Abstract
The joint detection of the gravitational wave (GW) GW170817 and its electromagnetic (EM) counterparts GRB170817A and kilonova AT 2017gfo has triggered extensive study of the EM emission of binary neutron star mergers. A parameter which is common to and plays a key role in both the GW and the EM analyses is the viewing angle of the binary’s orbit. If a binary is viewed from different angles, the amount of GW energy changes (implying that orientation and distance are correlated) and the EM signatures can vary, depending on the structure of the emission. Information about the viewing angle of the binary orbital plane is therefore crucial to the interpretation of both the GW and the EM data and can potentially be extracted from either side. In the first part of this study, we present a systematic analysis of how well the viewing angle of binary neutron stars can be measured from the GW data. We show that if the sky position and the redshift of the binary can be identified via the EM counterpart and an associated host galaxy, then for 50% of the systems the viewing angle can be constrained to uncertainty from the GW data, independent of electromagnetic emission models. On the other hand, if no redshift measurement is available, the measurement of the viewing angle with GWs alone is not informative, unless the true viewing angle is close to 90°. This holds true even if the sky position is measured independently. Then, we consider the case where some constraints on the viewing angle can be placed from the EM data themselves. We show that the EM measurements can then be used in the analysis of GW data to improve the precision of the luminosity distance, and hence of the Hubble constant, by a factor of 2–3.
- Received 26 July 2018
- Revised 14 July 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031028
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The collision of two neutron stars is one of the richest and most complex astrophysical events we can observe, the details of which could dramatically broaden our understanding of how matter behaves under extreme conditions. As the two stars approach and collide, they emit gravitational and electromagnetic waves, release huge amounts of energy, and produce heavy metals such as gold. This was shown with the detection of gravitational and electromagnetic waves from the binary neutron star merger GW170817 in 2017. However, many of the observed features still await a detailed explanation. Here, we focus on the orientation of the orbital plane, a critical parameter for interpreting the electromagnetic data.
Proposed models for describing these collisions offer different predictions for how the emitted radiation depends on the orientation of the system. Therefore, a precise measurement of the orbital orientation of the binary is key to pinpointing the physics of binary neutron star mergers. We show that gravitational-wave data alone do not usually provide any meaningful measurement of the orbital orientation. On the other hand, if an electromagnetic counterpart is found, which provides the sky position and the distance of the source, then the orientation can often be measured to high precision. Conversely, if the electromagnetic data constrain the orientation, that information can be used to obtain a more precise measurement of the distance to the source and hence of the expansion rate of the Universe.
As gravitational-wave observatories detect more binary neutron stars, some with electromagnetic counterparts, we will be able to understand much better the details of these collisions.
