The 19^th Australian Institute of Physics Congress was held last week right here in Melbourne, so of course I couldn't miss it. I chaired and presented in the astronomy session on Monday, and also caught a few of the gravitational wave talks. Bruce Allen gave a great plenary session talk on Einstein @ home, the distributed analysis project for LIGO (and Arecibo pulsar) data. Bruce also stepped in to give the public lecture on Wednesday evening, the original speaker Jocelyn Bell-Burnell could not make it. On Thursday we also had a "town hall" style open session to discuss the LIGO-Australia project.

Labels: 2010, /gravitational waves

In one of the public 2008 observations of bursters by RXTE we noticed a new event from a relatively quiet source, 2S 0918-549. On closer inspection, Jean in 't Zand noticed a remarkable interval of strong variability late in this long (~300 s) event. This type of variability, which is achromatic (i.e. energy-independent) and seems to be restricted to bursts with extremely strong radius-expansion, may arise from disruption and later resettling of the accretion disk. Our paper has been accepted by A&A and is also out at arXiv:1009.5359.

Labels: 2010, /thermonuclear bursts

Thermonuclear bursts that show photospheric radius-expansion (PRE) are pretty rare in most sources, so much so that Chandra has only observed a few of them in a decade of observations. In 2006 we made an observation of a prolific PRE burster, 4U 1728-34, to try to catch some bursts and search for spectral features near the peak. We got plenty of bursts, but only 4 of them were likely PRE, and we saw no features. But, we did find evidence for an extremely weak periodic variation in the X-ray intensity, which if orbital in nature, means the neutron star orbits its mass donor once every 10.77 minutes! Our paper was just accepted by ApJ, and is available at arXiv:1009.2296.

Labels: 2010, /spectroscopy

A bunch of us have been working on the X-ray data from a quite remarkable transient, IGR J17473-2721, which went into a 6-month long outburst beginning in 2008. The source went through some dramatic changes in the burst behaviour, some of which we think we understand, but some of which we don't. In particular at the beginning and the end of the outburst completely different burst behaviour is observed at roughly the same intensity (accretion rate). This effect may be due to the heating of the neutron star core which has taken place during the outburst. Jerome's paper is out now on astro-ph at arXiv.org:1007.5201.

Labels: 2010, /transients

This July was a very busy month for meetings. It kicked off with the annual Astronomical Society of Australia meeting which was held this year in my old home town of Hobart, Tasmania. I presented some preliminary results from the PEGS project in a talk titled "Tuning up for Gravitational Wave detection in Accreting Neutron Stars". As is traditional the Harley Wood Winter School was held the weekend before; the theme was "Powerful Objects" and I gave a talk on accretion-powered neutron stars. Back to Melbourne for a week, and then off to Bremen, Germany for cospar¹⁰. I revised my ASA talk in Event H02, and also gave a talk on the discovery of burst oscillations in EXO 0748-676 in Event E13 as well as presenting a poster on MINBAR.
Then it was off to Leiden, NL for the Lorentz Center workshop on X-ray bursts and burst oscillations, where I gave an invited talk on the status of burst observations. The workshop was a terrific event, an excellent chance to catch up with colleagues (see the group photo) and see the state-of-the-art of burst research, as well as see a bit of Leiden.

Accreting neutron stars — Tiny Galactic Powerhouses — HWWS talk
Tuning up for Gravitational Wave detection in Accreting Neutron Stars — ASA 2010/COSPAR H02 talk
A 552 Hz Burst Oscillation in EXO 0748-676 — COSPAR E13 talk
The Multi-INstrument Burst ARchive — MINBAR — COSPAR E13 poster
Status of thermonuclear burst observations — Lorentz Center Workshop talk

Labels: 2010, /meetings

IGR J00291+5934, as well as being the fastest known accretion-powered millisecond pulsar, is unusual in that it's shown repeated outbursts that have been observed by RXTE. Jake Hartman, Deepto and I had been close to submitting a terrific paper doing the phase connection between the 2004 and 2008 outbursts, when to our dismay Alessandro Patruno posted a paper (arXiv:1006.0815) with the same timing analysis. Now in a rush to get ours out, three days later came a separate analysis from Papitto et al. (arXiv:1006.1303). Finally, two days later, ours was submitted, to the Astrophysical Journal, and to arXiv:1006.1908. Of course, I think ours is best, not least because Jake did a really nice analysis which interprets the shape of the unusual double 2008 outburst in terms of the disk ionisation structure. Now it is a race to see who will get published first!

