Liverpool Telescope Technical Note 1: Telescope and IO:O Throughput

We measure the absolute photon efficiency of the Liverpool Telescope using observations of photometric standard stars and a throughput model of the IO:O instrument. Over a period of 1 year following mirror re-coating the telescope transmission is in the range 73-79% over 4000-10000 Angstroms. Transmission in the u'-band (~3500 Angstroms) is slightly lower at 65%. Immediately following mirror re-coating the throughput is shown to be consistent within 4% to predictions for two reflections from bare-aluminium-coated optics. The telescope throughput (two reflections) degrades at a rate ~0.0002 mag/day. Also presented are the effective wavelengths and widths for the u',g',r',i',z',B,V broad-band filters, taking into account all optical components of the system

MOPTOP: A multi-colour optimised optical polarimeter

We present the design and science case for the Liverpool Telescope's fourth-generation polarimeter; MOPTOP: a Multicolour OPTimised Optical Polarimeter which is optimised for sensitivity and bi-colour observations. We introduce an optimised polarimeter which is as far as possible limited only by the photon counting efficiency of the detectors. Using a combination of CMOS cameras, a continuously rotating half-wave plate and a wire grid polarising beamsplitter, we predict we can accurately measure the polarisation of sources to ∼ 1% at ∼19th magnitude in 10 minutes on a 2 metre telescope. For brighter sources we anticipate much low systematics (0.1%) than our current polarimeter. The design also gives the ability to measure polarization and photometric variability on timescales as short as a few seconds. Overall the instrument will allow accurate measurements of the intra-nightly variability of the polarisation of sources such as gamma-ray bursts and blazars (AGN orientated with the jet pointing toward the observer), allowing the constraint of magnetic field models revealing more information about the formation, ejection and collimation of jets. © 2016 SPIE

Spectroscopy of candidate electromagnetic counterparts to gravitational wave sources

A programme of worldwide, multi-wavelength electromagnetic follow-up of sources detected by gravitational wave detectors is in place. Following the discovery of GW150914 and GW151226, wide field imaging of their sky localisations identified a number of candidate optical counterparts which were then spectrally classified. The majority of candidates were found to be supernovae at redshift ranges similar to the GW events and were thereby ruled out as a genuine counterpart. Other candidates ruled out include AGN and solar system objects. Given the GW sources were black hole binary mergers, the lack of an identified electromagnetic counterpart is not surprising. However the observations show that is it is possible to organise and execute a campaign that can eliminate the majority of potential counterparts. Finally we note the existence of a "classification gap" with a significant fraction of candidates going unclassified

Confirmation of Monoperiodicity Above 20 Seconds for Two Blue Large-Amplitude Pulsators

Blue Large-Amplitude Pulsators (BLAPs) are a new class of pulsating variable star. They are located close to the hot subdwarf branch in the Hertzsprung-Russell diagram and have spectral classes of late O or early B. Stellar evolution models indicate that these stars are likely radially pulsating, driven by iron group opacity in their interiors. A number of variable stars with a similar driving mechanism exist near the hot subdwarf branch with multi-periodic oscillations caused by either pressure (p) or gravity (g) modes. No multi-periodic signals were detected in the OGLE discovery light curves since it would be difficult to detect short period signals associated with higher-order p modes with the OGLE cadence. Using the RISE instrument on the Liverpool Telescope, we produced high cadence light curves of two BLAPs, OGLE-BLAP-009 ($m_{\mathrm{v}}=15.65$ mag) and OGLE-BLAP-014 ($m_{\mathrm{v}}=16.79$ mag) using a $720$ nm longpass filter. Frequency analysis of these light curves identify a primary oscillation with a period of $31.935\pm0.0098$ mins and an amplitude from a Fourier series fit of $0.236$ mag for BLAP-009. The analysis of BLAP-014 identifies a period of $33.625\pm0.0214$ mins and an amplitude of $0.225$ mag. Analysis of the residual light curves reveals no additional short period variability down to an amplitude of $15.20\pm0.26$ mmag for BLAP-009 and $58.60\pm3.44$ mmag for BLAP-014 for minimum periods of $20$ s and $60$ s respectively. These results further confirm that the BLAPs are monoperiodic

Using dummy and pseudo-dummy amplifiers to correct for common mode CCD noise

Some modern CCD designs provide a dummy readout amplifier that is designed to be operated with the same clock and bias signals as the true amplifier in order to provide a measurement of clock induced and other common-mode noise signals in the true amplifier readout. In general the dummy output signal is subtracted electronically from the true output signal in a differential input preamplifier before digitization. Here we report on an alternative approach where both signals are digitized and the subtraction done in software. We present the results of testing this method of operation using the ARC SDSU generation III CCD controllers and an e2v CCD231 device and find it works well, allowing a noise figure of ~ 2:2 electrons to be reached in the presence of significantly higher (~ 6 electrons) pickup noise. In addition we test the effectiveness of using unused (but still genuine) readout amplifiers on the detector to provide a pseudo-dummy output, which we also find effective in cancelling common mode noise. This provides the option of implementing noise reduction on CCDs that are not equipped with dummy outputs at the expense of overall readout speed

