What is a gamma-ray burster? What is the evidence that gamma-ray bursters are not located in the disk of our Galaxy or in a halo surrounding our Galaxy?

Understanding Gamma-Ray Burster (GRB) distances is critical to appreciating their significance in the cosmos. Despite their name, these bursts aren't confined to the Milky Way; rather, they're extragalactic, originating from cosmological distances.

Gleaning evidence from GRB afterglows allows astronomers to measure their redshifts—an effect similar to the Doppler shift experienced with sound, but with light. A higher redshift means the object is further away, as the universe's expansion stretches the wavelengths of light. GRBs have been found with high redshifts, indicating they occur in distant galaxies billions of light-years away. What does this mean? Essentially, we're seeing these bursts from the distant past of the universe.

As GRBs are uniformly distributed across the sky and not along the Milky Way plane, we can infer they are not bound to our galaxy. This distribution paves the way for using them as cosmic yardsticks. By examining the light curves and redshifts of GRBs, we can map the universe's large-scale structure and gauge the cosmos's expansion rate.

The immensity of a Gamma-Ray Burst's (GRB) energy release is almost incomprehensible. The phenomenon can emit as much energy in a few seconds as the sun is expected to release over its entire 10-billion-year lifetime. This juxtaposition not only underscores the GRBs' power but also challenges us to understand the underlying mechanisms.

The energy emission of GRBs is so extreme, it's evident across the observable universe. These titanic eruptions result from cataclysmic events such as the collision of neutron stars or the implosion of massive stars into black holes. Understanding the energy spectrum of GRBs is no easy feat; it involves examining gamma rays, the most energetic form of electromagnetic radiation, which is detected by space-based instruments.

Astonishingly, the total energy output, even by conservative estimates, ranges from 10^44 to 10^54 ergs. To put that into context, that's up to a billion trillion times the annual global energy consumption on Earth! Just how these bursts manage to concentrate such colossal amounts of energy into gamma rays remains a hotly debated topic in astrophysics.

Astronomers gather evidence about Gamma-Ray Bursts (GRBs) using various observational methods. Initially, these bursts were detected serendipitously by satellites originally designed to monitor nuclear test ban treaties, which showed the universe was not as tranquil as once thought.

Observations today predominantly come from dedicated satellite missions like the Swift Observatory and the Fermi Gamma-ray Space Telescope. These instruments capture the fleeting gamma-ray light from GRBs, which allows astrophysicists to pinpoint the bursts' locations in the sky.

Following the initial gamma-ray detection, telescopes across the electromagnetic spectrum join in to observe the afterglow - the lingering emission in X-rays, optical light, and radio waves. This sustained effort often reveals GRBs' host galaxies, illuminating the bursts' origins and distances. The diverse evidence gleaned – from gamma-ray light curves to afterglow spectral lines – furnishes a comprehensive picture of these mysterious cosmic events and their place in the universe's vast expanse.