# Surface Radioactivity or Interactions? Multiple Origins of Early-excess Type
Ia Supernovae and Associated Subclasses

Ji-an Jiang  Institute of Astronomy, Graduate School of Science, The
University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan  Department
of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan  Mamoru Doi  Institute of Astronomy,
Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka,
Tokyo 181-0015, Japan  Research Center for the Early Universe, Graduate School
of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan  Kavli Institute for the Physics and Mathematics of the Universe (WPI),
The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
Keiichi Maeda  Department of Astronomy, Kyoto University, Kitashirakawa-
Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan  Kavli Institute for the Physics
and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5
Kashiwanoha, Kashiwa, Chiba 277-8583, Japan  Toshikazu Shigeyama  Research
Center for the Early Universe, Graduate School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

(Received 2018 June 25; Revised 2018 August 16; Accepted 2018 August 16)

###### Abstract

Early-phase Type Ia supernovae (SNe Ia), especially those with luminosity
enhancement within the first few days of explosions (“early-excess SNe Ia”),
play an irreplaceable role in addressing the long-standing progenitor and
explosion issue of SNe Ia. In this paper, we systematically investigate 11
early-excess SNe Ia from subluminous to luminous subclasses. Eight of them are
selected from 23 SNe Ia with extremely early-phase optical light curves
(“golden” early-phase SNe Ia), and three of them are selected from 40 SNe Ia
(including 14 golden samples) with early-phase UV/NUV light curves. We found
that previously discovered early-excess SNe Ia show a clear preference for
specific SN Ia subclasses. In particular, the early-excess feature shown in
all six luminous (91T- and 99aa-like) SNe Ia is in conflict with the viewing
angle dependence predicted by the companion-ejecta interaction scenario.
Instead, such a high early-excess fraction is likely related to the explosion
physics of luminous SNe Ia; i.e. a more efficient detonation happening in the
progenitor of luminous SNe Ia may consequently account for the early-excess
feature powered by the radiation from a 56Ni-abundant outer layer. The
diversity of early-excess features shown in different SN Ia subclasses
suggests multiple origins of the discovered early-excess SNe Ia, challenging
their applicability as a robust progenitor indicator. Further understanding of
the early-excess diversity relies not only on multiband photometry and prompt-
response spectroscopy of individual early-excess SNe Ia but also on
investigations of the general early-phase light-curve behavior of each SN Ia
subclass, which can be realized through ongoing/forthcoming transient survey
projects in the near future.

supernovae: general

##  1 Introduction

Type Ia supernovae (SNe Ia) have been thought to originate from the
thermonuclear explosion of a carbon-oxygen white dwarf (WD) in a binary
system. Even though great success was achieved by using them as a cosmic
distance indicator in the 1990s (Perlmutter et al., 1997, 1999; Riess et al.,
1998), the progenitor and the physics leading to the explosion are still under
debate (Hillebrandt et al., 2013; Maoz et al., 2014; Maeda & Terada, 2016). In
the last decades, a tremendous number of SNe Ia have been found through
various kinds of transient surveys, and a growing number of SNe Ia discovered
within a few days of their explosions provide unique information about the
progenitor system and the explosion mechanism of SNe Ia (Nugent et al., 2011;
Foley et al., 2011; Cao et al., 2015; Shappee et al., 2016; Marion et al.,
2016; Hosseinzadeh et al., 2017; Jiang et al., 2017).

Kasen (2010) proposed that a prominent brightening in the first few days of
the explosion can be observed under specific viewing directions due to the
interaction between the expanding ejecta and a nondegenerate companion star,
which makes SNe Ia with additional luminosity enhancement in the early time
(“early-excess SNe Ia”; EExSNe Ia) a powerful indicator for the single-
degenerate (SD) progenitor system (Kasen, 2010; Maeda et al., 2014; Kutsuna &
Shigeyama, 2015). Since then, surveys for EExSNe Ia have become particularly
popular in time-domain astronomy. The first reported EExSN Ia, iPTF14atg, has
been suggested as strong evidence for the companion-ejecta interaction (Cao et
al., 2015). Soon after, the companion-interaction scenario was also proposed
as the likely origin of early-excess features of SN 2012cg and the recently
discovered SN 2017cbv (Marion et al., 2016; Hosseinzadeh et al., 2017).

However, whether the early light-curve excesses of these SNe Ia are
exclusively attributed to the companion interaction is still under debate.
Theoretically, the interaction between dense circumstellar matter (CSM) and SN
ejecta (Shen et al., 2012; Levanon et al., 2015, 2017; Tanikawa et al., 2015;
Piro & Morozova, 2016; Maeda et al., 2018) and vigorous mixing of radioactive
56Ni in the outermost region of SN ejecta (Piro & Morozova, 2016) may produce
a similar early-excess feature to that predicted by the companion interaction.
For instance, Kromer et al. (2016) claimed that the spectra of iPTF14atg
cannot be reproduced by the SD scenario but show good consistency with the
prediction of merging two sub-Chandrasekhar-mass WDs. If the progenitor system
of iPTF14atg is not an SD system, then the early light-curve excess of
iPTF14atg cannot be attributed to the companion-interaction scenario, and
other effects should come into play. Different physical mechanisms have also
been proposed to explain the early excess shown in SN 2012cg and SN 2017cbv
(Shappee et al., 2018; Hosseinzadeh et al., 2017; Miller et al., 2018). Other
than these “companion-interaction-like” EExSNe Ia, the early light-curve
excesses of several SNe Ia have been initially explained by different
scenarios (e.g. iPTF14bdn, Smitka et al. 2015; iPTF16abc, Miller et al. 2018).
In particular, Jiang et al. (2017) reported a normal-brightness EExSN Ia,
MUSSES1604D (SN 2016jhr), that shows a prominent but red early excess and
strong Ti ii absorptions around the $B$-band maximum. The early excess of this
peculiar object can be well explained by the radiation emitted from short-
lived radioactive elements that were generated by a precursory detonation at a
thin helium shell of its primary WD, which is the first robust evidence of the
multiple origins of EExSNe Ia.

As opposed to the EEx discoveries in several SN Ia subclasses, there is no
clear evidence of EEx detection in normal SNe Ia so far. For example, a
smooth-rising light curve of SN 2011fe from the very beginning (with
brightness $\sim$1000 times fainter than its peak) provides a significant
constraint on the companion type (Nugent et al., 2011; Li et al., 2011). The
nondetection of EEx in normal SNe Ia can be explained by either an
intrinsically faint EEx due to a small nondegenerate companion star or an
unfavorable viewing angle by chance under the companion-interaction scenario.
However, such an observational fact is in contrast to the high EEx
fraction111For SNe Ia in each subclass, an EEx fraction is defined as the
number fraction of EExSNe Ia over EExSNe Ia and well-observed early-phase SNe
Ia that do not show EEx features (non-EExSNe Ia; selected from papers and
public resources) in the same subclass. discovered in 91T/99aa-like SNe Ia,
which may suggest differences in their natures between some SN Ia subclasses
(see §4.2).

With the largest sample of early-phase SNe Ia discovered until 2018, we
present new evidence to further prove multiple origins of EExSNe Ia and
discuss the explosion mechanism and progenitor of specific SN Ia subclasses
from the EEx perspective. This paper is organized as follows. General
information on 23 well-observed early-phase SNe Ia (“golden early-phase SNe
Ia”) from the subluminous to luminous subclass are summarized in §2, and
further investigations of previously reported and unnoticed EExSNe Ia are
shown in §3. Discussions about the multiple origins of EEx and their
associated subclasses are given in §4, and our conclusions are summarized in
§5.

##  2 Golden early-phase SNe Ia

In this section, we systematically investigate 23 “golden early-phase SNe Ia”
from published papers. In general, we define spectroscopically classified SNe
Ia with single-band or multiband photometry taken from $\sim$16 days before to
15 days after the $B$-band maximum (in the rest frame) as golden early-phase
SNe Ia. Given that some SNe Ia have intrinsically short rising phases, An SN
Ia discovered at $\sim$15 days before the maximum but with a large photometric
depth is also denoted as a golden sample. Here this “large photometric depth”
is quantitatively defined in a way that a magnitude difference between the
detection and the maximum in the same band is larger than 4 mag (LSQ12hxx,
LSQ13ry, iPTF13ebh, and SN 2016coj), or a rising rate derived from the
earliest photometry and the nondetection limiting magnitude is higher than
about 1 mag day-1 (ASASSN-14lp). Note that SN 2014J (observations from early-
phase to post-maximum in the same band are missing; Goobar et al., 2014) and
two early-phase SNe Ia discovered by the $Kepler~{}Space~{}Telescope$, KSN
2012a and KSN 2011b (without spectral information; Olling et al., 2015), do
not meet the criteria of golden early-phase SNe Ia.

