Photometric and polarimetric observations of fast declining Type II supernovae 2013hj and 2014G

Abstract

1 INTRODUCTION

Core-collapse supernovae (CCSNe) originate from massive stars with M_ZAMS > 8 M_⊙ (Burrows 2013), which, at the end of the nuclear burning phase, having insufficient thermal energy, start to collapse under self-gravity. The massive core overcomes the electron degeneracy state, after which the process of neutronization takes place, releasing a large amount of energetic neutrinos that helps to drive the explosion. CCSNe are subclassified into hydrogen-rich (Type II) and hydrogen-deficient (Type I) SNe. The retention of a significant amount of hydrogen in SNe II at the time of the explosion is visible as characteristic Balmer emissions in their spectra.

CCSNe are further divided into various subtypes (see Filippenko 1997, for a review of SN classifications). Among these, the most common subtype is IIP, which constitutes ∼60 per cent of all CCSNe (Li et al. 2011). SNe IIP have an extended hydrogen envelope, which is left almost fully ionized at the time of shock breakout. As the expanding envelope cools down, the ionized envelope starts to recombine with the recombination front moving inward through the ejecta, which is responsible for a sustained plateau of almost constant brightness for ∼80–100 d. At the end of the recombination phase, SNe IIP experience a rapid drop in luminosity, settling on to a slow declining tail phase that is powered by gamma-rays emitted from the radioactive decay of ⁵⁶Co to ⁵⁶Fe, which in turn depends upon the amount of short-lived radioactive ⁵⁶Ni synthesized in the explosion.

According to the historical classification based on the light-curve properties (Barbon, Ciatti & Rosino 1979), the fast declining counterpart of Type IIPs are SNe IIL, which are supposed to show an almost linear decline in the light curve until it reaches the tail phase, without any sort of flattening during the early phases of the light curve. Over time, it has been realized that such events that could fit the criteria of SNe IIL are extremely rare. Studies by Anderson et al. (2014) and Sanders et al. (2015) on a large sample of SNe II disprove the existence of any bimodality in light-curve properties, which can be grouped into distinct subclasses. Rather, they found a continuum in the light-curve slopes and physical parameters. Moreover, it is now also believed that a steep declining plateau-nebular transition-like phase can be detected even in typical SNe IIL if they are observed long enough. The plateau slopes of SNe II light curves are primarily governed by the amount of hydrogen present in the ejecta. As in the case of SNe IIP, where the hydrogen content is high enough with an extended envelope, the energy deposited instantaneously from shock and also from the initial decay of ⁵⁶Ni is released slowly over a longer period of time, which sustains the flat plateau phase. In contrast, if the amount of intact hydrogen is relatively less, then energy will be released at a faster rate, resulting in a fast declining light curve but with a brighter initial peak luminosity. Considering the vast diversity of progenitor properties (Smartt 2009), a variable amount of hydrogen mass intact in the ejecta would give rise to a continuum distribution of light-curve slopes. Faran et al. (2014) proposed a criterion to re-establish SNe IIL classification, according to which a SN with a decline of 0.5 mag in the V-band light curve during the first 50 d would qualify as a Type IIL. However, this criterion is slightly too rigid for Type IIL classification as it would classify more than 50 per cent of all Type II SNe as Type IIL, considering the sample of Anderson et al. (2014). Therefore, rather than using this criterion, we use the SNe IIL light-curve template presented by Faran et al. (2014) for the purpose of classification. It should also be noted that, even by following the scheme of template matching, many of the earlier studies that comprise the collective sample of SNe IIP might also include some reclassified SNe IIL.

There have been numerous theoretical and numerical attempts to understand progenitor properties and to relate them to other observable parameters of explosion (Litvinova & Nadezhin 1985; Arnett & Fu 1989; Hamuy 2003). Stellar evolutionary models suggest that these SNe can originate from red supergiant (RSG) stars of masses ∼9–25 M_⊙ (e.g. Heger et al. 2003). However, progenitors directly identified from pre-SN archival Hubble Space Telescope (HST) images for a decent number of IIP SNe suggest an upper mass limit of 16.5 ± 1.5 M_⊙ (Smartt et al. 2009). One of the many proposed explanations for the lack of high-mass RSG progenitors is that stars above 17 M_⊙ might end up as SNe IIL or IIn (Smartt 2009).

Extragalactic SNe are spatially unresolvable and hence it is difficult to address the structure and geometry of explosions. Polarimetric observations have proved to be a valuable probe when investigating the asymmetry of SN explosions. In SNe, the source of polarization is primarily the geometry of the electron scattering atmosphere within the ejecta. Any asymmetry in this envelope would give rise to net polarization of observed light. CCSNe are often found to show a significant degree of polarization in optical and infrared (IR) wavelengths (Wang & Wheeler 1996; Leonard et al. 2001; Leonard & Filippenko 2001; Wang et al. 2002a; Pereyra et al. 2006). SNe with a hydrogen envelope stripped (Type Ib/c) or partially stripped (Type IIb or IIn) exhibit a higher level of polarization compared to SNe with an intact H envelope, such as SNe IIP (typically about 0.5–0.8% polarization). The thick H envelope obscures the observed asymmetry, whereas probing deeper towards the central part of the explosion, more polarization is observed, which implies a higher degree of asymmetry in the electron scattering atmosphere (Leonard & Filippenko 2005). However, in SNe IIP, polarization enhancement is observed towards the end of the plateau as the SN starts to enter the nebular phase, where the hydrogen recombination is close to completion and, with decreasing opacity, the inner asymmetric core is being revealed (e.g. SN 2004dj; Leonard et al. 2006). Generally, a moderate asphericity of ∼20 per cent would result in a linear polarization of ∼1% (Leonard & Filippenko 2005). Besides asphericity in the electron scattering atmosphere, some amount of polarization might also originate from scattering by the dust environment (Wang & Wheeler 1996), clumps in ejecta, the asymmetric distribution of radioactive ⁵⁶Ni (Chugai 2006) or the asymmetric ionization of the outer envelope due to shock produced in the circumstellar medium (CSM) interaction.

