Observations Of Extended Sources Research Articles

Polarization Leakage and the IXPE Point-spread Function

By measuring photoelectron tracks, the gas pixel detectors of the Imaging X-ray Polarimetry Explorer satellite provide estimates of the photon detection location and its electric vector position angle (EVPA). However, imperfections in reconstructing event positions blur the image, and EVPA-position correlations result in artificial polarized halos around bright sources. We introduce a new model describing this “polarization leakage” and use it to recover the on-orbit telescope point-spread functions, useful for faint-source detection and image reconstruction. These point-spread functions are more accurate than previous approximations or ground-calibrated products (Δχ 2 ≈ 3 × 104 and 4 × 104 respectively for a bright 106-count source). We also define an algorithm for polarization leakage correction substantially more accurate than existing prescriptions (Δχ 2 ≈ 1 × 103). These corrections depend on the reconstruction method, and we supply prescriptions for the mission-standard “Moments” methods as well as for “Neural Net” event reconstruction. Finally, we present a method to isolate leakage contributions to polarization observations of extended sources and show that an accurate PSF allows the extraction of sub-PSF-scale polarization patterns.

CLASSY. II. A Technical Overview of the COS Legacy Archive Spectroscopic Survey* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to provide the community with a spectral atlas of 45 nearby star-forming galaxies that were chosen to cover similar properties to those seen at high z (z > 6). The prime high-level science product of CLASSY is accurately coadded UV spectra, ranging from ∼1000 to 2000 Å, derived from a combination of archival and new data obtained with HST’s Cosmic Origins Spectrograph (COS). This paper details the multistage technical processes of creating this prime data product and the methodologies involved in extracting, reducing, aligning, and coadding far-ultraviolet and near-ultraviolet (NUV) spectra. We provide guidelines on how to successfully utilize COS observations of extended sources, despite COS being optimized for point sources, and best-practice recommendations for the coaddition of UV spectra in general. Moreover, we discuss the effects of our reduction and coaddition techniques in the scientific application of the CLASSY data. In particular, we find that accurately accounting for flux calibration offsets can affect the derived properties of the stellar populations, while customized extractions of NUV spectra for extended sources are essential for correctly diagnosing the metallicity of galaxies via C iii] nebular emission. Despite changes in spectral resolution of up to ∼25% between individual data sets (due to changes in the COS line-spread function), no adverse affects were observed on the difference in velocity width and outflow velocities of isolated absorption lines when measured in the final combined data products, owing in part to our signal-to-noise regime of S/N < 20.

Spatial field reconstruction with INLA: application to IFU galaxy data

Astronomical observations of extended sources, such as cubes of integral field spectroscopy (IFS), encode auto-correlated spatial structures that cannot be optimally exploited by standard methodologies. This work introduces a novel technique to model IFS datasets, which treats the observed galaxy properties as realizations of an unobserved Gaussian Markov random field. The method is computationally efficient, resilient to the presence of low-signal-to-noise regions, and uses an alternative to Markov Chain Monte Carlo for fast Bayesian inference, the Integrated Nested Laplace Approximation (INLA). As a case study, we analyse 721 IFS data cubes of nearby galaxies from the CALIFA and PISCO surveys, for which we retrieve the maps of the following physical properties: age, metallicity, mass and extinction. The proposed Bayesian approach, built on a generative representation of the galaxy properties, enables the creation of synthetic images, recovery of areas with bad pixels, and an increased power to detect structures in datasets subject to substantial noise and/or sparsity of sampling. A snippet code to reproduce the analysis of this paper is available in the COIN toolbox, together with the field reconstructions of the CALIFA and PISCO samples.

Calibration of the OHREX high-resolution imaging crystal spectrometer at the Livermore electron beam ion traps.

