X-ray Astrophysics - The high energy cosmos

7 November 2012

X-ray Astrophysics
-          The High Energy Cosmos

Professor Carolin Crawford

Armchair astronomers enjoy browsing the many fantastic pictures of space produced by the Hubble Space Telescope and modern ground-based telescopes. But there is a familiarity about many of these images –most are taken in visible light, and are readily recognisable as the light of stars, glowing gas clouds and where they are grouped together to form galaxies. The visible waveband, however, only give us a very limited view of the Universe. In particular, it does not show us matter that is either incredibly hot (at temperatures of tens of millions of degrees), extremely cold (at only a few degrees above absolute zero), or that is produced in violent cosmic events. Emission peaks in the optical waveband when radiated by matter within the narrow temperature range of a few thousand to a few tens of thousands of degrees; there are other forms of light which are produced via very different physical processes, and which illuminate very different characteristics of the Universe. Much of the X-ray emission that astronomers detect, for example, is powered by material falling under the pull of extreme gravity around compact objects such as white dwarfs, neutron stars and black holes.

The Electro-Magnetic Spectrum

Wavelengths
First, let’s be absolutely clear about what is meant by ‘X-ray light’. A prism can be used to split visible light into the constituent colours of the rainbow to create what is known as a spectrum. Different colours are characterised by their wavelength: if we view light as travelling in the form of a wave, the wavelength is the distance between repetitions of features in the signal, such as a peak or a trough. Blue light has a shorter wavelength than red light. However, there isn’t really that much difference between red and blue – the wavelength of blue light is around forty thousandths of a mm, and red light is around sixty thousandths of a mm. Wavelengths can cover a much wider range – they vary from thousands of km down to a fraction of the size of an atom, but the colours such wavelengths correspond to are invisible to our eyes, and thus form the ‘other’ light beyond the visible rainbow: the radio, infrared, ultraviolet, X-ray and  gamma-rays. All colours of light are collectively known as the electromagnetic spectrum, and travel through a vacuum at the speed of light. All electromagnetic radiation simply represents the transfer of energy from one place to another in the form of changing electric and magnetic fields.

Photons
The quantum physics interpretation of light describes it as a stream of elementary particles known as photons, which can display properties of both waves and particles … depending on the experiment and how it’s being observed. A photon has no mass, travels at the speed of light, and carries an amount of energy that is inversely proportional to its wavelength. Thus we can describe light not only in terms of wavelength or frequency, but also in terms of energy: radio and infrared photons have far less energy than in the X-ray and gamma-ray wavebands. Crucially, more energetic photons are produced in more energetic events.

Temperature Dependence
The amount of energy a photon possesses can be most simply related to the temperature of the object giving off the radiation. The molecules and/or atoms making up any substance (as long as it’s not at a temperature of absolute zero) are in continuous motion; the warmer that substance, the more the particles move about, and the more energetic the photons given off. Thus as a metal is heated, the colour of the light it emits will change from red-hot to the bluer white-hot as its temperature increases.

Stars
This principle can be extended to stars, many of which have surfaces at tens of thousands of degrees in temperature and so radiate most strongly in the visible waveband. But not all stars are the same temperature or colour: whilst blue stars are at 15,000-80,000 K, yellower stars like our Sun are at 5,000-8000 K and the coolest, reddest stars have temperatures of 2,000-4000K. Of course, many young massive stars are sufficiently hot to produce not just the blue visible light, but also copious amounts of more energetic ultraviolet light. As soon as you have matter at much more extreme temperatures of millions of degrees, most of the light given off is in the X-ray waveband.  (There are other ways of making X-ray light though, which we’ll return to later.)

X-radiation

The Discovery of X-rays
X-radiation was discovered in 1895 by Wilhelm Röntgen, a German physics professor at the University of Würzburg, and for this he was awarded the first Nobel Prize for physics, in 1901. The (originally temporary) name of ‘X-rays’ reflects their unknown and mysterious nature at the time. X-rays can pass through materials that block visible light, and are only stopped by much denser substances – as shown in one of the very first X-ray images Röntgen recorded, which was of his wife’s hand. Where the film has been exposed to X-rays it is white, and remains dark where the radiation has been blocked from falling on the film. Most of the X-rays travel through the flesh; some are stopped by the bone, and most by the metal of the wedding ring. Images such as these are the kind of X-rays we are most familiar with, as the ability to use the radiation to obtain a silhouette of the bone structure within the body immediately lends itself to medical use. However, the study of astronomical X-rays are fundamentally different from medical X-rays, as we are less interested in the shadow cast by intervening object, and more in the original source producing the X-ray emission.

