GRB-VREESWIJK · Gamma-Ray Burst afterglows: illuminating the early Universe

Final Activity Report Summary - GRB-VREESWIJK (Gamma-Ray Burst Afterglows: Illuminating the early Universe)

Gamma-ray bursts (GRBs) are brief flashes of very energetic photons, lasting from milliseconds to around 10 minutes. They occur about once a day in the universe and do not have a preferred location in the sky. Two classes of GRBs are known, namely the short-duration, lasting less than two seconds, and the long-duration, i.e. over 2 seconds, classes. Since the long-duration GRBs are accompanied by a bright so-called afterglow at other wavelengths, such as the optical and near-infrared wavelengths, we know much more about this class. For instance, we are aware of their distance distribution. Some occur relatively nearby, in nearby galaxies, while the most distant one is the 2econd most distant object which has been detected by astronomers so far. Their origin is believed to be the death of a massive star, with a mass of 30 to 100 times that of our Sun, which typically explodes in a supernova, while some also produce a gamma-ray flash. Nevertheless, only a small fraction of massive stars make a GRB and it still remains unclear what exactly triggers the explosion so as to also release a considerable fraction of the energy in gamma rays. One ingredient that is thought to be important is the metallicity, or the fraction of metals other than hydrogen, of the star-forming region in which the massive star was born. Given that the GRB afterglow can be so bright, we can use it both as a means to find out about the mechanism that produces GRBs, as well as a bright background light to study the universe between the GRB and ourselves. For instance, we can learn about the kind of metals that are present in the region immediately surrounding the GRB, in the galaxy in which it occurred, and anything that is along the line of sight. As we know from the laboratory, the metals will reveal themselves in a spectrum of the afterglow, at the exact wavelengths in which a particular metal will absorb the afterglow light. This analysis can be done up to any distance, including the so-called Dark Ages when all hydrogen in the universe was mostly neutral and the first sources of light began to illuminate it. One of the outstanding issues in contemporary astronomy is the exact way in which this transition from dark to light, also known as the epoch of re-ionisation, took place. One of the two main aims of this Marie Curie project was to use the new X-shooter spectrograph at the ‘very large telescope’ (VLT) of the European Southern Observatory to try to catch GRBs that occurred during the epoch of re-ionisation so as to learn more about the evolution of the neutral gas fraction and metallicity. Unfortunately, the X-shooter was delayed by two years. It was planned to start observing in 2007, but was offered to the community to be used only in October 2009. The second main aim was less risky, as it was based on an existing program, led by myself, using another spectrograph at the same VLT. With this program we surprisingly found that some of the absorption lines imprinted on the GRB afterglow spectrum changed in time. Modelling showed that this observation could be naturally explained by the GRB ultraviolet and optical photons illuminating a gas cloud along the line of sight. Using the modelling we could, for the first time, determine the distance between the exploding massive star and the absorbing gas cloud and surprisingly found that it was not located in the immediate surroundings of the GRB. We found a puzzling distance of 1 700 parsec, which was must larger than the typical star-forming region. During this Marie Curie project we also applied our modelling to two other cases. In one of them we found even larger distances, around 3 000 parsec, while the other case showed 500 parsec. These results showed that our novel technique might be a useful way to find out about the distribution of gas clouds in very distant galaxies. During the years to come I am planning to pursue this line of research, which this Marie Curie project allowed me to further develop.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз