Ron Burman

Pulsar magnetic alignment and the pulsewidth-age relation

Using pulsewidth data for 872 isolated radio pulsars, we test the hypothesis that pulsars evolve through a progressive narrowing of the emission cone combined with progressive alignment of the spin and magnetic axes. The new data provide strong evidence for the alignment over a time-scale of about 1 Myr with a log standard deviation of around 0.8 across the observed population. This time-scale is shorter than the time-scale of about 10 Myr found by previous authors, but the log standard deviation is larger. The results are inconsistent with models based on magnetic field decay alone or monotonic counter-alignment to orthogonal rotation. The best fits are obtained for a braking index parameter, n γ ≈ 2.3, consistent with the mean of the six measured values, but based on a much larger sample of young pulsars. The least-squares fitted models are used to predict the mean inclination angle between the spin and magnetic axes as a function of log characteristic age. Comparing these predictions to existing estimates, it is found that the model in which pulsars are born with a random angle of inclination gives the best fit to the data. Plots of the mean beaming fraction as a function of characteristic age are presented using the best-fitting model parameters.

Where are the missing gamma‐ray burst redshifts?

In the redshift range z = 0‐1, the gamma-ray burst (GRB) redshift distribution should increase rapidly because of increasing differential volume sizes and strong evolution in the star formation rate (SFR). This feature is not observed in the Swift redshift distribution and to account for this discrepancy a dominant bias, independent of the Swift sensitivity, is required. Furthermore, despite rapid localization, about 50 per cent of Swift and pre-Swift GRBs do not have an observed optical afterglow and 60‐70 per cent of GRBs are lacking redshifts. We employ a heuristic technique to extract this redshift bias using 69 GRBs localized by Swift with redshifts determined from absorption or emission spectroscopy. For the Swift and HETE + BeppoSAX redshift distributions, the best model fit to the bias at z 2. To achieve this high KS probability requires increasing the GRB rate density at small z compared to the high-z rate. This provides further evidence for a low-luminosity population of GRBs that are observed in only a small volume because of their faintness.

The Zadko Telescope: A Southern Hemisphere Telescope for Optical Transient Searches, Multi-Messenger Astronomy and Education

The new 1-m f/4 fast-slew Zadko Telescope was installed in June 2008 about 70 km north of Perth, Western Australia. It is the only metre-class optical facility at this southern latitude between the east coast of Australia and South Africa, and can rapidly image optical transients at a longitude not monitored by other similar facilities. We report on first imaging tests of a pilot program of minor planet searches, and Target of Opportunity observations triggered by the Swift satellite. In 12 months, 6 gamma-ray burst afterglows were detected, with estimated magnitudes; two of them, GRB 090205 (z = 4.65) and GRB 090516 (z = 4.11), are among the most distant optical transients imaged by an Australian telescope. Many asteroids were observed in a systematic 3-month search. In September 2009, an automatic telescope control system was installed, which will be used to link the facility to a global robotic telescope network; future targets will include fast optical transients triggered by high- energy satellites, radio transient detections, and LIGO gravitational wave candidate events. We also outline the importance of the facility as a potential tool for education, training, and public outreach.

ACIGA's high optical power test facility

Advanced laser interferometer detectors utilizing more than 100 W of laser power and with ~106 W circulating laser power present many technological problems. The Australian Consortium for Interferometric Gravitational Astronomy (ACIGA) is developing a high power research facility in Gingin, north of Perth, Western Australia, which will test techniques for the next generation interferometers. In particular it will test thermal lensing compensation and control strategies for optical cavities in which optical spring effects and parametric instabilities may present major difficulties.

