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

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

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• 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

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So do interstellar clouds

Draco nebula

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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.

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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.

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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.

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Photoelectric absorption

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

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• 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

• 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

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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.

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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

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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

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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.

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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

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• 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

• 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:

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• 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

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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.

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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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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.

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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.

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