An MHD gadget for cosmological simulations

Various radio observations have shown that the hot atmospheres of galaxy clusters are magnetized. However, our understanding of the origin of these magnetic fields, their implications on structure formation and their interplay with the dynamics of the cluster atmosphere, especially in the centres of galaxy clusters, is still very limited. In preparation for the upcoming new generation of radio telescopes (like Expanded Very Large Array, Low Wavelength Array, Low Frequency Array and Square Kilometer Array), a huge effort is being made to learn more about cosmological magnetic fields from the observational perspective. Here we present the implementation of magnetohydrodynamics (MHD) in the cosmological smoothed particle hydrodynamics (SPH) code gadget . We discuss the details of the implementation and various schemes to suppress numerical instabilities as well as regularization schemes, in the context of cosmological simulations. The performance of the SPH–MHD code is demonstrated in various one- and two-dimensional test problems, which we performed with a fully, three-dimensional set-up to test the code under realistic circumstances. Comparing solutions obtained using athena , we find excellent agreement with our SPH–MHD implementation. Finally, we apply our SPH–MHD implementation to galaxy cluster formation within a large, cosmological box. Performing a resolution study we demonstrate the robustness of the predicted shape of the magnetic field profiles in galaxy clusters, which is in good agreement with previous studies.     

Radioactivity induced background noise in space-borne astronomical gamma-ray telescopes employing inorganic scintillation spectrometers

Based on monoenergetic proton beam tests of both NaI and CsI, a method for estimating the spallation background, due to the cosmic ray protons and trapped protons, has been developed. Comparisons between the calculated background noise and those measured by telescopes on board satellites are made. Good agreements of these comparisons demonstrate that the approach is capable of providing realistic estimates for specific space borne gamma-ray telescopes in well defined orbits.     

Stellar intensity interferometry: Prospects for sub-milliarcsecond optical imaging

Using kilometric arrays of air Cherenkov telescopes at short wavelengths, intensity interferometry may increase the spatial resolution achieved in optical astronomy by an order of magnitude, enabling images of rapidly rotating hot stars with structures in their circumstellar disks and winds, or mapping out patterns of nonradial pulsations across stellar surfaces. Intensity interferometry (once pioneered by Hanbury Brown and Twiss) connects telescopes only electronically, and is practically insensitive to atmospheric turbulence and optical imperfections, permitting observations over long baselines and through large airmasses, also at short optical wavelengths. The required large telescopes (～10 m) with very fast detectors (～ns) are becoming available as the arrays primarily erected to measure Cherenkov light emitted in air by particle cascades initiated by energetic gamma rays. Planned facilities (e.g., CTA, Cherenkov Telescope Array) envision many tens of telescopes distributed over a few square km. Digital signal handling enables very many baselines (from tens of meters to over a kilometer) to be simultaneously synthesized between many pairs of telescopes, while stars may be tracked across the sky with electronic time delays, in effect synthesizing an optical interferometer in software. Simulated observations indicate limiting magnitudes around m_V = 8, reaching angular resolutions ～30 μarcsec in the violet. The signal-to-noise ratio favors high-temperature sources and emission-line structures, and is independent of the optical passband, be it a single spectral line or the broad spectral continuum. Intensity interferometry directly provides the modulus (but not phase) of any spatial frequency component of the source image; for this reason a full image reconstruction requires phase retrieval techniques. This is feasible if sufficient coverage of the interferometric (u, v)-plane is available, as was verified through numerical simulations. Laboratory and field experiments are in progress; test telescopes have been erected, intensity interferometry has been achieved in the laboratory, and first full-scale tests of connecting large Cherenkov telescopes have been carried out. This paper reviews this interferometric method in view of the new possibilities offered by arrays of air Cherenkov telescopes, and outlines observational programs that should become realistic already in the rather near future.     

Earth and space-based NEO survey simulations: prospects for achieving the spaceguard goal

