Techniques for finding black holes

Formation of extragalactic jets from a black hole's accretion disk

Accretion disks and gas jets:

Most accretion disks and gas jets are not clear proof that a stellar-mass black hole is present, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks and gas jets to form and to behave in the same ways as those around black holes. But they can often help by telling astronomers where it might be worth looking for a black hole.On the other hand, extremely large accretion disks and gas jets may be good evidence for the presence of supermassive black holes, because as far as we know any mass large enough to power these phenomena must be a black hole.

Strong radiation emissions:

Steady X-ray and gamma ray emissions also do not prove that a black hole is present, but can tell astronomers where it might be worth looking for one - and they have the advantage that they pass fairly easily through nebulae and gas clouds.But strong, irregular emissions of X-rays, gamma rays and other electromagnetic radiation can help to prove that a massive, ultra-dense object is not a black hole, so that "black hole hunters" can move on to some other object. Neutron stars and other very dense stars have surfaces, and matter colliding with the surface at a high percentage of the speed of light will produce intense flares of radiation at irregular intervals. Black holes have no material surface, so the absence of irregular flares around a massive, ultra-dense object suggests that there is a good chance of finding a black hole there.Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars or by collisions between neutron stars, and both types of event involve sufficient mass and pressure to produce black holes. But it appears that a collision between a neutron star and a black hole can also cause a GRB,so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away so the black holes associated with them are actually billions of years old.Some astrophysicists believe that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.Quasars are thought to be the accretion disks of supermassive black holes, since no other known object is powerful enough to produce such strong emissions. Quasars produce strong emission across the electromagnetic spectrum, including UV, X-rays and gamma-rays and are visible at tremendous distances due to their high luminosity. Between 5 and 25% of quasars are "radio loud," so called because of their powerful radio emission.

Gravitational lensing:

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a black hole) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of the general theory of relativity. According to this theory, mass "warps" space-time to create gravitational fields and therefore bend light as a result.A source image behind the lens may appear as multiple images to the observer.

Lensing by a black hole. Animated simulation of gravitational lensing caused by a going past a background galaxy. A secondary image of the galaxy can be seen within the black hole Einstein ring on the opposite direction of that of the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole. The surface brightness of the two images remain constant, but their angular size vary, hence producing an amplification of the galaxy luminosity as seen from a distant observer. The maximum amplification occurs when the background galaxy (or in the present case a bright part of it) is exactly behind the black hole.

In cases where the source, massive lensing object, and the observer lie in a straight line, the source will appear as a ring behind the massive object.
Gravitational lensing can be caused by objects other than black holes, because any very strong gravitational field will bend light rays. Some of these multiple-image effects are probably produced by distant galaxies.

Determining the mass of black holes:

Quasi-periodic oscillations can be used to determine the mass of black holes.The technique uses a relationship between black holes and the inner part of their surrounding disks, where gas spirals inward before reaching the event horizon. As the gas collapses inwards, it radiates X-rays with an intensity that varies in a pattern that repeats itself over a nearly regular interval. This signal is the Quasi-Periodic Oscillation, or QPO. A QPO’s frequency depends on the black hole’s mass; the event horizon lies close in for small black holes, so the QPO has a higher frequency. For black holes with a larger mass, the event horizon is farther out, so the QPO frequency is lower.

Objects orbiting possible black holes:

Objects orbiting black holes probe the gravitational field around the central object. An early example, discovered in the 1970s, is the accretion disk orbiting the putative black hole responsible for Cygnus X-1, a famous X-ray source. While the material itself cannot be seen directly, the X rays flicker on a millisecond time scale, as expected for hot clumpy material orbiting a ~10 solar-mass black hole just prior to accretion.

A Chandra X-ray spectrum of Cygnus X-1 showing a characteristic peak near 6.4 keV due to ionized iron in the accretion disk, but the peak is gravitationally red-shifted, broadened by the Doppler effect, and skewed toward lower energies

The X-ray spectrum exhibits the characteristic shape expected for a disk of orbiting relativistic material, with an iron line, emitted at ~6.4 keV, broadened to the red (on the receding side of the disk) and to the blue (on the approaching side).Another example is the star S2, seen orbiting the Galactic center. Here the star is several light hours from the ~3.5×10⁶ solar mass black hole, so its orbital motion can be plotted. Nothing is observed at the center of the observed orbit, the position of the black hole itself——as expected for a black object.