The previous post described the gravitational wave event GW170817 (which took place on August 17, 2017) and how it was ultimately identified as a binary neutron star merger. In addition, it was associated with a gamma ray burst (designated GW170817) and imaged across the electromagnetic spectrum, an unprecedented and landmark event in the field of multi-messenger astronomy. Though it is intrinsically of interest to be able to both "see" (EM waves) and "hear" (gravitational waves) an astrophysical event, what are some other conclusions to be drawn from the merger?
One simple conclusion requires nothing more than a quick calculation, but verifies a foundational principle of physics that while almost universally assumed, had never been directly proven. This principle states that both electromagnetic waves and gravitational waves travel at the speed of light in a vacuum, about 3*108 m/s. Recall that the merger is estimated to have taken place about 130 million light-years away. This means that both the gravitational wave signal and the gamma ray burst both took about 130 million years to travel from the source to detectors on Earth. Despite their long journey, they arrived within a few seconds of each other. Now, we cannot be certain exactly when the gamma ray burst was emitted, due to our incomplete understanding of how a binary neutron star merger would work. However, it is likely that the neutron stars must first collide (marking the end of the gravitational wave signal) before emitting a burst of gamma radiation. Moreover, this initial high-energy burst was estimated by most models to occur no more than a few minutes after the merger. Therefore, dividing the amount by which the signals could have drifted apart over their travel time, we obtain bounds on the "speed of gravity" relative to the speed of light. Even with conservative assumptions, these observations prove that the two speeds very likely differ by no more than one part in a trillion (0.0000000001%) and probably several orders of magnitude less than this. Theoretically, they are equal, but never before has this been measured with such incredible precision.
In a similar vein, the merger allowed other tests of various aspects of general relativity and field theory, such as the influence of gravitational waves on the propagation of electromagnetic field. The data all confirmed the current understanding of general relativity and set very tight bounds on possible deviations, better than those ever achieved in the past.
The detection of the merger also taught us about the very structure of neutron stars. Unlike black holes, which (to our knowledge) are effectively points of mass, neutron stars are on the order of a few miles across. Considering their mass (usually 1-2 solar masses), they are exceedingly dense, but nevertheless their physical size affects how the gravitational wave event unfolds. When the two objects get very close to one another, their mutual gravitational attraction is expected to cause tidal deformations, i.e. warping of their shape and mass distribution. In theory, information concerning the deformation is encoded in the measured waveforms.
The figure above (click to enlarge), while rather technical, gives some idea as to how exactly the gravitational wave data constrain the structure of the neutron star. The statement |χ| < 0.05 in the diagram indicates that the entire figure is made presupposing that the neutron stars were not spinning too fast (which our knowledge of neutron star systems suggests is a very reasonable assumption). The two axes measure the magnitude of two parameters Λ1 and Λ2 that measure how much the larger and the smaller neutron stars, respectively respond to tidal deformation. In other words, smaller values of the parameters (toward the lower left) mean denser and more compact neutron stars, as indicated. More on what these parameters actually mean can be found in the original paper here.
Next, the darker shades of blue represent values considered more likely given the shape of the gravitational wave signal. This is a probability distribution, and lighter shaded areas were not ruled out with certainty, but simply deemed less likely. The uncertainty in the original masses contributes to the uncertainty in this diagram. Finally, the gray shaded "stripes" indicate the predictions of several different theoretical models of neutron stars. These are distinguished by their different equations of state, which specify how mass, pressure, density, and other properties of neutron stars relate to one another. The varying predictions of these models show just how little was definitively known about neutron stars. Analysis of the merger event suggested that the "SLy" and "APR4" models were more accurate than the rest (at 50% confidence) and that the "MS1" and "MS1b" models are unlikely to be correct (with more than 90% confidence). No model was ruled out for sure, but the data above suggest that neutron stars are more compact than most models predicted.
The gamma ray burst that followed the merger also contained some information concerning how these mergers actually occur and the physics of when and why high-energy radiation is released. Notably, the gamma ray burst was a single short pulse (lasting under a second) with no discernible substructure. It was difficult to draw conclusions from this limited sample, but explaining the nature of the pulse and the delay may require a dense layer of ejecta from each of the neutron stars to momentarily impede electromagnetic radiation from the merger. It would take some time for gamma rays to penetrate this cloud of debris until they finally burst through.
Moreover, among the known population of gamma ray bursts, GW1701817A was relatively dim. This may have been due to the main "jets" of energy not being along our line of sight; most of the burst is hypothesized to have been released along the original axis of rotation of the two bodies. The discrepancy may in part have been due to observational bias, since brighter events are more likely to be observed. In such cases, the Earth likely was directly facing the angle of peak gamma ray emission. Detecting an event "off-center" elucidates somewhat the structure and extent of the these jets.
The image above (click to enlarge) was originally from this paper. It demonstrates schematically some different theories explaining the relative dimness of the gamma ray burst event and the structure of the jets along the axis of rotation. Earlier theories postulated a relatively uniform jet, as shown in the first scenario. If this is the case, our line of sight may have been outside the jet, but relativistic effects allowed us to see a smaller amount of the radiation. Other explanations postulate that the jet has some internal structure and "fades" with increasing angle from the axis (ii) or that the interaction of the jet with surrounding matter produces a secondary cocoon of radiation (iii). A final scenario is simply that this event was a few orders of magnitude dimmer than other known gamma ray bursts for some intrinsic reason, although the authors deem this unlikely.
Without the background information provided by the gravitational wave signal (the component masses of the merger, the timing of the merger, etc.), little of the above could be gleaned from the gamma ray signal. Nor would it be possible with only one of the two to conduct the precision tests of fundamental physics described earlier. These are examples of the power of multi-messenger astronomy. Having both an eye and an ear to the cosmos will continue to yield fundamental insights into the nature of our universe.
Sources: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.119.161101, https://arxiv.org/pdf/1710.05834.pdf, http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9/pdf
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