PSR J0337+1715 Triple System
Credit: Jason Hessels; ASTRON/UvA
An international team of astronomers using the
Green Bank Telescope (GBT) has discovered a pulsar that is orbited by two white
dwarf stars. Pulsars are rapidly rotating neutron stars that can be used like
precision astronomical clocks. This is
the first time that astronomers have found a triple star system that contains a
pulsar, and the discovery team has used the pulsar's clock-like properties to
turn the system into an unparalleled precision laboratory for studying the
effects of gravitational interactions.
The necessary data came from an intensive
observational program using several of the world's largest radio telescopes:
the GBT, the Arecibo radio telescope in Puerto Rico, and ASTRON's Westerbork
Synthesis Radio Telescope in the Netherlands. The team reports their findings
in the online edition of Nature on January 5 and will present them at the 223rd
meeting of the American Astronomical Society in Washington DC on January 6.
All three stars orbit each other in a space
smaller than the Earth's orbit around the Sun. This close proximity, combined
with the fact that all three stars are much denser than our Sun, together provide
the necessary conditions to test the true nature of gravity - in particular, the 'Strong Equivalence Principle'
postulated in Einstein's theory of General Relativity. "This triple star
system gives us the best-ever cosmic laboratory for learning how such
three-body systems work, and potentially for detecting problems with General
Relativity, which some physicists expect to see under such extreme
conditions", says first author Scott Ransom of the National Radio
Astronomy Observatory (NRAO).
West Virginia University graduate student
Jason Boyles originally uncovered the millisecond pulsar - some 4,200 light-years from Earth, spinning
nearly 366 times per second - as part of a large-scale search
for pulsars with the GBT. To use the pulsar as a gravity probe, the astronomers
needed to record as many of its pulses as possible. Then, by measuring how the 'tick of the
pulsar clock' varied with time, they were able to determine the orbital
geometry and the masses of the three stars.
"It was a monumental observing
campaign", comments Jason Hessels, of ASTRON (the Netherlands Institute
for Radio Astronomy) and the University of Amsterdam. "For a time we were
observing this pulsar every single day, just so we could make sense of the
complicated way in which it was moving around its two companion
stars." Hessels led the frequent monitoring of the system with the
Westerbork Synthesis Radio Telescope.
While the astronomers were busy processing
hundreds of terabytes of data, they were also building a precision model of the
system. "Our observations of this system have made some of the most
accurate measurements of masses in astrophysics," says Anne Archibald,
also from ASTRON. "Some of our measurements of the relative positions of the stars in the system are accurate to
hundreds of meters, even though
these stars are about 10,000 trillion kilometers from Earth" she adds. Archibald led the effort to use the measurements to
build a computer simulation of the system that can predict its motions. Archibald
and the team used techniques dating back to those developed by Isaac Newton to
study the Earth-Moon-Sun system, combined with the 'new' gravity of Albert
Einstein, which was required to make sense of the data. Moving forward, the
system gives the scientists the best opportunity yet to discover a violation of
a concept called the Strong Equivalence Principle. This principle is an
important aspect of the theory of General Relativity, and states that the
effect of gravity on a body does not depend on the nature or internal structure
of that body.
Two famous illustrations of the equivalence
principle are Galileo's reputed dropping of two balls of different weights from
the Leaning Tower of Pisa (possibly
an apocryphal story) and Apollo 15 Commander Dave Scott's
dropping of a hammer and a falcon feather while standing on the airless surface
of the Moon in 1971. Lunar laser ranging measurements, using mirrors left on
the Moon by the Apollo astronauts, currently provide the strongest constraints
on the validity of the equivalence principle. Here the experimental masses are
the stars themselves, and their different masses and gravitational binding
energies will serve to check whether they all fall towards each other according
to the Strong Equivalence Principle, or not. "Using the pulsar's
clock-like signal we've started testing this", Archibald explains.
"We believe that our tests will be much more sensitive than any previous
attempts to find a deviation from the Strong Equivalence Principle." "We're
extremely happy to have such a powerful laboratory for studying gravity,"
Hessels adds. "Similar star systems must be extremely rare in our Galaxy,
and we've luckily found one of the few!"
Ransom, Archibald and Hessels are part of the
international team of researchers that reports their findings today. The team
also includes ASTRON's Adam Deller, who led observations that precisely
determined the position of the pulsar on the sky, as well as Vlad Kondratiev
and Joeri van Leeuwen who performed part of the GBT pulsar survey.
Triple System
Credit: Bill Saxton; NRAO/AUI/NSF
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More Information
ASTRON is the Netherlands Institute for Radio
Astronomy. Its main mission is to make discoveries in radio astronomy happen,
via the development of new and innovative technologies, the operation of
world-class radio astronomy facilities, and the pursuit of fundamental
astronomical research. Engineers and astronomers at ASTRON have an outstanding
international reputation for novel technology development, and fundamental
research in galactic and extra-galactic astronomy.
NOVA is the
Netherlands Research School for Astronomy, a federation of the astronomical
institutes at the universities of Amsterdam,
Groningen, Leiden and Nijmegen. NOVA's mission is two-fold: to carry out
front-line astronomical research in the Netherlands, and to train young
astronomers at the highest international level.
Science Contact:
Jason Hessels, ASTRON/University of Amsterdam
E-mail: hessels@astron.nl
Phone: +31 (0)610260062
Anne Archibald, ASTRON
E-mail: archibald@astron.nl
Phone: +31 (0)638938333
PIO Contact:
Marieke Baan, Dutch Research School for Astronomy (NOVA)
Phone: +31 (0)614322627
E-mail: h.m.baan@uva.nl
Article:
A millisecond pulsar
in a stellar triple system. S. M. Ransom, I. H. Stairs, A. M. Archibald, J.W.
T. Hessels, D. L. Kaplan, M. H. van Kerkwijk, J. Boyles, A. T. Deller, S.
Chatterjee, A. Schechtman-Rook, A. Berndsen, R. S. Lynch, D. R. Lorimer, C.
Karako-Argaman, V. M. Kaspi, V. I. Kondratiev, M. A. McLaughlin, J. van
Leeuwen, R. Rosen, M. S. E. Roberts, K. Stovall. DOI:
http://dx.doi.org/10.1038/nature12917
Simulation by Anne Archibald: http://www.astron.nl/~archibald/video.html
