Patrick J. Sutton

X-Pipeline: an analysis package for autonomous gravitational-wave burst searches

Autonomous gravitational-wave searches—fully automated analyses of data that run without human intervention or assistance—are desirable for a number of reasons. They are necessary for the rapid identification of gravitational-wave burst candidates, which in turn will allow for follow-up observations by other observatories and the maximum exploitation of their scientific potential. A fully automated analysis would also circumvent the traditional by hand setup and tuning of burst searches that is both labourious and time consuming. We demonstrate a fully automated search with X-Pipeline, a software package for the coherent analysis of data from networks of interferometers for detecting bursts associated with gamma-ray bursts (GRBs) and other astrophysical triggers. We discuss the methods X-Pipeline uses for automated running, including background estimation, efficiency studies, unbiased optimal tuning of search thresholds and prediction of upper limits. These are all done automatically via Monte Carlo with multiple independent data samples and without requiring human intervention. As a demonstration of the power of this approach, we apply X-Pipeline to LIGO data to compute the sensitivity to gravitational-wave emission associated with GRB 031108. We find that X-Pipeline is sensitive to signals approximately a factor of 2 weaker in amplitude than those detectable by the cross-correlation technique used in LIGO searches to date. We conclude with comments on the status of X-Pipeline as a fully autonomous, near-real-time-triggered burst search in the current LSC-Virgo Science Run.

The transient gravitational-wave sky

Interferometric detectors will very soon give us an unprecedented view of the gravitational-wave sky, and in particular of the explosive and transient Universe. Now is the time to challenge our theoretical understanding of short-duration gravitational-wave signatures from cataclysmic events, their connection to more traditional electromagnetic and particle astrophysics, and the data analysis techniques that will make the observations a reality. This paper summarizes the state of the art, future science opportunities, and current challenges in understanding gravitational-wave transients.

Identifying the host galaxy of gravitational wave signals

One of the goals of the current LIGO-GEO-Virgo science run is to identify transient gravitational wave (GW) signals in near real time to allow follow-up electromagnetic (EM) observations. An EM counterpart could increase the confidence of the GW detection and provide insight into the nature of the source. Current GW-EM campaigns target potential host galaxies based on overlap with the GW sky error box. We propose a new statistic to identify the most likely host galaxy, ranking galaxies based on their position, distance, and luminosity. We test our statistic with Monte Carlo simulations of GWs produced by coalescing binaries of neutron stars and black holes, one of the most promising sources for ground-based GW detectors. Considering signals accessible to current detectors, we find that when imaging a single galaxy, our statistic correctly identifies the true host {approx}20% to {approx}50% of the time, depending on the masses of the binary components. With five narrow-field images the probability of imaging the true host increases from {approx}50% to {approx}80%. When collectively imaging groups of galaxies using large field-of-view telescopes, the probability improves from {approx}30% to {approx}60% for a single image and from {approx}70% to {approx}90% for five images. For the advanced generation ofmorexa0» detectors (circa 2015+), and considering binaries within 100 Mpc (the reach of the galaxy catalogue used), the probability is {approx}40% for one narrow-field image, {approx}75% for five narrow-field images, {approx}65% for one wide-field image, and {approx}95% for five wide-field images, irrespective of binary type.«xa0less

Multimessenger astronomy with the Einstein Telescope

Gravitational waves (GWs) are expected to play a crucial role in the development of multimessenger astrophysics. The combination of GW observations with other astrophysical triggers, such as from gamma-ray and X-ray satellites, optical/radio telescopes, and neutrino detectors allows us to decipher science that would otherwise be inaccessible. In this paper, we provide a broad review from the multimessenger perspective of the science reach offered by the third generation interferometric GW detectors and by the Einstein Telescope (ET) in particular. We focus on cosmic transients, and base our estimates on the results obtained by ET’s predecessors GEO, LIGO, and Virgo.

Vacuum polarization in the Schwarzschild spacetime and dimensional reduction

A massless scalar field minimally coupled to gravity and propagating in Schwarzschild spacetime is considered. After dimensional reduction under spherical symmetry the resulting 2D field theory is canonically quantized and the renormalized expectation values 〈Tab〉 of the relevant energy-momentum tensor operator are investigated. Asymptotic behaviors and analytical approximations are given for 〈Tab〉 in the Boulware, Unruh and Hartle-Hawking states. Special attention is devoted to the black-hole horizon region where the WKB approximation breaks down.

PROSPECTS FOR JOINT GRAVITATIONAL WAVE AND SHORT GAMMA-RAY BURST OBSERVATIONS

We present a detailed evaluation of the expected rate of joint gravitational-wave and short gamma-ray burst (GRB) observations over the coming years. We begin by evaluating the improvement in distance sensitivity of the gravitational wave search that arises from using the GRB observation to restrict the time and sky location of the source. We argue that this gives a 25% increase in sensitivity when compared to an all-sky, all-time search, corresponding to more than doubling the number of detectable gravitational wave signals associated with GRBs. Using this, we present the expected rate of joint observations with the advanced LIGO and Virgo instruments, taking into account the expected evolution of the gravitational wave detector network. We show that in the early advanced gravitational wave detector observing runs, from 2015-2017, there is only a small chance of a joint observation. However, as the detectors approach their design sensitivities, there is a good chance of joint observations provided wide field GRB satellites, such as Fermi and the Interplanetary Network, continue operation. The rate will also depend critically upon the nature of the progenitor, with neutron star--black hole systems observable to greater distances than double neutron star systems. The relative rate of binary mergers and GRBs will depend upon the jet opening angle of GRBs. Consequently, joint observations, as well as accurate measurement of both the GRB rate and binary merger rates, will allow for an improved estimation of the opening angle of GRBs.

