GRB 170817A: a peculiar low-luminosity short gamma-ray burst associated with a NS-NS merger gravitational wave signal

Short-duration gamma-ray bursts (sGRBs) have long been proposed to be produced in systems involving the coalescence of double neutron stars (NS-NS), and the observations of sGRB afterglows and host galaxies are consistent with such a conjecture. Based on the estimated event rate density derived from previously observed sGRBs at cosmological distances, the chance of detecting a sGRB within a small volume for detectable NS-NS mergers by advanced LIGO is very low. On August 17, 2017, coinciding with a double neutron star merger gravitational wave event detected by the LIGO-Virgo gravitational wave detector network, a short-duration GRB 170817A was detected by both Gamma-Ray Monitor (GBM) on board NASA's Fermi Gamma-Ray Observatory and INTEGRAL. We also detected it in the optical with the BOOTES-5/JGT at San Pedro Martir (Mexico). Here we show that the gamma-ray fluence ($\sim 4.46 \times 10^{-7} \ {\rm erg \ cm^{-2}} $) and spectral peak energy ($\sim 158$ keV) of this sGRB fall into the lower portion of the distributions of known sGRBs. With a very small distance from Earth, its peak isotropic luminosity ($\sim 1.7\times 10^{47}~{\rm erg \ s^{-1}}$) is abnormally low. With its detection, the estimated event rate density above this luminosity is at least $190^{+440}_{-160} {\rm Gpc^{-3} \ yr^{-1}}$, which is close to but still below the NS-NS merger event rate density. The low-luminosity sGRB may originate from a structured jet viewed from a large viewing angle. Alternatively, not all NS-NS mergers produce energetic jets. There are similar faint soft GRBs in the Fermi GBM archival data, a small fraction of which might belong to this new population of nearby, low-luminosity sGRBs.