Ray F Butler

We report I-band photometric observations of the radio-detected M9.5 dwarf BRI 0021-0214, obtained with the Galway Ultra Fast Imager (GUFI) on the 1.8m Vatican Advanced Technology Telescope VATT at Mt. Graham International Observatory, Arizona. In total, 19 hours of observations over a 73 day baseline were obtained. BRI 0021-0214 was shown to exhibit modulated emission with a period of 3.052 ± 0.004 hours with a mean amplitude variability of 0.0044 mag. When combined with rotational velocity data obtained from previous work, our newly discovered rotation period gives an inclination angle of 51.7$^{+5.0}_{-4.5}$ degrees for the rotation axis of BRI 0021-0214 relative to our line of sight. Previous studies have reported that the most plausible cause for optical variability from this dwarf is a consequence of suspended co-rotating dust clouds in its atmosphere. However reports of enhanced Hα and intermittent coherent radio emission suggest the possibility of auroral activity in its mag...

Brown dwarfs and low-mass stars together comprise the group of galactic objects known as ultracool dwarfs. A number have been detected as radio sources – in some cases, emitting periodic radio pulses synchronised to the dwarf\u27s known rotational period. Detections of optical variability in ultracool dwarfs have been primarily attributed to stellar rotation, with the modulations observed a consequence of either magnetic spots on the surface, the presence of atmospheric dust, or auroral emission. A combination of these mechanisms may be required to explain some dwarf lightcurves. Rotational estimates of ultracool dwarfs are typically obtained spectroscopically; however accurate estimates of true rotation velocities require knowledge of the dwarf\u27s rotational inclination axes. Direct measurement of a rotational signature in photometric data however provides an unambiguous rotational period, and this information can be used to constrain dwarf inclination geometries. In this thesis, we report on over ~ 160 hrs monitoring in I-band of multiple epochs from four ultracool dwarfs, spanning M tight binary dwarf 2MASS J1314203+132001A and L tight binary dwarf 2MASS J0746425+200032AB, the M9.5 dwarf BRI 0021-0214, and the L3.5 dwarf 2MASS J00361617+18211. This photometric campaign was carried out using the Galway Ultra-Fast Imager (GUFI) on the 1.8m Vatican Advanced Technology Telescope (VATT), on Mt. Graham, Arizona. All selected dwarfs exhibit periodic optical variability, where periods of both secondary components for our binary samples were newly discovered. This thesis discusses the use of two photometric analysis tools with the explicit aim of improving the quality of ground-based photometric measurements. Each data set was used to test the performance of the two systems. We find the LuckyPhot technique has obvious benefits to high precision photometry by reducing photometric errors, where the mean RMS error was reduced by ~ 47% with respect to the errors produced by the more standard GUFI pipeline method. This thesis also outlines a novel tool, Light Curve Fitter, which we apply to the binaries to investigate the presence of periodic photometric modulation in both binary members: refining the dominant member variability parameters, and searching for an elusive period of the weaker member. Light Curve Fitter is a python-based program, capable of detecting superposition of two sinusoidal waves to untangle the weaker components variability signature from that of the dominant source variability. We identify a newly discovered optical variability in the primary and secondary components of ultracool dwarf binary 2MASS J1314203+132001AB and 2MASS J0746425+200032AB, respectively. The optical data presented for both systems shows strongly correlated emissions in terms of phase and temporal variability. We have also shown the A and B variability signals of both dwarf binaries 2MASS J0746425+200032AB and 2MASS J1314203+132001AB, respectively, to be extremely consistent and stable over multiple epochs. This stability had seen in both radio and spectroscopic data, and the mechanism driving these processes in different parts of the electromagnetic spectrum could perhaps be fundamentally linked. We also investigate the orbital coplanarity of both binary dwarfs. Here the ability to deconvolve the inclination angle from the spectroscopic radial velocities, using direct estimates of the dwarf rotational periods, allows us to constrain the spin-orbit coupling of the binary system. In the case of the L dwarf binary 2MASS J0746425+200032AB, we calculate the equatorial inclination angle of the binary rotation axes are in alignment with the orbital plane of the system to within 10 degrees, consistent with solar-type binary formation mechanisms. For the M7 dwarf binary 2MASS J1314203+132001AB, due to missing parameters for the primary component, we investigate a tentative alignment of the spin-orbital axes of the A component. We find that the equatorial inclination angle of the secondary member spin axes is largely consistent with being aligned perpendicularly to the orbital plane. Finally, we find the rotation axes of the two single dwarfs are not perpendicular to our line of sight

The Galway Ultra-Fast Imager (GUFI) located on the 1.8m Vatican Advanced Technology Telescope (VATT) was tasked to monitor tight brown dwarf binaries. However, due to the close separation between the components in these binaries, the GUFI photometer could not image each component of binary systems as a point source in our campaign. Therefore, we developed and employed our novel application, the 'Light Curve Fitter', which is capable of distinguishing two superimposed sinusoidal waves. This application allows users to untangle the secondary component's variability signature from that of the dominant primary variability.

