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 hours 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 years, which is sufficiently stable in phase to allow us to establish a period of 1.95958 ± 0.00005 hours. 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.

Many astronomical objects emit polarised light, which can give information both about their source mechanisms, and about (scattering) geometry in their source regions. To date (mostly) only the linearly polarised components of the emission have been observed in stellar sources. Observations have been constrained because of instrumental considerations to periods of excellent observing conditions, and to steady, slowly or periodically-varying sources. This leaves a whole range of interesting objects beyond the range of observation at present. The Galway Astronomical Stokes Polarimeter (GASP) has been developed to enable us to make observations on these very sources. GASP measures the four components of the Stokes Vector simultaneously over a broad wavelength range 400-800nm., with a time resolution of order microseconds given suitable detectors and a bright source -this is possible because the optical design contains no moving or modulating components. The initial design of GASP is presented and we include some preliminary observational results demonstrating that components of the Stokes vector can be measured to <1% in conditions of poor atmospheric stability. Issues of efficiency and stability are addressed. An analysis of suitable astronomical targets, demanding the unique properties of GASP, is also presented.

Abstract. The Galway Astronomical Stokes Polarimeter (GASP) is an ultra-high-speed, full Stokes, astronomical imaging polarimeter based upon a Division of Amplitude Polarimeter. It has been developed to resolve extremely rapid stochastic (~ms) variations in objects such as optical pulsars, magnetars and magnetic cataclysmic variables. The polarimeter has no moving parts or modulated components so the complete Stokes vector can be measured from just one exposure -making it unique to astronomy. The time required for the determination of the full Stokes vector is limited only by detector efficiency and photon fluxes. The polarimeter utilizes a modified Fresnel rhomb that acts as a highly achromatic quarter wave plate and a beamsplitter (referred to as an RBS). We present a description of how the DOAP works, some of the optical design for the polarimeter. Calibration is an important and difficult issue with all polarimeters, but particularly in astronomical polarimeters. We give a description of calibration techniques appropriate to this type of polarimeter.

The Galway Astronomical Stokes Polarimeter (GASP) is an ultra-high-speed, full Stokes, astronomical imaging polarimeter based upon a Division of Amplitude Polarimeter. It has been developed to resolve extremely rapid (~microsecond) variations in objects such as optical pulsars and magnetic CVs. The polarimeter has no moving parts or modulated components, so the complete Stokes vector can be measured from just one exposure -making it unique to astronomy -and the time required for the complete determination of the Stokes vector is limited only by detectors and photon fluxes. The polarimeter utilizes a modified Fresnel rhomb, which acts as a highly achromatic quarter wave plate and a beamsplitter (referred to as an RBS). We present a detailed description of the design process for the RBS and other optics of the polarimeter, and give a theoretical analysis of its expected performance. Calibration is an important, and difficult, issue with all polarimeters, but particularly in astronomical polarimeters. We give a detailed description of calibration techniques appropriate to this type of polarimeter -particularly a version of the Eigenvalue Calibration Method of Compain & Drevillon. ASTRONOMICAL IMAGING POLARIMETRYPolarimetry is a powerful diagnostic tool in astrophysics enabling scattering, source asymmetries, magnetic field configurations, and longitudinal magnetic field strengths, for example, to be investigated. High precision and high speed in astronomical polarimetry is difficult to achieve but such polarimetry would be beneficial for the examination of optical pulsars and magnetic CVs in timescales shorter than they have been observed to date.Currently, the commonest technique used in astronomical imaging polarimeters employs a half waveplate (HWP), then a polarizing beamsplitter, to produce two beams, the intensities of which are measured [1]. The HWP is then rotated in steps of 22.5° and the intensities re-measured. A data reduction process can then give I, Q and U of the Stokes vector (as in Eq.1, where I n is the intensity of the polarization state and T denotes the transpose). If the HWP is replaced by a quarter waveplate (QWP), V can be measured.

We present an overview of the work currently ongoing into the detector system used on the Galway Astronomical Stokes Polarimeter (GASP). GASP is based upon a Division of Amplitude Polarimeter and has no moving parts or modulated components. The complete Stokes vector is measured from just one exposure. This means that the timing resolution depends only on the frame rate of the detector system. GASP has been used on the 2.2 m Telescope at Calar Altro (2009) and on 4.2 m WHT (2010). The system consists of the instrument box to which either an L3Camera system or an APD detector system is attached. A GPS Time and Frequency system controls all timing aspects for both detector systems. The choice of detector depends on the science target and objectives. For frame rates up to 650 Hz the L3Camera system can be used, but for faster varying targets, the system must be used with APD detectors. The latter can operate at resolutions of < 1 µsec with a timing accuracy of 25 nsec. Data loads for the L3Camera system are typically 86 GB per Hr per camera (for full frame). The APD system only stores data if a photon is detected and each photon event is individually time tagged to a 40 MHz clock (25 nsec). For the APDs, 1TB of disc space would provide a minimum of 8.5 hours recording time.