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SN 2017fzw: A Fast-expanding Type Ia Supernova with Transitional Features

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16 May 2023

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Abstract
In this study, we analyzed the optical observations of a subluminous Type Ia supernova (SN Ia) 2017fzw, which exhibited high photospheric velocity (HV) at B-band maximum light. The absolute B-band peak magnitude was determined to be MBmax = -17.77  0.10 mag, similar to 91bg-like SNe Ia. The decline rate of the B-band light curve was estimated to be Dm15(B) = 1.60  0.06 mag. The spectra of SN 2017fzw were similar to those of 91bg-like SNe Ia, with prominent Ti II and Si II l5972 features at early phases, gradually transitioning to spectra resembling normal (mainly HV subclass) SNe Ia at later phases, with a stronger Ca II NIR feature. Notably, SN 2017fzw exhibited spectral evolution features similar to those of HV SNe Ia at all phases, with a maximum-light Si II l6355 velocity of 13, 786  414 km s
Keywords: 
Subject: Physical Sciences  -   Astronomy and Astrophysics

1. Introduction

Type Ia supernovae (SNe Ia) are highly valued cosmological distance indicators [1] that have played a pivotal role in discovering the accelerating expansion of the universe [2] due to their uniform light curves and high luminosities. These supernovae are widely believed to arise from carbon-oxygen white dwarfs (WDs) experiencing explosive thermonuclear runaways [3]. Typically, their close-binary companion star is considered a non-degenerate object, known as the single-degenerate scenario (SD) [4,5], or another WD, known as the double-degenerate scenario (DD) [6]. Observations support both the SD and DD scenarios, which are both used to explain the observed diversity among SNe Ia [7,8].
The majority of SNe Ia ( ∼ 70 % ) are classified as Branch normal groups, which exhibit relatively uniform photometric and spectroscopic evolution [9,10]. The remaining SNe Ia are grouped into various subclasses based on differences in their photometric and spectroscopic evolution compared to normal SNe Ia. For instance, the overluminous SN 1991T-like subclass is characterized by broad light curves, relatively weak Si ii/S ii and obvious Fe ii/Fe iii absorption features around the maximum light [11,12,13]. On the other hand, the subluminous SN 1991bg-like subgroup, such as SN 1999by [14], display a rapid decline rate in their light curves and strong absorption features of intermediate-mass elements (IMEs) [15,16]. Approximately 15 − 20 % of all SNe Ia belong to the rapid decliner class, accompanied by low peak luminosity in most cases [17,18,19]. Moreover, the rapid decliner class has a single I-band maximum delayed a few days with regard to the B-band maximum. For the 91bg-like group, the above behaviour is mainly due to their lower mass of 56 Ni [20,21]. Transitional SNe Ia have photometric and spectroscopic properties between normal and 91bg-like [22]. These transitional SNe Ia appear with a frequency as high as 91bg-like SNe Ia, such as SNe 1986G [23], 2011iv [24], and 2012ij [25].
Understanding the origin of transitional SNe Ia is crucial to gain insights into their properties lying between normal and 91bg-like SNe Ia. The identification of transitional SNe with a continuous distribution of observed properties supports the theory that normal and 91bg-like SNe share a common origin [26]. Transitional SNe Ia are characterized by a large light curve shape parameter ( Δ m 15 ( B ) , the decline rate in B-band light curve over 15 days after the peak, [27,28]) and low luminosity, which are similar to 91bg-like SNe [24,26]. However, their near-infrared (NIR) light curves resemble those of normal SNe Ia that exhibit two maxima [24,29]. Spectroscopically, transitional SNe Ia show similarities to both normal and 91bg-like SNe Ia, which demonstrates their intermediate properties. Previous studies have reported that the range of the Phillips parameter for transitional groups is typically 1.5 − 1.8 mag [19,30,31]. However, it is important to note that some SNe Ia beyond this range are also classified as transitional SNe, while some SNe Ia within this range do not exhibit any clear intermediate properties [20,32].
In this paper, we report on the photometric and spectroscopic observations of SN 2017fzw, which exhibits characteristics of a transitional SN Ia however with lower luminosity and high velocity (HV). In Section 2, we provide a detailed description of the observations and data reduction process. In Section 3, we present the optical light curves, color curves, estimation of reddening, and quasi-bolometric light curves of SN 2017fzw. In Section 4, we predict its mean spectral sequences and compare the spectral evolution with other SNe Ia. We discuss our findings in Section 5 and present our conclusions in Section 6.

