Raymond F. Butler

Pulsed Optical Emission from PSR 0656+14

Using data collected with the Special Astrophysical Observatory (SAO) 6 m telescope and the University College Galway (UCG) Transputer Instrument for Fast Image Deconvolution (TRIFFID) imaging photometer, we show that the radio, X-ray, and γ-ray pulsar PSR 0656+14 exhibits pulsed optical emission. We observed that the pulsed fraction was consistent with 100% and that the flux was higher than that expected from a thermal source. The magnitude of PSR 0656+14 in the B band was observed to be 25.1 ± 0.3, consistent with previous CCD observations. The peak of the optical signal is at phase 0.2 compared to the radio and in phase with the weak γ-ray pulse.

Rotational Modulation of M/L Dwarfs Due to Magnetic Spots

We find periodic I-band variability in two ultracool dwarfs, TVLM 513-46546 and 2MASS J00361617+1821104, on either side of the M/L dwarf boundary. Both of these targets are short-period radio transients, with the detected I-band periods matching those found at radio wavelengths (P = 1.96 hr for TVLM 513-46546 and P = 3 hr for 2MASS J00361617+1821104). We attribute the detected I-band periodicities to the periods of rotation of the dwarfs, supported by radius estimates and measured v sin i values for the objects. Based on the detected period of rotation of TVLM 513-46546 (M9) in the I band, along with confirmation of strong magnetic fields from recent radio observations, we argue for magnetically induced spots as the cause of this periodic variability. The I-band rotational modulation of the L3.5 dwarf 2MASS J00361617+1821104 appeared to vary in amplitude with time. We conclude that the most likely cause of the I-band variability for this object is magnetic spots, possibly coupled with time-evolving features such as dust clouds.

PERIODIC OPTICAL VARIABILITY OF RADIO-DETECTED ULTRACOOL DWARFS

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, conducted 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 hr of monitoring, delivering photometric precision as low as ~0.15%. Five of the six targets exhibit clear periodicity, in all cases likely associated 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/early-L dwarfs, although the underlying physical cause of this correlation remains unclear. In one case, we have multiple epochs of monitoring of the archetype of pulsing radio dwarfs, the M9 TVLM 513–46546, spanning a period of 5 yr, which is sufficiently stable in phase to allow us to establish a period of 1.95958 ± 0.00005 hr. This phase stability may be associated with a large-scale stable magnetic field, further strengthening the correlation between radio activity and periodic optical variability. Finally, we find a tentative spin-orbit alignment of one component of the very low mass binary, LP 349–25.

Spin-orbit alignment in the very low mass binary regime The L dwarf tight binary 2MASSW J0746425+200032AB

Studies of solar-type binaries have found coplanarity between the equatorial and orbital planes of systems with <40 AU separation. By comparison, the alignment of the equatorial and orbital axes in the substellar regime, and the associated implications for formation theory, are relatively poorly constrained. Here we present the discovery of the rotation period of 3.32 ±  0.15 h from 2MASS J0746+20A – the primary component of a tight (2.7 AU) ultracool dwarf binary system (L0+L1.5). The newly discovered period, together with the established period via radio observations of the other component, and the well constrained orbital parameters and rotational velocity measurements, allow us to infer alignment of the equatorial planes of both components with the orbital plane of the system to within 10 degrees. This result suggests that solar-type binary formation mechanisms may extend down into the brown dwarf mass range, and we consider a number of formation theories that may be applicable in this case. This is the first such observational result in the very low mass binary regime. In addition, the detected period of 3.32 ± 0.15 h implies that the reported radio period of 2.07 ± 0.002 h is associated with the secondary star, not the primary, as was previously claimed. This in turn refutes the claimed radius of 0.78 ± 0.1 R_J for 2MASS J0746+20A, which we demonstrate to be 0.99 ± 0.03 R_J.

Parallel Astronomical Data Processing with Python: Recipes for multicore machines

Abstract 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 is astronomers who use 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 multiprocessing approaches—Pool/Map, Process/Queue and Parallel Python. Our test codes are freely available and can be downloaded from our website.

