Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf

6.2. Two-dimensional analysis: rotation measure and depolarization 117
IMAGE: M87.RMRAL_BLANK.FITS
DECLINATION (J2000)
12 23 45
12 23 30
12 23 15
12 23 00
;2000 0 2000 4000 6000
RAD/M/M
1 kpc
12 22 45
12 30 52
12 30 51
12 30 50 12 30 49 12 30 48
RIGHT ASCENSION (J2000)
12 30 47
Figure 6.7: Rotation measure of M 87 computed from fits to the position angle –λ 2 rotation at
eight frequencies as described in the text. The angular resolution is 0.4 arcsec FWHM.
The new map, shown in Fig. 6.7, has an average fitting errorσ RMfit = 15 rad m −2 . The RM
distribution has a mean of roughly 650 rad m −2 withσ RM ≈900 rad m −2 .
The main features of the RM image are as follows.
• The RM values are large and almost always positive, requiring a magnetic field pointing
towards us across most of the source. They range from≈−2000 up to≈6000 rad m −2 . This
compares with the much larger range of−1000-8000 rad m −2 found by Owen, Eilek & Keel
(1990). The most extreme positive values found by are Owen, Eilek & Keel (1990) not
consistent with the X-band position angles.
• The highest RM values (≈3500-6000 rad m −2 ) are seen only in a restricted patch in the E
lobe.
• The jet appears as a uniform RM structure characterized by low values (≈100 rad m −2 ), as
already pointed out by Owen, Eilek & Keel (1990).
• The RM distribution is asymmetrical: positive and smoothly varying in the W lobe, patchier
and associate with field reversals in the E lobe.
As in 0755+37, the low RM seen across the jet is surprising. Similar explanations may hold
for the two sources.
117