Probably the most important property of a synchronizer is that it should, in some sense, be obvious and so likely to be used by both the transmitter and receiver. Arguably the key property in making a phenomenon obvious is its luminosity. In addition, it is desirable for the phenomenon to occur at a sufficiently rapid rate to facilitate the use of a large number of these events. Further, it should be possible to determine their time of occurrence precisely to make it possible to use a brief artificially transmitted signal.
After GRBs, supernovae are the next most energetic phenomena known in the universe. However, it is considerably more difficult to detect all supernovae down to a specified flux limit than is the case for GRBs. The positions of supernovae are typically derived from optical observations and are thus known to high precision. However, the optical light curves of supernovae are relatively slowly varying and so are significantly less sharp time markers. Supernovae could perhaps be of better use if larger numbers could have their exact times of onset measured via, for example, a neutrino pulse (cf. SN 1987A Bionata et al. 1987, Hirata et al. 1987). Note that it has been suggested that a small subset of GRBs may be caused by the unusual Type Ic supernova class based on a possible association between SN1998bw, which had a redshift of only 0.008, and GRB980425 (e.g. Kippen et al. 1998).
Novae and other variable stars have also been proposed as possible synchronizers. Novae are significantly less energetic than supernovae and, in the case of Galactic novae, it is also typically necessary to accurately know the distance to the nova as well as the transmitter to be able to calculate time delays. Variable stars form a broad diverse ``class'' of objects. Their much lower luminosities and varied nature makes them less obvious as the synchronizers that would be universally used.
Until recently, the relatively poor precision with which gamma-ray burst locations were determined was arguably a problem with their use as synchronizers as this yields a large uncertainty in the time delay (eqn. 1). However, this limitation is disappearing with new generations of instruments augmented by the discovery of, in particular, optical afterglows. As the most energetic of all currently known natural phenomena this alone draws attention to GRBs. They occur at rapid rates, one a day even with BATSE level technology, are easily detectable, have short durations making them ``sharp'' time markers, and also occur at sufficiently large distances that it is not necessary to actually know these distances to calculate time delays. Hence, no other class of proposed phenomenon appears to posses any obvious properties which makes it a better candidate than GRBs for use as a synchronizer.