Mr.
Galaxy's Supernovae
Mr.
Galaxy's Supernovae
On April 12, 1991 (after I had completed my taxes!) I went out to the Orange County Astronomer's Anza Observatory to use the Kuhn 22-inch telescope to get some ideas for an article. I was visually searching through the southern part of the Virgo Cluster of Galaxies as part of an ongoing search for exploding stars (supernovae) in other galaxies. At about 11:30 pm that night I had just moved the telescope to a group of three galaxies that were more or less arranged in a north-south line with a bright, very wide, vertically aligned pair of stars between the northern and middle members.
The northern galaxy in the group, NGC 4527, I will not forget for a long time! It is a medium brightness object and is nearly edge-on, reminiscent of the Andromeda Galaxy (M31), although at magnitude 10.4 and 6.3 X 1.5 arcminutes in diameter it is admittedly a miniature version of that galaxy. The nucleus is pretty small and a little brighter than the rest of the galaxy. I noticed a faint star on the northeast edge of the galaxy and decided to look at our set of Supernova Search Charts by Thompson and Bryan to see whether we had one for NGC 4527. Fortunately, we did and when I compared the chart field to the telescope field I saw that the star was missing on the chart! I double-checked and triple-checked to be sure that I had the correct chart, the correct object in the telescope, the correct orientation of the chart and telescope, etc. Everything seemed to be in order.
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| Magnitudes: Star A = 12.1, Star B = 14.2, Star E = 10.7 |
Around 11:45 pm I made a magnitude and position estimate of the intruder just in case it was actually a supernova. I determined it to be about 50 arcseconds north and 30 arcseconds east of NGC4527 nucleus. Unfortunately, the Supernova Chart for this galaxy did not have photometrically determined magnitudes for any of the field stars. I chose several stars on the 4527 chart to bracket the object. These are labeled on the accompanying figure. When I first saw the suspected supernova, it was about 60% of the brightness of star A compared to star B. In other words, it was almost midway in brightness between my two comparison stars but it was a little closer in brightness to star A. As is true for almost all hand-plotted charts, Thompson bins the magnitudes in half-magnitude steps. Hence, my estimate could not be any more accurate than half a magnitude. Since none of the stars I was estimating from had labeled magnitudes, I could only estimate from the diameter of the stars as they were plotted. This method seemed to indicate that the newcomer was about magnitude 13.5. As it turned out, my magnitude estimate was about half a magnitude less than the other preliminary reports. I guess I need to calibrate my eye better for the next time!
I had both the Thompson chart and a CCD image of the galaxy to consult. Neither one of them showed a star at the position where I had seen mine. By now the adrenaline was really flowing! I had been watching the object for nearly an hour to be sure that it was not an asteroid passing through the field. The only other possibility that I could think of was that it could be an uncharted variable star. I had exhausted the arguments against it not being something unusual that could be proven with my resources; now it was time to make phone calls!
Of course, I did not have any of the correct phone numbers with me so I tried dialing information for the numbers of the various observatories in the area. But all I got were the taped 24 hour general information lines, nothing in the domes where the live human observers were. Who could I call at midnight on a workday that would have the necessary information? I phoned John Sanford my compatriot in the Orange County Astronomers' semi-organized supernova search program. Fortunately, he was awake and he gave me the phone numbers for the 18-inch and 48-inch Schmidt domes at Palomar Mountain Observatory.
I contacted the 18-inch dome and found Gene Shoemaker (of comet and asteroid fame) there. After I convinced him that I was not making a crank call (and asked him what it was like appearing in the National Enquirer!), he took the information on the alleged supernova and said he would find out if there were any reports of it. He called back within a few minutes to offer congratulations saying that Brian Marsden from the Minor Planets Center had contacted Jean Mueller at the 48-inch dome a little earlier to ask for confirmation of a supernova in NGC 4527!
At 3:15 am I estimated that the supernova had brightened by several tenths of a magnitude so that now it was approximately 90% of the brightness of star A compared to star B. It is highly unusual to catch a supernova still on the rise so that I was really enjoying the small change in the newcomer's brightness every time I examined the field. I continued looking at about a dozen other galaxies for the rest of the night and went to bed about 6 am content that our efforts were beginning to pay off.
I called Brian Marsden at the Minor Planets Center after an unrestful morning and found out the supernova was indeed still on the rise and that there had been three other groups besides me that had reported the object. Steve Knight, who lives in Maine, was the first to see it a couple of days before anybody else, but his report was delayed. The Reverend Bob Evans (who does not miss a supernova) saw it next in Australia. Two observers in Italy, Villi and Cortini, saw it before me but did not report it until after I did.
I made several more magnitude estimates in the meantime (up to April 20, 1991) and found the supernova continuing to increase in brightness. I saw the supernova as equal in brightness to star A at 11 pm PDT on April 17. Then on April 19 at 11:45 pm PDT I saw the supernova as brighter than star A by 30% using star E as the new upper comparison star. Later that night on April 20 at 3 am PDT, I estimated that the supernova was again a little brighter, now about 40% brighter than star A.
I have had a couple close calls in finding supernovas where they were discovered and reported before I could or where I had looked at the galaxy a week before the eventual supernova occurred. I also have made many comet "discoveries" that turned out to be uncharted galaxies. These were exciting at the time they occurred, but none of them were as satisfying as the real thing—discovering my first supernova!