INTEGRAL catches dead star exploding in a blaze of glory

Supernova explosion (annotated)

Astronomers studying SN2014J, a Type Ia supernova discovered in January 2014, have found proof that this type of supernova is caused by a white dwarf star reigniting and exploding.

This finding was made by using ESA’s Integral observatory to detect gamma rays from the radioactive elements created during the explosion.

This sequence shows some of the steps leading up to and following the explosion.

A white dwarf, a star that contain up to 1.4 times the mass of the Sun squeezed into a volume about the same size as the Earth, leeches matter from a companion star (image 1).  The Integral measurements suggest that a belt of gas from the companion star builds up around the equator of the white dwarf (image 2). This belt detonates (image 3) and triggers the internal explosion that becomes the supernova (image 4). Material from the explosion expands (image 5) and eventually becomes transparent to gamma rays (image 6). Copyright: ESA/ATG medialab. Hi-Res Image

 

Supernova SN2014J in nearby galaxy M82

In January 2014, a supernova was discovered in the nearby galaxy M82. At a distance of about 11.5 million light-years from Earth, SN2014J as it is known, is the closest of its type to be detected in decades. 

This composite Hubble image shows the supernova in visible light, obtained on 31 January with Hubble’s Wide Field Camera 3, superimposed on a mosaic of the entire galaxy taken in 2006 with Hubble’s Advanced Camera for Surveys. Copyright: NASA, ESA, A. Goobar (Stockholm University), and the Hubble Heritage Team (STScI/AURA).  Hi-Res Image

Astronomers using ESA’s INTEGRAL gamma-ray observatory have demonstrated beyond doubt that dead stars known as white dwarfs can reignite and explode as supernovae. The finding came after the unique signature of gamma rays from the radioactive elements created in one of these explosions was captured for the first time.

The explosions in question are known as Type Ia supernovae, long suspected to be the result of a white dwarf star blowing up because of a disruptive interaction with a companion star. However, astronomers have lacked definitive evidence that a white dwarf was involved until now. The ‘smoking gun’ in this case was evidence for radioactive nuclei being created by fusion during the thermonuclear explosion of the white dwarf star.

“INTEGRAL has all the capabilities to detect the signature of this fusion, but we had to wait for more than ten years for a once-in-a-lifetime opportunity to catch a nearby supernova,” says Eugene Churazov, from the Space Research Institute (IKI) in Moscow, Russia and the Max Planck Institute for Astrophysics,in Garching, Germany.

Although Type Ia supernovae are expected to occur frequently across the Universe they are rare occurrences in any one galaxy, with typical rates of one every few hundred years. 

INTEGRAL’s chance came on 21 January 2014, when students at the University College London’s teaching observatory at Mill Hill, UK detected a type Ia supernova, later named SN2014J, in the nearby galaxy M82.

According to the theory of such explosions, the carbon and oxygen found in a white dwarf should be fused into radioactive nickel during the explosion. This nickel should then quickly decay into radioactive cobalt, which would itself subsequently decay, on a somewhat longer timescale, into stable iron. 

Because of its proximity – at a distance of about 11.5 million light-years from Earth, SN2014J is the closest of its type to be detected in decades – INTEGRAL stood a good chance of seeing the gamma rays produced by the decay. Within one week of the initial discovery, an observing plan to use INTEGRAL had been drawn-up and approved.  

Using INTEGRAL to study the aftermath of the supernova explosion, scientists looked for the signature of cobalt decay – and they found it, in exactly the quantities that the models predicted. 

“The consistency of the spectra, obtained by INTEGRAL 50 days after the explosion, with that expected from cobalt decay in the expanding debris of the white dwarf was excellent,” says Churazov, who is lead author of a paper describing this study and reported in the journal Nature. 

With that confirmation in hand, other astronomers could begin to look into the details of the process. In particular, how the white dwarf is detonated in the first place. 

White dwarfs are inert stars that contain up to 1.4 times the mass of the Sun squeezed into a volume about the same size as the Earth. Being inert, they can’t simply blow themselves up. Instead, astronomers believe that they leech matter from a companion star, which builds up on the surface until a critical total mass is reached. At that point, the pressure in the heart of the white dwarf triggers a catastrophic thermonuclear detonation.

