High energy Astrophysics

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High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
9. The cosmic X-ray and g-ray background
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9. The cosmic X-ray and g-ray background
Slide 2

• This lecture
• Discovery of the background?
• How isotropic, what is its spectrum?
• Photoelectric absorption
• Resolving the background
• Synthesis of the background
• Growth of black holes
• Latest developments

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Discovery
Slide 3

• 1962 Rocket flight.
• 2nd attempt door didnt open first time
• Giacconi got Nobel prize 2002!
• First cosmic background to be discovered
• Before the microwave background.

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Discovery data
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The moon casts a shadow
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So do interstellar clouds
Slide 6

• Draco nebula

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So what does the background look like?
Slide 7

• We need to understand what comes from our galaxy
  and what comes from beyond.
• Sensible to use Galactic coordinates.

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So what is going on?
Slide 13

• Isotropic background in the 2-10 keV band.
• At higher energy (i.e. g-rays), the galaxy
  becomes brighter in diffuse radiation.
• At lower energy the galaxy becomes progressively
  more cut out.

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The answer is simple.
Slide 14

• There is more material in the Galactic plane.
• At low energies the X-rays are absorbed by
  material.
• At high energies g-rays are produced by the
  material.

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Photoelectric absorption
Slide 15

• A photon which has gt the binding energy of an
  electron is absorbed the electron escapes.
  (hence photoionization)

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Slide 16

• For most elements, inner shell transitions most
  important at X-ray energies.
• Greatest absorption at soft X-ray energies
• He, C and O are most important at soft energies.
• Fe important at higher energy

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Softest energies
Slide 18

• Galactic poles brightest in soft X-rays
• Least material in these directions
• But we still find X-rays close to plane in the
  softest band.
• Much of soft X-ray background is local
• We live in a hot bubble probably the remnant
  of a supernova
• Some of soft X-ray background now even thought to
  come from within the solar system due to charge
  exchange in the solar wind.

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Hardest energies
Slide 19

• High energy cosmic rays interact with the nuclei
  of atoms or ions
• their energies are much higher than electron
  binding energies
• Smash them to pieces. Various particle decays
  produce g-rays
• The more material, the more interactions so the
  Galactic plane is bright.

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Spectrum of the background
Slide 20

• Soft X-rays lines, thermal emission
• 1-50 keV harder than AGN
• peak energy around 30 keV
• At higher energies power law

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What makes the highest energy background?
Slide 21

• Gamma-ray imaging extremely difficult
• Blazars are leading contenders.
• There are enough of them
• They have been detected in g-rays
• They have the right spectrum
• Also expected to be a contribution from SN1a
  between 200keV and 2500 keV
• Radioactive decay of unstable isotopes produced
  in the supernova

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Why is there a peak at 30 keV?
Slide 22

• Possibility was bremsstrahlung the Universe is
  full of hot gas?
• Ruled out by COBE microwave background
  spectrum/isotropy
• Left with lots of individual sources producing
  the background as leading hypothesis.

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AGN?
Slide 23

• AGN the most X-ray productive source population
  known.
• But their spectra are too soft ruled out?
• Back to photoelectric absorption
• Greatest absorption at soft energies absorbed
  spectra are hard.
• AGN run out of steam at 100 keV
• This will appear as 30 keV in z2 AGN
• With the right population of AGN the background
  can be synthesized.

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Slide 24

• AGN only emit 10 as X-rays
• BUT
• If UV radiation absorbed as well, quite a lot of
  energy could be hidden from us.
• If we correct XRB spectrum for absorption, we
  can work out how much energy we are missing.
• Use normalisation at 30 keV where photoelectric
  opacity minimal.
• Could be a significant amount of radiation
  re-emitted in the infrared.

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X-ray background is the history of accretion
Slide 26

• Recent dynamical measurements of galaxy centres
  imply that 0.2 of a galaxy spheroids mass in
  the form of supermassive black hole.
• The rest is stars.
• If the black hole built up its mass by accretion

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Slide 27

• Energy released by stars is mass in stars (99) x
  fraction turned into helium (10) x efficiency of
  hydrogen burning (0.7).
• Energy released by accretion is mass in black
  hole (0.15) x efficiency of accretion (10).

Estars 0.99 x 0.1 x 0.007
5

Eaccretion 0.0015 x 0.1
Accretion really is an important source of energy
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Resolving the background
Slide 28

• To truly find out whether the background is made
  by sources, we need to resolve it into sources.
• Biggest problem historically is angular
  resolution faint sources are blurred together.

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Slide 29
Improved angular resolution of ROSAT all sky
survey 1000 sources to 77000 sources
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About 80 of the soft X-ray background resolved
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Slide 31
Chandra X-ray observatory deep field
90 of the soft X-ray background resolved
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Where do we stand now?
Slide 32

• At low energy about 90 of the background is
  resolved
• Biggest source of uncertainty is the measurement
  of the diffuse background itself
• The faint sources have hard spectra, as expected.
  
• A variety of evidence suggests that they are
  absorbed.
• Redshifts are a little less than expected.
• So by resolving the X-ray background we are
  learning about the evolution of accretion power
  over cosmic history.

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Some key points
Slide 33

• The X-ray background was the first cosmic
  background to be discovered.
• Early hypothesis that it is produced by diffuse
  hot gas has been proved wrong.
• Material in our galaxy
• absorbs the soft X-ray background
• interacts with cosmic rays to produce a strong
  signal in g-rays
• Most of cosmic X-ray and g-ray background comes
  from AGN
• tells us about the history of accretion
• we see a universe full of massive black holes
• Most of background at lt 10 keV now resolved.