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TOPIC: Neutron starquakes
Blobrana


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RE: Neutron starquakes
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Title: A dynamical description of neutron star crusts
Authors: V. de la Mota, F. Sébille, Ph. Eudes

Neutron Stars are natural laboratories where fundamental properties of matter under extreme conditions can be explored. Modern nuclear physics input as well as many-body theories are valuable tools which may allow us to improve our understanding of the physics of those compact objects.
In this work the occurrence of exotic structures in the outermost layers of neutron stars is investigated within the framework of a microscopic model. In this approach the nucleonic dynamics is described by a time-dependent mean field approach at around zero temperature. Starting from an initial crystalline lattice of nuclei at subnuclear densities the system evolves toward a manifold of self-organised structures with different shapes and similar energies. These structures are studied in terms of a phase diagram in density and the corresponding sensitivity to the isospin-dependent part of the equation of state and to the isotopic composition is investigated.

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Blobrana


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Research by a theoretical physicist at Indiana University shows that the crusts of neutron stars are 10 billion times stronger than steel or any other of the earth's strongest metal alloys.
Charles Horowitz, a professor in the IU College of Arts and Sciences' Department of Physics, came to the conclusion after large-scale molecular dynamics computer simulations were conducted at Indiana University and Los Alamos National Laboratory in New Mexico. The research will appear Friday (May 8) in Physical Review Letters.

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Blobrana


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Simulations have shown that a superdense neutron star crust is much harder than expected and can support big 'mountains'. This raises hope for registering gravitational waves soon.

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Blobrana


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The crust of neutron stars is 10 billion times stronger than steel, according to new simulations. That makes the surface of these ultra-dense stars tough enough to support long-lived bulges that could produce gravitational waves detectable by experiments on Earth.
Neutron stars are the cores left behind when relatively massive stars explode in supernovae. They are incredibly dense, packing about as much mass as the sun into a sphere just 20 kilometres or so across, and some rotate hundreds of times per second.

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Blobrana


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For postdoctoral researcher Andrew Steiner and other astrophysicists, the shockwave that jolted the Earth in late December 2005 was a lucky find, providing new data on neutron stars
Really, you didn't feel it? In the closing days of 2005, the shockwave of a gigantic quake jolted the Earth's atmosphere, and you didn't detect the tremors?
There is this minor detail: the shakeup happened light years away, on the crust of a neutron star, and the wave rolled by our planet as X-rays, not ground-rocking rumbles. So unless you can see in the ultra-ultraviolet, the failure is not exactly your fault.

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Starquake!


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Blobrana


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Neutron stars not just rocky planets and moons can boast topographical features such as plateaus or mountains, a new computer simulation suggests. As the stars rotate, these structures should ripple the surrounding fabric of space, producing gravitational waves that astronomers have long hoped to detect.

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Blobrana


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Title: Pulsar slow glitches in a solid quark star model
Authors: C. Peng (PKU), R. X. Xu (PKU)
(Version v2)

A series of five unusual slow glitches of the radio pulsar B1822-09 (PSR J1825-0935) were observed over the 1995-2005 interval. This phenomenon is understood in a solid quark star model, where the reasonable parameters for slow glitches are presented in the paper. It is proposed that, because of increasing shear stress as a pulsar spins down, a slow glitch may occur, beginning with a collapse of a superficial layer of the quark star. This layer of material turns equivalently to viscous fluid at first, the viscosity of which helps deplete the energy released from both the accumulated elastic energy and the gravitation potential. This performs then a process of slow glitch. Numerical calculations show that the observed slow glitches could be reproduced if the effective coefficient of viscosity is ~10² cm²/s and the initial velocity of the superficial layer is order of 10^{-10} cm/s in the coordinate rotating frame of the star.

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Blobrana


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Title: Quark Matter in Neutron Stars: An apercu
Authors: Prashanth Jaikumar, Sanjay Reddy, Andrew W. Steiner

The existence of deconfined quark matter in the superdense interior of neutron stars is a key question that has drawn considerable attention over the past few decades. Quark matter can comprise an arbitrary fraction of the star, from 0 for a pure neutron star to 1 for a pure quark star, depending on the equation of state of matter at high density. From an astrophysical viewpoint, these two extreme cases are generally expected to manifest different observational signatures. An intermediate fraction implies a hybrid star, where the interior consists of mixed or homogeneous phases of quark and nuclear matter, depending on surface and Coulomb energy costs, as well as other finite size and screening effects. In this brief review article, we discuss what we can deduce about quark matter in neutron stars in light of recent exciting developments in neutron star observations. We state the theoretical ideas underlying the equation of state of dense quark matter, including colour superconducting quark matter. We also highlight recent advances stemming from re-examination of an old paradigm for the surface structure of quark stars and discuss possible evolutionary scenarios from neutron stars to quark stars, with emphasis on astrophysical observations.

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Blobrana


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Pulsars Quakes
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Scientists have discovered how to predict earthquake-like events in pulsars, the dense remains of exploded stars. These are violent episodes that likely crack a pulsar's dense crust and momentarily increase its spin rate.

John Middleditch of Los Alamos National Laboratory led the discovery team and presents these findings today at the American Astronomical Society Meeting in Calgary.
Middleditch and his team have discovered that for one particular pulsar, named PSR J0537-6910, the time until the next quake is proportional to the size of the last quake. Using this simple formula, the scientists have been able to aim NASA's Rossi X-ray Timing Explorer at the pulsar a few days before a quake to watch the event unfold.
Using the Rossi Explorer, the team has tracked about 20 "starquakes" in this pulsar over the past eight years and uncovered a remarkably simple, predictive pattern.

