PASJ: Publ. Astron. Soc. Japan 61, 713–720, 2009 August 25 c 2009. Astronomical Society of Japan.
Early Spectral Evolution of the Rapidly Expanding Type Ia Supernova 2006X
Masayuki YAMANAKA,1,2,3 Hiroyuki NAITO,4 Kenzo KINUGASA,5 Naohiro TAKANASHI,6 Masaomi TANAKA,7 Koji S. KAWABATA,2 Shinobu OZAKI,8 Shin-ya NARUSAWA,4 and Kozo SADAKANE3 1Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 2Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526 3Astronomical Institute, Osaka Kyoiku University, Asahigaoka, Kashiwara, Osaka 582-8582 4Nishi-Harima Astronomical Observatory, Sayo-cho, Hyogo 679-5313 5Gunma Astronomical Observatory, Takayama-mura, Gunma 377-0702 6National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 7Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 Downloaded from https://academic.oup.com/pasj/article/61/4/713/1538768 by guest on 27 September 2021 8Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Kamogata-cho, Asakuchi, Okayama 719-0232 [email protected]
(Received 2008 August 17; accepted 2009 April 13) Abstract We present optical spectroscopic and photometric observations of Type Ia supernova (SN Ia) 2006X from 10 d before the B-band maximum to 91 d after. This object exhibits one of the highest expansion velocities ever published for SNe Ia. At its premaximum phases, the spectra show strong and broad features of intermediate-mass elements, such as Si, S, Ca, and Mg, while the O I 7773 line is weak. The extremely high velocities of the Si II and S II lines and the weak feature of the O I line suggest that an intense nucleosynthesis might take place in the outer layers, favoring a delayed detonation model. Interestingly, the Si II 5972 feature is quite shallow, resulting in an unusually low depth ratio of Si II 5972 to Si II 6355, R(Si II). The low R(Si II) is usually interpreted as being a high photospheric temperature. However, the weak Si III 4560 line suggests a low temperature, in contradiction with an interpretation on the low R(Si II). This could imply that the Si II 5972 line might be contaminated by underlying emission. We propose that R(Si II) may not be a good temperature indicator for a rapidly expanding SN Ia in the premaximum phases. Key words: stars: supernovae: general — stars: supernovae: individual (SN 2006X) — techniques: spectro- scopic
1. Introduction samples and divided them into three groups (HVG, LVG, and FAINT) according to the luminosity and the temporal Type Ia supernovae (SNe Ia) are thermonuclear explosions gradient of the Si II velocity. SNe Ia with both a high velocity of C+O white dwarfs. It is thought that the explosion is gradient (HVG) and low velocity gradient (LVG) have normal triggered when the mass of the white dwarf approaches the luminosity, while the FAINT group is dimmer than the two Chandrasekhar limit ( 1.4 Mˇ). Their peak luminosity, cali- classes mentioned above, as represented by the subluminous brated using a correlation with the light curves and/or color SN Ia 1991bg (Filippenko et al. 1992). Branch et al. (2006) curves, is quite uniform, and thus SNe Ia have been used studied near-maximum spectra of SNe Ia and defined four as “standard candles” for measuring extragalactic distances groups according to the equivalent width ratio of Si II 6355 (Phillips 1993; Phillips et al. 1999; Wang et al. 2005). and Si II 5972, and also the profile of the Si II 6355 line near Observations of distant SNe Ia led to the discovery of an accel- the maximum light. These studies show that the physics of erating expansion of the universe (Riess et al. 1998; Perlmutter SNe Ia cannot be represented by a family of one parameter, 1 et al. 1999). such as the luminosity decline rate, Δm15.B/ (Benetti et al. Spectroscopic properties and their time evolution had been 2004). thought to be homogeneous in SNe Ia (e.g., Branch et al. Nugent et al. (1995) found that the depth ratio of Si II 5972 1993; Filippenko 1997). However, recent spectroscopic obser- to Si II 6355, R(Si II), is correlated with the absolute lumi- vations show that the spectra of SNe Ia during the premaximum nosity of SNe Ia at the maximum brightness. It is thought that phases show diversity in both their line velocities and profiles R(Si II) is an indicator of the temperature of the photosphere. (Fisher et al. 1997; Benetti et al. 2004; Mazzali et al. 2005a, During the premaximum phases, R(Si II) shows considerable 2005b; Quimby et al. 2006b; Altavilla et al. 2007; Garavini diversity. Benetti et al. (2005) showed that, during the premax- et al. 2007; Phillips et al. 2007; Sahu et al. 2008; Wang et al. imum phases, R(Si II) in HVG SNe is high in the earliest 2009). In particular, the Doppler velocity of the Si II 6355 phases and declines rapidly, while that of LVG SNe does not feature exhibits a large dispersion, and the velocity is not corre- lated with the peak luminosity (Hatano et al. 2000). 1 The difference between the B-band magnitude at the maximum and that at Benetti et al. (2005) collected some well-observed SNe Ia 15 d after the maximum (Phillips 1993). 