Nancay Radio Telescope Research Articles

EXPERIMENTAL SET-UP FOR DETECTING VERY FAST AND DISPERSED MILLISECOND PULSARS

We have developed observational and data processingtechniques for detecting millisecond pulsars. Themethod of data processing consists in correlating timeseries of data by folding them in two steps accordingto a trial period. While very time consuming,especially for very short periods, this methods allowsus in principle to detect very fast and dispersedmillisecond pulsars. The experimental set-up has beenintegrated into a data acquisition system developed atCentre national d'etudes spatiales (CNES).Observations of known millisecond pulsars atNancay radio observatory are used to validate thesystem and assess the sensitivity of the survey, whichis 3.5 mJy for 240 sec of integration. Futureimprovements in the radio-telescope and the systemshould bring it down to below 1 mJy. The applicationof the method to a millisecond pulsar survey is underway at Nancay radio observatory in collaborationbetween the Bureau international des poids et mesure(BIPM), Observatoire Midi-Pyrenees (OMP), and CNES.

Overshoot of main-line spectral extent in circumstellar OH masers

ABSTRA C T Several OH/IR sources were reported by David et al. and Dickinson & Turner to show a slight overshoot of the 1667-MHz OH line relative to the presumably terminal expansion velocity indicated by the 1612-MHz line. From the David et al. data, we selected a representative sample of 21 objects which we observed with the Nancay radio telescope with a good velocity resolution in both lines simultaneously. Our data clearly confirm the overshoot phenomenon. Shifts range from a few tenths to a few km s ˇ1 . Prominent overshoots, by at least 3 velocity channels, are found in 65 per cent of the measurements; only 17 per cent do not show a clear overshoot. Overshoots in the front and back parts of the envelope occur independently. No strong correlation was found with the front/back asymmetry, expansion velocity, IRAS colours, pump efficiency or line ratios. It is argued that in normal envelopes, slight changes in the medium can broaden main lines, or can quench satellite line components, so that main lines appear to be the most extended. Overshoots can also be produced, however, by departure from the currently accepted circumstellar processes. Zeeman broadening, clumpiness, velocity fluctuations or axisym- metric winds appear as the most likely processes able to produce overshoots. Monitoring, mapping and observations of other maser species are required in order to identify firmly the nature and the importance of these overshoots.

Study of new sample of candidate Blue Compact Dwarf Galaxies. HI observations of 73 objects at nancay radio telescope

HI observations are presented for a new sample of 73 candidates BCDGs. HI emission has been detected from 36 and possible emission from 11 galaxies. For detected galaxies HI radial velocities, HI line width measured at 50% and 20% of the maximum intensity, HI fluxes, and HI masses are determined. The distributions of several parameters are given.

Nançay "blind” 21 cm line survey of the Canes Venatici group region

Submitted to: Astron. Astrophys. Abstract: A radio spectroscopic driftscan survey in the 21cm line with the Nancay Radio Telescope of 0.08 steradians of sky in the direction of the constellation Canes Venatici covering a heliocentric velocity range of -350 < V_hel < 2350 km/s produced 53 spectral features, which was further reduced to a sample of 33 reliably detected galaxies by extensive follow-up observations. With a typical noise level of rms = 10 mJy after Hanning smoothing, the survey is -- depending on where the detections are located with regard to the center of the beam -- sensitive to M(HI)=1-2x10^8 Msun at 23 Mpc and to M(HI)=4-8x10^7 Msun throughout the CVn Groups (Ho=100). The survey region had been previously examined on deep optical plates by Binggeli et al. 1990 and contains loose groups with many gas-rich galaxies as well as voids. No galaxies that had not been previously identified in these deep optical surveys were uncovered in our HI survey, neither in the groups nor the voids. The implication is that no substantial quantity of neutral hydrogen contained in gas-rich galaxies has been missed in these well-studied groups. All late-type members of our sample are listed in the Fisher and Tully (1981b) optically selected sample of nearby late-type galaxies; the only system not contained in Fisher and Tullys' Catalog is the S0 galaxy NGC 4203. Within the well--sampled CVn group volume with distances corrected for flow motions, the HI mass function is best fitted with the Zwaan et al. (1997) HI mass function (alpha=-1.2) scaled by a factor of f=4.5 in account of the locally overdense region.