Early INTEGRAL observations of SN2014J tell a somewhat different story, and have been the focus of a separate study, reported online in Science Express by Roland Diehl from the Max Planck Institute for Extraterrestrial Physics, Germany, and colleagues.

Diehl and his colleagues detected gamma rays from the decay of radioactive nickel just 15 days after the explosion. This was unexpected, because during the early phase of a Type Ia supernova, the explosion debris is thought to be so dense that the gamma rays from the nickel decay should be trapped inside.

“We were puzzled by this surprising signal, and some from the group even thought it must be wrong,” says Diehl. “We had long and ultimately very fruitful discussions about what might explain these data.”

A careful examination of the theory showed that the signal would have been hidden only if the explosion had begun in the heart of the white dwarf. Instead, Diehl and colleagues think that what they are seeing is evidence for a belt of gas from the companion star that must have built up around the equator of the white dwarf. This outer layer detonated, forming the observed nickel and then triggering the internal explosion that became the supernova.

“Regardless of the fine details of how these supernovae are triggered, INTEGRAL has proved beyond doubt that a white dwarf is involved in these stellar cataclysms,” says Erik Kuulkers, ESA’s INTEGRAL Project Scientist. “This clearly demonstrates that even after almost twelve years in operation, INTEGRAL is still playing a crucial role in unraveling some of the mysteries of the high-energy Universe.”  

Notes for editors
 

“⁵⁶Co gamma-ray emission lines from the type Ia supernova SN2014J” by E. Churazov et al., is published in the 28 August 2014 issue of Nature; DOI: 10.1038/nature13672 

“Early ⁵⁶Ni decay γ rays from SN2014J suggest an unusual explosion” by R. Diehl et al., appeared online in Science Express on 31 July 2014; DOI: 10.1126/science.1254738

Some of the observations of SN2014J were obtained as part of an INTEGRAL Target of Opportunity programme led by Principal Investigator Jordi Isern (ICE-CSIC/IEEC, Spain). The INTEGRAL Project Scientist, Erik Kuulkers, made additional observing time available, on request of the INTEGRAL supernova community, to maximise the scientific return. This was supplemented by a contribution from the Russian guaranteed time on the recommendation of the Russian INTEGRAL Advisory Committee. 

Type Ia supernovae are particularly important because they are used to gauge distances across much of the visible Universe. In the 1990s, their study led to the discovery of the cosmic acceleration that is now thought to be powered by a mysterious form of energy called ‘dark energy’. The Nobel Prize for Physics in 2011 was awarded to Saul Perlmutter, Adam Riess, and Brian Schmidt for their role in the discovery of dark energy. 

The International Gamma-ray Astrophysics Laboratory (INTEGRAL) was launched on 17 October 2002. It is an ESA project with the instruments and a science data centre funded by ESA Member States (especially the Principal Investigator countries: Denmark, France, Germany, Italy, Spain, Switzerland), and with the participation of Russia and the USA. The mission is dedicated to the fine spectroscopy (E/∆E = 500) and fine imaging (angular resolution: 12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV with concurrent source monitoring in the X-ray (4-35 keV) and optical (V-band, 550 nm) wavelengths.

For further information, please contact:
 
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Phone: +31 71 565 6799
Mobile: +31 61 594 3 954
Email: markus.bauer@esa.int

Eugene Churazov
Space Research Institute (IKI), Moscow, Russia
Phone: +7-495-3333377
Email: churazov@hea.iki.rssi.ru
and Max Planck Institute for Astrophysics, Germany
Phone: +49-89-30000-2219
Email: churazov@mpa-garching.mpg.de

Roland Diehl
Max Planck Institute for Extraterrestrial Physics, Germany
Phone: +49-89-30000-3850
Email: rodmpe.mpg.de

Erik Kuulkers
INTEGRAL Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Phone: +34-91-8131-358
Email: Erik.Kuulkers@sciops.esa.int

Source: ESA  

Igniting a supernova explosion

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

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 

Source: Max Planck Institute for Astrophysics