High performance computing has been used in various fields of astrophysical research. But most of it is implemented on massively parallel systems (supercomputers) or graphical processing unit clusters. With the advent of multicore processors in the last decade, many serial software codes have been re-implemented in parallel mode to utilize the full potential of these processors. In this paper, we propose parallel processing recipes for multicore machines for astronomical data processing. The target audience are astronomers who are using Python as their preferred scripting language and who may be using PyRAF/IRAF for data processing. Three problems of varied complexity were benchmarked on three different types of multicore processors to demonstrate the benefits, in terms of execution time, of parallelizing data processing tasks. The native multiprocessing module available in Python makes it a relatively trivial task to implement the parallel code. We have also compared the three mult...

We present methods which comprise a highly realistic exposure time calculator (ETC) for planning observations, and a simulator for the creation of realistic astronomical images - both of which are designed for any combination of location, telescope, filter and detector. The unique principle is that we begin with the physical properties of the stellar sources, as output by N-body simulations and evolutionary codes. Their fluxes (energy spectra) are computed, scaled for distance, and fed through a series of propagation stages applying the effects of the interstellar medium, the Earth's atmosphere, and the passband throughputs of the telescope optics, filters and the camera/detector, including all expected noise sources. We maintain flux propagation as a function of wavelength - not as an approximate integration over typical passbands. The simulator uses the fluxes output by the ETC to construct the actual images. Either single images or a time-series data set can be simulated; the...

A fraction of very low mass stars and brown dwarfs are known to be radio active, in some cases producing periodic pulses. Extensive studies of two such objects have also revealed optical periodic variability and the nature of this variability remains unclear. Here we report on multi-epoch optical photometric monitoring of six radio detected dwarfs, spanning the ∼M8- L3.5 spectral range, con-ducted to investigate the ubiquity of periodic optical variability in radio detected ultracool dwarfs. This survey is the most sensitive ground-based study carried out to date in search of periodic optical variability from late-type dwarfs, where we obtained 250 hours of monitoring, delivering photometric precision as low as ∼0.15%. Five of the six targets exhibit clear periodicity, in all cases likely associ-ated with the rotation period of the dwarf, with a marginal detection found for the sixth. Our data points to a likely association between radio and optical periodic variability in late-M/ea...

ABSTRACT Low Light Level CCD (L3‐CCD) cameras have received much attention for high cadence astronomical imaging applications. Efforts to date have concentrated on exploiting them for two scenarios: post‐exposure image sharpening and “lucky imaging”, and rapid variability in astrophysically interesting sources. We demonstrate their marked superiority in a third distinct scenario: observing in the high‐flux and wide dynamic range regimes. We realized that the unique features of L3‐CCDs would make them ideal for maximizing signal‐to‐noise in observations of bright objects (whether variable or not), and for high dynamic range scenarios such as faint targets embedded in a crowded field of bright objects. Conventional CCDs have drawbacks in such regimes, due to a poor duty cycle—the combination of short exposure times (for time‐series sampling or to avoid saturation) and extended readout times (for minimizing readout noise). For different telescope sizes, we use detailed models to show that a range of conventional imaging systems are photometrically out‐performed across a wide range of object brightness, once the operational parameters of the L3‐CCD are carefully set. The cross‐over fluxes, above which the L3‐CCD is operationally superior, are surprisingly faint—even for modest telescope apertures. We also show that the use of L3‐CCDs is the optimum strategy for minimizing atmospheric scintillation noise in photometric observations employing a given telescope aperture. This is particularly significant, since scintillation can be the largest source of error in timeseries photometry. These results should prompt a new direction in developing imaging instrumentation solutions for observatories.

ABSTRACT A high cadence imaging system, based on a Low Light Level CCD (L3CCD) camera, has been developed for photometric and polarimetric applications. The camera system is an iXon DV‐887 from Andor Technology, which uses a CCD97 L3CCD detector from E2V technologies. This is a back illuminated device, giving it an extended blue response, and has an active area of 512×512 pixels. The camera system allows frame‐rates ranging from 30 fps (full frame) to 425 fps (windowed & binned frame). We outline the system design, concentrating on the calibration and control of the L3CCD camera. The L3CCD detector can be either triggered directly by a GPS timeserver∕frequency generator or be internally triggered. A central PC remotely controls the camera computer system and timeserver. The data is saved as standard ‘FITS’ files. The large data loads associated with high frame rates, leads to issues with gathering and storing the data effectively. To overcome such problems, a specific data management approach is used, and a Python∕PYRAF data reduction pipeline was written for the Linux environment. This uses calibration data collected either on‐site, or from lab based measurements, and enables a fast and reliable method for reducing images. To date, the system has been used twice on the 1.5 m Cassini Telescope in Loiano (Italy); we present the reduction methods and observations made.

