Peremennye Zvezdy

Peremennye Zvezdy (Variable Stars) 29, No. 2, 2009

Received 22 October; accepted 25 November.

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Photometric Observations and Modeling of Type IIb Supernova 2008ax

D.Yu. Tsvetkov1, I.M. Volkov1,2, P.V. Baklanov3, S.I. Blinnikov3,1, O. Tuchin4

  1. Sternberg Astronomical Institute, University Ave. 13, 119992 Moscow, Russia

  2. Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranska Lomnica, Slovak Republic

  3. Institute for Theoretical and Experimental Physics, Bol'shaya Cheryomushkinskaya Str. 25, 117218 Moscow, Russia

  4. Samara Palace of Children's Creativity, Kuibyshev Str. 151, 443010 Samara, Russia


CCD UBVRI photometry covering about 320 days is presented for the type IIb SN 2008ax. Its photometric behavior is typical of core-collapse SNe with low amount of hydrogen. The main photometric parameters are derived and a comparison with SNe of similar types is reported. Preliminary modeling is carried out, and the results are compared to the observed light curves. The main parameters of the hydrodynamical models are close to those used for SN IIb 1993J.



Introduction

The Supernova SN 2008ax was discovered independently by Mostardi et al. (2008) and Itagaki (Nakano and Itagaki 2008) on March 3.45 UT and March 4.62 UT, respectively. The magnitude of the SN at discovery, estimated on unfiltered CCD frames, was about 16. The first detection was only 6 hours after the image with limiting magnitude about 18.5 and showing no sign of the SN was obtained by Arbour (2008). The offsets from the nucleus of the host galaxy NGC 4490 are 53 1E, 25 8S. The projected distance from the center is 2.8 kpc, while the radius of the galaxy is about 9 kpc. NGC 4490 is a barred spiral galaxy of type SBcd. SN II-P 1982F was discovered earlier in this galaxy (Tsvetkov 1984). The positions of the two SNe are quite close, SN 1982F occurred 19 (0.9 kpc) closer to the nucleus at about the same positional angle (116 for SN 2008ax, 120 for SN 1982F).

Crockett et al. (2008) identified a source coincident with the position of SN 2008ax in pre-explosion HST observations in three optical filters. The possible progenitor may be a single massive star (initial mass ), which loses most of its H-rich envelope and explodes as an 11-12 helium-rich Wolf-Rayet star, or an interacting binary producing a stripped progenitor.

Photometric and spectroscopic observations of SN 2008ax covering first 2 months past discovery were reported by Pastorello et al. (2008) (hereafter P08). The object displayed typical spectral and photometric evolution of a type IIb supernova, consistent with the explosion of a young Wolf-Rayet star.

Roming et al. (2009) (hereafter R09) presented UV, optical, X-ray and radio properties of SN 2008ax. They detected initial fading in UV light curves followed by a rise, reminiscent of the dip seen in type IIb SN 1993J.

Observations and reductions

We started photometric monitoring of SN 2008ax 4 days after its discovery and continued observations until 2009 January 23. CCD images in filters were obtained with the following instruments: the 50-cm reflector of Astronomical Institute of Slovak Academy of Sciences at Tatranska Lomnica with SBIG ST-10XME CCD camera (hereafter S50); the 50-cm meniscus telescope and the 60-cm reflector of Crimean Observatory of Sternberg Astronomical Institute equipped respectively with Meade Pictor 416XT and Apogee AP-47 cameras (C50, C60); the 70-cm reflector of Sternberg Astronomical Institute in Moscow with Apogee AP-7 CCD camera (M70); the 1-m reflector of Simeiz Observatory with AP-47 camera (C100). The images on 2009 January 23 were obtained at the 2-m Faulkes Telescope North (F200).

The standard image reductions and photometry were made using IRAF1.

The galaxy background around SN 2008ax is strong and non-uniform, and we applied image subtraction for most of the frames. Our observations did not allow us to construct good template frames, and we used for subtraction the images of NGC 4490 downloaded from the ING archive2.

After subtraction, the magnitudes of the SN were derived by PSF fitting relative to a sequence of local standard stars. The comparison stars are shown on Fig. 1, and their magnitudes are reported in Table 1.

Fig. 1. SN 2008ax with local standard stars.

