Astron. Astrophys. 344, L67-L70 (1999)

3. The magnitude of the optical transient

We estimated the total AB magnitude of the OT by performing aperture photometry with an aperture radius of . The local sky was measured in an annulus with an inner radius of and a width of . The photometry aperture and sky annulus were chosen to maximize the signal-to-noise from the OT and to minimize the contamination from the host galaxy and the nebulosity seen near the OT. The integrated magnitude depends slightly ( mag) on the sizes of the aperture and sky annulus since the OT is situated near several bright knots in the outer regions of the host galaxy. No aperture correction was applied since the uncertainties in the background subtraction are larger than the aperture correction. The STIS Instrument Handbook gives a zero point in the AB system of 26.386 for the 50CCD (clear) imaging mode, which corresponds to a zero-point flux of µJy. This zero point gives a calibrated AB magnitude for the OT of , which corresponds to a flux of µJy. We assume that the OT has a power-law spectrum of the form where (Bloom et al. 1999a) and the zero-point flux for the STIS in its 50CCD (clear) imaging mode yields µJy. Fruchter et al. (1999b) note that the STIS magnitude of an object with a power-law spectrum is largely independent of due to the symmetric shape of the 50CCD (clear) bandpass. We converted the AB magnitude to the Johnson V-band and Kron-Cousins R-band using this power-law spectrum and the photometric zero-points from Fukugita et al. (1995). The resulting calibration equations are and , where C is the observed count rate in ADU/s on the drizzled image.

The OT is at , . Schlegel et al. (1998) find the Galactic reddening in this direction to be . We used (Rieke & Lebofsky 1985) and (Cardelli et al. 1989) to derive extinction corrected magnitudes of , and for the OT. These correspond to fluxes of µJy and µJy respectively.

© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999
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