The Issues of Mismodelling Gravitational-Wave Data for Parameter Estimation

Bayesian inference is used to extract unknown parameters from gravitational wave signals. Detector noise is typically modelled as stationary, although data from the LIGO and Virgo detectors is not stationary. We demonstrate that the posterior of estimated waveform parameters is no longer valid under the assumption of stationarity. We show that while the posterior is unbiased, the errors will be under- or overestimated compared to the true posterior. A formalism was developed to measure the effect of the mismodelling, and found the effect of any form of non-stationarity has an effect on the results, but are not significant in certain circumstances. We demonstrate the effect of short-duration Gaussian noise bursts and persistent oscillatory modulation of the noise on binary-black-hole-like signals. In the case of short signals, non-stationarity in the data does not have a large effect on the parameter estimation, but the errors from non-stationary data containing signals lasting tens of seconds or longer will be several times worse than if the noise was stationary. Accounting for this limiting factor in parameter sensitivity could be very important for achieving accurate astronomical results, including an estimation of the Hubble parameter. This methodology for handling the non-stationarity will also be invaluable for analysis of waveforms that last minutes or longer, such as those we expect to see with the Einstein Telescope.Comment: 15 pages, 5 figures. Comments welcom

Improving the Sensitivity of Advanced LIGO Using Noise Subtraction

This paper presents an adaptable, parallelizable method for subtracting linearly coupled noise from Advanced LIGO data. We explain the features developed to ensure that the process is robust enough to handle the variability present in Advanced LIGO data. In this work, we target subtraction of noise due to beam jitter, detector calibration lines, and mains power lines. We demonstrate noise subtraction over the entirety of the second observing run, resulting in increases in sensitivity comparable to those reported in previous targeted efforts. Over the course of the second observing run, we see a 30% increase in Advanced LIGO sensitivity to gravitational waves from a broad range of compact binary systems. We expect the use of this method to result in a higher rate of detected gravitational-wave signals in Advanced LIGO data.Comment: 15 pages, 6 figure

Electromagnetic follow-up of gravitational wave candidates

Observations of astrophysical systems in different wavelengths can reveal insights in to systems which are not available from a single wavelength. The same can be expected from multi-channel observations of systems which also produce gravitational waves (GWs). The most likely source of strong, detectable GWs, which will also produce an electromagnetic (EM) signature, is the merger of compact objects containing neutron stars (NS) and black holes (BH), namely NS-NS and NS-BH systems. The focus of this thesis is to summarise current and past efforts to detect an EM counterpart of a GW event, with emphasis on compact merger sources. To begin, the formulation of GWs in general relativity is brie y discussed, as well as the main classes of GW sources. The global networks of GW interferometers in the recent past and near future are described, together with brief explanations of operational principles and the main challenges GW detectors face to make a confident detection. Current literature is reviewed to give a brief summary of the most promising sources which produce both GW and EM signals. Emphasis is given to gamma-ray bursts (GRBs), their afterglows, and kilonovae. In addition a brief description of GW searches triggered by an external source (such as a GRB) is given. A new form of search is then discussed in which GW events are used to point conventional EM telescopes, with emphasis on rapidly slewing, wide field of view optical telescopes. The main challenge in this form of search is that timing information from a network of GW interferometers yields large error regions for the source sky direction making it diffcult to locate an EM transient. Therefore a new statistic is presented in which galaxies (taken from a galaxy catalogue) within this search region are ranked. The probability of identifying the host galaxy of a GW signal from NS-NS and NS-BH systems is investigated and results presented for past and future GW detector configurations. The ROTSE-III telescope system took part in this first search for EM counterparts of GW triggers. With four identical robotic telescopes located across the world it responded to five GW events. Presented is an automation of the ROTSE image processing pipeline which allows large-scale processing and automated validation and classification of candidates. A background study was conducted to better understand the optical transient background and to determine the statistical significance of candidates. Pipeline performance is tested by inserting simulated transients following kilonova and GRB lightcurves in to images; an efficiency study is described. Finally the results of the images taken in response to the five GW events are presented and discussed

The c-terminal extension of a hybrid immunoglobulin A/G heavy chain is responsible for its Golgi-mediated sorting to the vacuole

We have assessed the ability of the plant secretory pathway to handle the expression of complex heterologous proteins by investigating the fate of a hybrid immunoglobulin A/G in tobacco cells. Although plant cells can express large amounts of the antibody, a relevant proportion is normally lost to vacuolar sorting and degradation. Here we show that the synthesis of high amounts of IgA/G does not impose stress on the plant secretory pathway. Plant cells can assemble antibody chains with high efficiency and vacuolar transport occurs only after the assembled immunoglobulins have traveled through the Golgi complex. We prove that vacuolar delivery of IgA/G depends on the presence of a cryptic sorting signal in the tailpiece of the IgA/G heavy chain. We also show that unassembled light chains are efficiently secreted as monomers by the plant secretory pathway

Large-Scale Image Processing with the ROTSE Pipeline for Follow-Up of Gravitational Wave Events

Electromagnetic (EM) observations of gravitational-wave (GW) sources would bring unique insights into a source which are not available from either channel alone. However EM follow-up of GW events presents new challenges. GW events will have large sky error regions, on the order of 10-100 square degrees, which can be made up of many disjoint patches. When searching such large areas there is potential contamination by EM transients unrelated to the GW event. Furthermore, the characteristics of possible EM counterparts to GW events are also uncertain. It is therefore desirable to be able to assess the statistical significance of a candidate EM counterpart, which can only be done by performing background studies of large data sets. Current image processing pipelines such as that used by ROTSE are not usually optimised for large-scale processing. We have automated the ROTSE image analysis, and supplemented it with a post-processing unit for candidate validation and classification. We also propose a simple ad hoc statistic for ranking candidates as more likely to be associated with the GW trigger. We demonstrate the performance of the automated pipeline and ranking statistic using archival ROTSE data. EM candidates from a randomly selected set of images are compared to a background estimated from the analysis of 102 additional sets of archival images. The pipeline's detection efficiency is computed empirically by re-analysis of the images after adding simulated optical transients that follow typical light curves for gamma-ray burst afterglows and kilonovae. We show that the automated pipeline rejects most background events and is sensitive to simulated transients to limiting magnitudes consistent with the limiting magnitude of the images

Cost-benefit analysis for commissioning decisions in GEO600

Gravitational wave interferometers are complex instruments, requiring years of commissioning to achieve the required sensitivities for the detection of gravitational waves, of order 10^-21 in dimensionless detector strain, in the tens of Hz to several kHz frequency band. Investigations carried out by the GEO600 detector characterisation group have shown that detector characterisation techniques are useful when planning for commissioning work. At the time of writing, GEO600 is the only large scale laser interferometer currently in operation running with a high duty factor, 70%, limited chiefly by the time spent commissioning the detector. The number of observable gravitational wave sources scales as the product of the volume of space to which the detector is sensitive and the observation time, so the goal of commissioning is to improve the detector sensitivity with the least possible detector down time. We demonstrate a method for increasing the number of sources observable by such a detector, by assessing the severity of non-astrophysical noise contaminations to efficiently guide commissioning. This method will be particularly useful in the early stages and during the initial science runs of the aLIGO and adVirgo detectors, as they are brought up to design performance.Comment: 17 pages, 17 figures, 2 table