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X-rays from AGNs

Many galaxies have unusually bright cores. These galaxies are lumped together as Active Galactic Nuclei (AGN). This class covers a broad range objects. You may have heard of one type, the quasar (quasi-stellar object); these AGNs are very distant, so that when they were first seen, only the point-like, bright cores of the galaxies were visible, leading to their name. But the AGN class also includes other objects sporting such names as Seyfert galaxies, blazars, and LINERs, that are closer and are clearly identifiable as galaxies. The cores of these galaxies are unusually bright, much brighter than would be produced by the stars within their cores. Much of the radiation is non-thermal (not a black body), meaning that it is produced by something other than stars. Depending on the type of AGN, these objects can produce light that has strong emission lines, strong absorption lines, or no lines at all.

Some AGNs produce jets; there is either one jet, or there are two jets traveling in opposite directions. The jets are narrow streams of gas that move at close to the speed of light; they are strong emitters of radio waves. The jets can extend several megaparsecs away from the galaxy (this length is several hundred times the visible size of the galaxy.

AGNs are generally bright at x-ray energies, with the brightest objects emitting as x-ray 10 billion times the power of the Sun. But this power can vary rapidly, as fast as a third of an hour. From the time it takes light to travel across the emission region, we can say the region producing x-rays must be smaller than several AU to create such rapid variability.

Brilliant and blazing, AGNs are the dragons of astronomy. But not every dragon is awake. One belief in the astronomical community is that the core of every galaxy has the potential to be an AGN. Sleeping dragons may abound in our universe, including at the core of our own galaxy. Was the dragon in our own galaxy was ever awake? Perhaps.

While the different classes of AGN are based on differences in appearance, they are all thought to be manifestations of one single object: an accretion disk flowing onto a massive black hole. What do I mean by massive? Up to several billion times the mass of the Sun, which is up to 0.1% of the mass of the host galaxy; a black hole of 1 billion solar masses would permit one-hour variability.

If you have read the pages on accreting black hole candidates in x-ray binary systems, the theory of active galactic nuclei will sound familiar. The theory is that gas that is either orbiting freely in the galaxy or is stripped from stars within the core of the galaxy is pulled into orbit around the black hole. The gas in the disk orbits the central black hole, and viscosity within the disk converts the gravitational potential energy of the gas into thermal energy, which is then radiated away as infrared, optical, ultraviolet, and x-ray light. The inner edge of the disk is defined by the point where the orbital speed equals the speed of light: any matter than falls inside this radius is unable to orbit the black hole, since matter cannot travel faster than the speed of light, so it falls onto the black hole. Once matter moves inside this radius of last stable orbit it ceases to emit radiation, even though it is still above the event horizon of the black hole. To produce the power we see in the brightest AGNs, the black hole must eat 10-4 of a solar mass every year.

The accretion disk defines an axis of rotation, and along this axis it can accelerate material to high velocity. This effect is seen in some x-ray binary systems within our galaxy, and it is believed to create the relativistic jets seen in many AGNs. This would explain why only two jets at most are ever seen in these systems. Special relativity provides one explanation of why only one jet is often seen: the radiation from the jet moving away from us is redshifted, which causes the brightness of the jet to fall relative to that of the jet moving towards us.

The complication in studying AGNs is that interstellar gas shrouds the central object from view at many energies. This shroud is responsible for the absorption and emission lines seen in many AGNs. X-rays and gamma-rays are in many instances able to penetrate this shroud, making these wavelengths important in the study of AGNs.

All of these features of the AGN are present in some x-ray binary systems, which is why one often hears of binary systems such as Cygnus X-1 referred to as laboratories for studying AGNs. Despite the tremendous difference in size—109 solar masses versus 10 solar masses—the basic physics is similar between AGNs and x-ray binaries. The study of AGNs is simply an extension of the study of x-ray binaries containing black holes.

As in the study of x-ray binaries, the study of AGNs in the x-ray attempts to discern the physics at the inner edge of the accretion disk around the black hole, particularly the motion of the gas. Two types of motion are expected: the rotation of the accretion disk around the black hole, and the acceleration of gas into a jet along the disk's axis of rotation. This motion is measured looking for the effects of the Doppler shift on atomic lines. Both the Chandra X-ray Telescope and the Newton-XMM are suited for this work. The difficulty encountered is that the x-rays we see come from regions moving in a variety of directions, and these different regions are too close together to separate with imaging telescopes. So, for instance, if the accretion disk is tilted, we see blended together areas moving towards us, areas away from us, and areas moving perpendicular to our line of sight. The x-ray emission lines are therefore blends of lines that are blueshifted and redshifted. The effect is a broad line, with the precise shape of the line set by the variety of motions that are present. As a practical matter, the modeling of a line by an accretion disk and a wind is difficult, because the theory permits many free parameters.

Another aspect of the theory that x-ray observatories are testing is whether the x-ray region is obscured in part by interstellar gas away from the disk. The idea is that the inner edge of the accretion disk is visible if we are looking down the axis of rotation, but it is block by clouds of gas several parsecs from the disk if we are looking along the plane of the accretion disk. The gas in the clouds would be cool when compared to the accretion disk. When light from the accretion disk passes through a cloud, much of the light is absorb, and the spectrum acquires absorption lines. The energy that is absorb by the cloud is reradiated in all directions as line radiation. Someone looking down the axis of this system would see the direct radiation from the accretion disk with emission from the clouds, so that the spectrum would be a continuum with emission lines superimposed. Because the absorption lines and emission lines are related by the conservation of energy, we can test whether the spectra of AGNs showing emission lines are consistent with the spectra of AGNs showing absorption lines. This consistency has been found in the studies using current x-ray observatories.

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