(OCR+proof, and reformatting by H.Andernach 1/96)
Key words: Galaxies: active - Galaxies: magnetic fields - Polarization - Radio Sources
Tabara and Inoue (1980) compiled polarization data and derived 637 RMs for radio sources. In deriving RMs, they set an upper limit of |RM| to avoid the n*pi ambiguity, otherwise a large value of RM could be often fitted to the oserved data by chance coincidence due to the limited number of observed wavelengths and their finite errors. in fact, the upper limit of 200 rad/m^2 was set for high galactic latitude sources. Because of certain amount of Galactic contribution (Inoue, Tabara 1981), the upper limit was increased with decreasing galactic latitude. More than 70% of the sources have |RM| < 50 rad/m^2 (Tabara, Inoue 1980). However, it has been known that some radio sources certainly show |RM| > 200 rad/m^2 (e.g., Tabara, Inoue 1980; Simard-Normandin et al. 1981; Dreher et al. 1987). Therefore, in order to distinguish the real high RM from false value of high RM, a system that is free from the nPI ambiguity is greatly needed. With such system, we could see high RM and study if there exists an upper limit to RM or not.
We have therefore built a multichannel polarimeter at 10 GHz attached to the Nobeyama 45-m telescope. Gauss and Goldstein (1973) first made a multichannel polarimeter at around 1300 MHz. High frequency observations are essential to study high RM because (1) high degree of polarization is expected because of low depolarization, (2) the amount of Faraday rotation is small, which could reduce the n*pi ambiguity even for high RM, (3) by adding a new data point to a plot of polarization angle vs. wavelength square, the n*pi ambiguity is very much reduced, and (4) more active, dense regions such as hot spots and nuclear regions can be seen, where high RM is expected. In fact, this system eventually enabled us to find high RMs in the Galactic center region (Inoue et al. 1984; Tsuboi et al. 1986), and in four extragalactic radio sources of |RM|>1000 rad/m^2 (Kato et al. 1987). Since it is HOW obvious that at least several sources show intrinsically high RM, we have made a survey for high RM sources using the polarimeter. In this paper we describe the results of this survey. In section 2, source selection for the survey is described, and in section 3 the observation system including the multichannel polarimeter, the calibration method, and data reduction is given in detail. The results of the observations on 103 sources are given in section 4. The results are incorporated in the revised catalogue of Tabara and Inoue (1980). Using this new catalogue, we have derived much reliable RMs applying a new method on the Or ambiguity. The results will be given in Part 11 (Aizu et al. 1995 in preparation). Time variation of RM will be also published elsewhere.
(a) Sources that are not well fitted to small RMs on polarization angle vs. wavelength square plot by the existing data. These sources are referred to as CPA (Candidates based on the Polarization Angle).
(b) Sources with a strong depolarization at longer wavelengths. Although the strong depolarization is not always associated with high RM (e.g., Dreher et al. 1987), the sources deserve to be examined. These sources are re ferred to as CDP (Candidates based on the DePolarization).
(c) Sources with compact steep spectrum (CSS). Two of four sources listed by Kato et al. (1987) were CSS sources, and they suggested that the dense medium around the nucleus, which stops the jets and hence prevents the formation of extended radio structure, is responsible for the high RM. CSS lists of Perley (1982), Peacock and Wall (1982), and Pearson et al. (1985) were surveyed.
These three criteria for the high RM candidates are not exclusive with
each other. We further imposed the following three conditions to these candidates:
(i) Due to the sensitivity and different polarization natures of each
criteria, we set limits in different way; the polarized flux density at 5 GHz
is above 30 mJy for CPA, and for CDP the expected polarized flux density at 10
GHz is above 20 mJy. For CSS, the total flux density S at 5 GHz is S > 750 mJy
and the spectral index alpha < -0.2 (S ~ freq^alpha).
(ii) The galactic latitude is above 20d so that the galactic RM due to the
interstellar plasma should be negligibly small (moue, Tabara 1981),
and also alpha < -0.2.
(iii) The polarized flux density changes no more than 30% from the average, or
the polarization angle is no longer than 20d, irrespective of the time of
observation. The variation in polarization was taken from Berge & Seielstad
(1972), Altschuler & Wardle (1976), and Aller et al. (1985).
