Monday, July 4, 2022

What Are Gamma-ray Bursts?

Discovered in the 1960s by U.S. military satellites looking for covert nuclear tests, gamma-ray bursts are short-lived explosions of gamma rays, the most energetic form of light. Lasting from a few milliseconds to several hours, they shine hundreds of times brighter than a typical supernova and about a million trillion times as bright as the Sun. Observed in distant galaxies, they are the brightest electromagnetic events known to exist in the universe. A typical burst releases as much energy in a few seconds as the Sun will in its entire 10 billion year lifetime.

A longer-lived afterglow of X-ray, ultraviolet, optical, infrared, microwave, and radio emissions usually follows the initial flash of gamma rays. Evidence from recent satellites like NASA’s Swift and Fermi observatories indicate that the bright bursts come from the collapse of matter into a black hole. On average, approximately one gamma-ray burst is detected every day.

Gamma-ray bursts do not come from any particular direction in space, though they are associated with very faint galaxies at enormous distances. The explosions are thought to be highly focused, with most of the energy collimated into a narrow jet traveling near the speed of light. We can only detect the gamma-ray bursts of jets pointed directly at us.

Astronomers classify gamma-ray bursts into long- and short-duration events. While the two types of events are likely created by different processes, both result in the creation of a new black hole. Long-duration bursts last anywhere from 2 seconds to several hours. Although they are associated with the deaths of massive stars in supernovas, not every supernova results in a gamma-ray burst.  Short-duration bursts last less than 2 seconds.  They appear to result from the merger two neutron stars into a new black hole, or the merger of a neutron star and a black hole to form a larger black hole.

The keen resolution of Hubble helps study the environments of gamma-ray bursts. Hubble images showed that one type of gamma-ray burst arises from far-flung galaxies, which are forming stars at enormously high rates. This confirmed that the bursts of light originated from the collapse of massive stars. Hubble’s unique ultraviolet spectroscopy will play a pivotal role in understanding how elements are forged in these massive explosions.

In 2017, NASA’s Fermi telescope observed a short-duration gamma-ray burst tied to the gravitational waves detected by the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO). A pair of smashing neutron stars was thought to have created an immensely explosive kilonova along with the gamma-ray burst and the gravitational waves. Hubble set out to observe the kilonova and capture its near-infrared spectrum, which revealed the motion and chemical composition of the expanding debris. The spectrum looked exactly how theoretical physicists had predicted the outcome of the merging of two neutron stars would appear.

0 commenti:

Post a Comment