In Santa Fe: The Shed

Ed Brown discusses burning models with Nevin Weinberg and Andrew
Cumming. The Shed is a "new Mexican" restaurant, one block from La
Fonda. Map.

Milan Matos: Mass Measurements of Exotic Nuclei

Milan gave a nice summary of mass measurement techniques. Milan mentioned the importance of masses to neutron star modeling. He then summarized current techniques including direct measurements such as Penning traps, Schottky method, time-of-flight, and indirect measurements done with transfer reactions. Milan also mentioned capabilities of different facilities producing isotopes in different regions of the chart of nuclides.

Dmitry Yakovlev: Pyconuclear reactions in the crust

Dmitry Yakovlev gives an interesting and entertaining overview of pyconuclear reactions. These density sensitive nuclear fusion processes are manifestations of the QED vacuum, which involves Kindergarten physics and the problem is therefore explained by a young Russian girl. The reaction rate of pyconuclear reactions is independent of temperature, but increases exponentially with increasing density. These processes are thought to allow for the formation of large nuclei in the neutron star crust.

(The image shows George Gamow at young age).

Dany Page: Crust cooling of strange stars vs neutron stars

Dany described how as cooling depends on the thickness of the crust it should be possible to distinguish between neutron stars and strange stars - a strange star has a much thinner crust (a few hundred metres as opposed to km).  So, relaxation of the crust should be a good test.  However, he hasn't yet had the chance to do the calculations.......answers may be coming in the near future.

Sanjib Gupta: Nuclear reactions in the crust and implications for superburst ignition and cooling

Some talked about nuclear reactions near the stellar surface; others talked about those in the core.   Sanjib talked about reactions in between.  Such reactions in the crust are a figurative bridge between surface heating and core neutrino cooling.

These nuclear reactions, electron captures, neutron emissions, and pycnonuclear reactions, are of utmost importance because they set both the heating and compositional profile.

In the outer crust, electrons can capture into excited states of nuclei; the subsequent radiative de-excitation can release ~4 times more energy than previous models predicted, which assumed electron capture into the ground state.  Interestingly, Daligault & Gupta (2009) find that the outer crust is amorphous, i.e. it does not form a lattice!  This result is exciting, but conflicts with previous calculations (like Chuck Horowitz's simulations).  Does it conflict with observations?  Unclear right now, but it would be useful to find out (hint, hint...).

A multi-component inner crust may form a lattice, as some leaked, free neutrons redistribute themselves among the other nuclei and thereby "homogenize" the plasma.  This neutron rearrangement can affect pycnonuclear heating as well.

Nathalie Degenaar: Crust cooling of EXO 0748-676.

Nathalie shows us new observations of crustal cooling for the source EXO 0748-676. This source just turned to be in quiescence. It would be interesting to compare the cooling behavior of KS 1731 and MXB 1659. Data are just coming out. I find this very promising.  Theorists have now more data to compare with current models (see Ed Brown and Cackett talks). This of course if the source is really in quiescence.

Nevin Weinberg: Carbon detonation and shock-triggered helium burning in neutron star superbursts

Nevin told us about the physics of the superburst rise.  This is relevant because the rise both (1) determines the energy released during the superburst and thereby the light curve morphology and (2) sets the composition of the deep ocean and crust. 

The burning during the rise eventually occurs on such a small timescale that it becomes hydrodynamic; Nevin convincingly argues that a detonation ensues.  The detonation-induced shock triggers rapid heating of the overlying H/He layer and thus an X-ray burst-like event.  Such events are likely the detonation's observational signature.

The existence of the burst-like events may set an upper limit on the superburst ignition depth independently of other methods.

Randall Cooper: The 12C + 12C Reaction Rate and Superburst Ignition

Thermal properties of crust  and core sets ocean temperature and thereby the superburst ignition depth, so superbursts are probes of NS interiors.  But there is a problem......observationally inferred ignition depths are shallower than all theoretical superburst models predict (in models ocean is too cold)

Randy's proposed solution: 12C +12C rate is unmeasured at relevant energies and the rate used in models assumes reaction rate is nonresonant.  But, 12C + 12C rate may be much higher if a resonance exists......if a resonance exists observations agree with a range of NS models.

Randy's task for nuclear experimentalists - measure 12C + 12C reaction rate at astrophysically relevant energy.

Ed Brown relayed a new experimental result from the CLAUS2009 workshop.....recent experimental efforts suggest a large resonance.

Jean in 't Zand: New phenomena in thermonuclear X-ray bursts and Superbursts

Jean described two newly discovered X-ray burst phenomena: (1) long burst tails, (2) superexpansion bursts.

Long burst tails:
-- by stacking bursts from the clocked burster GS 1826-24, can now detect ~hour long tails in its ~100 sec. long type I X-ray bursts. These tails are due to heat propagating out on long time scales and match tails seen in the Heger et al. 2007 burst calculations.
-- puzzle: some bursts show tails with a fluence that is larger than the fluence of the flash itself, suggesting that more heat goes down than goes out. Is this due to strange variations in the surface conduction properties?