2. Observations and Data Reduction

SN 2017fzw was first detected on August 9, 2017, at 0.41-m PROMPT-5 telescope, located at Cerro Tololo Inter-American Observatory (CTIO) during the Distance Less Than 40Mpc survey (DLT40) conducted by Tartaglia et al. (2018) [33]. Its coordinates were measured to be α = 06 h 21 m 34 s . 820 and δ = − 27 ∘ 12 ′ 53 ′ ′ . 57 (J2000), and its initial clear-filtered magnitude (AB) was recorded as approximately 17.17 mag [34,35]. Hosseinzadeh et al. (2017) [36] conducted a subsequent spectroscopic study, which identified SN 2017fzw as a 91bg-like SN Ia and determined its host galaxy to be NGC 2217 at a redshift of z = 0.0054 . This corresponds to a distance modulus ( μ ) of μ = 31.66 ± 0.15 mag, or a distance of 21.45 Mpc (taking into account only the Virgo Infall) with an assumed Hubble constant of 67.8 km s − 1 Mpc − 1 [37,38]. 2017fzw is located approximately 100”away from the center of its host galaxy. Figure 1 displays the finder chart of SN 2017fzw.
The optical photometry ( B V g r i ) of SN 2017fzw was mainly collected by the Swope 1-m telescopes of Las Cumbres Observatory (LCO) [39,40] network for the Global Surpernova Project [41]. The data of LCO was reduced using lcogtsnpipe[42] and a PyRAF-based pipeline. The instrumental magnitudes of LCO are calibrated relative to Landolt (1992) [43] ( B V ) and Smith et al. (2002) [44] ( g r i ) standard stars observed over multiple photometric nights, which are detailedly introduced by Krisciunas et al. (2017) [45] and Phillips et al. (2019) [46]. Ultraviolet (UV) and optical observations of this SN were also observed with the N e i l G e h r e l s S w i f t O b s e r v a t o r y ( S w i f t ) [47] in six bands, including U V W 1 , U V W 2 , U V M 2 , U, B and V filters [48]. The U V M 2 -band data could be ignored due to only four observations. According to the zero points of Breeveld et al. (2011) [49] in the Vega magnitudes, we obtained the S w i f t UV/optical light curves using the data-reduction pipeline of the S w i f t UV/optical Supernova Archive (SOUSA) [50]. We measured the source counts using a 3 ′ ′ aperture and corrected by an average point-spread function. Thus, the final flux-calibrated LCO and S w i f t light curves of SN 2017fzw are listed in Table A1, and shown in Figure 2.
After the discovery, optical observations were conducted using FLOYDS spectrographs mounted on the 2-m Faulkes Telescope North and South of the LCO [40,51], which are from the Global Supernova Project [41]. A total of 11 low-resolution optical spectra of this SN was obtained between − 9 to + 168 days. The spectral flux of SN 2017fzw are calibrated by standard stars observed with a comparable air mass as the SN on the same night. The LCO extinction curves and telluric correction are utilized to correct for the effects of atmospheric extinction and telluric absorption lines in the spectral data of SN 2017fzw.

