had been observed in the vicinity of the PSR B1937+21. If PSR B1937+21 was that young, it would not have had time to move far from the site at which it formed. As neutron stars are formed as the result of supernova explosions, evidence of the explosion should be nearby for a young pulsar. If it was that young, it would also be expected to still be hot, in which case the thermal radiation from PSR B1937+21 would be observable at x-ray wavelengths. Venkatraman Radhakrishnan and G. Srinivasan used the lack of observed supernova remnant to argue that PSR B1937+21 had not formed with such a fast period, but instead had been "spun up" by a companion star which essentially gave the pulsar its angular momentum, a mechanism now generally used to explain millisecond pulsars. They also made a theoretical estimate of the necessary spin down rate to be 1 seconds per second. Backer et al. revised their estimate of the upper limit of the spin down rate just a month after the initial discovery, to 1 seconds per second, but the currently measured value is more nearly in line with the theoretical estimate, at 1.05 seconds per second. The age of PSR B1937+21 was also later determined to be 2.29 years, a value which is consistent with the observational evidence. The companion which is supposed to have spun-up PSR B1937+21 is no longer present, making it one of few millisecond pulsars which does not have a stellar mass companion. The generally high occurrence of companions to millisecond pulsars is to be expected, considering a companion is necessary to spin-up millisecond pulsars to their short periods. However, millisecond pulsars do not actively accrete matter from a companion, but instead need to have only done this at some time in the past, and thus the lack of companion for PSR B1937+21 is not seen as a being in disagreement with the spin-up model. Possible mechanisms for creating isolated millisecond pulsars include evaporation of the donor star or tidal disruption of the system. Pulses During one period of rotation for PSR B1937+21, there are two peaks observed, known as the pulse and interpulse. PSR B1937+21 is unusual among pulsars in that it occasionally produces pulses far brighter than an average pulse. Until 1995, the sole other pulsar known to produce giant pulses was the Crab pulsar, and by 2006, there were 11