Texas in Australia

The XXIIIth Texas Symposium, held here in Melbourne, concluded last week. All in all a terrific success, with a very interesting program full of great talks by stellar (sorry) attendees. There were plenty of international visitors, many of whom commented on how much they enjoyed their visit despite the long trip. I managed to catch up with quite few people I hadn't seen for a while, but for most the social highlight was Thursday's dinner at the NGV's Garden Restaurant.

Magnetic Field Burial Observed in an Accretion-Powered Millisecond Pulsar? (poster presented at the symposium, .PDF format)

2006 Transit of Mercury

On November 9^th we had a fun morning following the progress of the transit of Mercury. We climbed up onto the roof of the Physics building before dawn, but sadly missed first and second contact. Later on we moved down to the court east of the Student Union, where we had a steady stream of onlookers curious about what we were doing with a large telescope and an assortment of whiteboards. My own photos of the projected image didn't come out too well, but Ivy's were much better, and she put together this composite image illustrating Mercury's progress across the disk of the sun.

See also the writeup in the School of Physics Alumni Newsletter issue 5, Dec 2006

Intermittent pulsations in HETE J1900.1-2455

The Energizer Bunny of the accretion-powered millisecond pulsars is unusual not just because it has been active for more than a year since its discovery in June 2005 ("he's still goin'!"). Pulsations were only detected in the first few months of the outburst; since then, the source has been indistinguishable from an ordinary low-accretion rate LMXB. In all the other millisecond X-ray pulsars (and, for that matter, all other accretion-powered pulsars) pulsations are always present when the source is active. Our paper on the properties of the pulsations is out now at astro-ph/0609693.

HEAT is... well, hot

We just had the first High-Energy Astrophysics Teleconference (HEAT) this afternoon, hosted by yours truly. The idea is to bring observers and theorists interested in high-energy astrophysics together on a regular basis, to share information, present new results, and generally build our small, diverse, and highly dispersed community. We had a round of introductions followed by a little science discussion, and suggestions for future topics and presentations; everthing went very well. The next meeting is scheduled for Friday October 13th at noon — please email me if you are interested in taking part.

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)

Helium-rich bursts and the distance to SAX J1808.4-3658

Our paper on the outburst and thermonuclear burst properties of SAX J1808.4-3658 was just accepted by ApJ. We used RXTE observations to constrain the distance, deriving a likely range of 3.4-3.6 kpc. We also compared the burst properties to Andrew C's ignition model to deduce the H-fraction at ignition; these bursts are the first confirmed He-rich bursts which have been studied in detail.
Read the paper

>> Full abstract Abstract:
We analysed Rossi X-ray Timing Explorer observations of the accretion-powered 401 Hz pulsar SAX J1808.4-3658, in order to precisely determine the source distance. While the fluences for the five transient outbursts observed from 1996 were constant to within the uncertainties, the outburst interval varied signficantly, so that the time-averaged flux (and accretion rate) decreased by around 40%. By equating the time-averaged X-ray flux with the expected mass transfer rate from gravitational radiation, we derived a lower limit on the distance of 3.4 kpc. Combined with an upper limit from assuming that the four radius-expansion thermonuclear bursts observed during the 2002 October outburst reached at most the Eddington limit for a pure He atmosphere, we found that the probable distance range for the source is 3.4-3.6 kpc. The implied inclination, based on the optical/IR properties of the counterpart, is i<~30 degrees.

We compared the properties of the bursts with an ignition model. The time between bursts was long enough for hot CNO burning to significantly deplete the accreted hydrogen, so that ignition occurred in a pure helium layer underlying a stable hydrogen burning shell. This is the first time that this burning regime has been securely observationally identified. The observed energetics of the bursts give a mean hydrogen fraction at ignition of approx. 0.1, and require that the accreted hydrogen fraction X_0 and the CNO metallicity Z_CNO are related by Z_CNO approx. 0.03(X_0/0.7)^2. We show that in this burning regime, a measurement of the burst recurrence time and energetics allows the local accretion rate onto the star to be determined independently of the accreted composition, giving a new method for estimating the source distance which is in good agreement with our other estimates.

Thermonuclear bursts from the candidate ultracompact binary 1A 1246-588

Some activity recently from an old Ariel V source, 1A 1246-588. It's listed in SIMBAD as a high-mass binary, but recent optical observations instead suggest that it is an ultracompact binary with a very low-mass donor. Then, in late May, the RXTE/ASM detected a flare, which subsequent analysis showed to be consistent with a long thermonuclear burst. (An earlier burst was detected by BeppoSAX). These types of bursts occur in very low-accretion rate sources where a deep fuel layer can build up prior to ignition. There are only a handful of ultracompact binaries in the Galaxy, and substantial interest in their properties and evolution.

See also ATel 875

MIRAX workshop proceedings

Here's the paper "Accretion-powered Millisecond Pulsar Outbursts" I submitted to the proceedings of the MIRAX workshop last December.

>> Click for the abstract Abstract: The population of accretion-powered millisecond pulsars has grown rapidly over the last four years, with the discovery of six new examples to bring the total sample to seven. While the first six discovered are transients active for a few weeks every two or more years, the most recently-discovered source HETE J1900.1-2455, has been active for more than 8 months. We summarise the transient behaviour of the population to estimate long-term time-averaged fluxes, and equate these fluxes to the expected mass transfer rate driven by gravitational radiation in order to constrain the distances. We also estimate an upper limit of 6 kpc to the distance of IGR J00291+5934 based on the non-detection of bursts from this source.

X-ray data analysis

For those in the local Astro group who may have an interest or a need to analyse some X-ray data from Chandra or RXTE, I have installed some analysis software (Linux machines only) and related calibration files in /data/software. For Chandra data analysis I have installed CIAO version 3.3.0.1, as well as the ISIS spectral fitting package version 1.3.1 and the CALDB files. For RXTE analysis I have installed HEASOFT version 6.0.3 and the appropriate CALDB files.

If you missed the introduction to X-ray astronomy I gave on April 5^th, here is a PDF file of the slides and notes

>> Read more To initialise CIAO:

source /data/software/ciao/bin/ciao.csh

For ISIS, add /data/software/isis/bin to your path and invoke with "isis". See the CXC for analysis threads and other documentation.

To initialise HEASOFT (which also has general tools to manipulate FITS files) for analysis of RXTE data:

setenv HEADAS /data/software/headas/i686-pc-linux-gnu-libc2.2.4
source $HEADAS/headas-init.csh
setenv CALDB /data/software/caldb
source $CALDB/software/tools/caldbinit.csh

HEASOFT also includes the popular spectral fitting package Xspec. See the HEASARC site for more info or RXTE data analysis recipes

I also plan to install the SAS software for analysis of XMM-Newton data. Let me know if you are interested in getting started with these packages or need any help.