Our International Space Science Institute (ISSI) team on thermonuclear bursts called for — as with all such teams — two meetings in Bern, separated by about a year. For this team, featuring a strong contingent of members from Monash, we decided to have a mid-term meeting in Melbourne, sponsored (in part) by the Monash Centre for Astrophysics (MoCA). This inaugural "ISSI-South" meeting, held August 19—23, featured 9 team members from Germany, Canada, Netherlands, USA, Denmark and Spain, in addition to the five local team members and students.

The meeting was as usual a great chance to share recent progress and catch up with the team's activities. Highlights included the workshop dinner on the Colonial Tramcar Restaurant, and the windup tag-team seminar to MoCA members by team leader Andrew Cumming and myself.

Labels: 2013, /meetings

The Astronomical Society of Australia held its Annual Scientific Meeting at Monash's own Clayton campus last month. The ASM has not been held at Monash since 1991, so it has been a long time coming. The Harley Wood Winter school was held, as is traditional, immediately prior to the ASM on Phillip Island. The winter school was a great success, attracting 76 attendees; kudos to the organisational team led by Dane Kleiner (SOC chair) and Jake Crossett (LOC chair).

The meeting itself kicked off on Sunday evening with a very civilised opening reception, and continued throughout the week. Highlights for me included the great opening speech by Monash Chancellor Dr. Alan Finkel; the special session on science with the Mileura Widefield Array (MWA) and the Australian Square Kilometre Array Pathfinder (ASKAP); and the conference dinner in Chinatown featuring the very smooth Leon de Bruin quintet.

The local organising committee was chaired by yours truly, and the rest of the meeting ran very smoothly with the help of the rest of the LOC; the excellent SOC (chaired by Michael Brown); and the expert conference organisation team of Danielle, Bree & Kristie from ASN Events. Many thanks to everyone who helped make it a success, including of course the speakers, poster presenters, tweeters, and all the other attendees!

Labels: 2013, /meetings

Since the 1970s the standard approach for analysing the X-ray spectrum emitted during thermonuclear bursts included subtracting off the persistent spectrum prior to the burst, implicitly assuming that this component (which arises from accretion) could not vary during the burst. On the other hand, predictions of the response of the accretion disk to the sudden X-ray brightening during a burst suggest that the accretion rate should increase.
In a study of several hundred bright bursts observed by the Rossi X-ray Timing Explorer, Monash PhD student Hauke Worpel found evidence that the accretion rate does increase during the burst, and by a substantial amount, up to a factor of 20. This result was also corroborated by the recent simultaneous observation of a burst by Chandra and RXTE. Hauke's paper was just accepted by ApJ.

Read the paper arXiv:1303.4824

Labels: 2013, /thermonuclear bursts

Back in November 2011, the 401-Hz millisecond pulsar SAX J1808.4-3658 went into another of it's regular (every ~3 yr) outbursts. SRON's Jean in 't Zand had a target-of-opportunity Chandra program targeting bright PRE bursts of another source, and managed to convince the TAC to observe SAX J1808.4-3658 instead.

We only detected one burst in the Chandra observation, but it was a beauty — perhaps the brightest ever seen from the source, and it was observed simultaneously by RXTE! Although we found no evidence for discrete spectral features in the burst (the goal of the Chandra observation), the simultaneous observation gave us a unique view on the broad-band X-ray spectrum during the burst. Remarkably, we found evidence that the persistent flux level varies during the burst, increasing by a factor of up to 20 while the burst is active.

This result echoes the findings of my student Hauke Worpel, who found the same effect in a much wider study of several hundred such bursts observed with RXTE (see arxiv:1303.4824). Jean's paper was just accepted by A&A.

Read the paper arXiv:1301.2232

Labels: 2013, /thermonuclear bursts

The Rapid Burster is unique for its so-called type-II X-ray bursts, during which the X-ray intensity brightens dramatically, and which can recur as often as every few tens of seconds. The precise mechanism is unknown, but the bursts are thought to be episodes of enhanced accretion mediated (somehow) by the magnetic field. SRON PhD student Tullio Bagnoli has just completed a study of the type-I (thermonuclear) bursts which are also seen from the Rapid Burster, which are detected up to an unusually high accretion rate, compared to the other neutron stars in low-mass X-ray binaries. In this respect, the Rapid Burster appears similar to the enigmatic source Terzan 5 X-2, which suggests that this system may also rotate unusually slowly. The paper was just accepted by MNRAS

Read the paper arXiv:1302.4286

Labels: 2013, /thermonuclear bursts

In January I was back to Europe for the X-ray Binaries 13 meeting, a "thinkshop" on the present and future of studies of X-ray binaries organised by Sergio Campana (Brera) &co. The meeting featured a terrific overview of the current state of knowledge of neutron star and black hole binaries, and prospects for the future. I gave an invited talk on thermonuclear bursts and the prospects for the future.

Of course, in a setting as beautiful as Bormio, we also enjoyed the skiing, although it did get a bit cold (-20° at the summit at one point!)

Thermonuclear (type-I) X-ray bursts (1.5 MB PDF)

Labels: 2013, /meetings