A synthesis map of the sky at 34.5 MHz

This paper describes a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude 13°36′12′′N). This telescope was used in the transit mode and by per forming 1-D synthesis along the north-south direction the entire observable sky was mapped in a single day. This minimized the problems that hinder wide-field low-frequency mapping. This survey covers the declination range of-50° to + 70° (- 33° to +61° without aliasing) and the complete 24 hours of right ascension. The synthesized beam has a resolution of 26′ x 42′ sec (δ- 14°. 1). The sensitivity of the survey is 5 Jy/beam (1σ). Special care has been taken to ensure that the antenna responds to all angular scale structures and is suitable for studies of both point sources and extended objects This telescope is jointly operated by the Indian Institute of Astrophysics, Bangalore and the Roman Research Institute, Bangalore.     

Gamma rays from millisecond pulsars

We estimate the contribution of millisecond pulsars to the diffuse gamma-ray background of the Galaxy, and show that a significant fraction of the Galactic background may originate from them. A small number of the unidentified COS-B point sources may, in fact, be millisecond pulsars. It is argued that several hundred millisecond pulsars may be detectable as point sources by the GRO satellite A preliminary version of these results was presented at the NATO Advanced Study Institute on Neutron stars, AGNs and Jets, Erice, September 5–17, 1988. On leave from Raman Research Institute, Bangalore, India.     

Far infrared mapping of three Galactic star forming regions: W3(OH), S209 & S187

Three Galactic star forming regions associated with W3(OH), S209 and S187 have been simultaneously mapped in two trans-IRAS far infrared (FIR) bands centered at ≈140 and 200μm using the TIFR 100 cm balloon borne FIR telescope. These maps show extended FIR emission with structures. The HIRES processed IRAS maps of these regions at 12, 25, 60 & 100 ìm have also been presented for comparison. Point-like sources have been extracted from the longest waveband TIFR maps and searched for associations in the other five bands. The diffuse emission from these regions have been quantified, which turns out to be a significant fraction of the total emission. The spatial distribution of cold dust (T < 30 K) for two of these sources (W3(OH) & S209), has been determined reliably from the maps in TIFR bands. The dust temperature and optical depth maps show complex morphology. In general the dust around S209 has been found to be warmer than that in W3(OH) region.     

A digital correlation receiver for the GEETEE radio telescope

A 128-channel digital correlation receiver has been built for the GEETEE 1, the low-frequency radio telescope situated at Gauribidanur, South India, (latitude 13°36′12′′ N). The receiver uses a modified doublesideband (DSB) technique. The quadrature samples required for a DSB system are obtained by sampling the digitized intermediate frequency (I.F.) signals by two clocks which are separated in time by one quarter of the period of the I.F. The visibilities required for one-dimensional synthesis are measured using one-bit correlators. A technique to measure amplitude information for the signal using a threshold detector and a one-bit correlator has been developed. The receiver has been successfully used for continuum, spectral-line and pulsar observations. The antenna system of GEETEE and its configuration for one dimensional synthesis are also described in this paper This telescope is jointly operated by the Indian Institute of Astrophysics, Bangalore and the Raman Research Institute, Bangalore.     

Stochastic data in astronomy. II. Search for harmonic components of time series with very large gaps

This is an analysis of certain aspects of using the CLEAN algorithm for Fourier analysis of short segments of time series and of time series consisting of short segments of length ΔT separated by very long irregular gaps. It is assumed that the time series contain a harmonic component of amplitude A with a period longer than the length of the longest of the segments of the time series plus white noise with dispersion N². Reliability plots are constructed for determining the ranges of the parameters (ν, ϕ) for which the CLEAN procedure can be used to determine the values of ν, the frequency, and ϕ, the phase of the harmonic component, with a given accuracy. The results of this analysis are used to search for harmonic components in the variation of the Hβ line profile in spectra of the triple star δ Ori A obtained in 2004 with the BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences. __________ Translated from Astrofizika, Vol. 50, No. 2, pp. 281–297 (May 2007).     

Simultaneous microwave observations of solar flares at 6 and 20 cm wavelengths using the VLA

Using the Very Large Array, solar burst observations have been carried out simultaneously at 6 and 20 cm. Structural changes and preheating have been observed in the flaring regions on time scales of minutes to tens of minutes before the onset of the burst impulsive phase. The 6 cm burst sources are located close to the neutral line, or near the legs of a flaring loop. The 20 cm burst sources show complex and extended structures spatially separated from both the preburst emission and the gradual decay phase of the burst. We interpret the observations in terms of a two-component flare model (bulk heating as well as acceleration of particles) and derive the physical parameters of the burst sources.On leave of absence from Indian Institute of Astrophysics, Bangalore, India.     

