Swings between rotation and accretion power in a millisecond binary pulsar
A new transient source, identified as IGR J18245-2452, was first
detected in X-rays on 28 March 2013 by INTEGRAL in the globular cluster M28,
which lies in the constellation Sagittarius. Observations by XMM-Newton
determined the pulsar's spin period to be 3.9 milliseconds clearly identifying
it as an X-ray-bright millisecond pulsar powered by accretion of material from a
nearby low-mass star companion. The spin period and other key characteristics
was found to match perfectly those of a pulsar in M28 that had been observed in
2006, but only at radio wavelengths. This is then the first ever fast-spinning
'millisecond pulsar' caught in a crucial evolutionary phase, as it swings
between emitting pulses of X-rays and radio waves.
It is thought that neutron stars in low-mass binary systems can accrete matter
and angular momentum from the companion star and be spun-up to millisecond
rotational periods. During the accretion stage, the system is called a low-mass
X-ray binary, and bright X-ray emission is observed. When the rate of mass
transfer decreases in the later evolutionary stages, these binaries host a radio
millisecond pulsar whose emission is powered by the neutron star’s rotating
magnetic field. This evolutionary model is supported by the detection of
millisecond X-ray pulsations from several accreting neutron stars and also by
the evidence for a past accretion disc in a rotation-powered millisecond pulsar.
It has been proposed that a rotation-powered pulsar may temporarily switch on
during periods of low mass inflow in some such systems. Only indirect evidence
for this transition has hitherto been observed. Here we report observations of
accretion-powered, millisecond X-ray pulsations from a neutron star previously
seen as a rotation-powered radio pulsar. Within a few days after a month-long
X-ray outburst, radio pulses were again detected. This not only shows the
evolutionary link between accretion and rotation-powered millisecond pulsars,
but also that some systems can swing between the two states on very short
Animation of a pulsar spun-up by accretion from a companion star
This animation shows a slowing down radio pulsar which is then
reaccelerated by accretion of gas from a companion star.
the full animation on the ESA site.
The discovery of IGR J18245-2452
Two IBIS/ISGRI images of the region on the sky where IGR J18245-2452 was discovered (20-40 keV energy band). The top panel shows how the region appeared on 18 February 2013, the bottom panel from 29 March 2013 to 6 April 2013. Both images have an exposure time of about one day and show how the hard X-ray sky is variable.
Credits: ISDC/C. Ferrigno