Wanpeng told us about three reactions, alpha captures on waiting points, that are important for type I X-ray bursts: 14O(a,p), 15O(a,g), and 26Si(a,p); such reactions are some of the first that occur at ignition.
15O(a,g) is particularly crucial for burst ignition. Tan's analysis of the measurements result in a rate that is sufficiently precise to constrain the critical accretion rate above which burning is thermally stable to within 10%; the critical rate is roughly the Eddington limit.
However, several workshop participants called the accuracy of his analysis into question. Barry Davids asserted that "it's not a detection at the two sigma level."
Tuesday, May 19, 2009
In Santa Fe: Dinner at Coyote Cafe, and an unexpected tour of the town
It's Monday evening just after the final talk, and I soon join a group of nuclear theorists to go to dinner. Where should we go? How do we get there? Certainly these are difficult questions for Santa Fe neophytes, but luckily our sextet includes Andrew Steiner, a former resident. Excellent! We unanimously decide to follow are intrepid local expert to what certainly will be a great restaurant.
We pass by numerous shops, legal offices, and restaurants. I start to get worried. "Andrew, do you know where we're going?" "No, I'm not really familiar with the area. I just figured we'd walk down this street until we found something good." Having now reached what looks like the end of town, I suspect that won't happen.
Luckily, we run into Sanjay Reddy. Great! He lives here, he must know where we should go! He tells us of a great restaurant, somewhere over that way on a road that probably starts with the letter "G." Oh dear. Thankfully, he mentions the Coyote Cafe as well, which is located on this very street and in the direction of La Fonda. That sounds safe, let's go there.
Coyote Cafe turns out to be an excellent choice: The food is somewhat pricey (~$30 entree), but it's delicious, and the wine list is extensive. Andrew redeems himself with his choice of Chardonnay, and we have a thoroughly enjoyable dinner.
We pass by numerous shops, legal offices, and restaurants. I start to get worried. "Andrew, do you know where we're going?" "No, I'm not really familiar with the area. I just figured we'd walk down this street until we found something good." Having now reached what looks like the end of town, I suspect that won't happen.
Luckily, we run into Sanjay Reddy. Great! He lives here, he must know where we should go! He tells us of a great restaurant, somewhere over that way on a road that probably starts with the letter "G." Oh dear. Thankfully, he mentions the Coyote Cafe as well, which is located on this very street and in the direction of La Fonda. That sounds safe, let's go there.
Coyote Cafe turns out to be an excellent choice: The food is somewhat pricey (~$30 entree), but it's delicious, and the wine list is extensive. Andrew redeems himself with his choice of Chardonnay, and we have a thoroughly enjoyable dinner.
Anuj Parikh: Nuclear Physics Uncertainties in Models of Type I X-Ray Bursts
Anuj started with the quote "Are those original x-ray bursts or RI's" referring to a Gibson guitar. Anuj then continued describing another x-Ray burst sensitivity study.The study explored sensitivity of model to peak temperature, duration, and initial composition. Then, using one-zone post processing models the study varied individual reactions (>2000), simultaneous varied multiple reaction rates, and individual variation of Q-values (<1MeV).
Varying individual rates identified as65(p,g) and ga61(p,g) as having the strongest effect on the models used.
Anuj also showed that Q-values are important for calculating forward reactions from theory and for detailed balance in reverse rate calculation. Q-value uncertainty is most important when the Q-value is small. The sensitivity study identified the Q-value from ge64(p,g) as having the most significant impact.
In Santa Fe: Tomasitas
If you are thinking about (affordable) dinner for tonight, I would suggest Tomasitas. This is a Mexican restaurant (not surprising in NM). I have enjoyed the food but overall I like to go there on Tuesday nights because they have Mariachi at 8 p.m (no cover). Siting can be limited at that time so I recommend to arrive early. Here is a link with the address and more information.
http://local.yahoo.com/info-20219154-tomasitas-santa-fe
http://local.yahoo.com/info-20219154-tomasitas-santa-fe
At Crust09: Bob's Take Away
Something for the observers: still wondering what exactly Bob's famous quest is?
Since June 2007 Bob's quest is available for download. Try it out, and prepare your answer, before Bob asks!
Many thanks to Erik Kuulkers for the mp3.
Richard Cyburt: X-ray burst reaction flows and ash composition
Richard compared results for XRB lightcurves coming from two different methods: Multizone and single zone calculations. Overall the two methods agree quite well, both for composition and light curves. Both compositions peak in the interval A =60,80.
Richard shared us a very useful piece of information about the JINA REACLIB project. REACLIB is a huge data base with nuclear reaction rates. Users can create their own libraries there too.
Richard shared us a very useful piece of information about the JINA REACLIB project. REACLIB is a huge data base with nuclear reaction rates. Users can create their own libraries there too.
