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Mon Apr 09, 2018

The influence of stellar spin on thermonuclear runaways

Many burst sources exhibit inexplicable decreases in burst rate even as the accretion rate increases. However, not all burst sources show the same type of behaviour, which hasn't been understood to date. Using the MINBAR sample, we measured the burst rate as a function of accretion rate, from seven neutron stars with known spin rates. At the highest accretion rates, the burst rate is lower for faster spinning stars. The observations imply that fast (> 400 Hz) rotation encourages stabilization of nuclear burning, suggesting a dynamical dependence of nuclear ignition on the spin rate. This dependence is unexpected, because faster rotation entails less shear between the surrounding accretion disk and the star. Although we don't yet fully understand this behaviour, it's hoped that further analysis and simulations will help to explain it. Our paper has now been accepted by ApJL.

Read the paper arXiv:1804.03380

Labels: 2018, /thermonuclear bursts

Tue Mar 20, 2018

Simulating X-ray bursts during a transient accretion event

Thermonuclear burst modelling has to date focused on stable accretion rates, but bursts are also observed during episodes of transient accretion. The accretion-powered millisecond pulsar SAX J1808.4—3658 exhibited four helium-rich X-ray bursts during the outburst of 2002 October. Monash student Zac Johnston undertook the first multizone simulations of X-ray bursts throughout the outburst, incorporating a time-varying accretion rate, to try to match the observations. The model, with an accreted hydrogen fraction of X = 0.44 and a CNO metallicity of 0.02, reproduces the observed burst arrival times and fluences well, and offers the prospects of more detailed future simulations of such outbursts. Zac's paper reporting the modeling has now been accepted by MNRAS.

Read the paper arXiv:1711.03970

Labels: 2018, /thermonuclear bursts

Fri Dec 15, 2017

Thermonuclear bursts in the spotlight

It's been a few years now since a review of thermonuclear bursts came out, so a little while ago Tomaso Belloni and Mariano Mendez commissioned Laurens Keek and I to write one. It's a big job to try to cover the literature in such a broad field, and this took (unfortunately) a lot longer than we expected. Tomaso and Mariano were very patient, and I'm pleased to announce that the review is finally ready! You can read it for yourself on the arXiv, or wait for it as part of "Timing Neutron Stars: Pulsations, Oscillations and Explosions", in Springer's ASSL series, edited by Tomaso and Mariano

Read the paper arXiv:1712.06227

Labels: 2017, /thermonuclear bursts

Wed Nov 29, 2017

Cracking the code of burst rates

It's been a long-standing mystery, how for some burst sources, the burst rate decreases even as the accretion rate increases. Theory and numerical models both predict that as you dial up the accretion rate, the burst ignition point should be reached earlier and earlier (up to the point when burning becomes stable and bursts stop), so that the burst rate should always increase. Yuri Cavecchi recently came up with an alternative explanation, related to the way that the burst ignition point moves from the equator to higher latitudes at higher accretion rates. If this movement is sufficiently fast, a decrease in burst rate can result even as the accretion rate continues to increase. Yuri's paper (also with longtime burst expert Anna Watts) was just accepted by ApJ

Read the paper arXiv:1711.04389

Labels: 2017, /thermonuclear bursts

Tue Sep 05, 2017

Analysing the cooling tails of thermonuclear bursts

When the upper layer of an accreting neutron star experiences a thermonuclear runaway of helium and hydrogen, it exhibits an X-ray burst with a cool-down phase of typically 1 minute. Analysis of light curves of 1254 X-ray bursts observed by the Rossi X-ray Timing Explorer shows that the decay is described as a power-law with index in the range 1.3—2.1, with a Gaussian component also required for half of the bursts. The Gaussian appears consistent with being due to the rp process, which consists of rapid proton captures and slow beta-decays of proton-rich isotopes, and is expected to be prominent in bursts with a significant fraction of hydrogen in the fuel. The Gaussian fluence fraction suggests that the layer where the rp process is active is underabundant in H by a factor of at least five with respect to cosmic abundances. Jean's paper reporting the analysis is now accepted by A&A.

Read the paper arXiv:1708.08644

Labels: 2017, /thermonuclear bursts

Sat Jul 22, 2017

MINBAR meetings lead to new burst source

This (northern) summer I visited colleagues at DTU Space, Denmark and SRON, Netherlands to continue work on the Multi-INstrument Burst ARchive (MINBAR). We're currently preparing for the first data release (DR1), and we made a lot of progress with the data analysis and assembly of the companion paper. We also got to celebrate the deployment of NICER to the International Space Station, although because of a delay our celebration was a little early!

Unexpectedly, the careful eye of Jean in 't Zand identified a handful of bursts observed by RXTE from a 2008-9 transient, XTE J1812-182, that was not previously known to be a burster. We reported our discovery in Astronomer's Telegram #10567 and will shortly present a more detailed analysis via a paper. While this is a very pleasing outcome from the meeting, it now means there are even more bursts to analyse for MINBAR!

Labels: 2017, /thermonuclear bursts

Fri Mar 24, 2017

Reference bursts for model comparisons

Thermonuclear (type-I) X-ray bursts arise on the surface of neutron stars in binary systems, and offer a powerful probe of the neutron star environment as well as the nuclear reactions that power them. Efforts to match observed burst to numerical simulations have been fairly limited to date, partially because of the dearth of high-quality, well characterised burst measurements. To address this issue, we have assembled a set of "reference" bursts featuring examples of a number of different types of bursts, presented in a paper which has just been accepted by PASA. We also hope that the observed bursts will serve as test cases for numerical codes in order to assess the variations between those codes, in order to quantify the fundamental uncertainty of burst simulations.

Read the paper arXiv:1703.07485

Labels: 2017, /thermonuclear bursts

Mon Oct 31, 2016

Understanding burst oscillations

Some thermonuclear burst sources exhibit temporary "burst oscillations", periodic variations in the X-ray intensity at frequencies characteristic of each source. It has been shown that these oscillaions trace the neutron star spin, but much is still not known about the detailed mechanism, and many puzzles remain. UvA student Laura Ootes led a comprehensive study of the oscillations detected in the entire Rossi X-ray Timing Explorer sample, which has just been accepted for publication by ApJ. The paper presents a comprehensive comparison of the observed properties with burst oscillation models, as summarised in a series of 14 (!) tweets by co-author Anna Watts. Although we still cannot unambiguously identify the oscillation mechanism, this analysis is going to be definitive for many years, until a new X-ray timing mission is launched.

Read the paper arXiv:1610.08995

Labels: 2016, /thermonuclear bursts