Gamma-ray bursts
Gamma-ray bursts (GRBs) are the most luminous explosions known to man. They are unique physics laboratories in which the radiation is produced in ultra-relativistic outflows, traveling with bulk Lorentz factors of hundreds and harboring extreme particle acceleration, something hard to produce elsewhere in the Universe let alone in laboratories here on Earth. GRBs are thought to originate in massive stars, either through the collapse of a massive, rapidly rotating star, or from the merger of two compact objects such as neutron stars or black holes. This progenitor division appears visible in observations by the duration and spectral properties of the actual burst of gamma-rays itself, and by the placement of the burst with respect to the centre of their host galaxy. With their origin in massive stars and visible from enormous distances (redshifts of more than 8 have been measured), GRBs are an invaluable tool to probe star formation across the ages, but they can also be used as a backlight for observations of their host and intervening galaxies.
Amsterdam has been a long-time center for research into GRBs, mainly their afterglows, which is the broadband radiation across the whole electromagnetic spectrum produced after the initial gamma-ray event. The first optical afterglow was discovered in Amsterdam in 1997, and one year later the connection between GRBs and supernovae was also discovered in Amsterdam (which confirmed their origin in massive stars). Researchers at the Anton Pannekoek Institute, also as part of large international collaborations, have continued to contribute significantly to the GRB field with observations of their host galaxies and afterglows at radio, optical and X-ray frequencies, and theoretical studies of the relativistic outflows and emission processes giving rise to these enigmatic events.
The research on GRBs in Amsterdam currently focuses on the following topics:
- The optical spectra obtained from afterglows, which provide a wealth of information on the direct surroundings of the GRBs and their host galaxies. For this research we mainly perform observations with the William Herschel Telescope on La Palma and the new-generation X-Shooter spectropgraph on the Very Large Telescope in Chili. The emission and absorption lines in their spectra reveal the redshifts and metallicities of the host galaxies, and high-resolution spectroscopy provides unique information on the effects that GRBs have on the dust in their immediate environments.
- Broadband modeling of GRB afterglows, using observations at radio, millimeter, infrared, optical and X-ray frequencies, which sheds light on the macro- and microphysical processes that are playing a role in the relativistic outflows causing the radiation that is observed. The parameters that are derived include the energetics and collimation of the outflow, the density and structure of the circumburst medium, and the magnetic field strength and energy in the relativistic electrons which are producing the observed radiation. These physical parameters give insight in the amount of energy produced in the collapse of massive stars or compact binary mergers, particle acceleration and magnetic field generation.
- Radio observations of GRBs with the Westerbork Synthesis Radio Telescope and in the near future with the new-generation Low Frequency Array. These observations provide unique information to constrain the physical parameters by following the behaviour until much later times than any other wavelength, sometimes even for years. At these late times the outflow is not relativistic anymore and the true energetics can be determined free of any relativistic effects. The radio band is also used to search for off-axis emission from a spreading GRB outflow that is originally pointed away from us in some particular supernovae.
- Modeling the evolution of the relativistic outflow using detailed hydrodynamics and radiation codes, which forms the basis to understand the kinematics that go into the explosion. While the traditional afterglow modeling adopts several simplifications, these studies use powerful computers to determine the evolution of the outflow and the observed radiation in great detail, taking into account for instance the structure of the outflow and the surrounding medium, and the angle under which the collimated outflow is observed.
-- The progenitors of short gamma-ray bursts and their effects on the observed gamma-ray and afterglow emission. Short GRBs are thought to originate in the merger of two neutron stars or a neutron star and black hole, but the end product of that merger is unclear. It is predicted that an unstable millisecond pulsar with an extremely high magnetic field is formed, which will then collapse into a black hole. The imprint of this so-called millisecond magnetar is being investigated, mainly with gamma- and X-rays that are observed at early times.
Contact persons are:
The following people are working on this topic: