## 2. Analysis and resultsPresently we have data for three pulsars, where a clear change from
linear to circular polarization towards high frequencies is observed
(see Fig. 5 in von Hoensbroech et al. 1998b). It is obvious from
Fig. 1 that the three objects have very different rotational periods
between the degrees of linear ()
and the circular () polarization as
an depolarization independent parameter.
The polarization data we used for this analysis were accessed
through the online EPN-database
(
The theoretical functional dependence of Here is the local electron gyro frequency at the PLR, the angle between the propagating wave and the direction of the local magnetic field at the PLR and the streaming velocity of the plasma. Higher order terms can be neglected as long as the wave frequency is different from . Obviously, a -frequency dependence
of
There are a couple of parameters which indirectly enter Eq. (2) as scaling factors. is proportional to the local magnetic field strength . This value again depends on basic pulsar parameters such as the period, its time derivative and, if known, on the inclination angle between the rotation- and the magnetic dipole axis. Furthermore the angle between the propagating wave and the local magnetic field depends on the chosen field line and the assumed emission height. For the background plasma Lorentz factor we made the assumption . Finally we chose the PLR at 20% of . As the values of the PLR and the emission height (2% of are given as fractions of , their absolute values depend on the period. The combination of and PLR was chosen without restriction of generality as various other combinations yield the same result (see Fig. 5).
However, apart from the known intrinsic pulsar parameters ## 2.1. PSR B0144+59This pulsar is the first one in which we found the effect of increasing circular polarization to high frequencies. Its rotational parameters are ms and , yielding to a weak surface magnetic field of `only' T, an average value for W and the characteristic age yrs. Reasonable data in full polarization was available between 610 MHz and 4.85 GHz. Fig. 2 shows the measured values and the theoretical curve for the
change of ## 2.2. PSR B1737-30PSR B1737-30 has a spin period of ms and a period derivative of . The resulting spin down energy loss places it amongst the top 10% of the pulsar sample. The very low characteristic age yrs and the very high surface magnetic field T make this one an extreme object. Note that in terms of the surface magnetic field, this object is at the opposite end of the "normal" pulsar sample compared to PSR B0144+59. The change of ## 2.3. PSR B1913+10With a spin period ms and its temporal derivative this is an average pulsar. This is also reflected by its parameters W, T and yrs. Fig. 4 shows the measured values for ## 2.4. OutridersThe systematic deviation of the low frequency points is certainly a draw back of these observations. However they can be understood through the following argument: von Hoensbroech et al. (1998a) have shown that the polarization properties in general are much less systematic at low radio frequencies compared to higher ones. This indicates that the polarization of pulsars undergo some sort of randomization at low radio frequencies. This can be caused either through intrinsic variations - e.g. non constant PLR at low frequencies - or through additional propagation effects in the highly magnetized medium close to the pulsar, which mainly affect low radio frequencies. © European Southern Observatory (ESO) 1999 Online publication: February 23, 1999 |