In December we kicked off a second international team with a meeting at Bern's International Space Science Institute. This time the theme is Thermonuclear Bursts: Probing Neutron Stars and their Accretion Environments, and we focus on photospheric radius-expansion (PRE) thermonuclear bursts, and burst oscillations.

PRE bursts are so bright that the pressure from the outgoing radiation field exceeds even the ultrastrong gravitational field of the neutron star on which they occur. These events are key diagnostics for measuring neutron star properties, but much of the observed behaviour is not well understood. The same can be said for burst oscillations, which occur during brief intervals in some bursts, and allow us to measure the neutron-star spin. Despite more than a decade of study, there is yet no consensus on the origin of the oscillations.

Bern itself was in full Christmas mode, with the Weihnachtsmarkt in full swing, and the streets flowed with gluwein. Michael Zamfir and I managed a day skiing up at Grindlewald, in the shadow of the magnificent Eiger.

Labels: 2012, /meetings

The high-energy (and gravitational wave) astrophysics community in Australia has always been a bit under the radar, with a relatively small number of internationally recognised (but widely-dispersed) researchers. From 2006—9 I scheduled regular teleconferences to bring together researchers in the field, which provided a great way for this community to interact and share information. With the loss of a few key participants in 2009, these telecons ceased. However, with the influx of new researchers to astrophysics over the last few years, thanks to the Super Science fellowship scheme and other initiatives, the number of researchers with some interest in this field is larger than perhaps any time before. As a result, I'm happy to announce the first High-Energy Astrophysics Telecon (or HEAT) for 2012. Curtin's James Miller-Jones will report on recently-discovered evidence for intermediate-mass black holes in M22.

HEAT website next telecon 2pm AEDT 23^rd October 2012

Labels: 2012, /meetings

I'm in Toulouse this week to attend the second science meeting dedicated to ESA's proposed Large Observatory for X-ray Timing (LOFT) mission. Almost a year has passed since the last meeting, and detailed studies by the various science teams in that time have begun to clarify the observational prospects offered by the mission. I presented a poster describing some simulations of spectral features which might be observed from thermonuclear bursts.

Thermonuclear burst spectroscopy with LOFT (0.7 MB PDF)

Labels: 2012, /missions

The third accreting millisecond pulsar discovered in 2011 August by ESA's INTEGRAL satellite, IGR J17498-2921 was found by RXTE to pulse at 401 Hz. Although this might seem fast (401 times every second!), it is nowhere near the record for a millsecond X-ray pulsar (599 Hz, for IGR J00291+5934) or even for a radio pulsar (716 Hz, for PSR J1748-2446ad) Subsequent observations revealed Doppler modulations of the pulse indicating a 3.8 hr orbit, as well as thermonuclear burst activity. ISSI's Maurizio Falanga led a paper on the properties of the source and its behaviour throughout the outburst. The paper has just been accepted by A&A.
Read the paper (arXiv:1208.1384)

Labels: 2012, /pulsars

This month we were lucky enough to have a sizeable group of thermonuclear burst experts in town, both visitors and attendees to the August Nuclei in the Cosmos meeting. We arranged a workshop-of-opportunity at Monash on July 27^th, with a day of talks and discussions on a wide range of topics of interest for thermonuclear burst aficionados. The workshop dinner at the palatial Notting Hill Hotel rounded out a very enjoyable day. I gave a talk on the status of burst observations, while MoCA students Nathanael Lampe and Hauke Worpel also presented their work.
Workshop website

Labels: 2012, /meetings

The 39^th COSPAR Scientific Assembly was held this week at the remarkable Infosys Global Education Centre, Mysore, India. I gave an invited talk on observations of thermonuclear bursts to session E1.3, Multi-wavelength Studies of Compact Objects with Focus on ASTROSAT. ASTROSAT is a multi-wavelength astronomy mission under development by the Indian Space Research Organisation. The satellite features a large-area proportional counter (similar to the RXTE/PCA), as well as two imaging instruments with good low-energy X-ray sensitivity, an all-sky monitor, and an optical instrument. The capabilities are well suited to burst studies, and it will be very exciting to see what new discoveries will follow the launch (expected in 2013). I also gave a talk in session E1.7, Beyond the Continuum: X-ray Line Spectroscopy of Compact Objects. Although a rather difficult trip, it was also productive, and I managed to catch up with many collaborators and friends, as well as visiting India for the first time.
Observations of thermonuclear bursts (2.0MB PDF)

Labels: 2012, /meetings

The Astronomical Society of Australia held its Annual Scientific Meeting at UNSW's Kensington Campus in Sydney this week. It is always a good chance to catch up with old friends and colleagues, and this instalment was no exception. I was particularly pleased to see a strong showing from MoCA, including students Hauke Worpel, Sammanani (Shakya) Premachandra, and Nathanael Lampe, who all presented posters. Perhaps it is unreasonable of me to complain about having to compete with the announcement of the likely discovery of the Higgs boson which occurred during my talk — at least it should never happen again!