We report the calibration of the Orion High-Resolution X-ray (OHREX) imaging crystal spectrometer at the EBIT-I electron beam ion trap at Livermore. Two such instruments, dubbed OHREX-1 and OHREX-2, are fielded for plasma diagnostics at the Orion laser facility in the United Kingdom. The OHREX spectrometer can simultaneously house two spherically bent crystals with a radius of curvature of r = 67.2 cm. The focusing properties of the spectrometer allow both for larger distance to the source due to the increase in collected light and for observation of extended sources. OHREX is designed to cover a 2.5°-3° spectral range at Bragg angles around 51.3°. The typically high resolving powers at these large Bragg angles are ideally suited for line shape diagnostics. For instance, the nominal resolving power of the instrument (>10 000) is much higher than the effective resolving power associated with the Doppler broadening due to the temperature of the trapped ions in EBIT-I. The effective resolving power is only around 3000 at typical EBIT-I conditions, which nevertheless is sufficient to set up and test the instrument's spectral characteristics. We have calibrated the spectral range for a number of crystals using well known reference lines in the first and second order and derived the ion temperatures from these lines. We have also made use of the 50 μm size of the EBIT-I source width to characterize the spatial focusing of the spectrometer.

Submillimeter Polarimetry with PolKa, a Reflection-Type Modulator for the APEX Telescope

Imaging polarimetry is an important tool for the study of cosmic magnetic fields. In our Galaxy, polarization levels of a few up to ∼10% are measured in the submillimeter dust emission from molecular clouds and in the synchrotron emission from supernova remnants. Only few techniques exist to image the distribution of polarization angles as a means of tracing the plane-of-sky projection of the magnetic field orientation. At submillimeter wavelengths, polarization is either measured as the differential total power of polarization-sensitive bolometer elements, or by modulating the polarization of the signal. Bolometer arrays such as LABOCA at the APEX telescope are used to observe the continuum emission from fields as large as in diameter. Here we present PolKa, a polarimeter for LABOCA with a reflection-type waveplate of at least 90% efficiency. The modulation efficiency depends mainly on the sampling and on the angular velocity of the waveplate. For the data analysis, the concept of generalized synchronous demodulation is introduced. The instrumental polarization toward a point source is at the level of ∼0.1%, increasing to a few percent at the -10 db contour of the main beam. A method to correct for its effect in observations of extended sources is presented. Our map of the polarized synchrotron emission from the Crab nebula is in agreement with structures observed at radio and optical wavelengths. The linear polarization measured in OMC1 agrees with results from previous studies, while the high sensitivity of LABOCA enables us to also map the polarized emission of the Orion Bar, a prototypical photon-dominated region.

Different concepts of next generation IACT arrays

Having matured over a few decades, the IACT technique is now ready to widen its energy domain as well as to increase sensitivity in its core region (0.1-50 TeV) by as much as one order of magnitude as compared to existing facilities. This will allow to use astronomy to search for dark matter, probe intergalactic space and quantum gravity as well as to make efficient observations of extended sources. The design of a new-generation IACT array is moti- vated by the key science questions to be addressed and con- strained by the cost of the experiment. A number of perfor- mance characteristics impact the scientific capabilities, not just point-source sensitivity, but also field-of-view, angular resolution, energy threshold and others, and the optimization of the parameters at fixed cost defines the approach to simu- lation studies. In this paper, I will highlight IACT array design studies carried through to optimize the array configuration, optical and mechanical designs as well as possible designs for tele- scope cameras. systems and OB associations, as well as to study extragalac- tic objects like AGNs and starburst galaxies. The number of detected VHE -ray sources now numbers 100, with the source count roughly doubling in a time scales of '3 years (Colin and LeBohec, 2009). Most importantly, many of these 100 sources have been studied in great detail with detailed imaging-spectrometry information or with light curves and energy spectra on minute time scales. The sensitivity of current experiments like VERITAS, H.E.S.S. and MAGIC is 1% of the flux of the Crab Neb- ula, angular resolution for single -rays of 5 0 above 1 TeV,