3. Photometric Properties

3.1. Optical Light Curves and Time of First Light

Figure 2 shows the UV/optical light curves of SN 2017fzw. These light curves have a nearly daily cadence from about − 10 to + 125 days relative to B-band maximum light. Like normal SNe Ia, the r/i-band light curves of SN 2017fzw clearly show a shoulder/secondary maximum, and its light curve peak reached slightly earlier in i-band relative to B-band. These multi-band light curves were fit by SuperNovae in the object oriented Python code SNooPy2[52,53] to determine the time of B-band maximum light and other important light curve parameters, as shown in the left panel of Figure 3. We found that SN 2017fzw reached the B-band maximum light ( T m a x ( B ) ) on MJD 57987.90 ± 0.35 , corresponding to a peak magnitude of B m a x = 14.08 ± 0.02 mag and a decline rate Δ m 15 ( B ) = 1.60 ± 0.06 mag estimated in the rest frame of this SN.
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Figure 2. UV and optical ( U B V g r i ) light curves of SN 2017fzw obtained by S w i f t and LCO. The vertical dashed line indicates its B-band maximum, and the light curves have been shifted vertically for clarity.
Figure 2. UV and optical ( U B V g r i ) light curves of SN 2017fzw obtained by S w i f t and LCO. The vertical dashed line indicates its B-band maximum, and the light curves have been shifted vertically for clarity.
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Figure 3. Left panel: best-fit light curve model (solid black lines) from SNooPy2 [52,53] for SN 2017fzw. The dashed black lines indicate the 1- σ uncertainty (in many cases smaller than the line width) with respect to the best-fit light curve templates, and the light curves have been shifted vertically for clarity. Right panel: ideal fireball model [54] fits (dashed lines) to the multi-band early light curves of SN 2017fzw (markers with error vars) during the ∼ 7 days before the B-band maximum. The bottom panel displays the residual of the best-fit curves, and the horizontal black dashed line represents zero residual.
Figure 3. Left panel: best-fit light curve model (solid black lines) from SNooPy2 [52,53] for SN 2017fzw. The dashed black lines indicate the 1- σ uncertainty (in many cases smaller than the line width) with respect to the best-fit light curve templates, and the light curves have been shifted vertically for clarity. Right panel: ideal fireball model [54] fits (dashed lines) to the multi-band early light curves of SN 2017fzw (markers with error vars) during the ∼ 7 days before the B-band maximum. The bottom panel displays the residual of the best-fit curves, and the horizontal black dashed line represents zero residual.
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The time of first light (FLT) was estimated as MJD 57972.00 ± 0.24 by fitting the early multi-band light curves of SN 2017fzw during the ∼ 7 days before the B-band maximum with the ideal expanding fireball model [54], as shown in the right panel of Figure 3. Thus, its rise time is given as 15.9 ± 0.4 days, which is comparable to the average value of normal SNe Ia (i.e., 16.0 days, [55]) and also close to the upper limit value of 91bg-like SNe Ia ( ∼ 13 − 15 days, [20]). The basic photometric parameters of this SN are listed in Table 1, which are consistent with results of Graham et al. (2022) [56] within the uncertainties. For the following discussions, the phases are all given in reference to the B-band maximum of SN 2017fzw.
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Figure 4. B V -band light curves of SN 2017fzw from LCO, compared with SNe 1999by (91bg-like subgroup), 2011fe, 2017fgc (normal subgroup), 2004eo, 1986G, and 2012ij (transitional subgroup). All light curves of comparison SNe Ia have been shifted to match SN 2017fzw in peak magnitudes.
Figure 4. B V -band light curves of SN 2017fzw from LCO, compared with SNe 1999by (91bg-like subgroup), 2011fe, 2017fgc (normal subgroup), 2004eo, 1986G, and 2012ij (transitional subgroup). All light curves of comparison SNe Ia have been shifted to match SN 2017fzw in peak magnitudes.
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In Figure 4, we compare the B V -band light curves of SN 2017fzw with those of some normal and subluminous SNe Ia, including SNe 1986G [23,28,57], 1999by [14,58], 2004eo [59], 2011fe [60], 2012ij [25], and 2017fgc [61]. One can see that the light curves of SN 2017fzw like other transitional SNe Ia are all similar to those of SN 1999by in terms of their morphology. Especially, the light curve of SN 2017fzw in V-band shows the strongest resemblance to that of transitional SN 1986G having similar Δ m 15 ( B ) .

