A Gamma Ray Burst From the Primeval Universe
Gamma ray bursts (GRBs) are some of the most powerful explosions in the Universe and now a new GRB, detected by NASA’s Swift X-ray Telescope, has been spoted from a time in the Universe’s history when galaxies were just beginning to form.
Long duration GRBs occur when very massive stars run out of fuel for nuclear fusion in their cores. The collapse and subsequent violent explosion can rip a star completely to shreds in a hypernova. Twin beams of gamma rays burst forth from the event, and if Earth is in the path of one of those beams, we see the gamma ray burst. The Swift satellite catches these energetic events as soon as they happen all over the sky so that astronomers can quickly follow up with other telescopes.
This was the case with GRB 130606A, detected on June 6, 2013. A group of astronomers led by Robert Chornock of the Harvard Smithsonian Center for Astrophysics used the 6.5-meter Multiple Mirror Telescope in Arizona and the 8-meter Gemini North telescope in Hawaii to observe the afterglow of the explosion. With these two huge telescopes, the spectra of the GRB afterglow was observed.
The galaxy that hosted the burst turned out to be at a redshift of 5.91, converting to a distance of 12.7 billion light-years away. Only a few GRBs have been seen at this distance, in galaxies as they were in the first billion years of the Universe’s existence. This period of history is near the end of the epoch of reionization. Our telescopes are not yet powerful enough to measure the light from many galaxies at this time, but it is crucial in understanding how galaxies formed and evolved. (It is sometimes erroneously called the ‘dark ages,’ however, that is a term better used to describe the time before these galaxies even formed, when a few rare stars began to light up the dark, empty Universe.)
To date, quasars have been our only direct probe into this period of time, though low frequency radio telescopes are closing in on the signal of the hydrogen emissions between galaxies from this epoch. This GRB is the first of its kind to produce so much information from its spectrum, telling us much about the host galaxy. The astronomers used absorption lines to measure how many heavy elements (those heavier than helium, that is) were present in this galaxy. It turns out to have about 10 percent of the heavy elements as a star like our sun. That’s not quite ideal for forming rocky planets around stars just yet.
The GRB afterglow could also probe how much neutral hydrogen exists between galaxies at this time. Today, just about all of the hydrogen in between galaxies is ionized, but it only became this way during the first billion years of history when ultraviolet light from young stars in young galaxies ripped the electrons away from the hydrogen nuclei. The astronomers did find a bit of neutral hydrogen still around this GRB’s host galaxy, consistent with findings from quasars. This is promising, since multiple methods of probing the same astrophysical setting help reduce certain errors or biases. It’s always good to have a check!
As GRB130606A is one of the furthest GRBs detected, it certainly had much to offer in the way of science results. These massive explosions are terrifying but wonderful, and help us to understand the weird and huge Universe in which we live.
Image: This artist’s illustration depicts a gamma-ray burst illuminating clouds of interstellar gas in its host galaxy and intergalactic gas between the distant galaxy and us. The gas absorbs certain frequencies of light in the spectrum, allowing us to determine the makeup and amount of gas in the galaxy and along the line of sight. Credit: Gemini Observatory/AURA, artwork by Lynette Cook
This research will be published in Astrophysical Journal, and a preprint is available on arXiv.org.