Supergiant Fast X-ray Transient Binaries are Actually Quite Common in Our Galaxy
Supergiant Fast X-ray Transients (SFXTs) are a baffling type of X-ray binary system discovered with the European Space Agency's INTEGRAL (International Gamma-Ray Astrophysics Laboratory) space telescope. These binaries were first noticed because of their occasional flare-ups in the X-ray portion of the spectrum. The transient activity, which lasts only a few hours, enhances the brightness of these otherwise very faint sources dramatically. Over the years, INTEGRAL has discovered numerous SFXTs. Previously thought to be extremely rare, it turns out that these are actually quite common in our galaxy.
INTEGRAL is the first space observatory that can simultaneously observe objects in gamma rays, X-rays, and visible light. Its principal targets are violent explosions known as Gamma Ray Bursts (GRBs), powerful phenomena such as supernova explosions, and regions in the Universe thought to contain black holes.
The new class of double star systems is characterized by:
1) a very compact object that produces highly energetic, recurrent and fast-growing X-ray outbursts, and
2) a very luminous ‘Supergiant’ companion.
The compact object can be an accreting body such as a black hole, a neutron star, or a pulsar. Scientists have called such class of objects ‘Supergiant Fast X-ray Transients’. These ‘Transients’ are systems that display periods of enhanced X-ray emission.
Over the years, INTEGRAL has shown that ‘Supergiant Fast X-ray Transients’, characterized by fast outbursts and supergiant companions, form a wide class of double star system that lies hidden throughout the Galaxy.
Due to the transitory nature, in most cases these systems were not detected by other observatories because they lacked the combination of sensitivity, continuous coverage, and the wide field of view of INTEGRAL.
These 'Transients' show short outbursts with very fast rising times – reaching the peak of the flare in only a few tens of minutes – and typically lasting only a few hours. This is the main difference between 'Transients' and most other observed transient X-ray binary systems, which display longer outbursts, lasting typically a few weeks up to months.
“We are pretty confident that the fast outbursts are associated to the mass transfer mode from the supergiant star to the compact object,” says Ignacio Negueruela from the University of Alicante in Spain.
“We believe that the short outbursts cannot be related to the nature of the compact companion, as we observed fast outbursts in cases where the compact objects were very different - black holes, slow X-ray pulsars, or fast X-ray pulsars.”
Studying sources such as ‘Supergiant Fast X-ray Transients’, and understanding the reasons for their behavior, is providing valuable insight into the evolution paths that lead to the formation of high-mass X-ray binary systems.