Sky Observations Research Articles

Absolute calibration and beam reconstruction of MITO(a ground-based instrument in the millimetric region)

An efficient sky data reconstruction derives from a precise characterization of the observing instrument. Here we describe the reconstruction of performances of a single-pixel 4-band photometer installed at MITO (Millimeter and Infrared Testagrigia Observatory) focal plane. The strategy of differential sky observations at millimeter wavelengths, by scanning the field of view at constant elevation wobbling the subreflector, induces a good knowledge of beam profile and beam-throw amplitude, allowing efficient data recovery. The problems that arise estimating the detectors throughput by drift scanning on planets are shown. Atmospheric transmission, monitored by skydip technique, is considered for deriving final responsivities for the 4 channels using planets as primary calibrators.

Cosmic Microwave Background Anisotropy Measurement from Python V

We analyze observations of the microwave sky made with the Python experiment in its fifth year of operation at the Amundsen-Scott South Pole Station in Antarctica. After modeling the noise and constructing a map, we extract the cosmic signal from the data. We simultaneously estimate the angular power spectrum in eight bands ranging from large (l ~ 40) to small (l ~ 260) angular scales, with power detected in the first six bands. There is a significant rise in the power spectrum from large to smaller (l ~ 200) scales, consistent with that expected from acoustic oscillations in the early universe. We compare this Python V map to a map made from data taken in the third year of Python. Python III observations were made at a frequency of 90 GHz and covered a subset of the region of the sky covered by Python V observations, which were made at 40 GHz. Good agreement is obtained both visually (with a filtered version of the map) and via a likelihood ratio test.

Feasible study on the integration system for the space monitoring of major earthquakes and volcanoes in terrestrial land

With the rapid development of space technology, earth observation technology and sky observatory technology, they have played a more and more important part in monitoring and predicting of earthquakes and volcanoes in the terrestrial land. In recent years, the related agencies have done the experiments and researches on monitoring and predicting of earthquakes and volcanoes in the forewarning period by means of many approaches, such as satellite thermal infrared remote sensing (TIRS), Global Positioning System (GPS), differential interferometric synthesis aperture radar (D-INSAR), astronomical time-latitude residual anomaly, and Geographic Information Systems (GIS), etc. A quite large number of research foundation has been built in the fundamental theories and application methods. The experiments and researches have shown that these technology is efficient methods for high frequency crust movement. If the existed separate scientific forces and results are possibly assembled together to form a more complete integration monitoring system with the combination of space, sky observation, ground, deep geology and macro anomaly, it will come into a new stage of monitoring and predicting of earthquakes and volcanic eruptions.

Applications of ultraviolet absolute radiometry in satellite and surface-based remote sensing of atmospheric ozone

Side-by-side comparisons of Langley-type Dobson AD double wavelength pair direct sun observations between SBUV/2, SSBUV flight models, and the NOAA world primary standard Dobson spectrophotometer 83 and 61 show that the SBUV/2-type instruments yield column ozone amounts that are 2% higher than the NOAA Dobson spectrometers. Similar results have been obtained with a radiometrically stable multifilter spectroradiometer (MFS) under less than ideal conditions. A new approach based on a modeled table look-up method for using zenith sky radiances to derive total column ozone from zenith clear sky conditions has been tested using SBUV/2, SSBUV flight models and the MFS equipped with narrowband interference filters at the Dobson AD wavelength pairs. These clear zenith sky observations yield total column ozone that is in good agreement with ozone from the NOAA Dobson spectrophotometer direct sun observations. New model calculations on the relationship between satellite nadir radiances and surface-based zenith clear sky radiances suggest that the combination of a very radiometrically stable surface-based spectroradiometer combined with a compact high performance double monochromator could be used to derive a common radiometric scale among satellite-borne ozone monitoring instruments.