All golden early-phase SNe Ia are further classified into “normal”,
“luminous,” “transitional,” “subluminous,” and “hybrid” (or “He-det”)
subclasses depending on their general photometric and spectroscopic
characteristics. Their basic information is listed in Table 1. We adopt
${\Delta}m_{15}(B)$ value and $B$-band apparent magnitudes at peak
($m_{B,max}$) from relevant publications. Absolute $B$-band magnitudes at peak
($M_{B,max}$) are derived by adopting a Hubble constant $H_{0}$ = 70 km s-1
Mpc-1 and Galactic and host extinction values from Schlegel et al. (1998) and
relevant publications, respectively. $M_{B,max}$ are plotted against
${\Delta}m_{15}(B)$ in Figure 1.

Table 1: Characteristics of Golden Early-phase SNe Ia

Namea | Subclass | ${\Delta}m_{15}(B)$ | Peak | Redshift | Discovery | Photometric | Early Excess? | Data
---|---|---|---|---|---|---|---|---
|  |  | Magnitudeb |  | Phase (days)c | Depth (mag)d | Optical/UV | Sourcese
SN 2009ig | Normal | 0.89 |  $-$19.07 | 0.008797 |  $-$16.4 (Unf) | 4.0 | N/N | (1)
SN 2010jn | Normal | 0.90 |  $-$19.49 | 0.02602 |  $-$15.9 ($R48$) | 3.4 | N/– | (2)
SN 2011fe | Normal | 1.10 |  $-$19.15 | 0.000804 |  $-$17.3 ($g$) | 7.3 | N/N | (3), (4)
SN 2012fr | Normal | 0.85 |  $-$19.45 | 0.005453 |  $-$15.9 (Unf) | 3.9 | N/N | (5), (6)
LSQ12hxx | Normal | 1.01 |  $-$19.34 | 0.069 |  $-$15.6 ($gr$) | 4.4 | N/– | (7), (8)
LSQ12hzj | Normal | 1.16 |  $-$19.31 | 0.029 |  $-$15.9 ($gr$) | 3.7 | N/– | (7), (8)
LSQ13ry | Normal | 1.27 |  $-$19.46 | 0.030 |  $-$15.7 ($gr$) | 4.5 | N/– | (7), (8)
iPTF13dge | Normal | 1.18 |  $-$19.19 | 0.0159 |  $-$18.2 ($R48$) | 4.1 | N/N | (9)
SN 2013dy | Normal | 0.92 |  $-$18.93 | 0.003889 |  $-$17.6 (Unf) | 5.9 | N/– | (10), (11)
ASASSN-14lp | Normal | 0.80 |  $-$19.48 | 0.005101 |  $-$14.6 ($V$) | 3.5 | N/N | (12)
SN 2015F | Normal | 1.35 |  $-$19.36 | 0.004846 |  $-$16.3 ($V$) | 4.8 | N/N | (13)
SN 2016coj | Normal | 1.32 |  $-$18.81 | 0.004523 |  $-$15.4 (Unf) | 5.1 | N/– | (14), (15)
iPTF14bdn | Luminous/91T-like | 0.84 |  $-$19.44 | 0.01558 |  $-$18.0 ($R48$) | 4.1 | Y/Y | (16)
SN 2012cg | Luminous/99aa-like | 0.86 |  $-$19.64 | 0.00162 |  $-$17.1 ($B$) | 5.2 | Y/Y | (17)
iPTF16abc | Luminous/99aa-like | 0.95 |  $-$19.56 | 0.0234 |  $-$17.8 ($g$) | 5.5 | Y/Y | (18)
SN 2017cbv | Luminous/99aa-like | 1.06 |  $-$20.11 | 0.00399 |  $-$18.9 ($B$) | 4.3 | Y/Y | (19)
iPTF13asv | Luminous/peculiar | 1.03 |  $-$19.93 | 0.036 |  $-$16.2 ($R48$) | 3.7 | N/– | (20)
LSQ12gpw | Luminous/peculiar | 0.76 |  $-$19.93 | 0.058 |  $-$19.9 ($gr$) | 3.2 | Y/– | (7), (8)
SN 2012ht | Transitional | 1.39 |  $-$18.40 | 0.003559 |  $-$15.8 (Unf) | 4.3 | N/N | (21)
iPTF13ebh | Transitional | 1.79 |  $-$18.95 | 0.0133 |  $-$14.5 ($R48$) | 5.9 | N/N | (22)
PTF10ops | Subluminous/02es-like | 1.12 |  $-$17.77 | 0.062 |  $-$15.6 ($R48$) | 3.2 | Y/– | (23)
iPTF14atg | Subluminous/02es-like | 1.20 |  $-$17.78 | 0.0213 |  $-$18.5 ($R48$) | 3.8 | Y/Y | (24)
MUSSES1604D | Hybrid/He-det | 0.95 |  $-$18.80 | 0.11737 |  $-$19.6 ($g$) | 5.2 | Y/– | (25)

Note. —

a Names of EExSNe Ia are denoted in boldface.

b The peak absolute magnitude in the $B$ band. Galactic extinction has been
corrected for all golden early-phase SNe Ia. Reddening corrections of host
galaxies are performed for SN 2010jn, SN 2012cg, SN 2013dy, SN 2015F,
iPTF13ebh, iPTF16abc, and ASASSN-14lp by following related papers. For
ASASSN-14lp, we tentatively adopt $R_{V}$ = 1.7 to give a reasonable peak
magnitude for this spectroscopically normal SN Ia.

c The “discovery phase” is the detection (the corresponding filter is shown in
parentheses) time relative to the SN’s $B$-band maximum in the rest frame.

d The “photometric depth” is defined as a magnitude difference between the
detection and the maximum in the same band. Due to the lack of unfiltered
(Unf) peak magnitude of SN 2012fr and SN 2012ht, their photometric depths are
calculated by using the $R$-band peak magnitude.

e (1) Foley et al. (2011), (2) Hachinger et al. (2013), (3) Nugent et al.
(2011), (4) Pereira et al. (2013), (5) Zhang et al. (2014), (6) Klotz &
Conseil (2012), (7) Firth et al. (2014), (8) Walker et al. (2015), (9)
Ferretti et al. (2016), (10) Zheng et al. (2013), (11) Pan et al. (2015), (12)
Shappee et al. (2016), (13) Cartier et al. (2017), (14) Zheng et al. (2017),
(15) Richmond & Vietje (2017), (16) Smitka et al. (2015), (17) Marion et al.
(2016), (18) Miller et al. (2018), (19) Hosseinzadeh et al. (2017), (20) Cao
et al. (2016b), (21) Yamanaka et al. (2014), (22) Hsiao et al. (2015), (23)
Maguire et al. (2011), (24) Cao et al. (2015), (25) Jiang et al. (2017).

###  2.1 Early-phase normal and transitional SNe Ia

There are 12 normal SNe Ia in our golden sample. In general, their spectral
features and luminosities well match those of normal SNe Ia. For three objects
(SN 2010jn, SN 2012fr, and ASASSN-14lp) that show relatively high brightness
but with normal-like spectral evolution, we classify them as normal SNe Ia at
the bright end of the Phillips relation (Phillips, 1993) by following previous
studies (Hachinger et al., 2013; Childress et al., 2013; Shappee et al.,
2016). In addition to early optical observations, prompt $Swift$/UVOT follow-
up observations were successfully triggered for six normal SNe Ia (§3).
However, none of the 12 normal SNe Ia show early light-curve excess in optical
and UV wavelengths.

SN 2012ht and iPTF13ebh are categorized as “transitional” type based on their
fast-decline light curves and relatively high luminosities as opposed to
subluminous/ 91bg-like SNe Ia (Yamanaka et al., 2014; Hsiao et al., 2015).
Although two transitional golden early-phase SNe Ia show quite different
spectral features, smooth-rising light curves are found in both events at
early phases.

###  2.2 Early-phase luminous SNe Ia

Our golden early-phase SN Ia sample includes six SNe Ia with bright, slow-
evolving light curves and shallow absorptions by intermediate-mass elements
(IME), which are grouped into “luminous/91T-like,” “luminous/99aa-like,” and
“luminous/peculiar” subclasses in terms of their luminosities and specific
spectral features.

