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Arecibo Telescope Used to Study Neutron Star Twice as Massive as the Sun, Orbiting Every Two Hours: Einstein's Theory of Gravitation Passes with Flying Colors

An international research team led by astronomers from the Max Planck Institute for Radio Astronomy (Germany) has used a variety of large optical and radio telescopes including the world's largest, the U.S. National Science Foundation's Arecibo radio telescope in Puerto Rico to study PSR J0348+0432, an extreme stellar system. The observations of this pulsar/white dwarf binary revealed a neutron star that weighs twice as much as the Sun, making it the most massive measured to date. Together with the unusually short orbital period of only 2.5 hours, this makes the system a strong emitter of gravitational radiation. The energy loss caused by this radiation has been measured in radio observations of the pulsar, particularly due to the exquisite sensitivity of the Arecibo 1,000-foot (305-meter) diameter telescope. These results, which so far are consistent with expectations from Einstein's theory of General Relativity, make the pulsar system a laboratory for gravity in extreme conditions not previously accessible.

The results are published in an extended report in the current issue of "Science"(26 April 2013, Vol. 340, #6131).

A National Science Foundation (NSF)-sponsored facility, the Arecibo Observatory is operated by SRI International in alliance with Ana G. Méndez-Universidad Metropolitana and the Universities Space Research Association (USRA) whose research focus includes astronomy.

Fernando Camilo (not part of the team presenting this work), director of astronomy at Arecibo, comments: "This beautiful work from an international collaboration is a testament to what can be achieved using state-of-the-art facilities operating at multiple wavelengths. These results also show that, 40 years after the discovery of the first binary pulsar, certain fundamental advances in our understanding of relativistic gravity are still being led by the most sensitive single-dish radio telescopes, such as the National Science Foundation's Arecibo and Green Bank radio telescopes."

Fig. 1: An artist's impression of the PSR J0348+0432 binary system. The white-dwarf companion, with 17% of the mass of the Sun, is the larger object in white; the pulsar, with a mass twice that of the Sun and spinning 25 times per second, is represented with radio beams originating along the direction of its magnetic poles. The movement of the binary emits gravitational waves, ripples in space-time (illustrated by the mesh) that propagate outwards at the speed of light. Credit: Luis Calçada/ESO.

Half a million Earths packed into a sphere 15 miles in diameter, spinning faster than a kitchen blender: these almost unimaginably extreme conditions are met in a neutron star a type of stellar remnant formed in the aftermath of a supernova explosion (which occurs when a star at least 8 times more massive than the Sun runs out of nuclear fuel). Neutron stars (also known as pulsars when detected via regular pulses emitted under steady rotation) often catch the attention of astronomers because they offer the opportunity to test physics under unique conditions.

PSR J0348+0432 is a pulsar with a white-dwarf companion, discovered in 2007 using the National Science Foundation's Green Bank Telescope in West Virginia in an ongoing global effort to find more of these rare objects. The pulsar and the white dwarf in this system are close enough to emit a significant amount of gravitational waves, making the orbit shrink, as predicted by general relativity.

"I was observing the system with the European Southern Observatory's Very Large Telescope in Chile, trying to detect changes in the light emitted from the white dwarf caused by its motion around the pulsar," says John Antoniadis, a PhD student at the Max Planck Institute for Radio Astronomy (MPIfR) in Germany and leading author of the paper presenting the recent results. "Together with existing radio data, this allows us to weigh both the white dwarf and the pulsar.

After a quick on-the-spot analysis I realized that the pulsar was quite a heavyweight: a mass twice that of the Sun, making it the most massive neutron star we know of."

Fig. 2: A comparison of the size of the PSR J0348+0432 system with other systems used for tests of General Relativity: PSR B1913+16 (the Hulse-Taylor binary pulsar) and PSR J0737-3039 ("the double pulsar"). All orbits are shown to scale relative to our Sun. Credit: Norbert Wex/MPIfR.

