Relevamiento de quasares en el hemisferio sur

La primera etapa de este survey comprende la observación de 700 objetos extragalácticos del tipo QSO que fueron seleccionados del "A catalogue of Extragalactic Radio Source Identifications, Second Edition" publicado en 1983 por Veron-Cetty y Veron (Astron. Astrophys. Suppl. Ser., Vol. 53, Nº 2, p. 219-221). El trabajo observacional es realizado con la antena de 30 m del IAR operando el sistema receptor en el continuo centrado en 1410 MHz. Estas radiofuentes han sido detectadas en los dos grandes relevamientos del Hemisferio Sur, el de Parkes (Australia) realizado en 408, 1410 y 2650 MHz durante el período 1965-1968 y en fecha más reciente por el Observatorio de Molonglo (Australia) donde se trabajó en 408 MHz (Large et al., Mon. Not. R. Astron. Soc. 194, 693, 1981). Nuestro interés está centrado en el armado de espectros, la determinación de índices espectrales y las posibles variaciones de flujo que podamos detectar por medio de la comparación con otros datos observacionales.Asociación Argentina de Astronomí

Radio spectra of quasars. I

We present 1410 MHz flux density for 243 quasars by Veron-Veron (1983), in "A Catalogue of Extragalactic Radio Source Identification. Second Edition". Using observational data at different frequency we have calculated spectral index and the curvature of spectra. The 243 observed quasars occupy a sky area from 00h to 05h 58m on right ascension and from -9.5° to -90° in declination. All of the are part of a complete sample of 700 quasars.Asociación Argentina de Astronomí

Spectra of southern QSO

Spectral indices distribution of 684 radio-sources identified as quasars by Veron-Cetty and Veron (1983) located at south of declination -10 degrees, are analyzed. The spectral indices studied are between frequencies 408 and 1410 MHz, 1410 and 2700 MHz, and 2700 and 5009 MHz. About 33% of the sources with spectral indices measured are normal steep spectra radio-sources. The distribution of spectral indices show significative energy loss between 1410 and 2700 MHz for about 15% of the sample. From the literature it is known that QSO radio spectra turn over near 1 GHz due to synchrotron self-absorption (Kapahi 1981, Peacock and Wall 1981, Downes et al. 1981). These radio-sources have attracted considerable attention because they seem to be associated with quasars of high redshift (Gopal-Krishna et al. 1983). From the present sample we found 17 GPS sources (2.4%). With these sources a total of 67 GPS sources are known.Asociación Argentina de Astronomí

X-ray Spectral Survey of WGACAT Quasars, II: Optical and Radio Properties of Quasars with Low Energy X-ray Cut-offs

We have selected quasars with X-ray colors suggestive of a low energy cut-off, from the ROSAT PSPC pointed archive. We examine the radio and optical properties of these 13 quasars. Five out of the seven quasars with good optical spectra show associated optical absorption lines, with two having high delta-v candidate systems. Two other cut-off quasars show reddening associated with the quasar. We conclude that absorption is highly likely to be the cause of the X-ray cut-offs, and that the absorbing material associated with the quasars, not intervening along the line-of-sight. The suggestion that Gigahertz Peaked Sources are associated with X-ray cut-offs remains unclear with this expanded sample.Comment: 17 pages, LaTeX, including 2 Tables and 1 figure. Ap.J. in pres

The SPECFIND V2.0 catalogue of radio cross-identifications and spectra. SPECFIND meets the Virtual Observatory

The new release of the SPECFIND radio cross-identification catalogue, SPECFIND V2.0, is presented. It contains 107488 cross-identified objects with at least three radio sources observed at three independent frequencies. Compared to the previous release the number of entry radio catalogues is increased from 20 to 97 containing 115 tables. This large increase was only made possible by the development of four tools at CDS which use the standards and infrastructure of the Virtual Observatory (VO). This was done in the framework of the VO-TECH European Design Study of the Sixth Framework Program. We give an overview of the different classes of radio sources that a user can encounter. Due to the increase of frequency coverage of the input radio catalogues, this release demonstrates that the SPECFIND algorithm is able to detect spectral breaks around a frequency of ~1 GHz.Comment: 11 pages, 10 figures, accepted for publication in A&