The first broad-band polarimetric observation was done for SN 1987A (Barrett 1988), demonstrating significant polarization and its time evolution. Late-time HST observations of spatially resolved ejecta revealed elongation in a direction consistent to the polarization angle inferred from polarimetry (Wang et al. 2002b), which validated our understanding of the use of polarimetry as a probe for SN explosion geometry. SNe 1999em and 2004dj are extremely well studied and observed spectropolarimetrically, which provides detailed insight on the nature of these explosions (Leonard et al. 2001, 2006). While, on the one hand, spectropolarimetry has the advantage of wavelength-resolved polarization information, on the other hand, broad-band polarimetry is useful to extract overall polarimetric information and to infer any asymmetry without the requirement of bright SNe or powerful observational resources.

In this paper, we present broad-band photometric and polarimetric observations of two fast declining Type II SNe 2013hj and 2014G. A brief introduction of these events is given in the following subsection. The rest of the paper is organized as follows. In Section 2, we describe the photometric and polarimetric observations and the data reduction procedure. The adopted distance and estimated extinction values for both SNe are discussed in Section 3. In Section 4, we analyse the photometric light-curve properties of SNe 2013hj and 2014G and compare with other SNe II to establish its position in Type II diversity. In this section, we also estimate ⁵⁶Ni mass from computed bolometric light curves. The explosion parameters and progenitor properties are estimated by modelling the light curve, which are described in Section 5. We discuss the polarimetric analysis and its temporal evolution in Section 6. Finally, in Section 7, we summarize the results obtained in this work.

SNe 2013hj and 2014G

Both SNe 2013hj and 2014G are moderately bright events that have been detected very young. SN 2013hj was discovered on 2013 December 12.3 utc, by Antezana et al. (2013) in the galaxy MCG-02-24-3 (∼30 Mpc). Unfortunately, there is no reported non-detection prior to SN explosion, which can be used to observationally constrain the explosion epoch. The classification spectrum from Asiago on 2013 December 13.1 utc shows broad and very shallow Hα and Hβ P-Cygni profiles on top of a featureless blue continuum. Comparison of this spectrum with well-studied SNe templates, using gelato (Harutyunyan et al. 2008) and snid (Blondin & Tonry 2007), is found to be similar to Type II SNe and matches best with the SN 2012A spectrum at 2.6 d after the explosion. Moreover, from visual comparison with other SNe II spectra (e.g. Valenti et al. 2014; Bose et al. 2015b) at early epochs, the December 13 spectrum looks no older than 4 d post-explosion. Therefore, we adopt an explosion epoch (0 d) of 2013 December 10.5 utc (JD = 245 6637.0 ± 1.5 d) for SN 2013hj.

SN 2014G was discovered on 2014 January 14.5 utc in NGC 3448 (∼24 Mpc) and was reported in Central Bureau for Astronomical Telegrams (CBAT) Transient Object Follow-up Reports.¹ They also reported an earlier faint detection (∼17.4 mag) in the CCD image taken on January 13.6 utc with a limiting magnitude of 18.5. Denisenko et al. (2014) reported the last confirmed non-detection of the SN on 2014 January 10.9 utc up to a limiting magnitude of 19.4. Thus, we adopt the mid-time between the first faintest detection and the last non-detection as the explosion epoch (0 d) for SN 2014G, which is 2014 January 12.2 utc (JD = 245 6669.7 ± 1.4 d). Ochner et al. (2014) reported an optical spectrum taken on 2014 January 15.1 utc, showing the H i line with narrow Hα emission and an unusually strong He ii line, which was found to be similar to Type IIn SN 2013cj. However, it was again reclassified as Type IIL by Eenmae et al. (2014) from a spectrum taken several weeks after the peak. The basic parameters of these two SNe and their host galaxies are tabulated in Table 1.

2 OBSERVATION AND DATA REDUCTION

2.1 Photometry

Broad-band photometric observations of SNe 2013hj and 2014G have been carried out in UBVRI bands. Rigorous follow-up observations for both SNe were initiated soon after their discovery. The observations in the nebular phase continued until the objects went behind the Sun. High cadence monitoring was carried out using the ARIES 104-cm Sampurnanand Telescope (ST) and the 130-cm Devasthal Fast Optical Telescope (DFOT) located at Nainital, India. Additional BVRI data of SN 2013hj were also collected from the 50-cm telescope at Osaka Kyoiku University, Japan. The Swift ultraviolet optical telescope (UVOT) also observed SN 2014G for nearly a month in all six UVOT bands.

The routine data cleaning, reduction, extraction of light curve and field calibration to generate local standards follows the same procedure as outlined in Bose et al. (2013). Figs 1 and 2 show the fields of SNe 2013hj and 2014G, respectively. As seen in the figures, both SNe are fairly embedded in their host galaxies and the estimated SN flux measurements will be contaminated by the background, especially during late phases. It is therefore necessary to correct for the host galaxy fluxes and in order to do so galaxy templates have been observed. Template images of the host galaxies in all five photometric bands were taken during 2015 May 7–10, when the SNe had become much fainter below the detection limit (at an age of ∼515 and ∼480 d for SN 2013hj and SN 2014G, respectively). In order to do the template correction, we adopted the aperture flux subtraction method, in which the SN flux and host galaxy background flux (from the template image) are extracted using the same aperture and at the exact same location. The SN fluxes are then subtracted by the estimated background flux, resulting in the true SN flux free from galaxy contamination. This entire process is achieved with a set of self-written scripts, which employs daophot within it. In principle, the technique of template image subtraction should be adopted for the best results (Alard & Lupton 1998; Alard 2000). However, in this technique, there are a number of shortcomings with data obtained from small ground-based telescopes. The stellar point spread function (PSF) can often be degraded in terms of minor elongation and skewness due to a defect in passive telescope optics, which can further vary depending on the telescope's altitude-azimuth. In such cases, the PSF matching becomes non-trivial and can leave behind significant and arbitrary residual in flux, resulting in larger errors in photometry. In our case, such defects are prevailing in several images when the SNe were observed at high zenith angles.

Figure 1.

SN 2013hj in MCG-02-24-3. The V-band image taken from the 104-cm ST covering a subsection of about 11 × 11 arcmin² is shown. The secondary field standards are marked with circles.

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Figure 2.

SN 2014G in NGC 3448. The V-band image taken from 104-cm ST covering a subsection of about 11 × 11 arcmin² is shown. The secondary field standards are marked with circles.

