Pedagoguery

When it comes to the sizes of astronomical objects, by and large, Nature likes variety. Planets and stars run a whole continuum of sizes, from tiny specks like your smaller asteroids, to behemoths like Eta Carinae (which is anywhere from 50 to 100 times the size of our sun). However, in the case of black holes, there appeared to be a gap. We have managed to find stellar mass black holes, which range from about 4 to 20 times the mass of the sun, and super massive black holes, which range in the millions to billions of solar masses, but nothing in between. Now, since the launch of two X-ray observatories, Japan's ASCA and NASA's Chandra, we may have candidates to fill that gap.

Most black holes are observable only when they are accreting matter. A black hole by itself is, by its very nature, extremely difficult to observe. However, feed a steady stream of matter into it, and the matter forms into an accretion disk that generates enough energy to put out significant amounts of X-rays. The bigger the black hole, the more luminous the accretion disk. What both ASCA and Chandra have observed are Ultra Luminous X-Ray objects, or ULXs. Many of these ULXs are located in star-forming regions of galaxies. In addition, they also tend to vary their luminosity on short time scales – an indication that the objects themselves must be small in size.

The largest black hole that can form from the collapse of a star is thought to be about 20 solar masses. Such a black hole, if it were to accrete the maximum amount of matter, would shine with an X-ray luminosity of about 5 x 10³⁹ ergs per second, if it were radiating uniformly in all directions. (Feeding the black hole more matter past a certain point would only cause the accretion disk to grow, and would potentially cause it to blow off more matter, so the process tends to be somewhat self-limiting.) The ULXs observed tend to be more luminous than this by a factor of 10 to 100. Does this indicate an intermediate mass black hole (IMBH)? The evidence is inconclusive.

The first assumption made is that the black holes are radiating uniformly. Most black holes that we observe beam most of their radiation out as jets streaming from the rotational poles. As a result, many ULXs could well be stellar-mass black holes whose jets happen to be aimed toward us. Only one object, a very bright ULX in the M82 galaxy in Ursa Major (called M82 X-1) appears to unambiguously be an IMBH. How can we get a more definitive answer about the others? Obtains a more direct measurement of their mass.

The most famous black hole, Cygnus X-1, has had its mass measured because it has a companion star. By observing the motion of that star, we can obtain a combined mass of the system, and knowing stellar physics as we do, we can also get a good idea of the mass of the companion by direct observation. Subtract the one from the other, and we get a value of about 6 solar masses for the black hole. Astronomers are attempting to do the same thing with UHXs. Since most of them appear to be in star forming regions, odds are good that they have companions. Observations of M82 X-1 indicate, for example, the presence of an evolved companion star in a 62-day orbit. The difficulty is that all of the UHXs are very distant. Matching up a Chandra X-ray image with a Hubble optical image is a start, but the X-ray object can only be pinned down to about 1 arc second on the Hubble image, and there are frequently several possible stars within that area of the sky. While difficult, this is the most promising mechanism, since it involves the fewest assumptions.

Another mechanism involves the variability of the UHX itself. The luminosity of an accretion disk is “quasi-periodic”, which means that it flickers at not-quite periodic intervals as bright blobs within the disk disappear behind the black hole as they orbit. Since the variability depends somewhat on the orbital period of the disk, in theory we could use it to determine the mass of the black hole. Some assumptions have to be made in this method, however, so the results are not as reliable.

So assuming IMBHs exist, how would they be formed, and why do there appear to be so few of them? The best clue to their formation lies in where they are found: very active star forming regions. In such regions, very large numbers of stars are formed in very tight quarters. Collisions between stars are common in such conditions, and can lead to mergers of stars. If conditions are right, it can lead to a runaway merger, yielding a star with between 800 to 3000 solar masses, which would quickly collapse to form a black hole. Simulations indicate that the entire process would take place in about 3 million years, which is very short in cosmic time scales.

Once an IMBH is formed, it will interact gravitationally with the stars around it. A typical interaction would result in the star getting ejected from the cluster, and possibly from the host galaxy as well. As a result, the black hole would sink a little closer to the central black hole of the galaxy. Eventually, the two would merge. This steady loss of IMBHs to the central black hole could be a reason why so few are observed.

Next issue: how to protect interplanetary travelers from cosmic rays.