Eclipsing Pulsar Promises Clues to Crushed
ScienceDaily (Aug. 18, 2010) —
Astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) have found the
first fast X-ray pulsar to be eclipsed by its companion star. Further studies
of this unique stellar system will shed light on some of the most compressed
matter in the universe and test a key prediction of Einstein's relativity
The pulsar is a rapidly spinning neutron star -- the crushed core of a
massive star that long ago exploded as a supernova. Neutron stars pack more than
the sun's mass into a ball nearly 60,000 times smaller. With estimated sizes
between 10 and 15 miles across, a neutron star would just span Manhattan or the
District of Columbia.
"It's difficult to establish precise masses for neutron stars, especially
toward the higher end of the mass range theory predicts," said Craig Markwardt
at NASA's Goddard Space Flight Center in Greenbelt. "As a result, we don't know
their internal structure or sizes as well as we'd like. This system takes us a
step closer to narrowing that down."
Known as Swift J1749.4-2807 -- J1749 for short -- the system erupted with an
X-ray outburst on April 10. During the event, RXTE observed three eclipses,
detected X-ray pulses that identified the neutron star as a pulsar, and even
recorded pulse variations that indicated the neutron star's orbital motion.
J1749 was discovered in June 2006, when a smaller eruption brought it to the
attention of NASA's Swift satellite. Observations by Swift, RXTE and other
spacecraft revealed that the source was a binary system located 22,000
light-years away in the constellation Sagittarius and that the neutron star was
actively capturing, or accreting, gas from its stellar partner. This gas gathers
into a disk around the neutron star.
"Like many accreting binary systems, J1749 undergoes outbursts when
instabilities in the accretion disk allow some of the gas to crash onto the
neutron star," said Tod Strohmayer, RXTE's project scientist at Goddard.
The pulsar's powerful magnetic field directs infalling gas onto the star's
magnetic poles. This means that the energy release occurs in hot spots that
rotate with the neutron star, producing fast X-ray pulses. How fast? J1749 is
spinning 518 times a second -- a city-sized sphere rotating as fast as the
blades of a kitchen blender.
In addition, the pulsar's orbital motion imparts small but regular changes in
the frequency of the X-ray pulses. These changes indicate that the stars revolve
around each other every 8.8 hours.
During the week-long outburst, RXTE observed three periods when J1749's X-ray
emission briefly disappeared. Each eclipse, which lasts 36 minutes, occurs
whenever the neutron star passes behind the normal star in the system.
"This is the first time we've detected X-ray eclipses from a fast pulsar that
is also accreting gas," Markwardt said. "Using this information, we now know the
size and mass of the companion star with unprecedented accuracy."
By comparing RXTE observations across the theoretical mass range for neutron
stars, the astronomers determined that J1749's normal star weighs in with about
70 percent of the sun's mass -- but the eclipses indicate that the star is 20
percent larger than it should be for its mass and apparent age.
"We believe that the star's surface is 'puffed up' by radiation from the
pulsar, which is only about a million miles away from it," Markwardt explained.
"This additional heating probably also makes the star's surface especially
disturbed and stormy."
Writing about their findings in the July 10 issue of The Astrophysical
Journal Letters, Markwardt and Strohmayer note that they have all but one
orbital variable needed to nail down the mass of the pulsar, which is estimated
to be between about 1.4 and 2.2 times the sun's mass.
"We need to detect the normal star in the system with optical or infrared
telescopes," Strohmayer said. "Then we can measure its motion and extract the
same information about the pulsar that the pulsar's motion told us about the
However, a pioneering X-ray measurement well within the capability of RXTE
may make a hunt for the star irrelevant.
One consequence of relativity is that a signal -- such as a radio wave or an
X-ray pulse -- experiences a slight timing delay when it passes very close to a
massive object. First proposed by Irwin Shapiro at the Massachusetts Institute
of Technology (MIT) in Cambridge, Mass., in 1964 as a new test for predictions
of Einstein's relativity, the delay has been demonstrated repeatedly using radio
signals bounced off of Mercury and Venus and experiments involving spacecraft
"High-precision measurements of the X-ray pulses just before and after an
eclipse would give us a detailed picture of the entire system," Strohmayer said.
For J1749, the predicted Shapiro delay is 21 microseconds, or 10,000 times
faster than the blink of an eye. But RXTE's superior timing resolution allows it
to record changes 7 times faster.
With only three eclipses observed during the 2010 outburst, RXTE didn't
capture enough data to reveal a large delay. However, the measurements set a
limit on how massive the normal star can be. The study shows that if the star's
mass was greater than 2.2 times the sun's, RXTE would have seen the delay.
"We believe this is the first time anyone has set realistic limits for this
effect at X-ray wavelengths outside of our solar system," Markwardt noted. "The
next time J1749 has an outburst, RXTE absolutely could measure its Shapiro
Launched in late 1995, RXTE is second only to Hubble as the longest serving
of NASA's currently operating astrophysics missions. RXTE discovered the first
accreting millisecond pulsar -- SAX J1808.4-3658 -- in 1998 and continues to
provide a unique observing window into the extreme environments of neutron stars
and black holes.