HD 49798 is a star 650 pc (about 2,120 light years) away from us. In 1997, evidence was found of something emitting bursts of x-rays every 13 seconds in data taken by the ROSAT telescope. Now the XMM-Newton X-ray space telescope has pointed its more sensitive eye towards the star and found the culprit – a white dwarf star close to its mass limit.
White dwarfs are the cores of massive collapsed stars. The heavier the star, the stronger the force of the matter collapsing onto a more massive core. Very heavy stars will leave behind cores greater than 1.4 solar masses. At this point, the crushing forces become so great that the core collapses further than the white dwarf stage and end up as neutron stars. This means there is an inverse mass size relationship, with heavier white dwarfs being smaller than lighter ones.
An average white dwarf has a mass of 0.6 that of the Sun, crammed into the diameter of the Earth. From watching the orbits of the companion star and white dwarf, it is known that this one now has a mass of 1.3 times that of the Sun in half the average diameter. The white dwarf has been attracting material from the outer envelope of its companion and allowing it to accrete on its surface. As well as providing extra mass, the gas spinning round the white dwarf on the way down has transferred angular momentum, spinning the object up to form the x-ray pulses.
It is estimated that the white dwarf has a million or so years to live before either collapsing into a neutron star or exploding in a type 1a supernova. Calculations suggest to anyone on Earth, this would appear as intense as the full Moon and be visible in daylight.