Binary Black Hole Mergers in the first Advanced LIGO Observing Run

TL;DR

This paper reports the first Advanced LIGO observing run results, including two definitive binary black hole mergers (GW150914 and GW151226) and a marginal third candidate (LVT151012). Using two independent, GR-based matched-filter analyses (PyCBC and GstLAL), the study provides detailed parameter estimates, stringent tests of general relativity, and a population-driven inference of BBH merger rates. The findings show no GR violations within current uncertainties and reveal BBH mergers across a broad mass range, with implications for formation channels and expectations for numerous detections in future observing runs. The work also discusses sky localization, distances, spins, and the potential stochastic gravitational-wave background, illustrating the transformative impact of LIGO's initial observations on gravitational-wave astronomy.

Abstract

The first observational run of the Advanced LIGO detectors, from September 12, 2015 to January 19, 2016, saw the first detections of gravitational waves from binary black hole mergers. In this paper we present full results from a search for binary black hole merger signals with total masses up to $100 M_\odot$ and detailed implications from our observations of these systems. Our search, based on general-relativistic models of gravitational wave signals from binary black hole systems, unambiguously identified two signals, GW150914 and GW151226, with a significance of greater than $5σ$ over the observing period. It also identified a third possible signal, LVT151012, with substantially lower significance, and with an 87% probability of being of astrophysical origin. We provide detailed estimates of the parameters of the observed systems. Both GW150914 and GW151226 provide an unprecedented opportunity to study the two-body motion of a compact-object binary in the large velocity, highly nonlinear regime. We do not observe any deviations from general relativity, and place improved empirical bounds on several high-order post-Newtonian coefficients. From our observations we infer stellar-mass binary black hole merger rates lying in the range $9-240 \mathrm{Gpc}^{-3} \mathrm{yr}^{-1}$. These observations are beginning to inform astrophysical predictions of binary black hole formation rates, and indicate that future observing runs of the Advanced detector network will yield many more gravitational wave detections.

Figures (15)

• Figure 1: Left panel: Amplitude spectral density of the total strain noise of the H 1 and L 1 detectors,$\sqrt{S(f)}$, in units of strain per $\sqrt{\mathrm{Hz}}$, and the recovered signals of GW150914, GW151226, and LVT151012 plotted so that the relative amplitudes can be related to the SNR of the signal (as described in the text). Right panel: Time evolution of the recovered signals from when they enter the detectors' sensitive band at 30 Hz . Both figures show the $90 \%$ credible regions of the LIGO Hanford signal reconstructions from a coherent Bayesian analysis using a nonprecessing spin waveform model [48].
• Figure 2: The four-dimensional search parameter space covered by the template bank shown projected into the component-mass plane, using the convention$m_{1}>m_{2}$. The colors indicate mass regions with different limits on the dimensionless spin parameters $\chi_{1}$ and $\chi_{2}$. Symbols indicate the best matching templates for GW150914, GW151226, and LVT151012. For GW150914 and GW151226, the templates were the same in the PyCBC and GstLAL searches, while for LVT151012 they differed. The parameters of the best matching templates are consistent, up to the discreteness of the template bank, with the detector frame mass ranges provided by detailed parameter estimation in Sec. IV.
• Figure 3: Search results from the two analyses. The upper left-hand plot shows the PyCBC result for signals with chirp mass$\mathcal{M}> 1.74 M_{\odot}$ (the chirp mass of an $m_{1}=m_{2}=2 M_{\odot}$ binary) and $f_{\text{peak }}>100 \mathrm{~Hz}$, while the upper right-hand plot shows the GstLAL result. In both analyses, GW150914 is the most significant event in the data, and it is more significant than any background event in the data. It is identified with a significance greater than $5 \sigma$ in both analyses. As GW150914 is so significant, the high significance background is dominated by its presence in the data. Once it has been identified as a signal, we remove it from the background estimation to evaluate the significance of the remaining events. The lower plots show results with GW150914 removed from both the foreground and background, with the PyCBC result on the left and the GstLAL result on the right. In both analyses, GW151226 is identified as the most significant event remaining in the data. GW151226 is more significant than the remaining background in the PyCBC analysis, with a significance of greater than $5 \sigma$. In the GstLAL search, GW151226 is measured to have a significance of $4.5 \sigma$. The third most significant event in the search, LVT151012, is identified with a significance of $1.7 \sigma$ and $2.0 \sigma$ in the two analyses, respectively. The significance obtained for LVT151012 is not greatly affected by including or removing background contributions from GW150914 and GW151226.
• Figure 4: Posterior probability densities of the masses, spins, and distance to the three events GW150914, LVT151012, and GW151226. For the two-dimensional distributions, the contours show$50 \%$ and $90 \%$ credible regions. Top left panel: Component masses $m_{1}^{\text{source }}$ and $m_{2}^{\text{source }}$ for the three events. We use the convention that $m_{1}^{\text{source }} \geq m_{2}^{\text{source }}$, which produces the sharp cut in the two-dimensional distribution. For GW151226 and LVT151012, the contours follow lines of constant chirp mass ( $\mathcal{M}^{\text{source }}=8.9_{-0.3}^{+0.3} \mathrm{M}_{\odot}$ and $\mathcal{M}^{\text{source }}=15.1_{-1.1}^{+1.4} \mathrm{M}_{\odot}$, respectively). In all three cases, both masses are consistent with being black holes. Top right panel: The mass and dimensionless spin magnitude of the final black holes. Bottom left panel: The effective spin and mass ratios of the binary components. Bottom right panel: The luminosity distance to the three events.
• Figure 5: Posterior probability distributions for the dimensionless component spins$c S_{1} /\left(G m_{1}^{2}\right)$ and $c S_{2} /\left(G m_{2}^{2}\right)$ relative to the normal to the orbital plane $L$, marginalized over the azimuthal angles. The bins are constructed linearly in spin magnitude and the cosine of the tilt angles, and therefore have equal prior probability. The left plot shows the distribution for GW150914, the middle plot is for LVT151012, and the right plot is for GW151226.