Speed Distributions of Merging X-Ray Sources During Chromospheric Evaporation in Solar Flares

We explore the speed distributions of X-ray source motions after the start of chromospheric evaporation in two Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) flares. First, we make CLEAN images at 15 energy bands with a 12 second integration window; then, we outline a flaring loop geometry to cover the looptop and footpoint sources as much as possible. Consistent with the previous steps, we find converging motion of the double footpoint sources along the flaring loop in these two events. This motion is dependent on the energy band and time and is typically seen at 3 – 25 keV, indicating a chromospheric evaporation origin. The speed distributions at various energy bands are measured for the 10 September 2002 flare, which exhibits a separation-to-mergence motion pattern well correlated with the rising-to-decay phases at 50 – 100 keV.     

HI shells in the Leiden-Dwingeloo HI survey

An algorithm of an automatic search for HI shells is presentedand results of identifications in the Leiden-DwingelooHI survey are shown. An identification algorithm searches forlocal minima in channel maps, then creates 3-dimensional objects,HI holes. There is no a priori constraint on the shape of an HI hole, only the dimension criterion: dimensions of the structuremust lie in a preselected interval.When this algorithm is applied to the Leiden-Dwingeloo HI survey,it finds about 1000 structures, which are seen in 4 or more consecutivevelocity channels (Δ v ≥ 4 kms^-1. A total energy contained in all structures in the Milky Way is of theorder of 10⁵⁵ erg, which is less than the energy correspondingto the observed SNR. 80% of the energy contained in holes resides at low galactic z-coordinates, |z|<2 kpc.     

Anisotropy of the space orientation of radio sources. I: The catalog

A catalog of extended extragalactic radio sources consisting of 10461 objects is compiled based on the list of radio sources of the FIRST survey. A total of 1801 objects are identified with galaxies and quasars of the SDSS survey and the Veron-Veron catalog. The distribution of position angles of the axes of radio sources from the catalog is determined, and the probability that this distribution is equiprobable is shown to be less than 10⁻⁷. This result implies that at Z equal to or smaller than 0.5, the spatial orientation of the axes of radio sources is anisotropic at a statistically significant level.     

A search for faint giant radio galaxies in the NVSS survey

Giant radio galaxies are the most extended radio sources in the universe having the sizes of groups and clusters of galaxies (about 1 Mpc). Their total number is comparable with the amount of clusters, revealing the Zeldovich-Sunyaev effect, which may lead to a biased estimation of the angular power spectrum for the ZS effect. To assess a possible contribution to the power spectrum, we need to collect the data on all the observed giant radio galaxies. We propose an algorithm for the selection of large sources (sized over 4′) from the NVSS survey catalog, using the parameters of possible components of radio sources, cataloged as compact objects. As a result of the first stage of our work, we have selected 61 new giant radio galaxy candidates, of which 35 have a pronounced morphological type FRII, and 26 galaxies—the FRI type.     

Adaptive scale model reconstruction for radio synthesis imaging

A sky model from CLEAN deconvolution is a particularly effective high dynamic range reconstruction in radio astronomy,which can effectively model the sky and remove the sidelobes of the point spread function(PSF)caused by incomplete sampling in the spatial frequency domain.Compared to scale-free and multi-scale sky models,adaptive-scale sky modeling,which can model both compact and diffuse features,has been proven to have better sky modeling capabilities in narrowband simulated data,especially for large-scale features in high-sensitivity observations which are exactly one of the challenges of data processing for the Square Kilometre Array(SKA).However,adaptive scale CLEAN algorithms have not been verified by real observation data and allow negative components in the model.In this paper,we propose an adaptive scale model algorithm with non-negative constraint and wideband imaging capacities,and it is applied to simulated SKA data and real observation data from the Karl G.Jansky Very Large Array(JVLA),an SKA precursor.Experiments show that the new algorithm can reconstruct more physical models with rich details.This work is a step forward for future SKA image reconstruction and developing SKA imaging pipelines.     

RADIO MAPS OF THE SOLAR WIND TRANSITION REGION

Occultation studies of near-Sun plasmas using several natural sources simultaneously result in large-scale patterns, radio maps of the solar wind flow. Large radio telescopes of the P.N. Lebedev Physical Institute, Pushino, were used. Previously it had been shown that the plasma acceleration and traverse of the sound barrier proceed in an extended region, the transition region of the solar wind, located at radial distances of about 10–40 solar radii from the Sun. The 1989–1994 experiments showed that the evolution of the transition region geometry is very close to that of the optical corona. On the other hand, the plasma flow structures characteristic of the transition region persist in the course of the 11-year cycle, which demonstrates the existence of some specific mechanism of the solar wind acceleration, independent of wide variations of the general solar activity state. These experimental facts are discussed in connection with the existing theoretical approaches.     