The total number of the candidates thus selected were 96:
28 CPAs, 19 CDPs, and 64 CSSs.
The total bandwidth of 2 GHz was divided into four contiguous channels with 500-MHz bandwidth each at IF stage of 5 7 GHz. Each channel was sampled simultaneously, and converted by a 12-bit A/D converter in the receiver room. Figure 1 shows the block diagram of this system.
The HPBW at 10 GHz was 2.7', and the position of the telescope was switched by 5' in the azimuthal direction every 15 s to remove the sky fluctuation (i.e., ON-OFF switching). The pointing error was less thank 30", which did not cause effective error in polarization measurements. The direction of the received polarization was rotated by 22.5d from 0d to 180d for every 2 min. or four ON-OFF cycles, so that one sequence took 18 min. Just before or aver this sequence, the total intensity was measured by the Dicke switching mode. The differential polarizations from 0d to 180d were fitted by a sinusoidal curve, which gives the polarization angle and the polarized intensity. The one-sigma detection limit for the polarization is 10 mJy.
The instrumental polarization due to the phase shifter had a half-period component. This had a fixed phase with respect to the coordinate of the phase shifter against one period rotations of the polarizations angle (180d, or 90d rotation of the half-wavelength phase shifter) for a linearly polarized input. The residual after calibration of this component was less than 0.1% of the input intensity. The offset of the polarization angle was calibrated using 3C 286, the assumed RM and intrinsic polarization angle being 0 rad/m^2 and 33.0d +/- 0.1d, respectively. The amount of the correction was 3.7d. These calibrations were made at each observing run for each channel by observing 3C 84 with a wide range of elevation angles. Since the phase shifter was sometimes removed and re set, the correction parameters change from time to time. The flux densities of the calibrators are based on Baars et al. (1977), and they are summarized in table 1. The receiving band was so wide that the observations sometimes suffered from interference. In particular, the second lowest channel was affected seriously. In that case, we only give results of the remaining channels in the next section.
Columns 1-4 repeat those in table 2, and column 5 gives RM arid its error. The percentage of CSS decreases from 66% in the original sample (table 2) to 43% in table 3, as polarization of CSS is, in general, weak. Figure 3 shows the histogram of RM distribution. More than 30% of the sources have |RM| > 500 rad/m^2. Among them, 5 sources (14%) show |RM| > 1000 rad/m^2. Although the error of RM is large, this distribution of large ratio of high RM is remarkably different from those of usual radio sources (e.g., Tabara, Inoue 1980). The number of sources, however, decreases rapidly and becomes rare above |RM| > 1000 rad/m^2. No clear trend of concentration for any source criteria can bee seen between RM distributions above and below 500 rad/m^2. However, it should be noted that more than 40% of CSS sources show |RM| > 500 rad/m^2.
TABLE
Codes for selection criteria for sources:
SS = compact steep spectrum sources
DP = candidates based on depolarization
PA = candidates based on polarization angle
Table 2. Results of the four-channel polarimetry
|----- 2.84 cm / 10.56 GHz ---|----- 2.98 cm / 10.07 GHz ----|----- 3.14 cm / 9.55 GHz -----|----- 3.31 cm / 9.06 GHz -----| RM dRM
IAUname Others ID Criteria S10.56/mJy p% +-% PA/d +-d | S10.07/mJy p% +-% PA/d +-d | S9.55/mJy p% +-% PA/d +-d | S9.06/mJy p% +-% PA/d +-d | rad/m^2
----+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8----+----9----+----0----+----1----+----2----+----3----+----4----+----5----+----6----
0019-000 4C+00.02 G SS 535 26 1.7 1.0 114.5 17.3 483 25 0.8 1.6 168.5 54.6 542 28 2.0 0.7 56.6 9.3 540 27 1.7 1.0 106.5 16.2
0023-263 SS 1712 75 0.5 0.4 162.1 26.3 1798 79 0.4 0.3 121.1 20.9 1928 84 0.6 0.2 68.9 10.7 2082 89 0.3 0.3 17.8 33.1