  Future x-ray observatories will require imaging detectors with fast readout
speeds that simultaneously achieve or exceed the other high performance
parameters of x-ray charge-coupled devices (CCDs) used in many missions over
the past three decades. Fast readout will reduce the impact of pile-up in
missions with large collecting areas while also improving performance in other
respects like timing resolution. Event-driven readout, in which only pixels
with charge from x-ray events are read out, can be used to achieve these faster
operating speeds. Speedster-EXD550 detectors are hybrid complementary
metal-oxide semiconductor (CMOS) detectors capable of event-driven readout,
developed by Teledyne Imaging Sensors and Penn State University. We present
initial results from measurements of the first of these detectors,
demonstrating their capabilities and performance in both full-frame and
event-driven readout modes. These include dark current, read noise, gain
variation, and energy resolution measurements from the first two
engineering-grade devices.
