View Full Version : Discussion: Gravitational Waves Could Define ...
2003-Jul-08, 08:24 PM
SUMMARY: It's possible that the spin rate of pulsars is limited by gravitational radiation according to new data gathered by NASA's Rossi X-ray Timing Explorer - a phenomenon predicted by Albert Einstein. Pulsars are the core remains of exploded stars, no larger than 15 kilometres across, and some rotate as fast as once/millisecond. Scientists believe that as a pulsar speeds up, it flattens out, and the distortions in its shape cause it to emanate waves of gravity which stop it from rotating so fast it flies apart.
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2003-Jul-10, 12:24 AM
Could someone please explain in laymans terminology what dark energy is and how it relates to the GUT?
2003-Jul-10, 04:00 AM
This is easy... nobody knows. ;-)
Okay, longer answer, and bear in mind that I'm not a real astronomer, so I might get my science all mixed up.
A few years ago some astronomers did a careful measurements of some distant supernovae to determine the speed that the universe was slowing down. This would help calculate if the universe would keep on expanding forever, or collapse back in on itself.
What they discovered, was that not only did the universe have enough momentum to not collapse, but that it was actually speeding up.
So, some unknown force, "dark energy", is causing every object in the universe to push away from each other. It's a constant force accelerating everything away.
But, nobody actually knows what this force is. One theory is that random particles are being spontaneously created in the vaccuum of space and their interaction with other particles is pushing everything out.
Nobody knows exactly what's causing it, but in the coming years, scientists are going to be working hard to gather more data and experiment.
Should be exciting times.
2007-Sep-23, 09:08 PM
Gravitational Waves Could Define Pulsar Spin (http://www.universetoday.com/2003/07/03/gravitational-waves-could-define-pulsar-spin/) is the UT story; "Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars" is the Nature paper (http://adsabs.harvard.edu/abs/2003Natur.424...42C) (you can read the arXiv preprint here (http://arxiv.org/abs/astro-ph/0307029); the "accreting millisecond pulsar" is SAX J1808.4−3658); the abstract:
Millisecond pulsars are neutron stars (NSs) that are thought to have been spun-up by mass accretion from a stellar companion. It is unknown whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many NSs that are accreting from a companion exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond brightness oscillations during bursts from ten NSs (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here, we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting NSs, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most NS models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.This paper has generated quite a buzz!
ADS lists 114 citations to it (http://adsabs.harvard.edu/cgi-bin/nph-ref_query?bibcode=2003Natur.424...42C&refs=CITATIO NS&db_key=AST), of which eight refer to SAX J1808.4−3658 in the title, 25 refer, in the title, to other accreting millisecond pulsars (XTE J1814-338 (five times), 4U 1728-34 (GX 354-0) (twice), PSR J1939+2134, EXO 0748-676, IGR J00291+5934 (four times), 4U 1820-30, 4U 1636-53 (twice), XTE J0929-314, HETE J1900.1-2455 (twice), A1744-361 (twice), XTE J1807-294, XTE J1739-285 (twice), and XB 1254-690).
13 of these papers include 'gravitation' (or similar) in the their titles.
Two immediate questions then:
1) to what have these subsequent papers confirmed this paper's "This [the observations reported] firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting NSs"?
2) how much further support is there, now, for "theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars"?
More generally, why is this such a hot topic for astronomers?
Maybe one of the UT team could do a follow-up interview with one of the 2003 Nature paper's authors?
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