Weisberg Part of Team That Solves Disappearance of the Cosmic Spinning Top
Joel Weisberg, the Herman and Gertrude Mosier Stark Professor of Physics and Astronomy and the Natural Sciences, is part of a team that calculated the mass of a neutron star and its binary companion, using Einstein’s General Theory of Relativity, and recently published the findings in The Astrophysical Journal. Weisberg was included in the team due to his expertise in binary pulsars, which he has intensively studied for more than 35 years.
Northfield, Minn.—Joel Weisberg, the Herman and Gertrude Mosier Stark Professor of Physics and Astronomy and the Natural Sciences, is part of a team that calculated the mass of a neutron star and its binary companion, using Einstein’s General Theory of Relativity, and recently published the findings in The Astrophysical Journal. Weisberg was included in the team due to his expertise in binary pulsars, which he has intensively studied for more than 35 years.
The observed member of the binary system, a pulsar known as PSR J1906+0746 (or J1906 for short), spins and emits a lighthouse-like beam of radio waves once every 0.144 seconds. The pulsar, a neutron star, orbits another neutron star — or maybe a white dwarf — in a little under four hours. Masses been measured only in a handful of other double neutron stars (including another one measured by Weisberg), and J1906 is by far the youngest. As the supernova explosion that formed J1906 occurred only 100,000 years ago (the blink of an eye in the lifetime of a star), the binary is in a remarkably pristine and unevolved state. Normal pulsars live to be around 10 million years old; they can then be “recycled” (spun-up) by a binary companion to live for yet another 1 billion years. If the companion to J1906 is a neutron star, it is likely recycled, although it appears to not be shining our way.
The team measured the space-time warp caused by the gravity of the pair of stars and determined their masses — just before the pulsar member disappeared. “These two stars each weigh more than the Sun, but are still over 100 times closer together than the Earth is to the Sun,” notes co-author Ingrid Stairs, professor of physics and astronomy at The University of British Columbia, Canada.
“Our result is important because weighing stars while they freely float through space is exceedingly difficult,” said Joeri van Leeuwen, an astrophysicist at ASTRON, The Netherlands Institute for Radio Astronomy, and University of Amsterdam, The Netherlands, who led the study. “That is a problem because such mass measurements are required for precisely understanding gravity, the force that is intimately linked to the behavior of space and time on all scales in our Universe.”
The resulting extreme gravity causes many remarkable effects, said Stairs. One of these is geodetic precession. When you start a spinning top, it not only rotates – its spin axis also wobbles. According to general relativity, neutron stars, too, should start to wobble as they move through the gravitational well of a massive, nearby companion star. Orbit after orbit the pulsar travels through a space-time that is curved, which leaves an imprint on the spin axis. Weisberg and colleagues were the first ever to measure this phenomenon, in another binary pulsar. The current team now measured this geodetic precession in J1906. Over the course of an Earth year, this adds up to a change of 2.2 degrees in the orientation of the pulsar rotation axis. “Through the effects of the immense mutual gravitational pull, the spin axis of the pulsar has now wobbled so much that the beams no longer hit Earth,” said van Leeuwen. “The pulsar is now all but invisible to even the largest telescopes on Earth. This is the first time such a young pulsar has disappeared through precession. Fortunately this cosmic spinning top is expected to wobble back into view, but it might take as long as 160 years.” An animation of the orbiting stars and their relativistic phenomena is available on Weisberg’s personal website.
The pulsar was discovered in 2004 with the Arecibo Observatory, the world’s most sensitive radio telescope due to its large, 305-meter dish. Weisberg has taken numerous Carleton students to observe pulsars with this telescope, including Yuping Huang ’17 (Shantou, Guangdong, China) and Carolyn Raithel ’15 (Cincinnati) in December. From that moment on, the team monitored the pulsar almost daily with the five largest radio telescopes on Earth: the Arecibo Telescope (USA), the Green Bank Telescope (USA), Nançay Telescope (France), the Lovell Telescope (UK) and the Westerbork Synthesis Radio Telescope (The Netherlands). For more than five years the campaign kept exact score of all rotations of the pulsar — an astounding one billion in total. Each few-minute chunk of these data enabled the team to measure the pulse times of arrival with a precision of less than 50 millionths of a second, until the pulsar started to fade away due to geodetic precession.
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“The Binary Companion of Young, Relativistic Pulsar J1906+0746,” by
Joeri van Leeuwen, Laura Kasian, Ingrid H. Stairs, D. R. Lorimer, F. Camilo, S. Chatterjee, I. Cognard, G. Desvignes, P. C. C. Freire, G. H. Janssen, M. Kramer, A. G. Lyne, D. J. Nice, S. M. Ransom, B. W. Stappers, and J. M. Weisberg,
Astrophysical Journal, Vol 798, p. 118, January 10, wi2015, http://iopscience.iop.org/0004-637X/798/2/118/