ASA 2008, Perth WA

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)

New outburst of IGR J17473-2721

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

Burst catalog paper accepted

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

US/Netherlands tour

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

Biases for neutron-star mass, radius and distance measurements

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)

The Clocked Burster is running fast

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)

Thermonuclear bursts observed by RXTE

The long-awaited catalog of bursts observed by RXTE has finally been accepted by ApJS, and is also out on astro-ph/0608259. The preprint version omits the extended figures; click below for a full abstract, more complete versions, and data tables.

>> Read more Abstract

We have assembled a sample of 1187 thermonuclear (type-I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than ten years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate.
We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that in general the peak flux of such bursts varies typically by 13%, We classified sources into two main groups based on the burst properties: both long and short bursts (indicating mixed H/He accretion), and consistently short bursts (primarily He accretion), and calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at 0.06 Mdot_Edd (>~2x10³⁷ erg/s) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He-burning. We found many examples of bursts with recurrence times <30 min, including burst triplets and even quadruplets.
We describe the oscillation amplitudes for 13 of the 16 burst oscillation sources, as well as the stages and properties of the bursts in which the oscillations are detected. The burst properties are correlated with the burst oscillation frequency; sources spinning at <400 Hz generally have consistently short bursts, while the more rapidly-spinning systems have both long and short bursts. This correlation suggests either that shear-mediated mixing dominates the burst properties, or alternatively that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution.

Preprint with complete figures (10 Mb PDF)
ASCII version of the burst data (table 5 in the accepted version) + IDL template for use with READ_ASCII
ASCII version of table 9 (burst oscillation properties)