Orphan Afterglows of Collimated Gamma‐Ray Bursts: Rate Predictions and Prospects for Detection

We make a quantitative prediction for the detection rate of orphan gamma-ray burst (GRB) afterglows as a function of flux sensitivity in X-ray, optical, and radio wave bands, based on a recent model of collimated GRB afterglows. We find that the orphan afterglow rate strongly depends on the opening angle of the jet (roughly ?), as expected from simple geometrical consideration, if the total jet energy is kept constant as suggested by recent studies. The relative beaming factor brel, i.e., the ratio of all afterglow rates including orphans to those associated with observable prompt GRBs, could be as high as brel 100 for searches deeper than R ~ 24, depending on afterglow parameters. To make the most plausible predictions, we average the model emission for 10 sets of afterglow parameters obtained through fits to 10 well-observed, collimated GRB jets, weighted by the sky coverage of each jet. Our model expectations are consistent with the results (or constraints) obtained by all past searches. We estimate the number of orphan afterglows in the first 1500 deg2 field of the Sloan Digital Sky Survey (SDSS) to be about 0.2. The relative beaming factor brel is rapidly increasing with the search sensitivity: brel ~ 3 for the SDSS sensitivity to transient objects in the northern sky (R ~ 19), ~14 for the past high-z supernova searches (R ~ 23), and ~50 for the sensitivity of the Subaru Suprime-Cam (R ~ 26). Predictions are made for the current facilities and future projects in X-ray, optical, and radio bands. Among them, the southern sky observation of the SDSS (sensitive to transients down to R ~ 23) could detect ~40 orphan afterglows during the 5 yr operation. The Allen Telescope Array would find about 200 afterglows in a radio band at ~0.1-1 mJy with brel ~ 15.

All-sky astrophysical component separation with Fast Independent Component Analysis (FASTICA)

We present a new, fast, algorithm for the separation of astrophysical components superposed in maps of the sky. The algorithm, based on the Independent Component Analysis (ICA) technique, is aimed at recovering both the spatial pattern and the frequency scalings of the emissions from statistically independent astrophysical processes, present along the line-of-sight, from multi-frequency observations, without any a priori assumption on properties of the components to be separated, except that all of them, except possibly one, must have non-Gaussian distributions. The analysis starts from very simple toy-models of the sky emission in order to assess the quality of the reconstruction when inputs are well known and controlled. In particular, we study the dependence of the results of separation conducted on and off the Galactic plane independently, showing that optimal separation is achieved for sky regions where components are smoothly distributed. Then we consider simulated observations of the microwave sky with angular resolution and instrumental noise, supposed to be white and stationary, at the mean nominal levels for the Planck satellite. The angular response function is assumed to be identical at each frequency since this is, up to now, one of the Fast Independent Component Analysis (FastICA) limitations. We consider several Planck observation channels containing the most important known diffuse signals: the cosmic microwave background (CMB), Galactic synchrotron, dust and free–free emissions. A method for calibrating the reconstructed maps of each component at each frequency has been devised. The spatial patterns of all the components have been recovered on all scales probed by the instrument. In particular, the CMB angular power spectra is recovered at the per cent level up to ℓmax≃2000. Frequency scalings and normalization have been recovered with better than 1 per cent precision for all the components at frequencies and in sky regions where their signal-to-noise ratio ≳1.5; the error increases at ∼10 per cent level for signal-to-noise ratios ≃1. Runs have been performed on a Pentium III 600-MHz computer; although the computing time slightly depends on the number of channels and components to be recovered, FastICA typically took about 10 min for all-sky simulations with 3.5-arcmin pixel size. Although the quoted results have been obtained under a number of simplifying assumptions, we conclude that FastICA is an extremely promising technique for analysing the maps that will be obtained by the forthcoming high-resolution CMB experiments.