X-ray Production

X-ray Colours
X-rays are emitted at a range of colours – in fact, there is a far wider variety of X-ray colours than there are in the visible waveband. In general, X-ray colours are referred to as ‘hard’ and ‘soft’ for high- and lower-energy photons respectively.

Thermal X-rays
X-rays are very energetic photons, and can most simply be produced by very hot matter in what is known as thermal emission. To get something to glow in the optical, it needs to be heated to temperatures of thousands of degrees; as an X-ray photon has over a thousand times more energy than an optical photon, so to get matter to glow with thermal X-radiation requires it to be heated to temperatures of millions of degrees.

The Sun
The nearest astronomical X-ray emitter is our nearest star, the Sun. It may not be an obvious radiator of X-rays, as its surface is ‘only’ at a temperature of 6000K, and far too cold to give off X-ray emission – it radiates most strongly in the visible. Even though the Sun was the first known source of cosmic X-rays, understanding the emission mechanism is still not entirely straightforward. The X-rays are soft thermal emission coming not from the disc, but instead from the very tenuous upper layers of the Sun’s atmosphere, known as the corona. This diffuse plasma extends over 1 million km above the surface of the Sun, and is the origin of the Solar wind of charged particles that flows out into the Solar System. The temperature of the corona is 1-2 million degrees, which is surprising given that it is so far from the nuclear fusion in the core, the ultimate source of energy generation in the Sun.  An extra and continual input of heat is required to raise the temperature of the corona, and this energy is thought to be supplied by reconnection processes in the strong and rapidly changing magnetic field of the Sun. This is why that X-ray images of the Sun show its disc to be dark, except for the most active magnetic regions on the surface, such as those associated with sunspots.

Bremsstrahlung Radiation
Within a hot plasma, many atoms have been stripped apart to create a population of free electrons and charged atoms (known as ions), which can then produce emission through a process known as bremsstrahlung (meaning ‘braking’) radiation. A freely-moving electron will have its path deflected if it passes close to a positively charged ion. The acceleration will cause the electron to lose kinetic energy, which is then given off as a photon. [This is also sometimes referred to as ‘free-free’ emission, because the charged particles remain free of each other during the whole process.] Bremsstrahlung can emerge at all wavebands from radio right through to X-rays; how energetic the emitted photons are will depend on how fast the electrons were originally moving, and how much energy has thus been lost. The X-rays are produced from the very highest energy circumstances, where the plasma is at temperatures of millions of degrees, or alternatively where the particles are moving at close to relativistic speeds - characteristic of situations where a lot of energy is released suddenly, such as in a supernova explosion.

Clusters of Galaxies
X-rays are the dominant form of radiation observed from giant clusters of galaxies. The individual galaxies, and the stars they contain, are again to cool to produce X-ray emission. However, a giant reservoir of hot gas occupies the space between the galaxies, and is known as the intracluster medium. From its spatial extent and luminosity, we find that there is on average about seven times more mass in this hot gas than that in all the stars in the galaxies added together. The intracluster medium has a smooth spatial distribution centred on the core of the cluster. The gas is again at temperatures of millions of degrees, and forms a charged plasma that radiates strongly in the X-rays from thermal bremsstrahlung.  Indeed, this again presents a puzzle, as the central parts of the intracluster medium radiates so much energy through its X-ray emission that it should have long since cooled down. The fact that it remains so hot requires a continual heating mechanism within the cluster – most likely related to the mechanical energy of sound waves travelling through the cluster gas and originating from disturbances caused by the active supermassive black hole that can be found at the centre of the most massive galaxy. (I refer you to my ‘Sounds of the Universe’ lecture for more details!).

Compton Scattering
Photons can also have their energy boosted by collisions with fast-moving particles, converting them from a low-energy to a high-energy photon in a process known as Compton scattering. This can, for example, happen to the microwave photons originating in the cosmic background all over the sky, as they pass through a foreground screen of fast-moving material. The particles involved in the collisions themselves need to have been accelerated to almost relativistic speeds by immense explosions or the effect of intense gravitational fields, so Compton scattering X-ray light can trace some of the more violent and dramatic regions of our Universe.

