The discovery was made on June 12.82 UT, in the morning twilight at Australia with a 31-cm reflecting telescope. The precise position, measured from the image taken on the next night with 1.0-m Siding Spring Observatory by Robert H. McNaught of the Australia National University, is:
R.A. = 1h36m42s.65, Decl. = +15d44'20".9 (J2000.0),
which is about 13" east and 161" south of the center of the spiral galaxy M74, just on the southern spiral arm. There is a foreground star (red mag 12.4) with the position end figures 32s.00, 45'08".7.
The first spectrum was taken on the same night at America, i.e. June 13.46 UT = 0.64 days after discovery, in twilight by Peter Garnavich, University of Notre Dame, and E. Bass, Cornell University. The IR spectrum shows a series of strong and broad hydrogen Paschen emission lines, which indicates that is it a type II SN around maximum (IAUC 8150). The explosion may have occured when M74 was behind the Sun. We are possibly able to enjoy the bright SN for some weeks.
Apparently, this supernova was discovered already in its fading phase when discovered. Presumably it had occurred about one month earlier and was probably somewhat brighter, but at that time, M74 was invisible because it was near its conjunction with the Sun. As of June 30 and July 4, 2003, it was estimated at magnitude 14.1, on July 12 at 14.6, and on July 30 had dropped to mag 16.2.
The image at the right was obtained by Australian amateur astronomer Ted Dobosz just shortly after the SN's discovery.
In January 2004, the progenitor star of SN 2003gd was identified on images taken with the Hubble Space Telescope (with WFPC2 and ACS) and Gemini North telescope (with GMOS camera), taken of this region in M74 less than a year before the supernova event; it shows up particularly clear on a Hubble/ACS image taken about 200 days before the explosion. On this image, a red star of visual mag +25.8 is seen just in the right place. At the distance of M74, this corresponds to an absolute magnitude of -6M, or a luminosity of about 10,000 times that of our sun. From these data, astronomers have identified the progenitor as a red supergiant star, and calculated a mass of 8 to 10 solar masses with an extended hydrogen envelop, which probably started its life as a main sequence star of up to 12 solar masses (Smartt et.al 2004). This was only the third case where the progenitor of a supernova could be identified, after supernovae SN 1987A in the Large Magellanic Cloud and SN 1993J in M81.
Last Modification: August 11, 2005