Gamma Ray Bursts
A gamma-ray burst, or GRB, is well-described by its name. They are first detected as superluminous blasts of intense radiation in the gamma-ray wavelengths, which are more energetic than X-rays. The bursts sometimes last for just a few seconds, up to a few hours. The burst is followed by an afterglow at increasing longer, less energetic wavelengths. Afterglows can normally be detected for a few days to a week before they become too dim. GRBs are thought to be caused by a supernova, when a high mass star implodes to form a neutron star or a black hole. The implosion forms narrow high speed jets of matter and radiation. When one happens to be pointed towards Earth, we may detect it as a GRB, even if the explosion took place billions of light years away in a distant galaxy.
GRB 161023A
The light curve of GRB 161023A peaks very quickly and fades slowly. From the moment the burst was first detected to its peak brightness is only about 4 minutes, during which it brightens from magnitude 16.5 to 13, about a factor 300. Over the next 5 days, the blast slowly dims to about magnitude 23, about ten thousand times fainter than its brightest.
The brightenting occurs so quickly in a gamma-ray burst that light curves are usually plotted this way, using the logarithm of the time since detection, which spreads out the many data points observed early in the burst as it is rapidly evolving.
A spectrum of the GRB was taken just a few hours after the burst was first noticed, but it had already faded from about magnitude 13 to 17.5, dimming by a factor of about 60 in just a few hours.
The overall shape of the spectrum is smooth and is brightest in the blue, dimming towards the red end. But a number of strong absorption lines of ionized atoms affect the spectrum. These are caused by gas in the distant galaxy that is host to the GRB. The very deepest line at the blue end of the spectrum is from hydrogen in the host galaxy. There are also many dips in the spectrum caused by absorption from silicon, oxygen, carbon, iron and other elements. Not all of these lines originate in the host galaxy at redshift 2.7 - some of them come from other gas in other galaxies between the GRB and Earth.
From the GRB spectrum, we know that it took place in a galaxy at redshift 2.7. Using well-known parameters for the Big Bang expansion of the universe, we know that the GRB actually happened 11 billion years ago, and the light just reached Earth in 2016. We also see signatures of several intervening galaxies through which the light passed on its way here. The closest one is only 8 billion light years from Earth. Because they change so quickly, many people across the world help observe these exciting GRBs. The light curve and spectrum shown here were provided and published by Antonio de Ugarte Postigo et al. 2018, Astronomy & Astrophysics, 620, A119 (link).