It was the detection of the higher frequency QPO at 450 Hz that implied spin. Strohmayer's finding also marks the first detection of paired QPOs from a black hole. Neutron stars often exhibit paired QPOs, and this is thought to be a result of radiation coming from the solid neutron star surface. Strohmayer's detection of paired QPOs from an object with no solid surface, therefore, challenges these important theories of how neutron stars produce these QPOs. The spin of a black hole would be caused by the angular momentum of the star that formed it, Strohmayer said, particularly if that progenitor is a spinning neutron star.
Neutron stars can theoretically collapse into black holes if they accrete enough matter.
Alternatively, a black hole could form directly during a supernova. Compared to a non-spinning black hole, a spinning black hole also has a smaller event horizon, the theoretical boundary around a black hole beyond which nothing can escape. The Rossi Explorer, launched in December 1995, is a unique X-ray satellite designed to record rapidly fluctuating objects, such as pulsars and neutron stars, which can spin over a thousand times per second.