Synchrotron Emission
X-rays can also be produced by charged particles that are moving fast and accelerated when trapped to spiral around magnetic field lines. Such acceleration continually supplies the particle with extra energy, which it then releases as synchrotron radiation. Again, this can be radiated all across the electromagnetic spectrum, with the X-rays tracing the most extreme situations.

Tycho’s Supernova Remnant
When a very massive star runs out of sufficient fuel to sustain nuclear fusion, it ends its life in a spectacular manner. The core collapses inwards under gravity to form either a neutron star or a black hole, while the outer parts of the star are blasted apart in a supernova explosion. The hot debris is propelled outwards at high speed, to crash into the surrounding interstellar medium in an expanding bubble of hot gas; this is preceded by the intense blast wave, the energy of which accelerates charged particles.  An X-ray image of the remains of a supernova explosion − such as the one observed by Tycho Brahe in 1572 − demonstrates a range of X-ray emission processes. The red colours in the image portray the thermal emission given off by the expanding clouds of incredibly hot debris from the exploded star. These clouds are then surrounded by a shell of much harder (higher energy) emission shown in blue, generated where electrons are accelerated behind the rapidly outward-moving shockwave from the supernova blast (to energies a hundred times higher than achieved by the LHC on Earth). Most of this is due to high-energy bremsstrahlung radiation. Distinct striped areas within the non-thermal emission mark out regions where the turbulence behind the shock wave is greater and the magnetic fields consequently the most tangled. The X-ray emission in these regions is synchrotron emitted by electrons forced to spiral around the magnetic field lines.

Continuum Emission
All these processes result in X-ray light being produced over a wide range of energies. If we plot the intensity of the emission against the energy of the photon we have what is known as a spectrum, which when spread out into a smooth curve is called a continuum. The shape of the curve (ie the way the light is distributed across the energies) generated is a characteristic of the production mechanism.

Absorption and Emission lines
Many electrons are not free, however, but orbit around the nucleus of a neutral or charged atom. Such electrons are constrained to occupy only specific energy levels which are exactly prescribed by the unique atomic structure of each chemical element. If given energy, the electrons can move between these levels, but like books on a bookshelf they cannot occupy intermediate levels.

If an ion (or atom) absorbs a photon, the energy is given to one (or more) of the orbiting electrons, allowing it to jump to a higher level and thus storing the energy within the ion. When the electron later reverts to the lower energy state, energy is released in the form of a photon. But as the energy levels are at specific intervals, the amount of energy this photon possesses is completely constrained to be a specific value – and thus colour. The light emitted is then observed as excesses at one particular colour, sharply peaked to form an emission line, usually superimposed on the underlying continuum emission. Similarly absorption lines at very specific colours indicate where particular energies of light have been absorbed from the general continuum emission by particular atoms and ions. The spectrum of any charged hot gas (known as a plasma) will show many such emission lines, each one indicative of a unique transition of an electron between two energy levels in an element. Thus detailed study and modelling of the emission line spectrum of a source will reveal much about its chemical composition to astronomers.

A similar process is observed in the visible wavebands – for example, the pink glow from excited hydrogen atoms is a key feature of interstellar clouds known as nebulae – but the optical emission lines are predominantly from lighter elements, and only sample specific transitions. All the main electron transfers between the inner energy levels of all the heavier elements (Carbon onwards…) produce emission lines in the X-ray waveband. The spectrum of a hot gas (or plasma) will show many such emission lines, and thus reveal much about its chemical composition – particularly in the heavier elements – to astronomers.

Cas A Supernova Remnant
Many of the heavy elements are produced not only at the core of the most massive stars through nuclear fusion, but also during the final supernova explosion, when a flood of neutrons are released to combine with the existing elements in the plasma. Supernova remnants such as Cas A are thus a rich source of emission lines, and X-ray observations permit the mapping the distribution of the elements across the remnant. The distribution of sulphur, silicon, oxygen, magnesium and neon are all similar to each other, and found at the outer edges of the still-expanding debris.  However, the iron – which was originally at the inner core of the pre-supernova giant star – has be expelled right to the outer edges of the supernova remnant, with little remaining at the centre. It’s almost as if the supernova explosion has turned the star inside out.

