The source dubbed '4U 1735-44' originates in a close binary star system which is known since the earliest days of X-ray astronomy. The X-radiation is produced mainly in an accretion disk around a compact neutron star (i.e., a star as massive as the Sun but the size as the city of Amsterdam). This neutron star is also known to exhibit thermonuclear flashes which show themselves as bursts of X-rays. These bursts always last shorter than about 10 minutes, like for any other burster. They are always characterized by a thermal spectrum with a temperature of 20 million degrees, which cools during the decay.
On August 22nd, 1996, the WFC made detected an unusual event from 4U 1735-44: a flare with a temperature of 20 million degrees, which cools during the decay. Taking into account also the peak flux, the flare showed all characteristics of a thermonuclear flash except one: it lasted about 1000 times as long. No other physical mechanism for such a flare is known thus far, and it was proposed by Remon Cornelisse et al. (from SRON and Utrecht University) that this is an extraordinary case of a thermonuclear flash where perhaps not hydrogen or helium make up the fuel by higher-Z elements. Since their discovery, two more such flares have been discovered in two different sources.