Optical and Near-Infrared Polarimetry of Highly Reddened Type Ia Supernova 2014J: Peculiar Properties of Dust in M82
We presented optical and near-infrared multi-band linear polarimetry of the highly reddened Type Ia SN 2014J appeared in M82. SN 2014J exhibits large polarization at shorter wavelengths, e.g., % in band, and the polarization decreases rapidly at longer wavelengths, with the position angle of the polarization remaining at approximately over the observed wavelength range. These polarimetric properties suggest that the observed polarization is likely to be caused predominantly by the interstellar dust within M82. Further analysis shows that the polarization peaks at a wavelengths much shorter than those obtained for the Galactic dust. The wavelength dependence of the polarization can be better described by an inverse power law rather than by Serkowski law for Galactic interstellar polarization. These suggests that the nature of the dust in M82 may be different from that in our Galaxy, with polarizing dust grains having a mean radius of m .
Subject headings:dust, extinction — circumstellar matter — galaxies: individual (Messier 82) — supernovae: individual (SN 2014J) — polarization
The homogeneity in photometric properties of normal Type Ia supernovae (SNe Ia) is expected to be related to common physical properties during the onset of thermonuclear explosions in the progenitor white dwarfs with a mass close to Chandrasekhar’s limiting mass (Hillebrandt & Niemeyer, 2000, for review). The continuum light from normal SNe Ia is intrinsically weakly polarized (%), although the absorption features, including Si ii 6355 and Ca ii IR triplets, are often polarized by –% (Wang et al., 1996; Wang, Wheeler, &Höflich, 1997; Wang et al., 2003; Leonard et al., 2005; Wang et al., 2006; Wang and Wheeler, 2008; Chornock & Filippenko, 2008; Zelaya et al., 2013; Maund et al., 2013). In fact, a Si ii 6355 absorption line with a polarization of % and an equivalent width of m (for SN 2014J near the maximum) gives an additional polarization of only % for typical band polarimetry (m). This practically allows us to use SNe Ia as a unique bright unpolarized-light source within distant galaxies for broadband polarimetry. Thus, a SN Ia has the potential to project the interstellar polarization (ISP) along the line of sight inside the host galaxy when subject to a substantial amount of interstellar reddening, as is commonly seen in our Galaxy (e.g., Whittet, 2003).
SN 2014J is the closest SN Ia in this quarter century. It was discovered in M82 (at a distance Mpc; Sakai & Madore 1999) on 2014 Jan 21.81 (UT dates are used throughout this Letter) at a magnitude of mag (Fossey et al., 2014; Goobar et al., 2014), probably a week after the explosion (Zheng et al., 2014). The apparent brightness provides us with an opportunity for various studies, including constraining early light curve model (Goobar et al., 2014; Zheng et al., 2014), studying progenitor systems (Kelly et al., 2014), and evaluating properties of extragalactic interstellar/circumstellar media (Amanullah et al., 2014; Foley et al., 2014; Welty et al., 2014). In addition, this SN provides a rare opportunity to probe the ISP within the starburst galaxy M82 because it suffers significant reddening from the host galaxy ( mag). Prior to SN 2014J, there were only three reddened SNe Ia ( mag) for which the wavelength dependence of optical polarization had been measured, which are SN 1986G in the peculiar giant S0 galaxy NGC 5128 (Cen A) (Hough et al. 1987; mag and Mpc), SN 2006X in the Virgo Cluster spiral galaxy NGC 4321 (Patat et al. 2009; – mag and Mpc), and SN 2008fp in the peculiar spiral galaxy ESO 428-G14 (Cox & Patat 2014; mag and Mpc).
The Galactic ISP at ultraviolet (UV) to near-infrared (NIR) wavebands can be approximated by Serkowski law (Serkowski, Mathewson, & Ford 1975), a smooth function of wavelength given by
where is the peak polarization degree occurring at wavelength and is a parameter describing the width of the peak. The polarization observed for SNe 1986G and 2006X can be described by Serkowski law at optical wavelengths; however, the derived parameters are peculiar, i.e., the wavelengths m (SN 1986G) and m (SN 2006X) are significantly shorter than the Galactic value (m; Vrba, Coyne, & Tapira 1981), and (SN 2006X) is not consistent with the value expected from the Wilking law, i.e., , for the Galactic ISP (Wilking et al. 1980; Whittet et al. 1992). SN 2008fp exhibits interstellar polarization similar to the downscaled one of SN 2006X (Cox & Patat, 2014). For the Galactic ISP, this – correlation may be interpreted as a narrowing in the size distribution with grain growth (e.g., Whittet, 2003). The shorter with these SNe suggests that the size of the dust grains polarizing light in the host galaxies is, on average, smaller than those in the Milkyway. This is also consistent with the smaller values of the total-to-selective extinction ratio, i.e., –, obtained for some reddened SNe Ia (Phillips et al., 2013, and references therein). If the empirical relation of Galactic ISP, (Serkowski, Mathewson, & Ford, 1975; Whittet, 2003), still holds for small , the observed values of – corresponds to –m, which is comparable with observed in SNe 1986G and 2006X.
