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

The annual meeting of the Astronomical Society of Australia was held this July in Perth, WA. I've never spent much time in Perth, so it was good to spend a little time over there. The UWA campus at Crawley is beautiful, right on the shore of the Swan river, and with fantastic architecture and gardens. We stayed at St. George's College, which is also very picturesque, and backs onto King's Park. The meeting was very well attended and a lot of fun. I gave an invited talk on searches for spectral features from radius-expansion bursts observed by RXTE

ASA 2008, Perth WA
ASA talk (2.9MB PDF)

Labels: 2008, /thermonuclear bursts

This system was discovered back in 2005, and was detected in outburst again this March. Since then, it's been observed by Swift, INTEGRAL and RXTE, and even SuperAGILE detected a thermonuclear burst (ATel #1445). Following reports of the bursting activity we triggered some observations with RXTE to search for burst oscillations; we caught some bursts, but no oscillations. Since then, public RXTE observations revealed kHz QPOs and radius expansion bursts, from which we estimated the distance (ATel #1651). Observations are continuing.
See also
INTEGRAL monitoring of IGR J17473-2721
Diego Altamirano's IGR J17473-2721 page
IGR J17473-2721 at SIMBAD

Labels: 2008, /transients

The burst catalog paper has just been accepted by ApJS! The referee report, by a team of referees, stretched to 11 pages; it took quite a while to address all the (generally constructive) criticisms. In addition, the authors, all at MIT when the project started, are now scattered to the four corners of the US (as well as Australia). The accepted version is (I think) a big improvement on the original, the extra time spent has really helped our understanding of the global burst properties. Check it out for yourself at astro-ph/0608259v2

Click here to read the abstract and download the full version, as well as data tables

Labels: 2008, /thermonuclear bursts

I just got back from a very busy trip around the US, with a stop in Amsterdam on the way home. I presented a couple of posters at the AAS/High Energy Astrophysics Division Meeting in Los Angeles, and went on to visit Ed Brown at JINA, where I gave a lunch talk on the following Monday.

Then it was on to Boston for a brief visit to MIT to catch up with collaborators and friends, as well as a pilgrimage to Toscaninis (sadly the Harvard Square location has closed). After that I had a couple of nights in St. Louis for the APS meeting, and gave an invited talk on burst observations in a special session on the physics of X-ray bursts (L3).

Finally I headed home via Amsterdam for another invited talk at the Decade of Accreting Millisecond X-ray Pulsars workshop at UvA.

HEAD posters 1 2 (0.24/1.22 MB PDF)
APS talk (10.2 MB PPT)
AMSPs workshop talk (1.5 MB PPT) and the proceedings paper

Labels: 2008, /thermonuclear bursts

Our paper on the unusually low "touchdown" fluxes for radius-expansion bursts from high-inclination sources was just accepted by MNRAS. Usually the touchdown flux is thought to equal the Eddington flux, but we found that in sources that show X-ray dips — likely arising from structure at the edge of the accretion disk passing across the line of sight, implying that we see these systems almost edge-on — the touchdown flux could be less than half the maximum flux seen earlier in the same burst. The low touchdown fluxes also likely arise from interactions with the disk material, which have some implications for neutron-star distance (but not mass and radius) determination following the method of Özel (2006)

Read the paper (arxiv.org:0712.0412)

Labels: 2008, /neutron star EOS

Summer 2007 was a big season for new results on GS 1826-24, the "Clocked Burster". UCSD student Tommy Thompson and I just submitted a new analysis of the X-ray flux–recurrence time relationship in this system. We found a few instances where the thermonuclear bursts — albeit still very regular — occurred more frequently that would be predicted by the relationship derived from a previous study. XMM-Newton observations during one of these episodes revealed the likely presence of an additional soft component, which may account for the "missing" flux. This source may also be useful in future for precision studies of the X-ray flux–accretion rate relationship, which can be measured precisely thanks to the regular bursts.

Read the (accepted) paper (arXiv:0712.3874)

Labels: 2008, /thermonuclear bursts