The Intracluster Medium
Most of the high temperature plasma comprising an intracluster medium radiates strongly as thermal bremsstrahlung continuum, but superposed on this are emission lines - which depend on the temperature of the plasma as well as the absolute elemental abundance within the gas. From such X-ray spectra it’s possible to determine the abundances of heavy elements within the intracluster medium. Such studies show that it is not composed solely of primordial hydrogen and helium, but slightly enriched by heavier elements, indicating that some fraction of the gas must have been processed at the cores of stars and then expelled. Studies of the relative abundance of different elements thus can help scientists model the past star formation history of galaxies within the cluster - how and when the heavy elements are introduced into the intracluster medium, and what kinds of supernovae might have been responsible. 

X-ray Fluorescence
X-ray emission lines can also produced if a very high-energy particle or photon collides with an atom and knocks an electron free of the very innermost energy levels of an atom. This creates an unstable atom and an electron from an outer energy level immediately jumps to fill the vacated lower energy state, producing a photon with that energy. We will see later that such fluorescence is produced by the collision of high-energy X-rays produced in the vicinity of a black hole with iron atoms in the accretion disc.

Moon
Surprisingly, the illuminated portion of the Moon is a source of X-ray emission due to fluorescence, where high-energy X-ray photons from the Sun bombard the lunar surface. Measurements of this radiation can give direct measurements of the amount and distribution of different elements and minerals present, such as oxygen, magnesium, aluminium and silicon.

Charge Exchange
Emission lines can also be produced when an electron is exchanged between an ion and a neutral atom or molecule (such as carbon or oxygen) during a collision. A photon is emitted as the captured electron drops down into a lower energy state in its new home.

Comets
This process of ‘charge exchange’ can be seen in the X-radiation from comets, where highly-charged ions in the Solar wind collide with neutral atoms in the comet’s extended atmosphere, or corona. X-ray observations can trace the elements within both the solar wind and the comet, and the structure of the comet’s halo as it expands away from the nucleus.

The Need for X-ray astronomy
X-ray astronomy enables us to study processes and regions in the Universe that cannot be observed any other way –where matter is extremely hot, and highly energized. For example, when we look at a comparison of the night sky around Orion in both visible and X-ray light, there are few sources in common. The visible image shows mainly the light of stars, and the primary source of all the radiation being the nuclear fusion at their core. In the X-ray image the radiation is powered by material falling under their strong gravitational fields of compact objects such as black holes and neutron stars.

Another feature of the X-ray sky that is very different from the visible is that the X-ray brightness of many sources varies rapidly with time. This is because we are viewing far more energetic objects than ordinary stars, most of which are powered by unpredictable flows of hot matter. The brightness of the sources often depends on the fuel supply – some are relatively steady, others may go through cycles of accretion and dormancy lasting for a few weeks or months at a time, and others may flicker on and off as they undergo explosive or eruptive behavior.

X-ray Observations
The collection of X-rays is not a simple matter. Röntgen’s original X-ray image shows how most are easily absorbed by matter – whether just centimetres of bone or a few metres of air. X-rays do not make it through Earth’s atmosphere, as all are naturally absorbed well before ground level. To detect any cosmic X-rays, an experiment has to be flown at altitudes of at least 100km. The earliest X-ray detectors were thus flown on simple rockets – some even on captured V-2 rockets in the late 1940’s.

The birth of X-ray astronomy
The Sun was always expected to be a source of X-rays, and it was originally thought that everything else would be too far away, and too faint to detect. The fact that there were astronomical X-ray sources beyond the Sun wasn’t discovered until 1962. A team of astronomers led by Riccardo Giacconi designed and flew an experiment with the intention of detecting X-ray fluorescence emission from the action of Solar X-ray irradiation of the Moon’s surface. The scientific payload was relatively primitive, using of a couple of Geiger counters as detectors; the experiment scanned around the sky during the 5 minutes and 50 seconds that the rocket was over 80 km above the Earth’s surface. The main X-ray source detected was not the Moon, but originated from the direction of the constellation of Scorpius. This is the source labeled Sco X-1, though it was several years before an accurate identification of its position could be made. It’s now known to be a stellar binary system in our Galaxy, and it was only the first of many more bright X-ray sources waiting to be discovered. This flight marked the start of the development of modern X-ray astronomy fifty years ago, and in recognition Giacconi was awarded the Nobel Prize for physics in 2002.

