English (UK)
Gamma-ray bursts (GRBs) are brief and intense flashes of gamma-ray radiation that occur randomly in any direction of the sky. They have been related to several processes linked to catastrophic stellar events. The durations of the gamma-ray emissions range from a few milli-seconds up to half an hour or more. They are so energetic that they can be detected over distances of thousands of millions of light-years. Given that the Earth’s atmosphere is opaque to gamma-ray photons, GRBs can only be caught thanks to spaceborne gamma-ray detectors onboard satellites such as NASA’s Swift spacecraft. As soon as Swift localizes a GRBs, it reacts and distributes the coordinates (mainly through the internet) to astronomers all over the world, who then follow-up these explosive events using ground-based telescopes. Observations done from the ground have shown that GRBs are accompanied by a fading optical-infrared-radio emission called the "afterglow", which can be explained as synchrotron radiation: light coming from electrically charged particles in powerful magnetic fields moving at ultra-fast, relativistic speeds (velocities above 99% of speed of light).
On Christmas day 2010 a peculiar GRB occurred, GRB101225A, nicknamed “the Christmas Burst”. It lasted more than 2000 seconds, much longer than most other GRBs. Its low-energy emission (i.e., all the radiation measured below the gamma-ray regime) was dominated by a strong hot thermal component, a classical black-body spectrum, that was cooling down with time. This means that heat radiation was the primary source of the afterglow in this event, in contrast to all GRBs detected so far, which were dominated by synchrotron radiation.
An international group of researchers, led by Christina Thöne and Antonio de Ugarte Postigo of the Astrophysical Institute of Andalusia (IAA-CSIC, Granada, Spain), has just published an article on GRB101225A in the journal Nature investigating the physics of this rare explosion. Based on a large number of observations, both from space and from the ground, they propose a new scenario to explain this exotic cataclysmic event. To date, the two most popular models were the collapsar and the compact binary merger, explaining GRBs lasting longer or shorter than 2 s, respectively (hence called “long-” and “short-duration” GRBs). The peculiar properties of GRB101225A, however, require a third, completely different, model.
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| An artist’s impression of the process that caused the Christmas Burst, the anomalous explosion GRB101225A. A. Simonnet, NASA E/PO, Sonoma State University.
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The collaboration, including astronomers from all around the world, proposes that the Christmas Burst is the result of a merger of a neutron star and an evolved giant star burning helium in its core, placed at a distance of ~5.5 thousands of million light-years (redshift z ~ 0.3). This exotic binary system underwent a common envelope phase, when the neutron star reached the atmomsphere of the helium star, in which most of the hydrogen of the helium star was expelled. Later, when the two stars merged, the explosion created a GRB-like jet, which became thermalised by its interaction with the pre-existing common envelope. This interaction then gave rise to the observed, unusual black body spectrum dominated by radiation generated by hot material.
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| The optical counterpart of GRB101225A as observed with the Zeiss 1.23 m reflector of Calar Alto Observatory, approximately one day after the explosion. Note its blue colour, atypical for standard GRB afterglows. The colour can be explained by a thermal source with a temperature of ~21000 K.
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The observations performed with the Calar Alto Zeiss 1.23 m telescope (Almería, Spain), taken approximately 1 day after the explosion, were among the first displaying the visible light connected to the GRB event, and they were essential to realize that GRB101225A involved a different kind of stellar death. The 1.23 m VRI-band observations revealed colours atypical for a standard GRB, and the spectral shape that could not be explained by synchrotron emission. The image shows the optical counterpart of GRB101225A as detected by the 1.23 m telescope. Note the blue colour of the optical emission (indicated by an arrow), consistent with a thermal source at a temperature of ~21000 K, which was later cooling down to ~5000 K. In contrast to the standard GRBs, where the ejected material is extremely fast, ultra-relativistic in fact, the outflow of GRB101225A detected by the 1.23 m CAHA telescope would be “only” mildly-relativistic, with an expansion velocity below 25% of the speed of light. Still, this is well over 70 000 km/h! (40 000 miles/h).
The non-relativistic character of the outflow, that is a consequence of the dense environment (slowing down down the high-velocity output of the explosion), make it difficult to detect these exotic events at extreme distances, as it is possible for the ultra-relativistic, standard GRBs. This may explain why this rare kind of stellar death had not been observed before.
Many more scientific secrets lurk in the world of gamma ray bursts. This innovative field of modern astrophysics requires close coordination among several ground-based observatories, working in fast connection with spaceborne instruments, and world-wide collaboration among many scientists. Calar Alto telescopes and instruments will continue to take part in this challenge.
© Calar Alto Observatory, November 2011