THE AFTERGLOW and EARLY-TYPE HOST GALAXY of the SHORT GRB 150101B at z = 0.1343

We present the discovery of the X-ray and optical afterglows of the short-duration GRB 150101B, pinpointing the event to an early-type host galaxy at z = 0.1343 ±0.0030. This makes GRB 150101B the most nearby short gamma-ray burst (GRB) with an early-type host galaxy discovered to date. Fitting the spectral energy distribution of the host galaxy results in an inferred stellar mass of ≈7 × 10¹⁰ M_⊙, stellar population age of ≈2-2.5 Gyr, and star formation rate of ≲0.4 M_⊙ yr^-1. The host of GRB 150101B is one of the largest and most luminous short GRB host galaxies, with a B-band luminosity of ≈4.3L^∗ and half-light radius of ≈8 kpc. GRB 150101B is located at a projected distance of 7.35 ±0.07 kpc from its host center and lies on a faint region of its host rest-frame optical light. Its location, combined with the lack of associated supernova, is consistent with an NS-NS/NS-BH merger progenitor. From modeling the evolution of the broadband afterglow, we calculate isotropic-equivalent gamma-ray and kinetic energies of ≈1.3 × 10⁴⁹ erg and ≈ (6-14) × 10⁵¹ erg, respectively, a circumburst density of ≈ (0.8-4) × 10^-5 cm^-3, and a jet opening angle of ≳9°. Using observations extending to ≈30 days, we place upper limits of ≲(2-4) ×10⁴¹ erg s^-1 on associated kilonova emission. We compare searches following previous short GRBs to existing kilonova models and demonstrate the difficulty of performing effective kilonova searches from cosmological short GRBs using current ground-based facilities. We show that at the Advanced LIGO/VIRGO horizon distance of 200 Mpc, searches reaching depths of ≈23-24 AB mag are necessary to probe a meaningful range of kilonova models.