We have used an improved model of the orbit and absolute magnitude distribution of Near Earth Objects (NEOs) to simulate the performance of asteroid surveys. Our results support general conclusions of previous studies using preliminary Near Earth Asteroid (NEA) orbit and magnitude distributions and suggest that meeting the Spaceguard Goal of 90% completion for Near Earth Objects (NEOs) greater than 1 km diameter by 2008 is impossible given contemporary surveying capabilities.The NEO model was derived from NEO detections by the Spacewatch Project. For this paper we developed a simulator for the Catalina Sky Survey (CSS) for which we had a complete pointing history and NEO detection efficiency. The good match between the output of the simulator and the actual CSS performance gives confidence that both the NEO model and simulator are correct. Then, in order to determine if existing surveys can meet the Spaceguard Goal, we developed a simulator to mimic the LINEAR survey, for which detailed performance characteristics were unavailable. This simulator serendipitously provided an estimate for the currently undiscovered population of NEOs upon which we base all our estimates of time to 90% completion. We also developed a set of idealized NEO surveys in order to constrain the best possible survey performance in contrast to more realistic systems.A 100% efficient, all-sky, every night survey, subject only to the constraints of detection above a specified air mass and when the Sun is 18° below the horizon provides a benchmark from which to examine the effect of imposing more restrictions and the efficacy of some simple survey strategies. Such a survey must have a limiting V-magnitude of 20.1 ± 0.2 to meet the Spaceguard Goal.More realistic surveys, limited by latitude, the galaxy, minimum rates of NEO motion, etc., require fainter limiting magnitudes to reach the same completion. Our most realistic simulations, which have been normalized to the performance of the LINEAR detector system’s operation in the period 1999-2000, indicate that it would take them another 33 ± 5 years to reach 90% completeness for the larger asteroids (?1 km diameter). They would need to immediately increase the limiting magnitude to about 24 in order to meet the Spaceguard Goal.The simulations suggest that there may be little need for distributing survey telescopes in longitude and latitude as long as there is sufficient sky coverage from a telescope or network of telescopes which may be geographically close. An idealized space-based survey, especially from a satellite orbit much interior to Earth, would offer an advantage over their terrestrial counterparts. We do not consider a cost-benefit analysis for any of the simulations but suspect that a local-area network of telescopes capable of covering much of the sky in a month to V ∼ 21.5 may be administratively, financially, and scientifically the best compromise for reaching 90% completion of NEOs larger than 1 km diameter.     

Phasing segmented mirrors using defocused images at visible wavelengths

Plans for future optical telescopes of diameter more than 10 m are based on segmented mirrors, made up of hundreds or even thousands of segments. A challenge for these telescopes is the alignment in piston (cophasing) where phase differences between individual segments have to be reduced to a small fraction of the observing wavelength in order to avoid degradation of image quality.
Based on the phase discontinuity sensing method used at the Keck telescopes to measure small piston errors using infrared wavelengths, we develop a new method that allows fast high-precision measurements of large piston errors even at visible wavelengths.     

The ORAC‐DR data reduction pipeline

The ORAC‐DR data reduction pipeline has been used by the Joint Astronomy Centre since 1998. Originally developed for an infrared spectrometer and a submillimetre bolometer array, it has since expanded to support twenty instruments from nine different telescopes. By using shared code and a common infrastructure, rapid development of an automated data reduction pipeline for nearly any astronomical data is possible. This paper discusses the infrastructure available to developers and estimates the development timescales expected to reduce data for new instruments using ORAC‐DR. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Small observatories for the UV

We describe concepts for small space telescopes that are able to provide significant UV science and can be realized with small (but realistic) budgets. The concepts are based on nano-satellites carrying small optics, with no redundancy, without producing intermediate models prior to flight model, and using COTS (custom off-the-shelf) components. We describe a few concepts of deployable optics that could provide large collecting areas and high angular resolution while packaged in the small volume of a nano-satellite. We point out areas where technological development is still required.     

Exploring the gamma-ray horizon with the next generation of gamma-ray telescopes. Part 2: Extracting cosmological parameters from the observation of gamma-ray sources

The potential of the new generation Cherenkov telescopes to measure the energy spectrum of both, the already established extragalactic very high energy γ-ray emitters and the best very high energy candidates from the EGRET catalogue is discussed. By a realistic simulation of the analysis of the expected extrapolated energy spectra, it is shown that the foreseen capability and precision of these instrument to measure the gamma-ray horizon may open the door to competitive measurements of the cosmological parameters.     

Examining the feasibility of air shower core-location from polarization properties of associated Cherenkov light

Based on this exploratory investigation involving CORSIKA simulation code generated Cherenkov photons and a linearly polarized, hypothetical photon beam, we make a case here for exploiting polarization properties of atmospheric Cherenkov events for providing an independent method for locating air-shower cores by a TACTIC-like array of atmospheric Cherenkov telescopes. Preliminary results based on simulations indicate that for a 3 TeV γ-ray having ∼30% degree of polarization for its associated Cherenkov light at a core distance of ∼100 m, core location can be found with an error of ∼27 m. Deceased This revised version was published online in July 2006 with corrections to the Cover Date.     

Detecting neutrino transients with optical follow-up observations

A novel method is presented which will enhance the sensitivity of neutrino telescopes to identify transient sources such as Gamma-Ray Bursts (GRBs) and core-collapse Supernovae (SNe). Triggered by the detection of high energy neutrino events from IceCube or other large scale neutrino telescopes, an optical follow-up program will allow the identification of the transient neutrino source. We show that once the follow-up program is implemented, the achievable sensitivity of IceCube to neutrinos from SNe and GRBs would increase by a factor of 2–3. The program can be realized with a small network of automated 1–2 m telescopes and has rather modest observing time requirements.     

