Mr.
Galaxy's Supernovae
Mr.
Galaxy's Supernovae
The night of May 15/16, 1996 is one date that will always provide a source of astonishment for me. It is not everyday that somebody finds one exploding star in an evening, let alone two. I think only one professional has ever done it before: Rob McNaught at Siding Springs Observatory in Australia using film in a wide-angle Schmidt camera. According to Brian Marsden at the Central Bureau for Astronomical Telegrams (CBAT) in Cambridge, Massachusetts, I am the first amateur to accomplish that feat. I think I am also the first person ever (professional or amateur) to find two supernovae (SNe) in a night using a CCD camera. Most other surveys use film for their large field coverage, while the incomparable Rev. Bob Evans in Australia still hunts SNe visually with his own 16-in telescope. He also uses a 40-in telescope owned by the Australian National University at the Siding Springs location near the small town of Coonabarabran. Even though I am the leading amateur SN hunter in the northern hemisphere, Bob leaves me a distant second with about 30 SNe (at last count) to his credit. In addition, he is now examining and finding SNe and comets on European Southern Observatory (ESO) Schmidt photos so that he is not ready to relinquish his lead for a long, long time!
My supernova hunting exploits began in January of 1989 after noticing, but not reporting, a stellar knot in the nice galaxy, M66, the day before it was found to be SN1989B by Evans. As an aside, standard naming practice for identifying SNe includes the letters 'SN' followed by the year it was found. Capitalized letters of the alphabet after the year indicate the sequential order ('A' to 'Z') in which a SN has been found. It is now common that over 26 SNe are found annually so that 'aa' and 'ab' are the 27th and 28th SNe of the year and so on. Following the progress of the SN in M66 for several months, I plotted the resulting light-curve, which compared favorably with that of the professionals. My first two SN finds (SN1991T in NGC 4527 and SN1994I in M51) were shared discoveries and were both found in the month of April. My third SN (SN1995J in NGC 4512) was a solo discovery and was also found in April. I began to think that the only month I could find a SN was in April. When this year's April went by, I thought that was the end of my SN discovery prospects for the year. Fortunately, I was proved wrong and in an unexpectedly spectacular way!
I wanted to squeeze in a night or two of observing during a favorable phase of the moon in between job hunting, honey-do's, helping organize two upcoming amateur astronomy conferences (RTMC and IAPPP), and a trip to New Mexico. Wednesday and Thursday nights, May 15/16, was the only time-slot available to go to the OCA's Anza Observatory. The first night, I had just finished initializing the coordinates on the Kuhn 22-in telescope's computer using the nice galaxy, NGC 2903 in Leo. This telescope has been used for all of my SN discoveries, while our Patterson Electronics CCD camera was responsible for 3 out of 5 of my finds. I decided to image NGC 2935 in Hydra after looking at a rather poor rendition of it in John Vickers' mostly excellent "Deep Space CCD Atlas: North". Curious about the stars near the nucleus of the galaxy, I compared my 60 second image to the one in the book. You can imagine my surprise and delight when I saw an extra star on the edge of the galaxy about 70 arcseconds almost due south of the nucleus. I thought the new star was approximately magnitude 16 since it could not be detected visually in the 22-in. However, the galaxy was sinking into the pretty bright southwestern skies of our site.
I started checking my other books, like Vickers and Alex Wassilief's nice collaboration "Deep Space CCD Atlas: South" and Wassilief's solo atlas "CCD Images of Southern Galaxies", to see if there were other pictures for comparison. Fortunately, I found several good images of NGC 2935, which turns out to be a very nice barred spiral (though it is not apparent in my images). After taking another image of the galaxy, I convinced myself that it was a good SN prospect. Since the night was still pretty young, I called Jean Mueller who works at the 48-in Schmidt telescope on Palomar Mountain to ask her if she could check a Palomar Sky Survey photograph for an independent confirmation. After a little hesitation, because the star field was complex, Jean finally agreed that there was something worthwhile reporting. I then called John Sanford to see if he could report a probable SN in NGC 2935 to CBAT since we do not have Internet access at the Anza Observatory. This galaxy is a southern object and was several hours past meridian when I chose to look at it early in the evening. It had set further in the southwest and was almost inaccessible when I made my second observation to be sure the suspect was not an asteroid or something else.
