Stars caught in the circle dance

Pair of neutron star and white dwarf turns out to be a test candidate for General Relativity Theory

Dance around the center of gravity: This simulation shows the orbits of the pulsar J1952 + 2630 and its companion, probably a white dwarf. The orbits are almost circular, the apparent ellipticity arises from the viewpoint. The mass of the White Dwarf in such a binary system is exceptionally high at 95 percent of the mass of our Sun. © AEI
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Neutron stars are made of matter, which is much more densely packed than usual and rotate at high speed around its own axis. They emit radiation and are often visible as pulsars in the radio wave range. Researchers have now discovered a pulsar that, together with a white dwarf - a burnt out sun - performs a perfect circular dance.

Based on the so-called Shapiro effect, the scientists now want to weigh the pair, reports the journal "Astrophysical Journal Letters".

Deep view into space

To answer tricky questions of the general theory of relativity, researchers usually only look deep into space. And even there, it is often difficult to filter out the appropriate astrophysical objects from the data jungle. That's why scientists can help out with time-consuming data analysis by volunteers who provide unused computing power to their home or office computers for projects such as .

With this support, the team led by Bruce Allen from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute / AEI) in Hanover, together with colleagues from the PALFA collaboration, found the radio pulsar J1952 + 2630. The researchers found in the data of the Arecibo telescope.

"I am very excited that tracked down another exotic radio pulsar. These amazing objects are really extreme, squeezed to one third of their size, they would collapse to a black hole, "says Allen. "A big thank you goes to the thousands of volunteers without whom we would not have made the discovery." Display

One flash every 20.7 milliseconds

J1952 + 2630 flashes once every 20.7 milliseconds and is located at a distance of about 31, 000 light-years from Earth. From the modulation of the radio pulses, the astronomers concluded that the pulsar has a partner star with a minimum mass of 95 percent of the solar mass. The dance of both celestial bodies once around the common focal point lasts 9.4 hours and is almost perfectly circular.

The astrophysicists derive important conclusions about the nature and developmental history of the companion from this orbit, which they can not directly see: he is probably a - relatively heavy - white dwarf, a disused one Stern, who once led a quite ordinary existence, as well as our sun. At the end of his life, he blossomed into a red giant and shed the outer layer of matter. Part of this matter then absorbed the neutron star.

Rare combination

The two stars also exchanged (orbit) angular momentum, turning their orbits into a perfect circle, the researchers said. If the companion star used to have significantly more mass than the sun possessed, it would have turned into a neutron star at the end of its life in a supernova explosion. And by the resulting impulse he would have been kicked asymmetrically into an elliptical orbit according to the scientists.

The combination of a neutron star and a fairly massive white dwarf in circular orbit is rare. Usually, white dwarfs have only 0.1 to 0.3 solar masses for such orbit orbits. And just half a dozen of the hundred or so known dual star systems with pulsar have these properties. So far, the astronomers know 1, 900 pulsars, including single genes.

Delay of light

Because of the relatively high mass of the companion, this binary star system is probably suitable for testing a general relativistic phenomenon, namely the propagation delay of light, says Allen's colleague Benjamin Knispel. "That way, we could also determine the masses of the two components exactly."

This effect, also known as the Shapiro delay, occurs when visible light or radio waves pass a gravitational field, such as that of a star, on their way through space. The gravitational field deflects the rays from the straight path. According to the scientists, however, the light needs a little more time for this detour. While a white dwarf pushes itself into the line of sight between the pulsar and the earth, the radio pulses regularly emitted by the neutron star have to cover an ever further distance.

Researchers weigh stars

In this way, the pulses arrive at the observer one after the other at a respective greater time interval. To measure this, we need to look at the side of the system as much as possible, that is, at the edge of the orbital plane so that the neutron star's radio pulse will, in certain spatial constellations, be the gravitational field of the white As Dwarf passes on the way to us, "says Knispel. With this method, the two stars could be weighed. Knispel is already planning the next observations together with his colleagues. (Astrophysical Journal Letters, 2011; http://lanl.arxiv.org/abs/1102.5340)

(MPG, 07.04.2011 - DLO)