A metamorphic pulsar caught in the act of changing its appearance
Millisecond pulsars are neutron stars which emit radiation modulated
by their extremely quick rotation. Such a large rotation rate is thought
to be the outcome of a gigayear-long evolutionary phase of accretion of
mass from a low-mass companion star. At the end of this X-ray bright
phase, the magnetic field of the pulsar is thought to reactivate a
pulsed emission ranging from the radio to the gamma-ray band, and the
system is observed as a millisecond pulsar which taps energy from its
rotation.
After ten years of INTEGRAL contribution to the study of these fascinating
systems, it recently discovered a source which proved to be what
astrophysicists were waiting for for more than 40 years: a millisecond
pulsar showing at different times either rotation (radio) or accretion
powered (X-ray) pulsar activity.
This source, IGR J18245-2452, is an X-ray transient discovered by
IBIS/ISGRI on 28 March 2013 in the globular cluster M28. It was identified
as an accreting millisecond pulsar spinning at a period of 3.9 ms thanks to
follow-up observations with XMM-Newton. Cross-referencing with catalogues
of radio rotation-powered pulsars of the cluster, it was realized that the
source had already been seen in 2006 as a radio pulsar. Even more surprising,
as the X-ray emission faded, it took just a couple of days for the radio
pulsar to reactivate its rotation powered emission.
This source demonstrates beyond any doubt the link shared by rotation (radio)
and accretion powered (X-ray) millisecond pulsars. Moreover it proves the
existence of an intermediate, unstable phase during which radio and X-ray
pulsar states alternate on very short time scales. These transitions reflect
the interplay between the pulsar magnetosphere and the matter in-falling
towards the neutron star.
Panel (a) and (b) shows IBIS/ISGRI images of the neighbouring sky before
and after the switch on of the X-ray transient, respectively. The power
density spectrum of the X-ray emission observed by XMM-Newton, and the 11 hr
orbital modulation of the coherent signal of the pulsar are given in panel (c)
and (d), respectively. Panel (e) shows the 20-60 keV pulse profile accumulated
by IBIS/ISGRI between 28 March and 15 April 2013, for a total exposure of
446 ksec.
The study is based on data from a number of space-based high-energy
observatories and ground-based radio telescopes: ESA's INTEGRAL and XMM-Newton,
NASA's Swift and Chandra, CSIRO's Australia Telescope Compact Array and Parkes
radio telescope, NRAO's Robert C. Byrd Green Bank Telescope, and ASTRON's
Westerbork Synthesis Radio Telescope.
References:
Swings between rotation and accretion power in a binary millisecond
pulsar, Nature 501, 517-520
A. Papitto et al. http://dx.doi.org/10.1038/nature12470