XPOL—the Correlation Polarimeter at the IRAM 30-m Telescope

ABSTRACTXPOL, the first correlation polarimeter at a large-millimeter telescope, uses a flexible digital correlator to measure all four Stokes parameters simultaneously, i.e., the total power I, the linear polarization components Q and U, and the circular polarization V. The versatility of the back end provides adequate bandwidth for efficient continuum observations as well as sufficient spectral resolution (40 kHz) for observations of narrow lines. We demonstrate that the polarimetry-specific calibrations are handled with sufficient precision, in particular the relative phase between the Observatory’s two orthogonally linearly polarized receivers. The many facets of instrumental polarization are studied at 3 mm wavelength in all Stokes parameters: on-axis with point sources and off-axis with beam maps. Stokes Q, which is measured as the power difference between the receivers, is affected by instrumental polarization at the 1.5% level. Stokes U and V, which are measured as cross-correlations, are very minimally affected (maximum sidelobes 0.6% [U] and 0.3% [V]). These levels critically depend on the precision of the receiver alignment. They reach these minimum levels set by small ellipticities of the feed horns when alignment is optimum (≲0.3′′). A second critical prerequisite for low polarization sidelobes turned out to be the correct orientation of the polarization splitter grid. Its cross-polarization properties are modeled in detail. XPOL observations are therefore limited only by receiver noise in Stokes U and V even for extended sources. Systematic effects set in at the 1.5% level in observations of Stokes Q. With proper precautions, this limitation can be overcome for point sources. Stokes Q observations of extended sources are the most difficult with XPOL.

Evidence for Solar-Wind Charge-Exchange X-Ray Emission from the Earth’s Magnetosheath

Abstract We report an apparent detection of the C VI 4p to 1s transition line at 459 eV, during a long-term enhancement (LTE) in the Suzaku north ecliptic pole observation of 2005 September 2. The observed line intensity is comparable to that of the C VI 2p to 1s line at 367 eV. This is strong evidence for the charge-exchange process. In addition, O VII, O VIII, Ne X, and Mg XI lines showed clear enhancements. There are also features in the 750–900–eV range that could be due to some combination of Fe L lines, higher order transitions of O VIII (3p to 1s and 6p to 1s), and a Ne IX line. From the correlation of the X-ray intensity with the solar-wind flux on time scales of about half a day, and from the short-term ($\sim 10 \,\mathrm{minutes}$) variations of the X-ray intensity, these lines most likely arise from solar-wind heavy ions interacting with neutral material in the Earth’s magnetosheath. A hard power-law component is also necessary to explain the LTE spectrum. Its origin is not yet known. Our results indicate that solar activity can significantly contaminate Suzaku cosmic X-ray spectra below $\sim 1 \,\mathrm{keV}$. Recommendations are provided for recognizing such contamination in observations of extended sources.

Evidence for Solar-Wind Charge-Exchange X-Ray Emission from the Earth's Magnetosheath

We report an apparent detection of the C VI 4p to 1s transition line at 459 eV, during a long-term enhancement (LTE) in the Suzaku north ecliptic pole (NEP) observation of 2005 September 2. The observed intensity of the line is comparable to that of the C VI 2p to 1s line at 367 eV. This is strong evidence for the charge-exchange process. In addition to the C VI lines, emission lines from O VII, O VIII, Ne X, and Mg XI lines showed clear enhancements. There are also features in the 750 to 900 eV range that could be due to some combination of Fe XVII and XVIII L-lines, higher order transitions of O VIII (3p to 1s and 6p to 1s), and a Ne IX line. From the correlation of the X-ray intensity with solar-wind flux on time scales of about half a day, and from the short-term (~10 minutes) variations of the X-ray intensity, these lines most likely arise from solar-wind heavy ions interacting with neutral material in the Earth's magnetosheath. A hard power-law component is also necessary to explain the LTE spectrum. The origin of this component is not yet known. Our results indicate that solar activity can significantly contaminate Suzaku cosmic X-ray spectra below ~1 keV. Recommendations are provided for recognizing such contamination in observations of extended sources.

