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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

Fri Dec 18, 2015

Kepler predictions for thermonuclear bursts

Monash researcher Alexander Heger has been for many years exploring the dependence of thermonuclear burst properties on the source conditions, including accretion rate and fuel composition. A year or two back Honours student Nathanael Lampe took all of the models Alexander had compiled over the years, and performed a common analysis, measuring the burst recurrence time and energetics, for the purposes of comparing broadly to observations, as well as understanding the stability properties of the bursts. As part of this analysis, Nathanael assembled a table giving the properties of bursts in each available run. Nathanal's paper has now been accepted by ApJ, and it is anticipated that the burst table will become a useful resource for observers (and theorists) in years to come.

Read the paper arXiv:1512.05769

Labels: 2015, /thermonuclear bursts

Mon Jan 12, 2015

Varying accretion rate during bursts: the sequel

In a study of several hundred "photospheric radius-expansion" bursts observed by the Rossi X-ray Timing Explorer, Monash PhD student Hauke Worpel found evidence that the accretion rate increases by a substantial amount during the burst (see the previous post). As a followup, Hauke also analysed the much larger sample of almost 1800 less-intense bursts, that did not exhibit radius expansion. The additional analyses confirm the original result, and further support the interpretation that the accretion rate is increasing in response to the bursts. Interestingly, even including the amplified contribution from the persistent (accretion) emission does not result in a formally good fit to all the spectra, suggesting that the burst spectrum deviates significantly from a blackbody. Hauke's paper was just accepted by ApJ.

Read the paper arXiv:1501.02070

Labels: 2015, /thermonuclear bursts

Mon Jun 23, 2014

Influence of accretion geometry on bursts

One of the interesting outcomes of our ISSI international team on thermonuclear bursts was the work by Juri Poutanen (U. Turku) and his then-student Jari Kajava (now an ESAC research fellow), showing that the spectral evolution during the burst proceeded in characteristically different ways, depending upon the persistent spectral state of the source at the time of the burst. The persistent X-rays from burst sources arises from accretion, and these sources are found in two broad states: hard and soft, which are thought to indicate different physical conditions in the accretion flow. Juri's paper (arXiv.org:1405.2663) concerned the well-known burster and transient 4U 1608—52, while a companion paper led by Jari (arXiv.org:1406.0322) extended this analysis to 10 other sources. Those who study bursts are increasingly concerned with the interaction of bursts and the accretion environment, and these results are helping to determine just how these two phenomena are relatied. Juri's paper was just accepted by MNRAS, and Jari's paper has been submitted.

Read the papers arXiv:1405.2663 & arXiv:1406.0322

Labels: 2014, /thermonuclear bursts