A Search for the Optical Counterpart of the triple pulsar system PSR B1620–26 in M4

We have performed photometry in B and V of the field of the triple system PSR B1620-26 in the globular cluster M4. Our images were taken with two two-dimensional imaging photon-counting detectors, which permitted deep exposures to be made without saturation. Photometry of the proposed optical counterpart of the second companion of PSR B1620-26 was obtained, yielding a magnitude V = 21.30 ± 0.08 and a color B - V = 1.32 ± 0.20. The color index has not been successfully determined previously. These values locate the counterpart on the main sequence of the cluster color-magnitude diagram and lead to a mass determination that is consistent with a 0.48 M☉ cluster main-sequence star.

A modular camera system for high time-resolution photometry

A new modular high time resolution imaging camera system with sub-microsecond timing accuracy has been built in the Physics Dept. of NUI, Galway. The system was designed to be mounted on large telescopes for observing the temporal, spectral and polarisation characteristics of faint astronomical objects, such as optical pulsars. The camera system developed allows simultaneous and independent observing of multiple wavebands of emission from the target objects. This is achieved using optics that split images into their different spectral or polarisation components. The system currently incorporates a multi-anode microchannel array (MAMA) photon detecting and imaging camera with a time resolution of up to 100ns. This is combined with three high quantum efficiency avalanche photodiodes (APDs) with count rates of up to 16 million photons per second. The high time resolution recording system can allow for the removal of telescope tracking inaccuracy and wind shear off-line. This yields better PSFs for bright objects such as crowded globular star clusters. This combination of different detectors allows the system to be operated as a multi purpose, high QE, high time resolution system. The modular nature of the design electronics also allows the addition and removal of detectors without limiting the performance of other elements within the system. The data path is also designed so that archiving integrity is maintained while the data path is simultaneously used for real-time analysis and display systems. Future applications in the bio-medical imaging sector are envisaged for high time resolution fluorescence imaging, and astronomical polarisation studies.

A search for the optical counterpart to PSR B1821-24 in M 28

We have analysed archival HST/WFPC2 images in both the F555W & F814W bands of the core eld of the globular cluster M 28 in an attempt to identify the optical counterpart of the magnetospherically active millisecond pulsar PSR B1821-24. Examination of the radio derived error circle yielded several potential candidates, down to a magnitude of V 24.5 (V0 23.0). Each were further investigated, both in the context of the CMD of M 28, and also with regard to phenomenological models of pulsar magnetospheric emission. The latter was based on both luminosity-spindown correlations and known spectral flux density behaviour in this regime from the small population of optical pulsars observed to date. None of the potential candidates exhibited emission expected from a magnetospherically active pulsar. The fact that the magnetic eld & spin coupling for PSR B1821-24 is of a similar magnitude to that of the Crab pulsar in the vicinity of the light cylinder has suggested that the millisecond pulsar may well be an ecient nonthermal emitter. ASCAs detection of a strong synchrotron-dominated X-ray pulse fraction encourages such a viewpoint. We argue that only future dedicated 2-d high speed photometry observations of the radio error-circle can nally resolve this matter.

Detection of new optical counterpart candidates to PSR B1951+32 with HST/WFPC2

There remain several definitive-ray pulsars that are as yet undetected in the optical regime. A classic case is the pulsar PSR B1951+32, associated with the complex CTB 80 SNR. Previous ground based high speed 2-d optical studies have ruled out candidates to mV 24. Hester (2000a) has reported an analysis of archival HST/WFPC2 observations of the CTB 80 complex which suggest a compact synchrotron nebula coincident with the pulsars radio position. Performing a similar analysis, we have identified a possible optical counterpart within this synchrotron nebula at mV 25:5 26, and another optical counterpart candidate nearby at mV 24:5. We assess the reality of these counterpart candidates in the context of existing models of pulsar emission.

Development and use of an L3CCD high‐cadence imaging system for Optical Astronomy

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...