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Ground based high time resolution astronomical (HTRA) observations require specialised detectors. Photon counting detectors that combine high spatial resolution imaging with fast event timing capability have been a significant technical challenge. We have developed a high-temporal and -spatial resolution, high-throughput sealed tube microchannel plate detector with electronic readout as a tool for HTRA applications. The design is based on a 25 mm diameter S20 photocathode followed by a microchannel plate stack, read out by a cross delay line anode with timing and imaging electronics. The detector supports 500 kHz global count rate, 10 kHz /100 μm2/sec local count rate, and 100 ps timing resolution. We also describe high time resolution astronomical observations currently being obtained with the NASA GALEX UV satellite, as well as preliminary visible wavelength imaging results obtained on the Crab pulsar at the Lick Observatory. [ABSTRACT FROM AUTHOR]

When one spends 108-8.5 euros on a large aperture telescope, it behoves one to make maximal use of the photons concentrated at the focus. One popular path is to use very large focal plane arrays, for a high multiplex advantage; this is a natural driver in extragalactic survey science. The other avenue to efficiency applies to single source science, where a large field of view is not necessary, but energy resolution over a broad wavelength range, high time resolution and, possibly, polarization measurements wring maximum benefit from the collected photons. Cryogenic energy-resolving sensors are proving to be promising detectors for many areas of astronomy. Significant astronomical observations have, in fact, already been made using two detector schemes: Superconducting Tunnel Junction (STJ) and Transition-Edge Sensor (TES). TES instruments, in particular, have found application as bolometric detectors from the sub-mm to the hard X-ray range. We focus here on TES applications in the near IR through UV, reviewing the basic technology and describing current work toward effective imaging arrays. A TES camera using such arrays can address the need for broad-band single source efficiency at large telescopes noted above. A review of early test observations and a description of a few science goals are presented to illustrate this potential. [ABSTRACT FROM AUTHOR]

Investigative studies have designed concept instruments for a proposed 42 m European Extremely Large Telescope (E-ELT) facility suite. An ELT will aid, at its most basic level, detection of the faintest stellar sources and instrinsic features, and detailed quantitative analysis of relatively bright sources. The development of such a telescope thus plays to the strengths of high time resolution (HTR) detectors which provide single photon detection capabilities and/or extremely high refresh rates—they can maximize detection of faint sources through photon counting and they can cope with the high photon flux from relatively bright objects building up extremely high S/N. Specific considerations and fast detectors then need to be adopted into the ELT design to accommodate high time resolution astrophysics (HTRA) efficiently. No one instrument or detector can cover all HTRA timescales (nanoseconds to seconds and ‘slower'), and a dedicated facility HTRA instrument on the ELT is unlikely as yet, but chosen facility instruments can include HTRA options in their packages. Currently there are numerous HTRA options in existing telescope facilities around the world as well as many visiting HTRA instruments. This demonstrates an active HTRA community with worldwide support and interest in HTRA observation capabilities, which underscores the need for HTR capabilities to be designed into the ELT. [ABSTRACT FROM AUTHOR]

Geiger-mode Avalanche Photodiodes (GM-APDs) are establishing themselves as potential candidates for the broad temporal range covered in high time resolution astrophysics (HTRA). These detectors have already been employed in astronomical instrumentation and significant results have been obtained to date. Their high time resolution and quantum efficiency make these single photon event counting detectors ideal for observations of stochastic phenomena, and ultimately for extreme HTRA observations. In this chapter, we review the technology and to illustrate their potential we briefly touch on specific science goals and astronomical applications. We then focus on the fabrication and characterisation of GM-APDs, and discuss the development and challenges posed in designing array devices. [ABSTRACT FROM AUTHOR]

