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Gamma-Ray Bursts (GRBs) are brief and intense flashes of gamma-ray
radiation that can occur randomly from any direction of the sky. Their
durations range from a few milliseconds up to over half an hour. They
are so energetic that we can detect them even at distances of
thousands of millions of light years. Since our atmosphere is opaque
to gamma-ray photons, GRBs are detected by gamma-ray detectors on
board spacecraft such as NASAs Swift satellite. Follow-up observations
using ground-based telescopes have shown that GRBs are accompanied by
fading emissions from ultraviolet to radio wavelengths, the so-called
"afterglow". This afterglow emission is usually produced by
synchrotron radiation emitted by charged particles moving in magnetic
fields at ultra-relativistic speeds (velocities above 99% of the speed
of light).
On Christmas Day 2010 a very peculiar GRB occurred, designated
GRB101225A according to the date of its discovery, also nicknamed
”the Christmas Burst“. It lasted more than half an hour,
much longer than most GRBs detected so far. Its low-energy emission
(i.e., all radiation measured below the gamma-ray regime) was
dominated by a hot thermal component - a classical blackbody spectrum,
challenging the long-standing paradigm that GRB afterglows are
produced by synchrotron radiation.
An international group of researchers, led by Christina Thöne and
Antonio de Ugarte Postigo from the Instituto de Astrofisica de
Andalucia (IAA — CSIC, Granada, Spain) recently published an
article in Nature on the Christmas Burst. The collaboration of
researchers also included scientists at the Max Planck Institute for
Astrophysics, who contributed to the theoretical interpretation of the
data and its explanation by a feasible model. Based on a large set of
space and ground-based observations, the team proposes a new scenario
to explain this exotic explosive event. The current standard models to
explain the two broad types of GRBs that have been observed are the
"Compact Binary Merger" model, for short duration (<2 sec) GRBs and
the "Collapsar" model, for long duration (>2 sec) GRBs. However,
according to Thöne et al, the peculiar properties of GRB101225A
require a different model altogether.
They propose that GRB101225A is the result of a neutron star merging
with the helium core of an evolved giant star. This somewhat exotic
binary system underwent a common envelope phase when the neutron star
entered the atmosphere of the giant star, during which the giant star
expelled most of its hydrogen envelope. The final explosion created a
GRB-like jet, which became thermalized by its interaction with the
dense, previously ejected envelope, giving rise to the observed black
body spectrum. This ejected material was cooling down progressively
from 1 million K immediately after the burst, to ~ 5,000K at 20 days
after the event.
Finally, about 10 days after the explosion a faint supernova component
started to emerge, reaching its maximum 40 days after the GRB and
dominating the fading blackbody radiation. The best fit to this
supernova component is a faint broadline Type Ic-like supernova at a
distance of 5.5 billion light-years (redshift z~0.3).
The high velocities and the high density of the material make it
difficult to observe such an event the large distances, where GRBs are
normally seen. This could explain why such an event has been seen only
recently.
Original publication:
Thöne et al.,
"The unusual gamma-ray burst GRB 101225A from a helium star/neutron
star merger at redshift 0.33",
 Nature, 480, (issue 7375), 72-74 (2011)
Contact:
Dr. Hannelore Hämmerle
Press officer
Max Planck Institute for Astrophysics
Tel.: +49 89 30000-3980
Email: pr mpa-garching.mpg.de
Dr. Christina C. Thöne
Scientist
IAA - CSIC
Tel.: +34 958 230 612
Email: cthoeneiaa.es
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