H is eventually accreted to the surface of the neutron star, where it is immediately converted into He by the extreme temperatures and pressures that exist there. A thin surface layer of He builds up, and once a critical mass is reached, the He ignites explosively, heating the entire surface of the neutron star and releasing a sudden burst of X-rays. After the burst, the system returns to a quiescent state and the neutron star begins to re-accumulate the He surface layer. The process repeats resulting in recurrent X-ray bursts. The mechanisms that produce X-ray bursts and recurrent novae are similar. Recurrent novae form when a white dwarf accretes a surface layer of H that undergoes explosive burning.
"We have witnessed a star's spectacular and prolonged demise," said Dacheng Lin, a research scientist at UNH's Space Science Center and the study's lead author. "Dozens of these so-called tidal disruption events have been detected since the 1990s, but none that remained bright for nearly as long as this one."
The X-ray source containing this force-fed black hole, known by its abbreviated name of XJ1500+0154, is located in a small galaxy about 1.8 billion light years from Earth.