Imaging detectors with high quantum efficiency over a wide spectral range, low noise, and fast readout will prove to be very useful in many applications in astronomy and other fields. In 2005 we have started to develop pnCCDs with high quantum efficiency for fast imaging of single optical photons in the wavelength range ∽350-1100 nm. The concept is based on fully depleted, back-illuminated pnCCDs with a sensitive thickness of 450 μmum which were originally developed for fast single photon X-ray imaging and spectroscopy. The pnCCD with column parallel readout shall be modified such that the on-chip JFET amplifiers are replaced by avalanche amplifier (AA) cells operated in the linear regime with a gain up to a few thousand. An additional on-chip n-channel MOSFET manages the suitable change of impedance such that the resulting signal can be handled by the following CAMEX readout ASICs. The amplification of the avalanche process must be sufficiently low to prevent optical crosstalk between the detector channels. The anticipated format in the imaging area will be of the order of 256× 256 pixels with a size of ∽50 μmum. The frame rate should be as high as 1000 frames per second. [ABSTRACT FROM AUTHOR]

We discuss the advantages and limitations of Electron Multiplying CCD technology in high precision photometry on short timescales, with special emphasis on probing the smallest structures in active galactic jets. Factors external to the EMCCD, rather than the architecture of the EMCCD itself, most often limit the precision of photometry that can be reached with groundbased observations. Although EMCCDs can be used in photon-counting mode, reliable photometry can still only be achieved when careful observing procedures are employed. Observations of a small sample of blazars taken with an EMCCD are briefly discussed. An innovative two-colour photometer, based on simultaneous observations with two EMCCDs is described and the science drivers are discussed. Novel opportunities for the use of EMCCDs in lucky photometry are outlined and it is clear that certain scientific questions cannot be addressed with groundbased observations taken with conventional CCDs. [ABSTRACT FROM AUTHOR]

Optical polarization is a valuable diagnostic tool in astrophysics, frequently enabling asymmetries in source regions, magnetic field configurations, and magnetic field strengths to be investigated. In the context of high time resolution astrophysics polarimetry is particularly valuable in understanding the nature of the optical pulsations from pulsars, and whilst the averaged (over many cycles) linear polarization of the Crab nebula pulsar was determined many years ago, no attempts have been made to determine the Stokes vector of single, individual, pulse cycles. This may be very important information in the investigation into the nature of the enhanced optical pulses associated with random giant radio pulses. In the case of the Crab nebula this requires an instrumental response time of no more than 100 μs, and, possibly, the capability of tagging individual photons with nanosecond resolution. This imposes certain constraints on the design of the polarimeter, which rules out all of the popular designs. We describe the design of a novel polarimeter which will be able to meet these constraints.This is based upon a design by Compain and Drevillon [1] in which their design is modified, by the use of a different glass and prism angles, to work over a wide bandwidth of 400-800 nm and with a high polarimetric efficiency. [ABSTRACT FROM AUTHOR]

Current astronomical instrumentation essentially exploits only spatial or temporal coherence properties of the incoming photon stream. However, beyond this first-order coherence, and encoded in the arrival times of the individual photons, information lies about the details of emission mechanisms such as stimulated emission or of subsequent scattering. The Extremely Large Telescopes of the future could provide the high photon flux needed to export to the astronomical field the photonic techniques currently applied in the laboratory. These ideas were developed in a conceptual study of a focal plane instrument (QuantEYE) for the 100 m OverWhelmingly Large Telescope of the European Southern Observatory. QuantEYE would be a novel astronomical photometer capable to push the time tagging capabilities toward the pico-second region. We are now building a prototype of QuantEYE for the Asiago 182 cm telescope (AquEYE), to be followed by a larger instrument for existing 8-10 m telescopes. This paper expounds the adopted technological solutions and the first steps performed to develop such a prototype [ABSTRACT FROM AUTHOR]