3.2. Reddening and Color Curves

According to the NASA/IPAC Extragalactic Database (NED) [38], the Galactic extinction toward SN 2017fzw is A B G a l = 0.16 [62]. Since the Na i D absorption feature was nondetectable in this SN spectra, the reddening of the host galaxy was assumed to be negligible. Fitting the multi-band light curves of SN 2017fzw with SNooPy2 estimate its μ as 31.71 ± 0.15 mag (see the left panel of Figure 3). Taking μ of its host galaxy as 31.66 ± 0.15 mag into account, we adopt their average value of μ = 31.69 ± 0.11 mag and an extinction law R V = 3.1 [63] in the following analysis. Then, the B-band absolute peak magnitude of SN 2017fzw was deduced as M m a x B = − 17.77 ± 0.10 mag, which is comparable to that of 91bg-like SNe Ia (about − 16.5 to − 17.7 mag [64]).
In Figure 5, we compare the SN 2017fzw B − V color curve with those of normal and 91bg-like SNe Ia. One can see that the color curve of SN 2017fzw is very similar to that of SN 2012ij in morphology, both are systematically much redder than those of normal SNe Ia. In particularly, SN 2017fzw tends to be even redder than all comparison SNe Ia after the reddest color. After ∼ + 50 days from the B-band maximum, all color curves became indistinguishable. On the other hand, the observed B − V color of SN 2017fzw is significantly bluer than 91bg-like SNe Ia (i.e., 0.4 − 0.7 mag [20]) at the B-band maximum light. It is also observed that SN 2017fzw reached the red peak earlier than normal SNe Ia.
The color stretch factor ( S B V ) defined as S B V = t m a x B − V / 30 days [53], where t m a x B − V presents the peak time of the B − V color curve, is proportional to t m a x B − V . Burns et al. (2014) [53] presented a relation between S B V and Δ m 15 ( B ) to estimate the factor S B V , but found that the factor t m a x B − V was better than Δ m 15 ( B ) in the case of fast-evolving 91bg-like SNe Ia. Thus, we utilize the former relation to calculate the S B V of SN 2017fzw in this work, and obtain its S B V = 0.63 with t m a x B − V ≈ 19 days. The estimated S B V is close to some transitional SNe Ia (e.g., ∼ 0.65 for SN 1986G, [23,31]).
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Figure 5. B − V color curve of SN 2017fzw, and those of normal SNe 2011fe and 2017fgc, and transitional SNe 2004eo and 2012ij. All SNe Ia have been dereddened for the Milky Way and host galaxies.
Figure 5. B − V color curve of SN 2017fzw, and those of normal SNe 2011fe and 2017fgc, and transitional SNe 2004eo and 2012ij. All SNe Ia have been dereddened for the Milky Way and host galaxies.
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3.3. Bolometric Light Curves

According to response curves of different filters, the quasi-bolometric flux of SN 2017fzw was constructed by trapezoidal integration of flux densities in UV/optical ( U B V g r i ) photometry, covering the emission range of 3000-9700 Å, as shown in Figure 6. Comparing its quasi-bolometric light curve with those of 91bg-like SNe 1991bg [16] and 1999by [14], transitional SNe 2004eo [59] and 2012ij [25], and normal SNe 2011fe [60] and 2017fgc [61], we find that SN 2017fzw most closely resembles SN 2012ij. Moreover, the modified radiation diffusion model of Arnett implemented in Minim Code [65,66,67] is employed to estimate the radioactive nickel mass of SN 2017fzw as M N i = 0.18 ± 0.03 M ⊙ . This is comparable to transitional SNe Ia within reasonable uncertainties (i.e., ∼ 0.14 M ⊙ , SN 1986G [31] and SN 2012ij [25]). We estimate a maximum luminosity of L = ( 4.6 ± 0.5 ) × 10 42 erg s − 1 .

