  Fast Radio Bursts must be powered by uniquely energetic emission mechanisms.
This requirement has eliminated a number of possible source types, but several
remain. Identifying the physical nature of Fast Radio Burst (FRB) emitters
arguably requires good localisation of more detections, and broadband studies
enabled by real-time alerting. We here present the Apertif Radio Transient
System (ARTS), a supercomputing radio-telescope instrument that performs
real-time FRB detection and localisation on the Westerbork Synthesis Radio
Telescope (WSRT) interferometer. It reaches coherent-addition sensitivity over
the entire field of the view of the primary dish beam. After commissioning
results verified the system performed as planned, we initiated the Apertif FRB
survey (ALERT). Over the first 5 weeks we observed at design sensitivity in
2019, we detected 5 new FRBs, and interferometrically localised each of these
to 0.4--10 sq. arcmin. All detections are broad band and very narrow, of order
1 ms duration, and unscattered. Dispersion measures are generally high. Only
through the very high time and frequency resolution of ARTS are these
hard-to-find FRBs detected, producing an unbiased view of the intrinsic
population properties. Most localisation regions are small enough to rule out
the presence of associated persistent radio sources. Three FRBs cut through the
halos of M31 and M33. We demonstrate that Apertif can localise one-off FRBs
with an accuracy that maps magneto-ionic material along well-defined lines of
sight. The rate of 1 every ~7 days next ensures a considerable number of new
sources are detected for such study. The combination of detection rate and
localisation accuracy exemplified by the 5 first ARTS FRBs thus marks a new
phase in which a growing number of bursts can be used to probe our Universe.
