Integral points to the fastest spinning neutron star (Forwarded)

ESA News
http://www.esa.int

16 February 2007

Integral points to the fastest spinning neutron star

Astronomers using ESA's gamma-ray observatory, Integral, have detected
what appears to be the fastest spinning neutron star yet. This tiny
stellar corpse is spinning 1122 times every second. If confirmed, the
discovery gives astronomers the chance to glimpse the insides of the dead
star.

The neutron star, known by its catalogue number XTE J1739-285, was
discovered during one of its active phases on 19 October 1999 by NASA's
Rossi X-Ray Timing Explorer (RXTE) satellite. In August 2005, while
Integral was monitoring the bulge of the Galaxy, XTE J1739-285 started to
come back to life. About a month later Integral discovered the first short
bursts of X-rays from the object.

Erik Kuulkers of the ESA Integral Science Operations Centre, Spain, who
leads the Galactic bulge monitoring programme, informed Philip Kaaret,
University of Iowa, via email that things were still hotting up near the
end of October. Kaaret arranged for the RXTE satellite to observe XTE
J1739-285 between 31 October and 16 November. Together the two satellites
recorded about twenty bursts between September and November.

Just because a star dies, it doesn't mean its life is over. A neutron star
is the tiny heart of a collapsed star. Measuring about 10 kilometres
across, yet containing something like the mass of the Sun, the interior of
a neutron star is the most exotic realm that astronomers can imagine.
According to their calculations a thimbleful of neutron star material
weighs a hundred million tonnes.

When a neutron star orbits another star, its strong gravitational field
can pull gas from the other star. This coats the surface of the neutron
star. When the coating reaches a height of between 5-10 metres, the gas
ignites in a thermonuclear explosion. This massive release of energy
generally lasts from between several seconds to several minutes and a
burst of X-rays is released.

Previous observations of other neutron stars have shown that the X-rays
emitted during bursts display oscillations that correspond to the rotation
rate of the neutron stars. So the team began analysing the XTE J1739-285
bursts for oscillations. What they found was astounding. In the brightest
burst, which RXTE recorded on 4 November, there were indeed oscillations
but they were nearly twice as fast as any previously observed.

"It was quite a surprise to us," admits Kuulkers. However, after running a
series of checks, the team satisfied themselves that the oscillations were
indeed taking place 1122 times a second (1122 Hz).

Previously, the fastest neutron stars were known to spin with frequencies
between 270-619 Hz. This had led some astronomers to estimate, using
statistical arguments, that the fastest a neutron star could spin was 760
Hz. If the new observations are confirmed, XTE J1739-285 smashes this
limit.

"Our detection is just above the level where we think there is something
real. We definitely need more observations. If we see the signal again,
then everyone will believe it," says Kuulkers.

This doesn't mean that neutron stars can spin as fast as they like. If the
spin is too fast, even the crushing gravity of the star will be unable to
hold matter to the surface and the star will break up. The exact break-up
speed depends on the internal conditions of a neutron star and as yet,
astronomers do not know these precisely.

"Our putative 1122 Hz detection places a serious constraint on neutron
star models. If we can find more stars that spin in this range, it will
certainly allow us to exclude some models of their interior structure,"
says Kuulkers.

So, now it is just a matter of patience. The astronomers will keep watch,
not only for XTE J1739-285 to burst again, but also for other
fast-spinning X-ray neutron stars.

Notes for editors

"Evidence for 1122Hz X-Ray burst Oscillations from the Neutron-Star X-Ray
Transient XTE J1739-285", by P. Kaaret et al., will be published in The
Astrophysical Journal on 10 March 2007.

For more information:

Philippe Kaaret, University of Iowa, USA
Email: philip-kaaret @ uiowa.edu

Erik Kuulkers, ESA Integral Science Operations Centre, Spain
Email: erik.kuulkers @ sciops.esa.int

Christophe Winkler, ESA Integral Project Scientist, Netherlands
Email: cwinkler @ rssd.esa.int

[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/SPECIALS/Integral/SEMPADBE8YE_1.html ]