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Mon Jun 08, 2020

Observing outburst onset in a low-mass X-ray binary

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 236th 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

Thu Apr 30, 2020

MINBAR paper accepted!

In late February we submitted the paper describing data release 1 (DR1) of the Multi-Instrument Burst Archive (MINBAR) to Astrophysical Journal Supplements. In a remakably quick turnaround we got a referee's report in less than a month, and the lightly revised version of the paper is now accepted!

Accompanying the paper is the online repository hosted at Monash's Bridges (formerly figshare) service. Here you can download the ASCCI files with the complete contents of the catalog, and (in future) will also provide links to software used to access and analyse the data.

Another way to access the data is via the web interface at burst.sci.monash.edu, which allows basic searching and plotting facility, and you can download subsets of the data for offline analysis too.

Read the paper (arXiv:2003.00685)

Labels: 2020, /thermonuclear bursts

Mon Sep 16, 2019

Fingerprinting X-ray bursters

The first accretion-powered millisecond pulsar ever found, SAX 1808.4—3658 is a remarkable object, also showing thermonuclear bursts during it's infrequent outbursts. Over several years PhD student Adelle Goodwin has been trying to match the bursts observed during it's 2002 outburst with the predictions of a numerical model. This exercise can be thought of as searching exhaustively for a set of parameters which (in the model) reproduce the observed properties of the bursts. Now that we have an improved formula for determining the energy of these bursts, she built a Markov-chain Monte-Carlo (MCMC) code to constrain system parameters including neutron star mass and radius for the source. Her paper describing the study, and it's results, has now been published by MNRAS

Read the paper (MNRAS 490, 2228, 2019)

Labels: 2019, /postgrads

Mon Jul 01, 2019

Bursting the Bubble workshop

In June I attended the Lorentz Center workshop Bursting the Bubble: Connecting Thermonuclear Burst Research to a Wider Community in Leiden, NL. This meeting is the continuation of a series of meetings on the same topic, most recently with the BERN'18 meeting, but originally a series of ISSI international teams since about 2010. This year's meeting featured a selected group of attendees with the goal of bringing our burst research to a wider community, most notably those observers interested in novae, the WD analogues of thermonuclear bursts. A highlight for me was Bill Wolf's tutorial session on simulating bursts and novae with MESA. The centre proved terrific facilities and support for the great presentations and discussions we had over the week. As with previous meetings, JINA-CEE was a generous sponsor of the event, with support specifically aimed at enabling attendance by early-career scientists. Thanks also go to the dedicated team of organisers led by the irrepressible Anna Watts!

Labels: 2019, /meetings

Tue Apr 16, 2019

A brand-new bursting X-ray binary

While examining RXTE data for the MINBAR project two summers ago, Jean in 't Zand unexpectedly discovered a brand-new bursting source, XMMU J181227.8—181234. Now PhD student Adelle Goodwin's paper on the source has been accepted by MNRAS. The source is quite unusual; distant and hence low-intensity, but apparently a high accretion rate leading to frequent (but weak) bursts. The shape of the bursts strongly implies H-poor accreted composition, but we also found evidence for short recurrence-time bursts, which previously have been seen only in H-rich accretors. Hopefully the source will go into outburst again sometime soon and we can gather more information on it's properties.

Read the paper (MNRAS 486, 4149, 2019)

Labels: 2019, /postgrads

Fri Apr 05, 2019

Australia-China Workshop on Astrophysics

The 5th Australia-China Workshop on Astrophysics was held in Healesville, VIC at RACV's Country Club resort this April. The meeting continues a series of events intended to foster cooperation between the two countries, and as my first attendance, I was very impressed with the level of cooperation alredy. A host of prominent Chinese and Australian astronomers were in attendance, and presentations were also made by students undertaking projects enabled made possible via cooperative programs. I gave a talk on X-ray binaries and the possibilities for joint observational programs, particularly with Insight-HXMT and the proposed eXTP. Exciting times ahead for cross-country cooperation!

Labels: 2019, /meetings

Mon Jan 21, 2019

Cavecchi visit

Longtime collaborator and friend of bursts Yuri Cavecchi visited Monash this month, supported by the MoCA Distinguished Visitor Program. Yuri and I have been working for a few years now on the global properties of thermonuclear bursts, particuarly the ignition conditions and how they respond to changes in accretion rate, motivated by observations with MINBAR. Yuri had already come up with a possible explanation for the drop in burst rates that is observed as accretion rates increase in some sources; during the visit we plan to work on matching the models in more detail to the observations. Stay tuned for exciting results coming soon!

Labels: 2019, /visits

Mon Nov 05, 2018

Measuring neutrino losses in thermonuclear bursts

PhD student Adelle Goodwin has been working hard trying to reconcile results of our 1-D time-dependent thermonuclear burst code Kepler with those of a much simpler (and much faster) code while trying to model the bursts from a millisecond pulsar, SAX 1808.4—3658. Continuing failure of this effort led to suspicion about the formula for the nuclear energy yield, and some careful experiments with Kepler revealed that the widely-used approximation for the energy generation adopted for the simpler code substantially overestimated the contribution lost as neutrinos. Adelle's paper on the study has now been published by ApJ. Adelle's work was also highlighted in the 2018 December edition of the JINA-CEE Newsletter.

Read the paper (ApJ 870, 64, 2019)

Labels: 2018, /postgrads