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Measurement of the Integrated Faraday Rotations of BL Lac Objects A Astronomy Reports, Vol. 45, No. 9, 2001, pp. 667–675. Translated from AstronomicheskiÏ Zhurnal, Vol. 78, No. 9, 2001, pp. 771–779. Original Russian Text Copyright © 2001 by Pushkarev. Measurement of the Integrated Faraday Rotations of BL Lac Objects A. B. Pushkarev Astro-Space Center, Lebedev Physical Institute, LeninskiÏ pr. 53, 117924 Russia Received November 10, 2000 Abstract—We present the results of multi-frequency polarization VLA observations of radio sources from the complete sample of northern, radio-bright BL Lac objects compiled by H. Kühr and G. Schmidt. These were used to determine the integrated rotation measures of 18 sources, 15 of which had never been measured previ- ously, which hindered analysis of the intrinsic polarization properties of objects in the complete sample. These measurements make it possible to correct the observed orientations of the linear polarizations of these sources for the effect of Faraday rotation. The most probable origin for Faraday rotation in these objects is the Galactic interstellar medium. The results presented complete measurements of the integrated rotation measures for all 34 sources in the complete sample of BL Lac objects. © 2001 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION must be known: first, the orientation of the electric vec- BL Lac objects are an extreme class of active galac- tor of its linear polarization, second its optical depth, tic nuclei characterized by a number of peculiar proper- and finally, its rotation measure. ties: weak or absent optical line emission, rapid and The first two properties flow directly from any strong variability in all parts of the electromagnetic multi-frequency polarization observations, and yield spectrum, a high degree of linear polarization, and the orientation of the magnetic field in the radiating compact radio and pointlike optical structure. region relative to the observed polarization electric vec- In 1990, Kühr and Schmidt [1] compiled a complete tor. When a sufficiently high degree of polarization is sample of radio-bright BL Lac objects in the Northern sky, detected at even one frequency, this indicates that the radiating plasma must be optically thin, since optically consisting of 34 sources satisfying the following selection Ӎ criteria: (1) total flux at 5 GHz I ≥ 1 Jy at the epoch at thick radiation cannot be more than 10–15% polar- which the sample was compiled, (2) two-point spectral ized [2]. If the plasma is optically thin, the magnetic α ≥ ν+α and electric fields are mutually perpendicular, whereas index between 11 and 6 cm 11Ð6 Ð0.5 (S ~ ), and ≤ the two fields are parallel if the plasma is optically (3) equivalent width of emission lines W 5 Å. The thick. The core of the source is usually considered to be m sample included sources brighter than 20 in the optical optically thick, while the jet becomes increasingly with declinations δ > 20¡ located outside the Galactic transparent—optically thin—with distance from the plane (|b| > 10¡). We are currently analyzing multi-fre- core. This is demonstrated by the presence of highly quency, polarization-sensitive Very Long Baseline Inter- polarized jet components [3] (isolated regions of ferometry (VLBI) data for all sources in the sample. brightening of the total intensity), whose degree of VLBI polarization observations can yield radio polarization can reach 60–70%—the limit for optically images of compact sources such as BL Lac objects with thin synchrotron radiation [2]. The high degrees of angular resolution of the order of a millisecond of arc. polarization that are sometimes observed in the VLBI Such observations contain information about all four jets of BL Lac objects testify that the magnetic fields Stokes parameters, i.e., both the total intensity and the are well ordered in these regions. As a rule, the jet emis- linear and circular polarization of the radiation. The sion has a steep spectrum, confirming that we are deal- emission of BL Lac objects is characteristically linearly ing here with an optically thin plasma. polarized, since it is synchrotron radiation. Thus, polar- Faraday rotation of the plane of linear polarization ization-sensitive observations carry information not occurs when polarized radiation passes through a only about the energetics and kinematics of the sources region containing thermal plasma and a magnetic field observed, but also about the degree of order and orien- with a non-zero component along the line of sight. The tation of the magnetic fields in their jets, which are very rotation measure can only be derived from multi-fre- important when analyzing the physical conditions in quency polarization observations. However, wide-band these objects. polarimetry (1.4, 5, 8, 15, 22 