Kinematics of the local universe

This paper presents 2112 new 21-cm neutral hydrogen line measurements carried out with the meridian transit Nancay radiotelescope. Among these data we give also 213 new radial velocities which complement those listed in three previous papers of this series. These new measurements, together with the HI data collected in LEDA, put to 6 700 the number of galaxies with 21-cm line width, radial velocity, and apparent diameter in the so-called KLUN sample.

Multiwavelength monitoring of GRS 1915 +105

Since its discovery in 1992, the superluminal X-ray transient GRS 1915+105 has been extensively observed in an attempt to understand its behaviour. We present here first results from a multiwavelength campaign undertaken from 1996 July to September. This study includes X-ray data from the RXTE All Sky Monitor and BATSE, two-frequency data from the Nancay radio telescope, and infrared photometry from the 1.8-m Perkins telescope at Lowell Observatory. The first long-term, well-sampled infrared light curve of GRS 1915+105 is presented herein; it is consistent with the interpretation of this source as a long-period binary. We compare the various light curves, searching for correlations in the behaviour of the source at differing wavelengths and for possible periodicities.

21-cm line observations of galaxies from Kazarian's lists

The 21-cm neutral hydrogen line has been measured for the rst time in 39 non-Seyfert type galax- ies from Kazarian list, with the Nan cay radiotelescope. The line proles, widths at 20% and 50% of the peak in- tensity, radial velocities as well as total fluxes are pre- sented. The values of radial velocity are in fairly good agreement with those obtained from optical spectra. The width values at 20% of the peak vary in wide range from 113 km s 1 (KAZ 579) to 608 km s 1 for KAZ 566. Nine of our objects remained undetected, whether their HI-flux was too faint, the integration time too short, or the fre- quency sighted wrong. However, for part of them, there were positive hint of detection. Comments on individual objects are given.

Observations of the OH Radical in Comet C/1995 O1 (Hale-Bopp) with the Nançay Radio Telescope

We present OH 18-cm observations of comet Hale-Bopp (C/1995 O1) at the Nancay radio telescope. On nucleus and offset position observations allowed us to obtain both OH production rates and quenching radii. The maximum OH production rate was reached around perihelion, at about1031 s-1.

A new dual-reflector feed system for the Nancay radio telescope

A new compact dual-reflector Gregorian feed system has been designed to improve the overall performance of the Nancay radio telescope. To operate continuously within the 1.0-3.5-GHz band, two wideband compact corrugated horns are used. The new feed system improves the sensitivity of the telescope by a factor between 2 and 3. To verify the theoretical design, a 1/20th-scale model was constructed and tested. The measured results were in excellent agreement with the predicted values.

A flare in the mira X oph

Before the observation of the 1974 U Ori eruption, it was considered that the Mira stars had only some regular OH variations. With this eruption, we realized that sometimes flares can occur in this type of star. In the course of an OH Mira star monitoring programme with the Nancay radio telescope, we have discovered a new eruptive type of OH maser emission in several sources. Especially, in early 1992, we observed a quickly rising 1665 Mhz emission in the Mira X Oph. The main characteristics of this flare were: large flux variations independent of the light curve; large degree of circular polarization; radial velocity emission close to the stellar velocity.

OH masers in envelopes of late type stars

Two studies of OH maser emission in envelopes of late type stars -miras (3) and OH/IR objects- have been performed with the Nancay radio-telescope. Mainly, the OH miras are found with thicker dust envelopes than the non OH ones. A sample of unidentified IRAS point sources selected on their colors has been observed. We have detected 46 new OH sources. The OH detection rate is a function of the galactic longitude and of the IRAS spectral classification.

Contagious B Star Formation in the Rho Ophiuchi Dark Cloud

Three young stars are still embedded in the small (0.7 pc wide) and densest region of the Rho Ophiuchi dark cloud. One of these stars coincides with the continuum source Oph 4 (Brown and Zuckerman, 1975). Their existence is indicated by the following: i) thermal continuum sources detected at 21 cm and 6 cm with the Westerbork Synthesis Radiotelescope (Netherlands) which display the presence of compact HII regions surrounding each of three stars; ii) peaks of CO emission (J = 1 → 0 transition) detected with the MWO antenna of MacDonald (Texas) which are associated with asymmetrically shaped lines; iii) C 158α (and S 158α for Oph 4) line emission mapped with the Nancay Radiotelescope. All these observations imply that the three stars have B3 - B2 spectral types.

