With energies far above those of optical light, X-rays andγ-rays represent the emis- sion generated by some of the most extreme environments the Universe has to offer, with emission mechanisms ranging from the peak energies of the highest temperature thermal flares to scattering from electrons in relativistic jets. However, the absorp- tion of these high energy photons through the photoionization of single atoms is so efficient in the Earth’s atmosphere as to make their observation from the ground all but impossible. As such, the pioneers of X-ray observations made use of instruments mounted on balloons lifted into the upper atmosphere. In the current era, X-ray and γ-ray observations are generally performed by dedicated platforms launched into or- bit. One of the earliest, a set of military satellites by the name of Vela, designed to police the then newly signed Nuclear Test-Ban Treaty, was instrumental in the discovery of the gamma-ray burst phenomenon (Bloom, 2011). Advancements in technology have enabled the application of more sophisticated detection techniques that are employed in the wide-area gamma/hard X-ray transient surveyors ofSwift
andFermi, and in instruments dedicated to the observation of lower energy X-rays including those installed onSwift,Chandra and XMM-Newton.
Numerous methods for the detection ofγ-rays and X-rays exist, from simple rate indicators like Geiger counters to more advanced methods capable of determin- ing the direction and incident energy of the photon. As the latter properties are essential in imaging and spectroscopy, the methods employed in current observing platforms tend to have components capable of both spatial and spectral resolution. While X-ray and gamma-ray emission is capable of penetrating conventional detectors used for longer wavelength emission, CCDs can nonetheless be used for X-ray observations. Unlike in the optical CCD case where in general a single photo- electron is produced by each incident (detected) photon, the much higher energies of X-ray and gamma-ray emission are capable of promoting numerous electrons through multiple secondary ionizations from the primary photolectron. Thus while optical observations require integrated exposures in order to provide a significant detection, single X-ray detections promoting hundreds to thousands of electrons are significant in of themselves. This enables the operation of the CCD in “photon- counting” mode where exposure times are set short enough to detect individual
photons. Not only does this produce fine temporal resolution, enabling customis- able binning of the data which is particularly useful in variability studies, the fact that each photoelectron represents the deposition of a known energy (typically a few eV depending on the detector properties), the number of counts received by the detector from each X-ray photon is effectively a measure of the energy of the photon. This means CCD observations of X-rays can act as both imaging and spec- troscopy simultaneously making them extremely versatile. This approach is used in
Swift’s Burst Alert Telescope (Swift-BAT) and X-ray Telescope (Swift-XRT), IBIS onINTEGRAL and the high energy instruments ofChandra and XMM-Newton.
A different approach was adopted for the Fermi gamma-ray burst detector, which instead makes use of the induced electron-positron production that occurs when a gamma-ray passes close to the nuclei of tungsten atoms in a series of thin foil layers. In this case, the energetics of the detectedγ-rays are determined through the interaction of the electron-positron pairs with a cesium iodide calorimeter at the base of the instrument, the luminosity of the resulting scintillation being a measure of the energy of the incident photon.
The properties of γ-ray emission also makes it resistant to the conventional methods of focussing used in the optics of longer wavelength observing platforms. Instead, wide angle γ-ray detectors such as Swift’s Burst Alert Telescope tend to determine the direction of the incoming photon through use of a coded aperture mask. These components consist of a random, non-repeating pattern of lead tiles that strongly attenuate the gamma-ray emission attempting to pass through them. This array of tiles covers the telescope aperture such that any incident emission will be partially blocked. Spatial information can then be determined by modelling the shadow left on the detector.
The somewhat lower energies observed with X-ray telescopes makes it possi- ble to use grazing incidence mirrors to focus the light in a similar way to the mirrors of optical telescopes. Hyperbolic and parabolic mirrors are arranged such that in- coming light strikes at a low angle of incidence, below the critical angle for X-rays. The resulting mirror can only adequately focus X-rays approaching across a narrow range of angles and thus one mirror set has a very limited collecting area, so coaxial nested mirrors are used to expand the available collecting area. This technology is used in the Swift-XRT satellite and as a result is able to accurately determine the position of GRB X-ray afterglows to of order 100 precision.
Figure 2.1: An example of an optical ground-based image before the process of re- duction is completed. The image has been inverted such that dark regions represent high counts. Note the uneven background and strong fringing which must be re- moved before the image can be analysed effectively. The image itself was taken with GMOS on Gemini in thei0-band.