Jordi Jose: Hydrodynamic simulations of type I X-ray bursts: metallicity effects
Jordi described his groups simulations of type I X-ray bursts using the 1D SHIBA hydro code coupled to a large network. In models with solar abundance of metals he finds very little 12C in the ashes (~1% by mass), peak nuclei with A~60, and, in general, results that approximately match those found by Heger et al. Sub-solar models (Z=0.001) had less energetic but longer bursts, lower alpha (alpha~30), synthesized heavier species (A~100), and smaller 12C mass fractions (~0.1%). More runs are in progress. Among the take home points (better constraints on Mdot, L, reaction rates), Jordi suggested we take home some local jewelry and handmade artwork :).
Jeff Blackmon: Nuclear Physics on Accreting Neutron Stars
Jeff Blackmon stressed the importance of experimental measurements of (p,g) and (a,p) reactions to accreting neutron stars. Current experiments are using indirect techniques to locate important resonances for these reactions. Specifically mentioned two direct measurements, the first was performed at Oak Ridge studying f17(p,g). Important resonance was measured, but direct capture rate still needs to be constrained. Secondly, he mentioned o14(a,p) measured at CRIB (RIKEN) which is 50-80% different from the recommended REACLIB rate.
To measure many of these reactions more low energy rare isotope beams are needed. Combined with new high resolution detectors such as ANASEN these important X-ray burst ignition rates can be better constrained.
Andrew Melatos: Crustal magnetic field in an accreting neutron star
Andrew gave us a lucid discussion of magnetic field evolution and its consequences in accreting neutron stars.
Magnetic field burial and flux freezing together distort the initial dipole and decrease the magnetic moment; however, the local field strength may increase! ZEUS says that the magnetic structures are Parker unstable but the instability is suppressed: In the end, the substrate exhibits global MHD oscillations. The interesting claim is that the end state could be a 10^15 G (!!!) magnetic field "wall" that could affect type I X-ray bursts, for example. The idea is exciting because there are severe discrepancies between observations and X-ray burst models that are difficult to explain through nuclear physics uncertainties alone, but the presence of such a structure, particularly at the ignition pressures of X-ray bursts (which are << B^2/8pi for B = 10^15 G), are difficult to believe.
A more plausible, yet no less exciting, idea is that asymmetric field burial could produce a quadrupole; the resultant gravitational radiation would limit the possible neutron star spin frequency. This could explain why all neutron stars have spin frequencies <~ 700 Hz, well below the break-up spin.
Alexander Heger: An Overview of X-Ray Burst Simulations
Alex began with an overview of type I X-ray bursts and their various accretion rate regimes. He then described his numerical simulations of bursts using the 1D Kepler hydro code which he coupled to a large reaction network. Runs with H/He accreting at high Mdot with low Z show long (~200s) tails due to rp-process burning and little 12C in the ashes of burning (X12~0.001). In runs with low Mdot and high Z, the ignition takes place in a pure He layer, leading to a very quick burst rise (~1 ms) and near Eddington luminosity at peak. Runs with very low Mdot undergo weak hydrogen flashes.
Some of the results may be sensitive to uncertainties in the nuclear physics and reaction rates and are a subject of future study.
Some of the results may be sensitive to uncertainties in the nuclear physics and reaction rates and are a subject of future study.
Coming Talks: Tuesday May 19
| Alexander Heger | An Overview of X-Ray Burst Simulations | 8:45 |
| Andrew Melatos | Crustal magnetic field in an accreting neutron star | 9:30 |
| Jeff Blackmon | Nuclear Physics on Accreting Neutron Stars | 9:55 |
| Jordi Jose | Hydrodynamic simulations of type I X-ray bursts: metallicity effects | 11:00 |
| Richard Cyburt | X-ray burst reaction flows and ash composition | 11:25 |
| Livius Trache | Nuclear reaction rates for explosive H-burning from experiments with rare nuclear beams. Indirect methods | 11:50 |
| K. Ernst Rehm | Recent Results for (p,g) and (a,p) Reactions on the rp-Procerss Path | 12:15 |
| Uwe Greife | Proton Capture Measurements with Radioactive Ion Beams | 2:00 |
| Anuj Parikh | Nuclear Physics Uncertainties in Models of Type I X-Ray Bursts | 2:25 |
| Giuseppe Lorusso | beta deay study in the Sn100 region | 2:50 |
| Wanpeng Tan | measurement of alpha-induced nuclear reactions for X-ray Bursts | 3:15 |
| Jerome Chenevez | The INTEGRAL view of intermediate long X-ray bursts | 4:10 |
| Laurens Keek | X-ray bursts with too short recurrence times | 4:35 |
| Craig Heinke | Progress & Problems from Quiescent LMXBs | 5:00 |
| Schatz, Brown, Haensel, Horowitz | Discussion, Accreting Neutron Stars | 5:30 |
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