In this Letter, we report our polarimetry of SN 2014J before and after the maximum light, along with our photometric and spectroscopic observations. Such multi-band polarimetry including NIR bands for reddened SNe Ia is still quite rare, and therefore this SN may provide us a valuable information on the interstellar and/or circumstellar media along the line of sight toward SN 2014J within M82.
2. Observations and Reduction
We performed imaging polarimetry of SN 2014J using Hiroshima One-shot Wide-field Polarimeter (HOWPol; Kawabata et al. 2008) on 2014 Jan 22.4 ( days relative to the -band maximum light; see §3.1) in bands and Hiroshima Optical and Near IR camera (HONIR; Akitaya et al. 2014) in bands on Jan 27.7 ( days), Feb 16.5 ( days), 25.6 ( days) and Mar 7.8 ( days). HOWPol employs a wedged double Wollaston prism, and is attached to the Nasmyth focus of the 1.5 m Kanata telescope at Higashi-Hiroshima Observatory. HONIR uses a cooled LiYF Wollaston prism and is attached to the Cassegrain focus of the same telescope. Each observation consisted of a sequence of exposures at four position angles (PAs) of the achromatic half-wave plates, , , , and for the HOWPol and the last HONIR observations, and at four PAs of the instrumental rotator at the Cassegrain focus of the telescope, , , and , for the first three HONIR observations. These data were calibrated using observations of unpolarized (HD 94851, HD 98281) and polarized standard stars (HD 30168, HD 150193, HDE 283701, Cyg OB 2 #11; Turnshek et al. 1990; Whittet et al. 1992), including measurements through a fully-polarizing filter or a wire grid. Using this procedure, the instrumental polarization (% in HONIR and –% in HOWPol) was vectorially removed.
In addition, we obtained photometry using HOWPol (), and HONIR () attached to the 1.5-m Kanata telescope, with MITSuME (Kotani et al., 2005) () attached to the 0.5 m telescope at Okayama Astrophysical Observatory (OAO) of National Astronomical Observatory of Japan, with a Peltier-cooled CCD () attached to the 0.51 m telescope at Osaka Kyoiku University (OKU), and with ISLE (Yanagisawa et al., 2008) () attached to the 1.88 m telescope at OAO, respectively. The magnitude in each band was determined relative to the nearby comparison star, BD+70587, which was flux-calibrated in bands using Landolt field stars (Landolt, 1992) on a photometric night. For NIR photometry, we used magnitudes of the same star in 2MASS Second Incremental Release Point Source Catalogue. We also collected low-resolution spectra with HOWPol (0.41–0.94 m, ) and HONIR (0.5–2.3 m, –) on the 1.5-m Kanata telescope. The flux was calibrated using observations of spectrophotometric standard stars obtained on the same nights.
Because SN 2014J is superimposed within the bright region of M82 and we cannot perform template subtraction in image reduction, the obtained flux should be more or less contaminated by the inhomogeneity and irregularity of the surface brightness of M82. However, the SN itself is sufficiently bright, and the polarizations obtained during the period from days to days from the maximum light should suffer minor effects from the galaxy light.
3.1. Photometric and Spectroscopic Properties
Figure 1 shows the obtained multi-band light curves (LCs). The apparent maximum magnitudes in bands are found to be mag on MJD 56690.4 (Feb 2.4) and mag on MJD 56691.7, respectively, as derived by a polynomial fit to the observed data around the maximum light. We also derived the observed -band magnitude decline rate mag. For the extinction toward SN 2014J, Amanullah et al. (2014) estimated the total reddening of mag and based on analysis of near-maximum-light spectral energy distribution from UV to NIR wavebands. This reddening is apparently dominated by the host galaxy component , because the IR Dustmap suggests that the Galactic component is only mag (Schlafly & Finkbeiner, 2011). We corrected for the extinction using the and values and the parameterized extinction curve (Cardelli, Clayton, & Mathis, 1989). The absolute magnitudes of mag and mag, as well as its color, are consistent with the empirical relations with within errors (e.g., Phillips et al., 1999), suggesting the photometric behavior in SN 2014J is not anomalous. We set the time of the band maximum as days, which is days after the epoch of the estimated first light (Zheng et al., 2014; Goobar et al., 2014).
