Targeted Signaling

It is not possible to know in advance the angle within which the transmitter will decide to broadcast to particular locations downstream from the GRB. The smaller the angle utilized the longer it will take to transmit to all targets. However, for large angles the time delays will be larger and harder to accurately calculate for the receiver and more targets may have to be transmitted to for any particular GRB. A non-exhaustive list of factors which could be used by the transmitter to decide which places to target, in likely order of the distance at which they could be detected by the transmitter, is:

• Detection of a solar type star
• Detection of a planetary system with Jupiter mass planet(s)
• Detection of terrestrial mass planet(s)
• Detection of life through the presence of, for example, an Oxygen atmosphere
• Detection of intelligent life through, for example, radio transmissions

The down-stream angle(s) used by a transmitter will presumably depend on the number of targets that are thought to be potential hosts for receivers for the signal. For a large number of possible targets small angles would perhaps be likely to be used. Conversely, at the small number extreme, the Earth would be the only target transmitted to even if the Earth was exactly upstream from the GRB event.

In order for the receiver to be able to calculate the time delay from a potential transmitter it is necessary to know the distance to that transmitter and the angular distance from the GRB. The current best set of stellar distances comes from HIPPARCOS parallax measurements (Perryman et al. 1997) which gave values accurate to $\sim$1 milli-arcsecond. In the the near future NASA's Space Interferometry Mission (SIM, Unwin et al. 1998) and ESA's GAIA (Gilmore et al. 1998) may yield large numbers of parallaxes with precisions better than $\sim$10 micro-arcseconds.

For an illustration of the resulting time delays, and the errors involved in calculating these, the three classes of objects in the Project Phoenix targeted survey are considered (Henry et al. 1995) and presented in Table 1. Time delays are given for the three maximum distances corresponding to the ``Nearest 100'' (D < 7.2 pc), ``Best & Brightest'' (D < 20 pc), and ``G Dwarf'' (D < 50pc) targets as well as for distances of 100 pc and 1000 pc. Errors in calculating time delays due to uncertainties in distance measurements from HIPPARCOS and a future GAIA/SIM class mission are listed. Time delay errors due to GRB position determinations accurate to 1 and 10 are also given as appropriate to future GRB missions. For the closest class it appears feasible to use even 10 accuracy GRB locations and HIPPARCOS distances for moderately large offset angles. For the largest distances considered in this table of 1000 pc, at angles of only 1$^{\circ}$ even parallax measurements accurate to 10 micro-arcsecond yield uncertainties in the arrival of a signal of almost 2 days. For all three of these Project Phoenix target classes GAIA/SIM class parallax measurements combined with good GRB measurements give errors on time delays that are modest (< 1 day) for offset angles up to 5$^{\circ}$.