Regular, mixed H/He thermonuclear bursts have long served as valuable comparisons for numerical ignition models. Such bursts are observed only from a few systems, and it remains a mystery why this mode of bursting is so rare. GS 1826-24 is well-known for it's consistently regular bursts, but similarly regular bursts have also been observed from the long-duration (now quiescent) transient KS 1731-26. These bursts may also serve as important test-cases for understanding the burst X-ray spectrum and how it may be used to constrain the neutron star EOS.
Recently Nathanael Lampe & I published a paper examining the consistency of radius measurements from samples of highly consistent, regular bursts. We found that the blackbody normalisation (from which the radius can be determined) can vary significantly between burst epochs. This may partly be attributed to variations in the spectral correction factor f_c during some bursts.
In parallel, Michael Zamfir and Andrew Cumming developed a way to constrain the surface redshift, and hence the neutron star mass and radius based on these bursts. (I had done something similar on the bursts from KS 1731-26 a few years ago with a 3rd-year student at Monash, Richard Linossi, but Michael & Andrew took this approach much further). There remains a lot of uncertainty about how best to extract meaningful measurements from bursts, but we are beginning to better understand the systematics that affect this approach. This is also timely as other groups have already derived mass and radius constraints from KS 1731-26 (Özel et al. 2012).

Galloway & Lampe 2012 (ApJ 747, #75)
Zamfir et al. 2012 (ApJ 749, #69)

Labels: 2012, /neutron star EOS

The Astronomical Society of Australia held it's meeting this week at U. Melbourne. This annual event brings astronomers from all over the country (and the world) together for a week to catch up, present their research, and also award prizes to researchers and students. I gave a poster describing work on using mixed H/He thermonuclear bursts to deduce the neutron star redshift. Zdenka gave a talk on preliminary results from her comparisons of burst properties and models in the candidate He accretor 4U 1728-34. It was nice to be back on campus at U.Melbourne for a week — great location and even better coffee!

Measuring neutron star parameters from mixed H/He thermonuclear bursts (1.1MB PDF)

Labels: 2009, /neutron star EOS

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