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Astron. Astrophys. 321, 513-518 (1997)

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2. Observations

We have recently performed the first study of radio pulsars at mm-wavelengths, using the 100-m Effelsberg radiotelescope of the MPIfR (Wielebinski et al. 1993, K96, X96). The data presented in this paper were obtained in December 1993 at 29.3 GHz ([FORMULA] 10.23mm), in July 1994 at 32.0 GHz ([FORMULA] 9.37mm) and in December 1995 at 35.4 GHz ([FORMULA] mm). For the December sessions we used a tunable receiver installed in the prime focus of the telescope providing signals of one linear polarization and 2 GHz bandwidth. In the observations in July 1994 we obtained both left and right hand circularly polarized signals. Here, a receiver of fixed centre frequency was installed in the secondary focus of the telescope and also provided a bandwidth of 2 GHz. While the weather conditions during the December sessions were rather unstable, i.e. involving cloudy sky, rain and even sometimes snow, the July session was made under near perfect and stable weather conditions where both atmospheric and differential telescope temperature varied a few degrees along the dish surface for long sessions during night and day time. Further details of the observing system are summarized in Table 1, and also given by K96.


[TABLE]

Table 1. System parameters for observations at mm-wavelengths.


The received signals were sampled every P/1024 s and folded synchronously to the pulsar topocentric period P. Sub-integrations of 15s were stored on disk for later analysis. A noise diode, which was directly coupled into the waveguide following the antenna horn, was used as a calibration signal and was switched on synchronously to the pulse period during the first fifty phase bins of each integration. The stability of the calibration signal itself was checked during the pointing observations of well-known flux calibrators; these were made regularly after each integration on a pulsar, which lasted typically 60 to 90 min. A detailed description of the applied flux calibration scheme can be found in Kramer (1995). The calibration runs on continuum sources enabled us to monitor the gain-elevation dependence which shows that significant corrections to the measured flux-densities need to be applied when our measurements are done at very high elevations (above [FORMULA]) and very low elevations (below [FORMULA]). The data presented here were however taken at elevation between [FORMULA] and [FORMULA] (see Table 2).


[TABLE]

Table 2. Modulation indices for the four strongest sources at mm-wavelengths. We quote the dispersion measure, DM, of the pulsar (column 2), distance (column 3) observing frequency, [FORMULA] (column 4), in GHz, the total observing time, T (column 5), in minutes, the average source elevation during scan (column 6) and observed modulation indices, [FORMULA] (column 7). All observation were done with 2 GHz bandwidth. The predicted scintillation parameters are transition frequency (column 8), scintillation index and time scale (column 9 and 10).


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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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