LIGO-GW170104

Advanced LIGO has detected another coalescing binary black hole (BBH). The new object (GW170104) was slightly less massive than the first-discovered BBH (GW150915). Both it and GW150914 share a common property: a quantity that measures the “net spin perpendicular to the orbit” (for short, the “net aligned spin”) is consistent with zero. This time, it’s less likely to be positive.

Now that we have observed two similar heavy" binary black holes, we are modestly more confident that these heavy black holes cannot be born with large, exactly parallel spins. (Some people had argued just that before Feb 2016, based on observations of smaller black holes made previously, with X-rays.) The LIGO team suspects that heavy black holes either (a) are individually born with small spins or (b) when they occur in binaries, have tilted spins so the net aligned spin” is small.

At RIT, our group also compared this data to supercomputer simulations, to assess the reliability of our inferences. We helped coordinate followup simulations to further corroborate our answers.

For more information

* [Abbott et al, "GW170104: Observation of a 50-solar-mass binary black hole coalescence at redshift 0.2"](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.221101) Physical Review Letters

* LIGO Open Science Center page for GW170104

Astrophysics The two leading models for forming binary black holes are from pairs of massive stars, or in dense stellar clusters. So far, all LIGO data remains consistent with both options. We’re still looking for a smoking gun. (For example, a binary black hole with a “net aligned spin” which is negative and inconsistent with zero would be harder to produce from stellar siblings.) Binary evolution can produce large tilts too. But in this case, it don’t need to – the spins of GW170104 and GW150914 could both be small or zero.

Inferring parameters If, however, nature provides many strongly precessing binary black holes, then our previous work suggests the models we’ve used for parameter inference might not reliable enough. (Modeling strongly precessing black holes with analytic approximations is hard!) So far these models (and LIGO’s inferences) have been done by making several approximations. We check these answers by performing followup simulations and by direct comparison of LIGO data to numerical relativity

Comparing GW170104 to numerical relativity Left: The following graphic compares GW170104 to 10 best-fitting NR simulations. RIT student Jacob Lange made the reconstructed yellow curves on this figure, using simulations of binary black holes provided by RIT (6), SXS (2), BAM (1), and Georgia Tech (1). The solid blue region is the inferred parameter range reconstructed by LIGO’s modeled inference [Image credit: LSC/James Clark]. Right: This movie compares the LIGO observations (shown in yellow and blue) to the expected response according to specific RIT simulations of binary black holes (shown in black). The inset in the upper left illustrates the properties of the black holes in each simulation. [Image credit: LSC/Andrew Williamson]

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For experts: My personal perspective : LIGO observations are now making reality a project I’ve worked on and proselytized about for the last decade: astrophysics enabled by measuring (precessing) binary black hole spins.

* Some of my pertinent older talks:

     Distinguishing between clusters and field binaries via isotropic vs random spins ([Syracuse & Amaldi, 2009](http://ccrg.rit.edu/~oshaughn/Talks/2009-05-29-Syracuse.pdf); [Stony Brook 2010](http://ccrg.rit.edu/~oshaughn/Talks/2010-06-StonyBrook-Talk.pdf); [UWM 2010](http://ccrg.rit.edu/~oshaughn/Talks/2010-12-UWM-Talk-New.pdf))

We’ve known for years that different formation models (clusters vs field binaries) made very different predictions for spin orientations (isotropic vs modestly misaligned). Since those predictions are so different, we can quickly distinguish between formation models so long as each observation provides some information about spin magnitudes and misalignments. [How? Statistics 101 methods on any parameter work fine eventually, though sharper methods yield robust results sooner.] The challenge is getting the data; making precise measurements of misalignments; and connecting those observed misalignments to astrophysical parameters.

* Some of my pertinent  papers:

    Modeling and understanding the distinctive physics of precessing binary black holes (e.g.,[Lundgren and ROS 2013, a precessing waveform model](http://adsabs.harvard.edu/abs/2014PhRvD..89d4021L); [ROS et al "Precession during merger", 2013](http://adsabs.harvard.edu/abs/2013PhRvD..87d4038O); [ROS et al 2011, on the corotating frame from outgoing radiation](http://adsabs.harvard.edu/abs/2011PhRvD..84l4002O))

   * Measuring spins of (precessing) binary black holes (e.g., [Trifiro et al 2016](http://adsabs.harvard.edu/abs/2016PhRvD..93d4071T); [Cho et al 2014](http://adsabs.harvard.edu/abs/2014PhRvD..89j2005O); the [2010 ``big dog" blind injection](http://www.ligo.org/news/blind-injection.php) and associated [Abbott et al 2013 PE paper](http://adsabs.harvard.edu/abs/2013PhRvD..88f2001A)), notably by using  numerical relativity (Lange et al 2017; [Abbott et al 2016](http://arxiv.org/abs/1606.01262))

    * Connecting the spins LIGO sees now  and the spin misalignments billions of years ago, when the BH was first formed  (e.g., [Gerosa et al 2015](http://arxiv.org/abs/1506.03492)).  [Since black hole spins evolve substantially over billions of years,  this work is particularly critical for interpreting LIGO measurements for astrophysics. We show how to evolve systems in time, emphasizing the role of the "net aligned spin" as a conserved constant.]

    * Connecting observed misalignments to astrophysics (e.g., [Gerosa et al 2013](http://adsabs.harvard.edu/abs/2013PhRvD..87j4028G); [ROS, Gerosa, Wysocki 2017](https://arxiv.org/abs/1704.03879))