Recent Results from the Milagro Gamma Ray Observatory

The Milagro gamma-ray observatory utilizes a large water Cherenkov detector to observe TeV extensive air showers produced by high energy particles impacting the Earth’s atmosphere. Milagro is different from Atmospheric Cherenkov Telescopes that are used to study TeV gamma-rays in that it views a wide field (2 steradian over-head sky) and it continuously operates (>90% live time). These factors give Milagro the potential for discovery of new sources with unknown positions and times, such as gamma-ray bursts, flaring AGN, and observation of extended sources like the Galactic plane or large supernova remnants. The Milagro detector consists of a 4800 m2 pond instrumented with 723 8” PMTs which detect Cherenkov light produced by secondary air-shower particles. A sparse array of 175 4000l water tanks surrounding the central pond detector has recently been added which will extend the physical area of Milagro to 40,000 m2 and substantially increase the sensitivity of the instrument. Based on three years of operation,consitutes Milagro has established its sensitivity through the detection of the Crab plerion and active galaxy Markarian 421. A summary of the recent results from the Milagro collaboration is presented with a focus on the first observation of the galactic plane in the TeV range and evidence for two newly observed TeV sources: diffuse emission from the Cygnus Region, and evidence for an extended TeV hot spot near the EGRET unidentified 3EG J0520+2556.

Astronomical results from SHARC-II

The Submillimeter High Angular Resolution Camera (SHARC-II) is a facility instrument for far-infrared (350μm) imaging at the Caltech Submillimeter Observatory (CSO). With 384 pixels, SHARC-II uses the world's largest bolometer array for astronomical observations. SHARC-II is most efficiently utilized for observations of extended sources and for deep sky surveys. The low 1/f detector noise allows total power measurements without the need to observe an emission free “off position”. This is possible because the sky emission can be distinguished from the celestial emission when the array scans over the sky at sufficient speed. Here we present a representative set of SHARC-II observations, which highlight the capabilities of the instrument. The observations show the submillimeter continuum emission from our own Galactic center, the nearby galaxy M51, and the gravitationally lensed high-z Cloverleaf galaxy H1413+1143.

Multi-fiber Coupling for Wide Field Fiber Interferometry

Adaptive Optics is now making it possible to efficiently use large telescopes in ground-based interferometers. Spatial filtering, especially with optical fibers, allows accurate interferometric measures to be done on these new interferometers (VLTI, Keck, OHANA). However, as telescope sizes increase and sensitivity improves, the problem of field of view of interferometers become more acute. In this paper, I focus on the observation of extended sources with fiber interferometers, and show that accurate visibility measurements are made difficult by the extent of the observed source. Out of the several solutions explored, one seems especially attractive: multi-fiber coupling. In this optical scheme, the pupil is subdivided in sub-pupils, each being coupled into a separate optical fiber. I show how this technique extends the field of view of fiber interferometers, reduces the errors in the interferometric measurements of extended sources, and provides flexibility which improves the sensitivity of fiber interferometers.

Wide field interferometric imaging with single-mode fibers

Classical single-mode fiber interferometers, using one fiber per aperture, have very limited imaging capabilities and small field of view. Observations of extended sources (resolved by one aperture) cannot be fully corrected for wavefront aberrations: accurate measurements of object visibilities are then made very difficult from ground-based fiber interferometers. These limitations are very severe for the new generation of interferometers, which make use of large telescopes equipped with adaptive optics, but can be overcome by using several fibers per aperture. This technique improves the wide field imaging capabilities of both ground-based and space interferometers.