A New Code for Nonradial Stellar Pulsations and its Application to Low-Mass, Helium White Dwarfs

We present a finite difference code intended for computing linear, adiabatic, non radial pulsations of spherical stars. This code is based on a slight modification of the general Newton-Raphson technique in order to handle the relaxation of the eigenvalue(square of the eigenfrequency) of the modes and their corresponding eigenfunctions. This code has been tested computing the pulsation spectra of polytropic spheres finding a good agreement with previous work. Then, we have coupled this code to our evolutionary code and applied it to the computation of the pulsation spectrum of a low mass, pure-helium white dwarf of 0.3 M _⊙ for a wide range of effective temperatures. In making this calculation we have taken an evolutionary time step short enough such that eigenmodes corresponding to a given model are used as initial approximation to those of the next one. Specifically, we have computed periods, period spacing, eigenfunctions, weight functions, kinetic energies and variational periods for a wide range of modes. To our notice this is the first effort in studying the pulsation properties of helium white dwarfs. The solution we have found working with these realistic white dwarf models are in good accord with the predictions of the asymptotic theory of Tassoul (1980) for high order modes. This indicates that the code presented here is able to work adequately also with realistic stellar models. This revised version was published online in July 2006 with corrections to the Cover Date.     

The history of radio telescopes, 1945–1990

Forged by the development of radar during World War II, radio astronomy revolutionized astronomy during the decade after the war. A new universe was revealed, centered not on stars and planets, but on the gas between the stars, on explosive sources of unprecedented luminosity, and on hundreds of mysterious discrete sources with no optical identifications. Using “radio telescopes” that looked nothing like traditional (optical) telescopes, radio astronomers were a very different breed from traditional (optical) astronomers. This pathbreaking of radio astronomy also made it much easier for later “astronomies” and their “telescopes” (X-ray, ultraviolet, infrared, gamma-ray) to become integrated into astronomy after the launch of the space age in the 1960s. This paper traces the history of radio telescopes from 1945 through about 1990, from the era of converted small-sized, military radar antennas to that of large interferometric arrays connected by complex electronics and computers; from the era of strip-chart recordings measured by rulers to powerful computers and display graphics; from the era of individuals and small groups building their own equipment to that of Big Science, large collaborations and national observatories.     

Wide-field tracking with zenith-pointing telescopes

Equipped with a suitable optical relay system, telescopes employing low-cost fixed primary mirrors could point and track while delivering high-quality images to a fixed location. Such an optical tracking system would enable liquid-mirror telescopes to access a large area of sky and employ infrared detectors and adaptive optics. Such telescopes could also form the elements of an array in which light is combined either incoherently or interferometrically. Tracking of an extended field requires correction of all aberrations including distortion, field curvature and tilt. A specific design is developed that allows a 10-m liquid-mirror telescope to track objects for as long as 30 min and to point as far as 4° from the zenith, delivering a distortion-free diffraction-limited image to a stationary detector, spectrograph or interferometric beam combiner.     

A means for improving telescope derives

In seeking to minimize local seeing, some large telescopes are being planned with less wind protection than has been traditional and some sites with very good seeing appear to suffer significantly from ground-transmitted vibration. A great care is needed in designing new telescopes to ensure that vibration does not become a dominant cause of image degradation. A supplementary drive system is proposed which could improve on past tracking performances even in such adverse conditions. The system may also reduce the costs of very large telescopes by easing the demands on the stiffness and accuracy of their mountings and drives.Paper presented at the Symposium on JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

A quantitative treatment of solar ‘seeing’, II

A previously described microthermal measurement technique is applied to diagnose the causes and locations of sources of harmful ‘seeing’ effects in solar telescopes. Examples of the investigation of a compact 30-cm refractor at Culgoora Observatory and the massive 60-in. McMath reflector at Kitt Peak Observatory are described. The separate contributions from various parts of the telescopes and their immediate environments to the overall modulation transfer function and to the rms wavefront deformation are determined. The effectiveness of ventilation equipment fitted to these telescopes is assessed. In particular, it is shown that rms wavefront deformations ranging from λ/8 to 2λ may originate within the immediate atmospheric environments of these telescopes. Since the locations of these disturbances are determined there is a possibility of effecting improvements for example by mounting a telescope adequately high above the floor of its supporting tower. In the McMath reflector the major contribution to bad ‘seeing’ generally arises in the upper part of the instrument in the vicinity of the heliostat.