On the nature of the FBS blue stellar objects and the completeness of the Bright Quasar Survey

The second part of the First Byurakan Survey is aimed at detecting all bright (B < 16.5) UV-excess starlike objects in a large area of the sky. By comparison with other major surveys such as the ROSA T All Sky Survey, the ROSAT WGACAT catalogue of point sources, the IRAS survey, the 6cm Green Bank, the 1.4GHz NRAO VLA, and the 92cm Westerbork Northern sky surveys and with the catalogue of mean UBVdata on stars, we estimate the number of AGNs present in the FBS survey and its completeness. We have made spectroscopic observations of nine of the most promising FBS candidates. We have found six new QSOs, bringing the total number of known QSOs in this survey to 42. By comparison with the Bright Quasar Survey, we found that the completeness of this last survey is of the order of 70% rather than 30–50% as suggested by several authors. Based party on observations collected at the Observatorie de Haute-Provence (CNRS, France). The APS databases are supported by the National Foundation, the National Aeronautics and Space Administration, and the University of Minnesota, and are available at hup:// aps.umn.edu. The Digitized Sky Survey was produced at the Space Telescope Science Institute (STScI) under U.S. Government Grant NAG W-2166. Published in Astrofizika, Vol. 42, No. 1, pp. 5–22, January–March, 1999.     

Isotropic wavelets: a powerful tool to extract point sources from cosmic microwave background maps

It is the aim of this paper to introduce the use of isotropic wavelets to detect and determine the flux of point sources appearing in cosmic microwave background (CMB) maps. The most suitable wavelet to detect point sources filtered with a Gaussian beam is the 'Mexican Hat'. An analytical expression of the wavelet coefficient obtained in the presence of a point source is provided and used in the detection and flux estimation methods presented. For illustration the method is applied to two simulations (assuming Planck mission characteristics) dominated by CMB (100 GHz) and dust (857 GHz), as these will be the two signals dominating at low and high frequencies respectively in the Planck channels. We are able to detect bright sources above 1.58 Jy at 857 GHz (82 per cent of all sources) and above 0.36 Jy at 100 GHz (100 per cent of all), with errors in the flux estimation below 25 per cent. The main advantage of this method is that nothing has to be assumed about the underlying field, i.e. about the nature and properties of the signal plus noise present in the maps. This is not the case in the detection method presented by Tegmark & Oliveira-Costa. Both methods are compared, producing similar results.     

On the supernova remnants produced by pulsars

We conclude that pulsar-driven supernova remnants (SNRs) are extremely rare objects. Indeed an analysis of the known sample of plerions suggests a very low birthrate ∼ 1 in 240 years. Long-lived and bright plerions like the Crab nebula are likely to be produced only when the pulsar has an initial period ∼ 10–20 milliseconds and a field ∼ 10¹² G. Such pulsars inside rapidly expanding shell remnants should also produce detectable plerions. The extreme rarity of SNRs with such hybrid morphology leads us to conclude that these pulsars must have been born with an initial period larger than ∼ 35–70 milliseconds. Joint Astronomy Program, Department of Physics, Indian Institute of Science, Bangalore 560012.     

Optical identifications of iras point sources based on low-dispersion fbs spectra. Stars. IV

A fourth list of point sources from the IRAS Point Source Catalog (PSC) that are optically identified with stars of late spectral types is given. The list contains data on 41 objects. The identifications were based on the Digital Sky Survey (DSS), the First Byurakan Survey, blue and red maps of the Palomar Observatory Sky Survey, and infrared fluxes at the wavelengths 12, 25, 60, and 100 μm in the region of +65° ≤ δ ≤ +69° and 05^h10^m ≤ α ≤ 18^h10^m. Of the 41 objects, which are given in the IRAS PSC as unidentified sources of infrared emission, 9 are associated with known stars in existing catalogs while 32 sources remained unidentified in the optical range, one of which is also a source in the deep IRAS survey (IRAS Serendipitous Source Catalog). The optical coordinates, their departures from the IR coordinates, the V stellar magnitudes, the color indices CI, and the preliminary spectral subtypes have been determined. The objects have optical magnitudes in the range of 8^m.5-14^m.5. Finder charts from the DSS are given for 32 of the objects. Translated from Astrofizika, Vol. 43, No. 3, pp. 361-368, July–September, 2000.     

