The results of recent observations suggest that the spectra of pulsars may flatten out at mm-wavelengths (Wielebinski et al. 1993; Kramer et al. 1996; Kramer et al. 1997b). While five out of eight pulsars detected at 32 GHz showed a continuation of the spectrum known from lower frequencies, three pulsars exhibited flux densities which were significantly larger than expected from extrapolations. It is believed that one of these cases is due to a moding behaviour still persisting at high frequencies (i.e. PSR B0329+54). The data for the remaining two sources (i.e. PSRs B1929+10 and B2021+51), however, led to the conclusion that an unexpected turn-up in the spectrum was being observed (Kramer et al. 1996). Such a change in the spectra of pulsars may be associated with other changes in their emission characteristics at millimeter wavelengths. In addition, we note that the polarization measured at 32 GHz is significantly lower than one would expect from observations at lower frequencies (Xilouris et al. 1996). Thus it was argued by Kramer et al. (1996) and Xilouris et al. (1996) that the emission properties at millimeter wavelengths are significantly different from those at longer wavelengths.
A change in the flux density spectrum could be related to the breakdown of coherence for the radiation process. At lower frequencies, only coherent emission can explain the high radiation temperatures of K at 400 MHz and generally strong polarization of the received radiation. It is to be expected that a critical frequency exists, above which a transition from the coherent radio emission to the incoherent emission occurs (cf. Michel 1982) (not necessarily caused by the same radiation process). Such a scenario is supported by the known spectrum of the Crab pulsar (e.g. Smith 1977): a steep radio spectrum obviously determined by a coherent process is followed by surprisingly high infrared and optical flux densities, exceeding the radio flux density by several orders of magnitude, and which is supposed to be created by an incoherent process. The determination of such a critical frequency has obviously great potential to contribute to the solution of the pulsar emission mystery (e.g. Melrose 1992).
Kramer et al. (1997b) have recently successfully detected PSRs B0355+54 and B2021+51 at 43 GHz. While PSR B0355+54 showed a continuation of its spectrum, PSR B2021+51 showed an excess in flux density consistent with a flattening of the spectrum. Since pulsars show much variety in their spectra at lower frequencies (Malofeev et al. 1994), it could be suspected that for other pulsars a similar peculiar behaviour occurs at even higher frequencies. In this letter we describe an attempt to observe PSR B0355+54 and PSR B2021+51 at 87 GHz, trying to further confirm the spectral behaviour of PSR B2021+51 and to investigate whether PSR B0355+54 shows a continuation of its spectrum at very high radio frequencies.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998