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

In December we kicked off a new international team supported by Bern's International Space Science Institute. This team is focussed on reconciling experimental, observational and numerical investigations on thermonuclear (X-ray) bursts and the nuclear reactions that power them. Due to the strong gravity and high temperature, these bursts probe nuclear physics and reactions not encountered elsewhere in nature, and are also influenced by the properties of the underlying neutron star - and hence the properties of matter at these extreme conditions. Our goals include identifying the key nuclear reactions which influence the burst lightcurve; taking advantage of new rare-isotope accelerator-based experiments and satellite observations to provide stringent tests of numerical models; identifying specific cases of ignition and burning from observation-model comparisons; and (ultimately) providing qualitatively new constraints on the properties of neutron stars and nuclear matter.

Although this is the third such team for me, it's the first I've led. Bern itself was once again celebrating Christmas, with the Weihnachtsmarkt in full swing, including the always popular gluhwein shack.

See the team home page for more information

Labels: 2015, /meetings

The first observations by a worldwide network of advanced interferometric gravitational wave detectors offer a unique opportunity for the astronomical community. At design sensitivity, these facilities will be able to detect coalescing binary neutron stars to distances approaching 400 Mpc, and neutron star-black hole systems to 1 Gpc. Eric Howell's paper, recently accepted by the Publications of the Astronomical Society of Australia, describes how Australian astronomical facilities and collaborations with the gravitational wave community can contribute to this new era of discovery, via contemporaneous follow-up observations from the radio to the optical and high energy. The preprint is now available at arXiv:1511.02959

Labels: 2015, /gravitational waves

It's an exciting time for Indian science at the moment, with the September 28^th launch of the ASTROSAT multiwavelength astronomy mission, and the development of a new interferometric gravitational-wave detector, IndIGO. Motivated by these developments, the International Centre for Theoretical Sciences hosted a joint worshop between November 4^th—6^th with the Albert Einstein Institute, at the new ICTS campus north of Bengaluru (Bangalore). The meeting featured a range of talks on searches for gravitational waves as well as X-ray, radio and optical followup. I gave an invited talk on our knowledge of the orbital parameters of the best continuous-wave candidate source, Scorpius X-1, based on our 2014 paper.

Labels: 2015, /meetings

Neutron stars are arguably the most exotic objects in the universe. Like one of those annoying friends who seemingly must overachieve in every aspect of life, neutron stars exceed in almost every category: surface gravity; magnetic field strength; density; and temperature.

>> Read more @ The Conversation

Labels: 2015, /outreach

My last appearance on the Australia Telescope Time Assignment Committee surprisingly was accompanied by an ABC News story on the role and operation of the committee. I really enjoyed my time on the TAC, despite the workload and deadlines; it's great to have such a broad overview of how our community chooses to utilise the national resources available to them. The other TAC members have been great fun to work with, and the ATNF staff are always helpful and responsive. I may have even learned a little about radio astronomy, although no proposals from me... yet!

Labels: 2015, /press

This June the terrific people at ESAC hosted a 3-day workshop to celebrate the 40th anniversary of the discovery of X-ray bursts. The workshop featured a comprehensive review of the state-of-the-art of burst physics and phenomenology, and plenty of opportunities for productive discussions. The organisers included a conference dinner at the fantastic Rstaurante Zerain which was a terrific (if very late) night out. It was also wonderful to spend some time in Madrid, and enjoy some sunshine! On behalf of my SOC co-chair, I had a lot of fun presenting the conference summary.

Labels: 2015, /meetings

Our Gravitational-wave Optical Transient Observer telescope prototype project was in the news this month, following up a 1-day workshop held in April to introduce the project to the Australian community. Warwick team members Danny Steeghs, Tom Marsh and Krzysztof Ulaczyk made the long trip down under to attend, and Danny introduced the project while Krzysztof discussed his experiences with his previous project, OGLE. The workshop was a big success, attracting attendees and speakers from all over the country, who presented work on a wide range of great transient science — from fast radio bursts to supernovae. For more information see the Monash Media story, the project homepage or the workshop site.

Labels: 2015, /press

Sco X-1 is the brightest X-ray source (excluding the sun) in the sky, and is potentially also the brightest source of continuous gravitational waves. Chris Messenger led a multi-year project aimed at determining the relative benefits of a number of search algorithms that will likely be applied to aLIGO data. The mock-data challenge provided simulated data including signals at a range of strains, and each algorithm then attempted to find the signals. Encouragingly, the faintest detected signal is only a factor of two stronger than the estimated torque balance limit, which indicates that this limit may be beaten with improved algorithms, data, and also perhaps system parameter precision.

Read the paper (arXiv:1504.05889)

Labels: 2015, /gravitational waves

I am proud and pleased to report the award of Hauke Wörpel's PhD degree was approved last week. Hauke's thesis, entitled "Radiation drag on the accretion disk in type-I X-ray bursts" was submitted last October. His thesis involved analysis of a few thousand thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer, showing that the accretion rate seems to increase during the bursts, and by a substantial amount, up to a factor of 20. Hauke also undertook numerical SPH modelling of accretion disks under the influence of bursts, with a view to better understanding the dynamics of the disk. Hauke has already left these shores and is now undertaking postdoctoral work at the Leibniz-Institut für Astrophysik Potsdam, where, on arrival, he unexpectedly grew an umlaut. Congratulations Hauke!

Labels: 2015, /postgrads

The Rapid Burster is a neutron-star binary that exhibits singular behaviour; extremely frequent "type-II" X-ray bursts, which cannot be thermonuclear in origin. The standard explanation of these bursts is that they arise from episodic accretion onto the neutron star, and it is this accretion which also fuels the (mostly) independent thermonuclear (type-I) bursts. The Rossi X-ray Timing Explorer made many observations (totalling 2.4 Ms, or almost a month of continuous observing!) of the source over multiple transient outbursts throughout the mission lifetime, building up an impressive database of burst behaviour. SRON PhD student Tullio Bagnoli, having already made a detailed study of the thermonuclear events in this source, has now completed a study of the type-II events, examining the demographics of the almost 8500 bursts detected by RXTE. The singular behaviour of this source may be explained by a special combination of magnetic field strength, stellar spin period and alignment between the magnetic field and the spin axis. Tullio's paper was just accepted by MNRAS.

Read the paper arXiv:1502.03941

Labels: 2015, /postgrads

The NCA has completed and released the first draft of Australian Astronomy's Decadal plan for 2016—25, which you can download here. Now is your last chance for feedback on the committee's recommendations! To provide plenty of opportunity for contributions, a series of meetings will be held around the country in February. Melbourne's meeting will be held on 19^th February at Swinburne.

Labels: 2015, /meetings

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

I am pleased and proud to report that the award of Sammanani (Shakya) Premachandra's PhD degree was approved last week. Shakya's thesis, entitled "Precision ephemerides of neutron star binaries to assist gravitational wave searches: Sco X-1 & Cyg X-2" was submitted last September. Her thesis involved extensive analyses of optical data of candidate gravitational wave sources Scorpius X-1 and Cygnus X-2, with a view towards improving the precision of binary orbital parameters and hence making more sensitive searches for gravitational waves in future with advanced-LIGO. The system parameters she has established for the two best candidate sources have already been utilised by the LSC's continuous-wave group in a mock data challenge to compare various search algorithms. These parameters will also serve as crucial input parameters to the actual searches, expected as soon as advanced-LIGO comes on line later this year. Congratulations Shakya!

Labels: 2015, /postgrads