GW170608: Observation of a 19-solar-mass Binary Black Hole Coalescence

TL;DR

GW170608 reports a low-mass binary black hole merger detected by the LIGO network during O2, with component masses approximately $m_1=12^{+7}_{-2} \mathrm{M}_\odot$ and $m_2=7^{+2}_{-2} \mathrm{M}_\odot$ (chirp mass $\mathcal{M}=7.9^{+0.2}_{-0.2} \mathrm{M}_\odot$) at a luminosity distance of $D_L=340^{+140}_{-140}$ Mpc. A coherent Bayesian analysis using both precessing and non-precessing waveform models yields consistent source parameters, including a modest effective inspiral spin $\chi_\mathrm{eff}=0.07^{+0.23}_{-0.09}$ and a final black hole with $M_\mathrm{f}\approx18\,\mathrm{M}_\odot$ and $a_\mathrm{f}\approx0.69$, while the distance and inclination leave some degeneracy. No deviations from General Relativity are detected; bounds on the graviton mass and dispersion are competitive with prior events. The event extends the observed BBH population into the low-mass regime and informs discussions of metallicity-dependent formation channels and BH-binary demographics for future population studies and rate estimates.

Abstract

On June 8, 2017 at 02:01:16.49 UTC, a gravitational-wave signal from the merger of two stellar-mass black holes was observed by the two Advanced LIGO detectors with a network signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with component masses $12^{+7}_{-2}\,M_\odot$ and $7^{+2}_{-2}\,M_\odot$ (90% credible intervals). These lie in the range of measured black hole masses in low-mass X-ray binaries, thus allowing us to compare black holes detected through gravitational waves with electromagnetic observations. The source's luminosity distance is $340^{+140}_{-140}$ Mpc, corresponding to redshift $0.07^{+0.03}_{-0.03}$. We verify that the signal waveform is consistent with the predictions of general relativity.

Figures (4)

• Figure 1: Time-frequency power maps of LIGO strain data at the time of GW170608. The characteristic upwards-chirping morphology of a binary inspiral driven by GW emission is visible in both detectors, with a higher signal amplitude in LHO. This figure, and all others in this Letter, were produced from noise-subtracted data (Section \ref{['s:parameters']}).
• Figure 2: Posterior probability densities for binary component masses ($m_1$, $m_2$), total mass ($M$), and chirp mass ($\mathcal{M}$) in the source frame. One-dimensional component mass distributions include posteriors for the effective precession (blue) and the non-precessing (red) waveform model, as well as their average (black). The dashed lines demarcate the $90\%$ credible intervals for the average posterior. The two-dimensional plot shows contours of the $50\%$ and $90\%$ credible regions overlaid on a color-coded posterior density function. For comparison, we show both one- and two-dimensional distributions of averaged component mass posterior samples for GW151226 (orange) GW151226-DETECTION. In the top panel, we further compare GW170608 and GW151226's source-frame total mass (left) and source-frame chirp mass (right). All other known binary black holes lie at higher chirp masses than GW170608 and GW151226.
• Figure 3: Top panel: Marginalized one-dimensional posterior density functions for the spin parameters $\chi_p$ and $\chi_\mathrm{eff}$ (blue) in comparison to their prior distributions (pink) as obtained from the effective-precession model. The dashed lines indicate the 90% credible interval. The two-dimensional plot shows the 50% and 90% credible regions plotted over the posterior density function. Bottom panel: Posterior probabilities for the dimensionless component spins $\mathbf{\chi}_i$ with $i=1,2$ relative to the Newtonian orbital angular momentum $\hat{\mathbf{L}}$, i.e. the normal of the orbital plane. The tilt angles are $0^\circ$ for spins parallel to $\hat{\mathbf{L}}$, and $180^\circ$ for spins anti-parallel to $\hat{\mathbf{L}}$. The posterior density functions are marginalized over the azimuthal angles. Each pixel has a prior probability of $\sim1.8 \times 10^{-3}$; they are spaced linearly in spin magnitudes and the cosine of the tilt angles.
• Figure 4: Left: Spectrogram of strain data from LHO around the time of GW170608. This plot shows variations in the noise spectrum of the detector over periods on the scale of minutes; unlike Figure 1, it is not designed to show short-duration transient events. The strain amplitude is normalized to the interval between $-6$ and $-2$ minutes relative to the event time. See Appendix \ref{['s:app_a2l']} for discussion of the feature around $20$ Hz due to an angular control procedure. Right: Amplitude spectral density of strain data at both LIGO observatories for 240 s around the event time, ($-2,2$) minutes on left panel, and for data before the start of the angular coupling minimization at LHO, ($-6,-2$) minutes. Excess noise is clearly visible around $20$ Hz but data above 30 Hz are unaffected.