Improving LIGO calibration accuracy by tracking and compensating for slow temporal variations

Calibration of the second-generation LIGO interferometric gravitational-wave detectors employs a method that uses injected periodic modulations to track and compensate for slow temporal variations in the differential length response of the instruments. These detectors utilize feedback control loops to maintain resonance conditions by suppressing differential arm length variations. We describe how the sensing and actuation functions of these servo loops are parameterized and how the slow variations in these parameters are quantified using the injected modulations. We report the results of applying this method to the LIGO detectors and show that it significantly reduces systematic errors in their calibrated outputs.Comment: 13 pages, 8 figures. This is an author-created, un-copyedited version of an article published in Classical and Quantum Gravity. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

The Advanced LIGO Photon Calibrators

The two interferometers of the Laser Interferometry Gravitaional-wave Observatory (LIGO) recently detected gravitational waves from the mergers of binary black hole systems. Accurate calibration of the output of these detectors was crucial for the observation of these events, and the extraction of parameters of the sources. The principal tools used to calibrate the responses of the second-generation (Advanced) LIGO detectors to gravitational waves are systems based on radiation pressure and referred to as Photon Calibrators. These systems, which were completely redesigned for Advanced LIGO, include several significant upgrades that enable them to meet the calibration requirements of second-generation gravitational wave detectors in the new era of gravitational-wave astronomy. We report on the design, implementation, and operation of these Advanced LIGO Photon Calibrators that are currently providing fiducial displacements on the order of $10^{-18}$ m/$\sqrt{\textrm{Hz}}$ with accuracy and precision of better than 1 %.Comment: 14 pages, 19 figure

Reconstructing the calibrated strain signal in the Advanced LIGO detectors

Advanced LIGO's raw detector output needs to be calibrated to compute dimensionless strain h(t). Calibrated strain data is produced in the time domain using both a low-latency, online procedure and a high-latency, offline procedure. The low-latency h(t) data stream is produced in two stages, the first of which is performed on the same computers that operate the detector's feedback control system. This stage, referred to as the front-end calibration, uses infinite impulse response (IIR) filtering and performs all operations at a 16384 Hz digital sampling rate. Due to several limitations, this procedure currently introduces certain systematic errors in the calibrated strain data, motivating the second stage of the low-latency procedure, known as the low-latency gstlal calibration pipeline. The gstlal calibration pipeline uses finite impulse response (FIR) filtering to apply corrections to the output of the front-end calibration. It applies time-dependent correction factors to the sensing and actuation components of the calibrated strain to reduce systematic errors. The gstlal calibration pipeline is also used in high latency to recalibrate the data, which is necessary due mainly to online dropouts in the calibrated data and identified improvements to the calibration models or filters.Comment: 20 pages including appendices and bibliography. 11 Figures. 3 Table

Calibration Uncertainty for Advanced LIGO's First and Second Observing Runs

Calibration of the Advanced LIGO detectors is the quantification of the detectors' response to gravitational waves. Gravitational waves incident on the detectors cause phase shifts in the interferometer laser light which are read out as intensity fluctuations at the detector output. Understanding this detector response to gravitational waves is crucial to producing accurate and precise gravitational wave strain data. Estimates of binary black hole and neutron star parameters and tests of general relativity require well-calibrated data, as miscalibrations will lead to biased results. We describe the method of producing calibration uncertainty estimates for both LIGO detectors in the first and second observing runs.Comment: 15 pages, 21 figures, LIGO DCC P160013

Constraining the p-Mode–g-Mode Tidal Instability with GW170817

We analyze the impact of a proposed tidal instability coupling p modes and g modes within neutron stars on GW170817. This nonresonant instability transfers energy fromthe orbit of the binary to internal modes of the stars, accelerating the gravitational-wave driven inspiral.We model the impact of this instability on the phasing of the gravitational wave signal using three parameters per star: an overall amplitude, a saturation frequency, and a spectral index. Incorporating these additional parameters, we compute the Bayes factor (lnBpg !pg) comparing our p-g model to a standard one. We find that the observed signal is consistent with waveform models that neglectp-g effects, with lnBpg !pg ¼ 0.03 þ0.70 −0.58 (maximuma posteriori and90%credible region). By injecting simulated signals that do not include p-g effects and recovering them with the p-g model, we show that there is a ≃50% probability of obtaining similar ln Bpg !pg even when p-g effects are absent.We find that the p-g amplitude for 1.4 M⊙ neutron stars is constrained to less than a few tenths of the theoretical maximum, with maxima a posteriori near one-tenth this maximum and p-g saturation frequency ∼70 Hz. This suggests that there are less than a few hundred excited modes, assuming they all saturate by wave breaking. For comparison, theoretical upper bounds suggest ≲103 modes saturate by wave breaking. Thus, the measured constraints only rule out extreme values of the p-g parameters. They also imply that the instability dissipates ≲1051 erg over the entire inspiral, i.e., less than a few percent of the energy radiated as gravitational waves

Search for Gravitational Waves from a Long-lived Remnant of the Binary Neutron Star Merger GW170817

