Theorists have found the field of jets to be a lucrative playground. Chimneys, funnels, tunnels, vents and nozzles evoke common structures that we know can be responsible for launching collimated flows. However, to work on cosmic scales requires alternative conditions and unfamiliar physical regimes to be explored. Severe constraints limit the number of tenable models to just a few.

There are three challenges to face: to launch, to accelerate and to collimate. Winds can be launched and accelerated relatively easily with thermal, radiative and magnetic driving from stars and accretion discs. Vast amounts of energy are released through either collapse, external heating, mass infall or nuclear fusion. Upon expansion into the surroundings, a fraction of the released energy is in some combination of released gas, radiation and magnetic field. During transport, it is converted into radially directed kinetic energy. The discovery of jets, however, indicates that the theory is far more complex than just demonstrating the existence of a driver.

This chapter will first describe pure hydrodynamical methods for driving jets as originally invoked for the extragalactic case, but now relevant to cometary and planetary jets. The observations we have so far discussed have indicated that accretion discs and jets are very strongly correlated except in the solar system. In light of this disc–jet connection, models have concentrated on the need for a rotating accretion disc and associated magnetic torques. Differences come in ascribing the origin of the magnetic fields to the disc itself or to the central star. The most developed of these models are described below in detail.

This chapter focuses on data analysis, a central component of gravitational wave astronomy. After a short introduction to the field we discuss the techniques used to search for the three classes of gravitational wave signals. These include well predicted signals such as coalescing compact binaries, less certain signals such as those from supernovae, and the stochastic signals from gravitational wave backgrounds. We will finish by briefly discussing issues relevant to network detection.

Introduction

Observing gravitational waves requires a data analysis strategy that is in many ways different from conventional astronomical data analysis. There are several reasons why this is so. Gravitational wave antennae are essentially omni-directional, with their response better than 50% of the root mean square over 75% of the sky. Hence, data analysis systems will have to carry out all-sky searches for sources. Additionally, gravitational wave interferometers are typically broadband, covering three to four orders of magnitude in frequency. While this is obviously to our advantage, as it helps to track sources whose frequency may change rapidly, it calls for searches to be carried out over a wide range of frequencies.

In Einstein's theory, gravitational radiation has two independent states of polarisation. Measuring polarisation is of fundamental importance as there are other theories of gravity in which the number of polarisation states is more than two; in some theories dipolar and even scalar waves exist (Will, 1993). Polarisation has astrophysical implications too. For example, gravitational wave polarisation measurements can be very helpful in resolving the degeneracy that occurs in the measurement of the mass and inclination of a binary system.

Rudimentary definitions and concepts

A man-made jet is a narrow stream forced out of a designed aperture or nozzle. Water fountains and jet engines provide everyday examples of liquid and gas jets. Skin penetration and rock drilling are high-technology applications. Jets also occur naturally on the Earth associated with geysers and some types of volcanic eruption. These terrestrial jets arise when material is raised to a high pressure below the surface and is forced to ascend through channels with rigid walls. In contrast, the astrophysical jet involves relatively unfamiliar physics, usually under extreme, but occasionally in exotic, conditions.

In astronomy, there is rarely a solid nozzle or tube to align the jet flow. The material is driven, with a few exceptions, through an interacting gas. In other words, an astrophysical jet is a slender channel of high-speed gas propagating through a gaseous environment. The exceptional jets are the nearest extraterrestrial jets associated with comets such as Hale-Bopp. In the latter case, as for those shown in Fig. 1.1, they are believed to form when a high-pressure mixture of gas and dust breaks through vents in a solid crust.

Astrophysical jets are driven from diverse objects on very different size and mass scales. They can be produced from the vicinity of supermassive black holes in the case of active galactic nuclei (AGN), by star-sized black holes in microquasars, by neutron stars in some X-ray binaries, by protostellar cores in young stellar objects, and by white dwarfs in symbiotic binaries and supersoft X-ray sources.

Directed flows of atoms and molecules are observed to stream away from young stars during their formation. Although the outflows are observationally prominent and suggest mass loss rather than gain, these early evolutionary stages are recognised as such by other signatures which indicate ongoing infall and mass accumulation. The infall continues from a surrounding envelope and through an accretion disc for the first few million years in the lifetime of a typical solar-mass or low-mass star, and probably on a shorter timescale for the formation of a massive star. At first, it appears paradoxical that infall should be so well signposted by outflow.

