The INTEGRAL gamma-ray satellite has detected the radioactive decay of an isotope of titanium, 44Ti, in the remnant of the nearby supernova SN 1987A. This observation confirms that 44Ti powers the infrared, optical and ultraviolet emission that is still being observed 25 years after the stellar explosion.
On 24 February 1987, two astronomers at the Las Campanas Observatory in Chile and an amateur astronomer in New Zealand were the first to notice an unexpected bright star in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way. They actually witnessed the first supernova to be visible to the naked eye since SN 1604, which was studied by Johannes Kepler (CERN Courier December 2004 p15, January/February 2006 p10). SN 1987A reached peak brightness in May that same year and slowly declined over the following months.
The shape of the light curve of a supernova – the evolution of the luminosity – is determined by the radioactive decay of elements produced during the explosion of the progenitor star. Nickel-56, with a half-life of six days, is responsible for the peak of the emission, while the radioactive decay of cobalt-56 to iron-56 slows down the subsequent decrease in brightness for several months (77 days of half-life). Over longer timescales, 44Ti is expected to dominate in sustaining the remnant emission of the explosion for decades (85 years of half-life).
The actual contribution of 44Ti to the late time emission of a supernova is poorly known. Indeed, the violent interaction of the stellar ejecta with the surrounding medium will lead to shock waves and additional emission blending with the contribution from this radioactive decay in the infrared to ultraviolet band. Theoretical simulations of SN 1987A suggest that the amount of 44Ti synthesized during the explosion is in the range 0.02–2.5 × 10–4 solar masses. This uncertainty by two orders of magnitude is because there are many unknowns in the physical properties of the stellar interior and of the explosive shock wave. Direct detection of 44Ti is thus important for improving the constraints on the physical conditions in this supernova explosion.
This breakthrough has now been achieved by a small group of astronomers led by Sergey Grebenev of the Space Research Institute in Moscow. His request for a long observation (around 40 days) of SN 1987A by ESA’s INTEGRAL gamma-ray satellite turned out to be highly fruitful. The decay of 44Ti can be directly detected by INTEGRAL through emission lines produced in both hard X-rays at energies of 67.9 keV and 78.4 keV and in gamma-rays at 511 keV and 1157 keV. While the observation of the latter lines yielded only upper limits, the former ones allowed a 4.7σ detection.
SN 1987A is visible in the energy band 65–82 keV, while it remains invisible in two adjacent bands. The emission corresponds to a mass of 44Ti of 3.1 ± 0.8 × 10–4 solar masses. This is slightly above the upper bound of the theoretical predictions but corroborates the results obtained for Cassiopeia A (1.6+0.6–0.3 × 10–4), the only other supernova remnant where 44Ti has been clearly detected. Both measurements favour theoretical models with important production of 44Ti during the stellar explosion.
This discovery arrives at the end of the INTEGRAL satellite’s 10th year in orbit. The anniversary of the launch was celebrated on 17 October during a conference in Paris. Among the highlights of the spacecraft’s mission so far are the mapping of electron–positron annihilations in the bulge of the Galaxy, as well as the detection of polarization in the Crab Nebula and in the black hole binary Cygnus X-1 (CERN Courier March 2008 p12, November 2008 p11, May 2011 p12). Over the years, INTEGRAL has detected and characterized hundreds of new, heavily obscured X-ray sources among which some – called super-fast X-ray transients – were observed to undergo extremely rapid and high-amplitude luminosity variations.