MINNEAPOLIS, MN - Two dense
stars whipping around each other at breakneck speed may be the strongest known
source of Einstein's space-trembling gravity waves.
The double star - called RX
J0806 - was discovered in 1994 in X-rays. Later shown to be blinking on and off
every 5.4 minutes, the two-star setup is believed to be a pair of white dwarfs
- the dense ashes of burnt-out stars - rotating around each other.
The implied separation is
just 50,000 miles - a mere one-fifth the distance between the Earth and the
Moon, making this the closest stellar pair ever observed. The tangled duo
should be booming out gravity waves - undulations in the fabric of
space and time predicted by Einstein's theory of general relativity.
"Those waves have
still not been detected directly, but there is indirect evidence," said
Tod Strohmayer, who presented the results here last week at a meeting of the
American Astronomical Society meeting.
Strohmayer, of NASA's
Goddard Space Flight Center, presented data from the Chandra X-ray Observatory
that shows the time between the X-ray blips is decreasing by 1.2 milliseconds
every year. The implication is that the dwarfs are orbiting faster and faster,
as they gradually fall into each other at a rate of one inch per hour.
This "spin-up" is
consistent with rotational energy being lost to gravity waves. The amount of
energy radiated in gravity waves in all directions could be 100 times the
energy our Sun puts out in light, according to Strohmayer.
The study will be detailed
in the Astrophysical Journal.
Gravitational ticking
This is not the first time
that two objects have been seen spinning faster over time. The radio pulses
from the Hulse-Taylor pulsar were found to be getting closer
together due to gravity wave emission between the pulsar and an orbiting
neutron star companion.
Also presented at this
meeting, Ingrid Stairs of the University of British Columbia and colleagues
found that the distance between a double-pulsar system (PSR J0737-3039A and B)
is shrinking at about two and a half inches per day, just as expected from
energy loss to gravity waves.
"Things are ticking
along just as Einstein would have predicted," Stairs said.
Although gravity waves have
yet to be detected directly, future missions will be sure to look at this
newly-identified white dwarf pair, which is 1,600 light years from Earth in the
constellation Cancer.
"When LISA [the Laser
Interferometer Space Antenna] searches for gravitational wave sources, this one
might stick out like a sore thumb," Strohmayer said.
LISA, scheduled for 2012 launch, will
involve three satellites orbiting 3 million miles (5 million kilometers) apart
in a triangle formation. As gravity waves - traveling at the speed of light -
wash up on the Earth's shores, the satellites can detect a change in their
separation far less than the width of an atom.
"As a gravitational
wave source J0806 is very bright," said Lee Finn, the director of the
Center for Gravitational Wave Physics at Penn State University, in an email
message.
Finn expects that -
relative to each other - the LISA satellites will bob one quadrillionth of a
meter (roughly the size of an atomic nucleus) every 160 seconds in the J0806
surf.
Opposing faces
The white dwarfs are
assumed to each have about half the mass of the Sun, but their radii are likely
comparable to that of the Earth, which means their matter is densely packed.
The compact size allows the dwarfs to orbit far closer than normal stars could.
Questions remain as to what
mechanism is causing the observed X-rays to go on and off. One possibility is
that a hot X-ray spot on one of the dwarfs comes in and out of our view as the
dwarfs rotate around each other.
Evidence for this
hypothesis comes from the fact that the pair blinks in visible light as well.
The timing of the visible flashes are opposite that of the X-ray flashes: just
as the X-rays turn on/off, the visible turns off/on.
This offset might be
because the hot X-rays from the spot heat up the opposing face of the other
dwarf - causing it to emit visible light. It is as if the white dwarfs are two
dancers twirling around each other - one with a smile beaming in X-rays, the
other reflecting that gaze in the visible.
When will this dance end?
Strohmayer said the two
dwarfs should continue losing energy to gravity waves and merge between 500,000
and one million years from now. That event, theory says, would unleash a
colossal burst of gravitational waves.
A movie depicting how the
merger might play out is available here.
This article is part of
SPACE.com's weekly Mystery Monday series.
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