Automated Spectral Reduction Pipelines

The Liverpool Telescope automated spectral data reduction pipelines perform both removal of instrumental signatures and provide wavelength calibrated data products promptly after observation. Unique science drivers for each of three instruments led to novel hardware solutions which required reassessment of some of the conventional CCD reduction recipes. For example, we describe the derivation of bias and dark corrections on detectors with neither overscan or shutter. In the context of spectroscopy we compare the quality of at fielding resulting from different algorithmic combinations of dispersed and non-dispersed sky and lamp flats in the case of spectra suffering from 2D spatial distortions. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

A Luminous Red Nova in M31 and its Progenitor System

We present observations of M31LRN 2015 (MASTER OT J004207.99+405501.1), discovered in M31 in January 2015, and identified as a rare and enigmatic luminous red nova (LRN). Spectroscopic and photometric observations obtained by the Liverpool Telescope showed the LRN becoming extremely red as it faded from its M(V) = -9.4 +/- 0.2 peak. Early spectra showed strong Halpha emission that weakened over time as a number of absorption features appeared, including Na I D and Ba II. At later times strong TiO absorption bands were also seen. A search of archival Hubble Space Telescope data revealed a luminous red source to be the likely progenitor system, with pre-outburst Halpha emission also detected in ground-based data. The outburst of M31LRN 2015 shows many similarities, both spectroscopically and photometrically, with that of V838 Mon, the best studied LRN. We finally discuss the possible progenitor scenarios

The Classification of Periodic Light Curves from non-survey optimized observational data through Automated Extraction of Phase-based Visual Features

We implement two hidden-layer feedforward networks to classify 3011 variable star light curves. These light curves are generated from a reduction of non-survey optimized observational images gathered by wide-field cameras mounted on the Liverpool Telescope. We extract 16 features found to be highly informative in previous studies but achieve only 19.82% accuracy on a 30% test set, 5.56% above a random model. Noise and sampling defects present in these light curves poison these features primarily by reducing our Periodogram period match rate to fewer than 5%. We propose using an automated visual feature extraction technique by transforming the phase-folded light curves into image based representations. This eliminates much of the noise and the missing phase data, due to sampling defects, should have a less destructive effect on these shape features as they still remain at least partially present. We produced a set of scaled images with pixels turned either on or off based on a threshold of data points in each pixel defined as at minimum one fifth of those of the most populated pixel for each light curve. Training on the same feedforward network, we achieve 29.13% accuracy, a 13.16% improvement over a random model and we also show this technique scales with an improvement to 33.51% accuracy by increasing the number of hidden layer neurons. We concede that this improvement is not yet sufficient to allow these light curves to be used for automated classification and in conclusion we discuss a new pipeline currently being developed that simultaneously incorporates period estimation and classification. This method is inspired by approximating the manual methods employed by astronomers

GRAPE: Genetic Routine for Astronomical Period Estimation

Period estimation is an important task in the classification of many variable astrophysical objects. Here we present GRAPE: A Genetic Routine for Astronomical Period Estimation, a genetic algorithm optimised for the processing of survey data with spurious and aliased artefacts. It uses a Bayesian Generalised Lomb-Scargle (BGLS) fitness function designed for use with the Skycam survey conducted at the Liverpool Telescope. We construct a set of simulated light curves using both regular survey cadence and the unique Skycam variable cadence with four types of signal: sinusoidal, sawtooth, symmetric eclipsing binary and eccentric eclipsing binary. We apply GRAPE and a frequency spectrum BGLS periodogram to the light curves and show that the performance of GRAPE is superior to the frequency spectrum for any signal well modelled by the fitness function. This is due to treating the parameter space as a continuous variable.We also show that the Skycam sampling is sufficient to correctly estimate the period of over 90% of the sinusoidal shape light curves relative to the more standard regular cadence.We note that GRAPE has a computational overhead which makes it slower on light curves with low numbers of observations and faster with higher numbers of observations and discuss the potential optimisations used to speedup the runtime. Finally, we analyse the period dependence and baseline importance of the performance of both methods and propose improvements which will extend this method to the detection of quasi-periodic signals

Early time optical polarization of GRB Afterglows: GRB 060418 and GRB 090102

RINGO on the Liverpool Telescope has now measured the optical polarization of GRB 060418 (where a 2 sigma upper limit of P<8% was determined) and GRB 090102 (when a detection of P = 10 +/-1 % was made). We discuss the implications of these observations for the various competing models of GRB jet magnetization and describe a possible unified model that can explain both measurements