The relatively weak IME absorptions, prominent calcium and iron lines in the
premaximum spectra of three luminous SNe Ia (SN 2012cg, iPTF16abc, and SN
2017cbv) are reminiscent of 99aa-like SNe Ia. Note that iPTF14bdn has been
classified as a 99aa-like SN Ia by Smitka et al. (2015). However, the early
spectra dominated by iron-group elements (IGE), together with inconspicuous
IME absorptions, indeed show a closer resemblance to those of 91T-like SNe Ia.
Therefore, we classify iPTF14bdn as a 91T-like SN Ia in this paper.

In recent years, several SNe Ia with excessively high luminosities and broad
light curves have been discovered. Since the 56Ni mass required to produce
such high luminosities is in conflict with that realized in the explosion of a
1.4 $M_{\odot}$ WD, they are frequently called “super-Chandrasekhar-mass” SNe
Ia (“super-$M_{\rm Ch}$” SNe Ia). Spectroscopically, super-$M_{\rm Ch}$ SNe Ia
commonly show persistent C ii absorptions and moderately low photospheric
velocities. Except for the unknown UV properties, optical photometry and
spectroscopy of the golden early-phase SN Ia LSQ12gpw (Firth et al., 2014;
Walker et al., 2015; spectra can be found in the Open Supernova
Catalog222https://sne.space/; Guillochon et al., 2017) indicate a high
similarity to super-$M_{\rm Ch}$ SN Ia SN 2009dc (Silverman et al., 2011). We
classify this 09dc-like super-$M_{\rm Ch}$ SN Ia into the luminous/peculiar
subclass. Although the high peak luminosity, prominent IME absorptions, and
especially the persistent carbon absorption of another golden luminous SN Ia,
iPTF13asv—resemble super-$M_{\rm Ch}$ SNe Ia, its light curve evolves much
faster than those of super-$M_{\rm Ch}$ and many 91T/99aa-like SNe Ia (Table
1). By following the main conclusion from Cao et al. (2016b), we classify
iPTF13asv as a luminous/peculiar SN Ia.

The UV/optical light-curve excess is found in the early phase of all 91T/99aa-
like SNe Ia and LSQ12gpw (Figures 3–5), indicating an extremely high EEx
fraction of luminous SNe Ia. Detailed information for all luminous EExSNe Ia
is given in §3.2.

###  2.3 Early-phase subluminous SNe Ia

The SN 1991bg-like, SN 2002cx-like, and SN 2002es-like SNe Ia are typical
branches of subluminous SNe Ia. Mainly due to the fast-evolving light curves
of 91bg-like SNe Ia, so far, none of them have been discovered at very early
time. The 02cx-like SNe Ia occupy a large fraction of subluminous SNe Ia.
Luminosities of 02cx-like SNe Ia show very large scatter, and their light
curves evolve much slower than those of 91bg-like SNe Ia (see Foley et al.
2013 for a review). Although several 02cx-like SNe Ia have been discovered
over 10 days before reaching their $B$-band maxima, we are still missing the
photometric information in the first few days after their explosions. The
02es-like SNe Ia are spectroscopically similar to 91bg-like SNe Ia, indicating
a low temperature and ionization for both subclasses. The main characteristics
of 02es-like SNe Ia are a moderately faint ($M_{B,max}$ $\sim$$-$17.7–$-$18.0)
but slow-evolving (${\Delta}m_{15}(B)$ $\sim$1.1–1.3) light curve and lower
ejecta velocity compared with those of 91bg-like SNe Ia (Ganeshalingam et al.,
2012; White et al., 2015). Currently, the number of reported 02es-like SNe Ia
is small ($\lesssim$10), but two of them have been discovered at very early
time. In total, only two 02es-like SNe Ia, PTF10ops and iPTF14atg, can be
qualified as golden early-phase SNe Ia among published early-phase subluminous
SNe Ia, and both of them show interesting early light-curve behavior (§3.1).

There is one early-phase supernova, iPTF14dpk, which was claimed as a 02es-
like SN Ia by Cao et al. (2016a). In §3.4, we argue that the limited
observational information cannot provide any crucial evidence for classifying
iPTF14dpk as a 02es-like SN Ia; however, its peculiar early light-curve
behavior indeed suggests that iPTF14dpk is more likely an early-phase core-
collapse supernova (CCSN).

###  2.4 Early-phase hybrid (He-det) SNe Ia

Hybrid SNe Ia are defined as SNe Ia that show normal-like light curves but
with strong Ti ii absorptions commonly seen in subluminous SNe Ia at
pre/around-maximum phase. Jiang et al. (2017) recently discovered a prompt and
red light-curve excess of a hybrid SN Ia, MUSSES1604D (SN 2016jhr), within
$\sim$1 day of the explosion. The observed characteristics of MUSSES1604D,
including the peculiar EEx and the Ti ii trough feature shown in this normal-
brightness SN Ia, can be naturally explained by assuming that the SN is
triggered by a precursory detonation of a thin He layer at the surface of a
progenitor WD (the so-called “He-shell-detonation” or “He-det” scenario).
Currently, MUSSES1604D is the only early-phase SN Ia belonging to this class.

##  3 Early-excess SNe Ia

The definition of EExSNe Ia is schematically illustrated in Figure 2. In
general, early-excess features discovered so far can be visually classified
into two types. The first type is a prominent and precursory EEx occurring a
few days earlier than the 56Ni-powered main-body light curve (a “spike-like”
EEx), which can be easily distinguished (panels a & b). The second type is a
persistent (several days to more than 1 week) EEx that is coupled with the
main-body light curve from the beginning (a “bump-like” EEx), which can be
distinguished when the early-excess feature is prominent (panel c) or the
56Ni-powered main-body light curve evolves quickly (panel d). In addition to
two clear early-excess types, light curves influenced by an EEx scenario may
not have clear spike- or bump-like features but show “slow-rising” behavior in
the early phase (the yellow region in panel e), which may originate from a
weak bump that is covered by the main-body light curve, a weak excess observed
in insensitive wavelengths or a relatively large survey cadence, and so on.
Since such inconspicuous EEx will make the early light curve observationally
broader, we hereafter classify these SNe as “early-broad EExSNe Ia.” Given
that SN 2012cg shows the weakest early-excess feature in all previously
reported EExSNe Ia333The UV light curves of SN 2012cg roughly meet the
condition of panel d in Figure 2, and optical light curves are similar to the
“yellow+green” region in panel e; therefore, we use SN 2012cg as the reference
for selecting early-broad EExSNe Ia., we compare our early SN sample with SN
2012cg to find out early-broad EExSNe Ia. Specifically, by shifting light
curves of SNe Ia to match with that of SN 2012cg at peak, an SN Ia showing a
broader rising-phase light curve and similar or narrow post-maximum light
curve compared with that of SN 2012cg (the “yellow+green” region in panel e)
in any wavelength will be classified as an early-broad EExSN Ia. In other
words, an SN Ia which shows a generally broader light curve than that of SN
2012cg but without a spike- or bump-like early light-curve feature (the
“yellow$+$red” region in panel e) or a fast-rising light curve (panel f) will
be classified as a non-EExSN Ia. In our EExSN Ia sample (11 in total), nine of
them show prominent early-excess features (Figures 3–5). The newly discovered
SN Ia, SN 2017erp, and the previously reported SN 2012cg show relatively
inconspicuous early-excess features.

There are eight EExSNe Ia (iPTF14atg, iPTF14bdn, iPTF16abc, LSQ12gpw,
MUSSES1604D, PTF10ops, SN 2012cg, and SN 2017cbv) in our golden sample. Since
a more prominent early light-curve excess in UV/NUV wavelengths is predicted
by various EEx scenarios, we further select three additional EExSNe Ia (SN
2011hr, SN 2015bq, and SN 2017erp) from 40 SNe Ia (14 of them are golden
early-phase SNe Ia) that have UV/NUV light curves at relatively early phase.
In addition, we introduce another two possible EExSN Ia candidates,
ASASSN-15uh and SN 2015ak, in §3.2 but do not include them in our final sample
due to incomplete information. Our final EExSN Ia sample includes seven
luminous (91T/99aa-like & LSQ12gpw), two subluminous (02es-like), one normal
(SN 2017erp), and one He-det (MUSSES1604D) SNe Ia. Their basic information is
shown in Table 2.

