Distrito de Bogotá, Secretaria Distrital de Ambiente

Since X-rays are absorbed by the atmosphere, observations must be made by facilities onboard satellites (usually dedicated astronomy facilities but occasionally as part of larger projects such as the International Space Station, ISS). Various observatories are currently operational, each having slightly different capabilities driven by their primary science objectives. We describe in detail the instruments which are most heavily used in this work.

1.3.1

XMM-Newton

XMM-Newton (X-ray Multi-Mirror, Jansen et al. 2001) is ESA’s current flagship X-ray mission and also has facilities for UV/optical observations. XMM-Newton has three similar co-aligned X-ray telescopes (hence Multi-Mirror) with identical mirror modules (Aschenbach, 2002; Gondoin et al., 1998a,b). Two of these also include a reflection grating assembly,

which diffracts around half of the incident photons onto the Reflection Grating Spectrometers (RGS, Brinkman et al., 1998; den Herder et al., 2001). The remaining non-dispersed photons are detected by the European Photon Imaging Camera (EPIC) MOS CCD detectors (Turner et al., 2001). In the third telescope, all of the flux is focussed onto a third EPIC detector, which uses pn CCDs (Strüder et al., 2001). Each EPIC camera operates over the 0.3 − 10 keV range; this covers from the lowest energies above Galactic absorption and high enough to include the iron Kα line at 6.4 keV. This can be used to measure the soft excess and warm absorbers as well as make spin measurements from the iron line. The RGS only operates up to ∼ 3 keV, but at much higher resolution due to the dispersion from the gratings so provides detailed measurements of narrow lines in the soft band.

The last telescope onboard XMM-Newton is the Optical Monitor (OM, Mason et al., 2001). This is a 30 cm diameter Ritchey-Chretien telescope providing coverage in various filters from 170 to 650 nm. This provides simultaneous measurements of wavelengths with significant contribution from the disc, so is useful when determining the SED of AGN.

1.3.2

NuSTAR

NuSTAR(Harrison et al., 2013) is the leading observatory for hard (> 10 keV) X-rays, with innovations which provide major improvements over previous generation instruments in observations of either very bright or very faint sources. It is the first focussing telescope to operate at_{≳ 10 keV. This greatly reduces the contribution from background so allows precise} measurements of faint sources. Additionally, NuSTAR uses a triggered readout instead of a CCD, reducing the time over which pile-up can occur from the frame readout time to the trigger response time, which is short enough that pile-up is irrelevant for any astrophysical source. However, the dead-time before the detector is again sensitive limits throughput to around 400 detected counts/s. This asymptotic limit to the detected rate reduces the accuracy of flux measurements for very bright sources, but this effect is only significant at over 10 000 counts/s.

NuSTARoperates over 3 − 78 keV, which in the context of accreting black holes provides simultaneous measurements of the iron Kα line and Compton hump and allows measurements of coronal temperatures in many sources.

1.3.3

Swift

The Neil Gehrels Swift Observatory(Swift) is a multi-wavelength mission principally designed to study gamma ray bursts; the high slew speed to achieve rapid follow-up of a gamma ray detection also makes Swift ideal for monitoring of other high-energy sources (as multiple

short observations can be made efficiently). The highest energy instrument on Swift is the Burst Alert Telescope (BAT), operating from 15 − 200 keV with a wide field of view. The X-ray telescope (XRT) operates at similar energies to XMM-Newton but with only around 10% of the collecting area. The final science instrument is the Ultra-Violet and Optical Telescope (UVOT). This is derived from XMM-Newton-OM and uses flight spares from the OM as its optics. The available filters are also equivalent in wavelength while offering higher sensitivity, especially in the UV.

While the sensitivity of Swift is significantly lower in any given band than current class- leaders, the frequency with which multiple observations may be made is unparalleled, so Swift is uniquely suited to studying variability of multi-wavelength processes, such as relations between the disc and corona in AGN.

1.3.4

Others

Several other X-ray observatories provide (or recently provided) performance in complemen- tary aspects of observing (the impact of these telescopes may be at least as significant as those described in more detail above; the distinction is based on their use in the following chapters).

Chandra(Weisskopf et al., 2002) covers a similar energy range to XMM-Newton and has higher spatial resolution, though at the expense of effective area and throughput. This means it is most useful for detection of very faint objects or observations of spatially extened objects (e.g. galaxy clusters).

Suzaku (Inoue, 2003; Mitsuda et al., 2007) was a Japanese mission which provided coverage from soft to very hard X-rays until its end-of-life in mid 2015. Suzaku included two functioning instruments, the X-ray imaging spectrometer (XIS, Koyama et al., 2007; Matsumoto et al., 2005) and Hard X-ray detector (HXD, Kokubun et al., 2004; Takahashi et al., 2007). The XIS consisted of four identical units, each similar to one of XMM-Newton’s X-ray telescopes. The HXD operated from 10 to 700 keV using two types of detector: silicon PIN diodes below 50 keV and GSO scintillators above. The intended primary isntrument was a calorimeter (Stahle et al., 2004) which would have provided imaging spectroscopy at much higher resolution (∼ 6.5 eV FWHM) than other technologies but this suffered a failure before any science data could be taken. Despite this, the remaining instruments provided valuable data across an exceptionally broad X-ray band.

MAXI (Matsuoka et al., 2009) is an all sky monitor on the ISS operating from 2 − 20 keV which is useful for early identification of transient outbursts.

The Neutron star Interior Composition ExploreR (NICER Gendreau et al., 2016) is a recent X-ray telescope designed to determine the mass-radius relation of neutron stars by

making measurements of their pulse profiles. The fast timing capability and high effective area this requires is also ideal for studying rapid changes in other bright sources and NICER has an extensive observatory programme taking observations of most X-ray binaries and many bright AGN.