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

The 599 Hz millisecond pulsar IGR J00291+5934 is a source I've been studying for more than a decade, having been involved in the discovery as well as observation of every transient outburst since. Although the first few outbursts displayed a remarkable predictability, the most recent, in 2015, was almost 3 years "late". In a detailed study of the INTEGRAL and Swift observations of the outburst, ISSI PhD student Vittorio De Falco examined the broad-band X-ray spectrum and the pulse properties, as well as data covering the first thermonuclear burst ever detected from the source. Remarkably, the estimated time-averaged accretion rate is decreasing on a timescale of only a few years, suggesting that we are unlikely to see another bright outburst in the next decade, if ever. Vittorio's paper was just accepted by A&A.

Read the paper arXiv:1611.08218

Labels: 2016, /pulsars

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

Following the approval of our Centre for Excellence in Gravitational-wave Astronomy, OzGRav, Monash has several postdoctoral positions available to contribute to research programs within the Centre. One of these is intended to work with the GOTO team hunting for optical counterparts to the hotly-anticipated mergers of neutron-star binaries detected by LIGO. Please visit the website to view the criteria and position description, or contact me if you have any enquiries. Applications close December 1st.

Labels: 2016, /opportunities

The Australian Research Council announced this morning the outcomes of the Centres of Excellence scheme, and I'm delighted to report that our Centre for Gravitational Wave Discovery, aka OzGRav, was approved! The Centre, led by Swinburne's Prof. Matthew Bailes, will support research into detecting gravitational waves with interferometric detectors like Advanced LIGO as well as the Parkes Pulsar Timing Array. Nineteen Chief Investigators from six Australian universities will join with 15 partners from Australia and all over the world, in a program extending for the next 7 years. A great time to be a gravitational wave astronomer!

Labels: 2016, /gravitational waves

Some years ago I noticed a peculiar dip in the X-ray intensity of a well-known transient, Aql X-1. Such dips have been seen from other systems, but there they occur once each orbital period, and are generally explained by material in the outer accretion disc coming into sight once each orbital period, and (partially) blocking the view of the neutron star. With the help of Honours sudents James Upjohn and Matthew Stuart, and vacation student Alishan Ajamyan, we made an exhaustive search of the available RXTE data on the source, and turned up another dip, six years earlier. We explained the behaviour as a new kind of "intermittent" dipping, which — due to it's apparent rarity — could be detectable in several tens of other systems. Our paper has now been published in MNRAS.

Labels: 2016, /transients

This June I visited Niigata, Japan for the 14th Nuclei in the Cosmos meeting. This biennial gathering attracts a mix of nuclear experimentalists, modelers, and astrophysicists. I presented a poster on our thermonuclear burst model-observation comparisons.
Following the meeting I stopped by RIKEN in Tokyo, where members of the MAXI instrument team were the very generous hosts of a meeting of our International Space Science Institute international team. The Monitor of All-Sky X-ray Image (MAXI) instrument, deployed on NASA's International Space Station, has been continuously monitoring the X-ray sky for several years, and has detected many rare, long-duration thermonuclear bursts, as well as transient outbursts of known and new sources.

Labels: 2016, /meetings

In May I travelled to Athens, OH for the JINA-CEE symposium on neutron stars. This was a great week of presentations on a very wide range of astrophysics topics relating to neutron stars, including my own presentation on thermonuclear bursts, but also discussions of new & upcoming missions like NICER and ASTROSAT, progress on mass-radius measurements from bursts and cooling transients, rotation-powered pulsars, gravitational waves, you name it. The week kicked off on Sunday with a satellite workshop on Experiments for X-ray Burst Nucleosynthesis, where we reviewed the prospects for better experimental constraints on the many nuclear reactions important to thermonuclear bursts. A big thank-you to JINA-CEE for supporting my trip!

Labels: 2016, /meetings