A Highly Spinning and Aligned Binary Black Hole Merger in the Advanced LIGO First Observing Run

TL;DR

This paper reports GW151216, a highly spinning, aligned binary black hole merger found in LIGO O1 data using a new noise-mitigation pipeline, with a detector-frame chirp mass in $M^{\rm det} \in [20,40]\,M_\odot$ and $P_{\rm astro} \sim 0.71$. Parameter estimation reveals an exceptionally large effective spin $\chi_{\rm eff} \approx 0.81$ under a flat $\chi_{\rm eff}$ prior, with a near-equal mass ratio; analyses with an isotropic spin prior yield a lower $\chi_{\rm eff}$ and a different mass breakdown, illustrating strong prior sensitivity. A precession analysis finds no evidence for spin-orbit precession, reinforcing the interpretation of a predominantly aligned-spin system. The authors argue that these properties favor isolated binary evolution with tidal locking over dynamical formation, and they discuss the event’s implications for the high-redshift BBH population and stellar-evolution channels.

Abstract

We report a new binary black hole merger in the publicly available LIGO First Observing Run (O1) data release. The event has an inverse false alarm rate of one per six years in the detector-frame chirp-mass range $\mathcal{M}^{\rm det} \in [20,40]M_\odot$ in a new independent analysis pipeline that we developed. Our best estimate of the probability that the event is of astrophysical origin is $P_{\rm astro} \sim 0.71\, .$ The estimated physical parameters of the event indicate that it is the merger of two massive black holes, $\mathcal{M}^{\rm det} = 31^{+2}_{-3}\,M_\odot$ with an effective spin parameter, $χ_{\rm eff} = 0.81^{+0.15}_{-0.21}$, making this the most highly spinning merger reported to date. It is also among the two highest redshift mergers observed so far. The high aligned spin of the merger supports the hypothesis that merging binary black holes can be created by binary stellar evolution.

Figures (5)

• Figure 1: The blue curve shows the cumulative number of expected background events above a given value of the coherent score $\rho_c^2$ per O1 run in the BBH 3 bank, estimated from 20000.0 timeslides of the data. The flattening at low values is an artifact of the threshold used while collecting background triggers. Vertical black lines mark candidates, i.e., triggers at physical shifts (with previously reported events and injections removed). The event GW151216 , marked in red, has a FAR of 1 in 52 O1.
• Figure 2: Upper panels show the whitened strains around the trigger time of GW151216 in LIGO Hanford/Livingston detectors (light colored curves). Overplotted are the maximum likelihood fits using the spin-aligned IMRPhenomD waveforms (dark colored curves). Lower panels show the corresponding spectrograms. Note that the best-fit gravitational waveform accumulates nearly the entire signal-to-noise in the frequency range $[30,\,300]\,$Hz.
• Figure 3: Posterior distributions for the detector-frame chirp mass $\mathcal{M}^{\rm det}$, the mass ratio $q=m_2/m_1$, and the effective aligned spin $\chi_{\rm eff}$ obtained using the IMRPhenomD waveform model. We compare results obtained using the flat $\chi_{\rm eff}$ prior (red) and the isotropic spin prior (blue). The contours in the off-diagonal panels enclose 68% and 95% quantiles for the joint posterior distributions for each pair of parameters, and black dots mark the maximum-likelihood solution. The diagonal panels show the marginalized posterior (thick curves) and prior (thin curves) distributions. The parameter values quoted are the median and the 90% credible uncertainty intervals obtained using the flat $\chi_{\rm eff}$ prior.
• Figure 4: Prior and posterior distributions for the effective spin-precession parameter $\chi_p$ obtained using the IMRPhenomPv2 waveform model. We compare the results obtained using all the spin parameters (magenta) and by passing zero in-plane spin components to the waveform generation routine, without changing the rest of the prior (green). The consistency of these two curves illustrates that the detected signal has no signs of precession. The value and range for $\chi_p$ are the median and the 90% credible uncertainty range, respectively.
• Figure 5: Marginalized likelihood contours enclosing $50\%$ and $90\%$ of the distribution for BBH mergers detected to date. Likelihoods are computed using the frequency-domain surrogate model SEOBNRv4_ROMBohe:2016gbl, which is in good agreement with the analysis using IMRPhenomD. The panel in the lower left contrasts the populations of the detected mergers, and persistent and transient X-ray binaries reported in Ref. mcclintock2013black, in the $(\mathcal{M}, \chi_{\rm eff})$ plane. Dashed-dotted lines in the right-hand panel mark the allowed parameter space when the aligned spins of the black holes take specific values, which are typical of scenarios in which the BBH progenitors are tidally locked, or formed dynamically.