4. Optical Spectroscopic Properties

Spectroscopic monitoring of SN 2017fzw are displayed in the left panel of Figure 7, spanning the period − 9 to + 168 days. The spectral evolution of SN 2017fzw is characterized by several key features found in 91bg-like SNe Ia discussed in the literature, of which we now further elaborate upon [56]. A spectrum of SN 2017fzw at pre-maximum light shows noticeable P-Cygni absorption features of IMEs, such as Si ii, S ii, Ca ii and Mg ii. Its spectra also has a prominent Ti ii absorption feature, which is characteristic of 91bg-like SNe, confirming that SN 2017fzw belongs to the subluminous subclass. Moreover, the Si ii λ 5972 and O i absorption features of the SN 2017fzw spectra are found to be much stronger relative to normal SNe Ia at similar phases. After ∼ + 10 days from the B-band maximum, the S ii absorption features are nearly undetectable. In particular, the Si ii λ 5972 feature gradually becomes invisible after ∼ + 20 days and is replaced by the Na i feature. However, the Si ii λ 6355 feature is still detectable up to about one month past B-band maximum light.
In Figure 8, detailed spectral comparisons among SN 2017fzw, normal SNe Ia, and transitional SNe Ia are displayed at four phases (i.e., − 10 d, + 10 d, + 20 d, and + 40 d). The comparison sample inlude SNe 2004eo [59], 2011fe [60], 2012ij [25], and 2017fgc [61]. Like SNe 2004eo and 2012ij, SN 2017fzw shows prominent Ti ii and iron-group elements (IGE) absorption features after ∼ + 10 days. One can see that SN 2017fzw is very similar to SN 2012ij in both overall shape and strength of characteristic spectral lines at each phase, except for Si ii λ 6355 and Ca ii NIR absorption features. According to Li et al. (2022) [25], the spectra of SN 2012ij resemble those of SN 1999by. Thus, we could say that SN 2017fzw is similar to SN 1999by. These two different features in the spectra of SN 2017fzw tend to resemble SN 2017fgc, which is included in the high-velocity (HV) subgclass due to the classification criteria introduced by Wang et al. (2009) [69]. As shown in Figure 9, at later phases ( ∼ + 107 and + 168 days) SN 2017fzw also tends to develop characteristics very similar to the HV SN 2017fgc. To summarize, the spectra of SN 2017fzw resembles the 91bg-like SNe Ia at early phases and evolves away from 91bg-like ones at later phases.
For SNe Ia, the velocity measured from Si ii  λ 6355 at B-band maximum light is often used to determine their subclass [69]. Unfortunately, SN 2017fzw does not have optical spectral data at around B-band maximum light. Thus according to Hu et al. (2022), long short-term memory (LSTM) neural networks [68] is employed to predict the spectra of SN 2017fzw in this work, and results are shown in the right panel of Figure 7. One can see that the predictions of SN 2017fzw at around 6355 Å roughly coincide with its observations, except for its spectra after + 23 days. For the predicted spectrum of SN 2017fzw at B-band maximum, its velocity of Si ii λ 6355 is measured as v = 13 , 786 ± 414 km s − 1 , significantly beyond the upper limit (i.e., ∼ 11800 km s − 1 introduced by Wang et al. 2009 [69]) of the normal-velocity (NV) subclass. We thus classify SN 2017fzw as belonging to the HV subclass like SN 2017fgc. Consistent with the HV SNe Ia behaviour, the Ca ii NIR absorption feature in the HV SNe 2017fgc and 2017fzw is much stronger than those of the NV SNe 2004eo, 2011fe, and 2012ij after one month past B-band maximum light [61].

5. Discussion

5.1. Transitional Photometric Properties

SN 2017fzw exhibits properties that are characteristic of transitional SNe Ia. Specifically, it has a fast decline rate with Δ m 15 ( B ) = 1.60 mag, and an earlier peak time in the i-band light curve compared to the B-band light curve [32]. Notably, this SN displays a significant shoulder/secondary maximum in r / i -band light curves, which distinguishes it from 91bg-like SNe Ia. Typically, normal SNe Ia have a stronger shoulder/secondary maximum in NIR light curves, which is associated with the recombination of IGE [70]. Taubenberger (2017) [64] suggested that fast-declining SNe Ia tend to display weaker and earlier shoulder/secondary maximum due to the earlier recombination of IGE. Furthermore, two NIR maxima may combine to form a delayed single NIR maximum in some 91bg-like SNe Ia. Transitional SNe Ia exhibit properties, including luminosity and temperature, that are intermediate to those of normal and 91bg-like SNe Ia. As such, it is reasonable to suggest that they have a weak shoulder/secondary maximum in their NIR light curves [25].
We estimate the B-band absolute peak magnitude ( M m a x B ) of SN 2017fzw to be approximately − 17.77 mag, which is comparable to the brightness of 91bg-like SNe Ia (approximately − 16.5 to − 17.7 mag [64]). The B − V color curve of SN 2017fzw is bluer than that of 91bg-like SNe Ia at B-band maximum, which is consistent with the findings of Taubenberger et al. (2008) [20]. They discovered that a SN Ia with Δ m 15 ( B ) ≤ 1.75 mag has a relatively small luminosity, a double maximum in NIR light curves, and a rapid decay.
Figure 10 displays a comparison between M m a x B ( M m a x B = − 17.77 ) of SN 2017fzw and color stretch factor ( S B V = 0.63 , left panel) as well as Δ m 15 ( Δ m 15 = 1.60 ,right panel), utilizing data from Krisciunas et al. (2017) [45] and Li et al. (2022) [25], encompassing overluminous SNe Ia, subluminous SNe Ia, transitional SNe Ia, and normal SNe Ia. It is evident that SN 2017fzw, along with several other SNe categorized as normal or subluminous, is located between the majority of the normal SNe Ia (right-pointing triangles) and the 91bg-like SN Ia (green square). This supports the hypothesis of a continuous distribution of SNe ranging from normal ( S B V ≤ ∼ 0.5 ) to 91bg-like ( S B V ≥ ∼ 0.8 ) [24]. This hypothesis implies that normal and 91bg-like SNe Ia may not originate from two entirely distinct populations.

