Use of GRBs for SETI

It is now considered how GRBs may be used as synchronizers in SETI and two basic transmission strategies are investigated:

(i) Targeted Signaling. In this case the transmitter sends a signal promptly after the detection of a GRB in one or more directions ``close'' (i.e. within an angle $\theta$) to the opposite direction of the GRB at target(s) previously decided to be of interest. The beam width is likely to be made as small as possible.

(ii) All-sky signaling. The transmitter sends a signal promptly after the detection of a GRB in a direction exactly opposite from the GRB with a beam half-width $\theta$.

For a GRB located at a very large distance from both the transmitter and receiver the time delay between the detection of the GRB and the prompt transmitted signal is given by the simplified expression:
$\Delta T = (1 - cos\theta) D/c$

For small angles $\Delta T = D \theta^2/2c$, for $\theta$in radians. Note that D and $\theta$ formally refer to the distance and angular separation when the burst arrives at the transmitter rather than the receiver.