A&A 427, 769-771 (2004)
DOI: 10.1051/0004-6361:20041289

Twenty years monitoring of extragalactic sources at 22, 37 and 87 GHz^,^,

H. Teräsranta¹ - J. Achren² - M. Hanski² - J. Heikkilä¹ - J. Holopainen¹ - O. Joutsamo² - M. Juhola¹ - K. Karlamaa¹ - S. Katajainen² - P. Keinänen² - P. Koivisto² - M. Koskimies² - P. Könönen¹ - M. Lainela² - A. Lähtenmäki¹ - K. Mäkinen² - T. Niemelä² - P. Nurmi² - T. Pursimo² - R. Rekola² - T. Savolainen² - M. Tornikoski¹ - J. Torppa¹ - T. Valtonen¹ - N. Varjonen² - E. Vilenius¹ - J. Virtanen¹ - S. Wiren²

1 - Metsähovi radio observatory, Helsinki University of Technology, Metsähovintie 114, 02540 Kylmälä, Finland
2 - Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland

Received 14 May 2004 / Accepted 17 June 2004

Abstract
Long term monitoring results from mid 1995 to the end of 2000 of quasar observations at 22, 37 and 87 GHz done at the Metsähovi radio observatory are presented. Approximately 15 700 observations are published here.

Key words: galaxies: active - radio continuum: galaxies - quasars: general - astronomical data bases: miscellaneous

1 Introduction

To better understand the behaviour of quasars, it is important to monitor them over a large range of frequencies. The sources selected in this study are radio loud, have a flat radio spectrum and have shown blazar flaring type behaviour at least in some frequency band. Due to the small size of our antenna (13.7 m), the sample is limited to stronger sources with flux density S>0.5 Jy. The Northen location of the antenna (latitude = 60N) limits the observed sources to those with declination higher than -5 degrees, with some exeptions. The variability time scale of these sources at 22 and 37 GHz is typically from months to years. Monthly monitoring would in most cases be enough in to produce representative flux curve. The sensitivity of the antenna, receivers and time given for the observations limit the yearly observations to under 5000, which would allow a sample of 200 sources to be observed monthly. Currently in the list there are over 200 sources, partly as a result of selecting new flat spectrum sources for monitoring prior to the next gamma-ray observatories AGILE and GLAST. The stronger sources, which are also used for pointing the antenna, are observed daily, while for the new candidate sources must be limited to 2-4 yearly to demonstrate their spectra and duty cycle.

Table 1: Number of observations at each frequency.

2 Observing system

The Metsähovi radio telescope is a 13.7 m diameter radome inclosed antenna. The surface accuracy of the antenna, 0.1 mm rms, allows observations at over 100 GHz, but the sea level location is not suitable for submillimeter observations, at least during the warm season. The receivers at 22 and 37 GHz have HEMPT front ends operating at room temperature. The total noise temperatures are in the range of 200-300 K DSB, depending on the season. The 1 sigma rms values for a 30 min integration, with good observing conditions are about 0.03-0.06 Jy. Both receivers operate with dual beam mode. The Metsähovi antenna was upgraded by changing the antenna panels and back structure in 1994. The effects of the better panels on the data reduction were described in Teräsranta et al. (1998).

3 Final remarks

The number of observations for each source is shown in Table 1. The data before mid 1995 were published in Salonen et al. (1987), Teräsranta et al. (1987), Teräsranta et al. (1992) and Teräsranta et al. (1998). The weekly mean fluxes of the 56 best sampled sources since the start of monitoring are shown in Fig. 1 at 22 and 37 GHz. The numerical data of flux densities from mid 1995 to the end of 2000 is published in Table 2 only at the CDS via anonymous ftp. The data format in Table 2 is the following: object designation, other name, observing frequency (GHz), observation date, time in UT hours, time in minutes, flux density (Jy), 1 sigma error estimate (Jy). The earlier data has been anylysed as in in Valtaoja et al. (1988), Valtaoja et al. (1992) and numerous other publications.

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{1289fi01.ps} \addtocounter{figure}{-1} \end{figure}$	Figure 1: Weekly mean flux density of observed sources at 22 and 37 GHz.
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Acknowledgements

Part of this work was supported by the Academy of Finland and the Wihuri foundation.

References

Salonen, E., Teräsranta, H., Urpo, S., et al. 1987, A&AS, 70, 409 [NASA ADS] (In the text)
Teräsranta, H., Valtaoja, E., Haarala, S., et al. 1987, A&AS, 71, 125 [NASA ADS] (In the text)
Teräsranta, H., Tornikoski, M., Valtaoja, E., et al. 1992, A&AS, 94, 121 [NASA ADS] (In the text)
Teräsranta, H., Tornikoski, M., Mujunen, A., et al. 1998, A&AS, 132, 305 [NASA ADS] (In the text)
Valtaoja, E., Haarala, S., Lehto, H., et al. 1988, A&A, 203, 1 [NASA ADS] (In the text)
Valtaoja, E., Teräsranta, H., Urpo, S., et al. 1992, A&A, 254, 71 [NASA ADS] (In the text)

Online Material

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