Comparison of aerosol parameter retrieval results from the analysis of ADEOS/POLDER and ADEOS/OCTS data

In this study we presented a new multi-directional R-P(Reflectance-Polarization) algorithm which retrieves aerosol's three optical parameters, namely, the real part of refractive index, optical thickness and Ångström exponent, by using ADEOS/ POLDER's single near-infrared band data. We validated our retrieved results, by comparing with the simultaneously measured sky observation data at the target site. We found a good agreement between them. Furthermore, the retrieval results by the proposed algorithm were compared with those by a multi-directional R-R(Reflectance-Reflectance) algorithm using the reflectance data of POLDER in a single infrared band and by a mono-directional R-R algorithm using the reflectance data of ADEOS/OCTS in two infrared bands. Although we had a good agreement in the aerosol optical thickness among these algorithms, we found that the retrieval results of Ångström exponent are not reliable in the cases of the latter two algorithms using the reflectance data alone.

The ROSAT North Ecliptic Pole Survey

In the ROSAT All-Sky Survey, an 80.7 deg region around the north ecliptic pole (NEP) constitutes the deepest observation of the X-ray sky ever achieved with such a large, contiguous solid angle (W. Voges et al. 2001, ApJ, 553, L119). Here, 445 unique sources are detected with fluxes measured at greater than 4 j significance. The median and maximum exposure times are approximately 5 and 38 ks, respectively, and the minimum flux limit is ∼ ergs s cm 2 (0.5–2.0 keV). 14 1 2 # 10 We have identified the physical nature of 443 (99.6%) of the ROSAT NEP X-ray sources through a comprehensive program of imaging and spectroscopy primarily with the University of Hawaii 2.2 m, Canada-France-Hawaii 3.6 m, and Keck II 10 m telescopes (J. P. Henry et al. 2001, ApJ, 553, L109). The main constituents of the resulting catalog are 217 active galactic nuclei (AGNs; 49.0%), 152 stars (34.3%), and 64 groups and clusters of galaxies (14.4%). Minor components include eight BL Lacertae objects, one isolated elliptical galaxy, and one planetary nebula. We have obtained spectroscopic redshifts for all of the extragalactic objects. The median and maximum redshifts of the cluster sample are 0.205 and 0.811, whereas for the AGNs the corresponding values are 0.408 and 3.889. We have used the complete samples of extragalactic X-ray sources to examine the evolution and distribution of large-scale structure in the universe (I. M. Gioia et al. 2001, ApJ, 553, L105). The low-redshift X-ray luminosity function (XLF; 0.02 ! z ! ) of the NEP clusters is consistent with previous determi0.3 nations of the local XLF. Conversely, the high-redshift XLF ( ) shows significant deviations from the local XLF 0.3 ! z ! 0.85 at high luminosities. Similarly, a comparison of the number of observed clusters versus the number expected assuming noevolution models indicates a deficit of clusters at high redshift and high luminosity. Thus, the NEP data show negative evolution in the cluster population at ergs s and 44 1 L 1.5 # 10 z 1 X at greater than approximately 4.2 j. This result agrees with 0.3 the most simple form of hierarchical, bottom-up structure formation that predicts fewer luminous/massive objects in the past as compared to the present. Using the ROSAT NEP survey data, we have detected a supercluster at (C. R. Mullis et al. 2001, ApJ, 553, z p 0.087 L115). The X-ray data greatly improve our understanding of this supercluster’s characteristics, approximately doubling our knowledge of the structure’s spatial extent and tripling the cluster/group membership compared to the optical discovery data. The supercluster is a rich structure consisting of at least 21 galaxy clusters and groups, 12 AGNs, 61 IRAS galaxies, and various other objects. A majority of these components were discovered with the X-ray data, but the supercluster is also robustly detected in optical, IR, and UV wavebands. Extending ( Mpc), the 1 129 # 102 # 67 h50 NEP supercluster has a flattened shape oriented nearly edge-on to our line of sight. Owing to the softness of the ROSAT X-ray passband and the deep exposure over a large solid angle, we have detected for the first time a significant population of X-ray–emitting galaxy groups in a supercluster. Also relevant to large-scale structure, and in particular cluster formation, is the distant cluster RX J1716.6 6708 ( ), z p 0.81 which we discovered during the ROSAT NEP survey (J. P. Henry et al. 1997, AJ, 114, 1293; I. M. Gioia et al. 1999, AJ, 117, 2608). The member galaxies have a filamentary distribution, with the X-rays coming from the midpoint of the filament. The X-ray contours have an elongated shape that coincides roughly with weak gravitational lensing contours. This distant system of galaxies could be an example of a protocluster where matter is flowing along filaments and the X-ray flux is maximal at the impact point of the colliding streams of matter. Working beyond coherent structures, we have measured the degree of clustering in the ROSAT NEP data by calculating twopoint spatial correlation functions. Accounting for the effects of the NEP supercluster, the correlation function of galaxy clusters is consistent with previous measurements, albeit with large uncertainties. More importantly, our measurement of the spatial correlation function for X-ray–selected AGNs provides the best constraints (3.8 j) reported so far. The maximum likelihood determination of the power-law fit indicates best-fit values of Mpc and . If we set , the 2.5 1 1.40 r p 6.6 h g p 1.65 g p 1.8 0 5.4 1.07 correlation length is Mpc. These results suggest 1.5 1 r p 6.8 h 0 1.6 that X-ray–luminous AGNs are spatially clustered in a manner similar to that of normal galaxies and optically selected AGNs. Additional details concerning the ROSAT NEP survey can be found at http://www.ifa.hawaii.edu/∼mullis/nep.html.