The HIgh-Time resolution (HIT) mode of FORS2 has 3 sub-modes that allow for imaging and spectroscopy over a range of timescales from milliseconds up to seconds. It is the only high time resolution spectroscopy mode available on an 8 m class telescope. In imaging mode, it can be used to measure the pulse of pulsars and spinning white dwarfs in a variety of high throughput broad- and narrow-band filters. In spectroscopy mode it can take up to 10 spectra per second using a novel "shift-and-wait" clocking pattern for the CCD. It takes advantage of the user-designed masks which can be inserted into FORS2 to allow any two targets within the 6.8' × 6.8' field of view of FORS2 to be selected. A number of integration, or more precisely ‘wait', times are available, which together with the high throughput GRISMs can observe the entire optical spectrum on a range of timescales. [ABSTRACT FROM AUTHOR]

A high-speed photo-polarimeter, "OPTIMA" short for Optical Pulsar Timing Analyzer, has been designed and developed in the group for gamma-ray astronomy of the Max-Planck-Institut für Extraterrestrische Physik. This sensitive, portable detector is used to observe optical emissions of sources that radiate mainly at X- and gamma-ray energies, like pulsars and other highly variable compact sources. The single photon counting instrument is based on fiber fed avalanche photodiodes (APDs), a GPS timing receiver,a CCD camera for target acquisition and a stand-alone data acquisition and control system. Several configurations are available: for photometry a hexagonal bundle with seven channels and one fiber offset for sky background monitoring; for polarimetry a rotating polarization filter in front of the photometer or a newly developed 4-channel double Wollaston system; and for coarse spectroscopy a 4-colour prism spectrograph. [ABSTRACT FROM AUTHOR]

Quantum optics potentially offers an information channel from the Universe beyond the established ones of imaging and spectroscopy. All existing cameras and all spectrometers measure aspects of the first-order spatial and/or temporal coherence of light. However, light has additional degrees of freedom, manifest in the statistics of photon arrival times, or in the amount of photon orbital angular momentum. Such quantum-optical measures may carry information on how the light was created at the source, and whether it reached the observer directly or via some intermediate process. Astronomical quantum optics may help to clarify emission processes in natural laser sources and in the environments of compact objects, while high-speed photon-counting with digital signal handling enables multi-element and long-baseline versions of the intensity interferometer. Time resolutions of nanoseconds are required, as are large photon fluxes, making photonic astronomy very timely in an era of large telescopes. [ABSTRACT FROM AUTHOR]

This chapter asks what high time resolution can bring to the study of stellar pulsation, with particular focus on the pulsations of subdwarf B stars. We give a brief introduction to the different types of pulsation found in stars, and identify which short-period oscillations might benefit from high-speed observations. We describe the basic properties of radial and non-radial oscillations, their effect on the stellar surface and emitted flux, and the consequences for the observer. The particular case of non-radial oscillations in subdwarf B stars and their potential for asteroseismology is outlined. Various campaigns to obtain high time resolution spectroscopy and photometry, and hence to identify pulsation modes in subdwarf B stars, are described in more detail. We close with a brief summary of problems in stellar pulsation likely to benefit from advances in high time resolution technology. [ABSTRACT FROM AUTHOR]

The large surveys and sensitive instruments of modern astronomy are turning ever more examples of variable objects, many of which are extending the parameter space to testing theories of stellar evolution and accretion. Future projects such as the Laser Interferometer Space Antenna (LISA) and the Large Synoptic Survey Telescope (LSST) will only add more challenging candidates to this list. Understanding such objects often requires fast spectroscopy, but the trend for ever larger detectors makes this difficult. In this contribution I outline the science made possible by high-speed spectroscopy, and consider how a combination of the well-known progress in computer technology combined with recent advances in CCD detectors may finally enable it to become a standard tool of astrophysics. [ABSTRACT FROM AUTHOR]