5.2. Transitional Spectroscopic Properties

The spectral evolution of SN 2017fzw is analyzed and compared with normal and transitional SNe Ia in Figure 8 and Figure 9. It is observed that its spectra shift from being more 91bg-like SNe (i.e., having a prominent Ti ii absorption feature) at early phases to more normal SNe (mainly HV SN 2017fgc) at later phases, suggesting its transitional characteristics. Notably, SN 2017fzw belongs to the HV normal subclass according to the classification criteria of Wang et al. (2009) [69], which distinguishes it from most transitional SNe Ia (listed in Table 2). This can explain why SN 2017fzw, like HV SN Ia, exhibits a stronger Ca ii NIR absorption feature and Si ii  λ 6355 velocity compared to the SNe 2004eo and 2012ij.
For instance, Li et al. (2022) [25] discovered that transitional SN 2012ij falls into the NV subclass according to the same classification criteria established by Wang et al. (2009) [69]. This SN serves as a link between the NV subclass of the normal group and the 91bg-like SNe Ia. On the other hand, SN 2017fzw connects the HV subclass of the normal group with the 91bg-like SNe Ia, which further supports Li et al.’s (2022) [25] finding of a continuous distribution between normal SNe, including the NV and HV normal subclasses, and 91bg-like SNe Ia. Moreover, some authors have also identified a continuous distribution between normal and 91bg-like SNe Ia in other parameter spaces [22,71,72].
Table 2. Photometric and spectroscopic properties of transitional SNe Ia.
Table 2. Photometric and spectroscopic properties of transitional SNe Ia.
name S B V Δ m 15 ( B ) (mag) M m a x B (mag) V ( × 10 4 km s − 1 ) SN Type reference
SN 1986G 0.65 1.69 -17.76 0.81 NV [23]
SN 2003hv 0.76 1.61 -18.99 - LVG1 [73]
SN 2004eo 0.83 1.46 -19.08 1.07 NV [59]
SN 2007on 0.57 1.96 -18.24 0.95 NV [24]
SN 2011iv 0.63 1.77 -18.84 0.95 NV [24]
SN 2012ij 0.53 1.86 -17.95 1.05 NV [25]
SN 2015bp 0.67 1.56 -18.46 1.06 NV [74]
SN 2017fzw 0.63 1.60 -17.77 1.38 HV this paper
1Note that low-velocity gradient (LVG) subclass is proposed by Benetti et al. (2005) [75] base on the velocity gradient of the Si ii  λ 6355 , and most LVG SNe Ia belong to the NV subclass in general [61,69].

6. Conclusions

In this study, we presented and analyzed the photometric and spectroscopic data of the HV SN Ia 2017fzw with transitional properties. Our analysis revealed that SN 2017fzw has an M m a x B of − 17.77 mag, which is close to the peak magnitude of 91bg-like SNe Ia. Based on its Δ m 15 ( B ) = 1.60 mag and color stretch factor ( S B V = 0.63 ), we identified SN 2017fzw as a transitional SN that bridges between normal and 91bg-like SNe, thus confirming its transitional nature. Additionally, we classified SN 2017fzw as belonging to the HV subclass with a velocity of v = 13 , 786 km s − 1 . The spectra of SN 2017fzw show similarities to those of 91bg-like SNe, with prominent Ti ii features at early phases, and gradually evolve to resemble normal SNe Ia (mainly HV subclass), with stronger Ca ii NIR features at later phases.
In conclusion, our analysis shows that SN 2017fzw is a transitional SN Ia with properties that bridge the gap between normal and 91bg-like SNe Ia. This supports the hypothesis that there may be a continuous distribution of SNe spanning from normal to 91bg-like SNe, suggesting that these two groups may have a common origin. The discovery of transitional SN Ia provides an opportunity to investigate the relationships between these two groups, and to gain a better understanding of their progenitors and explosion mechanisms.

