Radio Flares of Compact Binary Mergers: the Effect of Non-Trivial Outflow Geometry

TL;DR

This paper investigates how complex outflow geometries in neutron star mergers affect the late-time radio flares, finding that non-spherical shapes generally extend the emission timescale, potentially reducing detection chances.

Contribution

It introduces an approximate method to estimate radio light-curves considering non-trivial outflow geometries, providing bounds on the expected signals.

Findings

Non-spherical outflows can enhance radio emission but mainly increase the emission timescale.

Non-spherical geometries tend to weaken the radio signals, possibly lowering detection prospects.

The method offers upper limits to the effects of outflow shape on radio flares.

Abstract

The next generation gravitational waves (GW) detectors are most sensitive to GW emitted by compact (neutron star/black hole) binary mergers. If one of those is a neutron star the merger will also emit electromagnetic radiation via three possible channels: Gamma-ray bursts and their (possibly orphan) afterglows (Eichler et al. 1989), Li-Paczynski Macronovae (Li & Paczynski 1998) and radio flares (Nakar & Piran 2011). This accompanying electromagnetic radiation is vitally important in confirming the GW detections (Kochanek & Piran 1993). It could also reveal a wealth of information regarding the merger and will open a window towards multi-messenger astronomy. Identifying and characterizing these counterparts is therefore of utmost importance. In this work we explore late time radio flares emitted by the dynamically ejected outflows. We build upon previous work and consider the effect of the outflow's non-trivial geometry. Using an approximate method we estimate the radio light-curves for several ejected matter distributions obtained in numerical simulations. Our method provides an upper limit to the effect of non-sphericity. Together with the spherical estimates the resulting light curves bound the actual signal. We find that while non-spherical geometries can in principle lead to an enhanced emission, in most cases they result in an increase in the timescale compared with a corresponding spherical configuration. This would weaken somewhat these signals and might decrease the detection prospects.