Discovery of the 22nd accreting millisecond X-ray pulsar by INTEGRAL
Accreting millisecond X-ray pulsars are old neutron stars which have been spun up
by the torque exerted by the gas transferred from a companion star. This process
takes millions of years. Such pulsars can be sporadically detected when the companion
transfers enough material to heat up the environment of the neutron star to millions
of degrees. The study of them allows to understand the late stages of binary system
interactions, the interaction of radiation and matter in extreme conditions, and to
investigate the physical properties of ultra-dense matter. These objects are thought
to be the progenitors of isolated millisecond pulsars, but the transition from binary
to isolated systems has not been clarified, as yet.
After the first detection of the X-ray transient IGR J17591-2342 during INTEGRAL
observations of the Galactic center
(ATel #11941;
see the main image at the left), follow-up data were collected with NASA's NuSTAR and
NICER in order to characterize its broad-band energy emission. Pulsations at 527.4 Hz
were clearly detected (see the inset on the top right, which shows a power density
spectrum obtained from one of the NICER observations); the sinusoidal modulation of
the pulse arrival times provided a precise estimate of the system's orbital period:
8.8 hours (see the inset on the bottom right, using a number of pulse arrival times
measured during different segments of the NuSTAR observation).
Since the first discovery of an accreting millisecond X-ray pulsar in 1996, roughly
one new such object per year has been discovered, owing primarily to the appearance
of wide-field (hard) X-ray monitors. IGR J17591-2342 is a clear example of how the
synergy between INTEGRAL and the other currently operating X-ray facilities can be
exploited at best to discover and characterize X-ray transients.
Figure credits: E. Bozzo, C. Ferrigno & A. Sanna.
References:
"NuSTAR and NICER reveal IGR J17591-2342 as a new accreting millisecond X-ray pulsar",
A. Sanna et al.,
A&A, in press (arXiv:1808.10195)
