ROSAT observations of gamma-ray pulsars: Vela, PSR 1706-44 and PSR 1951+31

The number of rotation-powered pulsars detected in both the X- and gamma-ray wavebands has increased to seven. While the gamma-ray emission of all these pulsars is found to be strongly pulsed at the rotational period of the radio pulsar no pulsed X-ray emission was detectable in the ROSAT data available from PSR 1706-44. For the X-ray pulses of PSR 1951+32 only a similar to 2 sigma signal was claimed. In this paper we show that the absence of pulsed X-ray emission from these two pulsars can be understood in terms of a Vela-like X-ray emission characteristics.