An astrophysicist from the University of St Andrews has helped trace the high-speed motion of matter whipping around a pulsar, finding evidence of a warp in space and time similar to that detected around black holes.
Dr Rudy Wijnands, an advanced research fellow from the School of Physics and Astronomy at St Andrews, leads a group of scientists who present new findings about pulsars in the July 3 issue of Nature.
Their work will provide a new tool to help scientists measure key properties of pulsars and neutron stars, such as their spin periods, as well as test Einstein’s theory of general relativity under conditions of extreme gravity.
Dr Wijnands led the analysis and was joined by Michiel van der Klis of the University of Amsterdam; Jeroen Homan of the Osservatorio Astronomico di Brera; Deepto Chakrabarty and Edward Morgan of the Massachusetts Institute of Technology; and Craig Markwardt of NASA Goddard Space Flight Center.
“The strange behavior of energy and matter that we finally see in detail around a pulsar paints a new picture of how space and time behave in regions of extreme gravity,” said Wijnands. “This finding may change how scientists go about measuring basic properties of these types of objects.”
A pulsar is a type of neutron star that emits steady pulses of radiation with each rotation, funneled along strong magnetic field lines, much like a lighthouse beam sweeping across space. A neutron star is the core remains of a star once at least eight times as massive as the Sun.
When such a massive progenitor star depletes its nuclear fuel, it no longer has the energy (or, outward radiation pressure) to support its bulk. The surface layers of the star blast outward, a supernova explosion that ultimately forms the colorful patterns typical of supernova remnants. The core collapses, squeezing about 1.4 times the mass of the Sun into a sphere only about 20 kilometers across.
Like a black hole, a pulsar possesses great mass confined to a small volume. Thus, it creates a strong gravitational potential, pulling in (or, accreting) material in its vicinity.
Wijnands’ team studied several pulsars in binary systems, in which the pulsar is orbiting a companion star. Isolated pulsars are often dim. Yet binary pulsar systems are often the scene of fireworks — bright outbursts of X- ray light created as gas from the companion star falls towards the pulsar and crashes onto its surface. The outbursts are random, sometimes lasting just a few weeks. Then the system might grow dim or undetectable for years.
In a letter to Nature, the scientists describe an observation of a pulsar named SAX J1808.4- 3658, which they followed for nearly 200 hours using NASA’s Rossi X-ray Timing Explorer during the autumn of 2002. Scientists call this type of object an accreting millisecond pulsar (and only five such objects are known), because it is accreting gas from a companion star and it is particularly fast, spinning 401 times per second.
Wijnands and his colleagues for the first time detected a type of flickering in the X-ray light from the pulsar, called twin kilohertz quasi-periodic oscillations, or kHz QPOs. These QPOs are registered by the Rossi Explorer’s instruments as two peaks in the frequency of the X rays — for example, at 200 and 500 Hz. Such flickering, thought to arise from matter whipping around the neutron star at over 50 percent light speed, is seen in over 20 neutron star systems, none of which are pulsars.
Scientists have several competing theories about the significance of these QPOs, with many arguing that the difference between the peaks is a measure of the neutron star’s spin frequency. This leading theory is called the beat- frequency model. Because no pulsars showed the QPOs, this model could not be verified — until now.
Wijnands’ group found that the beat-frequency model is not true for SAX J1808.4-3658 and likely other pulsars and neutron stars as well. Because it is a pulsar, the spin frequency of SAX J1808.4-3658 is well known: 401 Hz (401 revolutions per second). The difference in the twin kHz QPOs is about 200 Hz, or approximately half of 401 Hz — not a one-to-one ratio as previously thought.
The finding is significant because it paints a new picture of what is happening to space, time and matter around a neutron star. Also, it is now known that neutron stars must be spinning twice as fast as scientists had thought.
“Our finding is fatal for the beat- frequency model,” said van der Klis. “Before, most people thought the matter in the disk orbited around the star more or less undisturbed, just feeling the star’s gravity and following Einstein’s laws. Clearly, that is not the way it is. The matter reacts to the spin of the star below it in an unexpected way. One possibility is that the orbits in the accretion disk resonate with the spin through an interaction involving the relativistic periastron precession. That would provide a link with resonances we see in black hole disks as well.”
This same effect, “periastron precession,” is seen in the precession of the elliptical orbit of Mercury, an effect first predicted by Einstein and a famous classical test confirming general relativity. Mercury’s orbit is quickened by 50 centemeters a year by gravity itself pulling the fabric of space, as if Mercury were riding a moving walkway.
The motion around a pulsar is 20 quadrillion times faster (2×10^16) than Mercury around the Sun, due to the strongly warped spacetime near the pulsar, like a well in space. Thus, neutron stars serve as a laboratory for studying general relativity in ways not possible on Earth.
The Nature letter by Wijnands et al. coincides with a letter by the same team, led by Chakrabarty et al., about the same 200-hour observation dealing with millisecond pulsar burst oscillations and gravitational radiation, the subject of a NASA press conference in Washington, D.C., on July 2 at 1:00 p.m. EST.
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