714 M. Yamanaka et al. [Vol. 61, change significantly. Tanaka et al. (2008) suggested that the star, except for measurements during the last two epochs. We photospheric temperature in HVG SNe is lower than that of performed PSF photometry for the last two-epoch images, and LVG SNe during the premaximum phases (& 1 week before derived magnitudes that are consistent with those in Wang maximum) from their spectrum analysis. et al. (2008a), within a 2 error. The photometric calibration h m s ı 0 00 SN 2006X (˛2000:0 = 12 22 53: 90, ı2000:0 = 15 48 32: 9) was performed by using the comparison stars in Wang et al. was discovered on 2006 February 4.75 UT by Suzuki and (2008a). A summary of our photometric observations is given Migliardi (2006), independently, near the center of the nearby in table 1. galaxy NGC 4321 (M 100). The distance to NGC 4321 is From our data, we derived the date of the B-band maximum derived to be = 30.91 ˙ 0.14 (distance modulus) by at Modified Julian Date (MJD) = 53785.1 ˙ 1.8 and the a Cepheid calibration of the Hubble Space Telescope Key maximum magnitude mB;max = 15.30˙0.09 by a polynomial Project (Freedman et al. 2001). Quimby et al. (2006a) classified fitting. Both values are consistent with those in Wang et al. SN 2006X as being Type Ia. They reported that the Si II 6355 (2008a), who derived the B-band maximum date and magni- line velocity of this supernova was very high (20700 km s 1) tude as being 53785.67 ˙0.35 and 15.40 ˙0.03, respectively. Downloaded from https://academic.oup.com/pasj/article/61/4/713/1538768 by guest on 27 September 2021 on February 8.35 (t = 11.6 d, hereafter, t denotes days In this paper, we assume MJD = 53785.67 to be t = 0for from the B-band maximum, MJD = 53785.67) (see subsec- convenience of comparison with their study. tion 2.1). Wang et al. (2008a) presented extensive optical and 2.2. Spectroscopic Observations NIR photometric observations. They derived the maximum absolute magnitude, MV;max = 19.06 ˙ 0.17; the decline Spectroscopic observations were performed on 22 nights rate of the B-band light curve, Δm15(B) = 1.17 ˙ 0.05; and during the period from t = 10 d to +84 d. Eighteen spectra 43 1 maximum bolometric luminosity, Lbol;max ' 1.0 10 erg s , were taken at NHAO, including four during the premaximum suggesting that this supernova is a normally luminous SN phases, with the 2 m NAYUTA telescope and a low-resolution Ia. Wang et al. (2008a) obtained spectra around and after the spectrograph, MALLS (Medium And Low-dispersion Long- maximum, and suggested that SN 2006X is characterized by slit Spectrograph: Ozaki & Tokimasa 2005). The typical expo- strong, highly blueshifted absorption lines of intermediate- sure time was 1800 s. The wavelength coverage was 4000– mass elements. The Si II 6355 absorption was very deep and 6800 A,˚ and the wavelength resolution was 17 A˚ , corre- broad during one week after the maximum, being similar to sponding to a velocity resolution of 850 km s 1 at 6000 A.˚ those in SN 1984A and SN 2002bo (Branch et al. 2008). Patat Four spectra were taken at Gunma Astronomical Observatory et al. (2007) reported on the detection of a temporal variation (GAO), including two during the premaximum phases, with of the Na I D absorption line, and suggested the presence of the 1.5 m telescope equipped with a low-resolution spectro- dense circumstellar materials (CSM) around SN 2006X. The graph, GLOWS (Gunma LOW resolution Spectrograph and possible detection of an inner echo might also be consistent imager). The wavelength coverage was 4000–8000Aandthe˚ with the circumstellar dust scenario (Wang et al. 2008b). wavelength resolution was 15 A˚ (750 km s 1). One late-time In this paper, we present spectroscopic and photometric spectrum (at t = 63 d) was obtained by using the 8.2 m Subaru observations of SN 2006X. In particular, we show optical Telescope (National Astronomical Observatory of Japan) and spectra during the premaximum phases, which are not FOCAS (Faint Object Camera And Spectrograph: Kashikawa presented by Wang et al. (2008a). The premaximum spectra et al. 2002), with the wavelength coverage being 4000–9000A˚ are useful for exploring the physical properties in the outermost and the wavelength resolution being 14 A˚ (700 km s 1). The layers of the SN ejecta. Our observations and data reduction log of our spectroscopic observations is given in table 2. are described in section 2. Results are shown in section 3. We Data reduction was performed by the standard procedure for discuss the spectroscopic properties of SN 2006X during the long-slit spectroscopy with IRAF tasks. The sky background premaximum phases in section 4, and finally give conclusions was subtracted by 1D interpolation along the focal slit direc- in section 5. tion. Wavelength calibration was obtained by using night-sky lines taken in the object frames (for NHAO data, Iye et al. 2. Observations and Data Reduction 1992), or comparison lamp data (for the others). The flux was calibrated with the data of spectrophotometric standard stars 2.1. Photometric Observations obtained on the same night. We performed BVRCIC photometric observations on In determining the central wavelength of the absorption 24 nights during the period from t = 10 d to +91 d with lines, we used a method described in Hachinger, Mazzali, and two telescopes: a 0.6 m reflector equipped with an ST-9 Benetti (2006). We first estimated the center of each feature by imager at Nishi-Harima Astronomical Observatory (NHAO) eye. Then, we performed a 1D Gaussian fitting to the feature and a 0.51 m telescope equipped with a CCD camera at Osaka several times, and derived the mean central wavelength and its Kyoiku University (OKU). Data reduction was performed standard deviation. The final uncertainty of the center wave- in a standard manner for aperture photometry using IRAF.2 length was taken as the root sum square of the standard devia- Although there is a star located at 100 east and 800 north of the tion and the wavelength resolution described above. supernova, the star is faint (V = 17.05: Wang et al. 2008a), and thus our photometry is not significantly affected by the 3. Results
2 IRAF is distributed by the National Optical Astronomy Observatories, oper- 3.1. Light Curves and Color Curves ated by the Association of Universities for Research in Astronomy, Inc., under contract to the National Science Foundation of the United States. We present light curves and color ones in figures 1 and 2, No. 4] Early Spectroscopy of SN 2006X 715
Table 1. BVRCIC photometry for SN 2006X.