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