Table 1. Magnitudes of local standard stars
Star
1 11.73 0.04 11.62 0.01 11.06 0.01 10.73 0.01 10.41 0.01
2 13.78 0.05 12.94 0.01 11.89 0.01 11.35 0.01 10.85 0.01
3 13.66 0.05 13.71 0.01 13.18 0.01 12.83 0.02 12.50 0.02
4 14.61 0.06 14.60 0.02 13.96 0.01 13.55 0.04 13.18 0.02
5     16.25 0.03 15.20 0.02 14.57 0.03 14.03 0.03
6 16.58 0.07 16.02 0.03 15.00 0.02 14.47 0.02 13.93 0.03

Stars 1-4 were measured photoelectrically in the , filters by Tsvetkov (1984); a comparison with the new CCD data reveals good agreement, the mean differences are , . We may conclude that both calibrations are sufficiently correct.

The results of observations of the SN are presented in Table 2.

Table 2. Observations of SN 2008ax
JD 2454000+ Tel.
532.51     17.25 0.08 15.92 0.03 15.35 0.04 15.09 0.04 S50
537.59     15.22 0.02 14.54 0.02 14.19 0.03 13.84 0.04 S50
541.37 14.43 0.06 14.46 0.03 13.88 0.03 13.67 0.05 13.37 0.03 S50
544.42 14.09 0.06 14.22 0.02 13.68 0.02 13.36 0.03 13.06 0.02 S50
546.39 14.06 0.06 14.13 0.02 13.56 0.02 13.24 0.04 12.93 0.02 S50
551.31 14.33 0.06 14.19 0.02 13.41 0.02 13.08 0.03 12.73 0.03 S50
552.53 14.56 0.05 14.25 0.02 13.45 0.01 13.07 0.02 12.73 0.02 S50
553.43 15.07 0.12 14.38 0.05 13.58 0.04 13.10 0.07 12.80 0.03 S50
555.52 15.21 0.08 14.67 0.06 13.64 0.05 13.12 0.06 12.86 0.05 S50
556.42 15.60 0.08 14.82 0.05 13.77 0.05 13.32 0.05 12.87 0.05 S50
557.41 15.87 0.07 15.02 0.02 13.87 0.02 13.27 0.05 12.90 0.03 S50
563.40 17.27 0.13 15.77 0.04 14.39 0.03 13.76 0.05 13.19 0.04 S50
564.47     15.81 0.07 14.41 0.03 13.75 0.03 13.18 0.03 S50
570.37     16.21 0.10 14.71 0.02 14.05 0.05 13.40 0.03 S50
579.38     16.45 0.04 14.97 0.02 14.33 0.02 13.68 0.03 M70
583.33     16.52 0.05 15.07 0.03 14.46 0.03 13.79 0.04 M70
585.35     16.49 0.05 15.05 0.03 14.42 0.03 13.77 0.04 M70
590.30     16.62 0.05 15.20 0.03 14.59 0.03 13.89 0.03 M70
601.47     16.84 0.09 15.41 0.04 14.86 0.04 14.08 0.03 S50
602.34     16.99 0.04 15.50 0.03 14.89 0.02 14.17 0.03 M70
613.32     16.91 0.04 15.65 0.02 15.08 0.02 14.35 0.03 M70
616.41     17.03 0.04 15.69 0.03 15.08 0.04 14.35 0.04 S50
623.33     16.96 0.05 15.87 0.03 15.31 0.02 14.56 0.03 M70
628.44     17.25 0.10 16.00 0.03 15.45 0.03 14.60 0.04 S50
643.34         16.37 0.03 15.73 0.03 14.97 0.06 C50
644.32         16.35 0.03 15.75 0.03 15.07 0.11 C50
647.31         16.35 0.04 15.80 0.03     C50
647.31     17.53 0.05 16.41 0.02 15.85 0.02 15.00 0.04 C100
649.30         16.41 0.03 15.83 0.03     C50
656.30         16.63 0.02 15.98 0.03     C50
658.30         16.63 0.10 16.00 0.05     C50
660.33         16.67 0.04 16.07 0.03 15.49 0.10 C60
674.34     18.25 0.14 17.01 0.06 16.33 0.04 15.55 0.03 S50
675.32         17.01 0.04 16.45 0.07 15.73 0.08 S50
677.32         17.19 0.05 16.36 0.05 15.60 0.05 S50
679.32         17.31 0.07 16.52 0.06 15.73 0.16 S50
699.28         17.57 0.20 16.78 0.07     M70
720.24             16.99 0.07     C60
781.63             17.90 0.05     C60
783.62         18.98 0.07 18.18 0.06     C60
855.05     20.90 0.18 20.45 0.07 19.22 0.03 18.77 0.03 F200


Light and color curves

Fig. 2. The light curves of SN 2008ax. The dashed lines show the decline at the tail with rates reported in the text.

The light curves of SN 2008ax are shown in Fig. 2. The premaximum rise and the peak have good coverage by observations, and we can determine the dates and magnitudes of maximum light in different bands: ; ; ; ;