Labels: 2012, /meetings

Last week I visited Hannover for the Gravitational Wave Physics and Astronomy workshop, hosted by AEI-Hannover. I first attended this series of meetings, previously GWDAW, back in 2011; the attendees are a very interesting confluence of astrophysicists and gravitational-wave experimentalists. I presented the recent work with Shakya Premachandra on the orbital period of Sco X-1. I was also called in at the last minute to give a talk on the status and capabilities of the Australian Square Kilometre Array Pathfinder and the Mileura Widefield Array, replacing David Kaplan, who had to leave. The prospects for radio followup of future detections of graviational waves with advanced interferometers are very exciting! We also had the chance to visit the GEO600 interferometric gravitational wave detector, just outside Hannover, which was great (although rainy).

Tuning up for Gravitational Wave Detection in Accreting Neutron Stars: a progress report (3.2MB PDF)
Searching for Radio Transients with ASKAP (and MWA) (18.7MB PDF)

Labels: 2012, /gravitational waves

Regular, mixed H/He thermonuclear bursts have long served as valuable comparisons for numerical ignition models. Such bursts are observed only from a few systems, and it remains a mystery why this mode of bursting is so rare. GS 1826-24 is well-known for it's consistently regular bursts, but similarly regular bursts have also been observed from the long-duration (now quiescent) transient KS 1731-26. These bursts may also serve as important test-cases for understanding the burst X-ray spectrum and how it may be used to constrain the neutron star EOS.
Recently Nathanael Lampe & I published a paper examining the consistency of radius measurements from samples of highly consistent, regular bursts. We found that the blackbody normalisation (from which the radius can be determined) can vary significantly between burst epochs. This may partly be attributed to variations in the spectral correction factor f_c during some bursts.
In parallel, Michael Zamfir and Andrew Cumming developed a way to constrain the surface redshift, and hence the neutron star mass and radius based on these bursts. (I had done something similar on the bursts from KS 1731-26 a few years ago with a 3rd-year student at Monash, Richard Linossi, but Michael & Andrew took this approach much further). There remains a lot of uncertainty about how best to extract meaningful measurements from bursts, but we are beginning to better understand the systematics that affect this approach. This is also timely as other groups have already derived mass and radius constraints from KS 1731-26 (Özel et al. 2012).

Galloway & Lampe 2012 (ApJ 747, #75)
Zamfir et al. 2012 (ApJ 749, #69)

Labels: 2012, /neutron star EOS

March saw me back in Switzerland taking part in the second of two meetings at the International Space Science Institute on the subject of thermonuclear bursts. The meeting was a lot more focused than the first and we made plenty of progress towards our project goals. I was tasked with preparing a catalog of long (intermediate-duration) bursts which is already taking shape on the team wiki. Highlights of the trip were the excursion to Gruyeres and Maison Cailler, the home of some spectacularly good chocolate!

Labels: 2012, /meetings

This week I visited Aspen, CO for the Physics of Astronomical Transients meeting, at the remarkable Aspen Center for Physics. There is an increasing focus on transient astronomy at present, and astrophysicsts are racing to understand the physics of new phenomena that are being discovered by wide-field, automated instruments and observing programs such as NASA's Swift satellite, the Pan-STARRS program, and others. I presented a poster on the Variables and Slow Transients program for the Australian Square Kilometre Array Pathfinder (ASKAP) which is expected to uncover a wealth of new transient discoveries when it commences in 2013 or so.

Survey Science with ASKAP: Variables and Slow Transients (VAST) (8.5 MB vector PDF)

Labels: 2012, /transients

Late last year I was interviewed by Pratap for his excellent science blog Brains Matter. Give it a listen for a brief intro to my research on neutron stars

Labels: 2012, /outreach

As reported by NASA, the Rossi X-ray Timing Explorer has made its last observation, on January 3^rd this year. The spacecraft, launched in December 1995, had a tremendous impact on the study of accreting neutron stars, which has been celebrated this week with a special session at the 219^th AAS meeting in Austin, Texas. RXTE played a very personal role for me, having practically made my career; my PhD thesis was largely based on early observations of a high-magnetic field accretion-powered pulsar. Since then I've analysed many thousands of hours of data on millisecond pulsars, thermonuclear burst sources, and other exotica, and most of my published work has been based on these analyses. The loss of RXTE means the end to fast X-ray timing for the time being, at least until the much-anticipated launch of India's ASTROSAT satellite.

Labels: 2012, /missions