Astron. Astrophys. 322, L17-L20 (1997)

3. Results

The average 87 GHz pulse profile (9 hours integration) for PSR B0355+54 shows a clear detection of the pulse at the phase expected from lower frequency observations. This is illustrated in Fig. 1 which compares the 87 GHz profile (bottom) with a selection of lower frequency observations made with the 100-m Effelsberg radiotelescope of the MPIfR (unpublished data and data presented by Kramer et al. 1997b). The 87 GHz profile has been smoothed by applying a 4 ms running mean to the data, and yields a detection. The profiles presented have been aligned in time, referring to time of arrival at the solar system barycentre calculated for an infinite frequency. A detailed description of this procedure can be found in Kramer et al. (1997a). The occurance of the 87 GHz pulse at phase zero confirms the detection convincingly.

Fig. 1. Observed pulse profiles of PSR B0355+54 at several radio frequencies between 1.4 GHz and 87 GHz. Flux density on an arbitrary scale, and different for each frequency,has been plotted vertically. The time resolution is for frequencies between 1.4 and 14.6 GHz, for 23.05 GHz and for the 43 GHz observations.The 87 GHz profile represents the Pico Veleta measurement smoothed to a time resolution of 4 ms.

We estimate the average flux density to be 0.5 mJy with a uncertainty of mJy. This error estimate is based on the observed noise level together with a contribution to allow for calibration uncertainties (). As a pulse width for PSR B0355+54 we estimate , which is consistent with observations at 43 GHz.If we assume a Gaussian pulse shape this corresponds to a pulse width at of pulse maximum of where the quoted error contains an allowance for the undetected trailing component of the pulse which is observed at lower frequencies.

In the case of PSR B2021+51 no pulse was detected in a total of 10 hours integration. In accordance with previously published work (e.g. Kramer et al. 1996), we estimated a () upper limit of 0.78 mJy for the flux density by assuming the pulse width as observed at 43 GHz (Kramer et al. 1997b).

The resulting spectra of the two pulsars are presented in Fig. 2, including data published by Malofeev et al. (1994), Lorimer et al. (1995), Kramer (1995) and Kramer et al. (1996). For PSR 0355+54 we note that, within the measurement errors, the present result for the flux density at 87 GHz appears to be the same as measured at 43 GHz (Kramer et al. 1997b). It is thus larger than expected from an extrapolation of a fit to the lower frequency points. However the errors are such that all points at frequencies greater than 1.2 GHz are just consistent with a single power law spectrum with a spectral index of and a -probability of 0.04.

Fig. 2. Pulse spectra for PSRs B0355+54 (top) and B2021+51 (bottom). The measurements made at 87 GHz are presented as an open triangle and as an upper limit (at a 5 level), respectively. For references of flux densities at lower frequencies see text.

Unfortunately the upper limit for the flux density of PSR B2021+51 provides no strong constraint on the form of its spectrum above 43 GHz.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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