Press Release

First View of a Newborn Millisecond Pulsar?

By SpaceRef Editor
February 13, 2002
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Caption: This composite shows an artist’s impression (center) of a millisecond pulsar and its companion with an insert of the ESA/NASA Hubble Space Telescope image of the region (upper left). The millisecond pulsar system lies in the globular cluster NGC 6397 in Ara (the Altar). In the Hubble image insert the companion star is marked with an arrow. The artist’s impression shows the enigmatic millisecond pulsar (seen in blue with two radiation beams) and its bloated red companion star. Scientists believe that the best explanation for seeing a bloated red star instead of a ‘quiet’ white dwarf in the system is that the pulsar only recently has been spun up to its current rotation speed of 274 times per second by the gases transferred by the red star. It is first time such a system has been observed. Image credit: European Space Agency & Francesco Ferraro (Bologna Astronomical Observatory)

Combining Hubble Space Telescope images with radio
observations has revealed a highly unusual system consisting of a fast
spinning pulsar and a bloated red companion star. The existence of the
system is something of a mystery – the best explanation so far is that
we have our first view of a millisecond pulsar just after it has been
`spun up’ by its red companion star.

Although more than 90 specimens of the exotic species of fast-spinning
`millisecond pulsars’ are known today, no observations have yet been
made to back up the theory of how they reached this state. A series of
observations of the millisecond pulsar PSR J1740-5340 (spinning at 274
times per second) and its companion star from the ESA/NASA Hubble Space
Telescope and the Parkes radio telescope seem to show the final stage
of the pulsar acceleration process for the first time.

The generally favoured `recycling scenario’ describing the creation of
millisecond pulsars proposes that an old, slowly rotating neutron star
begins to absorb matter from its elderly companion star, typically a
red giant. The matter hits the surface of the neutron star and
transfers energy to make it rotate faster. The process ends when the
pulsar has been revitalised and is rotating at hundreds of times per
second (hence a millisecond pulsar), and its companion almost emptied
of matter and turned into a white dwarf.

A team of scientists from Bologna Astronomical Observatory conducted a
series of Hubble observations of the pulsar-companion system in the
globular cluster NGC 6397. The observations show that the millisecond
pulsar’s companion is not the expected white dwarf, but a bloated red
star, whose radius is about 100 times greater than that of a white
dwarf and at least five times greater than a normal star of similar
mass! This unique couple orbit around each other in 1.35 days.

The observations also indicate the abnormal presence of large amounts
of gas in the system. This gas is released from the bloated companion
star and soon will be swept away by the recently accelerated pulsar.
Once the pulsar has been spun up it can no longer absorb gas from the
companion.

Lead astronomer Francesco Ferraro explains: “We have certainly
discovered a very unusual pair. A system consisting of a millisecond
pulsar and a star that is not a white dwarf has never been seen before.
Our favoured theory is that we are seeing the system before the bloated
red star has been `emptied’ of gas and turned into a white dwarf. If
this compelling hypothesis is wrong then the companion star could be a
normal star in the globular cluster that has been captured by the
pulsar by chance. Maybe it has expelled the white dwarf that we
normally find in such systems.”

At last astronomers have observations to back the theory of millisecond
pulsar births, and the discovery opens a new window on the evolution of
millisecond pulsars.

Image credit: European Space Agency & Francesco Ferraro (Bologna
Astronomical Observatory)

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Notes for editors

Members of the team of scientists include: Francesco Ferraro, Andrea
Possenti, Nichi D’Amico and Elena Sabbi (all from Bologna Observatory,
Italy).

The results are published in two papers in the November 1, 2001 issue
of Astrophysical Journal Letters.

This news release is issued jointly by ESA (Hubble European Space
Agency Information Centre) and NASA (STScI/Office of Public Outreach).

The Hubble Space Telescope is a project of international co-operation
between ESA and NASA.

Science Contact

Francesco Ferraro

Bologna Astronomical Observatory, Italy

Phone: +31-51-20-95-774

E-mail: ferraro@bo.astro.it

PR Contacts

Lars Lindberg Christensen

Hubble European Space Agency Information Centre, Garching, Germany
Phone: +49-89-3200-6306 (089 in Germany)

Cellular (24 hr): +49-173-38-72-621 (0173 in Germany)

E-mail: lars@eso.org

SpaceRef staff editor.