Posters: Sergey Postnikov

Posters: Gloria Sala

Posters: Karl Smith

Andrew Steiner: Elucidating the properties of dense matter from starquakes in neutron star crusts

Andrew told us about how oscillations detected in magnetar giant flares may quantitatively constrain nuclear physics, in particular the ubiquitous nuclear symmetry energy. The symmetry energy is one of the largest uncertainties of the crust parameters; this uncertainly leads to uncertainties in the shear modulus and shear speed.

Interestingly, Andrew suggested that the traditional mode assignments to the crust and core modes may be incorrect. If true, this could explain the implied magnetar mass of less than one solar mass: The actual mass could be higher, which would help many at the workshop sleep better.

Nils Andersson: Modeling realistic crust dynamics

Nils convinced us that non-magnetic relativistic model is not good enough to describe torsional oscillations of NS. Improvement could be done by including crust-core coupling. His model uses relativistic multi-fluid equations of motion. He also includes elasticity of the fluids and the effect of magnetic field. His model depends on several aspects. For example, this model requires a detail understanding of the superfluid in the crust. Nils also need to know how smooth is the transition from the crust to the core. Another key ingredient is the vortex dynamics.
One of the main aspects Nils pointed out is the connection between the microphysics and the mesoscopic scale,e.g if you a particular nuclear pasta shape, how this would behave if its dimensions is of the order of cm.

Yuri Levin: MHD aspects of magnetar oscillations

Magnetar quasi-periodic oscillations (QPO's) would be relatively easy to model, and thereby deduce properties of neutron stars, if all we needed to worry about is the crust. If only it were that easy.

There are two types of torsional modes in the neutron star: shear modes in the crust and alfven modes in the core. These two modes have similar frequencies, so we expect coupling between the modes. The timescale for energy exchange is much smaller than the duration of the QPO's, so we expect many exchanges to occur.

Therefore, we need a realistic model of the coupled crust and core to really understand magnetar QPO's.

In Santa Fe: La Plazuela

If you don't want to leave the hotel to go to lunch, the La Plazuela restaurant here in the La Fonda hotel is a good option.

Right: Mark Alford, Craig Heinke and Nathalie Degenaar pose for a picture after discussing the intricacies of strange stars.

Tod Strohmayer: Fast X-ray Oscillations During Magnetar Flares

Tod started the afternoon session with a detailed overview of the fascinating oscillations seen during giant flares on magnetars.  In the two objects where these oscillations have been seen, a wide range of frequencies have been detected.  It is highly suggestive that these are torsional oscillations due to vibrations in the crust.

With further understanding and detections there is the exciting possibility to constrain neutron star properties from these oscillations.  As Tod discussed, there are still a number of theoretical issues, and lots of complexity in the observations.    For example, does the crust fracture (analogy to earthquake fractures)?  And, while what we observe are modulations in X-ray flux - can these actually be produced by crust motions?

At Crust09: Audience

Credit: Erik Kuulkers

The crowd eagerly waiting for the 1st talk after the morning coffee break on May 21.

Achim Schwenk: Neutrinos rates from chiral effective field theory

Achim talked about how to calculate nuclear reaction rates determined by spin relaxation time. This method allows to calculate scattering of neutrinos from one or two nucleons.
The spin response is embedded in the dynamical structure factor of the nucleons. The nucleon interactions are described by chiral effective field theory. I found very interesting his comparison of the chiral EFT and different interactions, including the V- k-low potential, and that they reproduced similar spin relaxation rates at low density. At high density chiral 3n interactions suppresses this rate.

Jorge Piekarewicz: The Nuclear Physics of the Neutron Star Crust

Jorge started with a description of the correlation between neutron skin and pressure of pure neutron matter which then effects the neutron star radius. Then mentioned the PREX experiment wanted to clarify that it is not lead on lead, but a slight tap from an electroweak probe.

Also, mentioned the Garvey-Kelson Mass Relations which has very general assumptions but seems to work extraordinarily well. More general and robust than expected hinges on smoothness of M(N,Z).

Finally, Jorge was trying to understand if nuclear pasta is robust or model dependent. Do we have to go through pasta or is there another possibility? His work showed that there is no pasta formation if there are no long range forces or if neutron skin is large.

At Crust09: Waiting on the couch

Randy Cooper, Ed Brown, and Fang Peng wait for some colleagues to go out to dinner. Ed and Fang smile for the camera, while Randy is distracted by something shiny.

Duncan Galloway: Measuring neutron star parameters from mixed H/He thermonuclear bursts

Duncan discussed a promising approach to constrain the neutron star parameters from type-I X-ray bursts. Comparing observed burst lightcurves with theoretical models allows for an independent distance measurement, whereas the neutron star radius can be constrained from fitting the spectral data and the Eddington flux can be estimated from modeling the temperature profile in the neutron star crust. This combination allows one to put constraints on the neutron star mass and radius. The technique is applied to the bursters GS 1826-24 and KS 1731-26. Eddington limited X-ray bursts have been detected from the latter, which provides additional constraints.

James Lattimer: The Nuclear Equation of State and Properties of the Crust

Jim kicked off the third day of the workshop with a nice discussion of the application of the equation of state (EoS) to properties concerning the crust.

For instance, he discussed how the thickness of the crust has a simple dependence on mass and radius and the chemical potential at core/crust boundary.  He pointed to several types of observations that are dependent on the crust thickness, e.g., crustal cooling and neutron star seismology. 

Jim finished with a summary of measured neutron star masses.  What was particularly exciting is a recent well constrained mass measurement of  1.67 +/- 0.01 M_sun  (Champion et al. 2008).

Finally, Jim's important take away point: observations of crustal properties can have significant ramifications on EoS, not just the crust itself.