Realizing that a change in the orbital period should be visible in the radio signals of the pulsar, the research team turned its full attention to PSR J0348+0432 using the three largest single-dish radio telescopes on Earth. "Our observations with the National Science Foundation's Arecibo telescope are so precise that by the end of 2012 we could already measure a change in the orbital period of 8 microseconds per year, as Einstein's theory predicts", according to MPIfR scientist and former Arecibo staff scientist Paulo Freire..

Gravity on the surface of PSR J0348+0432 is 300 billion times stronger than that on Earth. At the center of the pulsar, more than one billion tons of matter are squeezed into a volume of a sugar cube. These numbers nearly double the ones found in other 'pulsar gravity labs'. In the language of general relativity, astronomers were able for the first time to precisely investigate the motion of an object with such a strong space-time curvature. "The most exciting result for us was that general relativity still holds true for such an extreme object", says Norbert Wex, a theoretical astrophysicist at MPIfR. There are alternative theories that make different predictions, and therefore are now ruled out. In this sense PSR J0348+0432 is taking our understanding of gravity into a much more extreme regime than that probed by the first binary pulsar, PSR B1913+16, discovered at Arecibo in 1974, that earned Russell Hulse and Joe Taylor the Nobel Prize in Physics in 1993.

Fig. 3: The radio and optical telescopes used to observe the pulsar-white dwarf binary system PSR J0348+0432. Upper row (from left): NSF's Green Bank Telescope (GBT), NSF's Arecibo observatory, and Effelsberg radio telescope; Lower row (from left): ESO Very Large Telescope (VLT), and William-Herschel Telescope (WHT). Image Credits: NRAO / NAIC / MPIfR / ESO / IAC.

These findings are also important for scientists who search for gravitational waves. On Earth, they are using large detectors, like the laser interferometers LIGO, GEO600, and VIRGO. One of the key signals they are looking for in their data are the gravitational waves emitted by two neutron stars during the last few minutes when they quickly spiral towards each other and finally collide. Decades of mathematical research in general relativity were necessary to calculate the expected signal from such a collision. The new results on PSR J0348+0432 provide added confidence in the equations needed to identify gravitational waves for the whole range of neutron star masses observed in nature. The first identification of such waves is expected within about five years.

The Telescopes:

The National Science Foundation's Green Bank telescope (GBT) in the US discovered the pulsar in 2007. The National Science Foundation's Arecibo telescope in Puerto Rico was used to measure the orbital period variation of the system, with Effelsberg in Germany providing an independent check of the timing stability of the Arecibo system. Together with these results, ESO's Very Large Telescope (VLT) in Chile was used to measure the masses of both the pulsar and the white dwarf. The William-Herschel Telescope (WHT) on La Palma, Spain, was used to monitor the stability of the white dwarf.

About Arecibo Observatory:

The Arecibo Observatory is operated by SRI International in alliance with Ana G. Méndez-Universidad Metropolitana and the Universities Space Research Association, under a cooperative agreement with the National Science Foundation (AST-1100968).

The Arecibo Observatory is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Original Publication: Results are published as "A massive pulsar in a compact relativistic binary" (J. Antoniadis et al.) in the April 26, 2013, issue of "Science".

Additional Information:

Arecibo Observatory:
Max-Planck-Institut für Radioastronomie:


Dr. Paulo Freire
Max-Planck-Institut für Radioastronomie, Bonn, Germany
Phone: +49-228-525-496

Mr. John Antoniadis
Max-Planck-Institut für Radioastronomie, Bonn, Germany
Phone: +49-228-525-181

Dr. Fernando Camilo
Arecibo Observatory, Arecibo, Puerto Rico, USA
Phone: +-410-302-0299 or 1-787-878-2612 (extension 361)

About USRA:
Universities Space Research Association (USRA) is an independent, nonprofit research corporation where the combined efforts of in-house talent and university-based expertise merge to advance space science and technology. USRA works across disciplines including biomedicine, astrophysics, and engineering and integrates those competencies into applications ranging from fundamental research to facility management and operations. USRA engages the creativity and authoritative expertise of the research community to develop and deliver sophisticated, forward-looking solutions to Federal agencies and other customers - on schedule and within budget.