A number of ultracool dwarfs have been unexpectedly detected as radio sources in the last decade, four of which have been found to be producing periodic pulses. More recently, two of these pulsing dwarfs have also been found to be periodically variable in broadband optical photometry. The detected periods match the periods of the radio pulses which have previously been associated with the rotation period of the dwarf. For one of these objects, it has also been established that the optical and radio periodic variability are possibly linked, being a consequence of magnetically-driven auroral processes. In order to investigate the ubiquity of the periodic optical variability in radio detected sources, the GUFI instrument (Galway Ultra Fast Imager) was commissioned on the 1.8m Vatican Advanced Technology Telescope, on Mt. Graham, Arizona, and has been obtaining data for the past eighteen months. More than two hundred hours of multi-epoch photometric monitoring observations of radio dete...

High-cadence imaging is required in several astronomical scenarios. These include: studies of rapidly varying sources, achieving maximum signal-to-noise observations of bright objects, and high dynamic range scenarios, such as faint objects embedded in a crowded field of bright objects. Conventional CCDs have drawbacks in this regime, because many short exposures are needed (either for timeseries sampling, or simply to avoid saturation), with extended readout times between exposures. Consequently, the duty cycle (ratio of exposure time to readout time) dramatically worsens as exposures get shorter. However, Low Light Level CCDs (L3-CCDs) offer low readout noise, high readout rates, and 100% duty cycle. Coupled with its fast frame-transfer mechanism (~2ms to shift the image to the storage area), an L3-CCD can sustain essentially continuous open-shutter time. Our models demonstrate that for a fixed observing time, the L3-CCD will deliver a better signal to noise performance in the high-cadence imaging regime when compared to similar CCDs, even when the latter's performance is optimised by windowing and binning. We also demonstrate that the improved duty cycle reduces the photometric impact of atmospheric scintillation, for any given aperture of telescope. We outline the integration of an L3-CCD into our camera system for high cadence imaging.

ABSTRACT There are several pieces of evidence that the sample of gamma-ray bursts (GRBs) is inhomogeneous and that these objects can be situated in the Galaxy as well as at cosmological distances. We consider single black holes (BHs) of 10-100 Msolar as parent objects for GRBs of both types. The origin of GRBs is connected to the release of energy generated by magnetic field inhomogeneities together with interstellar plasma accretion onto a regular magnetosphere around a BH. The percolation nature of the energy release for this kind of instability yields a universal power law for the GRB energy distribution, that is E-beta (with beta=1.7-1.8), which is the same as that for the flares observed in the Sun, in flare stars and in X-ray sources. The number of GRBs per galaxy is about 103 for both galactic (with luminosities up to 1038 erg s-1) and cosmological (with luminosities up to 1052 erg s-1) populations. We also discuss the possibility of detecting galactic BHs as parent bodies of GRBs in quiescent phase, as stars of magnitude 20 to 29 with continuum spectra and fast variability.

Using MERLIN we report the analysis of a recent 16 hr L band observation of the inner flat radio continuum of the CTB 80 remnant which successfully resolved both the pulsar PSR B1951+32 and fine structure within the `hot spot' previously identified at lower resolution radio &X-ray observations, towards which the pulsar is moving. This `hot spot' is believed to arise from particle acceleration caused by interactions between the pulsar wind and the CTB 80 remnant. Such structures have been seen in other pulsar wind synchrotron nebulae, but this is the first time such emission has been detected at150 mas resolution in the radio. We discuss the implications of this observational dataset for existing multiwavelength observations of PSR B1951+32 &CTB 80.

The GUFI (Galway Ultra Fast Imager) has been primarily developed for high throughput differential photometry, in order to study variability in challenging circumstances, such as near bright sources or within crowded fields. The instrument features a low light level charged coupled device (L3-CCD) that enhances detector speed and sensitivity but only covers small fields of view. This presents limitations on possible science targets when suitable differential photometry comparison stars are not in the immediate vicinity of the target. Conventional solutions for imaging larger portions of sky without sacrificing SNR include telescope focal reduction methods and large arrays of CCDs. Our alternative solution entails a two-path, `outrigger' optical design to image target and comparison stars separately. This new approach allows detection of variable targets that formerly were not reachable with smaller-field detectors. The mechanical design was originally generated with AutoCAD® dra...