150 keV Emission from PKS2149-306 with BeppoSAX

A BeppoSAX observation of the z=2.34 quasar PKS2149-306 produced a strong signal in the high energy PDS instrument up to a maximum observed energy of nearly 50 keV, 150 keV in the quasar frame. The Beppo-SAX spectrum spans almost 3 decades (0.3-150 keV, quasar frame) and shows an extremely hard (alpha=0.4+/-0.05) X-ray spectrum above 3 keV (comparable to the X-ray background slope), and either a softer (alpha=1.0(+0.6, -0.3)) low energy component, or an ionized absorber at zero redshift. No evidence is seen of an Fe-K emission line (EW<167 eV at 6.5 keV quasar frame) or a Compton hump (R<0.3). A bremsstrahlung fit gives kT(rest)=46(+32, -16) keV, similar to the X-ray background value, and a high energy cut-off power law requires E(cut)>120 keV (quasar frame). The SED of PKS 2149-306 shows two peaks at ~ 10(12+/-0.5}Hz and \~10(21+/-1.0)Hz (~ 0.3 mm and ~ 4 MeV), strongly resembling a low energy cutoff BL~Lac object (LBL). The ratio of the two peaks shows an extreme Compton dominance (C_D=1.4+/- 0.4), as in flat spectrum radio quasars (FSRQs). The presence of an additional `optical/UV big bump' component may provide photons that cool the jet, suppressing the radio emission.Comment: 18 pages, 3 figures. LaTeX, with AAS .sty file aasms4. Revised to correct a stupid mistake affecting the Fe-K EW. The results now agree with the ASCA data from Yaqoob et a

Radio Frequency Spectra of 388 Bright 74 MHz Sources

As a service to the community, we have compiled radio frequency spectra from the literature for all sources within the VLA Low Frequency Sky Survey (VLSS) that are brighter than 15 Jy at 74 MHz. Over 160 references were used to maximize the amount of spectral data used in the compilation of the spectra, while also taking care to determine the corrections needed to put the flux densities from all reference on the same absolute flux density scale. With the new VLSS data, we are able to vastly improve upon previous efforts to compile spectra of bright radio sources to frequencies below 100 MHz because (1) the VLSS flux densities are more reliable than those from some previous low frequency surveys and (2) the VLSS covers a much larger area of the sky (declination >-30 deg.) than many other low frequency surveys (e.g., the 8C survey). In this paper, we discuss how the spectra were constructed and how parameters quantifying the shapes of the spectra were derived. Both the spectra and the shape parameters are made available here to assist in the calibration of observations made with current and future low frequency radio facilities.Comment: Accepted to ApJ

Estudio de la variación de flujo de radiofuentes en 1420 Mhz

Con el objeto de estudiar la variación de flujo de radiofuentes se seleccionaron alrededor de 80 objetos del catálogo de Parkes, los cuales tienen una densidad de flujo S ≥ 1 y en 1420 MHz y un índice espectral α ≤ 0.5. Se discuten los resultados de las primeras mediciones (marzo-agosto 1982) y su comparación con las mediciones de Parkes.Asociación Argentina de Astronomí

Multiwavelength observations of PKS 2255-282

During high radio frequency active galactic nucleus monitoring, we observed a major millimeter wave outburst from PKS 2255-282 in 1997 and initiated a campaign for multifrequency radio monitoring and VLBI imaging of the source. In 1998 January the EGRET instrument on board the Compton Gamma Ray Observatory detected a bright high-energy γ-ray outburst from its direction. Thus, we had the unique opportunity to monitor the source in detail at various radio frequencies already before the detection of the γ-ray outburst. We also initiated multiwavelength follow-up observations. In this paper we present multiwavelength data for PKS 2255-282 before and after the γ-ray outburst. Further, based on the observations, we estimate some physical parameters for this source and discuss the various emission mechanisms possibly involved during this outburst.M. Tornikoski, S. J. Tingay, A. Mücke, A. Chen, V. Connaughton, D. L. Jauncey, M. Johnston-Hollitt, J. Kemp, E. A. King, P. McGee, F. Rantakyrö, D. Rayner, O. Reimer, and A. K. Tzioumi