Dimensional-reduction anomaly

In a wide class of D-dimensional spacetimes which are direct or semi-direct sums of a (D-n)-dimensional space and an n-dimensional homogeneous “internal” space, a field can be decomposed into modes. As a result of this mode decomposition, the main objects which characterize the free quantum field, such as Green functions and heat kernels, can effectively be reduced to objects in a (D-n)-dimensional spacetime with an external dilaton field. We study the problem of the dimensional reduction of the effective action for such spacetimes. While before renormalization the original D-dimensional effective action can be presented as a “sum over modes” of (D-n)-dimensional effective actions, this property is violated after renormalization. We calculate the corresponding anomalous terms explicitly, illustrating the effect with some simple examples.

Colloquium: Multimessenger astronomy with gravitational waves and high-energy neutrinos

Many of the astrophysical sources and violent phenomena observed in our Universe are potential nemitters of gravitational waves and high-energy cosmic radiation, including photons, hadrons, and npresumably also neutrinos. Both gravitational waves (GW) and high-energy neutrinos (HEN) are ncosmic messengers that may escape much denser media than photons. They travel unaffected over ncosmological distances, carrying information from the inner regions of the astrophysical engines from nwhich they are emitted (and from which photons and charged cosmic rays cannot reach us). For the same nreasons, such messengers could also reveal new, hidden sources that have not been observed by nconventional photon-based astronomy. Coincident observation of GWs and HENs may thus play a ncritical role in multimessenger astronomy. This is particularly true at the present time owing to the nadvent of a new generation of dedicated detectors: the neutrino telescopes IceCube at the South Pole and nANTARES in the Mediterranean Sea, as well as the GW interferometers Virgo in Italy and LIGO in the nUnited States. Starting from 2007, several periods of concomitant data taking involving these detectors nhave been conducted. More joint data sets are expected with the next generation of advanced detectors nthat are to be operational by 2015, with other detectors, such as KAGRA in Japan, joining in the future. nCombining information from these independent detectors can provide origin always of constraining the nphysical processes driving the sources and also help confirm the astrophysical origin of a GW or HEN nsignal in case of coincident observation. Given the complexity of the instruments, a successful joint nanalysis of this combined GW and HEN observational data set will be possible only if the expertise and nknowledge of the data is shared between the two communities. This Colloquium aims at providing an noverview of both theoretical and experimental state of the art and perspectives for GW and HEN nmultimessenger astronomy.

Bounding the Time Delay between High-energy Neutrinos and Gravitational-wave Transients from Gamma-ray Bursts

We derive a conservative coincidence time window for joint searches of gravitational-wave (GW) transients and high-energy neutrinos (HENs, with energies & 100GeV), emitted by gamma-ray bursts (GRBs). The last are among the most interesting astrophysical sources for coincident detections with current and near-future detectors. We take into account a broad range of emission mechanisms. We take the upper limit of GRB durations as the 95% quantile of the T90’s of GRBs observed by BATSE, obtaining a GRB duration upper limit of 150s. Using published results on high-energy (> 100MeV) photon light curves for 8 GRBs detected by Fermi LAT, we verify that most highenergy photons are expected to be observed within the rst 150s of the

Towards an optimal search strategy of optical and gravitational wave emissions from binary neutron star coalescence

Observations of an optical source coincident with gravitational wave emission detected from a binary neutron star coalescence will improve the confidence of detection, provide host galaxy localization and test models for the progenitors of short gamma-ray bursts. We employ optical observations of three short gamma-ray bursts, 050724, 050709 and 051221, to estimate the detection rate of a coordinated optical and gravitational wave search of neutron star mergers. Modelu2002R-band optical afterglow light curves of these bursts that include a jet-break are extrapolated for these sources at the sensitivity horizon of an Advanced LIGO/Virgo network. Using optical sensitivity limits of three telescopes, namely TAROT (mu2002= 18), Zadko (mu2002= 21) and an 8–10 m class telescope (mu2002= 26), we approximate detection rates and cadence times for imaging. We find a median coincident detection rate of 4 yr−1 for the three bursts. GRB 050724 like bursts, with wide opening jet angles, offer the most optimistic rate of 13 coincident detections per year, and would be detectable by Zadko up to 5 d after the trigger. Late-time imaging tou2002mu2002= 26 could detect off-axis afterglows for GRB 051221 like bursts several months after the trigger. For a broad distribution of beaming angles, the optimal strategy for identifying the optical emissions triggered by gravitational wave detectors is rapid response searches with robotic telescopes followed by deeper imaging at later times if an afterglow is not detected within several days of the trigger.