SIMBAD references

This article is the second in a series that analyzes about 1.87 million periods of the Crab pulsar, observed by the PCA detector aboard the RXTE X-ray observatory. At these energies, the pulsar displays none of the three phenomena that are often seen in normal radio pulsars - ``pulse nulling'', ``systematic sub-pulse drifting'' and ``mode changing''. The presence or absence of these three behaviors in the Crab pulsar at radio wavelengths, something that has not yet been rigorously established, might be important for a satisfactory understanding of the above three phenomena.