X-ray Telescopes
Today’s X-ray astronomers rely on telescopes that orbit on satellites high above the Earth’s atmosphere. The design of the telescope that collects the light is of necessity, very different from that of an optical telescope or a radio dish, because X-rays do not reflect off mirrors. The highly energetic X-rays are absorbed by a mirror – in much the same way that fast-moving bullets will slam into a wall, and will only reflect in a ricochet if they meet the wall at a very shallow ‘grazing’ angle. Incoming X-rays can only be reflected by mirrors if they are oriented almost parallel to the incoming direction, and are brought to a focus by successive shallow reflections off paraboloid and hyperboloid surfaces. Thus X-ray mirrors are shaped into long hollow cylinder rather than the usual dish form we associate with telescopes. This means, however, that only a tiny cross-section of mirror is presented to incoming X-rays, and an individual mirror can’t catch many photons. The collecting area of the telescope is increased by nesting many cylindrical mirrors one inside each other.

Not only do the mirrors need to be very precisely engineered to present the right surface to incoming X-rays, but as the telescopes have to be launched into space on rockets, the mirrors are thin to minimise their weight. They also have to be coated with very reflective yet inert metals such as gold and iridium, in order not to lose too many X-rays through absorption. The mirrors are physically large, with overall a typical telescope length of 10 m and a diameter of 1m. The next generation of X-ray telescopes with even wider, longer mirrors may require two satellite spacecraft to carry them, one containing the mirrors, and one containing the focus and detectors. The X-rays are recorded by CCD devices which can log the exact time of arrival, position and energy of every photon received.

Over the intervening years, X-ray astronomy has developed almost beyond recognition from Giacconi's first rocket experiments, with hundreds of thousands of cosmic X-ray sources catalogued – comprising both nearby and very distant objects.

Stars and Supernovae

Normal and Active Stars
The Sun is a relatively normal star, and bright in X-rays only because it’s so close to us. We can just about detect other ‘ordinary’ stars if they are nearby, such as our nearest star, Proxima Centauri, which shows a similar level of X-ray activity. Some much more luminous stars show variable X-ray emission, particularly if they are losing large amounts of matter in their stellar wind. Interactions between this wind and the surrounding atmosphere provide shocks to heat the plasma to then emit the X-rays. The strongest stellar emitters of X-rays are, however, very young and massive stars.

Nebulae, star formation and young stellar clusters

Nearby regions of active star formation, such as the Rosette or Great Orion nebulas show large swirls of gas and dust in the visible wavebands, which are too cold to be apparent in the X-ray waveband. Only the bright young blue massive stars at the core of the nebulae can be seen in X-rays, alongside the hot gas forming a ‘superbubble’ around them that has been heated by the stellar winds and supernova shock waves. Many of these stars are far more active than our Sun is today, often flaring periodically in intensity. It’s likely that all stars displayed these energetic outbursts when they were much younger, something that perhaps has to be taken into account in models that try to simulate the collapse and formation of a planetary system around a star in the earliest phases of its evolution.

DG Tau
For example, DG Tau is a star with a mass similar to that of the Sun, but with an age of only about a million years. The star is bright in X-rays, and shows two jets emanating from the star and stretching away to distances several hundred times the size of our Solar System. The X-ray light from the far jet is partially absorbed relative to the near-side jet, suggesting the presence of a protoplanetary disc around the young star (which is too cool to be directly detected in X-rays). So in some cases, we don’t just have to incorporate the luminous and variable X-ray brightness into our models, but also the influence of powerful X-ray jets that may complicate our understanding of the immediate environment of a young star. It’s not impossible that such effects may have bathed the environment of the early Earth in X-radiation.

Compact Objects
We have already mentioned the X-ray emission from supernova explosions; young systems such as those associated with both Tycho’s and the Cass A supernova remnants show a clear distinction between the hot stellar debris and the rapidly-moving shell of accelerated electrons accompanying the shockwave. X-ray observations of significantly older systems, such as GD292.0+1.8 (which is estimated to have an age of several thousand years rather than several hundred) lack the thin outer shell of high energy X-ray emission, suggesting that the acceleration of electrons is efficient primarily in very early stages of supernova remnant evolution.

