Low Lorentz Factor 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 (NS) or neutron-stars and black-holes (BH). If the Lorentz-factors $\Gamma$ of jets from compact-stellar-mergers follow a similar power-law distribution to those observed for other high-energy astrophysical phenomena (e.g. blazars, AGN), the population of jets would be dominated by low-$\Gamma$ outflows. These jets will not produce the 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 $\Gamma$ 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 $t_p\sim0.1-10~$days after a merger; flux peaks for x-ray $10^{-6}{\rm~mJy}\lesssim~F_x\lesssim10^{-2}~$mJy; and optical flux peaks at $14\lesssim~m_g\lesssim22$. X-ray transients are detectable by Swift XRT, and $\sim85\%$ of optical transients will be detectable by telescopes with limiting magnitude $m_g \gtrsim 21$, for well localized sources on the sky. X-ray/optical transients are followed by radio transients with peak times narrowly clustered around $t_p\sim10~$days, and peak flux of $\sim~10-100~$mJy at 10 GHz and $\sim~0.1~$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-GRB would 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.
Comment: 13 pages, 4 figures. Accepted for publication in ApJ