The discovery of pulsars in 1968 heralded an era where the temporal characteristics of optical detectors had to be reassessed. Up to this point detector integration times would normally be measured in minutes rather seconds and definitely not on sub-second time scales. At the start of the 21st century pulsar observations are still pushing the limits of detector/telescope capabilities. Flux variations on times scales less than 1 nsec have been observed during giant radio pulses. Pulsar studies over the next 10-20 years will require instruments with time resolutions below a microsecond, high-quantum quantum efficiency, reasonable energy resolution and sensitive to circular possible and linear polarisation of stochastic signals. This chapter is a review of temporally resolved optical observations of pulsars. It concludes with estimates of the observability of pulsars with both existing telescopes and into the ELT era. [ABSTRACT FROM AUTHOR]

Cataclysmic Variables (CVs) show a wide range of time-dependent phenomena which make them ideal objects of study with high time-resolution instruments. The most rapid timescales observed in CVs are of the order of a few seconds, and thus CVs are relatively slow objects compared to some phenomena studied by the high time-resolution community. However, the rich and varied processes which occur in CVs make high time-resolution observations powerful tests of cutting-edge astrophysics. Here we present a review of high time-resolution observations of CVs to date, and underline their relevance to modern astrophysics. [ABSTRACT FROM AUTHOR]

Neutron stars are compact stars which contain matter of supranuclear density in their interiors (presumably with a large fraction of neutrons). They have typical masses M ∼ 1.4M⊙ and radii R ∼ 10 km. Thus, their masses are close to the solar mass M⊙ = 1.989 × 1033 g, but their radii are ∼ 105 times smaller than the solar radius R⊙ = 6.96×105 km. Accordingly, neutron stars possess an enormous gravitational energy Egrav and surface gravity g, (1.1)$$ \begin{gathered} E_{grav} \sim GM^2 /R \sim 5 \times 10^{53} erg \sim 0.2Mc^2 , \hfill \\ g \sim GM/R^2 \sim 2 \times 10^{14} cm{\text{ }}s^{ - 2} , \hfill \\ \end{gathered} $$ where G is the gravitational constant and c is the speed of light. Clearly, neutron stars are very dense. Their mean mass density is (1.2)$$ \bar \rho \simeq 3M/(4\pi R^3 ) \simeq 7 \times 10^{14} g cm^{{\text{ - 3}}} \sim (2 - 3)\rho _0 , $$ where ρ0 = 2.8 × 1014 g cm−3 is the so called normal nuclear density, the mass density of nucleon matter in heavy atomic nuclei. The central density of neutron stars is even larger, reaching (10-20) ρ0. By all means, neutron stars are the most compact stars known in the Universe. [ABSTRACT FROM AUTHOR]

This chapter is different from others. It deals with objects which, at the time of this writing, are hypothetical. We do not know whether strange matter and strange stars exist or not. We start with the discussion of the strange matter hypothesis (§8.3). Its history is described in §8.4. Models of strange matter are presented in §§8.5-8.8. Finally, §8.9 is devoted to other speculative self-bound phases of super-dense matter. [ABSTRACT FROM AUTHOR]

As discussed in Chapter 5, the fundamental problem of the EOS of neutronstar cores cannot be solved on purely theoretical basis: there is no strict theory but many theoretical models instead. Therefore, one can try to select (constrain) the true model using observations. In the present Chapter we will describe some results of this activity. The methods to constrain the EOS are numerous. The activity started just from the discovery of neutron stars in 1967 (Chapter 1). The results obtained by the time of this writing (2006) are a tremendous challenge of observational astrophysics, but look like a failure in their essence: the EOS has been only weakly constrained by observations. [ABSTRACT FROM AUTHOR]

It has been recognized since the 1960s that with increasing density ρ above the standard nuclear-matter density ρ0 the matter can undergo phase transitions to states qualitatively different from the state at ρ ∼ ρ0. These high-density phases are exotic by the standards of terrestrial nuclear physics. Their possible existence above some threshold densities results from assumed specific features of strong (hadronic) interactions (pion condensation, kaon condensation) and the quark structure of baryons (quark deconfinement). [ABSTRACT FROM AUTHOR]

