The Discovery of SN 1996ai in NGC 5005.

Claudio Bottari.

One of the major activities going on at my Mira observatory at Sava, Italy, is to record deep sky CCD images for a personal atlas I'm currently working on. I'm interested especially in HII regions and dust lanes of nearby galaxies. Images are made using a SpectraSource CCD HPC 1 camera equipped with a Texas Instrument TC215 chip, 1024 square, 12 micron pixels size, at 16 bit. The telescopes are a 50cm (20 inch) Newtonian, focal length 2.5 meters and an Astrophysics EDT 180mm lens, focal length 1.6 meters. The chip has a high quantum effciency in the red and near infra-red spectral regions and is often used coupled to I.SI.S. filters; they are ideal for imaging internal details of galaxies. Image processing is done with MiPS, Prism and Imagine32 softwares. On the night of June 16 I was ready to begin my usual work at the observatory. My observing list included NGC 5033, a galaxy in Canes Venatici, close to M63. After having initialized the camera and the telescope, I tried to center it using its coordinates. After a 40sec exposure, the PC monitor showed a faint glow just above the camera termal noise. Some processing showed clearly and to my surprise that I had centered nearby NGC 5005, a nice spiral that I had imaged about a year earlier on May 18 1995. There must have been a mistake in the process of pointing the instrument! The image showed clearly a star in the galaxy core, of magnitude 14.5, some 50sec. ENE from the nucleus. The star was not seen in my archive image! I checked various references such as the BT Atlas, Sandage's Hubble Atlas, Wray's Color Atlas and Thompon's supernovae charts. These references, as well as my earlier image, showed only an HII regions close to the suspect star position. No asteroids were found in that position, besides no movement was seen during the night. The brightness of the star ruled out also cosmic rays or a camera defect. The suspect star must have been a supernova!!! The next morning I rushed to the local post office and after a few trials which seemed to last centuries, the employee was able to send a telegram to Dr. Marsden. The supernova was caught during its early stages; negative observations by Mark Armstrong and Piero Mazza should place the outburst on the 14th or 15th of June. A subsequent spectrum shows the supernova to be a type Ia, with its light strongly absorbed by the galaxy. By the way, because of the supernova I still have to image NGC 5033! I hope to get it right next time!! PS: since I still don't have an Internet access, should you have any questions please refer to Stefano Pesci or Mirko Villi