PhD student Adelle Goodwin has been working hard trying to reconcile results of our 1-D time-dependent thermonuclear burst code Kepler with those of a much simpler (and much faster) code while trying to model the bursts from a millisecond pulsar, SAX 1808.4—3658. Continuing failure of this effort led to suspicion about the formula for the nuclear energy yield, and some careful experiments with Kepler revealed that the widely-used approximation for the energy generation adopted for the simpler code substantially overestimated the contribution lost as neutrinos. Adelle's paper on the study has now been published by ApJ. Adelle's work was also highlighted in the 2018 December edition of the JINA-CEE Newsletter.

Read the paper (ApJ 870, 64, 2019)

Labels: 2018, /postgrads

The Large Synoptic Survey Telescope (LSST) is a truly impressive instrument for the next decade. The 8.4m telescope will illuminate a 3200 Mpix camera, and carry out a 10-year survey of the sky expected to provide millions of transient alerts... every night. Many astronomers in Australia are keen to get their hands on the data which is expected to start flowing around 2023. The Australian Science Lead, Sarah Brough (UNSW) along with Astronomy Australia Ltd. is coordinating a joint national membership which would allow access to the data for Australian astronomers, and this week my application to be one of the Principal Investigators was approved! Next step is to join one (or more) of the science groups and think about all the fun we can have when the telescope comes on-line. I'm also hoping to attend theaupcoming LSST@Asia meeting in Sydney, 20—23 May 2019.

Labels: 2018, /transients

Another possible site for the southern counterpart of our GOTO instrument on La Palma is USQ's Mt. Kent observatory. About 30 min drive from Toowoomba, Mt. Kent is significantly easier to get to compared to Siding Spring, via the recently-upgraded Wellcamp airport, and offers comparable observing conditions. Even better, they already have some spare concrete pads that might suit a GOTO-sized instrument! Earlier in the year we submitted a funding proposal to support development of the telescope from 2019 onwards. Fingers crossed!

Labels: 2018, /gravitational waves

The 15^th Nuclei in the Cosmos meeting was held at the Gran Sasso National Laboratory, near Assergi, Italy. Monash had a pretty good showing with PhD students Adelle Goodwin, Zac Johnston and Melanie Hampel all in attendance. I presented a talk bringing together some of our recent work on measuring neutron star properties from thermonuclear bursts. A highlight was the conference banquet, at the wonderful Magione Papale, al fresco dining on a beautiful summer's evening topped off with a remarkable fireworks show.

The meeting setting offered a terrific opportunity to explore the area; from the earthquake-damaged (and still not fully recovered) town of L'Aquila, to the mountains and valleys surrounding the nearby village of Assergi, an unlikely place to find an Australian hotel owner!

Measuring neutron star properties from thermonuclear bursts (8 MB PDF)

Labels: 2018, /meetings

This month I hosted the Burst Environments, Reactions and Numerical modelling workshop at Monash's Prato Centre, near Florence. Inspired by a series of ISSI international teams over the last few years, the meeting featured a selected group of attendees with the goal of bringing together researchers working on observational, numerical and nuclear experimental investigations of thermonuclear (type I) bursts. The centre proved an excellent venue for the great presentations and discussions we had over the week. I'm very grateful to JINA-CEE and MoCA for sponsoring the event, as well as the organising committee as well as Adelle and Zac for the website and their help organising. Hopefully these meetings will continue further, perhaps in Leiden in 2019!

Workshop website

Labels: 2018, /meetings

The legacy of NASA's Rossi X-ray Timing Explorer continues to grow with the discovery of another burst oscillation in archival data from bursts observed more than 10 years ago, from the faint transient SAX J1810.8--2609. These burst oscillations, revealed by the careful work of Anya Bilous, confirm the spin frequency of the neutron star in this system at 532 Hz (that's an amazing 532 rotations every second!). This discovery adds to only a few tens of such measurements amongst the more than 100 known bursting sources. Anya's paper is now accepted by ApJ.

A Millisecond Oscillation in the Bursting X-Ray Flux of SAX J1810.8-2609 (Bilous et al. 2018) via NASA ADS

Labels: 2018, /thermonuclear bursts

This year's meeting (the 8th in the series!) was held in lovely South Bend, IN. As usual the meeting covered a wide range of nuclear astrophysics topics as well as the excellent attendee-driven "unconference" sessions. Donald Clayton gave a great historical overview in his opening talk. I presented a poster on the progress in our efforts to reconcile burst models and observations.
We also enjoyed the excellent share bike facilities and the great bike path along the river, conveniently leading the way to the Crooked Ewe

Reconciling observations and models of thermonuclear bursts: a progress report (0.5 MB PDF)

Labels: 2018, /meetings

This week the (rapidly expanding) GOTO collaboration met at Warwick U. We heard talks about the exciting science following the detection of GW170817, as well as the instrument and software development. It was great to hear from new prospective collaboration partners Rene Breton (U. Manchester) and Seppo Mattila (U. Turku). There is still a lot to do before the LIGO observations resume (expected in early 2019), but we are on-track to be ready to detect optical counterparts!

Labels: 2018, /meetings

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

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

This week I'm visiting Coonabarabran to check out Siding Spring Observatory as a possible site of the southern counterpart of our GOTO instrument on La Palma. Siting a second instrument in Australia willdramatically improve our ability to cover the large sky regions that LIGO produces in response to the detection of a binary neutron star or black hole merger. Later this year we plan to submit a funding proposal to support development of the telescope from 2019 onwards. Stay tuned!

Labels: 2018, /gravitational waves