Astronomers using INTEGRAL have detected the first direct signature of titanium-44 in the remnant of the nearby supernova 1987A. The discovery reveals a large amount of this key isotope in the remnant, equivalent to 0.03 per cent the mass of the Sun.

This value is close to upper bounds from theoretical predictions and exceeds the amount of titanium-44 observed in Cassiopeia A - the only other supernova remnant where this isotope has been found. The amount of titanium-44 found in SNR 1987A demonstrates that its radioactive decay has been powering the source for the past 22 years.

Stars are the tireless furnaces of the Universe. They spend most of their lives burning hydrogen into helium in their cores. However, towards the end of their lives, the most massive stars reach such high temperatures that nuclear fusion of heavier elements becomes possible. This causes the star's core to collapse and eventually leads to a supernova explosion.

During these powerful explosions, the nuclei of many heavy elements that we find on Earth are synthesised: titanium, iron, cobalt, and nickel. Supernovae and their remnants represent a unique laboratory for astronomers to study how nucleosynthesis occurs in some of the most extreme cosmic environments.

When a star explodes as a supernova, it releases an amount of energy equivalent to the radiation output of an entire galaxy. Such a sudden boost in luminosity makes supernovae easy to spot, even in very distant galaxies.

After the explosion, the brightness of a supernova remnant starts declining: the source is no longer powered by the initial energy of the explosion, but by the radioactive decay of some of the atomic nuclei produced during the supernova phase.

Just after the peak of its brightness, the power of a supernova remnant derives mainly from the decay of an isotope of nickel (nickel-56, or 56Ni) and, later, of cobalt (cobalt-56, or 56Co). Both of these isotopes have a rather short half-life and start decaying on timescales of a few to several hundreds of days.

In the long run, however, astronomers believe that the source of power is the decay of an isotope of titanium (titanium-44, or 44Ti). With a half-life of about 60 years, titanium-44 is able to keep supernova remnants shining from about three years after the explosion until the ejected material begins to interact with the circumstellar matter.

The remnant of the famous supernova 1987A (SNR 1987A), first detected in February 1987, is an ideal test bed to monitor how the brightness of a supernova changes over a period of several years. It is located relatively close by, in the Large Magellanic Cloud, one of the Milky Way's satellite galaxies - at a distance of only about 166 000 light-years. For the past 25 years, it has been providing astronomers with plenty of data to test theoretical predictions about supernovae and their aftermath in great detail.

"We have now detected, for the first time, the direct signature of titanium-44 in SNR 1987A," comments Sergei Grebenev from the Space Research Institute at the Russian Academy of Science in Moscow, Russia. Grebenev is the first author of a paper reporting on the results, which is being published in the 18 October 2012 issue of Nature. "The discovery is based on over 1500 hours of observations performed at hard X-ray energies with ESA's INTEGRAL mission. Such a long exposure was crucial to achieve a clear detection," he adds.

The data analysed by Grebenev and his collaborators show a clear signal, corresponding to the position in the sky of this supernova remnant, in the spectral range between 65 and 82 keV, which brackets two emission lines produced during the decay of titanium-44, namely at 67.9 and 78.4 keV. In addition, the astronomers observed the same patch of the sky at slightly higher and at slightly lower energies, but did not detect any signal.

"This demonstrates that the signal detected by INTEGRAL at 65-82 keV does arise from line emission," notes Chris Winkler, INTEGRAL Project Scientist at ESA and co-author of the Nature paper.

Previous indications of the presence of titanium-44 in SNR 1987A were only indirect and had been inferred by combining observations in other portions of the electromagnetic spectrum with complex numerical modelling.

"Titanium-44 is synthesised, during the first instants of a supernova explosion, via the nuclear burning of silicon," explains Grebenev. From analysis of the INTEGRAL data, the astronomers have estimated the total mass of this isotope that must have been produced right after the collapse of the stellar core of the supernova's progenitor.

It amounts to about 0.03 per cent of the mass of the Sun. Such a large amount of titanium-44 demonstrates that the radioactive decay of this isotope has been powering the supernova remnant at ultraviolet, visible and infrared wavelengths for the past 22 years.

However, the interaction between the ejecta from the supernova and the surrounding material, which started several years after the explosion, represents an additional mechanism to power the remnant and produce emission at soft X-ray energies.

The amount of titanium-44 detected in SNR 1987A with INTEGRAL is very close to the highest values predicted from numerical simulations of nucleosynthesis in supernova explosions. Astronomers believe that the production of unusually large amounts of this isotope is possible in some particular configurations of the explosion - in particular, in supernova explosions with a complex and asymmetric geometry, such as SN1987A.

Before now, titanium-44 had only been seen in one other supernova remnant - Cassiopeia A, which is about 400 years older than SNR 1987A. That discovery, made in 1994 with NASA's Compton Gamma Ray Observatory, was later confirmed using the Italian/Dutch BeppoSAX satellite and INTEGRAL. The amounts of titanium-44 found in these two remnants are comparable, although SNR 1987A seems to contain more than Cassiopeia A.

"The discovery of large amounts of titanium-44 in SN1987A confirms our previous suspicions that this supernova might have produced a significant quantity of this isotope," notes Winkler.

Astronomers expect to find titanium-44 in more supernova remnants by analysing more data already collected with INTEGRAL. "These observations are broadening our understanding of the nucleosynthesis reactions that take place during the final stages of a massive star's life."

The study presented here is based on data gathered with the IBIS/ISGRI hard X-ray telescope on board ESA's INTEGRAL observatory in 2010 and 2011. The total exposure time amounts to about 6 million seconds, equivalent to about 1650 hours.

Observations were performed in three narrow energy bands: 48-65, 65-82 and 82-99 keV. A signal corresponding to the position in the sky of the supernova remnant SNR 1987A was detected only in the 65-82 keV band, which encompasses two of the emission lines produced during the radioactive decay of the isotope titanium-44, at 67.9 and 78.4 keV, respectively.

The signal detected at 65-82 keV can be translated into the initial amount of titanium-44 that has been synthesised during the supernova explosion: M44 ~ (3.5+/-0.8) x 10-4 MSun.

This value is close to the upper bounds of predictions from numerical simulations, suggesting that either nucleosynthesis in this supernova did not proceed in the canonical way to produce such a large amount of titanium-44 or that the accuracy of the simulations must be improved.

The results presented here are reported in the paper "Hard X-ray emission lines from the decay of 44Ti in the remnant of Supernova 1987A" by S. A. Grebenev, et al., to be published in the 18 October 2012 issue of the journal Nature.