"By monitoring the pulsar spin rate and changes in the spin, we can pin down a starquake event to within a couple of days. These and other details have helped to simplify what has, until now, appeared to be a bewildering assemblage of facts about starquakes in pulsars. If only predicting earthquakes were this straightforward" - John Middleditch.

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Blobrana


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Starquakes
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On 27 December 2004, radiation from the biggest starquake on a neutron star ever recorded reached Earth. Unique data obtained by Double Star TC-2 and Cluster satellites enabled a group of European scientists to find the first observational evidence of cracks in the neutron star crust, during the initial phase of the starquake.
This result, published 16 June 2005 in the Astrophysical Journal, discriminates between current theories on the physical origin of such massive starquakes.

Millions of neutron stars populate our Milky Way galaxy. A neutron star is the remaining core of a massive star, once it has exploded. Made almost entirely of neutrons (subatomic particles with no electric charge), this stellar corpse concentrates more than the mass of our Sun within a sphere of ~20 km diameter. It is so dense that a sugar cube of neutron star on Earth would weight as much as all of humanity!

Two other physical properties characterise a neutron star, their fast rotation (or spin) and their high magnetic field. Astronomers have found different classes of neutron stars based on these properties. Some of them are the fastest spinning stars in the Universe (up to hundreds of revolutions per second); named pulsars , as they generate regular pulses of electromagnetic radiation including radio, visible, X-ray and gamma-ray wavelengths. These pulses are often compared to a spinning lighthouse beacon which appears to flash on and off.



Another class of neutron stars is known as magnetars, due to their ultra-high magnetic field. Their magnetic field intensity is indeed about 100 gigaTesla (or 10^11 T), a thousand times more than an ordinary neutron star. By comparison, the Earth's magnetic field is about 50 microTesla (5×10^-5 T). Most media used for data storage can be erased if they are exposed to a magnetic field of milliTesla (10^-3 T) intensity.

So far, a dozen of magnetars have been found. Four of them are also known as soft gamma repeaters, or SGRs, because they sporadically release large bursts of low energy (soft) gamma rays and (hard) X-rays, usually during short time periods (~ 0.1 s).

On 27 December 2004, the radiation from an extremely powerful explosion on the surface of SGR 1806-20 (the numbers indicate its position in the sky) reached Earth and lasted more than 6 minutes. During the first 200 ms, the amount of energy released was equivalent to what our Sun radiates in 250 000 years. It is the brightest event known to have impacted the Earth from an origin outside our solar system.

SGR 1806-20 is located at around 50 000 light-years from Earth on the far side of our Milky Way galaxy, in the direction of the Sagittarius constellation . A similar blast within 10 light years would have destroyed the ozone layer and be similar to a major nuclear blast. Fortunately, the closest known magnetar is 13 000 light years away.


Several scientific satellites observed the giant flare experienced by SGR 1806-20, including ESA γ-ray observatory INTEGRAL. The intensity of the emissions received may be roughly described as a major peak followed by a modulated decrease. However, the intensity of this major peak was hundreds of times stronger than any other observed so far (only two other giant flares have been recorded in the past 35 years).

"For the first 200 ms it saturated almost all instruments on satellites equipped to observe γ-rays" - Prof. Steve J. Schwartz from Imperial College London (UK) in his 16 June 2005 Astrophysical Journal paper.

Although designed to study the Earth's magnetosphere, the thermal electron detectors onboard Double star TC-2 and Cluster satellites performed unsaturated observations of this initial flare rise and decay. As explained in his 16 June paper, Professor Schwartz and his co-authors show that these unique data provide the first observational evidence of three separate timescales within the first 100 ms of this event.
The characteristics of these timescales (such as the number, duration, shape) play an important role in actual theories of starquakes on magnetars, which allows discriminating between them.

Based on these measured timescales and current theoretical models, the following scenario is confirmed in this paper. The giant flare is produced when the crust of the magnetar can no longer respond almost plastically to internal magnetic stress and finally cracks. One of the three timescales even allows an estimation of the fracture size: about 5 km. This is a significant size considering that SGR 1806-20 has been estimated to be a sphere of few tens of km in diameter.
On 27 June 2005, the Astrophysical Journal published a related study on SGR 1806-20, this time led by Italian astronomer GianLuca Israel from INAF-Osservatorio Astronomico di Roma. His data analysis reveals the presence of quasi-periodic oscillations (or modes) at the end of the 27 December 2004 event.

"These modes are likely to be associated with global seismic oscillations. In particular, the large crustal fracturing inferred by us can easily excite toroidal modes with characteristic frequencies in the observed range" - Professor Schwartz in his 16 June paper.

Therefore, Double Star TC-2 and Cluster data have not only enabled to directly estimate crustal properties of magnetars, they have also linked interior magnetic processes and their external consequences during giant flares.

"Cluster and Double Star were designed to study the various boundary layers of the Earth's magnetosphere, including the physics of magnetic reconnection. Such boundary layer physics has application throughout the astrophysical plasma universe, and it is therefore appropriate that these missions contribute in a more direct way to the study of magnetic reorganisation in an astrophysical object outside the solar system" - Professor Schwartz.

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