Probing the low-luminosity gamma-ray burst population with new generation satellite detectors

We compare the detection rates and redshift distributions of low-luminosity (LL) gamma-ray bursts (GRBs) localized by Swift with those expected to be observed by the new generation satellite detectors on the Gamma-ray Large Area Space Telescope (GLAST )( nowFermi) and, in future, the Energetic X-ray Imaging Survey Telescope (EXIST). Although the GLAST burst telescope will be less sensitive than Swift’s in the 15–150 keV band, its large FoV implies that it will double Swift’s detection rate of LL bursts. We show that Swift, GLAST and EXIST should detect about 1, 2 and 30 LL GRBs, respectively, over a 5-yr operational period. The burst telescope on EXIST should detect LL GRBs at a rate of more than an order of magnitude greater than that of Swift’s BAT. We show that the detection horizon for LL GRBs will be extended from z � 0.4 for Swift to z � 1.1 in the EXIST era. Also, the contribution of LL bursts to the observed GRB redshift distribution will contribute to an identifiable feature in the distribution at z � 1.

A cosmological `probability event horizon¿ and its observational implications

Suppose an astronomer is equipped with a device capable of detecting emissions (whether they be electromagnetic, gravitational, or neutrino) from transient sources distributed throughout the cosmos. Because of source rate density evolution and variation of cosmological volume elements, the sources first detected when the machine is switched on are likely to be ones in the high-redshift Universe; as observation time increases, rarer, more local, events will be found. We characterize the observers evolving record of events in terms of a ‘probability event horizon’ (PEH), converging on the observer from great distances at enormous speed, and illustrate it by simulating neutron star (NS) birth events distributed throughout the cosmos. As an initial application of the concept, we determine the approach of this horizon for gamma-ray bursts (GRBs) by fitting to redshift data. The event rates required to fit the model are consistent with the proposed link between core-collapse supernovae (SNe) and a largely undetected population of faint GRBs.

Status of the Australian Consortium for Interferometric Gravitational Astronomy

We report the status of research and development being undertaken by the members of the Australian Consortium for Interferometric Gravitational Astronomy.

Vehicle-induced seismic effects at a gravitational wave observatory

The seismic signature of a vehicle traveling on the site of the Australian International Gravitational Observatory has been studied using a two-axis seismometer. The main laboratory strongly attenuated the vehicle-induced seismic noise across all frequencies. We find that the magnitude of the vehicle-induced seismic signal followed a d−γ power law, with d the source–detector distance and γ≈1.7±0.4. The results are indicative of strong attenuation of seismic disturbances and are consistent with other measurements of low-amplitude Rayleigh wave attenuation in similar sandy soil types, a highly desirable property for the site of a gravitational wave observatory. This analysis provides a tool for estimating the magnitude of vehicle-induced seismic noise at the laboratory buildings from more distant sources on and around the site.

An Improved Method for Estimating Source Densities Using the Temporal Distribution of Cosmological Transients

It has been shown that the observed temporal distribution of transient events in the cosmos can be used to constrain their rate density. Here we show that the peak flux-observation time relation takes the form of a power law that is invariant to the luminosity distribution of the sources, and that the method can be greatly improved by invoking time reversal invariance and the temporal cosmological principle. We demonstrate how the method can be used to constrain distributions of transient events by applying it to Swift gamma-ray burst data and show that the peak flux-observation time relation is in good agreement with recent estimates of source parameters. In addition, we show that the intrinsic time dependence allows the method to be used as a predictive tool. Within the next year of Swift observation, we find a 50% chance of obtaining a peak flux greater than that of GRB 060017—the highest Swift peak flux to date—and the same probability of detecting a burst with peak flux >100 photons s-1 cm-2 within 6 years.

Are GRB optical afterglows relatively brighter at high z

The redshift distribution of gamma-ray bursts (GRBs) is strongly biased by selection effects. We investigate, via Monte Carlo simulations, one possible selection effect that may be modifying the Swift GRB redshift distribution. We show how telescope response times to acquire a GRB redshift may, via the Malmquist effect and GRB optical afterglow (OA) brightness distribution, introduce a bias into the average of the observed redshift distribution. It is difficult to reconcile a recently reported correlated trend between telescope response time and average redshifts unless we employ a redshift-dependent OA distribution. Simulations of this selection effect suggest that GRB OAs may have been either intrinsically brighter early in the Universe or suffered less local host galaxy extinction.