As discovered by ESA's Integral observatory, many of these supergiant systems produce strong and exceptionally fast-rising X-ray outbursts lasting a few hours only, hence their name 'supergiant fast X-ray transients'.

The outbursts may depend on the way stellar material is exchanged between the supergiant star and the compact object.

The light curve at the bottom-right was retrieved by Integral from the supergiant fast X-ray transient source IGR J17544-2619 on 17 September 2003.

The curve shows a very fast X-ray outburst from the compact object, lasting about two hours only, with very fast rise and slow decay. The counterpart of this source is a luminous supergiant, unambiguously identified by ESA's XMM-Newton and NASA's Chandra X-ray observatories.

Credits: ESA