arXiv subjects

James Boardman

Publications and source records attributed to James Boardman.

5 recordsLinked to original sources

arXiv2022

Investigating the low-flux states in six Intermediate Polars

We present optical photometry of six intermediate polars that exhibit transitions to a low-flux state. For four of these systems, DW Cnc, V515 And, V1223 Sgr and RX J2133.7+5107, we are able to perform timing analysis in and out of the low states. We find that, for DW Cnc and V515 And, the dominant periodicities in the light curves change as the flux decreases, indicating a change in the sources' accretion properties as they transition to the low state. For V1223 Sgr we find that the variability is almost completely quenched at the lowest flux, but do not find evidence for a changing accretion geometry. For RX J2133.7+5107, the temporal properties do not change in the low state, but we do see a period of enhanced accretion that is coincident with increased variability on the beat frequency, which we do not associate with a change in the accretion mechanisms in the system.

astro-ph.SR↗

arXiv2021

IM Normae: The Death Spiral of a Cataclysmic Variable?

We present a study of the orbital light curves of the recurrent nova IM Normae since its 2002 outburst. The broad "eclipses" recur with a 2.46 hour period, which increases on a timescale of 1.28(16)x10^6 years. Under the assumption of conservative mass-transfer, this suggests a rate near 10^-7 M_sol/year, and this agrees with the estimated /accretion/ rate of the postnova, based on our estimate of luminosity. IM Nor appears to be a close match to the famous recurrent nova T Pyxidis. Both stars appear to have very high accretion rates, sufficient to drive the recurrent-nova events. Both have quiescent light curves which suggest strong heating of the low-mass secondary, and very wide orbital minima which suggest obscuration of a large "corona" around the primary. And both have very rapid orbital period increases, as expected from a short-period binary with high mass transfer from the low-mass component. These two stars may represent a final stage of nova -- and cataclysmic-variable -- evolution, in which irradiation-driven winds drive a high rate of mass transfer, thereby evaporating the donor star in a paroxysm of nova outbursts.

astro-ph.SR↗

arXiv2018

Coherent brightness modulations in the dwarf nova AT Cancri

Light curves of the Z Cam type dwarf nova AT Cnc observed during standstill in 2016 and 2018 are analyzed. On the time scale of hours, previous reports on periodicities, in particular the presence of negative superhumps, could not be confirmed. Instead, a modulation with a period equal to the spectroscopic orbital period was detected which we thus interpret as a manifestations of the binary revolution. It enables us to derive a more accurate value of 0.201634 $\pm$ 0.000005 days (or its alias of 0.021580 days) for the period. AT Cnc also exhibits a hitherto unreported modulation of 25.731 $\pm$ 0.005 min, stable in period but not in amplitude over the entire time base of two years of the observations. We tentatively interpret this modulation in the context of an intermediate polar model for the system.

astro-ph.SR↗

arXiv2018

A Distance Oriented Kalman Filter Particle Swarm Optimizer Applied to Multi-Modality Image Registration

In this paper we describe improvements to the particle swarm optimizer (PSO) made by inclusion of an unscented Kalman filter to guide particle motion. We demonstrate the effectiveness of the unscented Kalman filter PSO by comparing it with the original PSO algorithm and its variants designed to improve performance. The PSOs were tested firstly on a number of common synthetic benchmarking functions, and secondly applied to a practical three-dimensional image registration problem. The proposed methods displayed better performances for 4 out of 8 benchmark functions, and reduced the target registration errors by at least 2mm when registering down-sampled benchmark brain images. Our methods also demonstrated an ability to align images featuring motion related artefacts which all other methods failed to register. These new PSO methods provide a novel, efficient mechanism to integrate prior knowledge into each iteration of the optimization process, which can enhance the accuracy and speed of convergence in the application of medical image registration.

cs.NE↗

arXiv2015

Results of a campaign to observe outbursts of the dwarf nova CSS 121005:212625+201948

A monitoring programme of CSS 121005:212625+201948 covering nearly two observing seasons has shown that it is a typical SU UMa dwarf nova, but it has one of the shortest supercycles of its class, at 66.9(6) d. The superoutbursts are interspersed with 3 to 7 short duration (~2 days) normal outbursts each of which are separated by a mean interval of 11 days, but can be as short as 2 days. The most intensively studied superoutburst was that of 2014 November, which lasted 14 days and had an outburst amplitude of >4.8 magnitudes, reaching magnitude 15.7 at its brightest. Time resolved photometry revealed superhumps with a peak-to-peak amplitude of 0.2 magnitudes, later declining to 0.1 magnitude. The superhump period was Psh = 0.08838(18) d. Time resolved photometry was conducted during several other superoutbursts, which gave broadly similar results.

astro-ph.SR↗