Fig. 1:
Artist's conception of a binary system, where a mass overflow from a
donor star onto a white dwarf star may occur. Once enough accreted
matter has accumulated on the surface of the
dwarf star, this may initiate a nuclear explosion, which in turn would
ignite the catastrophic nuclear burning and disruption of the dwarf
star: a supernova of type Ia.
Credit: ESA Noordwijk
Abb. 2:
The INTEGRAL Space Observatory for gamma-rays from cosmic sources (up). Credit: ESA Noordwijk. The Spectrometer (SPI) instrument is optimized for spectroscopy of gamma-ray lines. Credit: CNES Toulouse
Fig. 3:
Detection of a nickel line in the Supernova SN2014J, some two weeks
after the explosion. The position of the signal agrees within the
measurement
error with the position of the supernova (indicated by the cross).
Adapted from R. Diehl,Th. Siegert,W. Hillebrandt et al.
Science 31 July 2014.
High-energy observations with the INTEGRAL space observatory have
revealed a surprising signal of gamma-rays from the surface of material
ejected by a
recent supernova explosion. This result challenges the prevailing
explosion model for type Ia supernovae, indicating that such energetic
events might
be ignited from the outside as well rather than from the exploding dwarf
star's centre. The scientists from the Max Planck Institutes for
Extraterrestrial
Physics and for Astrophysics present their findings in the current
edition of Science to the astronomical community.
In January, a supernova explosion, called SN2014J, was reported in a
nearby starburst galaxy, called M82. Just two weeks later, astronomers
were
able to take data with the INTEGRAL space telescope, revealing two
characteristic gamma-ray lines from a radioactive nickel isotope (56Ni).
Supernovae are giant nuclear fusion furnaces, and the atomic nuclei of
nickel are believed to be the main product of nuclear fusion inside the
supernova. Presumably this radioactive element is created mainly in the
centre of the exploding white dwarf star and therefore occulted from
direct
observation. As the explosion dilutes the entire stellar material, the
outer layers get more and more transparent, and after several weeks to
months
also gamma-rays from the nickel decay chain are expected to be
accessible to observation.
As the astronomers scrutinized the new data, however, they found traces
of the decay of radioactive nickel just 15 days after the presumable
explosion
date. This implies that the observed material was near the surface of
the explosion, which was a surprise.
"For quite a while, we were puzzled by this surprising signal", says
Roland Diehl from the Max Planck Institute for Extraterrestrial Physics,
the lead
author of the study and Principal Investigator of the INTEGRAL
spectrometer instrument. “But we could not find anything wrong, rather
the gamma-ray
lines from 56Ni faded away as expected after a few days, and clearly
came from the direction of the supernova”, he explains the outcome of
their analysis
of the observations. At MPE, an expert analysis team has been developing
special methods for high-resolution spectroscopy of gamma-ray lines for
many years.
This has been successfully applied to the study of nucleosynthesis
throughout our Galaxy as well as for the Cassiopeia A supernova remnant -
and now to
the recent supernova observations.
"We know that the supernova burns an entire white dwarf star within a
second, but we are not sure how the explosion is ignited in the first
place", explains
Wolfgang Hillebrandt, a co-author of the study from the Max Planck
Institute for Astrophysics. "A companion star's action seems required",
he continues, "and
for a while, we believed that only those white dwarfs explode, which are
loaded with material from the companion star until they reach a
critical limiting mass."
But then, the explosion would be ignited in the core of the white dwarf,
and no nuclear fusion products should be seen on the outside.
Diehl, Hillebrandt, and their colleagues had argued over the result for a
while, challenging the methods of data analysis as well as ideas about
supernova explosion
scenarios. They now report their finding, supported by statistical
arguments, and their descriptions of their methods to help scientists
judge this important discovery.
They conclude that those gamma-rays shed new light on how a binary
companion’s material flow can ignite such a supernova from outside, and
without demand for exceeding
a critical mass limit for white dwarf stars.
From the early appearance of the nickel gamma-rays it seems that some
modest amount of outer material accreted from the companion star
ignited, and was processed
to fusion ashes including the observed nickel. This primary explosion
then must have triggered the main supernova, which was also observed
with a variety of
telescopes at many other wavelength bands, and appears as a rather
normal supernova in these observations.
Gamma-rays from radioactive decay directly trace nuclear fusion ashes,
and thus make a unique contribution to what we can learn about such
explosions.
The scenario that the astrophysicists describe ties in with recent
belief that rather rapid material flows such as they occur in merging
white dwarfs
may often be the origins of supernovae of this type.
About INTEGRAL
The INTEGRAL gamma-ray space observatory was launched in 2002 for a nominal 3-year mission, and now, after almost 12 years, is still in good shape for many more years of observations. Together with the partner institute IRAP/CESR in Toulouse, MPE was responsible for one of the two main telescopes, the SPI spectrometer. INTEGRAL has discovered many new sources of the violent high-energy universe, among them active galaxies, new classes of accreting binary systems and pulsars, gamma-ray bursters, and surveys of nucleosynthesis gamma-rays from different sources plus a puzzling signal from annihilation of antimatter.
INTEGRAL is a mission of the European Space Agency ESA in cooperation with Russia and the United States. Website: http://sci.esa.int/integral/
Original publication: ScienceXpress Online-Publikation 31Jul 2014
R. Diehl,Th. Siegert,W. Hillebrandt et al. Early 56Ni decay γ-rays from SN2014J suggest an unusual explosion Science 31 July 2014
R. Diehl,Th. Siegert,W. Hillebrandt et al. Early 56Ni decay γ-rays from SN2014J suggest an unusual explosion Science 31 July 2014
Contact at MPE
Prof. Dr. Roland Diehl
Max-Planck-Institut für extraterrestrische Physik
E-Mail: rod@mpe.mpg.de
Tel. +49 89 30000 3850
Contact at MPA
Prof. Dr. Wolfgang Hillebrandt
Max-Planck-Institut für Astrophysik
E-Mail: wfh@mpa-garching.mpg.de
Tel. +49 89 30000 2200
