EVIDENCE OF ACCRETION-GENERATED X-RAYS IN THE YOUNG ...

than the front-illuminated CCDs.
Each of the 10 ACIS CCDs has square pixels 0.49 00 on a side, giving each CCD
afieldofviewof8.3 0 . The total field of view is 16.9 0 by 16.9 0 for ACIS-I and 8.3 0
by 50.6 0 for ACIS-S. A maximum of six CCDs may be used simultaneously during
an observation. A typical configuration uses all six of the ACIS-S CCDs or all four
ACIS-I CCDs along with the S2 and S3 CCDs (either to maximize the imaging area
for spatially extended X-ray sources or to increase the likelihood of serendipitous
observations of other X-ray events).
When an X-ray photon (with an energy of at least 3.7 eV) strikes the (mostly)
silicon CCD and is absorbed, electrons are liberated. The number of ejected electrons
depends upon the energy of the incident X-ray photon. Due to the removal of an
electron from a silicon atom, a net positive charge – a “hole” – is left behind. The
freed electrons are then confined close to the interaction site via an electric field so
as not to recombine with the holes. After an exposure, the confined charges are
then transferred via “gate structures,” each of which consists of three electrodes that
e↵ectively transfer confined charge from one pixel to another by rapidly varying the
voltages in the electrodes. The gates are also what actually define the pixel boundaries
in the CCD. When the transferred charge reaches the edge of the CCD, the charge
is then transferred to a serial readout that then transfers each pixel charge to a local
processor to determine the position and amplitude of any detected X-rays. Each full
frame exposure lasts 3.2 seconds, e↵ectively giving ACIS a cadence of ⇠3.2 seconds.
As opposed to a typical observation with an optical CCD where the user only sees a
total flux in one filter after an entire observation, the high candence of ACIS allows
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