Introduction

If it is desired to transmit a signal across the Galaxy so that another, unknown, recipient may detect it there are two basic types of transmission patterns, an omnidirectional signal that may be detected anywhere, or a beamed signal that can only be detected by those in the beam. Similarly, in the time domain, a signal may either be transmitted continuously or, with the same energy expenditure, a more powerful signal may be transmitted for a shorter period of time. Providing that the recipient knows where and when a signal is coming from, a beamed brief, and hence stronger, signal would be easier to detect. However, for such a transmission scheme to be feasible, the problem is for a transmitter and a recipient, one or both unknown to the other, to find a strategy that will enable the transmitter and receiver to transmit and observe at the right time and location.

A strategy to achieve transmitter/receiver synchronization that has been considered by a number of authors is to utilize natural astronomical events, see, for example, Pace & Walker (1975), Tang (1976), McLaughlin (1977, 1986), Makovetskii (1978), Pace (1979), Gruber & Pfleiderer (1982), Tang (1981), Siebrand (1982), and Lemarchand (1994). In the simplest scheme omnidirectional signals would be transmitted at the occurrence of some particular event such as a nova outburst, maximum flux of a long period variable, specific binary phase or supernova occurrence. A signal would then be detected at the Earth delayed by a time corresponding to the difference between the event/Earth distance and the event/transmitter + transmitter/Earth distances. The time delay is thus given by:

$$\Delta T = (D - R_s + (R_s^2 + D^2 - 2R_sD cos \theta )^{1/2})/c$$ (1)

where R_s is the distance to the synchronizing astrophysical event, D is the distance to the transmitter, and $\theta$ is the angular separation as viewed from the Earth.

A further refinement is to transmit in a direction exactly or approximately away from the event which both reduces the time difference and gives a preferred direction in space. The use of one such locally dramatic event in particular, SN 1987A, is considered by Lemarchand (1994). The increased probability of detecting a signal if synchronizers are used is considered by McLaughlin (1977).

To date there has been no definite detection of a signal in the Search for Extra-Terrestrial Intelligence (SETI). However, there have been a few detections of non-repeating signals that have generated some interest such as the ``wow" signal found at Ohio State Radio Observatory (Dixon 1985, Gray 1994) and the strongest events from the META survey which appear to preferentially lie in the Galactic plane (Horowitz & Sagan 1993). While these may simply be noise or arise from natural astrophysical phenomena they could conceivably be genuine extra-terrestrial artificial signals that are transient either because transmission is intermittent or caused by interstellar scintillation (Cordes, Lazio, & Sagan 1997).

In this paper the use of one particular type of natural synchronizing signal is considered - the phenomenon of gamma-ray bursts (GRBs). These appear to posses a number of important advantages over other possible astrophysical events and their use in SETI is advocated for, in particular, targeted observations of relatively nearby stars. A brief summary of the phenomenology of gamma-ray bursts and their observations is given in Section 2, followed in Section 3 by a brief comparison of the use of GRBs to some other possible synchronizers, and in Section 4 two possible strategies for utilizing gamma-ray bursts in extraterrestrial communication are discussed.