High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 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. –2 nd attempt – door didnt open first time –Giacconi got Nobel prize 2002! First cosmic background to be discovered –Before the microwave background. Discovery Slide 3

Discovery data Slide 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

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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. 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

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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 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

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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 spheroids 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%). E accretion x 0.1 E stars 0.99 x 0.1 x = ~ 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 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