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Wed Sep 08, 2021

Searching for GRB optical counterparts with GOTO

Our GOTO prototype, while optimised for gravitational-wave followups, is also well suited to detecting optical counterparts of gamma-ray bursts detected by Fermi. Monash PhD candidate Yik Lun (Travis) Mong recently completed his analysis of our followup of 93 such events, focusing on 53 with favourable observational conditions. Unfortunately we couldn't confirm any counterparts (although he did find some intriguing candidates!)

Perhaps more importantly, Travis' analysis offers some lessons for future followup campaigns to help improve our chances of success. Travis' paper, in collaboration with the GOTO team, has been accepted by MNRAS

Read the paper (arXiv:2108.11802)

Labels: 2021, /postgrads

Tue Oct 20, 2020

Keeping the "beat" of nuclear burning

Millihertz quasi-periodic oscillations (mHz QPOs) observed in neutron-star low-mass X-ray binaries are generally thought to arise from "marginally stable" thermonuclear burning, at the upper limit of accretion rates where thermonuclear bursts are possible. A long-standing puzzle is that they seem to occur at accretion rates about an order of magnitude lower than predicted theoretically, and while bursts are still occurring.

Since no-one knows how you can simultaneously have (quasi-) stable and unstable burning on the surface of a neutron star (except perhaps Yuri Cavecchi), we'd dearly like to better understand this phenomenon. Monash PhD student Ka Ho Tse discovered a new example of mHz QPOs, in the transient 1RXS J180408.9—342058. While the properties of the newly-discovered oscillations are similar to the other seven examples, we now have a good sample of objects to examine the properties and compare against (for example) numerical models. Ka Ho's preliminary work suggests the observed amplitude is much higher than would be predicted theoretically, which presents a challenge for the conventional interpretation.

His paper describing the discovery, and exploring some of the implications for the phenomenon across all sources, has now been published by MNRAS.

Read the paper (MNRAS 500, 34—39, 2021)

Labels: 2020, /postgrads

Fri Apr 17, 2020

Multi-epoch model-observation burst comparisons for GS 1826—238

The "Clocked burster", GS 1826—238 is one of the best studied burst sources, due to it's (mostly) consistent and regular bursts. However there's long been some uncertainty about the fuel composition, with some studies suggesting metal-rich fuel, and others metal-poor. (Here by "metals" we mean an even more restrictive set than usually meant by astronomers, of the CNO nuclei that contribute to steady H-burning prior to ignition).

Now Monash PhD graduate Zac Johnston has resolved this uncertainty, with the most comprehensive observation-model comparison to date. He assembled a grid of almost 4000 KEPLER runs and performed a comparison to the observational data, via a novel interpolation scheme which speeds up the calculations by many orders of magnitude. His results confirm the "metal rich" scenario, and also provide constraints on the source distance and system inclination. His paper was just accepted by MNRAS. If you want to do your own burst comparisons, you can also check out the simulation grid data.

Read the paper (arXiv:1909.07977)

Labels: 2020, /postgrads

Mon Sep 16, 2019

Fingerprinting X-ray bursters

The first accretion-powered millisecond pulsar ever found, SAX 1808.4—3658 is a remarkable object, also showing thermonuclear bursts during it's infrequent outbursts. Over several years PhD student Adelle Goodwin has been trying to match the bursts observed during it's 2002 outburst with the predictions of a numerical model. This exercise can be thought of as searching exhaustively for a set of parameters which (in the model) reproduce the observed properties of the bursts. Now that we have an improved formula for determining the energy of these bursts, she built a Markov-chain Monte-Carlo (MCMC) code to constrain system parameters including neutron star mass and radius for the source. Her paper describing the study, and it's results, has now been published by MNRAS

Read the paper (MNRAS 490, 2228, 2019)

Labels: 2019, /postgrads

Tue Apr 16, 2019

A brand-new bursting X-ray binary

While examining RXTE data for the MINBAR project two summers ago, Jean in 't Zand unexpectedly discovered a brand-new bursting source, XMMU J181227.8—181234. Now PhD student Adelle Goodwin's paper on the source has been accepted by MNRAS. The source is quite unusual; distant and hence low-intensity, but apparently a high accretion rate leading to frequent (but weak) bursts. The shape of the bursts strongly implies H-poor accreted composition, but we also found evidence for short recurrence-time bursts, which previously have been seen only in H-rich accretors. Hopefully the source will go into outburst again sometime soon and we can gather more information on it's properties.

Read the paper (MNRAS 486, 4149, 2019)

Labels: 2019, /postgrads

Mon Nov 05, 2018

Measuring neutrino losses in thermonuclear bursts

PhD student Adelle Goodwin has been working hard trying to reconcile results of our 1-D time-dependent thermonuclear burst code Kepler with those of a much simpler (and much faster) code while trying to model the bursts from a millisecond pulsar, SAX 1808.4—3658. Continuing failure of this effort led to suspicion about the formula for the nuclear energy yield, and some careful experiments with Kepler revealed that the widely-used approximation for the energy generation adopted for the simpler code substantially overestimated the contribution lost as neutrinos. Adelle's paper on the study has now been published by ApJ. Adelle's work was also highlighted in the 2018 December edition of the JINA-CEE Newsletter.

Read the paper (ApJ 870, 64, 2019)

Labels: 2018, /postgrads

Mon Mar 09, 2015

Hauke's thesis accepted; postdoctoral position

I am proud and pleased to report the award of Hauke Wörpel's PhD degree was approved last week. Hauke's thesis, entitled "Radiation drag on the accretion disk in type-I X-ray bursts" was submitted last October. His thesis involved analysis of a few thousand thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer, showing that the accretion rate seems to increase during the bursts, and by a substantial amount, up to a factor of 20. Hauke also undertook numerical SPH modelling of accretion disks under the influence of bursts, with a view to better understanding the dynamics of the disk. Hauke has already left these shores and is now undertaking postdoctoral work at the Leibniz-Institut für Astrophysik Potsdam, where, on arrival, he unexpectedly grew an umlaut. Congratulations Hauke!

Labels: 2015, /postgrads

Tue Feb 17, 2015

The remarkable RXTE observations of the Rapid Burster

The Rapid Burster is a neutron-star binary that exhibits singular behaviour; extremely frequent "type-II" X-ray bursts, which cannot be thermonuclear in origin. The standard explanation of these bursts is that they arise from episodic accretion onto the neutron star, and it is this accretion which also fuels the (mostly) independent thermonuclear (type-I) bursts. The Rossi X-ray Timing Explorer made many observations (totalling 2.4 Ms, or almost a month of continuous observing!) of the source over multiple transient outbursts throughout the mission lifetime, building up an impressive database of burst behaviour. SRON PhD student Tullio Bagnoli, having already made a detailed study of the thermonuclear events in this source, has now completed a study of the type-II events, examining the demographics of the almost 8500 bursts detected by RXTE. The singular behaviour of this source may be explained by a special combination of magnetic field strength, stellar spin period and alignment between the magnetic field and the spin axis. Tullio's paper was just accepted by MNRAS.

Read the paper arXiv:1502.03941

Labels: 2015, /postgrads