This is a somewhat ad-hoc dictionary of therms used in this research blog.
ACIS
The Advanced Camera for Imaging and Spectroscopy (ACIS) is one of two detectors on Chandra. There are 10 chips arranged into two arrays – ACIS-I and ACIS-S – and for the purposes of this analysis I am interested in the ACIS-I array. The other detector on Chandra is the High-Resolution Camera (HRC).
Chandra
The Chandra X-ray observatory is a satellite, orbiting Earth, and has the best imaging capabilities of any X-ray observatory (in that it provides sub-arcsecond Point-Spread Function for sources observed near the center of the field of view).
More information:
Event
The ACIS and HRC detectors on Chandra record information on each detected “event”; this should be a X-ray photon but can be due to high-energy particles passing through Chandra, which act as a source of background that we have to remove.
The basic quantities recorded for each event - at least when the ACIS detector is used in its most common set up, as it was for the data I am interested in - are time, position, and energy (these values are actually derived from the recorded properties but this difference does not really make any difference to the research here).
time - each chip is read out once every ∼ 3. 2 seconds (a frame), which causes complications for sources bright enough to have multiple events within the same pixel in a frame. Fortunately for me the source I am interested is a lot fainter than this (I measure ∼ 60 events, the observation time is ∼ 20000 seconds, so the measured count rate is 60 / (20000 / 3. 2), or ∼ 0. 01 count/frame).
position - each ACIS detector consists of 1024 by 1024 pixels, and the event location is recorded as the chip number and pixel location. The aspect solution is used to work out the celestial coordinates of each event (that is, where it came from on the sky). For bright sources near the center of the observation it is possible to use some of the data recorded about each event to improve the positional information - using the EDSER sub-pixel algorithm - but for faint, off-axis sources such as the one I am interested in this algorithm is not needed.
energy - X-ray photons pass through the ACIS detector and some get absorbed, creating a cloud of electrons whose charge can be measured. Calibration can then convert this to an estimate of the energy of the incoming photon, but unfortunately this is not a unique conversion - for example see page 18 of the “Introduction to X-ray Data Analysis” PDF by my boss - which means that care should be taken using these values. When fitting a model to the spectrum - to try and determine what physical processes are important in the source - we have to use our knowledge of the instrument response to account for this complexity (in fact spectral analysis does not use energy but a related quantity, the pulse height, instead).
Event file
The basic data file used in X-ray astronomy is the event file, which is a time-sorted list of events that were recorded by the detector.
Point Spread Function (PSF)
The Point Spread Function describes the image created by a point source observed by a system - in this case Chandra and ACIS. On axis (that is, at the center of the image), the PSF is small, with a width of ∼ 0. 5 arcseconds (the PSF size varies with energy, so that low-energy photons have a smaller PSF than high-energy photons). The PSF increases in size as you go further off axis (that is, for sources further away from the center of the image); this is relevant since the group I am interested in is about 8 arcminutes away from the center of the observation
More information:
ACIS-I PSF size versus off-axis angle (note that this link takes you to the figure caption, you have to scroll up the page to get to the plots).
Therm
A unit of energy. Also, more apropos to this document, a misspelling of the word term.
