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

The 599 Hz millisecond pulsar IGR J00291+5934 is a source I've been studying for more than a decade, having been involved in the discovery as well as observation of every transient outburst since. Although the first few outbursts displayed a remarkable predictability, the most recent, in 2015, was almost 3 years "late". In a detailed study of the INTEGRAL and Swift observations of the outburst, ISSI PhD student Vittorio De Falco examined the broad-band X-ray spectrum and the pulse properties, as well as data covering the first thermonuclear burst ever detected from the source. Remarkably, the estimated time-averaged accretion rate is decreasing on a timescale of only a few years, suggesting that we are unlikely to see another bright outburst in the next decade, if ever. Vittorio's paper was just accepted by A&A.

Read the paper arXiv:1611.08218

Labels: 2016, /pulsars

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

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

This has been a fun few months for those interested in accretion-powered millisecond pulsars (and who isn't?) We've been observing HETE J1900.1-2455 for more than 3 years now, it has had by far the longest period of activity of any of the 8 known systems. Then, in August, IGR J00291+5934 went into outburst for the first time since it's December 2004 outburst. The X-ray flux faded shortly after, but the system unexpectedly returned to activity about a month later. To top it all off, the first-ever AMSP, SAX J1808.4-3658, also went into outburst in late September. Three AMSPs active at the same time is a terrific coincidence.

But there's more. Back in 2007 I made some predictions for the time of the next outburst for these two systems, based on the observed outbursts to date (see the table below). These predictions were remarkably accurate, with errors of 11 and 20 days (or 0.7 and 1.7%), for the two sources. For the record, I'm now prepared to make my predictions for the next outburst for each of these systems; for SAX J1808.4-3658 on 2012 May 15 (MJD 56062) and for IGR J00291+5934 a few months later on 2012 September 23 (MJD 56193). I hope to be able to report here how these new predictions hold up!
Table: Predictions of 2008 outburst times for two AMSPs (arXiv.org:0711.4420)

	Outburst time (MJD)
Source	predicted	actual	Error (d)
IGR J00291+5934	54680	54691	11
SAX J1808.4-3658	54710	54730	20

Labels: 2008, /pulsars

Using NASA’s Swift and Rossi X-ray Timing Explorer (RXTE) satellites, astronomers have discovered one of the most bizarre planet-mass objects ever found. The object’s minimum mass is only about 7 times the mass of Jupiter. But instead of orbiting a normal star, this low-mass body orbits a rapidly (182 Hz) spinning pulsar. It orbits the pulsar every 54.7 minutes at an average distance of only about 230,000 miles (slightly less than the Earth-Moon distance). [ from the NASA press release ]
As part of a team led by Hans Krimm (Goddard), we analysed the RXTE observations following the discovery of this system, Swift J1756.9-2508. Hans' paper has now been accepted by ApJL and has been posted on arXiv.org. The story has shown up in a few places on the web, including the October 17 Monash Memo and even got a peripheral mention in a NY Times story

Read the paper (arXiv.org/0709.1693; ApJ 668, L147) or the
NASA press release

Labels: 2007, /pulsars

I'm not sorry to be wrong about HETE J1900.1-2455 returning to quiescence; Swift and RXTE observations following the nondetection last month indicate that the source is as bright as before. This is bad news for anticipated measurements of the quiescent flux with Chandra, but good news for our ongoing RXTE monitoring and pulsation searches.

Also last month the 8th accretion-powered millisecond pulsar was discovered, Swift J1756.9-2508. At 182 Hz the source is the slowest of the class (by 3 Hz); the 54-minute orbital period indicates an ultracompact (and thus likely H-poor) mass donor. The pulsar was only active for two weeks, and was undetected with Swift by June 21. A paper is in preparation by the Swift team.

Labels: 2007, /pulsars

The most recently-discovered millisecond pulsar, HETE J1900.1-2455, has finally begun to fade into quiescence. The last couple of observations in May saw the flux below 10^-10 erg cm^-2 s^-1, and fading, as reported in ATel #1086. Collaborators in the Netherlands, led by Rudy Wijnands, triggered a series of Swift observations, which confirmed the decline. Swift observations are ongoing, and a Chandra pointing is planned for the end of June. HETE J1900.1-2455 was active for almost 2 years — longer by far than any of the other accretion-powered millisecond pulsars. We're sorry to see it go!

Flux history (.PS) from RXTE

Labels: 2007, /pulsars