GHz, for example) is not There are a number of factors that can complicate suited to this purpose. As is discussed in [5], the best suited deduction of the direction of the magnetic field in a par- observations are those made with the NRAO VLA at ticular region in a source. Three properties of the region 1.3–1.7 GHz. When planning observations designed to 1063-7729/01/4509-0667 $21.00 © 2001 MAIK “Nauka/Interperiodica” 668 PUSHKAREV 15 10 5 0 Ð5 Ð10 Ð15 15 10 5 0Ð5 Ð10 Ð15 15 10 5 0Ð5 Ð10 Ð15 Fig. 1. Total-intensity maps of 0119 + 115 at 1.41 GHz (left) and 1.63 GHz (right) with linear polarization vectors superposed. The peak flux densities are 892 and 891 mJy, respectively, and the lowest contours are 0.35% left and 0.25% right of the peaks, respec- tively. The horizontal and vertical scales are in arcseconds. 15 10 5 0 Ð5 Ð10 10 5 0Ð5 Ð10 10 5 0Ð5 Ð10 Fig. 2. Total-intensity maps of 0235 + 164 at 1.41 GHz (left) and 1.66 GHz (right) with linear polarization vectors superposed. The peak flux densities are 833 and 1032 mJy, respectively, and the lowest contours are 0.125% of the peaks. The horizontal and vertical scales are in arcseconds. derive the rotation measures of compact extragalactic ing radiation from electrons with qualitatively different sources, several requirements on the frequency interval energies—i.e., from physical different regions in the to be used should be taken into consideration. First, source—at the different wavelengths. there should be a large enough spacing between the cor- responding wavelengths (more precisely, the squares of the wavelengths) to detect the rotation of the plane of 2. OBSERVATIONS linear polarization. Second, these wavelengths should We obtained VLA observations (C or D configura- be long enough not to give rise to large ambiguities tion) during four sessions: January 19, 1996 (Session 1), when calculating the rotation measures. Finally, the January 29, 1996 (Session 2), February 7, 1996 (Ses- wavelengths should not be very different from each sion 3), and June 26, 1998 (Session 4). The 1998 obser- other, since otherwise, there is the possibility of detect- vations were conducted in order to refine our initial ASTRONOMY REPORTS Vol. 45 No. 9 2001 MEASUREMENT OF THE INTEGRATED FARADAY ROTATIONS 669 100 50 0 Ð50 Ð100 Ð100 Ð50 0 50 100 Ð100 Ð50 0 50 100 Fig. 3. Coverage of the u–v plane for the VLA observations of 0119 + 115 (left, three scans) and 0235 + 164 (right, six scans). The horizontal and vertical scales are thousands of wavelengths. χ χ 0¡ 0¡ 0003 Ð 066 0048 Ð 097 Ð20 Ð10 Ð40 Ð60 Ð20 Ð80 350 400 450 500 350 400 450 500 0119 + 115 80¡ 0138 Ð 097 0¡ 75 Ð100 Ð200 70 300 350 400 450 500 350 400 450 500 0¡ 0235 + 164 0256 + 075 95¡ Ð20 Ð40 90 Ð60 Ð80 85 300 350 400 450 500 350 400 450 500 λ2, cm2 Fig. 4. Dependence of the polarization position angle χ (in degrees) on the square of the wavelength (in cm2) for the sample sources. ASTRONOMY REPORTS Vol. 45 No. 9 2001 670 PUSHKAREV determination of the rotation measure for 0119 + 115, were determined using sources with observations span- since the 1996 data for this source were of compara- ning a large interval (>90¡) in parallactic angle— tively low quality, leading to a large uncertainty in its 2150 + 173 (Session 1), 0917 + 624 (Session 2), rotation measure. All the observations were carried out 1147 + 245 (Session 3), and 0235 + 495 (Session 4). ≤ at six frequencies (1664.9, 1635.1, 1515.9, 1467.7, The unpolarized sources 2352 + 495 (Session 1, mL 1%), ≤ 1417.5, and 1365.1 MHz), with bandwidths of 50 MHz 1323 + 321 (Sessions 2 and 3, mL 0.6%), and 0134 + 329 ≤ for the observations at 1664.9 and 1635.1 MHz and (Session 4, mL 0.8%) were used to check the results 25 MHz for the remaining frequencies. The data were of the solutions for the instrumental polarizations. calibrated in the AIPS (Astronomical Image Processing System, NRAO) package. The primary flux calibrators were 3C 286 (Sessions 2 and 3) and 3C 48 (Sessions 1 3. RESULTS and 4). The polarization position angles were calibrated Structure on VLA (kiloparsec) scales was detected using 3C 286 (Sessions 2 and 3) and 3C 138 (Sessions 1 in the two sources 0119 + 115 and 0235 + 164. Figures 1 and 4), for which we assumed the polarization position and 2 show images of the distributions of total intensity angles χ = + 33¡ and χ = Ð9¡, respectively, at all fre- for these sources, with linear-polarization vectors quencies, since these sources do not have significant superposed. The remaining sources were unresolved at rotation measures [8]. The instrumental polarizations the sensitivity level reached in our observations, and so χ χ 0¡ 0745 + 245 0820 + 225 70¡ Ð50 60 50 Ð100 0283 + 033 1147 Ð 245 20¡ 0¡ 10 Ð5 0 1334 Ð 127 1732 + 389 0¡ 0¡ Ð20 30 Ð40 20 Ð60 300 350 400 450 500 300350 400 450 500 λ2, cm2 Fig.
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