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The Art of Recycling Pulsars

Stellar Astrophysics helps to explain the behaviour of fast rotating neutron stars in binary systems

The formation of millisecond pulsars is the result of stellar cannibalism where matter flows from a donor star to an accreting pulsar in a binary system. During this process the pulsar emits X-rays while being spun up to amazingly high rotational speeds.Thomas Tauris (Bonn Univ. & MPIfR) can show that millisecond pulsars loose about half of their rotational energy during the final stages of the mass-transfer before the pulsar turns on its radio beam. This is in agreement with current observations and also explains why radio millisecond pulsars appear to be much older than their companion stars - and perhaps why no sub-millisecond radio pulsars exist at all.

An artist's impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry

Millisecond pulsars are strongly magnetized, old neutron stars in binary systems which have been spun up to high rotational frequencies by accumulation of mass and angular momentum from a companion star. Today we know of about 200 such pulsars with spin periods between 1.4-10 milliseconds. These are located in both the Galactic Disk and in Globular Clusters.

Since the first millisecond pulsar was detected in 1982 is has remained a challenge for theorists to explain their spin periods, magnetic fields and ages. As an example, there is the "turn-off" problem, i.e. what happens to the spin of the pulsar when the donor star terminates its mass-transfer process?

"We have now, for the first time, combined detailed numerical stellar evolution models with calculations of the braking torque acting on the spinning pulsar", says Thomas Tauris, the author of the present study. "The result is that the millisecond pulsars loose about half of their rotational energy in the so-called Roche-lobe decoupling phase." This phase is describing the termination of the mass transfer in the binary system. Hence, radio-emitting millisecond pulsars should spin slightly slower than their progenitors, X-ray emitting millisecond pulsars which are still accreting material from their donor star. This is exactly what the observational data seem to suggest. Furthermore, these new findings can help explain why some millisecond pulsars appear to have characteristic ages exceeding the age of the Universe and perhaps why no sub-millisecond radio pulsars exist.

The key feature of the new results is that it has now been demonstrated how the spinning pulsar is able to brake out of its so-called equilibrium spin. At this epoch the mass-transfer rate decreases which causes the magnetospheric radius of the pulsar to expand and thereby expelling the infalling matter like a propeller. This causes the pulsar to loose additional rotational energy and thus slow down its spin rate.

"Actually, without a solution to the "turn-off" problem we would expect the pulsars to even slow down to spin periods of 50-100 milliseconds during the Roche-lobe decoupling phase", concludes Thomas Tauris. "That would be in clear contradiction with observational evidence for the existence of millisecond pulsars."

This work has profited from a recent effort to bridge the Stellar Physics group at the Argelander-Institut für Astronomie at University of Bonn (led by Norbert Langer) with the Fundamental Physics in Radio Astronomy group at the Max-Planck-Institut für Radioastronomie (led by Michael Kramer). The stellar evolution models used for this work were made using a state-of-the-art code developed by Norbert Langer. A significant part of the observational data was supplied by the pulsar group. Michael Kramer and his colleagues are using the 100-m Effelsberg Radio Telescope to participate in several ongoing searches and discoveries of millisecond pulsars.

Thomas Tauris has been working at the Argelander-Institut für Astronomie and the Max-Planck-Institut für Radioastronomie as a visiting research professor since 2010. Some of his recent work on the recycling of millisecond pulsars has been published in the journal "Monthly Notices of the Royal Astronomical Society" in joint publications with Norbert Langer and Michael Kramer. On February 27 they host an international one-day workshop in Bonn on the "Formation and Evolution of Neutron stars".

Norbert Junkes | Max-Planck-Institut
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