The radio structure of 45 quasars atz < 1.5

Radio maps at 5 GHz with an angular resolution of 1 to 2 arcsec and a dynamic range ≳ 200:1 are presented for a sample of 45 radio quasars at redshifts between 0.2 and 1.5. The sources were imaged from observations made with the Very Large Array with the aim of investigating the epoch dependence of misalignments and asymmetries in their extended radio structure. Maps of some of the larger radio sources are presented also at a frequency of 1.5 GHz with a typical angular resolution of ≈ 4 arcsec. The radio structure of most of the quasars reported here has been delineated in considerably greater detail than available in the literature.     

Optical identification of iras point sources based on low-dispersion fbs spectra. stars. iii

A third list of point sources from the IRAS Point Source Catalog (PSC), optically identified with late-type stars, is given. The list contains data on 34 objects. The identification was based on the Digitized Sky Survey (DSS), the First Byurakan Survey (FBS). blue and red maps of the Palomar Observatory Sky Survey, and infrared fluxes at wavelengths of 12, 25, 60, and 100 mm in the regions of +61° ≤ δ ≤ +65°, 06^h45^m ≤ α ≤ 17^h28^m and +69° ≤ δ ≤ +73°, 03^h50^m ≤ α ≤ 18^h10^m. Of 34 objects given in the IRAS PSC as unidentified sources of infrared radiation, 11 are associated with known stars in existing catalogs, 6 are objects from the FBS survey of late-type stars, and 17 sources remained unknown in the optical range, 3 of them also being sources in the IRAS Serendipitous Survey Catalog (SSC). The optical coordinates, their departures from the 1R coordinates, the V magnitudes, the color indices CI, and the preliminary spectral subtypes were determined. The objects have optical magnitudes in the range of 7^h.6-13^m.6. Finder charts from the DSS are given for 23 objects. Translated from Astrofizika, Vol. 43, No. 1, pp. 77-84, January–March, 2000.     

Large-Scale Active Coronal Phenomena in Yohkoh SXT Images – IV. Solar Wind Streams from Flaring Active Regions

We demonstrate limb events on the Sun in which growing flare loop systems are embedded in hot coronal structures looking in soft X-rays like fans of coronal rays. These structures are formed during the flare and extend high into the corona. We analyze one of these events, on 28–29 August 1992, which occurred in AR 7270 on the eastern limb, and interpret these fans of rays either as temporary multiple ministreamers or plume-like structures formed as a result of restructuring due to a CME. We suggest that this configuration reflects mass flow from the active region into interplanetary space. This suggestion is supported by synoptic maps of solar wind sources constructed from scintillation measurements which show a source of enhanced solar wind density at the position of AR 7270, which disappears when 5 days following the event are removed from the synoptic map data. We also check synoptic maps for two other active regions in which existence of these fan-like structures was indicated when the active regions crossed both the east and west limbs of the Sun, and both these regions appear to be sources of a density enhancement in the solar wind. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005033717284     

A study of the kinematics of the local dark clouds

Lack of reliable estimates of distances to most of the local dark clouds has, so far, prevented a quantitative study of their kinematics. Using a statistical approach, we have been able to extract the average spatial distribution as well as the kinematical behaviour of the local dark clouds from their measured radial velocities. For this purpose, we have obtained radial velocities for 115 southern clouds and used the data from the literature for the northern ones. In this paper we present this new data, analyse the combined data and compare our results with those arrived at by earlier studies. The local clouds are found to be expanding at a speed of ∼ 4 kms^-1 which is in general agreement with the estimates from optical and HI studies. However, it is found that the kinematics of the local clouds is not described by the model proposed for the local HI gas where a ring of gas expanding from a point gets sheared by the galactic rotation. Rather, the observed distribution of their radial velocities is best understood in terms of a model in which the local clouds are participating in circular rotation appropriate to their present positions with a small expansion also superimposed. This possibly implies that cloud-cloud collisions are important. The spatial distribution of clouds derived using such a model is in good agreement with the local dust distribution obtained from measurements of reddening and extinction towards nearby stars. In particular, a region of size ∼ 350 pc in diameter enclosing the Sun is found to be devoid of clouds. Intriguingly, most clouds in the longitude range 100‡ to 145‡ appear to have negative radial velocities implying that they are approaching us. Carried out under the auspices of the Joint Astronomy Program, Department of Physics, Indian Institute of Science, Bangalore in partial fulfillment of the requirements for the Degree of Doctor of Philosophy.