  The physical characterization of potentially hazardous asteroids (PHAs) is
important for impact hazard assessment and evaluating mitigation options. Close
flybys of PHAs provide an opportunity to study their surface photometric and
spectral properties that enable identification of their source regions in the
main asteroid belt. We observed PHA (357439) 2004 BL86 during a close flyby of
the Earth at a distance of 1.2 million km (0.0080 AU) on January 26, 2015, with
an array of ground-based telescopes to constrain its photometric and spectral
properties. Lightcurve observations showed that the asteroid was a binary and
subsequent radar observations confirmed the binary nature and gave a primary
diameter of 300 meters and a secondary diameter of 50-100 meters. Our
photometric observations were used to derive the phase curve of 2004 BL86 in
the V-band. Two different photometric functions were fitted to this phase
curve, the IAU H-G model (Bowell et al. 1989) and the Shevchenko model
(Shevchenko 1996). From the fit of the H-G function we obtained an absolute
magnitude H=19.51+/-0.02 and a slope parameter G=0.34+/-0.02. The Shevchenko
function yielded an absolute magnitude of H=19.03+/-0.07 and a phase
coefficient b=0.0225+/-0.0006. The phase coefficient was used to calculate the
geometric albedo (Ag) using the relationship found by Belskaya and Schevchenko
(2000), obtaining a value of Ag=40+/-8% in the V-band. With the geometric
albedo and the absolute magnitudes derived from the H-G and the Shevchenko
functions we calculated the diameter (D) of 2004 BL86, obtaining D=263+/-26,
and D=328+/-35 meters, respectively. 2004 BL86 spectral band parameters and
pyroxene chemistry are consistent with non-cumulate eucrite meteorites.