     

Light Curves of the Type II-P Supernova SN 2017eaw: The First 200 Days

We present the results of our UBVRI photometry for the type II-P supernova SN 2017eaw in NGC6946 obtained fromMay 14 to December 7, 2017, at several telescopes, including the 2.5-m telescope at the CaucasusHigh-Altitude Observatory of the SAIMSU. The dates andmagnitudes atmaximumlight and the light-curve parameters have been determined. The color evolution, extinction, and peak luminosity of SN 2017eaw are discussed. The results of our preliminary radiation–gasdynamic simulations of its light curves with the STELLA code describe satisfactorily the UBVRI observational data.     

Populating the Galaxy with pulsars – I. Stellar and binary evolution

The computation of theoretical pulsar populations has been a major component of pulsar studies since the 1970s. However, the majority of pulsar population synthesis has only regarded isolated pulsar evolution. Those that have examined pulsar evolution within binary systems tend to either treat binary evolution poorly or evolve the pulsar population in an ad hoc manner. Thus, no complete and direct comparison with observations of the pulsar population within the Galactic disc has been possible to date. Described here is the first component of what will be a complete synthetic pulsar population survey code. This component is used to evolve both isolated and binary pulsars. Synthetic observational surveys can then be performed on this population for a variety of radio telescopes. The final tool used for completing this work will be a code comprised of three components: stellar/binary evolution, Galactic kinematics and survey selection effects. Results provided here support the need for further (apparent) pulsar magnetic field decay during accretion, while they conversely suggest the need for a re-evaluation of the assumed typical millisecond pulsar formation process. Results also focus on reproducing the observed     diagram for Galactic pulsars and how this precludes short time-scales for standard pulsar exponential magnetic field decay. Finally, comparisons of bulk pulsar population characteristics are made to observations displaying the predictive power of this code, while we also show that under standard binary evolutionary assumption binary pulsars may accrete much mass.     

A blind deconvolution method for ground based telescopes and Fizeau interferometers

In the case of ground-based telescopes equipped with adaptive optics systems, the point spread function (PSF) is only poorly known or completely unknown. Moreover, an accurate modeling of the PSF is in general not available. Therefore in several imaging situations the so-called blind deconvolution methods, aiming at estimating both the scientific target and the PSF from the detected image, can be useful. A blind deconvolution problem is severely ill-posed and, in order to reduce the extremely large number of possible solutions, it is necessary to introduce sensible constraints on both the scientific target and the PSF.In a previous paper we proposed a sound mathematical approach based on a suitable inexact alternating minimization strategy for minimizing the generalized Kullback–Leibler divergence, assuring global convergence. In the framework of this method we showed that an important constraint on the PSF is the upper bound which can be derived from the knowledge of its Strehl ratio. The efficacy of the approach was demonstrated by means of numerical simulations.In this paper, besides improving the previous approach by the use of a further constraint on the unknown scientific target, we extend it to the case of multiple images of the same target obtained with different PSFs. The main application we have in mind is to Fizeau interferometry. As it is known this is a special feature of the Large Binocular Telescope (LBT). Of the two expected interferometers for LBT, one, LINC-NIRVANA, is forthcoming while the other, LBTI, is already operating and has provided the first Fizeau images, demonstrating the possibility of reaching the resolution of a 22.8 m telescope. Therefore the extension of our blind method to this imaging modality seems to be timely.The method is applied to realistic simulations of imaging both by single mirrors and Fizeau interferometers. Successes and failures of the method in the imaging of stellar fields are demonstrated in simple cases. These preliminary results look promising at least in specific situations. The IDL code of the proposed method is available on request and will be included in the forthcoming version of the Software Package AIRY (v.6.1).     

A protocol standard for heterogeneous telescope networks

The scientific need for a standard protocol permitting the exchange of generic observing services is rapidly escalating as more observatories adopt service observing as a standard operating mode and as more remote or robotic telescopes are brought on‐line. To respond to this need, we present the results of the first interoperability workshop for Heterogeneous Telescope Networks (HTN) held in Exeter. We present a draft protocol, designed to be independent of the specific instrumentation and software that controls the remote and/or robotic telescopes, allowing these telescopes to appear to the user with a unified interface despite any underlying architectural differences. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ESO and European astronomy: four decades of reciprocity

This paper describes and discusses some key facts in the early history of the European Southern Observatory: the founding phase leading to the establishment of the “nucleus” of telescopes in Chile, the role and impact of the “parallel line” of national telescopes, the mission of ESO and the impact of the organisation as a maecenas to European astronomy. With specific focus on the foundation of A & A as a refereed journal foregoing page charges, the support of national telescopes of all classes, the allotment of telescope time on the sole basis of proposal quality, the free travel and technical support, the free distribution of “The Messenger”, the development and free distribution of MIDAS, and the personal dedication of some early foster persons.