Table 2: General Information on EExSNe Ia and Possible Candidates

Name | Subclass | ${\Delta}m_{15}(B)$ | Discovery Phasea | Photometric Depth | EEx Shapeb | EEx Durationc | Host Type
---|---|---|---|---|---|---|---
SN 2011hr | 91T-like | 0.92 |  $-$13.6 ($B$) | 2.0 | c | Long | Sb (NGC2691)
SN 2015bq | 91T-like |  $\sim$0.8 |  $-$13.9 ($B$) | 2.1 | c | Long | –
iPTF14bdn | 91T-like | 0.84 |  $-$18.0 ($R48$) | 4.1 | c | Long | Irr (UGC8503)
SN 2012cg | 99aa-like | 0.86 |  $-$17.1 ($B$) | 5.2 | d/e | Short | Sa (NGC4424)
iPTF16abc | 99aa-like | 0.95 |  $-$17.8 ($g$) | 5.5 | c/d | Medium | Sb (NGC5221)
SN 2017cbv | 99aa-like | 1.06 |  $-$18.9 ($B$) | 4.3 | c/d | Medium | SABc (NGC5643)
LSQ12gpw | 09dc-like | 0.76 |  $-$19.9 ($gr$) | 3.2 | a/c | – | –
SN 2017erp | Normal |  $\sim$0.9 |  $-$17.3 (Unf) |  3.6d | e | Medium | SABc (NGC5861)
PTF10ops | 02es-like | 1.12 |  $-$15.6 ($R48$) | 3.2 | b/d/e | – | –
iPTF14atg | 02es-like | 1.20 |  $-$18.5 ($R48$) | 3.8 | a | Medium | E (IC 831)
MUSSES1604D | He-det | 0.95 |  $-$19.6 ($g$) | 5.2 | a | – | S0-like
ASASSN-15uh | 99aa-like |  $\sim$0.8 |  $-$9.0 ($B$) | 1.1 | – | – | Sa-like
SN 2015ak | Normal |  $\sim$1.1 |  $-$15.1 (Unf) |  2.2d | e | – | Sb (ESO 108-21)

Note. —

a The “discovery phase” is the detection (the corresponding filter is shown in
parentheses) time relative to the SN’s $B$-band maximum in the rest frame.

b The EEx shape is evaluated by using a single-band light curve, which
includes the most complete early-phase information. If more than one
photometric band meet the requirement, a light curve in the bluest wavelength
is used. Letters correspond to the panels in Figure 2, which denote the
possible EEx shape of each SN Ia.

c A rough estimation of the duration of early-excess features. Light curves
used for the EEx shape evaluation and duration estimation are the same.
“Long,” “Medium,” and “Short” correspond to the EEx duration $T_{EEx}$
$\gtrsim$1 week, 0.5 week $\lesssim$ $T_{EEx}$ $\lesssim$ 1 week, and
$T_{EEx}$ $\lesssim$ 0.5 week, respectively.

d Due to the lack of unfiltered peak magnitude of SN 2017erp and SN 2015ak,
the photometric depths of two SNe are calculated by using the $Swift$/UVOT
$U$-band light curves started 0.6 and 1.5 days after their discoveries,
respectively.

###  3.1 02es-like and He-det EExSNe Ia

The best-observed 02es-like SN Ia so far is iPTF14atg. The prominent early
UV/optical excess of iPTF14atg can be basically explained by the companion-
ejecta interaction scenario as proposed by Cao et al. (2015). However, it is
still unclear what kind of explosion mechanism under the SD progenitor
scenario can explain the faint but slow-evolving light curve and spectral
evolution of 02es-like SNe Ia. On the contrary, previous simulations by Kromer
et al. (2016) suggest that the merger of two sub-Chandrasekhar-mass WDs could
well reproduce the spectral evolution and light curves of iPTF14atg by
assuming a low-metallicity progenitor system. They thus argued that the early
light-curve excess of iPTF14atg cannot be attributed to the companion-
interaction scenario.

The other 02es-like golden early-phase SN Ia, PTF10ops, also shows an
interesting early-phase light curve. As shown in Figure 3, although only one
data point of PTF10ops was taken at early time (Maguire et al., 2011), a very
likely early light-curve excess can be distinguished in comparison to other
golden early-phase SNe Ia and our companion-interaction model (Kutsuna &
Shigeyama, 2015; the cyan dashed curve in Figure 3), which implies that 02es-
like SNe Ia may commonly show peculiar light-curve behavior in the early
phase.

Jiang et al. (2017) pointed out that the spike-like, red EEx of iPTF14atg is
reminiscent of the He-det EExSN Ia MUSSES1604D, which originates from the
supernova explosion triggered by the helium detonation at the surface of the
progenitor. Given a significant amount of Ti production and the weak viewing
angle effect predicted by the He-det scenario, the high EEx fraction (Table 3)
and strong Ti ii absorptions shown in 02es-like SNe Ia can be explained with
the He-det scenario. In addition, recent simulations also verify the
possibility of reproducing the early light-curve excess of iPTF14atg with the
He-det scenario (Maeda et al., 2018).

As the current sample size of early-phase 02es-like SNe Ia is still inadequate
for statistical studies, it is uncertain that detections of EEx in both 02es-
like SNe Ia is due to a viewing angle-independent EEx scenario for 02es-like
SNe Ia or is just by chance.

###  3.2 Luminous EExSNe Ia

Surprisingly, all previously discovered early-phase 91T- and 99aa-like SNe Ia
show early-excess features (Figures 3–5). Even though early-excess features in
optical wavelengths are inconspicuous for some 91T/99aa-like SNe Ia, all of
them show more prominent EEx in UV/NUV wavelengths. In particular, 91T-like
SNe Ia seem to have stronger early-excess features than 99aa-like SNe Ia
(Figures 4–6).

The early-phase $R$-band light curve of a luminous golden early-phase SN Ia,
iPTF13asv, does not show the early-excess feature. Since both simulations from
different EEx scenarios and previous observations suggest that observations in
the $R$ band are not sensitive for detecting EEx, and the earliest photometry
of iPTF13asv is about 1-2 days later than that of other luminous early-phase
SNe Ia, we cannot confirm that the nondetection of EEx is due to the intrinsic
distinction between iPTF13asv and other luminous EExSNe Ia or the
imperfectness of early-phase observations for iPTF13asv. Here, we
conservatively classify iPTF13asv as a non-EExSN Ia and do not make a
comparison with other luminous EExSNe Ia in §4.

Four luminous EExSNe Ia in our sample have been well investigated before.
Marion et al. (2016) claimed that the EEx of SN 2012cg originates from the
companion interaction in comparison to light curves predicted by Kasen (2010).
Note that such a conclusion is controversial due to the lack of simulation-
based comparisons with other EEx mechanisms. In the case of the early-excess
feature of SN 2017cbv, the companion-interaction scenario is preferred based
on the early-phase light-curve fitting result. However, other EEx scenarios
indeed cannot be completely excluded, as discussed by Hosseinzadeh et al.
(2017). Other than SN 2012cg and SN 2017cbv, the EEx of iPTF16abc has been
argued to be incompatible with the companion interaction based on an
inconsistent light-curve fitting result and strong C ii absorptions at the
early phase (Miller et al., 2018). Smitka et al. (2015) speculated that the
blue EEx shown in a 91T-like SN Ia, iPTF14bdn, may originate from the
radioactive decay in the outer-layer ejecta where the 56Ni abundance is
greater than normal. However, there is no attempt to fit such a prominent
early light-curve excess with any EEx scenario so far.

Two other luminous EExSNe Ia, SN 2011hr and SN 2015bq, and one possible
luminous EExSN Ia candidate, ASASSN-15uh, are selected from published
resources and the $Swift$ Optical/Ultraviolet Supernova Archive
(SOUSA444http://swift.gsfc.nasa.gov/docs/swift/sne/swift_sn.html. The
reduction is based on that of Brown et al. (2009), including subtraction of
the host galaxy count rates and uses the revised UV zero points and time-
dependent sensitivity from Breeveld et al. (2011).; Brown et al., 2014). SN
2011hr was classified as a luminous SN Ia which may bridge 91T-like SNe Ia and
a super-$M_{\rm Ch}$ SN Ia, SN 2007if, in terms of high luminosity (Zhang et
al., 2016). Although the expected 56Ni mass of SN 2011hr may be greater than
the mean value of 91T-like SNe Ia, it can still be explained by the explosion
of a Chandrasekhar-mass WD progenitor that experienced a more efficient
detonation phase than normal SNe Ia. Additionally, the extremely high spectral
similarity to SN 1991T from $-$13 days to 2 months after the $B$-band maximum
indicates the physical resemblance between the two objects. In Figure 5, a
prominent light-curve excess can be seen at $\sim$12 days before the $U$-band
maximum. We therefore classify SN 2011hr as a luminous/91T-like EExSN Ia.

