The origin of QPO sources

The problem of the origin of quasi-periodic X-ray oscillation (QPO) sources is examined, noting that direct collapse of a white dwarf to a neutron star is difficult to explain since mass-accreting white dwarfs approaching Chandrasekhar's limit are normally assumed to be progenitors of Type I supernovae and not in general to leave a neutron star remnant. QPO sources can be interpreted as rotating, mass-accreting neutron stars possessing a magnetic field of the order of 10 to the 9 gauss. This would suggest that the neutron star was recently formed. Such objects, however, are quite old - at least 5 billion years old. Thus it is very likely that the neutron star was produced from the collapse, with little or no explosion, of an old white dwarf.