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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 mpact 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

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

Mon Jul 10, 2017

GOTO La Palma site inauguration

This month saw the official inauguration of Warwick's La Palma observing site, now hosting the Gravitational-wave Optical Transient Observer (GOTO) instrument. The July 3rd event celebrated the successful deployment of the GOTO prototype in the previous month. The press releases from Monash and Warwick were reposted in Space.com and Phys.org, amongst other channels (TV La Palma!) The project is a collaboration between Warwick and Monash, along with UK partners Sheffield, Leicester, and Armagh Universities, and the National Astronomical Research Institute of Thailand (NARIT).

Labels: 2017, /press

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

Tue Dec 06, 2016

The last outburst of a millisecond pulsar?

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

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