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SN 2014G was also observed with UVOT (Roming et al. 2005) on board the Swift spacecraft (Gehrels et al. 2004) in all six UVOT bands (uvw2, uvm2, uvw1, uvu, uvb, uvv). The UVOT photometry was obtained from the Swift Optical/Ultraviolet Supernova Archive (SOUSA; Brown et al. 2014). The reduction procedure for UVOT photometry is based on Brown et al. (2009), which includes subtraction of background galaxy counts, and the adoption of the revised zero-points and time-dependent sensitivity from Breeveld et al. (2011). The UVOT photometry for SN 2014G is presented in Table A3.

2.2 Polarimetry

Broad-band polarimetric observations in the R band have been carried out for SNe 2013hj and 2014G using the ARIES Imaging Polarimeter (AIMPOL; Rautela, Joshi & Pandey 2004) mounted at the 104-cm ST. The unvignetted field of view of the CCD image is ∼8 arcmin in diameter. The stellar FWHM is found to vary within two to three pixels, and CCD read-noise and gain are 7.0 e⁻and 11.98 e⁻ADU⁻¹ respectively.

Along with SNe, a number of polarized and unpolarized standards (Schmidt, Elston & Lupie 1992) have also been observed to estimate instrumental polarization and to correct the zero-point polarization angle, which mainly originate from imperfection in the manual alignment of the instrument. For the polarized standards, the degree of polarization P measured from the instrument is found to be consistent within the limit of errors and any offset found in the measured and standard values of the polarization angle θ is corrected in the measurements for SNe. Using unpolarized standards, the instrumental polarization is found to vary within 0.03 to 0.10%, which is consistent with that found by other authors for AIMPOL (e.g. Rautela et al. 2004; Eswaraiah et al. 2013). Because the direction of polarization is not confined to any particular plane and the value itself is very small (within error limits), we have not subtracted this from our observational measurements.

To obtain an estimate of the polarization towards the direction of SNe 2013hj and 2014G due to Galactic dust, we also observed a number of bright field stars (on 2015 April 22) within an average radius of |${\sim }5\deg$| around corresponding targets. The V-band polarization observations of these bright field stars are also presented in Heiles (2000). We have observed 11 and 3 field stars in the direction of SNe 2013hj and 2014G, respectively.

The observed polarization measurements for SNe 2013hj and 2014G are listed in Table B1 and those for field stars are tabulated in Table B2.

3 DISTANCE AND EXTINCTION

Distance and total line-of-sight extinction are the two parameters that are important to study the intrinsic properties of SNe. Thus, it is essential to determine these parameters for each of the SNe, before we characterize and analyse them.

SN 2013hj

MCG-02-24-3 hosts SN 2013hj at the edge of the projected view of the diffused spiral galaxy. As there is no redshift-independent distance available in the literature, we adopt a redshift distance of 28.2 ± 2.0 Mpc from Hyperleda² (Makarov et al. 2014) corrected for Virgo infall velocity and assuming H₀ = 70 km s⁻¹ Mpc⁻¹, Ω_m = 0.27 and |$\Omega _\Lambda =0.73$|⁠. However, there are well-accepted techniques that utilize the observations of a Type II SN itself to determine its distance. epm (Kirshner & Kwan 1974; Jones et al. 2009; Bose & Kumar 2014) and scm (Hamuy & Pinto 2002; Olivares et al. 2010) are two such methods that have been successfully implemented on several SNe II to estimate redshift-independent distances. However, these also require extensive spectroscopic information, which restricts us from implementing these methods on both of these SNe.

SN 2014G

SN 2014G is hosted in the galaxy NGC 3448. From previous studies, only a single redshift-independent distance estimate is available using Tully–Fisher method (Tully & Fisher 1988) for the galaxy, which is 24.4 ± 9.0 Mpc. We adopt this distance for SN 2014G throughout the paper.

The value of reddening due to the Milky Way towards NGC 3448 is found to be E(B − V)_MW = 0.010 ± 0.000 mag from the all-sky Galactic dust-extinction survey (Schlafly & Finkbeiner 2011). To estimate the host galaxy extinction for SN 2014G, we implement the colour method (Olivares et al. 2010) as described in the case of SN 2013hj. Following their prescription, using (V − I) = 0.956 ± 0.038 mag (interpolated at 56 d after correcting for Galactic extinction) in equation (5), we obtain host visual extinction |$A_{V_{\rm host}}=0.755\pm 0.222$| mag. Thus, summing up, we adopt a total extinction of A_V = 0.787 ± 0.222 mag, corresponding to a reddening value E(B − V) = 0.254 ± 0.072 mag.

4 OPTICAL LIGHT CURVE

4.1 Apparent magnitude light curves

SN 2013hj

UBVRI broad-band photometric observations of SN 2013hj are available at 117 phases from 3 to 170 d. The data coverage is dense and almost continuous during this span. The resultant light curves are shown in Fig. 3.

Figure 3.

Photometric light curve of SN 2013hj in Johnson–Cousins UBVRI bands. The light curves are shifted for clarity as indicated on the plot.

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The early light curves show a sharp rise in RI bands, which is also visible in BV bands but relatively much flatter. After the initial peak, the light curves decline slowly until the plateau ends. SN 2013hj does not show a very sharp transition in the slope at the end of the plateau, as is seen in generic SNe IIP, such as SNe 1999em (Leonard et al. 2002a), 1999gi (Leonard et al. 2002b), 2004et (Sahu et al. 2006) and 2013ab (Bose et al. 2015b). Rather, the light curve shows linearity only up to ∼80 d, and thereafter it shows a gradual increase in slope until it reaches the end of plateau at around ∼96–100 d. At the end of the plateau, the light curve declines steeply to settle on to a relatively slow declining radioactive tail at ∼118 d, which continues until the end of our observations.

