R. Schild, 2 October 2001
Because I have seen statements by MACHO scientists claiming that microlensing was discovered by MACHO searches in 1993, I am recording my recollections of the discovery.
1979 Chang, K. and Refsdal, S. NATURE 282, 561. "Flux Variations
of QSO Q0957+561 A,B and image splitting by stars Near the Light Path."
This paper laid out the discovery that stars could affect the quasar image brightness, and might be considered the discovery of microlensing. Sjur Refsdal has told me that at the time he was very worried that because of the effect, it might be impossible to determine time delays to measure the Hubble Constant with gravitational lensing
1981 Young, P. ApJ 244, 756. "Q0957+561: Effects of Random Stars
on the Gravitational Lens."
This paper first presented brightness curves, which heavily influenced my thinking as I realized that I would expect to see these effects in my monitoring data. It is hard to realize today the difficulty in computing the brightness curves with the status of astronomical computing in 1980. Computing jobs were still submitted on punched paper cards. I rarely referenced the paper by Richard Gott, published at about the same time (1981 ApJ 243, 140. "Are Heavy Halos Made of Low-Mass stars? A Gravitational Lens test") Gott had the same back-of-the-envelope calculations, but I always felt that Young's brightness curves were critically important and I always referenced the Young paper; probably I should have referenced both, but we were always told to be as absolutely concise as possible.
1986 Schild, R. and Cholfin, B. ApJ 300, 209. "CCD Camera Brightness
Monitoring of Q0957+561 A,B."
This paper first determines the Q0957 time delay, but does not mention the microlensing because I thought that to mention disagreement of the brightness curves might cause readers to distrust the time delay. Unfortunately in the simple plot showing the correlation calculation I labeled the axis autocorrelation where I meant cross-correlation. A simple plot of the data in this paper shows that the microlensing is quite evident. Sjur Refsdal visited Cambridge in 1986 or 87 to discuss with Emilio Falco the Q0957 models, and with me the time delay data. I showed him a plot of the data on graph paper and commented, "of course the fit is better if you tilt the graph paper." "But what would you mean if you tilt the graph paper" he replied. "You would tilt the graph paper if there were microlensing" I replied. I could see the light bulb go on in his head.
1988 Grieger, B. Kayser, R. and Refsdal, S. A&A 194, 54.
After returning to Hamburg, Refsdal wrote me a letter asking me what was the amount of the microlensing, and I replied that since 1980 it was 0.025 magnitudes per year. He incorporated this in his paper with the remark, "According to Schild (1987, private communication) image B has brightened relative to A by about 0.025 per year during the last 7 years, and he interprets this change as being due to microlensing. We completely agree ...." This is then the first published discovery of microlensing, in the sense that it is the first report of a microlensing event underway. But the time delay had not yet been confirmed, and my own microlensing report would not be submitted until 199 0, the year after the time delay confirmation by Vanderriest et al.
1989 Vanderriest et al, A&A 215, 1. "The Value of the Time
Delay for the Double Quasar 0957+561 from Optical Photometric Monitoring."
This paper confirmed my time delay value and provided a microlensing plot that combined my CCD data up to 1985 with their own photographic photometry. Unfortunately there was a systematic error in the French photometry for the last year, and they believed the microlensing event had ended. I knew that the 80's event was continuing, and avoided referencing their conclusion about microlensing.
1989 Irwin, M., et al AJ 98, 1989. "Photometric Variations in
the Q2237+0305 system: First Detection of a Microlensing Event."
This paper has often been taken as the first microlensing detection, because the microlensing variations can be studied without knowing the time delays of the four images. This occurs because it has been presumed that the time delays are very short, about a day. However this assertion has never been proved. But in an era when the Q0957 time delay would be controversial for 6 years, astronomers eager for microlensing data turned to Q2237.
1991 Schild, R. & Smith, R. C. AJ 101, 813. "Microlensing
in the Q0957+561 Gravitational Mirage."
As soon as the Q0957 time delay was confirmed, our paper gave the microlensing evidence from 10 years of brightness monitoring. It showed that the microlensing event mentioned in 1988 by Grieger et al had continued, and that the rate of 0.025 mag/year quoted there was correct. It was also discovered that a rapid microlensing had also been found, but it was of a very low amplitude and would be difficult to confirm; however it was understood to be important because it might be the detection of the missing mass. We read: "... and other fine structure is also apparent. The existence of fine structure depends critically upon the precise value of the time delay, and so fine structure cannot be considered established until the 404-day time delay is confirmed." I organized a daily brightness monitoring program at Mt Hopkins to try to get the time delay accurate to a day to allow study of the rapid microlensing.
1991-96 The dark years.
The 1991 PhD thesis of Joseph Lehar was soon followed up with a NATURE paper and then a 1992 ApJ paper, all seeming to show that the time delay was closer to 1.5 years. A re-analysis of the Vanderriest data by Press et al seemed to show the same 1.5 year time delay from the optical data. This was so eagerly pushed that the original determination and confirmation were easily forgotten and in such a controversial environment nothing about the Q0957 would be taken seriously. Thus the microlensing interest shifted to Q2237 where time delay seemed to be of no concern. During this period the MACHO/OGLE/DUO/EROS programs began studying microlensing of distant stars by foreground stars, with attendant press releases claiming the detection of the missing baryonic mass in the halo of our galaxy.
With the publication of new optical results by Pelt et al, Kundic et al, Colley and Schild, and Oscoz et al came the realization that the original confirmed Schild and Cholfin time delay was correct, and that the microlensing detection was therefore correct also. A paper published immediately in Astrophysical Journal in 1996 then showed that rapid microlensing that could only be understood as the detection of the baryonic dark matter as a population of free-roaming planets, and numbering one million for each star, was quickly published. It was described in the October 1996 issue of ASTRONOMY Magazine in an article by Peter Catalano, which correctly described the discovery as controversial. Soon the basic observation of rapid microlensing was confirmed in other quasar lens systems.
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