The accompanying outflow can take many forms besides that of a pair of highly supersonic antiparallel jets (Bally et al., 1996). Often no jet or only one jet may be detected. Quite often, only a partly collimated outflow is observed in the form of two diffuse lobes, expanding in opposite directions: a so-called bipolar outflow. Then, jets are not necessary but there is a choice between jets and collimated winds to provide the thrust and supply the energy to drive the two large-scale reservoirs. In yet other cases, compact shocked knots are moving directly away from the young star. Often arc-shaped, they are interpreted to be bow shocks driven by jet-like flows (or jets which have dissolved into a chain of bullet-like projectiles; see Bachiller (1996)).

This chapter describes the theory of gravitational waves. We first introduce gravitational waves and describe how they are generated and propagate through space. We then show how the luminosity, frequency and amplitude of a gravitational wave source can be defined. A brief mathematical summary of how gravitational waves are a natural consequence of Einstein's general theory of relativity is then provided. To finish, we summarise some important quantities that are used to describe gravitational wave signal strengths and the response of detectors to different types of signal.

Listening to the Universe

Our sense of the Universe is provided predominantly by electromagnetic waves. During the 20th century the opening of the electromagnetic spectrum successively brought dramatic revelations. For instance, optical astronomy gave us the Hubble law expansion of the Universe. Radio astronomy gave us the cosmic background radiation, the giant radio jets powered by black holes in galactic nuclei, and neutron stars in the form of radio pulsars. X-ray astronomy gave us interacting neutron stars and black holes. Infrared astronomy gave us evidence for a massive black hole in the nucleus of our own galaxy.

Gravitational waves offer us a new sense with which to understand our Universe. If electromagnetic astronomy gives us eyes with which we can see the Universe, then gravitational wave astronomy offers us ears with which to hear it. We are presently deaf to the myriad gravitational wave sounds of the Universe. Imagine you are in a forest: you see a steep hillside, massive trees and small shrubs, bright flowers and colourful birds flitting between the trees.

Laser interferometers are quantum instruments. This chapter presents the quantum theory of laser interferometer gravitational wave detectors. We show the basics for analysing the quantum noise in the detector, and for deriving the associated standard quantum limit (SQL) for the sensitivity. By providing different perspectives on the origin of the SQL, we illustrate the motivations behind different approaches for surpassing the SQL.

Introduction

The most difficult challenge in building a laser interferometer gravitational wave (GW) detector is isolating the test masses from the rest of the world (e.g. random kicks from residual gas molecules, seismic activities, acoustic noises, thermal fluctuations, etc.) whilst keeping the device locked around the correct point of operation (e.g. pitch and yaw angles of the mirrors, locations of the beam spots, resonance condition of the cavities, and darkport condition for the Michelson). Once all these issues have been solved, we arrive at the issue that we are going to analyse in this chapter: the fundamental noise that arises from quantum fluctuations in the system. A simple estimate (following the steps of Braginsky, 1968) will already lead us into the quantum world – as it will turn out, the superb sensitivity of gravitational wave detectors will be constrained by the standard quantum limit (SQL), which relates to the fundamental heisenberg uncertainty principle. Further improvements of detector sensitivity beyond this require us to manipulate the quantum coherence of light to our advantage. In this chapter, we will introduce how to analyse GW detectors quantum mechanically, and will describe several advanced configurations to surpass the SQL.

Jets are amongst the most mysterious phenomena to be discovered in modern astronomy. They are able to form and propagate under almost all conditions associated with a vast range of astrophysical objects. This book is concerned with all the diverse jets which have so far been found beyond our own planet. It will be seen that our universe is replete with jets because they act as essential outlets or valves for regulating the birth and early development of discrete objects and their extended environments.

The purpose here is to assimilate all we know from the different disciplines in which they are encountered. I cannot try to review radio galaxies, star formation, comets or planetary nebula, but only the parts in which jets are essential to their understanding. We thus learn about the driving mechanisms involved and their consequent impact, and so learn to appreciate the diversity. The idea is to accumulate, perhaps possible for the last time, all the material which relates to the phenomenom referred to as jets. Hence this is not a series of reviews but a gathering of essential knowledge. And, consequently, by establishing their common properties, this book hopes to represent a turning point in what we have come to understood as jets and what we will go on to discover.

It will be attempted to make this book self-contained with a modicum of required knowledge. It should serve as a timely introduction for astronomy students who seek to develop a broad approach to understand the ‘bigger picture’.