The light-curve parameters of SN 2013hj are listed in Table 2 (rows 1–3). The times of the peak in early light curves (Table 2, row 1) are consistent with most typical and common fast-rising SNe IIP such as 2005cs (Pastorello et al. 2009), 2004et (Sahu et al. 2006) and 2013ab (Bose et al. 2015b), which is significantly faster than slow-rising SNe, such as SNe 2006bp (Quimby et al. 2007), 2009bw (Inserra et al. 2012) and 2012aw (Bose et al. 2013). After the initial peak, the light curves continue to decline during the entire plateau phase, but in the R and I  bands a break in the slope is visible at 23 and 24 d respectively. This break is possibly the mark of the emergence of recombination phase, as the shock-heated ejecta is cooling down and recombination starts to power the light curve. Such a feature, but more prominent, was also seen in VRI bands of the Type IIP SN 2012aw (Bose et al. 2013).

Since the initial peak SN 2013hj continues to decline, but does not show a constant slope towards the plateau end. During 80–100 d, a gradual increase in the light-curve slope can be seen. The plateau slope for the linear part is listed in Table 2 (row 2). This is steeper than the values reported for generic Type IIP SNe 1999em (Leonard et al. 2002a), 1999gi (Leonard et al. 2002b), 2012aw (Bose et al. 2013) and 2013ab (Bose et al. 2015b), but flatter than the fast declining SNe such as SN 2013by (Valenti et al. 2015), SN 2013ej (Bose et al. 2015a) and SN 2014G (this paper). For example, decline rates for SN 2012aw are 5.60, 1.74 and 0.55 mag (100 d)⁻¹ in UBV bands, and for SN 2013ab decline rates are 7.60, 2.72, 0.92, 0.59 and 0.30 mag (100 d)⁻¹ in UBVRI bands, whereas SN 2013ej decline rates are 6.60, 3.57, 1.74, 1.07 and 0.74 mag (100 d)⁻¹. Decline rates for the radioactive tail phase in VRI bands are listed in Table 2 (row 3), where U- and B-band slopes were not computed due to large errors in tail data points.

SN 2014G

Photometric measurements in UBVRI bands for SN 2014G were made at 63 phases during 4–319 d, where continuous and high-cadence observations were only during 4–166 d, with a late nebular data point at 319 d recovered only after galaxy background subtraction. Additionally, 15 phases of UVOT observations during 2–27 d are also available in all six UVOT bands (uvw1, uvw2, uvm2, uvu, uvb and uvv). The light curves are shown in Fig. 4.

Figure 4.

Photometric light curve of SN 2014G in Johnson–Cousins UBVRI and Swift UVOT bands. The light curves are shifted for clarity as indicated on the plot.

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The early parts of the light curves show a sharp rise in most of the optical bands (BVRI and uvb, uvv). The rise is detectable in UV bands as well except for uvm2 and uvw2 bands. After the initial peak, all the light curves decline linearly until the plateau end (∼77 d), like a generic SNe IIL (SN 1980K, Barbon, Ciatti & Rosino 1982; SN 2013by, Valenti et al. 2015). From ∼77 d, the light curve declines steeply to enter the relatively slow declining radioactive tail phase starting from ∼90 d.

The light-curve parameters for SN 2014G are tabulated in Table 2 (rows 4–6). The early rising of the SN 2014G light curve (Table 2, rows 4) is consistent with slow rising SNe, for example, the V-band maximum is delayed to 16 d in SN 2006bp (Quimby et al. 2007), 13 d in SN 2009bw (Inserra et al. 2012) and 15 d in SN 2012aw (Bose et al. 2013). This is significantly slower than most typical Type II SNe, such as SNe 2005cs, 2004et and 2013ab or the other SN that we present here (i.e. SN 2013hj).

After reaching maxima, SN 2014G light curves in all bands show linear decline throughout the plateau and their decline rates are listed in Table 2 (rows 5). These values are similar to that of Type IIL SNe such as SN 1980K and SN 2013by, but much steeper than all Type IIP SNe (decline rates as mentioned above for SNe 2012aw and 2013ab). For example, SN 2013by (Valenti et al. 2015) shows a decline rate of 3.62, 2.01, 1.42 and 1.26 mag (100 d)⁻¹ in BVRI bands and 19.53, 20.54 and 15.26 mag (100 d)⁻¹ in uvw2, uvm2, uvw1 bands. The decline rates for radioactive tail light curves are also listed in Table 2 (rows 6). The slope of the radioactive tail light curve is also found to be steeper than normal SNe IIP. For example, SN 2013ab tail slopes are 0.36, 0.97, 0.66 and 1.16 mag (100 d)⁻¹ in BVRI, and in SN 2012aw tail slopes are 0.88, 0.88, 0.81 and 0.95 mag (100 d)⁻¹ in BVRI). However, it is very similar to that found in SN 2013ej, where tail slopes are 1.22, 1.53, 1.42 and 1.55 mag (100 d)⁻¹ in BVRI bands (Bose et al. 2015a).

4.2 Absolute magnitude light curves

The absolute V-band (M_V) light curves of SNe 2013hj and 2014G are shown in Fig. 5 and are compared with a moderately sized sample of SNe II. In order to determine the light-curve characteristics of these two SNe and to mark their position in the Type II diversity, we selected the comparison sample with a diverse range of properties, comprising archetypal Type IIP SNe 1999em, 1999gi and 2012aw to prototypical Type IIL SN 1980K. The plateau slopes for the whole SNe II sample shown in Fig. 5 are listed in Table 4. This table and this figure are primarily adapted from Bose et al. (2015a).

Figure 5.

Absolute V-band light curves of SNe 2013hj and 2014G are compared with other SNe II. The exponential decline of the tail light curve following the radioactive decay of ⁵⁶Co → ⁵⁶Fe is shown with a dashed line. On the bottom left side, two pairs of dotted lines in grey and green represent the slope range for Type IIP and IIL SNe templates as presented by Faran et al. (2014). The adopted JD, distance, E(B − V) and reference for the V-band magnitude are listed in Table 3.

