LOW-Gamma JETS FROM COMPACT STELLAR MERGERS: CANDIDATE ELECTROMAGNETIC COUNTERPARTS TO GRAVITATIONAL WAVE SOURCES

Short gamma-ray bursts (GRBs) are believed to be produced by relativistic jets from mergers of neutron stars (NSs) or NSs and black-holes (BHs). If the Lorentz-factors Γ of jets from compact stellar mergers follow a similar power-law distribution to those observed for other high-energy astrophysical phenomena (e.g., blazars, active galactic nuclei), the population of jets should be dominated by low-Γ outflows. These jets will not produce prompt gamma-rays, but jet energy will be released as X-ray/optical/radio transients when they collide with the ambient medium. Using Monte Carlo simulations, we study the properties of such transients. Approximately 78% of merger jets <300 Mpc result in failed GRBs if the jet Γ follows a power-law distribution of index −1.75. X-ray/ optical transients from failed GRBs will have broad distributions of their characteristics: light-curves peak tp ~ 0.1-10 days after a merger; flux peaks for X-ray 10^-6 mJy <~ Fx <~ 10^-2 mJy; and optical flux peaks at 14 <~ mg <~ 22. X-ray transients are detectable by Swift XRT, and ~85% of optical transients will be detectable by telescopes with limiting magnitude mg >~ 21, for well localized sources on the sky. X-ray/optical transients are followed by radio transients with peak times narrowly clustered around tp ~ 10 days, and peak flux of ∼10–100 mJy at 10 GHz and ∼0.1 mJy at 150 MHz. By considering the all-sky rate of short GRBs within the LIGO/Virgo range, the rate of on-axis orphan afterglows from failed GRBs should be 2.6(26) per year for NS–NS (NS–BH) mergers, respectively. Since merger jets from gravitational-wave (GW) trigger events tend to be directed to us, a significant fraction of GW events could be associated with the on-axis orphan afterglow.