SN 2015bq was discovered by the intermediate Palomar Transient Factory (iPTF;
Law et al., 2009; Rau et al., 2009) and well followed by $Swift$/UVOT from
about 2 weeks before the $B$-band maximum. The high-luminosity, slow-evolving
light curve, together with early-phase spectra composed by prominent IGE and
inconspicuous IME absorptions, indicate that SN 2015bq is a typical 91T-like
SN Ia. Interestingly, the prominent early light-curve excess of SN 2015bq
shows very high similarity to two other 91T-like SNe Ia, iPTF14bdn and SN
2011hr, which indeed is much stronger than that of all 99aa-like EExSNe Ia. In
§4.2, we discuss the intrinsic connection between 91T-like and 99aa-like
EExSNe Ia and the possible explosion mechanism of these luminous objects.

ASASSN-15uh was reported as a 98es-like SN Ia by the Asiago Transient
Classification Program (Tomasella et al., 2014; Turatto et al., 2015). Due to
the limited photometric and spectroscopic information of both the SN and its
host galaxy, we took a low-resolution spectrum of the host galaxy with the
Line Imager and Slit Spectrograph (LISS; Hashiba et al., 2014) mounted on the
Nayuta 2-m telescope in December 2017. We derived a host redshift of about
0.0150 by using hydrogen emission lines, which is consistent with the reported
redshift of the supernova ($\sim$0.0135; Turatto et al., 2015). High UV
luminosity and slow-evolving light curve well match those of 91T/99aa-like SNe
Ia. Spectroscopically, shallow IME absorptions together with prominent IGE
absorptions around 10 days before the $B$-band maximum are also reminiscent of
99aa-like SNe Ia. For the early-phase light curve, a possible early-excess
feature can be marginally distinguished from the earliest observation at
$\sim$8 days before the $U$-band maximum (Figure 5). Since the early-phase
information of ASASSN-15uh is insufficient to determine the rising behavior,
we mark ASASSN-15uh as a possible EExSN Ia candidate and do not make a
quantitative comparison with other luminous EExSNe Ia in the next section.

###  3.3 SN 2017erp, the first normal EExSN Ia?

SN 2017erp was discovered at $\sim$17 days before its $B$-band maximum. Its
early-phase spectral features, such as clear IME absorptions and high Si ii
velocity, show a good similarity to those of a normal SN Ia, SN 2003W (Jha et
al., 2017). Particularly, early $Swift$/UVOT observations of SN 2017erp show
interesting rising behavior in UV/optical wavelengths (Figures 3–5; see Brown
et al. (2018) for more complete multiband light curves), which well meets our
criteria of an early-broad EExSN Ia, especially for the $UVW1$-band light
curve. Except for the peculiar early-phase light curve, a relatively slow-
decline light curve toward the $B$-band peak magnitude of $\sim$$-$18.9
(Galactic extinction corrected) and the spectral evolution (Brown et al.,
2018) of SN 2017erp resemble those of normal SNe Ia, such as SN 2009ig and SN
2013dy. No matter what kind of mechanism accounts for the peculiar early-phase
light curve of SN 2017erp, an early-broad EEx discovered in a normal SN Ia
inspires us that the early light-curve excess can also be expected in a
fraction of normal SNe Ia.

The normal-type EExSNe Ia may have been discovered even earlier. The prompt
$Swift$/UVOT follow-up observation of SN 2015ak, a 99ee-like normal SN Ia
identified by Hosseinzadeh et al. (2015), shows a similar early-phase light-
curve behavior to that of SN 2017erp, though the photometric uncertainty is
much larger. The limited post-maximum information suggests that the light-
curve evolution is normal-like and the $Swift$/UVOT $B$-band peak magnitude is
$\sim$$-$18.5 mag by adopting the redshift of the reported host galaxy. Due to
the uncertainties in both the SN classification and the EEx identification, we
conservatively mark SN 2015ak as a likely normal EExSN Ia candidate.

###  3.4 iPTF14dpk, a CCSN discovered at the shock-cooling emission phase?

Cao et al. (2016a) claimed that iPTF14dpk is a 02es-like SN Ia viewed from the
direction that is far away from a nondegenerate companion star based on the
nondetection of early light-curve excess and moderate similarities to
iPTF14atg. We argue that the identification of this object is disputable
because 1) a subluminous $R$-band light curve and limited spectral
information555Spectra of iPTF14dpk were obtained at $-$10, $+$20, and $+$50
days after the $R$-band maximum, which are also similar to spectra of some SNe
Ic at the same epochs, e.g. SN 2004aw (Taubenberger et al., 2006). prevent us
from identifying whether iPTF14dpk is a 02es-like, 02cx-like SN Ia or an SN
Ic; 2) a starburst host galaxy is different from the hosts of all previously
discovered 02es-like SNe Ia but commonly seen in 02cx-like SNe Ia, and also
CCSNe where massive stars are actively formed; 3) according to the limiting
magnitude of the last nondetection of iPTF14dpk, its $R$-band magnitude
increased to $\sim$$-$17 with a rising speed over $-$1.8 mag night-1 (Figure
3). Such a peculiar early light-curve behavior cannot be reproduced by any EEx
mechanism of SNe Ia but resembles the cooling tail of shock breakout of CCSNe,
the rising phase of which is too short to be caught by a day-cadence survey
(e.g. SN 2006aj, Campana et al., 2006; SN 2011dh, Arcavi et al., 2011;
iPTF15dtg, Taddia et al., 2016).

Ambiguous classifications between subluminous SNe Ia and stripped-envelope
CCSNe can happen sometimes. For example, although the spectral and light-curve
resemblance between LSQ14efd and subluminous SNe Ia have been noticed by
Barbarino et al. (2017), they classified LSQ14efd as an SN Ic based on the
early-phase light curve (Figure 3) and the incompatible Si ii velocity
evolution to that of normal SNe Ia and a 02cx-like SN Ia, SN 2008A. We note
that their argument about the velocity evolution can only exclude a part of
SNe Ia but some subluminous SNe Ia (e.g. iPTF14atg) may have similar Si ii
velocity evolution to that of LSQ14efd. However, given that both LSQ14efd and
iPTF14dpk show unique early-excess features as opposed to other EExSNe Ia and
the lack of crucial evidence for the SN classification, we argue that both
objects are more likely CCSNe discovered at the shock-cooling phase.

##  4 Discussions

###  4.1 The fraction of EExSNe Ia and its implications

Current EEx scenarios can be classified into two different types in terms of
the physical mechanism: interaction-induced (i.e. companion & CSM interaction)
and surface-radioactivity-induced (i.e. He-det & “surface-56Ni-decay”)
scenarios. Recent simulations suggest that multiband photometric information
during the EEx phase is crucial for differentiating EEx scenarios (e.g. the
color evolution predicted by CSM interaction is generally faster than other
scenarios, the companion-interaction may have relatively blue and slow color
evolution at the EEx phase, etc.; see Maeda et al., 2018 for details).
However, the scatter of 56Ni-powered early-phase light curves (Firth et al.,
2014), the viewing angle effect, and uncertainties of current simulations
(especially in the UV wavelength) may prevent us from differentiating EEx
scenarios in some cases. Therefore, further evidence from simulation-
independent perspectives is particularly important. Since different EEx
scenarios may not only give rise to different EEx fractions due to the viewing
angle effect but also relate to specific SN Ia subclasses, statistically
investigating the fraction of EExSNe Ia in each subclass can be a good
starting point (Table 3).

