Free Astronomy Magazine July-August 2014

STELLAR EVOLUTION C omparison between an image of Wd1 taken in the visi- ble spectrum (left) and one taken in the X- ray part of the same. The mag- netar is com- pletely invisible in the one on the left, but it turns out to be the sin- gle most brilliant object in the one on the right. [NASA/CXC/UCLA /M.Muno et al.] any gaseous residue of those events has never been discovered, since the high stel- lar density and strong winds generated in that environment can wipe out any vola- tile trace in a very short time (astronomi- cally speaking). Supernovae, though, do not leave behind just expanding gas, but also a collapsed star, which, depending on the initial mass, is either a neutron star or a black hole. If the progenitor has an initial mass not greater than 1.4 solar masses, at the time of the explosion it will give rise to a neu- tron star; over that limit (called Chandra- sekhar’s limit) it will create a black hole. Considering the great masses of the stars exploded so far in Wd1 (over 40 solar mas- ses), we can only expect black holes, which if they do not interact with other stars or interstellar gas remain imperceptible. Unexpectedly, in 2005 the Chandra X-ray satellite-observatory discovered a pulsar right inside Wd1, and the following year this pulsar was identified as the source of a gamma-ray burst. The observations in the X and gamma bands led astronomers to conclude that that object, called CXO J164710.2-455216 (J1647-45 for short), is an extremely rare variety of neutron star, known as a magnetar, i.e. a collapsed star entirely similar to a neutron star (hence a sphere the size of the Earth and heavy as the Sun), but characterized by the pres- ence of a extraordinarily intense magnetic field (one million billion billion times stronger than that of the Earth), which de- cays in an average period of about 10,000 years by surrendering its energy to the production and emission processes of X- rays and gamma-rays. In the entire Milky Way, only a couple of dozen magnetars have so far been discov- ered, and for decades astronomers have been debating on the mechanisms account- able for such strong magnetic field. Al- though the model more widely accepted today dates back to 1992 (that of Duncan- Thompson, winner of the Bruno Rossi Prize in 2003), it was precisely the discovery of J1647-45 that provided more concrete evi- dence on the origin of these exotic celes- tial objects. The difference from previous findings lies in the fact that J1647-45 should be a black hole, not a neutron star, and understanding why its collapse did not break the neutron barrier has provided the key to correctly interpret the origin of its powerful magnetic field. To explain what most likely happened was a small team of astrophysicists, led by Simon Clark (Open University, Milton Key- nes, UK), with an article published in May in Astronomy & Astrophysics . Their start- ing point was the hypothesis according to