The discovery or potential discovery of a SN is disruptive to any other observing for the rest of the night. You want to watch your 'baby' as much as possible and the adrenaline flow makes it difficult to concentrate. In this sense it was fortunate that NGC 2935 set relatively early so that I could continue my SN hunting without feeling like I was ignoring my newest find. At this stage it was not known whether the SN candidate was still increasing in brightness or not. No perceptible change in magnitude was seen on my second image. Meanwhile, I decided to continue searching for SNe and imaged a couple dozen more galaxies before coming to the rather nondescript elliptical NGC 5557 in the constellation of Bootes at around 3:00 a.m. I took two integrations of this galaxy, one the standard 60 second shot and another one of 15 seconds duration. There is a 14th magnitude star about 15" SSE of the galaxy's nucleus and I wanted to get a better separation between the galaxy and it. Without much more than a cursory examination of it, I proceeded to image a few more galaxies and then made some short exposures of globular clusters. Dawn light was quickly approaching and clouds were starting to roll in from the north. I went to bed for a few hours and then examined the images I acquired that night.
Looking at the images of NGC 5557 with a little processing, I saw a faint pimple on the NW side of the nucleus about 5" NW of the galaxy's geometric center. Nothing could be seen in the original image because the light of the galaxy overwhelmed the faint (magn ~17) possible SN. Many SNe near the nuclei of galaxies are probably lost for that reason. I was able to locate a fairly good photo of the galaxy in Allan Sandage's wonderful "Carnegie Atlas of Galaxies". However, the image was a little over-exposed in the middle and it was not obvious that there was or was not a star in the location of my suspect. I could not determine whether I should report it from the reference images I had at the observatory. The cloud cover was increasing and it did not look like the weather would clear. I stayed until mid-evening when it was certain the clouds were there to stay. NGC 2935 was setting so that I could not get another image of its SN suspect.
When I arrived home previous images of galaxies were checked and fortunately I had one of NGC 5557 from July of 1994. It was not the best quality image but it was adequate enough for me to determine by blinking the image files that something was there in my latest image that had not been present before. My short exposure also showed it. The 'pimple' had not moved so that it was not an asteroid.
With much hesitation I called Dan Green at CBAT to find out the progress of my first SN suspect in NGC 2935 and to report the second one in NGC 5557. It turns out that it was a good thing I did call Dan because no progress had been made in the confirmation process for the N2935 suspect. Apparently, Dan thought the position he received was not accurate enough and there was no time of observation in the report, etc. I assured him the position was pretty good, but that I would remeasure it. He also wanted to know whether I was familiar with the object. I said no, but that I had several independent pictures of it. The people at CBAT want to be as certain as possible that the reported object is real before spending valuable professional telescope time. I went to my wife, Arlene's, work to use her email facility to send Dan a more detailed report and even included the original CCD images of the SN candidates.
The hardest part in any of my SN discoveries has been the waiting involved for someone to officially confirm the suspect after making the report to CBAT. One of the Bureau's staff has to request a professional astronomer with an available good-sized telescope to take a spectrum of the suspect. As Brian Marsden likes to say, "No spectrum, no supernova!". It took two days to confirm SN1996Z in NGC 2935 and three days for SN1996aa in NGC 5557; that is fairly typical. Meanwhile, you are sitting on pins and needles waiting for someone else to find 'your' object. There is no feedback (by phone, anyway) to say congratulations or try again next time. It's like communicating with a black-hole at times. I understand from talking with Brian Marsden and Dan Green that they get a goodly number of calls from amateurs about comets and SNe. It is difficult for them to verify all of the suspects; 99% of the reports they get are false alarms. I am on Dan's alert list and occasionally he will call to see if I can help resolve a report, although he says that I have reported too many false alarms myself (no more than 3 and always as a last resort). Luckily some OCA members subscribe to CBAT's email service. Charlie Oostdyk was kind enough to send me copies of the IAU's Circulars with the announcements (IAUC Nos. 6401 and 6403). Both SNe were determined to be Type Ia events, which means the progenitor stars were in binary systems and there was no hydrogen present in either spectra. Unfortunately, both SNe were past their maxima, but SN1996Z was only five days past (and magnitude 14.2), while SN1996aa was seen 6-8 weeks after its maximum light. It is very difficult to catch a SN on the rise because so little of its time is spent there, but that is where all the interesting physics is occurring.
My first SN (SN1991T) was the only one so far that was caught on the rise and it has been the subject of several dozen journal articles. It is my intention to find more SNe on the rise but that demands much more time spent searching. I estimate that I spend about 5 days a month at the observatory and record about 50 galaxies a night, not really enough time to find much more than one SN a year, which is about what I am averaging. As the saying goes: so many galaxies, so little time! I am grateful for the moral support of friends around me.
Wayne P. Johnson (aka Mr. Galaxy)