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TOPIC: Pulsars


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Neutron stars
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Neutron stars are like layered candies, with different chemicals concentrated at different depths, reveal the first detailed computer simulations of the stars' chemistry. If correct, this could affect the strength of any gravitational waves the stars emit, and may help explain the origin of the spectacular nuclear explosions seen tearing across their surfaces.
Neutron stars are incredibly dense remnants of supernova explosions, with a billion tonnes of matter packed into every cubic centimetre at their cores. Scientists study them as natural laboratories to learn more about the behaviour of matter under extreme pressures and temperatures.
Inside their cores, the pressure is so great that individual atoms are crushed out of existence, leaving only a dense liquid of neutrons. Surrounding the core is a crystalline crust, where the pressure is low enough for atomic nuclei to retain their individuality.

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RE: Pulsars
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Neutron stars are created when a massive star runs out of fuel and collapses.  As the star collapses, the density becomes so immense that protons and electrons are squeezed tightly together to form neutrons. The end result is a star only 20 km across but weighing 1 1/2 times more than our sun and made up mostly of neutrons.



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Neutron Star superbursts
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Neutron star crusts are hotter than previously thought
The drawing illustrates the structure of a neutron star.  Although much of the star (red and yellow regions) is at greater than nuclear density, the crust (brown layer) is made of "normal" nuclei. Many neutron stars accrete hydrogen and helium from a stellar companion like our sun. As the hydrogen and helium accumulates, it fuses to heavier elements in an explosion known as type I X-ray bursts. In some systems there are superbursts that occur yearly and are about 1000 times more energetic than a type I X-ray burst. Put another way, a superburst releases as much energy as the sun radiates in a decade. Over millions of years of accretion, the crust of the neutron star si gradually replaced by these "ashes." JINA researchers at MSU, Los Alamos National Laboratory, and University of Mainz, Germany, have now computed the heating in the crust using a realistic model of the relavant nuclear reactions. Interestingly, the amount of heat deposited in the crust by these reactions is much larger (a factor of 510) than previously thought. This extra heating may partially explain why how some neutron stars are able to produce superbursts on a yearly timescale: a hot crust helps to ignite the superburst!

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Credit: S Gupta (LANL), E F Brown and H Schatz  (MSU, NSCL and JINA) , P Möller (LANL), and K-L Kratz (Univ. Mainz)

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Posts: 131433
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Neutron Star superbursts
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Title: Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition
Authors: Sanjib Gupta, Edward F. Brown, Hendrik Schatz, Peter Moeller, Karl-Ludwig Kratz

We perform a self-consistent calculation of the thermal structure in the crust of a superbursting neutron star. In particular, we follow the nucleosynthetic evolution of an accreted element from deposition into the atmosphere down to neutron drip density. We include temperature-dependent continuum electron capture rates and realistic sources of heat loss by thermal neutrino emission from the crust and core. We show that, in contrast to previous calculations, electron captures to excited states and subsequent gamma-emission significantly reduces the local heat loss due to weak-interaction neutrinos. Furthermore, temperature-sensitive (gamma,n) rates trigger further energy deposition as the distribution of nuclei evolves toward a lower neutron separation energy. Depending on the initial composition these reactions release up to a factor of ten times more heat at densities <10^11 g/cc than obtained previously. This heating reduces the ignition depth of superbursts. In particular, it reduces the discrepancy noted by Cumming et al. between the temperatures needed for unstable 12C ignition on timescales consistent with observations and the reduction in crust temperature from Cooper pair neutrino emission.

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XTE J1739-285
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Title: A compact star rotating at 1122 Hz and the r-mode instability
Authors: Alessandro Drago (Univ. Ferrara and INFN sez. Ferrara, Italy) Giuseppe Pagliara (Inst. Theoretische Physik, Goethe Universitaet, Frankfurt am Main, Germany and INFN Italy), Irene Parenti (Univ. Ferrara and INFN sez. Ferrara, Italy)

We show that r-mode instabilities severely constraint the composition of a compact star rotating at a submillisecond period. In particular, the only viable astrophysical scenario for such an object, present inside the Low Mass X-ray Binary associated with the x-ray transient XTE J1739-285, is that it has a strangeness content. Since previous analysis indicate that hyperonic stars or stars containing a kaon condensate are not good candidates, the only remaining possibility is that such an object is either a strange quark star or a hybrid quark-hadron star.

