afterglow is the emission that follows a gamma ray burst in other parts of the spectrum, ranging from radio waves to X-rays, and lasting from a few days to several years. The afterglows fade away over time in a well-understood manner. The discovery of the first afterglows in 1997 was made possible by the Italian-Dutch satellite BeppoSAX. This discovery (and the detection of approximately 50 afterglows over the past 4 years) has revolutionized the field of gamma ray burst astronomy.

we have seen previously (question 14) the gamma ray burst is formed when shells of energy and matter ejected by the newly-formed hole collide and merge ("internal shocks"). After the shells merge into a single shell, this shell continues to move away from the black hole and, like a snowplow, it gathers up material from interstellar space (even though it is assumed that space is empty, in reality it is full of protons and electrons).

Approximately 50 afterglows have been detected with X-ray telescopes (see question 15) so far in almost 4 years of observations. However, of these 50 afterglows only approximately 40% have also been detected with optical or radio telescopes. So, on average an afterglow is detected once a month.

was explained in the previous question, the expanding shell from the gamma ray burst gathers up material, and imparts energy to this material. Initially, the shell has more energy and it accelerates the material that it gathers up into high energies. As the shell loses energy it gives less and less energy to the material and therefore the emission becomes weaker.

Gamma ray burst astronomers use many telescopes around the world to look for and monitor gamma ray bursts. These telescopes inculde the Hubble Space Telescope and the Chnadra X-ray Observatory in space, the Keck telescopes in Hawaii, the Very Large Array radio telescope in New Mexico (featured in the movie Contact starring Jodie Foster), the Very Large Telescope in South America, and many other smaller telescopes.

Observations of the afterglows all across the spectrum tell us many things about gamma ray bursts. First, observations of the first afterglow in 1997 confirmed that gamma ray bursts occur in very distant galaxies (see question 2). Second, from observations of the afterglow we can determine how much energy was released in the gamma ray burst. Third, we can determine how much material was present in the vicinity of the gamma ray burst.

The first step in the detection of afterglows is always the detection of a new gamma ray burst by a satellite such as the IPN, BeppoSAX, and HETE-II (see qeustion 4). The information from the satellite is quickly sent down to Earth and is distributed to gamma ray burst astronomers by email, pagers, and cellular phones. When astronomers get the information, they observe the part of the sky where the gamma ray burst occured, and look for an object which fades quickly.

The twinkling of radio waves from afterglows (see question 23) has shown that afterglows start very small (about the size of the Earth's orbit around the Sun), and then expand and become larger than the Solar system.