Detectable Sources for ARISE

We assume that a ground telescope with a sensitivity comparable to that of a VLBA antenna (such as an element of the Australia Telescope) will be used in the Southern Hemisphere. Then, Table 2 shows that the total number of detectable sources is near 210 if a typical observed brightness temperature of K is assumed, or about 90 for an observed brightness temperature near K. A correction for incompleteness would raise the number of sources to more than 300 if K, but would have little effect if K. Table 3 summarizes the number of detectable sources as a function of data rate, given that all other parameters remain fixed, for the two different values of the observed brightness temperature. (No incompleteness correction has been made.) This table indicates that the number of detectable sources is reduced below the desirable threshold of 100 for a data rate less than 4 Gbit sec. In addition, the reduction in detectable sources is dramatic if the observed brightness temperatures are significantly lower than K, so the data-rate requirement depends critically on the assumed brightness temperature distribution. It might be possible to gain back a factor of 1.5-2 by using global fringe-fitting with the VLBA, but this improvement is not assumed because of the lack of experience at 86 GHz. Other options for recovery of a larger number of sources are discussed below.

Table 3. Detectable 86-GHz Sources for ARISE-VLBA Baseline
Data Rate	Detection Threshold	Number
(Gbit sec)	(mJy)	K	K
8	120 mJy	211	88
4	170 mJy	147	50
2	240 mJy	88	29
1	340 mJy	47	20

• Extended Integration Time
• More Sensitive Ground Telescopes
• Impact of a Smaller ARISE Antenna