Superexpansion bursts:
-- found 32 bursts in data catalog that show a very large photospheric radius expansion (>1000 km) just after burst onset. These superexpansions always lasts for a few seconds and are followed by a moderate expansion phase (10 < r_ph < 100 km) which lasts on a time of order the burst duration (10-1000 s).
-- puzzle: why is the superexpansion duration independent of burst duration? It's probably not due to variations in early time luminosity. Maybe due to ejection of a thin shell that becomes optically thin after a few seconds?

Introducing: Karl Smith

Hi,
I am currently a graduate student at Michigan State Univ. where I have been working on sensitivities of X-ray burst models to nuclear uncertainties. I will be blogging about the interesting talks during the workshop.

Introducing: Nathalie Degenaar

Hi all,

I am currently a phd student at the University of Amsterdam where I work on (X-ray) observations of accreting neutron stars. I will be one of your dedicated bloggers at this conference in Santa Fe.

Chuck Horowitz: Physics of the neutron star crust and its breaking strain

In accreting Neutron Stars the new crust formed is a perfect crystal with some impurities, which could increase pycnocuclear reactions. Perhaps this provides the heat needed to explain superburst ignition (see Cumming talk).

NS shear modulus seems to be independent of impurities.

NS nuclear pasta shear viscosity does not depend on the pasta shape.

NS crust does not fracture.

Liliana asks: If NS does not break, where are the star faults (star quake sources) located? 

Andrew Cumming: Long thermonuclear flashes and the heating and composition of neutron star crusts

Andrew Cumming kicked off the workshop with a germane review of type I X-ray bursts, thermonuclear flashes on accreting neutron stars.  X-ray bursts are of great interest because some bursts, specifically those for which hot CNO cycle burning does not set the temperature of the ignition region, can be useful probes of the interior temperature and thereby the crust and core properties.

There are both great successes and failures in comparisons between observations and model results of "normal" H- and He-triggered bursts.  Generally, models of individual bursts agree well with observations (e.g., GS 1826-24), whereas models of global bursting behavior disagree.

Superbursts, extremely energetic flashes thought to be triggered by carbon burning, may be particularly useful probes of crust physics.  Models predict that all crust & core parameters "must be turned to hot," to match observations, but the same predictions are inconsistent with complementary observations of other phenomena.  What's the solution: More exothermic electron captures? a resonance in the carbon fusion reaction rate? strange stars? a new source of shallow heating?

We'll more about these possibilities in the next couple of days.  Stay tuned!

Posting your talk or poster at the conference website

Presenters can post their talks or posters from the online talk schedule. Just log in (so the system can figure out which presentation is yours), then upload a PDF file.

Coming Talks: Monday May 18

The talks for Monday May 18 (full schedule):

Andrew Cumming	Long thermonuclear flashes and the heating and composition of neutron star crusts	9:00
C. J. Horowitz	The Breaking Strain (Strength) of Neutron Star Crust	9:45
Jean in 't Zand	New phenomena in thermonuclear X-ray bursts	11:00
Randall Cooper	The 12C + 12C Reaction Rate and Superburst Ignition	11:25
Nevin Weinberg	Carbon Detonation and Shock-Triggered Helium Burning in Neutron Star Superbursts	11:50
Edward Cackett	Crustal cooling in MXB 1659-29 and KS 1731-260	1:30
Edward Brown	Mapping neutron star crusts with the cooling lightcurves of quasi-persistent transients	1:55
N. Degenaar	Crust cooling of EXO 0748-676	2:20
Sanjib Gupta	New results in neutron star crust nucleosynthesis and implications for X-ray Superburst ignition	2:45
Dany Page	Cooling of the crust of strange stars vs neutron stars	3:30
D.G. Yakovlev	Pychnonuclear reactions in the neutron star crust	4:25
Milan Matoš	Nuclear Masses and their Importance for the Cooling of the Neutron Star Crust	5:10
Peter Moller	GLOBAL CALCULATIONS OF NUCLEAR STRUCTURE PROPERTIES.	5:55

Ed Cackett: Crustal cooling in MXB 1659-29 and KS 1731-260.

Ed Cackett reviewed the existing data, and described the coming data, regarding observed cooling of the long-duration transients MXB 1659-29 and KS 1731-260. These are two post-outburst low mass X-ray binaries of great interest to observers and theorists, because the total magnitude of accretion over their outbursts is high enough to cause an energy deposit in the crust (due to Deep Crustal Heating) of such magnitude that it will heat the crust out of thermal equilibrium out of the core. Seven observations of the cooling crust have taken place; more have been approved, and will be made in the near future -- so stay tuned for new observational results. Up to now, these objects have continued a monotonic decrease in their emission temperature, consistent with the expectations of neutron star crustal cooling theory -- although interestingly different from the simple, early calculations of such cooling.

This talk informed the talk which followed, given by Ed Brown, in which Ed presented his (with Andrew Cumming) analysis of the observed cooling curves of these two objects, using more detailed cooling calculations.