Peremennye Zvezdy (Variable Stars) 30, No. 3, 2010
Received 4 May; accepted 5 May.
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|CCD photometry is presented for the type Ia supernova 2008gy. The light curves match the template curves for a fast-declining SN Ia, but the colors appear redder than average, and the SN may also be slightly subluminous. SN 2008gy is found to be located far outside the boundaries of the three nearest galaxies, each of them has a nearly equal probability to be the host galaxy.|
SN 2008gy was discovered by P. Balanutsa on unfiltered CCD images taken with a 335-mm telescope near Moscow on October 30.99 UT. This telescope is a part of the large "MASTER Robotic Net" (Lipunov et al. 2010). The new object was located at , (J2000.0), which is west and north of the center of PGC 1584648 (Lipunov 2008). Confirmation images were taken by Kryachko and Korotkiy (2008) on November 2.94 UT with an 80-mm refractor at Karachay-Cherkessia, Russia. Folatelli et al. (2008) reported that they had obtained spectra (range 340-602 nm) of SN 2008gy with the New Technology Telescope (+EFOSC2) at La Silla on November 19.0-19.1 UT. The spectrum of SN 2008gy shows it to be a type-Ia supernova, two to three weeks after maximum light, at a redshift of 0.029.
SN 2008gy was imaged again with the 335-mm telescope near Moscow on November 7.73 UT. The photometric monitoring of the SN in filters was carried out at the 60-cm reflector of the Crimean Observatory of Sternberg Astronomical Institute between 2008 November 9 and November 25. All image reductions and photometry were made using IRAF1.
The image of SN 2008gy obtained at the 60-cm reflector in the band is shown in Fig. 1. It is evident that the SN is located in a group of three galaxies, which are labeled G1, G2, G3. We searched the NED2 and Hyperleda3 databases and found out that G1 is PGC 1584648, G2 is PGC 1584876, and G3 is IC 1890. Only for IC 1890 is the redshift known, it is 0.03412. The value reported for SN 2008gy differs by 1500 km/s from this estimate, which seems larger than possible peculiar velocities of galaxies in a group or the velocity of the presupernova inside its host galaxy. However, the redshift uncertainty for SN 2008gy was not announced, and it can possibly account for most of this difference. We calculated the angular distances from the SN to the three galaxies on our CCD frames, respectively for G1, G2, G3: , , . If we assume that the redshift 0.029 corresponds to the distance for SN 2008gy and adopt the distance modulus , as reported in NED, the linear projected distances of the SN from the centers of the galaxies are 12.2 kpc, 21.3 kpc, 35.7 kpc. According to these data, G1 is the most likely host galaxy. But other galaxies are significantly brighter and larger, and the relative projected radial distances, defined as angular separation divided by the isophotal radius of the galaxy, for the three galaxies are 2.27, 1.74, 2.06. Thus, from this point of view, G2 is the most likely host galaxy, and G1 is the least probable host. Taking into account that the projected distances are the lower limits to the spatial separations, we conclude that there is nearly equal probability for SN 2008gy to belong to any of the three galaxies, or to be located in the intergalactic medium.
The local standard stars are also marked on Fig. 1. The magnitudes of these stars are reported in Table 1, they were calibrated on four photometric nights in November. It is clear that the galaxy background has no effect on the photometry of SN 2008gy. The magnitudes of the SN were derived by PSF fitting relative to a sequence of local standard stars. We used the magnitudes to calibrate the unfiltered CCD frames obtained at the 335-mm and 80-mm telescopes.
The results are presented in Table 2 and the light curves are shown in Fig. 2.
Fig. 2. The light curves of SN 2008gy in the (blue), (green) and (red) bands. Dots show data obtained at the 60-cm telescope, circles are for the unfiltered frames exposed at the 355-mm and 80-mm telescopes. The dashed curves are the light curves of SN 1994D.
Fig. 3. The (green) and (red) color curves for SN 2008gy. The dashed curves are the color curves of SN 1994D.
The data are best fitted by the light curves of moderately fast-declining SN Ia 1994D (Richmond et al. 1995). From this fit, we estimate that maximum light was reached around JD 2454775 (November 4) with , , . The color curves are shown in Fig. 3. The colors of SN 2008gy are significantly redder than for SN 1994D, approximately by 044 in and 022 in . The galactic extinction for SN 2008gy is , , according to NED. We cannot expect any significant extinction in the parent galaxy because the SN is located outside the boundaries of all possible host galaxies. So, we suppose that SN 2008gy is intrinsically redder than SN 1994D at the phase of early decline by about 028 in and 011 in . Assuming and correcting only for the galactic extinction, the absolute magnitudes of SN 2008gy are . We may suggest that the rate of early decline for this SN is not much different from that for SN 1994D, , and then the luminosity of SN 2008gy at maximum is significantly fainter than can be expected from the relations of and versus as presented by Hicken et al. (2009). However, the distance modulus corresponding to the redshift of IC 1890 is by 035 larger, and with absolute magnitudes brighter by 035 mag SN 2008gy fits quite well to the relation between maximum luminosity and rate of decline. Thus, we cannot make a definite conclusion on the luminosity of SN 2008gy, but its intrinsic red color is much more probable.
The possible lower luminosity and red color may be connected with the location of the SN far away from the host galaxy, where the characteristics of stellar population may be different from those closer to the galaxy center (see e.g. Sullivan et al. 2010 and references therein).
Acknowledgements. This work was supported by the Ministry of Science of the Russian Federation (state contract No. 02.740.11.0249). The work of D.T. was partly supported by the Leading Scientific Schools Foundation under grant NSh.433.2008.2 and by the RFBR grant 10-02-00249a. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. We acknowledge the usage of the HyperLeda database.
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