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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

Thu Jan 18, 2018

Surveying candidate sites for GOTO-South

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

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

Fri Nov 24, 2017

IAU Symposium #339 on Time-Domain Astronomy

This month I visited South Africa (and the African continent for the first time!) to attend the Southern Horizons in Time-Domain Astronomy meeting in Stellenbosch. The programme featured an exhaustive roster of hot topics in the transient realm right now — from kilonovae through FRBs to supernovae and stellar pulsations. A wide range of instrument development activities were reported, both world-wide and in South Africa, which is of course the host of the Southern African Large Telescope (SALT) and the Square Kilometer Array precursor MeerKAT As well there were workshops on relevant topics including astroinformatics and machine learning. The setting, in the midst of SA's wine regions was pretty hard to beat, and the chance to visit Cape Town — surely one of the most geographically stunning cities in the world — was a nice bonus. The program and most of the talks are now online so check it out!

Labels: 2017, /meetings

Wed Oct 18, 2017

Detection of an optical counterpart to a binary neutron star merger

The milestones just keep coming this year for the burgeoning field of gravitational wave astronomy. Closely following the first "triple coincidence" detection of a black hole merger (with the two LIGO instruments joined by the Italian Virgo instrument), comes the announcement yesterday of the first detection of a binary neutron-star merger, which was — spectacularly — accompanied by a gamma-ray transient detected by Fermi GBM. Things just got more exciting when an optical counterpart was detected for the gamma-ray and gravitational-wave source, with the Swope Telescope in Chile. The counterpart, named SSS17a, was subsequently followed by an estimated 70 different observational groups (including a range of Australian facilities coordinated via the OzGRav Centre of Excellence). A literal frenzy of activity followed, culminating in the LIGO press release in the early hours (Australian time) yesterday, and followed by our own press event at Old Parliament House yesterday morning. It is hard to quantify the impact of this event; the announcement was accompanied by an estimated 76 papers (with likely more to come). Sadly, our own GOTO telescope missed out on this event, but the optical brightness indicates that future events will be easily detectable, and validates our instrumental design. The prospects for additional detections when O3 begins (in late 2018) are excellent.

Labels: 2017, /gravitational waves

Wed Oct 04, 2017

2017 Nobel Prize in Physics to LIGO team

Rainer Weiss, Barry Barish and Kip Thorne were last night awarded the 2017 Nobel Prize for Physics, "for decisive contributions to the LIGO detector and the observation of gravitational waves". This award caps a terrific year for the collaboration, which (by my observation) has scooped pretty much all the major science awards. Here at Monash we're celebrating our involvement in the project, and looking forward to the (hopefully many) surprises in store for the future.

Labels: 2017, /press