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Fastest spinning star may have exotic heart
A dense stellar corpse called a neutron star has been found spinning at an astonishing 1122 rotations per second – 1.5 times faster than any other star. If confirmed, the finding could bolster the possibility of exotic "soft" states of matter inside dense stars.
Until now, no neutron star has ever been found to spin faster than 716 times per second, which was the previous record. In fact, scientists had proposed that fast-spinning neutron stars would emit gravitational waves – or ripples in space-time – that would limit their spin rates to about that speed.
Now, new observations have revealed a neutron star that appears to be spinning much faster than that supposed speed limit. If confirmed, the fact that the star has not been ripped apart by its ultra-fast rotation means it must be relatively dense, perhaps made of an exotic type of matter.

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RE: Pulsars
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A star found spinning more than a thousand times every second is thought to be the fastest rotating star known.
The neutron star is a burned out corpse that's collapsed into an incredible density rivalled only by black holes. Using the European Space Agency's Integral satellite, astronomers watched these emissions to measure the spin rate of the star, catalogued as XTE J1739-285.
It is zipping around on its axis 1,122 times every second. That smashes the previous record of 760 spins per second for a neutron star.

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PSR B1957+20
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Title: XMM-Newton Observations of the Black Widow Pulsar PSR B1957+20
Authors: Hsiu-Hui Huang, Werner Becker

We report on XMM-Newton observations of the "Black Widow pulsar", PSR B1957+20. The pulsar's X-ray emission is non-thermal and best modelled with a single powerlaw spectrum of photon index 2.03(+0.51/-0.36). No coherent X-ray pulsations at the pulsar's spin-period could be detected, though a strong binary-phase dependence of the X-ray flux is observed for the first time. The data suggest that the majority of the pulsar's X-radiation is emitted from a small part of the binary orbit only. We identified this part as being near to where the radio eclipse takes place. This could mean that the X-rays from PSR B1957+20 are mostly due to intra-shock emission which is strongest when the pulsar wind interacts with the ablated material from the companion star.

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Slow Pulsars
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Title: Glitch observations in slow pulsars
Authors: G. H. Janssen, B. W. Stappers

We have analysed 5.5 years of timing observations of 7 ''slowly'' rotating radio pulsars, made with the Westerbork Synthesis Radio Telescope. We present improved timing solutions and 30, mostly small new glitches.
The most interesting results are: 1) The detection of glitches one to two orders of magnitude smaller than ever seen before in slow radio pulsars. 2) Resolving timing-noise looking structures in the residuals of PSR B1951+32 by using a set of small glitches. 3) The detections of three new glitches in PSR J1814-1744, a high-magnetic field pulsar. In these proceedings we present the most interesting results of our study. For a full coverage, we refer the reader to Janssen & Stappers (2006).

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RE: Pulsars
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Title: Pulsar spins from an instability in the accretion shock of supernovae
Authors: John M. Blondin and Anthony Mezzacappa

Rotation-powered radio pulsars are born with inferred initial rotation periods1 of order 300 ms (some as short as 20 ms) in core-collapse supernovae. In the traditional picture, this fast rotation is the result of conservation of angular momentum during the collapse of a rotating stellar core. This leads to the inevitable conclusion that pulsar spin is directly correlated with the rotation of the progenitor star. So far, however, stellar theory has not been able to explain the distribution of pulsar spins, suggesting that the birth rotation is either too slow or too fast. Here we report a robust instability of the stalled accretion shock in core-collapse supernovae that is able to generate a strong rotational flow in the vicinity of the accreting proto-neutron star. Sufficient angular momentum is deposited on the proto-neutron star to generate a final spin period consistent with observations, even beginning with spherically symmetrical initial conditions. This provides a new mechanism for the generation of neutron star spin and weakens, if not breaks, the assumed correlation between the rotational periods of supernova progenitor cores and pulsar spin.

Source


An N.C. State University astrophysicist has a new explanation for why remnants of some dying stars spin. The theory is published today in the journal Nature. If it's accepted by astronomers, college textbooks will require updating.

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