A nova is a star that suddenly becomes tens of thousands of times brighter in a matter of days, and then gradually fades. These outbursts are thought to take place in binary systems in which a star and a white dwarf – the dense, burnt-out remnant of a star – orbit their common centre of mass.

Astronomers believe that white dwarfs in such systems accrete matter from their companion stars until they have gathered enough matter to spark nuclear reactions. These reactions are thought to trigger a nova explosion that can be detected as a jump in the brightness of the white dwarf over a wide range of wavelengths.

Although the visible light usually dims within weeks of the nova explosion, X-rays can be detected for much longer – up to tens of thousands of years according to existing theories. But many recent observations have shown that this period can last just a few years, which suggests that our understanding of the accretion and burning processes is seriously flawed.

In their study, Hernanz and Sala used the XMM-Newton satellite to monitor the low-energy – or ‘soft’ – X-rays emitted by nova V2487 Ophiuchi after it exploded in 1998. These soft X-rays are generated by nuclear ‘hydrogen burning’ of the accreted matter, so the duration of the soft X-ray emission is linked to the amount of matter the white dwarf has collected. To their surprise, Hernanz and Sala found that hydrogen burning in V2487 ceased just 2.7 years after it exploded.

The researchers then compared their measurements of high-energy – or ‘hard’ – X-rays with observations made in 1990, in which a source of hard X-rays was spotted in the same place as nova V2487. Hernanz and Sala discovered that the spectra and the fluxes of the pre-explosion X-rays matched their own data, which strongly suggested that the earlier study saw nova V2487 before it exploded. “This is the first time that a nova has been seen in X-rays before and after its explosion, ” Hernanz told PhysicsWeb.

Since hard X-rays are thought to be generated by the accretion process, Hernanz and Sala concluded that nova V2487 started to accrete matter as soon as it stopped burning hydrogen. “We have unambiguously discovered the reestablishment of accretion onto a white dwarf, only 1000 days after its explosion as a nova,” says Hernanz.

Hernanz and Sala hope that their findings will help astronomers to refine current models of nova explosions, and the accretion of matter in the ‘cataclysmic variable’ systems that give rise to them.