra: excellent PSF (deep surveys on small fields) • XMM-Newton: excellent sensitivity and FOV (shallower surveys on larger fields)

  11. Chandra Lockman Hole

  12. Chandra Deep Field South

  13. XRB completely resolved ? Extremely deep X-ray images reveal 85% to 100% of the XRB in discrete sources. At the brightest level: classical AGN (most quasars) At the faintest level: different population of AGN, characterized by different amounts of intrinsic absorption (population II AGN) Dark grey: population II AGNMedium grey: broad-line AGNLight grey: unidentified sources

  14. Cosmic quasar density (revised) Remember: optical redshift surveys allowed to investigate the cosmic quasar density as a function of redshift. Main result: there were 100-1000 times more luminous quasars at z ≈ 2.5 than today… Deep X-ray surveys can provide an alternative method to investigate the cosmic quasar density

  15. Cosmic quasar density (revised) • X-ray quasar density in agreement with optical quasar density (100x more abundant at z = 2-3 than today) • Different cosmic evolution for low-luminosity AGN (Seyferts). • Seyfert peak at z = 1 • decline of only factor 10 X-ray based cosmic AGN density suggests an anti-hierarchical supermassive black hole growth scenario. This is not expected in most ΛCDM models…

  16. Cosmic gamma-ray background • First evidence for the existence of a diffuse CGB at E > 35 MeV: SAS-2 (1972-1973) • The usual two options • superposition of unresolved point sources • truly diffuse emission

  17. Cosmic gamma-ray background Until 1991: diffuse mechanisms preferred (only 4 extragalactic gamma-ray sources were known) Since 1991 (CGRO): detection of many AGN. Result: superposition of AGNs is the preferred explanation for CGB. CGRO EGRET all-sky map

  18. Cosmic gamma-ray background • Current situation (similar to CXB in early 1990s): • the diffuse emission starts to be resolved in point sources • properties of known point sources still poorly known • No population can explain the entire CGB • E < 500 keV: normal AGN • E > 20 MeV: blazars • in between: unclear… (SNe, normal galaxies, SB galaxies,GRBs, matter-antimatter annihilations, self-annihilatingDM, exotic particles…)

  19. Cosmic gamma-ray background

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