Geostationary radiatively-cooled telescope (GROT)

A possibility of the survey experiment onboard the geostationary GOMS-Electro spacecraft comprising the 15cm-telescope 1:1.8 and the in-flight-tested radiative cooler is considered. With presumed CCD performance the limiting stellar magnitudes V=16 and K=12 would allow regular daily monitoring of variable and high throughput observations of extended sources as well as environmental studies of the near-Earth and near-Earth-orbit space in the wide FOV 2 deg.

Trigger conditions and effective areas of imaging air Cherenkov telescopes

Abstract The contributions to the trigger rate of an imaging atmospheric Cherenkov telescope and the optimization of the trigger conditions are discussed. Among the trigger options studied are cameras with trigger regions of different sizes, triggers requiring a coincidence of 2, 3 and 4 pixels as well as triggers imposing additional conditions on topology of the trigger pixels. A two-pixel trigger based on neighboring trigger pixels is shown to provide full efficiency, at considerably reduced random trigger rates. Numerical examples of effective areas and rates are given based on the parameters of the new HEGRA Cherenkov telescopes. Especially addressed are trigger conditions for the observation of extended sources with angular size ≈ 1°.

Analysis procedures for ROSAT XRT/PSPC observations of extended objects and the diffuse background

Proper reduction of ROSAT observations of extended sources or the diffuse background requires techniques that are quite different from the standard point-source analysis. We describe in detail an appropriate set of procedures for this purpose for the X-ray Telescope (XRT), Position Sensitive Proportional Counter (PSPC). We define a standard set of pulse-height bands recommended for analyzing extended structure and describe the use of detector efficiency maps for proper flat-fielding of images. Sources of noncosmic background contamination and their modeling and subtraction are discussed and demonstrated. IDL routines for implementing these data reduction procedures are being made available to the community through the ROSAT IDL library maintained at Goddard Space Flight Center (GSFC).

GAIN CALIBRATING NONUNIFORM IMAGE-ARRAY DATA USING ONLY THE IMAGE DATA

ABSTRACT An algorithm is developed for calibrating the spatial nonuniformity of image-array (CCD-type) detectors. Like other techniques this approach uses multiple, spatially displaced images. In circumstances where high-precision flat fields are not available by other means (i.e., sky flats) this technique is advantageous as it uses the data frames for gain calibration even when the array images extended, nonuniform, sources. Numerical experiments and direct observations with intrinsically uniform and quite nonuniform detectors show that this algorithm is useful when data frames are crowded with sources - circumstance where 'median filtering' flatfielding techniques often fail. The algorithm described is robust and efficiently uses information from multiple data frames to determine pixel gain variations, using visible and IR array observations of extended sources.

1.2.3 Scattering in the Earth’S Atmosphere: Calculations for Milky Way and Zodiacal Light as Extended Sources

AbstractThe results of detailed calculations on first order Rayleigh- and Mie-scattering in the Earth’s atmosphere illuminated by the Milky Way and the Zodiacal Light are presented. The influence of various independent parameters, as optical depth of the Rayleigh- and Mie-component and position of Milky Way and Zodiacal Light, is discussed. Linear and circular polarization of the scattered light are considered. The results are compared uith the procedures commonly applied in reducing photometric observations of extended sources of the night sky. It is shown that a substantial part of the discrepancies between the brightness distributions of Zodiacal Light resp. Milky Way given by different authors is due to an inaccurate treatment of the atmospheric scattered light.

Ratio of the temperatures of the quiet Sun and the centre of the new moon at λ = 1.3 mm

As radio wave spectral line observations are made at ever shorter wavelengths, the need for precise calibration increases. This is because, as a larger portion of the frequency spectrum becomes available for observation, an increasing number of multiple molecular transistions will be detected. To obtain reliable excitation parameters from these detections they must be calibrated accurately at each frequency. Linsky1 has proposed the Moon as a radiometric standard for observations of extended sources in the infrared, millimetre and microwave portions of the spectrum. We therefore measured the ratio of the quiet Sun temperature to that of the centre of the new Moon at 231 GHz(λ = 1.3mm).