Sgr A 0 predetection 1 Sgr A 30

Sgr A* @ ≤ 0 (predetection) 1 Sgr. A* @ 30, Green Bank

Sgr A* @ ≤ 0 (predetection) Gotta have radio or x-ray telescopes… ≈30 magnitudes of optical extinction 2 Sgr. A* @ 30, Green Bank

Sgr A* was a discovery waiting for the right interferometer 1070 s I’ferometers didn’t have good enuf spatial resolution (≤ 1”) or uv coverage - separate Sgr A* from its complex, high-flux, knotty background - to determine that its brightness temperature was nonthermal (104 K) The “Prescients”: Clark & Hogg 1966, Ekers and Lynden-Bell 1970, Downes & Martin 1971 3 Sgr. A* @ 30, Green Bank

Sgr A* was a discovery waiting for the right interferometer Goss, Brown & Lo (2003) summarize the history… I add very little… 4 Sgr. A* @ 30, Green Bank

Green Bank- Huntersville Baseline • Look south (d ≈ -30˚) - 5 -6 hours of coverage - ground radiation issues • Sufficient resolution - 35 km baseline - 300, 000 l @ 2. 7 GHz - 900, 000 l @ 8. 1 GHz 5 Sgr. A* @ 30, Green Bank

Green Bank- Huntersville Baseline • Sufficiently high frequency • N-S baseline projection & uv coverage - Lower MW opacity - rising spectrum of Sgr A with freq. • Sensitivity - 45’ and 85’ dishes - 30 MHz bandwidth - noise 0. 2 Jy in 30 s 6 Sgr. A* @ 30, Green Bank

The right program …for dubious reasons • Initial motivation: Tb > 104 K 7 Sgr. A* @ 30, Green Bank

The right program …for dubious reasons • Initial motivation: Tb > 104 K V E R E N 8 N O C D E I F M R Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: 2 < t < 3 y 12/1970: Balick proposes a 11 cm long-baseline survey of “standard” northern HII regions to pursue the Miley results. 1/1972: Balick amends proposal. Adds • Bob Brown • Bright southern HII regions (including Sgr A West) • 3. 7 cm observations 9 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: 0. 5 < t < 2 y 6/1972: Downes & Goss submit proposal to map Sgr A. NRAO does not catch the source conflict Completion of the system is delayed. 1973: Goss and Downes unable to travel to U. S. for their scheduled observations. They could (would? should? ) have discovered Sgr A* Balick leaves NRAO for Lick Obs. Downes and Goss both move to new jobs and urgent responsibilities. Neither group is aware of the proposals of the other. 10 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: t = 0 2/13 & 15/1974: B&B observe Sgr A West • Clear, dry winter weather • Stable phases - Crucial since the nominal phase center was way off! 11 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: t = 0 2/13 & 15/1974: B&B observe Sgr A West Strong fringes appear on chart recorder. Simple structure 11 cm Phase ≈ 180˚ : weather or structure? 12 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: t ≈ 0. 1 y Source conflict recognized by Dave Hogg after B&B discover Sgr A*. Hogg asks parties to find a resolution. Nothing happens right away. 2/25/1974: Balick writes to Hogg to request 3 days to monitor Sgr A source for variability. He adds: The observations of Sgr A are, of course, subject to the resolution of the proposal conflict between ourselves and Downes and Goss. …I am yet to hear from [them]. I hope they write soon, for Bob and I feel that our results on Sgr A merit rapid publication. 13 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: t ≈ 0. 1 y Downes & Goss remain preoccupied. Goss later wrote: With [the pressures of our new jobs], the urgency to complete the Downes-Goss proposal … decreased. … Dave Hogg became aware of the proposal conflict in early 1974 and wrote a letter on 15 February 1974 (note the precise discovery date) proposing several ways to resolve the conflict. However, Goss and Downes seemed to have lost interest at this point. Goss, Brown & Lo, 2003, Astron. Nachr. S 1, 1. 14 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: t ≈ 0. 1 y 2/25/1974 ctd: Balick notes that he had sent the phase center position for the Sgr A point source to Fred Lo. Lo had observed Sgr A at 5 GHz using VLBI (G. B. to N. R. L. Maryland Point) but had been unable to “unwind” the phases using his presumed source position. He quickly succeeded. Balick to Hogg: …on the basis of our position for the fine structure, Fred Lo was able to reanalyze his VBI observations and find fringe amplitudes of 0. 3. f. u. on a baseline exceeding 107 l. 15 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: t ≈ 0. 25 y 3, 4, 5/1974: Balick & Brown got a green light from NRAO to publish. Data analysis and publication take center stage. (Bob Brown will discuss this in the next talk). 6/3/1974: B&B submit paper to Ap. J Letters. No revisions are requested by the Ap. J editor. 16 Sgr. A* @ 30, Green Bank

Timeline to Sgr A*: t ≈ 0. 8 y Sgr A* @ 0: 17 Sgr. A* @ 30, Green Bank

Special thanks! Thanks to Billie Rodgriguez and Dave Hogg for digging out the old correspondence. Thanks to Dave Hogg (and Bill Howard) for never forgetting but always forgiving. Special thanks to Miller Goss for this meeting! Extra Special Thanks to Westerbork and MPI for keeping Downes & Goss out of Green Bank in 1973! 18 Sgr. A* @ 30, Green Bank