X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

4.3. ACIS – ADVANCED CCD IMAGING SPECTROMETER 29• Continuous Clocking (CC) modeCC mode achieves high timing resolution (3 ms) but has only one dimension of spatial resolution.Data are continuously clocked through the CCD and framestore to minimize thereadout time, with one-dimensional (1 × 1024 pixels) images.Telemetry formatThree telemetry formats are available as follows. Table 4.3 lists the operating modes, associatedformats, and approximate event rates at the telemetry saturation. The faint and very faint formatswere used for obs-BF and obs-A, respectively (§5.1).• GradedThis provides event positions in detector coordinates, event amplitudes, arrival times, andevent grades.• FaintThis provides the same information as the Graded format other than the event grades. Alternatively,this supplies all information about 3 × 3 pixels around the event pixel. The biasmap (zero level of each pixel) is telemetered separately.• Very FaintThis is basically the same as the Faint format, but provides all information about 5 × 5 pixelsaround the event pixel. This format is used for the TE mode only and offers the advantagefor minimizing the background after ground processing, but is limited for low-count sources(Table 4.3).Event pileupFor bright sources, two or more X-ray photons are often registered in a single ACIS pixel within onereadout cycle, which is called pileup. It causes an artificial spectral hardening and underestimateof the X-ray counts. In fact, the brightest two sources in the ρ Ophiuchi cloud (A-2 and BF-64,see chapter 5) suffered the pileup effect. It can be reduced by selecting a proper observing mode(e.g., shorten the exposure time).