GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral

Abstract

On August 17, 2017 at 12:41:04 UTC the Advanced LIGO and Advanced Virgo gravitational-wave detectors made their first observation of a binary neutron star inspiral. The signal, GW170817, was detected with a combined signal-to-noise ratio of 32.4 and a false-alarm-rate estimate of less than one per $8.0\times10^4$ years. We infer the component masses of the binary to be between 0.86 and 2.26 $M_\odot$, in agreement with masses of known neutron stars. Restricting the component spins to the range inferred in binary neutron stars, we find the component masses to be in the range 1.17 to 1.60 $M_\odot$, with the total mass of the system $2.74^{+0.04}_{-0.01}\,M_\odot$. The source was localized within a sky region of 28 deg$^2$ (90% probability) and had a luminosity distance of $40^{+8}_{-14}$ Mpc, the closest and most precisely localized gravitational-wave signal yet. The association with the gamma-ray burst GRB 170817A, detected by Fermi-GBM 1.7 s after the coalescence, corroborates the hypothesis of a neutron star merger and provides the first direct evidence of a link between these mergers and short gamma-ray bursts. Subsequent identification of transient counterparts across the electromagnetic spectrum in the same location further supports the interpretation of this event as a neutron star merger. This unprecedented joint gravitational and electromagnetic observation provides insight into astrophysics, dense matter, gravitation and cosmology.

Figures (5)

• Figure 1: Time-frequency representations [65] of data containing the gravitational-wave event GW170817, observed by the LIGOHanford (top), LIGO-Livingston (middle), and Virgo (bottom) detectors. Times are shown relative to August 17, 2017 12:41:04 UTC. The amplitude scale in each detector is normalized to that detector's noise amplitude spectral density. In the LIGO data, independently observable noise sources and a glitch that occurred in the LIGO-Livingston detector have been subtracted, as described in the text. This noise mitigation is the same as that used for the results presented in Sec. IV.
• Figure 2: Mitigation of the glitch in LIGO-Livingston data. Times are shown relative to August 17, 2017 12:41:04 UTC. Top panel: A time-frequency representation [65] of the raw LIGO-Livingston data used in the initial identification of GW170817 [76]. The coalescence time reported by the search is at time 0.4 s in this figure and the glitch occurs 1.1 s before this time. The timefrequency track of GW170817 is clearly visible despite the presence of the glitch. Bottom panel: The raw LIGO-Livingston strain data (orange curve) showing the glitch in the time domain. To mitigate the glitch in the rapid reanalysis that produced the sky map shown in Fig. 3 [77], the raw detector data were multiplied by an inverse Tukey window (gray curve, right axis) that zeroed out the data around the glitch [73]. To mitigate the glitch in the measurement of the source's properties, a model of the glitch based on a wavelet reconstruction [75] (blue curve) was subtracted from the data. The time-series data visualized in this figure have been bandpassed between 30 Hz and 2 kHz so that the detector's sensitive band is emphasized. The gravitational-wave strain amplitude of GW170817 is of the order of$10^{-22}$ and so is not visible in the bottom panel.
• Figure 3: Sky location reconstructed for GW170817 by a rapid localization algorithm from a Hanford-Livingston ($190 \mathrm{deg}^{2}$, light blue contours) and Hanford-Livingston-Virgo ( $31 \mathrm{deg}^{2}$, dark blue contours) analysis. A higher latency Hanford-Living-ston-Virgo analysis improved the localization ( $28 \mathrm{deg}^{2}$, green contours). In the top-right inset panel, the reticle marks the position of the apparent host galaxy NGC 4993. The bottom-right panel shows the a posteriori luminosity distance distribution from the three gravitational-wave localization analyses. The distance of NGC 4993, assuming the redshift from the NASA/ IPAC Extragalactic Database [89] and standard cosmological parameters [90], is shown with a vertical line.
• Figure 4: Two-dimensional posterior distribution for the component masses$m_{1}$ and $m_{2}$ in the rest frame of the source for the lowspin scenario ( $|\chi|<0.05$, blue) and the high-spin scenario ( $|\chi|<0.89$, red). The colored contours enclose $90 \%$ of the probability from the joint posterior probability density function for $m_{1}$ and $m_{2}$. The shape of the two dimensional posterior is determined by a line of constant $\mathcal{M}$ and its width is determined by the uncertainty in $\mathcal{M}$. The widths of the marginal distributions (shown on axes, dashed lines enclose $90 \%$ probability away from equal mass of $1.36 M_{\odot}$ ) is strongly affected by the choice of spin priors. The result using the low-spin prior (blue) is consistent with the masses of all known binary neutron star systems.
• Figure 5: Probability density for the tidal deformability parameters of the high and low mass components inferred from the detected signals using the post-Newtonian model. Contours enclosing$90 \%$ and $50 \%$ of the probability density are overlaid (dashed lines). The diagonal dashed line indicates the $\Lambda_{1}=\Lambda_{2}$ boundary. The $\Lambda_{1}$ and $\Lambda_{2}$ parameters characterize the size of the tidally induced mass deformations of each star and are proportional to $k_{2}(R / m)^{5}$. Constraints are shown for the high-spin scenario $|\chi| \leq 0.89$ (left panel) and for the low-spin $|\chi| \leq 0.05$ (right panel). As a comparison, we plot predictions for tidal deformability given by a set of representative equations of state [156-160] (shaded filled regions), with labels following [161], all of which support stars of $2.01 M_{\odot}$. Under the assumption that both components are neutron stars, we apply the function $\Lambda(m)$ prescribed by that equation of state to the $90 \%$ most probable region of the component mass posterior distributions shown in Fig. 4. EOS that produce less compact stars, such as MS1 and MS1b, predict $\Lambda$ values outside our $90 \%$ contour.