Type I X-ray Bursts
Click on the picture to watch a cool movie of an X-ray burster:
See a sky map in Galactic coordinates of Type I X-ray bursts.
Type I burst profiles vary greatly, but characteristic features include
- a sudden, rapid (1 to 10 seconds) increase in X-ray flux, reaching 5 to 20 times quiescent values
- a more extended period of decay (10 seconds to several minutes), during which the energy of the X-rays diminishes from peak values
The thermonuclear flash model (Lewin, van Paradijs, & Taam 1993) successfully explains these general features as resulting from explosive nuclear ignition on the surface of neutron stars. In short, after gas from the accretion disk reaches the surface of the neutron star, it spreads out evenly over it and proceeds to burn in the extremely high pressure environment. At a critical point, degenerate hydrogen and/or helium burning ignites explosively, suddenly heating up the entire surface to around 3 x 107 K, enough to emit strong X-rays. This flash is observed as the start of an X-ray burst. After the explosion, the surface cools, a process observed as the decay of the burst profile. Moreover, as more nuclear fuel continues to accrete onto the surface, the process may repeat.
Thus, the flash model accounts for the general observational features of burst properties such as
- the energies involved (~1038 to 1039 ergs)
- their rise time (seconds)
- duration (~10 to 100 seconds)
- spectral softening
- recurrence intervals (several hours)
The following figure shows a typical Type I X-ray burst:
Figure from Lewin, W.H.G., van Paradijs, J. & Taam, R.E., 1993, Sp Sci Rev, 62, 223.