Table 3: Supplementary Information on Early-phase SNe Ia in Different
Subclasses and Their Early-excess Fractions

Subclass | Early-phase SNe Ia | EExSNe Ia | EEx Fraction | Observed Rising- | Photometric-depth | Host
---|---|---|---|---|---|---
| Golden (Finala) | Golden (Final) | Golden (Final) | time Limitb | Limitc | Type
Normal | 12 (17) | 0 (1) | 0 (5.9%) | 18.2 (iPTF13dge) | 7.3 (SN 2011fe; $g$) | Various
Luminous (91T/99aa) | 4 (6) | 4 (6) | 100% (100%) | 18.9 (SN 2017cbv) | 5.5 (iPTF16abc; $g$) | Late-type
Luminous (peculiar) | 2 (1) | 1 (1) | 50% (100%) | 19.9 (LSQ12gpw) | 3.7 (iPTF13asv; $R48$) | Late-type
Transitional | 2 | 0 | 0 | 15.8 (SN 2012ht) | 5.9 (iPTF13ebh; $R48$) | Various
02es-like | 2 | 2 | 100% | 18.5 (iPTF14atg) | 3.8 (iPTF14atg; $R48$) | Early-type
Hybrid | 1 | 1 | 100% | 19.6 (MUSSES1604D) | 5.2 (MUSSES1604D; $g$) | Early-type

Note. —

a Since the earliest $R$-band photometry of iPTF13asv is about 1-2 days later
than that of other luminous early-phase SNe Ia and there is no early UV/blue-
optical photometry for this object, we exclude iPTF13asv from the final sample
for the early-excess fraction calculation. Instead, we include five additional
normal SNe Ia, SN 2007af, SN 2008hv, SN 2013gy, SN 2015ak, and SN 2017erp, and
two 91T/99aa-like SNe Ia, SN 2011hr and SN 2015bq, which have the earliest
UV/blue-optical information among other non-golden $Swift$/UVOT samples, in
the final estimation.

b The “observed rising-time limit” is the longest time interval (rest-frame)
from the detection to the $B$-band maximum (the champion SN’s name is shown in
parentheses) in each SN Ia subclass.

c The “photometric-depth limit” is the largest magnitude difference between
the detection and the maximum in the same band (the champion SN’s name and the
corresponding filter are shown in parentheses) in each SN Ia subclass.

Under the companion-interaction scenario, the early-excess feature can be
observed only from specific viewing angles, which constrains the observable
EExSNe Ia fraction to $\sim$10$\%$ (Kasen, 2010). Then, the probability of
$\sim$10-6 for a 100% EEx detection of six 91T/99aa-like SNe Ia suggests that
the early light-curve excess of 91T/99aa-like SNe Ia should be attributed to a
different EEx scenario.

The CSM-ejecta interaction through the WD–WD merging channel is also difficult
to explain EEx of 91T/99aa-like SNe Ia for the following reasons: i) short-
lived carbon absorptions shown in 91T/99aa-like SNe Ia imply a low carbon-
abundance environment; ii) such a high EEx fraction suggests a spherical
distribution of CSM to eliminate the viewing angle effect, which may be
difficult to realize because the CSM-interaction EEx requires a short lag time
between the secondary WD disruption and the SN explosion (Raskin & Kasen,
2013; Tanikawa et al., 2015; Levanon et al., 2015, 2017); iii) the long EEx
durations of some luminous SNe Ia require a tremendous amount of CSM that is
difficult to be produced by disrupting a companion WD (Maeda et al., 2018).
Therefore, the detection of EEx in all 91T/99aa-like SNe Ia suggests the
physical connection between EEx and the explosion mechanism (which may also be
related to a specific progenitor system) of these luminous objects.

Although EEx is always shown in 91T/99aa-like SNe Ia, a large scatter of the
EEx strength can be found in Figures 4 and 5, which is barely related to the
luminosity of main-body light curves. Instead, a likely correlation between
the EEx strength and the equivalent width of the Si ii $\lambda$6355 line666In
order to maximize the sample size of 91T/99aa-like EExSNe Ia for comparison,
we use the magnitude variance from about $-$13 to $-$5 days after the peak
epoch in the $UVW1$/$U$ band to describe the EEx strength. The smaller the
magnitude variance, the stronger the early light-curve excess. Spectra used
for the pseudo-equivalent width (pEW) calculation were taken around
$-14$–$-13$ days after their $B$-band maxima. is shown in Figure 6: a
91T/99aa-like SN Ia with a shallower Si ii $\lambda$6355 absorption has more
prominent EEx. In other words, 91T-like SNe Ia generally have stronger early-
excess features than 99aa-like SNe Ia. Such a correlation is independent of
the peak luminosity (panels (c) and (d) in Figure 6).

Defining a “non-EExSN Ia” is tricky, as it depends not only on the performed
early-phase observations (cadence, imaging quality, wavelengths, etc.) but
also on the strength of the EEx itself. By using the golden early-phase SNe Ia
and other SNe Ia which have early-enough UV/NUV information, we try to present
the currently “best” non-EExSN Ia sample for the EEx fraction estimation.
However, we note that even for some golden early-phase SNe Ia (e.g. SN
2010jn), it still remains a possibility that the non-EEx detection is due to
the dimness of the EEx and/or unfavorable observing wavelengths (e.g. $V$ and
$R$ band). In other words, the EEx fraction shown in this paper is a somewhat
lower limit due to the probable incompleteness of EEx detections from previous
observations. Therefore, an EEx fraction of 5.9% (one of a total of 17 normal
early-phase SNe Ia, which include 12 golden SNe Ia and five additional SNe Ia,
SN 2007af, SN 2008hv, SN 2013gy, SN 2015ak, and SN 2017erp, that have the
earliest UV/blue-optical photometric data among other non-golden normal SNe
Ia777Given that some normal SNe Ia in our $Swift$/UVOT sample are not early
enough for the non-EEx classification, we only include 12 golden normal SNe Ia
and five additional normal SNe Ia, SN 2007af, SN 2008hv, SN 2013gy, SN 2015ak,
and SN 2017erp, which have UV/blue-optical information at about 10 days before
their maxima in the same band.) or 11.8% (by counting both SN 2017erp and SN
2015ak as normal EExSNe Ia) for normal SNe Ia cannot exclude any EEx scenario.

There were several attempts at searching for EExSNe Ia with datasets from
large supernova survey projects (Hayden et al., 2010; Bianco et al., 2011;
Ganeshalingam et al., 2011), but none of them reported a positive EEx
detection. We argue that the previous studies are not in conflict with the
high EEx fraction of 91T/99aa-like SNe Ia proposed in this paper for the
following reasons. i) Since the purpose of those studies is to find
significant early-excess features for testing the companion-interaction
scenario (especially for the red-giant companion condition) by comparing their
early-phase light curves with the model light curve predicted by Kasen (2010),
they do not further investigate the “small” early light-curve scatter in their
plots. ii) Although only six 91T/99aa-like SNe Ia with very limited early-
phase data points are included in Ganeshalingam et al. (2011; see Figure 3
therein), it is clearly shown that the large light-curve scatter at early
phase is related to 91T/99aa-like SNe Ia. iii) The early light-curve scatter
shown in Figure 1 of both Hayden et al. (2010) and Bianco et al. (2011) is
similar to the scatter in Figure 3 of this paper, which may also relate to
91T/99aa-like SNe Ia in their samples. Since the sample composition in both
papers is not clearly mentioned, further analysis of those samples may
increase the total number of EExSNe Ia and improve the EEx-fraction
estimation. Indeed, our finding about the correlation between early light-
curve excess and 91T/99aa-like SNe Ia can be further supported by a very
recent work by Stritzinger et al. (2018; accepted during the proof stage of
this paper), which suggests that 91T/99aa-like SNe Ia tend to show very blue
$B-V$ color evolution in the early phase. In addition to statistical studies,
early-phase photometries of two $Kepler$-discovered SNe Ia, KSN 2012a and KSN
2012b, give good constraints on the non-EEx detection of two SNe (Olling et
al., 2015), although we exclude them from the fraction calculation due to a
lack of the spectroscopic identification. Given that both are more like normal
SNe Ia in terms of the light-curve behavior, the non-EEx detections of two SNe
should not influence the EEx fraction of 91T/99aa-like SNe Ia but may slightly
decrease the EEx fraction of normal SNe Ia.

###  4.2 From normal to super-$M_{\rm Ch}$ SNe Ia, the diversity and their
intrinsic connections

In order to explain the IGE-dominant absorptions in the early spectra of
91T/99aa-like SNe Ia, a more efficient detonation reaching the outermost
region of the ejecta is frequently proposed (e.g. Zhang et al., 2016). This
may account for the early light-curve enhancement due to high 56Ni production
in the outer layer of the ejecta (Piro & Morozova, 2016). However, as opposed
to a spike-like EEx powered by a large amount of fast-decay radioactive
elements from a precursory detonation of a thin He-shell predicted by the He-
det scenario, a broadened, possibly bump-like early-phase light curve will be
formed due to the radioactivity of substantial 56Ni at the outermost region of
the ejecta. Such a prediction is indeed reminiscent of the early-rising
behavior of luminous EExSNe Ia (Figures 4 & 5).