Author Contributions

Conceptualization, X.-Y.Z. and S.Z.; methodology, X.-Y.Z. and S.Z.; software, J.-Y.H., Y.-Y.L., X.-Y.Z. and S.Z.; validation, X.-Y.Z. and S.Z.; formal analysis, J.-Y.H. and Y.-Y.L.; investigation, J.-Y.H. and X.-Y.Z.; resources, S.Z. and X.-Y.Z.; data curation, A.Iskandar., K.A.Bostroem., D.A.Howell., C.McCully., and L.H.; writing—original draft preparation, J.-Y.H. and Y.-Y.L.; writing—review and editing, J.-Y.H., S.Z. and X.-Y.Z.; visualization, S.A.Bird., A.Esamdin., W.-X.L., T.-M.Z., J.-J.Z., S.-G.Z., Y.-S.X., Y.H. and L.-F.W.; All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China 12203029 and U2031202.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The photometric and spectroscopic data analyzed for this study are downloaded from Las Cumbres Observatory (LCO) and N e i l G e h r e l s S w i f t O b s e r v a t o r y ( S w i f t ).

Acknowledgments

The authors thank the staffs of LCO network 1-m/2-m telescopes and S w i f t that obsevred and provided the data. And the LCO group is supported by NSF grants AST-1911225, AST-1911151, and NASA grant 80NSSC19K1639. And this work is supported by The National Natural Science Foundation of China (NSFC, grants 11803076), and the High Level Talent–Heaven Lake Program of Xinjiang Uygur Autonomous Region of China.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Table A1. UV and optical photometry of SN 2017fzw obtained from LCO telescopes and SWIFT.
Table A1. UV and optical photometry of SN 2017fzw obtained from LCO telescopes and SWIFT.
MJD 1 U B V g r i U V W 2 U V W 1
57974.36 ⋯ ⋯ ⋯ ⋯ 17.169(0.0092) ⋯ ⋯ ⋯
57978.02 16.475(0.054) 15.784(0.029) 15.578(0.045) ⋯ ⋯ ⋯ 18.338(0.090) 17.890(0.162)
57981.81 ⋯ 15.359(0.025) 15.124(0.024) 15.212(0.023) ⋯ ⋯ ⋯ ⋯
57980.17 ⋯ 14.982(0.025) 14.776(0.025) 14.853(0.021) 14.611(0.025) 15.144(0.026) ⋯ ⋯
57980.76 15.599(0.065) 14.973(0.037) 14.716(0.055) ⋯ ⋯ ⋯ 17.910(0.182) 17.079(0.058)
⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮
58156.61 ⋯ ⋯ ⋯ 18.990(0.045) ⋯ ⋯ ⋯ ⋯
58167.54 ⋯ 19.389(0.042) 19.429(0.057) 19.178(0.031) 19.852(0.068) ⋯ ⋯ ⋯
58193.51 ⋯ ⋯ ⋯ 19.725(0.046) ⋯ ⋯ ⋯ ⋯
58215.06 ⋯ ⋯ ⋯ 20.105(0.041) ⋯ ⋯ ⋯ ⋯
58226.39 ⋯ 20.449(0.078) ⋯ ⋯ ⋯ ⋯ ⋯ ⋯
1 Phases are relative to the epoch of B-band maximum brightness (MJD = 57987.90). 2 Uncertainties, in units of 0.001 mag, are 1 σ . Note. Measurements are calibrated to the AB magnitude system. (This table is available in its entirety in machine-readable form.)
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Figure 1. SN 2017fzw (red circle) and a local sequence of standard stars (green circles) in the field of NGC 2217 (i-band image, obtained by LCO on 07 September 2017 UT).
Figure 1. SN 2017fzw (red circle) and a local sequence of standard stars (green circles) in the field of NGC 2217 (i-band image, obtained by LCO on 07 September 2017 UT).
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Figure 6. Quasi-bolometric (UV/optical) light curve of SN 2017fzwcompared with those of normal SNe 2011fe and 2017fgc (Zeng et al. 2021 [61]), and transitional SNe 2004eo and 2012ij (Li et al. 2022 [25]).
Figure 6. Quasi-bolometric (UV/optical) light curve of SN 2017fzwcompared with those of normal SNe 2011fe and 2017fgc (Zeng et al. 2021 [61]), and transitional SNe 2004eo and 2012ij (Li et al. 2022 [25]).
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Figure 7. Left panel: observed (black curves) and smoothed (blue curves) spectra of SN 2017fzw obtained from LCO. The corresponding phases relative to its B-band maximum of this SN are marked on the right side. Right top panel: spectral sequence predicted from two spectra with phase difference ( Δ p = 20 days) using long short-term memory (LSTM) neural networks [68] for SN 2017fzw. The predictive mean spectra are marked by solid black curves, and their 2 σ standard deviations are indicated by the gray shaded areas. The corresponding observed spectra are plotted as green curves, except for the two input spectra in red. The vertical dashed line indicates the Si ii λ 6355 absorption at − 1.0 day. Right lower panel: flux ratios of predictions to observations for the spectral sequence. All spectra have been vertically shifted for better display.
