Astron. Astrophys. 321, 81-83 (1997)

2. Observations and photometry

The observations (see Fig. 1) presented here were taken as part of a larger study (Trentham et al. 1996); the discovery of this SN was serendipitous. On March 13 1995, IRAS 12112+0305 was imaged for 12 minutes with a filter (Wainscoat & Cowie 1992) using the QUIRC 1024 1024 HgCdTe array at the f/10 Cassegrain focus of the University of Hawaii 2.24 m telescope on Mauna Kea. On May 1 1995, IRAS 12112+0305 was imaged for 8 minutes with a Mould B filter using a thinned Tektronix 2048 2048 CCD at f/10 on the UH 2.24 m telescope. This image revealed a bright point-source in a southern extension of the galaxy that was not visible in the March image. We interpret this point-source as a SN for the reasons detailed at the beginning of the next section. On August 1 1995, we obtained Mould V (5 minutes), R (15 minutes), and I (5 minutes) images of IRAS 12112+0305 with the same observational setup as for the May images. We failed to detect at the 3 level any point-source at the position of the bright point-source in the B-band image.

Fig. 1. Images of IRAS 12112+0305 taken as described in the text. North is up and East is to the left in all images. Each image is a square of side 41 arcseconds. The SN appears clearly as a bright point source in the May image. The insert in the May image shows a psf star - the image is elongated because of heavy windshake of the telescope. Note also the secondary nucleus, which is clearly visible in the K -band image, but is completely obscured by dust in the galaxy in the B -band image.

The images were reduced using standard techniques. The photometry was converted to the UBVRI magnitude system of Landolt (1992) for the optical images and the system of Wainscoat & Cowie (1992) for the QUIRC image; the zero points are accurate to 2-3%. We used the psf-fitting algorithm DAOPHOT (Stetson 1987) to measure the total apparent magnitude of the SN in the May B -band image. We estimate an uncertainty in this magnitude of . This error is larger than one normally expects from psf-fitting, but the total light contributed by the background galaxy is uncertain. We obtained limiting magnitudes of the SN in the March and August images as follows. First, the sky noise and seeing in the vicinity of the galaxy were computed. We then calculated the brightest point source that we would fail to detect at the 3 level in an aperture of size given and as computed above. The total magnitudes we compute for such sources give the following limits for the magnitude of the SN: (March 1995), (August 1995), (August 1995), (August 1995).

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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