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Astron. Astrophys. 322, 846-856 (1997)

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5. Results

Results of profile alignments are presented in the following sections for all sources detected at mm-wavelengths.

5.1. PSR B0329+54

Some of our data for this pulsar have already been presented by Kramer et al. (1996) to demonstrate its moding activity at mm-wavelengths. In Fig. 1a we show results of those observations which are summarised in Table 1. The location of the chosen fiducial point is marked by the horizontal errorbar beneath each profile. The length of this error bar indicates the effective resolution, which in general also indicates the accuracy of the fiducial point, is the larger of either that of the original data (sampling time) or that imposed by interstellar dispersion or that obtained after smoothing the data (e.g. at mm-wavelengths). A dotted vertical line indicates the pulse longitude identified with pulse phase zero. The data are consistent with simultaneous arrival times at each frequency. A conservative upper limit derived for the total time delay (not taking the uncertainties in the location of the profile midpoint into account) corresponds to the largest TOA residual difference between two single measurements with respect to each other, i.e. 1590 µs. Upper limits determined for the following other sources are derived similarly.

[FIGURE] Fig. 1a-d. Time aligned profiles of a B0329+54, b B0355+54, c B0540+23 and d B1133+16. The chosen fiducial point is marked by the errorbar beneath each profile, indicating the corresponding resolution (see text). Phase zero is shown as the vertical dotted line and is identified with the TOA of the lowest frequency profile.

5.2. PSR B0355+54

The upper two profiles shown in Fig. 1a-d b were obtained in December 1994. During one of these observations at 32 GHz, we detected this pulsar in its abnormal mode which is fairly rare at high frequencies (cf. Morris et al. 1980 and Xilouris et al. 1995). The occurrence of mode changing apparently affects the location of leading component within the profile. While observations at 32 GHz (normal mode), at 8.5 GHz and at 10.55 GHz were made in July 1994 (see Table 1), the presented 4.75-GHz profile (whose resolution is limited by dispersion smearing) was obtained in December 1994. A pulse profile at 2.25 GHz was not available due to a contamination of the data by RF-interference. All profiles seem to arrive simultaneously. A corresponding upper limit for the total time delay is 300µs.

5.3. PSR B0540+23

In contrast to our normal analysis, we have chosen the profile peak as the fiducial point since the trailing part is visible at low frequencies, and undetectable at 32 GHz. This pulsar is already fairly weak at 4.75 GHz and almost not detectable at 32 GHz. The best profile available at mm-wavelengths was obtained in February 1994. In order to align it with lower frequency profiles of reasonable S/N, we added observations of April 1994 (1.410 GHz and 1.615 GHz profiles). Although we applied the timing model by only adjusting the phase offset, all profiles align perfectly well (Fig. 1c). Despite the low S/N of most of the data, an upper limit for the total time delay of 530 µs is derived.

5.4. PSR B1133+16

Similar to the lower frequency results published by PW92, this pulsar is a "text-book" example for perfect alignment over a large frequency range (Fig. 1d). In order to demonstrate the very little change in profile width and shape already above 1.41 GHz, we added a corresponding 21cm-profile measured in July 1995. Although this latter measurement was done one year after the observations at highest frequency profiles, again an adjustment of only the phase was sufficient (cf. CD85). Obviously, the data are consistent with simultaneous arrivals at all frequencies. As an upper limit for a time delay we find 1640 µs.

5.5. PSR B1706-16

For this pulsar we present pulse profiles at 2.25 GHz, 4.75 GHz, 8.5 GHz and 32 GHz, respectively (Fig. 2a). Data at 8.5 GHz were contaminated by RF-interference and therefore smoothed as the data at 32 GHz. We determine an upper limit for a time delay between different frequencies of 1260 µs.

[FIGURE] Fig. 2a-d. Time aligned profiles of a B1706-16, b B1929+10, c B2020+28 and d B2021+51. See Fig. 1 for details.

5.6. PSR B1929+10

Since the 2.25 GHz profile of this pulsar measured in July 1994 was contaminated with radio interference, we represent a 1.71 GHz pulse shape from August 1994 instead (Fig. 2b). An extended emission from the main pulse complicates the exact determination of the profile midpoint. The chosen fiducial point of the 10.55-GHz profiles arrives slightly later than expected. However, a careful inspection of the pulse shape suggests that this effect is spurious and due to the complicated waveform which exhibits a slowly declining trailing component. In fact, the first resolvable component brightens with frequency and remains (as far as it can be inferred from the lower S/N) as the only one detected at 32 GHz. At the same time, the strong middle component of the 1.71 GHz profile fades and is completely covered by the first component at high frequencies. However, this development in the pulse shape seems mainly to occur at frequencies below 4.75 GHz. Above this frequency almost no changes, i.e. neither in shape nor pulse width, are observed and all profiles seem to arrive simultaneously. In this case, the upper limit for the time delay is governed by the uncertainty of the location of the fiducial point. We estimate this uncertainty to be about 590 µs.

5.7. PSR B2020+28

This pulsar was reported to exhibit a peculiar timing behaviour at high frequencies (Kuzmin et al. 1986). The data presented by Kuzmin et al. obtained at 102.5 MHz, 4.6 GHz and 10.7 GHz suggested that the pulses observed at 10.7 GHz arrived 5 ms (i.e. the half width of the profile!) earlier than it would be expected from a normal dispersion delay. In order to compensate for this delay, a dispersion measure exceeding the usual low frequency value had to be applied to the data to align the profiles properly. As an alternative explanation for this excess in the [FORMULA] -value, it was argued that magnetic quadrupoles existing in the emission region near the star alter the field structure in a way that high frequency emission is beamed towards earlier times. PW92 reported a similar behaviour for B0525+21 and B1237+25. Again, pulses observed at high frequencies, i.e. 4.8 GHz, seemed to arrive earlier.

In Fig. 2c we now present pulse profiles of B2020+28 obtained in a frequency range between 1.41 GHz (we show a high resolution profile measured in April 1995) and 32 GHz. Adjusting only the phase offset, the data align perfectly, although the adopted [FORMULA] -value was derived at low frequencies. In particular, we do not detect any delay between 4.75 GHz and 10.55 GHz as Kuzmin et al. (1986). Our data are actually consistent with a simultaneous arrival at all frequencies. An upper limit of 1035 µs for the time delay was determined. Possible reasons for the result of Kuzmin et al. (1986) could be the use of insufficiently accurate ephemeris or an error in the clock system during observations. A similar analysis of the other peculiar cases presented by PW92 appears to be difficult, since B0525+21 and B1237+25 are very weak radio sources at high frequencies (Seiradakis et al. 1995).

5.8. PSR B2021+51

A simple inspection of the relative locations of the fiducial markers in Fig. 2d suggests that high frequency profiles arrive slightly earlier than low frequency ones. However, although the profiles look simple in their form, they show a significant profile development which affects the choice of the fiducial points as the profile midpoints. In fact, the profile develops from a double-peaked profile at 0.43 GHz (e.g. Gould 1994) to a simple Gaussian form above 4.75 GHz. Fig. 2d demonstrates that the single component remaining at high frequencies is certainly dominated by the first one of the lower frequency components. The arrival times of the data are then consistent with an upper limit of 1650 µs for a total time delay.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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