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The mid-plateau absolute magnitude for SN 2013hj is highest amongst the compared sample of events. However, it should be noted that the uncertainty in adopted reddening and distance values may alter this inference to some extent. The plateau decline rate of SN 2013hj is higher than most generic SNe IIP (e.g. SN 1999em, SN 2005cs, SN 2012aw) but lower than SNe IIL 1980K or 2000dc. According to Faran et al. (2014), SNe II with a plateau decline of at least 0.5 mag in the first 50 d would be classified as Type IIL, which would definitely classify SN 2013hj as Type IIL. However, this criterion proposed by Faran et al. (2014) is slightly too rigid, as it would reclassify all SNe up to slopes of SN 2012A (see Table 4) as Type IIL. Faran et al. (2014) also presented the template light-curve range for SNe IIP and IIL, which is shown in Fig. 5. The plateau slope for SN 2013hj is marginally smaller than the minimum slope for the SNe IIL templates but it is much larger than the steepest slope for the SNe IIP templates. Thus, if SN 2013hj has to be classified either between Type IIP or IIL, it would fit best in the class of Type IIL rather than IIP. The closest comparison to SN 2013hj in terms of plateau slope, shape and duration is SN 2013ej. However, the plateau decline of SN 2013ej is slightly steeper than SN 2013hj, making it compatible to SNe IIL light-curve templates.

However, because of the fast decline of SN 2014G, its mid-plateau magnitude is lower than SN 2013hj, but the peak luminosity is highest amongst the compared sample. The decline rate is higher than all SNe II in the sample, except for prototypical Type IIL SN 1980K and SN 2000dc. The plateau slope of SN 2014G is marginally over the SNe IIL template, thus clearly qualifying as a Type IIL SN. SN 2013by is the closest comparison to SN 2014G in terms of light-curve shape and magnitudes except for the fact that the SN 2014G plateau is steeper and the nebular tail is brighter. The decline rate of the tail light curve is 1.57 mag (100 d)⁻¹, which is higher than that expected for a light curve powered from the radioactive decay of ⁵⁶Co to ⁵⁶Fe. However, such a steepening of the tail light curve was also seen in other Type IIL SNe. For example, SN 2013ej exhibits a steeper decline rate of 1.53 mag (100 d)⁻¹ (Bose et al. 2015a), and similarly other SNe IIL, 1980K and 2013by, also show steeper declines like SN 2014G (see Fig. 5). In Fig. 6, we show the Swift UVOT absolute magnitude light curves for SN 2014G in UV bands and compare with other SNe II, which are well observed by UVOT. The UV light-curve evolution of SN 2014G is found to be similar to other SNe. The peak brightness closely matches that of SN 2013by but is higher than most other SNe in the sample.

Figure 6.

Swift UVOT UV absolute light curves of SN 2014G compared with other well-observed SNe II from UVOT. For the compared SNe, references for UVOT data, extinction and distance are: SN 2005cs – Brown et al. (2009) and Pastorello et al. (2009); SN 2006at – Brown et al. (2009); Distance 65 Mpc; E(B − V) = 0.031 mag (only Galactic reddening; Schlafly & Finkbeiner 2011); SN 2006bp – Dessart et al. (2008); SN 2012aw – Bayless et al. (2013) and Bose et al. (2013); SN 2013by – Valenti et al. (2015); SN 2013ej – (Bose et al. 2015a).

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Based on the light-curve parameters, SN 2014G is a perfect example of a generic Type IIL SN, whereas SN 2013hj belongs to an intermediate class with a plateau slope close but lower than typical SNe IIL. With the adopted reddening and distance values, both SNe lie on the luminous end of the comparison sample, which may eventually fall in the range of normal luminosity if reddening or distance values are lowered. Anderson et al. (2014) found an anticorrelation between the plateau slope and duration, which is consistent with the trend we see for SNe 2013hj and 2014G as well as other SNe in the sample.

4.3 Bolometric light curve

Pseudo-bolometric light curves for SNe 2013hj and 2014G are computed using photometric fluxes corrected for adopted reddening and distance values. We follow the same method as outlined in Bose et al. (2013) to compute bolometric luminosities, which involves semi-deconvolution of the broad-band filter response from photometric fluxes. Thermal emission from SNe mostly peaks in optical and near-optical wavelengths depending on the phase of SN evolution. At early phases (<30 d) when the SN is hot, UV wavelengths dominate bolometric luminosity. Likewise, at late phases (>100 d) when the SN is cool enough, bolometric light is dominated by IR fluxes. We compute pseudo-bolometric luminosity for both SNe in the optical domain, which include UBVRI fluxes. For SN 2014G, we also computed bolometric luminosity by taking UV fluxes into account, which include fluxes from uvw2 to I bands. The UV+optical luminosity yields a significantly higher value at early times. In Fig. 7, we compare bolometric light curves of SNe 2013hj and 2014G with other well-studied SNe. The bolometric light curves of the comparison sample were also computed using the same method and within the same wavelength range.

Figure 7.

UBVRI pseudo-bolometric light curves of SNe 2013hj and 2014G compared with those of other well-studied SNe. Light curves with added Swift UVOT UV contributions are also shown for SN 2013ej, SN 2013ab and SN 2012aw (labelled UVO). The adopted distances, reddening and time of explosion values are listed in Table 3. The exponential decline of the radioactive ⁵⁶Co decay law is shown with a dashed line.

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Bolometric light of SN 2013hj suffers almost a linear decline during 6–100 d with a luminosity drop of ∼0.85 dex. This decline rate is similar to SN 2013ej, but higher than most generic SNe IIP in the sample. An additional drop of 0.47 dex is seen at the end of plateau while the light curve settles on to the radioactive nebular tail phase after 118 d. The slope of the tail light curve is similar to those of SNe 1987A, 1999em, 2004et and 2013ab, which is consistent with that expected for the ⁵⁶Co to ⁵⁶Fe radioactive decay rate.

SN 2014G shows a decline rate higher than all compared SNe including SN 2013hj. SN 2014G shows a linear decline throughout the plateau phase and experiences a drop of 0.89 dex in bolometric luminosity during 13–80 d. A drop of 0.49 dex in luminosity is seen during the transition from the plateau to the nebular phase. The SN 2014G light curve settles on to the relatively slow declining radioactive tail phase after 92 d. The slope during this phase is found to be steeper, 0.39 dex (100)⁻¹d, than other generic SNe IIP (e.g. SNe 1987A, 1999em, 2004et, 2013ab), and also that expected for a light curve powered by the radioactive decay of ⁵⁶Co to ⁵⁶Fe. On careful examination of the absolute V-band and bolometric light curve (see Figs 5 and 7), such steepening of the tail light curve is also seen in other Type IIL SNe 1980K, 2013by and 2013ej. The steepening of the tail light curve would indicate inefficient trapping of gamma-rays in the ejecta. In SNe IIL, the hydrogen envelope is relatively more depleted than the IIP counterparts, resulting in lower gamma-ray optical depth that causes leakage of gamma photons. Such a scenario may explain the steeper tail light curve in SN 2014G, and this may also be true for all other fast declining SNe IIL. However, due to fast decaying brightness, there are only a handful of SNe IIL with observations extending up to the tail phase, and SN 2014G is the newest among such candidates.