The Pulsar Wind Nebula
Many such supernova remnants house a highly magnetized and rapidly rotating neutron star at their core, formed from the collapse of the giant progenitor. The neutron star is surrounded by a magnetosphere of highly charged particles of matter and antimatter; these can be accelerated by the extraordinarily strong magnetic field, and spew out into the surroundings as a pulsar wind. The accumulated material forms a large bubble of plasma around the neutron star, known as the pulsar wind nebula. This is only apparent in the X-ray waveband, and can take a variety of shapes, according to the density of its immediate surroundings. Some (such as that at the core of the Crab nebula) show a flaring interface where shocks are generated at the boundary between the outward flow of the wind and its environment.

Binary Systems
Eventually the supernova remnant will fade and/or expand away into obscurity, leaving behind the neutron star or black hole. These are compact objects that are almost impossible to spot if they live in complete isolation.  It is a different matter, however, if they formed within a stellar binary system, where their extreme gravitational force exerts an influence on their companion. The first black hole known was discovered through its X-ray emission, and is part of such a binary system.

Cygnus X-1 and Cir- X1
Cyg X-1 is the first X-ray source detected in the constellation of Cygnus, and it was discovered shortly after Sco X-1. The original detection only yielded an approximate location on the sky, but the precise spot was refined later when an X-ray flare in the source coincided with an observed radio flare in a source whose position was known. The identification was made with a blue supergiant star – but although it was almost 20 times more massive than the Sun, it still seemed unlikely as such a powerful source of X-rays. Follow-up optical observations revealed that the star was not completely still, but wobbling to and fro as it followed a 5.6-day orbit… but around what? There was nothing apparent in the visible waveband, but the motion of the star showed it to be responding to the gravity of an invisible companion with a mass of around 10 Solar masses. Such a large invisible mass coinciding with a bright X-ray source immediately suggested that a black hole formed a binary system with the supergiant. Similarly, another of the very first discovered X-ray sources, Cir X-1 was identified as a neutron star in orbit with an ordinary star a few times more massive than our Sun.

In these systems, one of the stars is appreciably more massive and luminous than the other, and runs through its fuel supply sooner to complete its life cycle earlier. Its supernova explosion does not always disrupt the binary system (whether or not it remains intact depends on the relative masses of the two stars), so even when it has formed the neutron star or black hole, its companion remains in orbit around it. Some millions of years later, the companion star also nears the end of its life and swells to form a giant star. The outer layers of gas are less tightly bound to the star, and now become vulnerable to being torn off and gravitationally accreted by the compact object. When this gas falls through the strong gravitational field, it gains energy, heating up to millions of degrees to emit profusely in X-rays. It will spiral towards the black hole in a flattened accretion disc (see my “Rotation” lecture for details about why…), where friction caused by collisions between the accreting particles also supplies local heating and flickering of X-ray light. The whole process is uneven, and as the fuel supply varies with time, so will the intensity of the light from the system. This accretion process around neutron stars and black holes reveals their presence through bright X-ray sources right across the disc of our Galaxy, and even in other galaxies.

In the Milky Way
X-ray surveys of the disc of our Galaxy show thousands of such point-like X-ray sources – some are young stars, but most are stellar remains such as accreting neutron stars, black holes and white dwarfs in binary systems. A much more diffuse glow of harder (higher-energy) X-ray light that extends along the disc for thousands of light years, and spectral analysis of its light shows it’s composed of gas at temperatures of 10 million degrees and above. This is most likely material accumulated in the central region of our galaxy that has been heated by the combined shock waves from many supernova explosions from massive stars over several billion years.

And Other Spiral Galaxies
Other ordinary spiral galaxies – such as the nearest major galaxy to the Milky Way, the Andromeda galaxy – are very similar when viewed in the X-ray waveband. We no longer see the starlight that makes up the structure of the disc and bulge, but a collection of compact objects in accreting binary star systems, with the centre of the galaxy surrounded by a diffuse cloud of hot gas.

Black Holes
But there are not just stellar-sized black holes in galaxies. All galaxies of a reasonable size – including both our own and the Andromeda galaxy – have massive black holes at their core, most of which are dormant. The supermassive black hole at the centre of the Milky Way has a mass of around 4 million Solar masses, and it is not currently actively accreting. It may not always have been so quiet, and X-ray emitting lobes of hot gas to either side of the black hole suggest that there have been powerful eruptions from this region several times over the last several thousand years.

