UK XMM Science

Our Earth orbits the Sun and our Sun is one of roughly 100 billion stars which make up our Galaxy, the Milky Way. The Milky Way is the swathe of stars you can see across the night sky on a good clear, dark night (it's even more impressive in the southern hemisphere, where you look towards the centre of the Galaxy).

Our Galaxy is one of several others known as the Local Group, and is part of the larger family of galaxies called the Virgo Cluster. Clusters of galaxies themselves are bound together in superclusters. There are billions of galaxies in the Universe each full of billions of stars, and each one is unique. But not all of them emit X-rays - the most important types are listed below.

Normal galaxies

X-ray observations of normal galaxies are made to look at individual objects as well as gases within the galaxy itself. One type of these individual objects is Seyfert nuclei, which are smaller, less luminous versions of quasars. Another type is high mass X-ray binaries, which are accreting black hole binary star systems. Both of these are the subject of intense study - see the section on black holes for more information.

The hot phase of the `interstellar medium', the gases which lie between the stars, also emit X-rays. The mechanism by which these gases are heated is not well known and will be investigated by XMM.

Starburst galaxies

In the hearts of some otherwise normal galaxies, there are regions where enhanced rates of star formation and evolution exist. When very massive stars are formed they are very hot and emit `stellar winds'. They burn at a fast and furious rate and eventually turn supernova, blasting their material into their surroundings. The most massive stars sometimes endure, and survive, several cataclysmic explosions, even before going supernova. Galaxies with active regions of this kind are called `starburst galaxies' and can reach high X-ray luminosities, approaching those of Seyfert galaxies (low-luminosity quasars). EPIC and RGS will provide important diagnostic information about the conditions in starburst galaxies, for example how they're heated, what the conditions of the gas are like and how many `metals' (elements other than Hydrogen and Helium) are present. The amount of metals in the gas is important for example, because these massive stars are the `factories' which manufacture the elements that compose our environment - and ourselves.

Clusters of galaxies

While lone galaxies do exist, many are gravitationally bound together in huge families called `clusters'. These huge systems have enormous gravitational potentials and gradually suck in cool gas from the galaxies towards the gravitational centre, which is normally the site of a giant elliptical galaxy. The gas is heated as it falls in, reaching temperatures of 10 to 100 million degrees. It emits very strongly in X-rays thus clusters are amongst the most X-ray luminous sources in the Universe.

X-ray observations will trace the temperature, density and `metallicity' of these `cooling flows'. This information will, in turn, allow astronomers to trace the distribution of matter in galaxies and clusters. This is important for studies of the `missing matter' problem, ie we can only `see' about 10% of the matter that there is believed to be in the Universe. Some of the missing 90% is this X-ray emitting gas which lies in clusters. XMM will gives us deeper and more sensitive searches for `missing' cluster gas.

Supernova remnants

When massive stars come, rapidly, to the end of their lives, they undergo a massive explosion known as a `supernova'. The star first begins to collapse in on itself, then throws off its outer shells, leaving behind a neutron star or perhaps a black hole. X-rays are emitted by the hot gas of the supernova remnant (SNR) and also when the SNR shock wave crashes into the surrounding interstellar medium. EPIC and the RGS on XMM will be used to examine SNRs in our Galaxy and beyond, measuring temperatures and shell velocities, and looking at material from the star which has now been torn apart.

Black holes

Neutron stars

Instead of a black hole however, a neutron star may form. These are very exotic objects - the material of which they are composed is like a `neutron soup'. Essentially, the forces which normally keep the electrons and protons apart have been overcome by gravity, so they have all been forced together to from neutrons. Neutron stars in binaries accrete matter from their companions, reaching very high temperatures and emitting X-rays. Lone neutron stars are also thought to emit in X-rays, but very faintly, thus XMM will be pushed to its limits to search for these.

Pulsars

Pulsars are very rapidly spinning neutron stars found in SNRs. They have very strong magnetic fields, and X-rays are thought to be produced due to the relativistic effects of the magnetic field lines moving at light speeds.

Pulsars in binary systems rotate on timescales as fast as a thousandth of a second. And they spin faster with time, effectively gaining energy from that which is lost by the secondary. The EPIC-pn camera can make very fast timing studies of these objects, and with its excellent sensitivity, will be able to search over much greater distances for X-ray pulsars.

Space is not a perfect vacuum - it is never completely empty of particles or energy, even in the deepest parts of the Universe. Just as there is a Cosmic Microwave Background (CMB) which pervades space, there is also a Cosmic X-ray Background (CXB). However, while the CMB is believed to be redshifted relic of the Big Bang, the origin of the CXB is thought to be dominated by the emissions of galaxies and quasars which are too faint for our X-ray telescopes to measure individually.

However, while this seems a natural and attractive explanation, when we compare the spectra of the galaxies and quasars that we can see with the spectrum of the CXB, there are significant differences. Are the very faint X-ray emitters different from the brighter ones that we can see? XMM will make very deep observations of the faintest X-ray emitting objects in the sky, revealing them for the very first time. These observations will either resolve the paradox or deepen the mystery.

Part of the scientific motivation behind the XMM Survey Science Centre (SSC; SSC at Leicester, SSC at MSSL) is to build up a high quality database for CXB studies. During XMM's expected lifetime, it will record X-rays from millions of `serendipitous' sources - sources which are not the main target under study yet still provide valuable information - and these will be processed by the SSC.

Interstellar material in our Galaxy

XMM will be used to study the hot gases in our own Milky Way as well as other galaxies. The picture we get from our Galaxy will give us a template with which to compare external galaxies of a similar type. It will also give us detailed information about the structure, density and temperature of hot gas between the stars in the Galaxy, and the distribution of the elements and how its related to the stellar population.

`Cool' gas

`Cool gas? But didn't you say that a gas had to be hot to emit X-rays?'

Cool gas doesn't emit X-rays, but it does absorb them, so essentially we can map out the distribution of cool gas in our galaxy by looking at how many X-rays are removed before they reach us. Metals in the gas imprint certain features on a spectrum as well, so we can also trace the fractions of different elements throughout our Galaxy, and the temperature of the absorbing material.

The hot coronae of stars

Did you see the total eclipse of the Sun on a cloudless sky? Then you would have seen the corona at the point of totality - a very hot gas reaching far beyond the Sun which emits strongly in X-rays. Even with a star on our doorstep, astronomers still don't fully understand how the corona is heated to and maintains such high temperatures. And since X-ray coronae probably exist around stars of all types, this is an important question to answer. XMM will be able to detect many more normal, X-ray emitting stars than ever before, increasing our sample of such objects many times so that significant progress can at last be made.

Comets

Even comets are sources of X-rays. Some of it is due to the interactions of the comet's dust with X-rays from the Sun, but not all. Observations of comets in X-rays are very few and far between however, since astronomers did not expect these results, so XMM will make the first detailed X-ray studies of comets. They promise some fascinating results.