4.4 Mass of nickel

In CCSNe, radioactive ⁵⁶Ni is produced by explosive nucleosynthesis at the time of explosion. The nebular phase light curve is mainly powered by the radioactive decay chain of ⁵⁶Ni to ⁵⁶Co and ⁵⁶Co to ⁵⁶Fe, with an e-folding lifetime of 8.8 and 111.26 d, respectively. Thus, the tail luminosity would be proportional to the amount of radioactive nickel synthesized at the time of explosion.

SN 1987A is one of the most well-studied SNe with a fair degree of accuracy in the estimation of ⁵⁶Ni mass (0.075 ± 0.005 M_⊙; Arnett 1980). By comparing the tail luminosity of a SN with that of SN 1987A, we can also estimate the mass of ⁵⁶Ni for that SN. Although, in principle, true bolometric luminosities (including UV, optical and IR) are to be used to compute ⁵⁶Ni mass, we do not have IR fluxes for either SN 2013hj or SN 2014G. For the uniformity in comparison, we also use UBVRI bolometric luminosity for SN 1987A, computed using the same method and wavelength range. We compute the ratio of SN 2013hj to SN 1987A luminosity at 150 d to be 1.152 ± 0.168. This corresponds to a ⁵⁶Ni mass of M_Ni(2013hj) = 0.086 ± 0.013 M_⊙. Similarly, the ratio of SN 2014G to SN 1987A luminosity at 150 d is 0.735 ± 0.552, which corresponds to M_Ni(2014G) = 0.055 ± 0.041 M_⊙.

The amount of ⁵⁶Ni masses estimated from both methods are consistent with each other within errors. Thus, we adopt a mean value of both results, which is M_Ni(2013hj) = 0.080 ± 0.008 M_⊙ for SN 2013hj and M_Ni(2014G) = 0.050 ± 0.027 M_⊙ for SN 2014G. Although the quoted errors in ⁵⁶Ni mass include uncertainty from adopted reddening and distance values, these do not take care of selection bias involved in these parameters. Estimated ⁵⁶Ni masses may be lowered if lower values of reddening or distance are assumed.

5 LIGHT-CURVE MODELLING

We model the bolometric light curves of SNe 2013hj and 2014G following the semi-analytical approach originally developed by Arnett (1980) and further refined in Arnett & Fu (1989). Such simplistic models (e.g. Arnett 1980, 1982; Arnett & Fu 1989; Popov 1993; Zampieri et al. 2003; Chatzopoulos, Wheeler & Vinko 2012) are useful to obtain a preliminary yet reliable estimate of explosion parameters without running more accurate but resource intensive hydrodynamical models (e.g. Falk & Arnett 1977; Utrobin 2007; Bersten, Benvenuto & Hamuy 2011; Pumo & Zampieri 2011). The model has been implemented by Nagy et al. (2014) and Bose et al. (2015a) for several SNe II to estimate explosion parameters and are found to be fairly consistent with results from hydrodynamical models.

To implement the model, we require true bolometric luminosities for the SNe. Because our data are limited only to UV and optical, we add IR flux contribution assuming SNe 2013hj and 2014G have similar optical to IR flux ratio as observed for SN 1999em at similar phases. Fig. 8 shows the computed bolometric light curves of SNe 2013hj and 2014G fitted with our model. Because these models are inefficient to reproduce post-breakout peaks of early light curves, we restricted the data only to the later part of the photospheric phase.

Figure 8.

Observed bolometric light curves of SNe 2013hj and 2014G are shown with the best-fitting model.

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From the best-fitting model of SN 2013hj, we estimate an ejecta mass of 9.6 M_⊙, a progenitor radius of 700 R_⊙ and a total explosion energy ∼2.1 foe. The mass of radioactive ⁵⁶Ni incorporated in the model is 0.075 M_⊙, which is consistent with the value estimated in Section 4.4. Assuming the mass of the compact remnant to be 1.5–2.0 M_⊙, the total progenitor mass adds up to ∼11 M_⊙.

Similarly for SN 2014G, the model-estimated ejecta mass is 7.0 M_⊙, the progenitor radius is 630 R_⊙ and the total explosion energy is ∼2.1 foe. The ⁵⁶Ni mass determined from the model is 0.052 M_⊙, which is almost equal to the value estimated in Section 4.4. Adding up a mass of 1.5–2.0 M_⊙ for the compact remnant, the total progenitor mass is ∼9 M_⊙. To fit the steeper tail light curve of SN 2014G, as already discussed in Sections 4.1 and 4.3, we increased the gamma-ray leakage in the model by lowering the effectiveness of the gamma-ray trapping parameter A_g to 4 × 10⁴ d². This parameter is related to gamma-ray optical depth as τ_g ∼ A_g/t².

6 BROAD-BAND POLARIMETRY

Broad-band polarimetric observations in the R band have been carried out for SN 2013hj at five phases during 23–86 d, whereas for SN 2014G only three phases of observations during 12–25 d are available. Fig. 9 plots the observed temporal evolution of P and θ – dashed lines indicate measurements that have not been corrected for any interstellar polarization (ISP) – for both SNe, along with the R-band absolute light curve (data are tabulated in Table B1).

Figure 9.

Degree of polarization P, polarization angle θ and absolute R-band magnitude M_R are plotted in three panels for SNe 2013hj and 2014G. The dashed lines in the polarization plots represent the observed polarization parameters, whereas the solid lines represent polarization parameters after subtracting for ISP_MW. The horizontal dotted lines in the top panel represent the maximum amount of host galaxy polarization (P_max < 9 × E(B − V); Serkowski, Mathewson & Ford 1975) component that could be present in SN polarization values (0.50 and 2.20% for SNe 2013hj and 2014G respectively; see Section 6.1.2), which may be linearly subtracted if the polarization angles of host and SN are co-aligned. The R-band light curves are truncated to only show phases where polarimetric data exist.