Jets
A characteristic of the accretion of material onto a black hole is the presence of powerful jets of material moving at almost the speed of lightaway from its immediate vicinity. Not all matter that comes close to a black hole is irrevocably doomed. The accreting material is hot and magnetized, and as it swirls round in the accretion disc, strong electro-magnetic forces are built up, forced into a funnel-like shape that can spit charged particles at high speed out in two opposing jets along the rotation axis of the black hole. Much of the energy available from material falling toward the neutron star is converted into powering this jet. The highly-energized charged particles within the jet are also strong sources of non-thermal emission, often bright in X-rays. Such jets can often reveal where an otherwise obscured black hole is actively accreting at the centre of a galaxy to form what is known as an active galactic nucleus (or AGN). 

Centaurus A
The nearest galaxy to Earth that contains a currently active supermassive black hole is Cen A, a powerful source of radiation at the centre of a massive elliptical galaxy. The black hole at its core is bright in X-rays, and produces a pair of jets of high-energy particles that are powerful enough to blast through and beyond the host galaxy. X-ray synchrotron emission is produced within the jets; as each X-ray photon is so energetic, this should rapidly remove energy from the electrons to slow them – the fact that they are still glowing suggests that the particles must be continually re-accelerated  to relativistic speeds, perhaps by shock waves within the jet.

Broad Iron Lines
The fact that X-rays are the most energetic light means that they are emitted from the very hottest – and innermost places – within an accretion disc around a black hole. The X-ray light thus originates from the regions of space most affected by the extreme gravity, and the resultant bending of space and time. Study of the X-rays emitted from the immediate vicinity of a black hole can provide an acute test of our understanding of general relativity.

In practice, most of the X-rays near a black hole aren’t emitted directly from the accretion disc, as it doesn’t always reach sufficiently high temperatures. X-rays are generated from regions just above and below the disc, most likely intimately connected with the (not very well understood!) production of the jets. Some of the X-ray photons travel straight towards us, and some are reflected by the accretion disc. In particular, some of the X-rays excite ions of iron within the material in the disc, which release the energy later at a well-defined transition energy.  Were this accretion line emitted by a stationary cloud of plasma, the iron emission would appear as a sharp emission line within the spectrum. What is observed is very different however, due to the way it is smeared out by all the relativistic motions so close to the black hole. First of all, even under Newtonian gravity, we expect simple Doppler effects because the disc is spinning, so light from the receding half of the disc is redshifted, the light from the other half is blueshifted.  This spread is then skewed because the blueshifted peak becomes enhanced through relativistic beaming, to appear more accentuated than the red. We then need to take factor in other relativistic effects, such as ‘gravitational redshift’, which is needs because a photon will lose energy (and thus move to redder wavelengths) as it climbs out of the strong gravitational field. Further smearing comes into play according to the inclination of the disc to our line of sight, the rate of rotation, and where in the disc the emitting iron ions are located.

The end result is that the iron emission is moulded into a very characteristic and lop-sided profile, which is much broader than would be otherwise expected. These broad iron lines match theoretical predictions of the effects of extreme gravity, and are seen as a clear signature for the presence a  black hole, and can be used to detail its properties such as its spin and inclination to the line of sight.

The X-ray Background
The first X-ray mission back in 1962 also made another fundamental discovery which is that the sky is not completely dark at X-ray energies. There is an X-ray background at both hard and soft energies, although different sources are thought to be responsible for each background.  

Soft X-ray Background
The diffuse light of lower-energy X-rays fills the sky, and is thought to be emitted from local bubbles of hot gas within the inter-stellar medium in the disc of the Milky Way, most likely from hot winds from massive young stars, and from supernovae.

Hard X-ray Background
The X-ray background at higher energies has taken a lot longer to understand. It appears as a diffuse and unresolved glow distributed uniformly across the sky, suggesting an origin outside of the Milky Way. Only very deep observations with the most recent generation of X-ray telescopes has finally resolved the light into many individual sources, each of which is emitted by a very distant and powerful active galaxy.  Many of these sources are so remote, they date from when galaxies were at a much younger, and formative phase of growth, when the central black hole would have been heavily enshrouded by surrounding gas and dust. This thick obscuring environment would act to preferentially absorb the lower-energy X-rays, leaving only the most energetic of X-ray light to escape to be collected by our telescopes.

© Professor Carolin Crawford 2012