Astronomers at the giant Arecibo radar in Puerto Rico pinged the space rock as it approached. Using a pulsed beam of radio energy at 2380 MHz, they found that 2016 AZ8 has a moon:

The radar team says the primary is nearly spherical with a diameter of 430 meters, while the asteroid’s moon is more elongated, shaped like a cigar measuring 180 meters on its long axis.

Binary asteroids are not as rare as you might suppose. Studies show that about 15% of the near-Earth asteroid population has a moon. In the geological past, these systems have struck Earth, creating double impact craters such as the twin crater in Jämtland, Sweden. Ultimately, space agencies may need to deflect or destroy an asteroid hurtling toward us. Knowing the properties of binary systems such as 2016 AZ8 could prove crucial to future planetary defense.

Source: Spaceweather