Apart from pulse-to-pulse modulations, pulsars are generally known to be stable radio sources (e.g. Stinebring & Condon 1990). Variations in the measured flux-density on time-scales of minutes or hours to days or months are mostly attributed to extrinsic reasons, viz interstellar scintillation. Interstellar scintillation (ISS) is caused by irregularities in the electron density, , of the interstellar medium (ISM) which span scales from - m (Armstrong et al. 1995). The time scale and depth of the modulations are strong functions of distance and frequency. At typical pulsar observing frequencies one distinguishes between two scintillation regimes (e.g. Rickett 1990). Small scale structures in the ISM ( - m) produce short term flux-density variations on time scales of minutes to hours, known as Diffractive Interstellar Scintillation (DISS, Cordes et al. 1985). Large scale structures ( - m) cause longer term flux-density variations on the order of days to months (Sieber 1982), known as Refractive Interstellar Scintillation (RISS, Rickett et al. 1984, Kaspi & Stinebring 1992). While most of the studies have been carried out at low frequencies where pulsars are strong emitters, very few observations of ISS have been made at higher frequencies. The detection of pulsars at mm-wavelengths (Wielebinski et al. 1993) and the subsequent studies (Kramer et al. 1996; Xilouris et al. 1996; hereafter K96 and X96) motivated the present work, which aims at understanding fluctuations of pulsar signals at mm-wavelengths - an unexplored spectral region for the behaviour of the ISM.
As a wavefront from the emitted pulse passes through a region of irregularities, it suffers random perturbations in the relative phases, due to variations in the local refractive index. The scattered waves interfere at the position of a distant observer resulting in a diffractive modulation of the measured flux-density as the telescope moves through the interference pattern. These short-term DISS variations appear to be further modulated by long-term RISS variations, caused by large scale focusing or defocusing along the average path. The depth of the RISS modulations is typically lower than that of the DISS modulations. The modulation index, m, is defined as the noise corrected normalized root mean square variation in pulse flux-densities,
where and denote the mean of the measured on and off the pulse emission, respectively; and the corresponding variances around the mean. Whereas for DISS m is about unity, the slower RISS variations can be seen with m substantially less than unity, when the signals are averaged to suppress the DISS.
The above description applies for strong scintillation. As the frequency increases or the path length decreases, the scales of the two regimes approach each other. At a critical frequency () they merge into a single modulation at the Fresnel scale (, for pulsar at distance L); for higher frequencies still, the scintillations are weak and m decreases strongly as . The actual value of the critical frequency increases with the distance of the pulsar and is generally located at around a few GHz for pulsars beyond about 1 kpc (Backer 1975, Pynzar' & Shishov 1980, Malofeev et al. 1996). The observations presented here were performed in a spectral region well above the critical frequency in a regime where weak ISS prevails and therefore very weak fluctuations are expected. In the following, the observations of unexpected pulsar fluctuations are described together with the procedures taken to reduce instrumental effects.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998