Are binary black holes the best model for the source of detected transient gravitational waves?

Taking the exciting announcement about GW151226 with a grain of salt?

... we have observed that the neutron star radii (in the sense of the end of the matter distribution, where the pressure drops to zero) are smaller in f(R) theories than in their Einsteinian counterparts. This is due to much of the apparent or effective mass-energy being distributed in the outer gravitational field in f(R) theories, and thus not needing additional matter shells in the star. While current measurements of neutron star radii [42] in quiescent X-ray binary systems do point to radii smaller than preferred in General Relativity (9-11 km versus 12- 13 km for typical masses), more accurate measurements are eagerly awaited.  

Finally, because the calculated neutron star masses can be much larger than in General Relativity, the energy available for gravitational-wave emission as well as the total system mass can well exceed what is assumed in Einsteinian gravity. We have provided the basic reasoning and performed a qualitative analysis on this issue. Thus, the conclusion of the LIGO collaboration [28] that the merging objects must be black holes because the measured total mass seems to be 70M_⊙ and the mass loss about 3M⊙ is restricted to standard General Relativity. If gravity is significantly modified at neutron star scale (which certainly remains to be seen) then f(R) theory may accommodate an emission of 3−4M⊙ in the merger of neutron stars. This is being investigated.

On neutron stars in f(R) theories: small radii, large masses and large energy available for emission in a merger

Miguel Aparicio Resco, Alvaro de la Cruz-Dombriz, Felipe J. Llanes Estrada, Victor Zapatero Castrillo

(Submitted on 11 Feb 2016 (v1), last revised 16 Feb 2016 (this version, v2))

Waiting for electromagnetic counterparts... 

Mergers of binary neutron stars and black hole-neutron star binaries produce gravitational-wave (GW) emission and outflows with significant kinetic energies. These outflows result in radio emissions through synchrotron radiation of accelerated electrons in shocks formed with the circum-merger medium. We explore the detectability of these synchrotron generated radio signals by follow-up observations of GW merger events lacking a detection of electromagnetic counterparts in other wavelengths. We model radio light curves arising from (i) sub-relativistic merger ejecta and (ii) ultra-relativistic jets. The former produces radio remnants on timescales of a few years and the latter produces γ-ray bursts in the direction of the jet and orphan radio afterglows extending over wider angles on timescales of a week to a month. The intensity and duration of these radio counterparts depend on the kinetic energies of the outflows and on circum-merger densities. We estimate the detectability of the radio counterparts of simulated GW merger events to be detected by advanced LIGO and Virgo by current and future radio facilities. The maximum detectable distances for these GW merger events could be as high as 1 Gpc. 20–60% of the long-lasting radio remnants arising from the merger ejecta will be detectable in the case of the moderate kinetic energy of 3 · 10⁵⁰ erg and a circum-merger density of 0.1 cm^-3 or larger, while 5–20% of the orphan radio afterglows with kinetic energy of 1048 erg will be detectable. The detection likelihood increases if one focuses only on the well-localizable GW events.

Radio Counterparts of Compact Binary Mergers detectable in Gravitational Waves: A Simulation for an Optimized Survey

Kenta Hotokezaka, Samaya Nissanke, Gregg Hallinan, T. Joseph W. Lazio, Ehud Nakar, Tsvi Piran

(Submitted on 30 May 2016)

... or correlated PeV to EeV neutrino signals

As the technology of gravitational-wave and neutrino detectors becomes increasingly mature, a multi-messenger era of astronomy is ushered in. Advanced gravitational wave detectors are close to making a ground-breaking discovery of gravitational wave bursts (GWBs) associated with mergers of double neutron stars (NS-NS). It is essential to study the possible electromagnetic (EM) and neutrino emission counterparts of these GWBs. Recent observations and numerical simulations suggest that at least a fraction of NS-NS mergers may leave behind a massive millisecond magnetar as the merger product. Here we show that protons accelerated in the forward shock powered by a magnetar wind pushing the ejecta launched during the merger process would interact with photons generated in the dissipating magnetar wind and emit high energy neutrinos and photons. We estimate the typical energy and fluence of the neutrinos from such a scenario. We find that ∼PeV neutrinos could be emitted from the shock front as long as the ejecta could be accelerated to a relativistic speed. The diffuse neutrino flux from these events, even under the most optimistic scenarios, is too low to account for the two events announced by the IceCube Collaboration, but it is only slightly lower than the diffuse flux of GRBs, making it an important candidate for the diffuse background of ∼PeV neutrinos. The neutron-pion decay of these events make them a moderate contributor to the sub-TeV gamma-ray diffuse background.

Possible High-Energy Neutrino and Photon Signals from Gravitational Wave Bursts due to Double Neutron Star Mergers

He Gao, Bing Zhang, Xue-Feng Wu, Zi-Gao Dai

(Submitted on 13 Jun 2013 (v1), last revised 4 Aug 2013 (this version, v2))

The existence of fast radio bursts (FRBs), a new type of extragalatic transients, has been established recently and quite a few models have been proposed. In this work we discuss the possible connection between the FRB sources and ultra-high energy (> 1018 eV) cosmic rays. We show that in the blitzar model and the model of merging binary neutron stars, the huge energy release of each FRB central engine together with the rather high rate of FRBs, the accelerated EeV cosmic rays may contribute significantly to the observed ones. In other FRB models including for example the merger of double white dwarfs and the energetic magnetar radio flares, no significant EeV cosmic ray is expected. We also suggest that the mergers of double neutron stars, even if they are irrelevant to FRBs, may play a non-ignorable role in producing EeV cosmic ray protons if supramassive neutron stars were formed in a good fraction of mergers and the merger rate is & 10³ yr^-1 Gpc^-3 . Such a possibility will be unambiguously tested in the era of gravitational wave astronomy.

Model-dependent estimate on the connection between fast radio bursts and ultra-high energy cosmic rays

Xiang Li, Bei Zhou, Hao-Ning He, Yi-Zhong Fan, Da-Ming Wei

(Submitted on 19 Dec 2013 (v1), last revised 23 Nov 2014 (this version, v2))