This chapter presents a mixed bag of objects and phenomena from planetary nebula to gamma-ray bursts. The data are fragmentary with results often representing the behaviour of selected attractive objects which may be contradicted by further observations. Nevertheless, the importance of stellar jets to the overall progress in the topic is now immense. This promotion has been earned by the short timescales involved and the known properties of the driving sources. This provides opportunities to track changes in jets, which are often transient, and to relate these changes to the launching accretion disc and star.

The evolutionary timescales associated with accretion discs and jets should scale with the luminosity and mass of the central object. Therefore, the changes that would take millions of years in quasars should occur over just hours and days in galactic sources. Much depends on the escape speed from near the surface of the particular star. Symbiotics and supersoft sources are accreting white dwarfs. Microquasars are radio-emitting X-ray binaries with a radio morphology like quasars and high X-ray luminosity. The primary can be a neutron star or black hole. Gamma-ray bursts are associated with collapsars or hypernovae, which generate black holes and ultra-relativistic jets.

For each set of objects, we define (1) the driving set-up, i.e. the stellar system; (2) the major discoveries; (3) the source activity behind or accompanying the jet launching, e.g. outbursts; and (4) the jet phenomena. The review strategy assumed here is one of speed: we begin with the slower jets and finish with superluminal motions.

The GEO 600 detector is a 600m baseline interferometer in Hannover, Germany. We begin by discussing the history of this detector and go on to describe the techniques used to achieve initial target sensitivity. We then describe a series of upgrades and advanced techniques that will increase the sensitivity of this instrument for frequencies above 500 Hz. We finish by summarising the future plans of GEO 600.

A bit of history

GEO 600 is a British/German gravitational wave detector (see Grote for the LIGO Scientific Collaboration, 2008) located in Germany close to the city of Hannover. The scientific goal of GEO 600, beside taking data for gravitational wave detection, is the demonstration and testing of techniques for advanced detectors. GEO evolved from a collaboration between the groups working on the Garching 30m and the Glasgow 10m prototypes. In 1989 these groups proposed to build an underground 3 km gravitational wave detector, ‘GEO’, in the Harz mountains in northern Germany (Hough et al., 1989) based on earlier proposals (Maischberger et al., 1985; Leuchs et al., 1987). Although reviewed positively, a shortage of funds on both ends, in the British Science and Engineering Research Council and after the German reunification also in the German funding bodies, made the realisation of this large project impossible. The collaborators decided to try obtaining funds for a shorter detector and compensate for the shortness by implementing more advanced techniques. A suitable stretch of land to build a 600m instrument was found 20 km south of the city of Hannover, owned by the Universität Hannover and the state of Lower Saxony.

This chapter discusses how mirrors at cryogenic temperature can be used to improve the sensitivity of advanced gravitational wave interferometers. We start by describing the most relevant physical parameters of sapphire substrates at low temperature. Then we discuss how lowering the temperature of the test masses can reduce thermal noise and suppress thermal aberration. We finish by describing plans for the Large Cryogenic Gravitational-Wave Telescope, an advanced cryogenic interferometer in Japan. Throughout, we will describe not only the advantages of cryogenic temperature for interferometers, but also the significant technical challenges that must be met.

Introduction

The strain sensitivity of advanced gravitational wave interferometric detectors is expected to be limited by quantum noise over most of the detection band. Unfortunately for room temperature interferometers, mirror thermal noise may be the dominant noise source in the hundreds of hertz region. This will result in degradation in the sensitivity and will prevent the successful use of squeezed light in this frequency band. One promising way to significantly decrease the magnitude of the thermal noise is to lower the temperature of the interferometer test masses. Lowering the sensor temperature has greatly extended the range of numerous astronomical detector, such as CCD camera and radio receivers. The technique can also be successfully applied to future gravitational wave detectors.

Cooling the detector mirrors will reduce the thermal noise and will also provide another essential benefit: the wavefront distortion induced by optical absorption will be greatly attenuated due to the properties of the mirror substrate at cryogenic temperature.

The material contained in the jet is the first property to ascertain. Almost all our information is derived from distant observation. We can only hope to send probes to rendezvous with comets and possibly intercept their jets. Further afield, we rely upon the radiation that reaches us. Therefore, a chapter to outline the radiative properties is a necessity to interface observations with the physical structure.

