Einstein@Home detects unusual stellar pair

The neutron star and its companion could prove helpful in testing the general theory of relativity

April 06, 2011

Neutron stars are quite unique: the material they are made of is packed much more densely than conventional matter. They rotate extremely fast about their own axis, emitting radiation in the process, so they are often visible as pulsars in the radio spectrum. Researchers at the Max Planck Institute for Gravitational Physics in Hanover, working as part of the international PALFA Collaboration, and with the help of participants in the Einstein@Home project, have now discovered a pulsar accompanied by a white dwarf – a burnt-out star. The researchers want to weigh the pair, using what is known as the Shapiro effect.

Astronomers currently know of 1,900 pulsars, single stars included.

The combination of a neutron star and a rather massive white dwarf orbiting one another in a perfect circle is rare. White dwarfs orbiting like this usually have a mass of just 10 to 30 percent of the mass of the Sun. And just half a dozen of the hundred or so known binary star systems with pulsars are like this.

“Thanks to the relatively high mass of the companion, this binary star system will presumably be suited to testing a phenomenon of general relativity, namely the gravitational time delay of light,” says Allen’s Ph.D. student Benjamin Knispel. “Which means we could also determine the precise mass of each component.”

This effect, also known as the Shapiro delay, occurs when visible light or radio waves pass through a gravitational field, for instance that of a star, on their way through the universe. The gravitational field diverts these beams from their straight path. But this means the light takes longer to make the detour. When the white dwarf moves into the line of sight between the pulsar and the Earth, the radio pulses which are emitted by the neutron star in regular intervals must travel further and further.

The pulses thus reach the observer at ever-increasing time intervals. “To measure this, we need to look at the system from the side, if possible – that is, at the edge of the plane of orbit – so that, in certain configurations, the radio pulses from the neutron star pass through the white dwarf’s gravitational field on their way to us,” says Knispel. This method can be used to weigh the two stars. Knispel and his colleagues are already planning the next observations to do just that.

Additional information:

The pulsar ALFA (PALFA) Consortium was founded in 2003 with the aim of conducting a large-scale pulsar survey with the Arecibo telescope. It includes astronomers from 20 universities, institutes and observatories worldwide.

Einstein@Home, with more than 290,000 participants, is one of the largest distributed computing projects in the world. It was set up in 2005 and has been searching for gravitational waves amongst the data from the detectors of the international LIGO/Virgo/GEO collaboration. Since 2009, it has been using 35 per cent of its available computing power to support the work of the PALFA Collaboration.

The two amateur scientists whose computers found the strongest signal in the data analysis are Vitaly V. Shiryaev (Moscow, Russia) and Stacey Eastham (Darwen, Great Britain), who are credited by name and thanked in the publication.

(FM / HOR)

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