# X-Ray Bursts

As neutron star accretes material from its companion, hydrogen and helium pile up on the surface of the neutron star.  Eventually, this layer becomes so thick that nuclear reactions begin in the layer, heating the layer.  The additional heat increases the nuclear reactions leading to a runaway thermonuclear explosion on the surface of the neutron star: a Type-I X-ray burst.

During the burst, emission from the surface dominates the X-rays from the system, so not only is the source extremely bright during the burst but also the radiation is a probe of the neutron star surface.  If we can identify spectral lines in the emission during the burst, we can measure the gravitational redshift of the surface, a key prediction of general relativity.  This will yield an estimate of the ratio of the mass to the radius of the neutron star.   The exquisite energy resolution and sensitivity of Colibrì will allow us to identify even weak spectral lines.

Furthermore, very often these accreting neutron stars rotate rapidly, so a measurement of the width of the spectral lines can yield an estimate of the radius of the neutron star as well, giving unique constraints of the properties of the ultra-dense matter within neutron stars.