RADIATIVE TRANSFER IN A SCATTERING SPHERICAL ATMOSPHERE

We have written a code called QDM_sca, which numerically solves the problem of radiative transfer in an anisotropically scattering, spherical atmosphere. First we formulate the problem as a second order differential equation of a quasi-diffusion type. We then apply a three-point finite differencing to the resulting differential equation and transform it to a tri-diagonal system of simultaneous linear equations. After boundary conditions are implemented in the tri-diagonal system, the QDM_sca radiative code fixes the field of specific intensity at every point in the atmosphere. As an application example, we used the code to calculate the brightness of atmospheric diffuse light(ADL) as a function of zenith distance, which plays a pivotal role in reducing the zodiacal light brightness from night sky observations. On the basis of this ADL calculation, frequent uses of effective extinction optical depth have been fully justified in correcting the atmospheric extinction for such extended sources as zodiacal light, integrated starlight and diffuse galactic light. The code will be available on request.

A hard X-ray telescope with multilayered supermirrors for balloon observations of cosmic X-ray sources

We have been developing a hard X-ray telescope utilizing multilayer supermirrors. The international focusing optics collaboration for μ-Crab sensitivity (InFOCμS) project is for balloon observations of cosmic hard X-ray sky with this new type of telescope. For the first flight scheduled in the next summer, we are preparing one full telescope with outermost diameter of 40 cm, focal length of 8 m, and energy band witgh of 20 – 40 keV as a first step. At the focal plane a CdZnTe imaging detector will be prepared. In this paper, the science objectives of this project, current status of the development of gondola, X-ray telescope and hard x-ray detector, and expected performance are described.

The [ITAL]ROSAT[/ITAL] North Ecliptic Pole Survey X-Ray Data

In the ROSAT All-Sky Survey, an 80.7 deg2 region near the north ecliptic pole (NEP) constitutes the deepest contiguous observation of the X-ray sky. Four hundred and forty-five unique sources have been detected above a flux of ~2 × 10-14 ergs s-1 cm-2 (0.5-2.0 keV). The X-ray data for each source provide a likelihood of existence, position and position error, count rate and error, background count rate at the source position, exposure time, two hardness ratios and their errors, and a measure of extent and likelihood of nonzero extent. We describe the range and statistics of X-ray properties exhibited by the NEP sources and the accuracy with which they have been determined, concentrating on the flux measurement of extended galaxy cluster X-ray sources. We show two examples of sources whose X-ray morphology is useful in their optical identification.

