IGR J17503-2636: a new supergiant fast X-ray transient discovered by INTEGRAL
When a neutron star orbits a supergiant star, it is capable of capturing
gas from the powerful stellar wind of the supergiant. While falling
towards the neutron star, the gas is accelerated at about half the speed
of light and produces powerful X-ray emission. Such systems are known by
astronomers as supergiant X-ray binaries. Due to an intrinsic
instability in the radiation driven stellar winds, the wind of an OB
supergiant is inherently structured, with over-dense regions that can
achieve the size of typically 10% of the stellar radius. One believes
that accretion of these structures onto the neutron star, alternated by
accretion from a less dense intra-clump medium, is at the origin of
pronounced X-ray variability occurring on time scales from a few tens to
thousand of seconds. A sub-group of these systems are the so-called
Supergiant fast X-ray Transients (SFXTs), which exhibit an even more
pronounced variability with a few hours to few days long periods of
enhanced X-ray activity that are sparsely interrupted by long periods of
a much fainter quiescent state.
On 11 August 2018, INTEGRAL captured emission from a previously unknown
source, which was dubbed IGR J17503-2636. The discovery was reported in
an Astronomical Telegram (see
ATel #11952)
that triggered follow-up observations with the Swift, NuSTAR, and NICER
high-energy facilities. These satellites investigated the faint emission
that followed the initial flare captured by INTEGRAL, and were used to
characterize the energy spectrum and the time variability of the source.
Thanks to the 17-year long data stretch available with INTEGRAL, one can
demonstrate that the source was never detected before. Combining the
information from all observatories, it is proposed that
IGR J17503-2636 belongs to the class of SFXTs.
References:
"IGR J17503-2636: a candidate supergiant fast X-ray transient",
C. Ferrigno et al.,
A&A, in press (https://arxiv.org/abs/1903.03210)