Figure 2 shows a time series of spectra from days to days. Compared with the normal SN Ia 2011fe, SN 2014J is characterized by the absence of spectral features due to C ii 6580 and O i 7774, as well as the existence of high-velocity components in Ca ii IR triplet ( km s) during the earliest phase days, as pointed out by Goobar et al. (2014). The line velocity and equivalent width of Si ii 6355 around maximum ( days) are km s (Figure 2 inset panel) and Å, respectively, which are marginal between those of ‘Normal’ and ‘HV’ SNe (Wang et al., 2009). However, the relation between -corrected and yielded in SN 2014J ( mag for and mag for ) is apparently consistent with the branch of HV group (; Wang et al. 2009). The nearly constant absorption strength (up to months after discovery) of the interstellar Na i D lines and diffuse interstellar bands (e.g., Welty et al., 2014) indicate that the dust responsible for the extinction towards SN 2014J is located at a site moderately separated from the progenitor ( cm).
3.2. Polarimetric Properties
The observed polarization is shown in Figure 3. The polarization is relatively strong in blue bands, e.g., reaching % in band, and it decreases rapidly with wavelength, while the polarization PA is approximately constant at around . In NIR bands, the polarization is less significant (%); however, it is likely that the same polarization component still dominates because of having almost the same PA as the optical bands. There is no significant temporal variation in the polarization measured during the period from days to days from the maximum light, and the polarization in optical bands appears to be consistent with the result of spectropolarimetry covering wavelengths from 380 to 880 nm (Patat et al., 2014). Hereafter, we discuss only the averaged polarization (Table 1).
The large polarization measured for SN 2014J suggests that it is predominantly produced within M82, because the Galactic ISP is, at most, 0.18% according to the measurements of six Galactic stars in the vicinity of SN 2014J within of the all-sky polarization map (Heiles, 2000). Furthermore, the almost constant polarization during the period of our observation would exclude the possibility that it is originated in the close proximity to the progenitor. This, together with the unchanged interstellar absorption lines, favors that that the significant polarization of SN 2014J is caused by dust grains at a site remote from the SN. It has been suggested that multiple scattering due to circumstellar (CS) dust may account for half of the total extinction (Foley et al., 2014). However, we argue that the CS dust could not be the principal origin of the observed polarization because multiple scattering would effectively depolarize the light and the resulting continuum polarization would also show significant changes with time (see also Patat et al. 2014). In the optical image of M82 (e.g., Ohyama et al., 2002), PA of seems to align with the direction of the local dust lanes around the SN position, which further strengthens the argument that the polarization is unrelated to the CS matter.
Figure 3 shows that the polarization peak appears outside the wavelength range of our observations, i.e., m. This is considerably smaller than the typical value determined from the Galactic ISP. For comparison, we also plotted the polarization curves determined from the Serkowski law with/without the Wilking law in Figure 3. For the Serkowski law, we fitted it with a constant , a typical one for Galactic ISP (Serkowski, Mathewson, & Ford, 1975), because the fitted parameters do not converge in case of free . We cannot obtain any good fit with the Wilking law. In general, with the Wilking law, a short leads to small (corresponding to broader peak in curve); however, the observed steep gradient of requires a large value. The Wilking law also fails to describe the continuum polarization measured for SN 2006X (Patat et al., 2009). In addition, we find that and m derived for the SN 1986G data (Hough et al., 1987) do not satisfy the Wilking law. The fact that 5 out of 105 Galactic reddened stars show considerable ISP with m and the Wilking law holds for 4 of the 5 stars within the errors (Whittet et al., 1992) suggests that the failure of the Wilking law to describe the data may be common for highly reddened SNe Ia and the dust properties of the host galaxies may differ from those of the Milkyway.