A statistical study of local interstellar matter based on the Nancay 21-cm absorption survey

The survey of 21-cm galactic absorption towards 819 extragalactic sources, recently carried out with the Nancay radio telescope (Crovisier et al. 1978) provides a sample of absorbing neutral hydrogen clouds which is an order of magnitude larger than those resulting from other available surveys (Hughes et al. 1971, Radhakrishnan et al. 1972, Lazareff 1975, and Dickey et al. 1978). A statistical study of this sample offers insight into the properties of local interstellar matter.

OH Observation of Comet Kohoutek (1973f) at 18 cm Wavelength

AbstractThe main lines of OH at 18 cm wavelength were observed in Comet Kohoutek (1973f) from December 1973 through February 1974 with the Nancay radio telescope. They were detected in absorption in early December and reappeared in emission around mid-January. In a preliminary approach these results are interpreted in terms of U. V. pumping by the sum when the Fraunhofer spectrum is taken into account.

21-cm Absorption Spectrum in Front of Cygnus X-3

WE have previously presented observations1 made with the Nancay radio telescope of the 21-cm absorption spectrum in front of the highly variable radio source which is believed to be associated with Cygnus X-3 (ref. 2). This spectrum was derived from drift scans in different frequency channels. A baseline was fitted to the drift scans outside the source and subtracted from the record of the source; in a region exhibiting large variations in the 21-cm emission, like the surroundings of Cyg X-3, this procedure may involve large uncertainties due to an insufficient knowledge of the HI emission at the location of the source. Fortunately, in the case of strongly variable radio sources such as pulsars or Cyg X-3, an absorption spectrum free of these uncertainties can be derived from observations at two epochs when the fluxes of the source are widely different. For Cyg X-3, during September 1972 we obtained a few preliminary observations to compare with the initial ones and we suspected the existence of absorption around −68 km s−1 (ref. 1). We recently obtained good comparison observations after the source had become indistinguishable from the surrounding galactic background. Comparing these with the initial observations of September 4 and 5 (Fig. 1) we find three changes.

A model for the chromosphere-corona transition region based on radio observations and on hydrodynamical conservation equations

In order to avoid the disagreements and difficulties now found in the interpretation of quiet-Sun UV observations concerning the chromosphere-corona transition region, 6- and 11-cm maps obtained at cycle maximum with the Nancay radiotelescope are used in combination with existing measurements of the central brightness temperatures at centimetric wavelengths and with the hydrodynamic conservation equations so as to obtain an equatorial, quiet solar atmosphere model between 10000 K and 300000 K, in the interspicular regions. This model introduces a large ascending velocity in these regions. Some of the consequences of the model concerning the heating and replenishment of the corona are discussed. An explanation of sudden disappearance of quiescent prominences is suggested.

Quiet-Sun center-limb observations at 6 and 11 cm during cycle maximum

Simultaneous 6 and 11 cm maps of the quiet Sun have been obtained with the Nancay radiotelescope during the 1968–69 cycle maximum.

VRO 42 22 01: a Variable Source with Circular Polarization

THE radio source VRO 42 22 01 was recently identified with a variable stellar object, BL Lac (ref. 1), and we have obtained results which show that the source is radio variable and probably circularly polarized. Since September 1967, a 2,695 MHz receiver has been in operation at the Nancay radiotelescope. It allows simultaneous measurements of flux densities with electric vectors at position angles 0° (V) and 90° (H), and of the products, in phase and through a quarter-wavelength delay, of the two corresponding voltages. It is thus possible to determine the complete polarization, linear or circular, of the most intense sources. One of the programmes in progress with this receiver is the measurement of flux variations for known variable sources, and of possible polarization variations. The radio source VRO 42 22 01 was included in that programme. This source has a flux density at 610.5 MHz of S610 = 4.0 flux units (ref. 2) (1 flux umt = 10−26 W/m−2 Hz−1), but it does not appear in the 4C catalogue3, so S178<2.0 flux units. Its flux density was measured at 1,417 MHz by Williams, and Stewart4, who give S1417 = 7.3 flux units. Its radio spectrum resembles that of the variable components of well known radio sources.