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6.1 Estimation of interstellar polarization

The observed polarization in SN light primarily has three components: intrinsic polarization due to the SN itself, interstellar polarization due to the Milky Way (ISP_MW) and interstellar polarization due to the host galaxy (ISP_HG). Thus, in order to interpret the intrinsic polarization of SNe, we need to subtract ISP components from the observed polarization. However, there is no completely reliable method to derive ISP and this poses an issue in SN polarimetric studies. A number of methods have been proposed to estimate ISP (Trammell, Hines & Wheeler 1993; Tran et al. 1997; Wang et al. 2001), but all involve some assumptions that are not always valid (Wang & Wheeler 2008). Having this limitation, the best possible attempt has been made to make a reasonable estimate of ISPs, as discussed in the following subsections.

6.1.1 Milky Way

Thus, we obtain median ISP_MW as P_MW = 0.13 ± 0.06% and |$\theta _{\rm MW}=12.4\pm 10.3\deg$| for SN 2013hj, while for SN 2014G, P_MW = 0.08 ± 0.22% and |$\theta _{\rm MW}=106.6\pm 68.5\deg$|⁠. These ISP_MW values are then vectorially subtracted from the observed SN polarization; they are listed in Table B1 and plotted in Fig. 9 (solid lines).

According to Serkowski et al. (1975), the ISP efficiency due to Galactic dust is highly correlated to line-of-sight reddening, which may vary from three to nine times the reddening value. As a reasonable approximation, we adopt the mean polarization efficiency as P_mean = 5 × E(B − V). Using this relation and Galactic reddening values adopted (refer Section 3) towards the direction of each SN, we obtain the P_mean values of ∼0.23 and ∼0.05% for SNe 2013hj and 2014G, respectively. These are roughly consistent with the ISP_MW values we computed from observations of field stars, which implies that the Galactic ISM follows the mean polarization efficiency.

6.1.2 Host galaxy

For an accurate estimation of ISP_HG, we need to know dust grain properties and magnetic field orientation along the line of sight within the host galaxy. In context to a few SNe studies, it has been found that the sizes of dust grains in the respective host galaxy were dissimilar to Galactic dust grains, for example, SN 1986G (Hough et al. 1987) and SN 2001el (Wang et al. 2003). Because of the very diverse nature of dust grains in various galaxies, it is non-trivial to adopt mean polarization efficiency without knowledge of the grain properties. However, in order to get an idea of ISP_HG and for the sake of understanding its possible implication in SN polarization measurements, we assume that the dust grain properties of galaxies MCG-02-24-3 (for SN 2013hj) and NGC 3448 (for SN 2014G) are similar to Galactic dust. Now, we can estimate the maximum degree of polarization due to host galaxies following the Serkowski et al. (1975) relation, P_max < 9 × E(B − V). In Section 3, we estimated host galaxy extinctions for both SNe using the colour method, which translates to E(B − V)_host values of 0.055 and 0.244 mag for SNe 2013hj and 2014G, respectively, assuming R_V = 3.1. Therefore, the corresponding P_max (for host) values are <0.50 and <2.20% for SNe 2013hj and 2014G, respectively.

To vectorially remove ISP components from SN polarization, we need to know the direction information as well. It is generally believed, and is also found to be true, that galactic magnetic fields run along the spiral arms (Scarrott, Rolph & Semple 1990; Scarrott et al. 1991; Han 2009), which is roughly perpendicular to the line joining the galactic centre and the position of the spiral arm (e.g. Leonard et al. 2001; Maund et al. 2007; Kumar et al. 2014). However, for both SNe 2013hj and 2014G, the host galaxies are almost irregular, not in face-on projection, and the SN location is not well resolved. This makes it difficult to make any reasonable estimate of the magnetic field orientation and effective line-of-sight polarization angle at SN locations.

Despite emphasizing all the critical aspects of ISP_HG, we do not subtract it from SN polarization measurements or make any speculative inference based on that. This was done mainly because of three uncertainties: (a) the reddening estimation of the host galaxy is quite uncertain and may suffer systematic bias due to the method itself (see Section 3); (b) unknown grain properties in the host galaxy may significantly vary the maximum polarization efficiency we are adopting; (c) without knowing the polarization angle of the host galaxy, vectorial subtraction will not be possible because a subtraction of the ISP component may increase or decrease polarization values depending on θ. Thus, we caution the reader while interpreting the quoted polarization values as well as Figs 9 and 10 in the paper.

Figure 10.

Broad-band polarimetric evolution of SNe 2013hj and 2014G (only ISP_MW subtracted values) are compared with other SNe IIP. The references for data and explosion epochs are SN 1987A (Barrett 1988), SN 1999em (Leonard et al. 2001, 2002a), SN 2005af (Pereyra et al. 2006; Filippenko & Foley 2005) and SN 2012aw (Kumar et al. 2014).

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6.2 Intrinsic polarization and its evolution

The evolution of degree of polarization and the polarization angle of SNe 2013hj and 2014G are represented in Fig. 9. The evolution of polarimetric parameters of both events is also compared with other well-sampled SNe IIP, as shown in Fig. 10. Here, SN 1999em observations are V-band polarimetry, although the temporal evolution in the R band is expected to be similar. In both figures, the polarization values for SNe 2013hj and 2014G are subtracted for ISP_MW only, and thus these measurements have components for intrinsic SN polarization as well as ISP_HG. In Fig. 9, the horizontal dotted lines are shown to represent the maximum degree of host polarization component, which may be removed if the orientation of the host and measured SN polarization are assumed to be co-aligned. For any other θ of ISP_HG, the component of P due to ISP_HG in observed light will be lower than this level and so the intrinsic SN polarization will increase.

