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Fri Nov 09, 2007

Model lightcurves for bursts from GS 1826-24

Alex's paper comparing the observed burst lightcurves from GS 1826-24 with time-dependent models of nuclear burning is finally out. The correspondence is really remarkable, and confirms the solar composition of the accreted fuel, as we suspected from the 2004 paper. However, in that paper we found that the drop in recurrence time with increased flux was more than predicted by Andrew's ignition models; this discrepancy is resolved with the new modelling, which indicates that thermal/compositional inertia play a role in setting the ignition column.

Read the paper (arXiv:0711.1195)

Labels: 2007, /thermonuclear bursts

Mon Oct 22, 2007

Netherlands visit

I just got back from a month-long visit to Europe, spending most of my time working at the Space Research Organisation of the Netherlands (SRON) in Utrecht on some thermonuclear burst work. The goal was to combine the RXTE burst catalog with the much larger sample accumulated by the Wide-Field Camera onboard the Italian-Dutch BeppoSAX mission during it's 7-year (1996-2002) mission. We now have a preliminary combined catalog, which we call MINBAR (Multi-INstrument Burst ARchive), and we are working on improving the data products from the BeppoSAX bursts. I also visited the JEM-X instrument team at the Danish National Space Center in order to investigate adding bursts detected by INTEGRAL to the archive.
But it wasn't all work — of course, we also had a nice time sightseeing and catching up with friends and family.

Labels: 2007, /visits

Tue Sep 25, 2007

The Variable Warm Absorber in Circinus X-1

Norbert's paper on the faint-state observations of Cir X-1 has just been accepted to ApJ. While the P-Cygni lines (present in earlier observations when the source was brighter) were absent, we found evidence for complex absorption effects, including warm (moderately ionized) absorbers with varying column density. This Chandra observation has proved popular lately, with another paper analysing the same data posted just weeks earlier.

Read the paper arXiv:0709.3336

Labels: 2007, /spectroscopy

Mon Sep 17, 2007

NASA Astronomers Find Bizarre Planet-Mass Object Orbiting Neutron Star

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 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 (; ApJ 668, L147) or the
NASA press release

Labels: 2007, /pulsars

Fri Jul 13, 2007

An X-ray jet in Circinus X-1

Sebastian did well to spot the X-ray jet in one of Norbert's 2005 zero-phase observations of the enigmatic X-ray binary Cir X-1. This source is thus the first neutron star for which an extended X-ray jet has been detected. Previously ultrarelativistic arcsecond-scale radio jets were detected by Rob Fender's group; see their 2004 Nature paper. The kinetic jet power that we infer is significantly larger than the minimum power required for the jet to inflate the large-scale radio nebula.

Read the paper (ApJ 663, L93)

Labels: 2007, /jets

Thu Jul 05, 2007

Two accretion-powered millisecond pulsars active in June

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

Wed Jun 06, 2007

HETE J1900.1-2455 is returning to quiescence

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

Mon May 07, 2007

Fe Kα Lines in MCVs

Aimee's paper on the Fe Kα line profiles from the accretion columns of magnetic CVs was accepted by MNRAS last month. The paper extends Jason Cullen's nonlinear Monte-Carlo code to predict the degree of broadening expected from lines arising from close to the white dwarf. We used a Chandra observation of GK Per in outburst for comparison, and while the relative line strengths in the model is not set by the physical conditions, the results were promising.

Read the paper (astro-ph/0704.1516)

Labels: 2007, /spectroscopy

Wed Apr 04, 2007

ASP2011 Measurement techniques

For the last three weeks I've been teaching the Measurement Techniques unit of 2nd year Astronomy (ASP2011) at Monash. Below are links to my notes and slides presented during the lecture.
Lecture notes (Word document)
Slides for lectures (PDF) 1-2, 3-4, 5-6 & 7-8

Labels: 2007, /teaching

Tue Jan 23, 2007

SINS summer school

Last week I spent two days at the first Stellar Interiors and Nucleosynthesis summer school, hosted by the CSPA at Monash U. I gave a talk on nuclear processes in thermonuclear bursts, which was good since it forced me to actually learn a little bit more about this subject. Most of the other talks were pretty hard-core stellar physics, but Jordi Jose gave a nice series of talks on classical novae and type 1a supernovae.

Nuclear burning on the surface of accreting neutron stars (6.7 Mb .ppt)

Labels: 2007, /meetings