The evidence is accumulated across the electromagnetic spectrum providing a bank of line strengths and continuum fluxes. These continuum values yield the spectral shape, perhaps a power law with excess emission in the form of broad bumps. The specific wavelengths of the line fluxes can often be confidently identified with the generating atoms or molecules, and the ratios of fluxes may directly yield the particle states. Here, we introduce the major radiation processes to which matter in various states is subject. In the chapters which follow, we may then work backwards to relate the spectral properties to the gas which would produce it.

That, however, is only half the story. We must still question whether or not we are detecting the jet itself. Instead, it may merely be the excited interface with ambient gas or it could be a minor component of the jet. In fact, we often detect just a few discrete regions where the density or temperature has been temporarily enhanced. This can be misleading, since these regions are atypical of the underlying continuous flow which has produced them. This has remained highly problematic for extragalactic jets where we only observe the effects of the relativistic electrons and magnetic field.

We have learnt about nearby jets by going out to meet comets as they come in to visit our neighbourhood. Jets of gas and dust appear as comets approach the Sun. Since Vega and Giotto's rendezvous with Halley in 1986, we have known that the jets are generated from within the nucleus of a comet, and that the nucleus is only a few misshapen kilometres of low-density ice and rock. Such jets change the trajectory of a comet, providing a rocket effect, which alters the comet orbit. In addition, mass loss through jets produces the spiral and shell structures seen in the comae and proceed to supply the tails. The mass loss also implies that several metres depth of material is lost from an active region per orbit around the Sun.

Our capability for close encounters has resulted in the only resolved images of the launching regions of astrophysical jets. Here, we first synthesise the observations of cometary jets and establish their properties. In Chapter 9, we will see how the probable launch mechanism compares with other models.

Solar jets have also come into prominence in the last decade through high-resolution satellite imaging. On all scales, from spicules to coronal jets, the Sun displays a bewildering array of jet-like phenomena. The uniting factor is that they are magnetically rooted in the photosphere. Some propagate into the chromosphere, while others shoot up into the corona. Apart from exploiting the same energy source, they really have nothing to do with cometary jets. Rather, being magnetically mediated, the physics is relevant to more distant jets although the obligatory accretion disc is missing.

From the first eight chapters, we conclude that the subject of astrophysical jets has seen observational advances over the last twenty years that take it almost beyond recognition in breadth and depth. The penultimate two chapters have shown how the major theories from the 1980s have been developed and adapted but not dismissed. Yet, there are long-standing debates which have not found a resolution and new issues which stretch our understanding.

The challenging themes can be listed under the following headings. We still need to gather evidence for:

• The process of formation and launch

• The collimation and acceleration

• The propagation, structure and stability

• The contents, both near and far from the driving source.

Here, rather than providing a long list of questions, we focus on some specific hot issues relevant to the general subject: the composition, regulation, feedback and unification.

Composition

The physical parameters, such as the energy, speed, mass flux and abundances of jet material, can often be constrained after identifying the radiation process. However, invisible components, whose presence has not yet been settled, may be flowing out adjacent to the radiating material or mixed in on the particle scale. For example, in the observed warm atomic and cold molecular jets from young stars and protostars, there may be more outflowing mass in the form of cold neutral atomic gas (Nisini et al., 2005). In addition, dust internal to the jet avoids direct detection (Dionatos et al., 2009).

Plans for a third generation interferometric gravitational wave (GW) detector are epitomised by the Einstein Telescope proposal. We start by describing the motivation for building third generation instruments, followed by a description of the different science objectives that can be achieved by such an observatory. In the next section we discuss the technological challenges that must be met to achieve third generation sensitivities. The final section outlines a possible timeline for the development of this detector and various detector configurations that are being considered.

Introduction to the third generation of GW observatories

As described in the previous chapters and based on the current models of GW sources, the next generation of advanced interferometric GW detectors (the ‘second’ generation of GW interferometers, such as ‘Advanced LIGO'and ‘Advanced Virgo’) promise the detection of GW in the first year of operation close to the target sensitivity. For example, at the nominal sensitivity of these apparatuses, it is expected that a few tens of coalescing neutron stars will be detected each year. But, apart from extremely rare events, the expected signal-to-noise ratio (SNR) of these events, in the advanced detectors, is too low for precise astronomical studies of the GW sources and for complementing optical and X-ray observations in the study of fundamental systems and processes in the Universe.

These evaluations and the need for observational precision in GW astronomy have led the GW community to start a long investigative process into the future evolution of advanced detectors to a new (‘third’) generation of apparatuses (Punturo et al., 2009), with a considerably improved sensitivity.