It should be noted in Fig. 9 that there is a small, yet significant amount of intrinsic polarization left in SN 2013hj even after considering the maximum possible subtraction of ISP_HG. About 0.3–0.5% of polarization can be detected above maximum ISP_HG during 57 and 86 d, which is well above the error bars of individual data points. The P and θ evolutions do not show a significant relative variation among individual data points and are all within the limit of errors. However, a subtle increasing trend is noticed in overall P evolution, but θ remains almost constant throughout. This might imply that there is some asymmetry in the electron scattering SN envelope, which is increasing according to envelope size, and the decrease in optical depth is revealing the asymmetric central part of explosion, but its geometrical orientation remains constant over time. It can be noted that the possible increase in polarization with time is almost linearly anticorrelated with the decline of SN brightness. Such a monotonic increase in polarization without change in polarization angle has also been observed in SN 1999em (see Fig. 10), as reported by Leonard et al. (2001) and Leonard & Filippenko (2005).

SN 2014G has only three measurements within the first 25 d. If the maximum amount of ISP_HG is considered, then the entire detection is possibly submerged within the ISP_HG (see Fig. 9). The polarization angle for ISP_HG would play an important role in this case, and only after knowing this we can conclude about the intrinsic polarization or asymmetry in the SN envelope or CSM. Any favourable misalignment of the ISP_HG angle can lead to significant residual SN polarization. A sudden and unusual increase in polarization is noticed at 15 d, which is coincident with the peak R-band light, although no change in polarization angle is observed. However, considering the large errors and low density of observations, we restrict ourselves from making any conclusive inference based on a single measurement, which may result from a spurious detection.

7 SUMMARY

In this paper, we present high-cadence photometric observations and a few epochs of broad-band polarimetric observations of two fast declining Type II SNe 2013hj and 2014G.

Because of the lack of a reliable handle on distance and reddening estimation, we adopted the distance from a single Tully–Fisher measurement or redshift distance available in the literature, whereas host galactic reddening is estimated using the colour method, which itself is not very reliable. Details of these methods and adapted values are discussed in Section 3. Implications of these uncertainties on the derived parameters and properties must be considered while making any interpretation. Although the derived parameters include the uncertainty from reddening and distance values, it does not take care of the possible selection bias, which might have been introduced while adopting those values.

The plateau slope of SN 2013hj is significantly steeper [1.5 mag (100 d)⁻¹ in the V band] than typical SNe IIP, but is similar to generic SNe IIL templates (Faran et al. 2014). SN 2013hj is found to lie at an intermediate position in the broad continuum of light-curve properties of SNe II diversity (see Anderson et al. 2014). If, at all, SN 2013hj has to be classified in the IIP/L taxonomy, then it will qualify better as a Type IIL. SN 2013hj does not show a sharp change in slope at the plateau end (∼98 d) as we see in typical SNe IIP/L, rather a gradual increase of slope is observed. However, SN 2014G is a perfect Type IIL candidate, showing high initial peak brightness and a steep plateau slope [2.55 mag (100 d)⁻¹ in the V band], which lies at the higher end of the SNe IIL light-curve template. A sharp end of the plateau phase is visible at ∼77 d for SN 2014G.

Because of the fast decline of brightness, most SNe IIL do not have post-plateau observations. SN 2014G is among the very few SNe IIL that have extensive observations covering all the light-curve phases until the nebular tail. The plateau–nebular transitional drop in luminosity is also prominent for the SN, which is expected but rarely detected for SNe IIL (Anderson et al. 2014). The radioactive tail of SN 2014G is found to be steeper than that expected for a light curve powered by the radioactive decay chain of ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe. Such steepening is explained by inefficient trapping of gamma-rays in the ejecta. The tail light curves of Type IIL SNe 1980K, 2013by and 2013ej also show a significant steepening. This possibly implies that all Type IIL SNe may exhibit such a steepening of the tail light curve, as SNe IIL have thinner hydrogen envelopes, which is responsible for gamma-ray leakage and incomplete thermalization of photons. More Type IIL SNe need to be observed rigorously until the nebular phase to confirm whether the steepening of the tail is a generic feature to all SNe IIL. The masses of synthesized radioactive ⁵⁶Ni estimated from the tail bolometric light curves are 0.08 ± 0.01 and 0.05 ± 0.03 M_⊙ for SNe 2013hj and 2014G, respectively.

We have performed semi-analytical modelling of the bolometric light curves of SNe 2013hj and 2014G to estimate explosion and progenitor properties. For SN 2013hj, we estimate a progenitor mass of ∼11 M_⊙ with a radius of ∼700 R_⊙, whereas for SN 2014G, the progenitor mass is estimated to be ∼9 M_⊙ with a radius ∼630 R_⊙. The total explosion energy (kinetic + thermal) for both SNe is approximately ∼2 foe. The steeper tail light curve of SN 2014G is fitted by reducing the effectiveness of the gamma-ray trapping parameter A_g in the model.

Broad-band polarimetric observations of SNe 2013hj and 2014G are also presented in this paper. Even after considering the maximum possible contamination in polarization due to the host galaxy, SN 2013hj is found to reveal some intrinsic polarization in its light (especially during mid- to late-plateau phases), which may become significant on adopting lower host reddening or by changing the host polarization angle. The temporal evolution of polarization shows a subtle but monotonic increment as the SN evolves towards the plateau end. It is generally expected for stripped or partially stripped SNe, namely Type Ib/c or IIb, to exhibit a higher degree of polarization compared to SNe with hydrogen envelopes intact as in SNe IIP. As both of our studied SNe are fast declining Type II events, some fraction of the H envelope may have already been shredded during pre-SN evolution in the form of stellar winds. Thus, the explosion will have a relatively lower amount of hydrogen retained, which will quickly start to reveal the central part that has a higher degree of asymmetry (Leonard & Filippenko 2005). Evidence of similar polarization enhancement in SNe IIP during the plateau to early nebular phase has been also reported by Leonard et al. (2001, 2006) for SNe 1999em and 2004dj. However, the polarization detected for SN 2014G in our observations is of very low significance, and it may be within the level of ISP due to the host galaxy.

We are grateful to the observers and observing staff of ARIES and the observatory of Osaka Kyoiku University for their kind cooperation in the observations of these objects. SB also acknowledges Dr C. Eswaraiah for a useful discussion on AIMPOL observations and data. We gratefully acknowledge the services of the NASA ADS, NED and SOUSA, which have been used to access data and references in this paper. We also thank the referee for useful comments and suggestions on the manuscript.

REFERENCES

APPENDIX A: TABLES OF PHOTOMETRY