Abstract
For gravitational wave (GW) detected neutron star mergers, one of the leading candidates for electromagnetic (EM) counterparts is the afterglow from an ultra-relativistic jet. Where this afterglow is observed, it will likely be viewed off-axis, such as the afterglow following GW170817/GRB 170817A. The temporal behaviour of an off-axis observed GRB afterglow can be used to reveal the lateral jet structure, and statistical model fits can put constraints on the various model free-parameters. Amongst these parameters is the inclination of the system to the line of sight. Along with the GW detection, the afterglow modelling provides the best constraint on the inclination to the line-of-sight and can improve the estimates of cosmological parameters, for example, the Hubble constant, from GW-EM events. However, modelling of the afterglow depends on the assumed jet structure and—often overlooked—the effects of lateral spreading. Here we show how the inclusion of lateral spreading in the afterglow models can affect the estimated inclination of GW-EM events.
Highlights
Electromagnetic (EM) counterparts to gravitational wave (GW) detected events (GWEM) are amongst the most promising multi-messenger astronomy sources
Where GWs are detected from neutron star mergers, the potential EM counterparts include macro/kilo/ mergernova, Gamma-Ray Bursts (GRBs), and GRB afterglows [1]
GRB afterglows as GW-EMs will probe the structure of the outflows and jets that produce a fraction of the cosmological population of GRBs [2,3,4,5]
Summary
Electromagnetic (EM) counterparts to gravitational wave (GW) detected events (GWEM) are amongst the most promising multi-messenger astronomy sources. For a GW detection of a neutron star merger, the EM afterglows that are typically associated with GRBs offer a unique opportunity to probe these systems via a new trigger, the GW signal. As GRBs are highly beamed, they are typically seen at very small system inclinations to the line-of-sight; a GW triggered GRB producing system will likely be seen at a much higher inclination. In such cases, the prompt GRB will likely be absent, or faint, and the afterglow will appear unique when compared to the cosmological population. GRB afterglows as GW-EMs will probe the structure of the outflows and jets that produce a fraction of the cosmological population of GRBs [2,3,4,5]
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