Such an efficient detonation is expected in an extreme case of the delayed-
detonation model, represented by the so-called gravitationally confined
detonation (GCD; Plewa et al., 2004; Townsley et al., 2007; Jordan IV et al.,
2008; Meakin et al., 2009). The buoyantly rising deflagration bubble ignited
near the stellar core will finally trigger a detonation at the opposite pole
from the breakout location of the deflagration bubble. Thanks to the mild
expansion before the bubble reaches the surface, the detonation is more
efficient than the traditional delayed-detonation scenario (Niemeyer &
Woosley, 1997) and thus results in a larger amount of 56Ni extended to the
ejecta’s surface. According to Jordan IV et al. (2012), the detonation happens
at the antipodal point from where the star ejected the ash from its interior,
giving rise to approximatly azimuthal symmetry upon the system. Therefore, the
diversity of element distribution due to the viewing angle effect can be
expected, e.g. more 56Ni will be generated at the surface on the opposite side
of the off-center ignition point. Since the asymmetry of the detonation
becomes smaller when getting to the inner region of the WD, the viewing angle
effect in the earlier phase is more prominent. Such a prediction is basically
in line with recent 3D simulations (Seitenzahl et al., 2016).

Although spectra produced by Seitenzahl et al. (2016) show clear distinctions
from SN 1991T after taking into account the viewing angle effect, shallower-
than-normal IME absorptions and luminous and slow-evolving light curves are
qualitatively consistent with typical features of 99aa-like SNe Ia.
Nevertheless, their simulation shows that only UV/NUV light curves and
spectral features at wavelengths $\lesssim$ 5000 Å highly depend on the
viewing angle, which suggests that a main spectral difference between 91T- and
99aa-like SNe Ia, the Si ii absorptions, cannot be completely attributed to
the viewing angle effect through the GCD scenario.

Simulations of the GCD scenario also indicate that by switching the offset
distance of the ignition point, the total 56Ni amount (and thus the EEx
strength) and spectral features can be varied. In order to explain the extreme
spectral features of 91T-like SNe Ia with the GCD scenario, an intuitive
extrapolation is that 91T-like SNe Ia that have the strongest EEx and
shallowest IME features correspond to the most extreme offset condition. By
taking into account both the off-center level and the viewing angle effect,
spectral differences between 91T- and 99aa-like SNe Ia, the correlation
between the EEx strength and the equivalent width of the Si ii $\lambda$6355
line, and the luminosity scatter shown in 91T/99aa-like SNe Ia could be
promisingly explained with the GCD scenario. In order to further testify to
the GCD scenario and its relation to luminous SNe Ia, it is encouraged to
investigate the upper limit of the initial offset and multiple-bubbles-
triggered GCD models in future simulations.

For the progenitor system of 91T/99aa-like SNe Ia, since the GCD-induced EEx
can be realized through both single- and double-degenerate channels if the
mass of merged WDs can reach the Chandrasekhar mass limit, early-excess
features of 91T/99aa-like SNe Ia may not provide direct information on their
progenitor systems.

A further question is whether both 91T/99aa-like and normal SNe Ia have the
same origin. In principle, the luminosity and spectral features of normal SNe
Ia can be reproduced by decreasing the offset distance of the ignition point
so that the GCD scenario will finally get back to the traditional delayed-
detonation scenario, which can also give rise to the smooth, small-scatter
rising light curves that we see in normal SNe Ia. For the progenitor system of
two SN Ia subclasses, nondetections of a surviving companion star and hydrogen
emission lines in the nebular phase spectra of normal SNe Ia so far are in
conflict with predictions by the SD progenitor scenario (Schaefer & Pagnotta,
2012; Mattila et al., 2005; Leonard, 2007; Shappee et al., 2013; Lundqvist et
al., 2013; Maguire et al., 2016), which is in opposition to the growing
evidence of the possible connection between 91T/99aa-like SNe Ia and the SD
progenitor scenario (Leloudas et al., 2015). Therefore, even with the same
explosion physics (note that GCD in nature is a special case of the delayed-
detonation scenario), normal and luminous SNe Ia may originate from different
progenitor systems (Fisher & Jumper, 2015).

If the amount of surface 56Ni is moderate compared with that of 91T/99aa-like
SNe Ia, we may see an early-broad EExSN Ia instead of a bump-like one, which
not only is in line with the early-phase light curve of SN 2017erp and
previous simulations (Piro & Morozova, 2016) qualitatively but may also
explain some “slow rising,” “fast decline” light curves found in previous
statistical studies (Hayden et al., 2010; Ganeshalingam et al., 2011).
However, if the surface-56Ni-decay scenario accounts for the EEx of the normal
SN Ia SN 2017erp and the candidate SN 2015ak, a further question is why the
56Ni radiation through the traditional delayed-detonation scenario can realize
such a large early light-curve scatter in normal SNe Ia. One possible
explanation is that a small offset degree of the ignition point may also exist
even under the traditional delayed-detonation scenario (Maeda et al., 2010).
In order to further understand the origin of such inconspicuous EEx shown in
normal SNe Ia, a comprehensive investigation of SN 2017erp and SN 2015ak is
highly recommended.

As we mentioned in §2.2, early light-curve excess is also found in a 09dc-like
SN Ia, LSQ12gpw, at $\sim$20 days before the $B$-band maximum. Although the
explosion mechanism and progenitor system of super-$M_{\rm Ch}$ SNe Ia are
still under debate, previous observations show evidence of dense CSM around
super-$M_{\rm Ch}$ SNe Ia. Yamanaka et al. (2016) reported the near-infrared
light echo from $\sim$40 to 110 days after the $B$-band maximum of a
super-$M_{\rm Ch}$ SN Ia candidate, SN 2012dn, suggesting the presence of a
wind from the pre-explosion system, possibly through the SD channel. If such a
dense CSM can be formed nearer to the explosion site, not only the companion
interaction but also the CSM interaction may explain the early light-curve
excess of LSQ12gpw under the SD progenitor scenario.

On the other hand, Taubenberger et al. (2013) pointed out that the discrepancy
between the extremely luminous peak and the faint light-curve tail of the
super-$M_{\rm Ch}$ SN Ia SN 2009dc may require additional energy injection at
early phase (see also Maeda et al., 2009), i.e. the interaction between SN
ejecta and nearby CSM. Such confined CSM could be realized through the double-
degenerate (DD) channel as the envelope predicted by the merger scenario would
be ejected soon after the disruption of the secondary WD (Levanon et al.,
2015; Tanikawa et al., 2015). Under the CSM-interaction scenario through the
DD channel, not only the prominent early light-curve excess of LSQ12gpw but
also the persistent C ii features that are commonly shown in super-$M_{\rm
Ch}$ SNe Ia could be explained. Furthermore, although only one early-phase
super-$M_{\rm Ch}$ SN Ia (i.e. LSQ12gpw) has been found, the discovery of EEx
in this super-$M_{\rm Ch}$ SN Ia may imply an EEx scenario that is weakly
affected by the viewing angle effect. Therefore, we speculate that the early
light-curve excess in super-$M_{\rm Ch}$ SNe Ia is more likely powered by the
CSM interaction through either the DD or the SD channel. Given that the CSM
components are possibly dominated by different materials in two progenitor
systems, spectroscopy at the EEx phase may provide new evidence of the
progenitor system of super-$M_{\rm Ch}$ SNe Ia.

###  4.3 The companion-interaction EExSNe Ia

The statistical study of EExSNe Ia not only further supports the multiple
origins of previously discovered EEx but also reduces the possibility of the
connection between luminous EExSNe Ia and the companion-interaction scenario
to a large extent.888Note that if 91T/99aa-like SNe Ia truly originate from
the SD progenitor system, it remains a possibility that the EEx of a small
fraction of luminous SNe Ia may have multiple components. However, as opposed
to the bump-like EEx of 91T/99aa-like SNe Ia, a spike-like EEx of the
subluminous EExSN Ia, iPTF14atg, cannot be interpreted by the 56Ni
radioactivity from the outermost layer of the ejecta. Additionally, recent
simulations by Maeda et al. (2018) also indicate a significant discrepancy
between the early-phase light curve of iPTF14atg and their CSM-interaction
models.

Although the EEx of iPTF14atg seems to be promisingly reproduced with the
companion interaction (Cao et al., 2015; Maeda et al., 2018), whether its
spectral evolution and general light-curve behavior can be reproduced through
the SD channel is still under debate (Kromer et al., 2016). Alternatively, as
we discussed in §3.1 (also see Jiang et al. 2017; Maeda et al. 2018), in terms
of the spectral and EEx resemblance between MUSSES1604D and iPTF14atg, the He-
det scenario is also promising for explaining 02es-like EExSNe Ia. As the
current early-phase information of 02es-like SNe Ia is too limited to make a
comparative analysis or give a stringent constraint from the statistical
perspective, finding more early-phase 02es-like SNe Ia through ongoing
transient surveys will shed light on the nature of this peculiar SN Ia
subclass.