Using only Serkowski law, we note that there is a systematic difference in the polarization of –% between the observed polarization and the fitted curve at longer wavelengths (m, Figure 3). This difference may be explained by considering an analog of the ‘IR polarization excess’ found in the Galactic ISP at longer wavelengths (m), which is characterized by an inverse power-law, i.e., (e.g., Nagata, 1990). The wavelength dependence of polarization of SN 2014J at m can be well described by with the index of (Figure 4). It should be noted that the polarization closely follows a power-law dependence even at the optical wavelengths for SN 2014J. For Galactic reddened stars, the index is typically in a relatively narrow range of 1.5–2.0 and is uncorrelated with the wavelength dependence of optical polarization, e.g., (Martin et al., 1992; Whittet, 2003). The index obtained for the ISP in M82 appears slightly steeper than that for the Galactic ISP. This may be related to the failure of the Wilking law, because the polarization peak observed in SN 2014J is clearly sharper than that expected for a Galactic ISP with a similar (see Figure 3).
As described above, SN 2014J is a highly reddened SN Ia, similar to SNe 1986G, 2006X and 2008fp. The blue continuum of these SNe Ia all exhibit significant polarization, which is atypical of a Galactic ISP. To a first-order approximation, a small corresponds to small mean size of the dust grains causing the observed polarization. This can be explained using Mie theory for dielectric cylinders, e.g., , where is the effective radius and is refractive index of the cylindrical grain (e.g., Whittet, 2003). Assuming m and (appropriate for silicates), then of the polarizing grains should be m. Although a small (and thus a small ) could be the result of a failure of alignment/asphericity only of larger grains, the small inferred for SN 2014J suggests that the effect is not significant and the grain size should be intrinsically small.
It is not known whether the empirical relation (m) determined for the Galactic ISP (Serkowski, Mathewson, & Ford, 1975; Whittet, 2003) holds in the host galaxies of these highly reddened SNe Ia; however, interestingly, it has been shown that is small for these SNe, e.g., for SN 1986G and for SN 2006X (Table 2 in Phillips et al., 2013) and the m obtained for SN 1986G (Hough et al., 1987) satisfies the empirical relation within the errors. For SN 2006X, is much smaller than that expected from m (i.e., ). Wang et al. (2008) derived a slightly larger value of from optical and NIR photometry of SN 2006X; however, this is still smaller than the expected value. It should be noted that is somewhat ambiguous because the observations did not cover the wavelengths shorter than 0.35 m (Patat et al., 2009), which makes it difficult to completely rule out the applicability of the empirical relation. For SN 2014J, (Amanullah et al., 2014), which leads to m, which is speculatively consistent with the result of our fit to Serkowski law with a constant . Therefore, although the Wilking law (i.e., – relation) no longer appears to be valid for extragalactic ISPs (see §3.2), the positive correlation of – may hold even for an ISP with m, at least, in part of the extragalactic environment. If this is indeed the case, the smaller values of seen in some moderately to highly reddened SNe Ia (e.g., Phillips et al. 2013) suggests that the ISP in the host galaxies has a smaller , and hence a smaller . The failure of the data to follow the Wilking law is harder to be explain using grain sizes only; a qualitative difference in the size distribution (and possibly in composition, shape, and degree of alignment) would be required for the dust grains between our Galaxy and those host galaxies of SNe Ia. A possible explanation for the atypical dust seen in the host galaxies of some SNe Ia is that the interstellar dust within our Galaxy is not in fact typical. The understanding of physical properties of extragalactic dust grains remains incomplete. Extensive data on the UV and optical polarimetry for SNe Ia may therefore be crucial for further understanding of the properties of extragalactic dust.
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|Band||(%)aaAveraged polarization and the position angle are shown (see §3.2). In bands the data are weighted means over five nights from days through days, and in other bands they are over four nights from days through days. The error is predominantly due either to the observational error () in bands or the uncertainty of the polarimetric calibration (instrumental polarization/depolarization) in bands.||PAaaAveraged polarization and the position angle are shown (see §3.2). In bands the data are weighted means over five nights from days through days, and in other bands they are over four nights from days through days. The error is predominantly due either to the observational error () in bands or the uncertainty of the polarimetric calibration (instrumental polarization/depolarization) in bands.||Band||(%)aaAveraged polarization and the position angle are shown (see §3.2). In bands the data are weighted means over five nights from days through days, and in other bands they are over four nights from days through days. The error is predominantly due either to the observational error () in bands or the uncertainty of the polarimetric calibration (instrumental polarization/depolarization) in bands.||PAaaAveraged polarization and the position angle are shown (see §3.2). In bands the data are weighted means over five nights from days through days, and in other bands they are over four nights from days through days. The error is predominantly due either to the observational error () in bands or the uncertainty of the polarimetric calibration (instrumental polarization/depolarization) in bands.|