Figure 7. Left panel: observed (black curves) and smoothed (blue curves) spectra of SN 2017fzw obtained from LCO. The corresponding phases relative to its B-band maximum of this SN are marked on the right side. Right top panel: spectral sequence predicted from two spectra with phase difference ( Δ p = 20 days) using long short-term memory (LSTM) neural networks [68] for SN 2017fzw. The predictive mean spectra are marked by solid black curves, and their 2 σ standard deviations are indicated by the gray shaded areas. The corresponding observed spectra are plotted as green curves, except for the two input spectra in red. The vertical dashed line indicates the Si ii λ 6355 absorption at − 1.0 day. Right lower panel: flux ratios of predictions to observations for the spectral sequence. All spectra have been vertically shifted for better display.
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Figure 8. Optical spectra of SN 2017fzw at four different phases (i.e., − 10 d, + 10 d, + 20 d, and + 40 d) are compared with those of normal SNe 2011fe and 2017fgc, and transitional SNe 2004eo and 2012ij. All of the spectra have been corrected for redshift and reddening of host galaxies and the Milky Way, and shifted vertically for clarity.
Figure 8. Optical spectra of SN 2017fzw at four different phases (i.e., − 10 d, + 10 d, + 20 d, and + 40 d) are compared with those of normal SNe 2011fe and 2017fgc, and transitional SNe 2004eo and 2012ij. All of the spectra have been corrected for redshift and reddening of host galaxies and the Milky Way, and shifted vertically for clarity.
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Figure 9. Comparison between spectra of SN 2017fzw (yellow curve) and SN 2017fgc (yellow curve) at around + 105 and + 170 days. As in Figure 8, both spectra have been corrected for redshift and reddening.
Figure 9. Comparison between spectra of SN 2017fzw (yellow curve) and SN 2017fgc (yellow curve) at around + 105 and + 170 days. As in Figure 8, both spectra have been corrected for redshift and reddening.
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Figure 10. Left panel: M m a x B of SNe Ia versus their corresponding S B V . Right panel: M m a x B of the same SNe Ia versus their corresponding Δ m 15 ( B ) . The sample includes SN 2017fzw (red star), transitional SN 2012ij (orange circle), 91T-like SN Ia (blue circle), 91bg-like SN Ia (green square), other subluminous SNe Ia, and normal SNe Ia. The sample is taken from Krisciunas et al. (2017) [45] and Li et al. (2022) [25].
Figure 10. Left panel: M m a x B of SNe Ia versus their corresponding S B V . Right panel: M m a x B of the same SNe Ia versus their corresponding Δ m 15 ( B ) . The sample includes SN 2017fzw (red star), transitional SN 2012ij (orange circle), 91T-like SN Ia (blue circle), 91bg-like SN Ia (green square), other subluminous SNe Ia, and normal SNe Ia. The sample is taken from Krisciunas et al. (2017) [45] and Li et al. (2022) [25].
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Table 1. The photometric and spectroscopic parameters of SN 2017fzw.
Table 1. The photometric and spectroscopic parameters of SN 2017fzw.
Parameter Value
B m a x 14.08 ± 0.02 mag
M m a x B − 17.77 ± 0.10 mag
Δ m 15 ( B ) 1.60 ± 0.06 mag
S B V 0.63 ± 0.04
T m a x ( B ) MJD 57987.90 ± 0.35
FLT MJD 57972.00 ± 0.24
Rise Time 15.9 ± 0.4 days
μ 33.71 ± 0.15 mag
Redshift 0.0054
v 13 , 786 ± 414 km s − 1
M N i 0.18 ± 0.03 M ⊙
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