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

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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 [FORMULA] filter (Wainscoat & Cowie 1992) using the QUIRC 1024 [FORMULA] 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 [FORMULA] 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 [FORMULA] level any point-source at the position of the bright point-source in the B-band image.

[FIGURE] 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 [FORMULA] 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 [FORMULA] of the SN in the May B -band image. We estimate an uncertainty in this magnitude of [FORMULA]. 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 [FORMULA] and seeing [FORMULA] in the vicinity of the galaxy were computed. We then calculated the brightest point source that we would fail to detect at the 3 [FORMULA] level in an aperture of size [FORMULA] given [FORMULA] and [FORMULA] as computed above. The total magnitudes we compute for such sources give the following limits for the magnitude of the SN: [FORMULA] (March 1995), [FORMULA] (August 1995), [FORMULA] (August 1995), [FORMULA] (August 1995).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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