Bright X-ray binaries are thought to be promising candidates for detecting continuous gravitational waves, because it is possible that the spin-up torques provided by accretion are offset by gravitational wave emission to maintain their equilibrium spin periods. A long-standing project led by Monash PhD student Shanika Galaudage has sought to use the most sensitive X-ray data available for the two best candidates, Sco X-1 and Cyg X-2, to make a deep search for pulsations that would measure the spin of the compact object in these systems. The analysis was extremely challenging, particularly for Sco X-1; that source is so bright that highly unusual data modes need to be selected for observations with the RXTE/PCA, which complicates the data reduction. Unfortunately we found no strong evidence of pulsations, but we did derive the deepest limits on pulse amplitudes ever. The paper describing our results has now been accepted by MNRAS.

Read the paper

Labels: 2021, /gravitational waves

Our GOTO prototype, while optimised for gravitational-wave followups, is also well suited to detecting optical counterparts of gamma-ray bursts detected by Fermi. Monash PhD candidate Yik Lun (Travis) Mong recently completed his analysis of our followup of 93 such events, focusing on 53 with favourable observational conditions. Unfortunately we couldn't confirm any counterparts (although he did find some intriguing candidates!)

Perhaps more importantly, Travis' analysis offers some lessons for future followup campaigns to help improve our chances of success. Travis' paper, in collaboration with the GOTO team, has been accepted by MNRAS

Read the paper (arXiv:2108.11802)

Labels: 2021, /postgrads

So-called AM CVn systems are the cataclysmic variables (CVs) with the shortest orbital periods, down to as short as 5 minutes. CVs in turn are binary systems consisting of a white dwarf accreting from a low mass companion. These systems are the white dwarf analogue of low-mass X-ray binaries, and exhibit much behaviour that is qualitatively similar, including accretion outbursts.

Armagh Observatory PhD student Chris Duffy used data from GOTO as well as several other sky surveys to examine the behaviour of a group of systems with basically identical orbital periods. Chris found that despite the common orbital periods, the outbursting behaviour was quite diverse, which has implications for the disk instability model thought to drive this behaviour.

Chris' paper was just accepted by MNRAS.

Read the paper (arXiv:2102.04428)

Labels: 2021, /white dwarf

The web interface to the MINBAR burst sample is the result of a collaboration with the Monash eResearch Centre. The interface offers data selection and filtering tools, as well as output as ASCII for offline analysis and basic plotting.

We'd always envisaged offering more visualisation capabilities, but had not managed to implement them, until now. When you search the burst interface, you'll see a "wiki" link for each entry. Clicking on this link will bring up a page listing all the analysis results for this event, along with a dynamic plot of the burst lightcurve, and link to the time-resolved spectroscopy (where available). Click here (#2582) for a beautiful example from GX 17+2.

The same lightcurve files are now also accessible via the Python repository, using the get_lc method. We're also working on a method to return the time-resolved spectroscopy.

Labels: 2021, /thermonuclear bursts

The 43^rd COSPAR Scientific Assembly was to be held in Sydney last August, but thanks to the COVID-19 pandemic, it was delayed until this week. Sudip Bhattacharya and I led a session on "Rapidly Rotating Neutron Stars", which featured a terrific set of presentations on radio and X-ray pulsars, as well as gravitational waves.

Monash PhD student Ka Ho Tse presented some of his recent work on the enigmatic mHz quasi-periodic oscillations observed in some burst sources, suggestive of quasi-stable nuclear burning but at accretion rates an order of magnitude below where we would expect them.

The COSPAR organisers imposed a very complex virtual program, with speakers required to pre-record full duration talks as well as 3-minute summaries. The platform was overly complicated, and plagued with technical issues; on top of that, having to go on your own time to view the full-duration talks kind of defeats the purpose of the session.

I'm not sure how well it worked for attendees, but for session organisers it was a nightmare, and I was glad when it was over. Some organisers of other sessions abandoned the platform altogether and switched to Zoom. These large virtual conferences are very challenging to set up, and it's all well and good to be ambitious with the format, but it would have been much better to keep it simple. It was also a bit galling for the organisers to blame the chairs and speakers in the closing address.

Labels: 2021, /meetings

This week GOTO postdoctoral researcher and control system guru Martin Dyer presented two papers at the (virtual) SPIE Astronomical Telescopes + Instumentation 2020 meeting. Like many 2020 meetings, this one had a challenging path to actually occurring, moving twice (from to Japan to California) and finally to the fully online format.

Martin's first paper offers a brief summary of the project, describing the hardware, software and future plans including the new "GOTO-South" node to be constructed at Siding Spring Observatory, NSW. The second paper describes development of the "G-TeCS" control system, which was much of Martin's thesis project, and serves as the critical "nervous system" of the facility, without which we could not operate.

Labels: 2020, /meetings

Millihertz quasi-periodic oscillations (mHz QPOs) observed in neutron-star low-mass X-ray binaries are generally thought to arise from "marginally stable" thermonuclear burning, at the upper limit of accretion rates where thermonuclear bursts are possible. A long-standing puzzle is that they seem to occur at accretion rates about an order of magnitude lower than predicted theoretically, and while bursts are still occurring.

Since no-one knows how you can simultaneously have (quasi-) stable and unstable burning on the surface of a neutron star (except perhaps Yuri Cavecchi), we'd dearly like to better understand this phenomenon. Monash PhD student Ka Ho Tse discovered a new example of mHz QPOs, in the transient 1RXS J180408.9—342058. While the properties of the newly-discovered oscillations are similar to the other seven examples, we now have a good sample of objects to examine the properties and compare against (for example) numerical models. Ka Ho's preliminary work suggests the observed amplitude is much higher than would be predicted theoretically, which presents a challenge for the conventional interpretation.

His paper describing the discovery, and exploring some of the implications for the phenomenon across all sources, has now been published by MNRAS.

Read the paper (MNRAS 500, 34—39, 2021)

Labels: 2020, /postgrads

SAX J1808.4—3658 is the first accretion-powered millisecond pulsar ever discovered, and still one of the best-studied, thanks to a quasi-regular series of outbursts since 1996. Based on the regularity of the outbursts, I'd predicted that it would return to activity in 2019 May. This prediction was a bit off, as it turned out, since we didn't hear a peep out of it until late July in the same year. Even so, thanks to the prediction and a series of optical and X-ray observations led by Monash PhD student Adelle Goodwin, we gathered data giving a view of the outburst onset of unprecedented detail.

Adelle presented her work at the 236^th AAS meeting last week, and also took part in a press event the same day. Our press release got picked up and distributed widely, at space.com, Science Daily,, phys.org,, CNET and even Nine news. Adelle also appeared on RRR's Einstein-a-go-go show and was interviewed by a number of other outlets.

On the science side, the unexpectedly long delay of 12d between the optical and X-ray activity may have implications for the most widely-accepted model which explains how these outbursts are triggered. Adelle's paper on the study has been submitted to MNRAS.

And the next outburst? You'll have to wait until 2023 November — or perhaps even later.

Read the paper arXiv:2006.02872

Labels: 2020, /pulsars