How Do We Know The Sources Near SMC X-3 Are Pulsars?
We can tell that the 5 outbursting sources discovered near SMC X-3 are pulsar because they have regular X-ray pulsations. The period of the sources, or frequency of pulsation are visible in the X-ray data. For example, the graph below, called a light curve, shows that the X-ray intensity varies over time. But more importantly, it varies in a consistent manner, showing dips every 59 seconds. This light curve actually belongs to the 59 second period pulsar that Drs. Lochner and Whitlock observed.
Also, BeppoSAX had observed this area of the sky fairly recently, allowing confirmation of their identities.