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High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
9. The cosmic X-ray and -ray
background
9. The cosmic X-ray and -ray background
• 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
Slide 2
• 1962 Rocket flight.– 2nd attempt – door didn’t open first time– Giacconi got Nobel prize 2002!
• First cosmic background to be discovered– Before the microwave background.
DiscoverySlide 3
Discovery dataSlide 4
The moon casts a shadow
Slide 5
So do interstellar clouds
Draco nebula
Slide 6
So what does the background look like?
• We need to understand what comes from our galaxy and what comes from beyond.
• Sensible to use Galactic coordinates.
Slide 7
Slide 8
Slide 9
Slide 10
Slide 11
Slide 12
So what is going on?
• Isotropic background in the 2-10 keV band.
• At higher energy (i.e. -rays), the galaxy becomes brighter in diffuse radiation.
• At lower energy the galaxy becomes progressively more cut out.
Slide 13
The answer is simple.
• There is more material in the Galactic plane.
• At low energies the X-rays are absorbed by material.
• At high energies -rays are produced by the material.
Slide 14
Photoelectric absorption
• A photon which has > the binding energy of an electron is absorbed: the electron escapes. (hence photoionization)
Slide 15
• 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
Slide 16
Slide 17
Softest energies
• 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
Slide 18
Hardest energies
• 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 -rays
• The more material, the more interactions – so the Galactic plane is bright.
Slide 19
Spectrum of the background• Soft X-rays – lines, thermal emission
• 1-50 keV – harder than AGN
• peak energy around 30 keV
• At higher energies: power law
Slide 20
What makes the highest energy background?
• Gamma-ray imaging extremely difficult• Blazars are leading contenders.
– There are enough of them– They have been detected in -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
Slide 21
Why is there a peak at 30 keV?
• 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.
Slide 22
AGN?• 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 z=2 AGN– With the right population of AGN we can
synthesize the background
Slide 23
• 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.
Slide 24
Slide 25
X-ray background is the history of accretion
• Recent dynamical measurements of galaxy centres imply that ~0.2% of a galaxy spheroid’s mass in the form of supermassive black hole.
• The rest is stars.
• If the black hole built up its mass by accretion:
Slide 26
• 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%).
Eaccretion 0.0015 x 0.1
Estars 0.99 x 0.1 x 0.007= ~ 5
Accretion really is an important source of energy
Slide 27
Resolving the background
• 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.
Slide 28
Improved angular resolution of ROSAT all sky survey: 1000 sources to 77000 sources
Slide 29
Slide 30
About 80% of the soft X-ray background resolved
Slide 31
90% of the soft X-ray background resolved
Chandra X-ray observatory deep field
Where do we stand now?• 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 a little less than expected.– So by resolving the X-ray background we are learning
about the evolution of accretion power over cosmic history.
Slide 32
Some key points:• 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 -rays
• Most of cosmic X-ray and -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 < 10 keV now resolved.
Slide 33