As we have seen in §3.5, nuclei cannot exist at densities exceeding ∼ (1.5 − 2.0) × 1014 g cm−3. At these densities the matter becomes a uniform plasma of neutrons, protons, and electrons. Up to ρ ∼ 2ρ0 its properties can be calculated in a rather reliable way using the methods of the nuclear many-body theory, which have been applied with some success for the microscopic description of ordinary nuclear structure. At such densities the only baryons present in the ground state of the matter are nucleons which are in beta equilibrium with electrons (and muons if the electron Fermi energy exceeds mμc2 = 105.7 MeV). The shell of the neutron-star core with densities ρ ≲ 2ρ0 is called the outer core. Its matter will be called the npeμ matter. At higher densities our assumption of the npeμ composition of dense matter can be invalid because the matter may contain hyperons. The derivation of the equation of state (EOS) of the matter composed of nucleons, hyperons, electrons, and muons will be the topic of the present chapter. [ABSTRACT FROM AUTHOR]

Neutron stars are relativistic objects. Their structure and evolution should be studied using the General Theory of Relativity. The importance of relativistic effects for a star of mass M and radius R is characterized by the compactness parameterrg/R given by Eq. (1.4), where rg is the Schwarzschild radius. Typically, one has rg/R ∼ 0.2 − 0.4 for a neutron star, whereas rg/R ≪ 1 for all other stars. For instance, rg/R ∼ 10-4 for white dwarfs and rg/R ∼ 10−6 for main-sequence stars of M ≃ M⊙. [ABSTRACT FROM AUTHOR]

Magnetic fields B ≳ 1012 G, typical for isolated neutron stars (§1.3.8), drastically modify many physical properties of the matter. Motion of free electrons and ions perpendicular to the field lines is quantized into Landau orbitals with a characteristic transverse scale equal to the magnetic lengtham = (ħc/eB)1/2. This brings to the scene an atomic field-strength parameter γ = (a0/am)2, where a0 is the Bohr radius. If this parameter is large, the Lorentz force acting on valence electrons in atoms exceeds the Coulomb force. The Landau energy levels of electrons are modified by relativistic effects if the field strength in the relativistic units, (4.1)$$ b = h--\omega _c /(m_e c^2 ) = B/B_r , $$ becomes b ≳ 1. Here, ωc = eB/(mec) is the electron cyclotron frequency and Br = me2}c{3}/(eħ) = 4.414 × 1013 G is often called the relativistic magnetic field. [ABSTRACT FROM AUTHOR]

After discussing the Coulomb properties of neutron star envelopes in Chapter 2, we will mainly focus on the nuclear part of the problem. Following a widespread convention we will often call an entire neutron star envelope as a crust (see a discussion on this point in the introductory part of Chapter 2). [ABSTRACT FROM AUTHOR]

After an overall introduction into the neutron star physics, we begin a systematic description of neutron star layers (Fig. 1.2). Chapters 2-4 are devoted to neutron star envelopes. These envelopes are non-uniform. The structure of an envelope for a star with the effective surface temperature ∼ 106 K is schematically shown in Fig. 2.1. We indicate characteristic densities and geometrical depths of interfaces between adjoining layers. One can distinguish the atmosphere, the ocean of ion liquid, followed by solidified layers denoted as the outer and inner crusts. At the bottom of the envelope there may be a mantle containing a liquid crystal of nonspherical nuclei (§3.7.2). These layers are already outlined in §1.3.1 and will be studied in more detail below. The properties of the atmosphere, ocean, and the upper edge of the solidified layer strongly depend on temperature. For the assumed surface temperature ∼ 106 K the internal temperature of a cooling (isolated) star would be ∼ 108 K; the main temperature gradient would occur in the ocean, while deeper layers would be nearly isothermal. When the star cools, the atmosphere and the ocean become thinner. In a sufficiently cold star they may be absent and the solidified matter will extend to the very surface. [ABSTRACT FROM AUTHOR]