##  5 Conclusions

In this paper, we present general information on published, well-observed
early-phase SNe Ia so far and summarize the characteristics of 11 (six
reported and five unnoticed) EExSNe Ia in different subclasses. In particular,
by investigating the connections and differences between these EExSNe Ia, new
evidence of multiple origins of early light-curve excess and the implication
for the explosion mechanism of 91T/99aa-like SNe Ia are presented.

We found that a 100% EEx detection in six early-phase 91T/99aa-like SNe Ia is
significantly in conflict with the prediction of the companion-ejecta
interaction scenario but can be promisingly explained by the radioactive decay
of a 56Ni-abundant outer layer or, alternatively (but less likely),
interacting with spherically distributed CSM. In addition, the discovered
correlation between the Si ii absorption feature and the strength of EEx also
suggests an intrinsic connection between the explosion mechanism and the early
light-curve excess of 91T/99aa-like SNe Ia. By investigating the early-excess
behavior, post-EEx photometric/spectroscopic properties, and explosion models
proposed for luminous SNe Ia, the surface-56Ni-decay scenario is preferred for
interpreting 91T/99aa-like EExSNe Ia. Specifically, we argue that the
gravitationally confined detonation is a promising scenario for producing
91T/99aa-like SNe Ia with such a high EEx fraction. Also, spectral and early-
excess differences between 91T- and 99aa-like SNe Ia and their intrinsic
luminosity scatters could be qualitatively explained by taking into account
the variation of the off-center distance of the initial ignition point and the
viewing angle effect. In contrast to the high EEx fraction of luminous SNe Ia,
early light-curve excess discovered in a normal SN Ia, SN 2017erp, and a
possible candidate, SN 2015ak, so far suggests that EEx may accompany with a
fraction of normal SNe Ia. Whether the EEx shown in normal SNe Ia is
attributed to surface 56Ni decay through the traditional delayed-detonation
mechanism requires further investigation.

Even though about a dozen of EExSNe Ia were successfully discovered in various
SN Ia subclasses, we have not found any crucial evidence to support the
companion-interaction scenario, even for the most promising candidate,
iPTF14atg. The multiple origins of early light-curve excess suggest that
EExSNe Ia may not be a superior indicator of the SD progenitor system as we
originally expected, and we need to be more cautious when interpreting any
newly discovered EExSNe Ia. Further understanding of the early-excess
scenarios relies not only on individual studies of well-observed EExSNe Ia but
also on systematical investigations of the early-phase light-curve behavior of
each SN Ia subclass, which can be realized with ongoing survey projects such
as the Zwicky Transient Facility (ZTF; Smith et al., 2014), the MUlti-band
Subaru Survey for Early-phase SNe Ia (MUSSES; Jiang et al. 2017; Miyazaki et
al. 2018), and forthcoming transient surveys with the Tomo-e Gozen Camera
mounted on the 1.05-m Kiso Schmidt telescope (Sako et al., 2018) and the Large
Synoptic Survey Telescope (LSST; Ivezić et al., 2008) in the near future.

We thank the anonymous referee for informative comments and suggestions. We
also thank G. Hosseinzadeh, A. Miller, and M. Yamanaka for promptly providing
access to published data of several SNe Ia in our sample. This work has been
supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grants
18J12714 (J.J.), 18H04342 and 16H01087 (J.J. and M.D.), 18H05223 (J.J., M.D.,
K.M., and T.S.), 18H04585 and 17H02864 (K.M.), and 16H06341, 16K05287, and
15H02082 (T.S.).

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Figure 1: Absolute $B$-band peak magnitude ($M_{B,max}$) vs. the light-curve
decline rate, expressed by the magnitude decline in 15 days from the $B$-band
peak ($\Delta$$m$${}_{15}(B)$), for 23 golden early-phase SNe Ia. Four
91T/99aa-like (blue triangles), one luminous/peculiar (blue open square), two
subluminous (red inverted triangles), and one He-det (green pentagon) SNe Ia
show early-excess features. Gray points are from CfA3 samples in Hicken et al.
(2009). The Phillips relation is highlighted in cyan.  Figure 2: Schematic
diagram of different shapes of early-excess features. Spike-like (panels a &
b), prominent bump-like (panel c), and weak bump-like early-excess features
but with a fast-evolving 56Ni-powered main-body light curve (panel d) can be
easily distinguished. We further use SN 2012cg (a sketchy dotted light curve
is shown in each panel, and all SN light curves are shifted to match with that
of SN 2012cg at peak), which shows the weakest early-excess feature as the
reference for selecting other early-broad EExSNe Ia. Specifically, an SN Ia
showing a broader rising-phase light curve and similar or narrow post-maximum
light curve compared with that of SN 2012cg (the “yellow+green” region in
panel e) in any wavelength will be classified as an early-broad EExSN Ia. SNe
Ia with light curves in the “yellow+red” region (panel e) and the red region
(panel f) are classified as non-EExSNe Ia. Figure 3: Early-phase light curves
of golden early-phase SNe Ia. Symbols are the same as in Figure 1. In order to
compare early light-curve behaviors at a comparable wavelength coverage with
the largest sample size, we select golden early-phase SNe Ia that have
photometric information in either the PTF $R48$ band, Cousins $R$ band or SDSS
$r$ band for comparison. Three normal SNe Ia with LSQ $gr$-band photometry are
plotted with small gray crosses. Early light curves of SN 2012fr and SN
2016coj are not included due to the lack of early photometric information in a
comparable wavelength coverage to other golden early-phase SNe Ia (note that
neither of them shows early-excess feature even in blue wavelengths; Zhang et
al., 2014; Zheng et al., 2017). As a reference of the early-excess feature of
normal SNe Ia, we attached the early $Swift$/UVOT $V$-band light curve of the
normal EExSN Ia, SN 2017erp. As can be seen, the early light-curve behavior of
two cooling-emission CCSN candidates (filled circles), iPTF14dpk ($R48$) and
LSQ14efd ($V$), are significantly different from those of golden early-phase
SNe Ia. A cyan dashed curve is predicted by the interaction between SN ejecta
and a 1.05 $M_{\odot}$ red-giant companion viewed from the companion side
(Kutsuna & Shigeyama, 2015). Figure 4: Early-phase $UVW1$-band light curves of
different SN Ia subclasses. Symbols are the same as in Figures 1 and 3 are
used (diagonal crosses are specified for normal SNe Ia with slow-decline light
curves). All light curves are shifted to the same peak magnitude for
comparison. The 91T- and 99aa-like SNe Ia have clear excess compared to the
rest, though with large scatter. In contrast, normal SNe Ia, except for SN
2017erp and SN 2015ak, show smooth, small-scatter, rising light curves though
some of them (diagonal crosses) have similar post-maximum light curves to
those of 91T/99aa-like SNe Ia. So far, we have not discovered any early-excess
features from transitional, 91bg-like, and 02cx-like SNe Ia, which may be due
to the extremely limited early-phase information of these SN Ia subclasses.
Figure 5: Early-phase $U$-band light curves of different SN Ia subclasses.
Symbols are the same as in Figure 4. All light curves are shifted to the same
peak magnitude for comparison. Early light-curve excess can still be clearly
discovered in 91T/99aa-like SNe Ia and a 02es-like SN Ia, iPTF14atg. Two
normal SNe Ia, SN 2017erp and SN 2015ak, show marginal early-excess features
in the $U$ band. The SNe Ia in other classes show smoothly rising light curves
in the early phase.  Figure 6: Early $UVW1$/$U$-band excess vs. the pseudo-
equivalent width (pEW) of the Si ii $\lambda$6355 line at about 13 days before
the $B$-band peak of 91T/99aa-like EExSNe Ia. In order to maximize the sample
size of 91T/99aa-like EExSNe Ia for the comparison, we use the magnitude
variance from about $-$13 to $-$5 days after the peak epoch in the $UVW1$/$U$
band to describe the EEx strength (panels (a) and (b)). Two normal SNe Ia that
have both photometric and spectroscopic information at a similar epoch are
included as references. In panels (c) and (d), the correlation between the pEW
of the early Si ii $\lambda$6355 line and the EEx strength is still clear
after normalizing the magnitude variance to the $U$-band peak magnitude for
each SN, suggesting that the proposed correlation is unlikely to be related to
the SN’s intrinsic brightness. The epoch (relative to the $B$-band maximum) of
each spectrum used for the pEW calculation is shown in the legend.