The estimation of aerosol optical parameters from ADEOS/POLDER data

A cross calibration analysis between POLDER and OCTS sensors on board ADEOS satellite in terms of the space reflectance was made. Space reflectance values, computed from the POLDER and OCTS data (acquired simultaneously on 26 April 1997) for the same ocean target, were examined and we found that they are in good agreement in all bands, when the new in-flight calibration gain factors for each sensor are assumed. Furthermore, we retrieved aerosol optical parameters (i.e., the optical thickness, Ångström exponent, and refractive index) by using several retrieving algorithms from using ADEOS/POLDER’s directional space reflectance and polarization data in infrared bands simultaneously. Retrieved results were presented by the form of distribution maps. In addition, good and bad points of each retrieval algorithm were discussed. Finally, we proposed an improved method for retrieving aerosol optical parameters, which utilizes theoretical P–SR (polarization–space reflectance) diagrams at given viewing angles in an 865 nm band. The estimated aerosol optical parameters by our proposed method were compared with the simultaneously measured sky observation data. We found an excellent agreement between them.

Evidence against the Sciama Model of Radiative Decay of Massive Neutrinos

We report on spectral observations of the night sky in the band around 900 A where the emission line in the Sciama model of radiatively decaying massive neutrinos would be present. The data were obtained with a high-resolution, high-sensitivity spectrometer flown on the Spanish satellite MINISAT. The observed emission is far less intense than that expected in the Sciama model.

Vertical distribution of dust particles in the Earth's atmosphere during the 1998 Leonids

Abstract— Photometric observations of the twilight sky were carried out during Leonids 1998. The obtained vertical distributions of aerosol between 20 and 140 km demonstrate the processes of the intrusion of fine meteor dust and its subsequent intra‐atmospheric dynamics. The characteristic radii of two fractions of the meteor dust particles were estimated by their sedimentation velocities. They varied within rp = 0.006–0.06 μm and rp = 19–81 μm limits depending on an assumed particle density within ρp = 0.4–4.0 g cm−3. The assumption of ρp = 2.0 g cm−3 gave radii of the two fractions to be 0.01 and 30 μm, respectively.

Anisotropy in the Cosmic Microwave Background at Degree Angular Scales: Python V Results

Observations of the microwave sky using the Python telescope in its fifth season of operation at the Amundsen-Scott South Pole Station in Antarctica are presented. The system consists of a 0.75 m off-axis telescope instrumented with a HEMT amplifier-based radiometer having continuum sensitivity from 37-45 GHz in two frequency bands. With a 0.91 deg x 1.02 deg beam the instrument fully sampled 598 deg^2 of sky, including fields measured during the previous four seasons of Python observations. Interpreting the observed fluctuations as anisotropy in the cosmic microwave background, we place constraints on the angular power spectrum of fluctuations in eight multipole bands up to l ~ 260. The observed spectrum is consistent with both the COBE experiment and previous Python results. There is no significant contamination from known foregrounds. The results show a discernible rise in the angular power spectrum from large (l ~ 40) to small (l ~ 200) angular scales. The shape of the observed power spectrum is not a simple linear rise but has a sharply increasing slope starting at l ~ 150.

Estimating Cloud Type from Pyranometer Observations

In this paper the authors evaluate an inexpensive and automatable method to estimate cloud type at a given location during daylight hours using the time series of irradiance from a pyranometer. The motivation for this investigation is to provide ground-based estimates of cloud type at locations where there are no human observations of sky condition. A pyranometer naturally measures the effect of intervening clouds along the solar beam path to the sensor. Because a daily time series of irradiance is nonstationary, it is appropriately scaled to yield a stationary time series. From the latter, the standard deviation and ratio of observed irradiance to clear-sky irradiance derived from a 21-min moving window are related to one of the following cloud types or conditions: cirrus, cumulus, cirrus and cumulus, stratus, precipitation or fog, no clouds, and other clouds. Comparisons with human observations at the Department of Energy Atmospheric Radiation Measurement Calibration and Radiation Testbed site in northern Oklahoma show that the pyranometer method and human observations are in agreement about 45% of the time. Many of the differences can be attributed to two factors: 1) the pyranometer method is weighted toward clouds crossing the sun’s path, while the human observer can view clouds over the entire sky, and 2) the presence of aerosols causes the pyranometer to overestimate the occurrence of cirrus and cirrus plus cumulus. When attenuation of the solar beam by aerosols is negligible or can be accounted for, the pyranometer method should be especially useful for cloud-type assessment where no other sky observations are available.

