The Hypervelocity Star SDSS J090745.0+024507 Is Variable^*

We present high-precision photometry of the hypervelocity star SDSS J090745.0+024507 (hereafter HVS), which has a Galactic rest-frame radial velocity of v = 709 km s^-1, and thus has likely been ejected from the supermassive black hole in the Galactic center. Our data were obtained on two nights using the MMT 6.5 m telescope and is supplemented by lower precision photometry obtained on four nights using the FLWO 1.2 m telescope. The high-precision photometry indicates that the HVS is a low-amplitude variable. Assuming a periodic, sinusoidal model for this variation, we obtain period P = 0.2-2 days and amplitude A = 2%-10%. Together with the known effective temperature of T_eff ≃ 10,500 K (spectral type B9), this variability implies that the HVS is probably a MS star, since BHB stars do not appear to be variable. Thus, we resolve the previously reported twofold degeneracy in the luminosity and distance of the star. The variability, effective temperature, and possible periodicity of the HVS indicate that it likely belongs to the class of slowly pulsating B-type main-sequence stars. The HVS has a heliocentric distance of 71 kpc and an age of ≲0.35 Gyr. The time of ejection from the center of the Galaxy is ≤100 Myr, and thus the existence of the HVS constitutes observational evidence of a population of young stars in the proximity of the central supermassive black hole ~0.1 Gyr ago. It is possible that the HVS was a member of a binary that was tidally disrupted by the central black hole; we discuss constraints on the properties of the companion's orbit.