One unanswered question about the binary neutron star coalescence GW170817 is the nature of its post-merger remnant. A previous search for post-merger gravitational waves targeted high-frequency signals from a possible neutron star remnant with a maximum signal duration of 500 s. Here, we revisit the neutron star remnant scenario and focus on longer signal durations, up until the end of the second Advanced LIGO-Virgo observing run, which was 8.5 days after the coalescence of GW170817. The main physical scenario for this emission is the power-law spindown of a massive magnetar-like remnant. We use four independent search algorithms with varying degrees of restrictiveness on the signal waveform and different ways of dealing with noise artefacts. In agreement with theoretical estimates, we find no significant signal candidates. Through simulated signals, we quantify that with the current detector sensitivity, nowhere in the studied parameter space are we sensitive to a signal from more than 1 Mpc away, compared to the actual distance of 40 Mpc. However, this study serves as a prototype for post-merger analyses in future observing runs with expected higher sensitivity

Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo

We present results on the mass, spin, and redshift distributions with phenomenological population models using the 10 binary black hole (BBH) mergers detected in the first and second observing runs completed by Advanced LIGO and Advanced Virgo. We constrain properties of the BBH mass spectrum using models with a range of parameterizations of the BBH mass and spin distributions. We find that the mass distribution of the more massive BH in such binaries is well approximated by models with no more than 1% of BHs more massive than 45M(circle dot) and a power-law index of alpha = 1.3(-1.7)(+1.4) (90% credibility). We also show that BBHs are unlikely to be composed of BHs with large spins aligned to the orbital angular momentum. Modeling the evolution of the BBH merger rate with redshift, we show that it is flat or increasing with redshift with 93% probability. Marginalizing over uncertainties in the BBH population, we find robust estimates of the BBH merger rate density of R = 53.2(-28.2)(+55.8) Gpc(-3) yr(-1) (90% credibility). As the BBH catalog grows in future observing runs, we expect that uncertainties in the population model parameters will shrink, potentially providing insights into the formation of BHs via supernovae, binary interactions of massive stars, stellar cluster dynamics, and the formation history of BHs across cosmic time

All-sky search for long-duration gravitational-wave transients in the second Advanced LIGO observing run

We present the results of a search for long-duration gravitational-wave transients in the data from the Advanced LIGO second observation run; we search for gravitational-wave transients of 2–500 s duration in the 24–2048 Hz frequency band with minimal assumptions about signal properties such as waveform morphologies, polarization, sky location or time of occurrence. Signal families covered by these search algorithms include fallback accretion onto neutron stars, broadband chirps from innermost stable circular orbit waves around rotating black holes, eccentric inspiral-merger-ringdown compact binary coalescence waveforms, and other models. The second observation run totals about 118.3 days of coincident data between November 2016 and August 2017. We find no significant events within the parameter space that we searched, apart from the already-reported binary neutron star merger GW170817. We thus report sensitivity limits on the root-sum-square strain amplitude hrss at 50% efficiency. These sensitivity estimates are an improvement relative to the first observing run and also done with an enlarged set of gravitationalwave transient waveforms. Overall, the best search sensitivity is h50% rss ¼ 2.7 × 10−22 Hz−1=2 for a millisecond magnetar model. For eccentric compact binary coalescence signals, the search sensitivity reaches h50% rss ¼ 9.6 × 10−22 Hz−1=2

Tests of General Relativity with GW170817

The recent discovery by Advanced LIGOand AdvancedVirgo of a gravitational wave signal from a binary neutron star inspiral has enabled tests of general relativity (GR) with this new type of source. This source, for the first time, permits tests of strong-field dynamics of compact binaries in the presence of matter. In this Letter, we place constraints on the dipole radiation and possible deviations from GR in the post-Newtonian coefficients that govern the inspiral regime. Bounds on modified dispersion of gravitational waves are obtained; in combination with information from the observed electromagnetic counterpart we can also constrain effects due to large extra dimensions. Finally, the polarization content of the gravitational wave signal is studied. The results of all tests performed here show good agreement with GR

Narrow-band search for gravitational waves from known pulsars using the second LIGO observing run

Isolated spinning neutron stars, asymmetric with respect to their rotation axis, are expected to be sources of continuous gravitational waves. The most sensitive searches for these sources are based on accurate matched filtering techniques that assume the continuous wave to be phase locked with the pulsar beamed emission. While matched filtering maximizes the search sensitivity, a significant signal-to-noise ratio loss will happen in the case of a mismatch between the assumed and the true signal phase evolution. Narrowband algorithms allow for a small mismatch in the frequency and spin-down values of the pulsar while coherently integrating the entire dataset. In this paper, we describe a narrow-band search using LIGO O2 data for the continuous wave emission of 33 pulsars. No evidence of a continuous wave signal is found, and upper limits on the gravitational wave amplitude over the analyzed frequency and spin-down ranges are computed for each of the targets. In this search, we surpass the spin-down limit, namely, the maximum rotational energy loss due to gravitational waves emission for some of the pulsars already present in the LIGO O1 narrow-band search, such as J1400 − 6325, J1813 − 1246, J1833 − 1034, J1952 þ 3252, and for new targets such as J0940 − 5428 and J1747 − 2809. For J1400 − 6325, J1833 − 1034, and J1747 − 2809, this is the first time the spin-down limit is surpassed