Hypervelocity stars ( HVSs ) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin .
Following the discovery of the first HVS by Brown and collaborators , we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs : SDSS J091301.0+305120 and SDSS J091759.5+672238 , traveling with Galactic rest-frame velocities at least +558 \pm 12 and +638 \pm 12 km s ^ { -1 } , respectively .
Assuming the HVSs are B8 main sequence stars , they are at distances \sim 75 and \sim 55 kpc , respectively , and have travel times from the Galactic Center consistent with their lifetimes .
The existence of two B8 HVSs in our 1900 deg ^ { 2 } survey , combined with the Yu & Tremaine HVS rate estimates , is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster .
The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major in-fall event at the Galactic Center in the last \sim 160 Myr .