Labels: 2010, /pulsars

Zdenka, Randy Cooper and I just completed a study of He-rich bursts from 4U 1728-34, which can occur as frequently as once every 2 hours. This rate cannot be matched by even the most optimistic ignition models. Although in principle these bursts could be used as indicators of additional heat sources contributed from the core or crust (that was our initial hope), it now seems more likely that some other mechanism is responsible for their frequency, perhaps because accretion does not cover the entire neutron star. Zdenka's paper has now been accepted by ApJ, and is available at arXiv:1005.4729.

Labels: 2010, /thermonuclear bursts

Laurens has just posted a nice paper looking at short recurrence time thermonuclear (type-I) bursts as seen by RXTE and BeppoSAX. These bursts occur after waiting times of as short as a few minutes, which is not sufficient to reach the critical conditions for ignition. The likely explanation is that the bursts ignite from fuel leftover from the previous event, but the details remain a bit of a mystery. Interestingly, short recurrence time bursts are not seen from sources (or candidate sources) that accrete primarily He. The paper was just accepted by ApJ.

Read the paper arXiv:1005.3302

Labels: 2010, /thermonuclear bursts

Yesterday George Hobbs from ATNF kindly hosted a teleconference for discussion of various approaches and tools available for astronomical databases. This was motivated by a number of queries he'd had about the ATNF Pulsar database and the related software. There are a variety of possible database management systems available (including the IDL solution I'd been using to date), so it is good to know a little bit more about their pros and cons. Also, there is much work being done, some in the context of the various virtual observatory efforts, in standardising astronomical data and methods of public access.
Below are some useful links and references that came up in the discussion.

• Tim Cornwall pointed to http://www.astrogrid.org which should be (as others also suggested) the first point of contact for making your data available/compatible to/with VO
• There is much software already available to view Virtual Observatory compatible data; see the VO-Software section at Euro-VO site, and the International Virtual Observatory Alliance (IVOA) for the data standards etc.
• Emil Lenc is using MySQL to store data related to the Australia Telescope Large Area Survey (ATLAS). Some suggested that his experience might be the most relevant in terms of people with small datasets wishing to manage them and/or make them public
• Russel Jurek (?) stood in for Baerbel Koribalski to talk about the HIPASS public data release; they have a nice webpage
• The other useful reference is last year's Astroinformatics School, which featured presentations on several topics relevant to this discussion

Labels: 2010, /meetings

I just got back from the American Astronomical Society's High-Energy Astrophysics Division meeting, which was held in the beautiful Hilton Waikoloa Village resort on the Big Island. On the work side it was very productive; I kicked off the conference on Monday morning with an invited talk on bursts, and also presented a poster on our recent EXO 0748-676 result. Jake Hartman also presented some excellent work on the 2008 outburst of IGR J00291+5934 (poster 42.15). Of course, there was also the chance to catch up with all our old friends and collaborators; this was the best attended HEAD meeting ever! But in a place this beautiful you can't spend your whole time working, so we also took some time to play in the lagoon, and with the dolphins, etc.
On the way back Jasmina & I visited some more friends and colleagues at University of Sydney, and gave some more talks, as well as visiting the amazing Powerhouse Museum, from which it was quite a challenge to extract Aleks...

Thermonuclear bursts and the neutron star equation of state (invited talk 01.01l)
A 552Hz burst oscillation in EXO 0748-676 (poster 42.23)

Labels: 2010, /meetings

Last December Danny Steeghs, Tom Marsh & I were awarded some funding to support a program of optical observations to measure (or improve) system parameters for bright X-ray binaries. These systems may produce detectable gravitational radiation, and optimal searches for these gravitational waves (e.g. with LIGO) depend on precise knowledge of the systems, including the orbital period and neutron star spin frequency. However, many of the best candidates are rather poorly known, so we hope to improve this situation. We hope to make our first observations this year, likely at La Palma, Canary Islands.

Labels: 2010, /gravitational waves