Flares, jets and quasi-periodic outbursts from neutron star merger remnants

Using numerical relativity simulations with a subgrid dynamo prescription to generate strong initial magnetic fields, we investigate the possibility of launching a jet-like outflow from the hypermassive neutron star (HMNS) during the early stages of the merger, prior to the remnants collapse to a black hole. We demonstrate that buoyant instabilities in the strongly magnetized HMNS can lead to a periodic emission of powerful electromagnetic flares shortly after the merger. These are followed by a collimated mildly relativistic outflow. Both types of outflows feature quasi-periodic kilohertz substructure. These early-time outflows may power precursors to short-duration gamma-ray bursts (SGRB) or in some cases the entire SGRB. While the overall temporal power spectrum we find broadly agrees with the one recently reported for quasi-periodic oscillations in the SGRB GRB910711, our simulations suggest that the periodic electromagnetic substructure is dominated by magnetohydrodynamic shearing processes rather than correlating with the corresponding post-merger gravitational wave signal.Comment: 10 pages, 6 figure

Electromagnetic precursor flares from the late inspiral of neutron star binaries

The coalescence of two neutron stars is accompanied by the emission of gravitational waves, and can also feature electromagnetic counterparts powered by mass ejecta and the formation of a relativistic jet after the merger. Since neutron stars can feature strong magnetic fields, the non-trivial interaction of the neutron star magnetospheres might fuel potentially powerful electromagnetic transients prior to merger. A key process powering those precursor transients is relativistic reconnection in strong current sheets formed between the two stars. In this work, we provide a detailed analysis of how the twisting of the common magnetosphere of the binary leads to an emission of electromagnetic flares, akin to those produced in the solar corona. By means of relativistic force-free electrodynamics simulations, we clarify the role of different magnetic field topologies in the process. We conclude that flaring will always occur for suitable magnetic field alignments, unless one of the neutron stars has a magnetic field significantly weaker than the other.Comment: 19 pages, 10 figure

Reconnection-powered fast radio transients from coalescing neutron star binaries

It is an open question whether and how gravitational wave events involving neutron stars can be preceded by electromagnetic counterparts. This work shows that the collision of two neutron stars with magnetic fields well below magnetar-level strengths can produce millisecond Fast-Radio-Burst-like transients. Using global force-free electrodynamics simulations, we demonstrate that electromagnetic flares, produced by overtwisted common flux tubes in the binary magnetosphere, collide with the orbital current sheet and compress it, resulting in enhanced magnetic reconnection. As a result, the current sheet fragments into a sequence of plasmoids, which collide with each other leading to the emission of coherent electromagnetic waves. The resulting millisecond-long burst of radiation should have frequencies in the range of $10-20\,\rm GHz$ for magnetic fields of $B^{\ast}=10^{11}\, \rm G$ at the stellar surfaces.Comment: 10 pages, 4 figures, version accepted by PR

Electromagnetic precursors to black hole - neutron star gravitational wave events: Flares and reconnection-powered fast-radio transients from the late inspiral

The presence of magnetic fields in the late inspiral of black hole -- neutron star binaries could lead to potentially detectable electromagnetic precursor transients. Using general-relativistic force-free electrodynamics simulations, we investigate pre-merger interactions of the common magnetosphere of black hole -- neutron star systems. We demonstrate that these systems can feature copious electromagnetic flaring activity, which we find depends on the magnetic field orientation but not on black hole spin. Due to interactions with the surrounding magnetosphere, these flares could lead to Fast Radio Burst-like transients and X-ray emission, with $\mathcal{L}_{\rm EM} \lesssim 10^{41} \left( B_\ast/ 10^{12}\, \rm G \right)^2\, \rm erg/ s$ as an upper bound for the luminosity, where $B_\ast$ is the magnetic field strength on the surface of the neutron star.Comment: 13 pages, 6 figures; accepted version, to appear in ApJ