DOAS Zenith Sky Observations: 2. Seasonal Variation of BrO Over Bremen (53°N) 1994-1995

Zenith sky observations of BrO over Bremen (53°N) are reported for the period of September 1994 to January 1996. BrO differential slant columns between 90° and 80° solar zenith angle showed a strong seasonal variation between a winter maximum of 1.9·1014 molec/cm2 and a summer minimum of 0.6·1014 molec/cm2. The seasonal variation in BrO twilight values is shown to be inversely correlated with NO2 columns in agreement with current knowledge of gas phase chemistry of bromine. In contrast to model predictions, no significant difference between morning and evening BrO measurements was observed. During a 6 day polar vortex excursion to mid-latitudes OClO could be measured above Bremen indicating chlorine activation in the vortex air. No significant increase in BrO differential slant columns was detected during this time.

天空観測用可搬型赤外放射計の開発と作動特性

天空観測用可搬型赤外放射計の開発と作動特性

Optical path modifications due to tropospheric clouds: Implications for zenith sky measurements of stratospheric gases

Potential influences of tropospheric clouds on zenith scattered light differential optical absorption spectroscopy (ZSL‐DOAS) of stratospheric gases, such as O3, NO2, OClO, BrO, and other species are discussed. It is argued that compared to clear sky, clouds can occasionally considerably change the geometrical path lengths in the troposphere, and hence change the relative contribution of the troposphere to the total atmospheric trace gas absorption. Major reasons for the geometrical path enhancements due to clouds are seen in the photon diffusion in optically thick clouds and in the multiple reflections of light between layers and patches of clouds. In addition, optically thin clouds in the upper troposphere (e.g., As and Ci clouds) may decrease the apparent absorption due to stratospheric species, whereby the magnitude of that effect largely depends on the solar zenith angle and wavelength. This is explained by the quite different air mass factors for the zenith scattered light observed under clear sky and cloudy sky conditions. Although opposite in sign, these “cloud effects” can combine and cause significant errors in ground‐based zenith sky observations of stratospheric gases. It is demonstrated that the proposed mechanisms can explain the recently reported cloudy sky absorption changes detected for O3, O4, NO2, BrO, and H2O.

Cloudy sky optical paths as derived from differential optical absorption spectroscopy observations

Recently, the influence of tropospheric clouds on zenith sky observations of atmospheric gases (O3, NO2, O4, H2O, and others) has been reported [Van Roozendael et al., 1994; Erle et al., 1995]. For measurements under heavy cloud cover, Erle et al. [1995] determined from simultaneously measured absorptions of different tropospheric species an average optical path for photons reaching the instrument of >100 km. This path length was much larger than expected from previous reports [Feigelson, 1981]. Thus an important question was whether the magnitude of the observation was a typical situation rather than a rare exception. Here we add a large set of new “cloud event” observations evaluated from data which were recorded in the Arctic, at midlatitudes, and the tropics. It is shown that the observed absorption enhancements of O3, H2O, and O4 perfectly coincide with indirect cloud indicators, i.e., changes in the average intensity and the color index as well as direct observations of cloud passages detected by satellite. In particular, we demonstrate that the observed optical paths have the same magnitude as the path lengths reported by Erle et al [1995], and thus optical paths under cloudy skies may indeed become much larger than previously reported. Besides these observations made during specific meteorological conditions we also studied optical paths continuously for several weeks in Arctic winter/spring conditions. We derived a frequency distribution of path lengths for cloudy skies with an average mean optical path of 29 km.