Optical Spectropolarimetry of SN 2002ap: A High-Velocity Asymmetric Explosion

Abstract

We present spectropolarimetry of the Type Ic supernova SN 2002ap and give a preliminary analysis: the data were taken at two epochs, close to and 1 month later than the visual maximum (2002 February 8). In addition, we present June 9 spectropolarimetry without analysis. The data show the development of linear polarization. Distinct polarization profiles were seen only in the O I λ7773 multiplet/Ca II IR triplet absorption trough at maximum light and in the O I λ7773 multiplet and Ca II IR triplet absorption troughs a month later, with the latter showing a peak polarization as high as ~2%. The intrinsic polarization shows three clear position angles: 80° for the February continuum, 120° for the February line feature, and 150° for the March data. We conclude that there are multiple asymmetric components in the ejecta. We suggest that the supernova has a bulk asymmetry with an axial ratio projected on the sky that is different from 1 by an order of 10%. Furthermore, we suggest very speculatively that a high-velocity ejecta component moving faster than ~0.115c (e.g., a jet) contributes to polarization in the February epoch.

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arXiv:astro-ph/0205414v3 16 Oct 2002
accepted for publication in the Astrophysical Journal (Letters)
Preprint typeset using LATEX style emulateapj v. 14/09/00
OPTICAL SPECTROPOLARIMETRY OF SN 2002AP: A HIGH VELOCITY ASYMMETRIC
EXPLOSION1
K. S. Kawabata2,3, D. J. Jeffery4, M. Iye2,5, Y. Ohyama6, G. Kosugi6, N. Kashikawa2,
N. Ebizuka7, T. Sasaki6, K. Sekiguchi6, K. Nomoto8,9, P. Mazzali9,8,10, J. Deng9,8,
K. Maeda8, K. Aoki6, Y. Saito2, T. Takata6, M. Yoshida11, R. Asai8, M. Inata11,
K. Okita11, K. Ota8,2, T. Ozawa12, Y. Shimizu11, H. Taguchi13, Y. Yadoumaru12,
T. Misawa8,2, F. Nakata8,2, T. Yamada2, I. Tanaka2, and T. Kodama14
accepted for publication in the Astrophysical Journal (Letters)
ABSTRACT
We present spectropolarimetry of the Type Ic supernova SN 2002ap and give a preliminary analysis:
the data were taken at two epochs, close to and one month later than the visual maximum (2002 February
8). In addition we present June 9 spectropolarimetry without analysis. The data show the development
of linear polarization. Distinct polarization profiles were seen only in the O I λ7773 multiplet/Ca II IR
triplet absorption trough at maximum light and in the O I λ7773 multiplet and Ca II IR triplet absorption
troughs a month later, with the latter showing a peak polarization as high as ∼ 2 %. The intrinsic
polarization shows three clear position angles: 80◦for the February continuum, 120◦for the February
line feature, and 150◦for the March data. We conclude that there are multiple asymmetric components
in the ejecta. We suggest that the supernova has a bulk asymmetry with an axial ratio projected on the
sky that is different from 1 by of order 10 %. Furthermore, we suggest very speculatively that a high
velocity ejecta component moving faster than ∼ 0.115c (e.g., a jet) contributes to polarization in the
February epoch.
Subject headings: polarization — supernovae: individual (SN 2002ap)
1. introduction
SN 2002ap was discovered in the nearby spiral galaxy
M74 (= NGC 628) on 29 January 2002 (Nakano et al.
2002) and reached its maximum of V ∼ 12.4 mag on Febru-
ary 8 (Gal-Yam, Ofek, & Shemmer 2002). It has been
classified as a Type Ic supernova (SN Ic) and suggested to
be a hypernova (but at the low-energy end of the sequence
of hypernovae; Mazzali et al. 2002 and references therein).
A SN Ic is thought to be the result of the core col-
lapse of a massive star that has either lost its hydrogen
and helium envelopes prior to the explosion or has an in-
visible helium envelope due to low excitation. The de-
tails of the explosion mechanism are still under discussion
(Nomoto, Iwamoto, & Suzuki 1995; Branch 2001 and ref-
erences therein). SN 1998bw, the most luminous and en-
ergetic Type Ic ‘hypernova’ to date, has been particularly
well studied, and its probable connection with the γ-ray
burst GRB 980425 has been pointed out (e.g., Galama et
al. 1998; Iwamoto et al. 1998; Nomoto et al. 2001). An
aspherical hyperenergetic explosion has been suggested to
explain the slowly-declining light curve of SN 1998bw and
the narrowness of the [O I] λ6300, 6363 emission line in
the nebular phase (Mazzali et al. 2001; Nakamura et al.
2001; Maeda et al. 2002).
Intrinsic polarization is zero for SNe (which are unre-
solved sources) if they are spherically symmetric: any in-
trinsic polarization thus reveals asymmetry. Core-collapse
supernovae are, in fact, generally polarized in the contin-
uum at levels of p ≃ 0.5–4 % due to electron scattering and
their polarization increases after optical maximum light
(e.g., Jeffery 1991b; Wang et al. 1996, 2001; Leonard et
al. 2001); however, the polarization falls to zero at very
late times, when the electron scattering opacity becomes
very low (e.g., Jeffery 1991b).
ization profile—actually mostly due to electron polarized
light interacting with a line—predicted theoretically (Jef-
fery 1989) and to some degree confirmed observationally in
SN 1987A and other supernovae (Jeffery 1991a,b; Leonard
The typical line polar-
1Based on data obtained at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan (NAOJ)
2Opt. & IR Astron. Div., NAOJ, Mitaka, Tokyo 181-8588, Japan
3E-mail: koji.kawabata@nao.ac.jp
4Dep. of Phys., New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA
5Dep. of Astron., Graduate Univ. for Advanced Studies, Mitaka, Tokyo 181-8588, Japan
6Subaru Telescope, NAOJ, 650 North A’ohoku Place, Hilo, HI 96720, USA
7RIKEN, Wako, Saitama 351-0198, Japan
8Dep. of Astron., Univ. of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
9Research Center for the Early Universe, Univ. of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
10Osservatorio Astronomico, Via Tiepolo, 11, 34131 Trieste, Italy
11Okayama Astrophys. Obs., NAOJ, Asakuchi-gun, Okayama 719-0232, Japan
12Misato Obs., Amakusa-gun, Wakayama 640-1366, Japan
13Dep. of Astron. & Earth Sci., Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan
14Theory Div., NAOJ, Mitaka, Tokyo 181-8588, Japan
1

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2Kawabata et al.
& Filippenko 2001; Leonard et al. 2001), is an inverted
P Cygni profile: strong polarization maximum at the flux
P Cygni trough feature and polarization minimum at the
flux P Cygni emission feature. Line blending and other
intrinsic effects can distort these profiles. For SN 1998bw,
an intrinsic optical polarization of 0.4–0.6 % was found,
suggesting an asymmetry of less than 2/1 in the axial ra-
tio of the ejecta (Patat et al. 2001; Kay et al. 1998). No
distinct line polarization features were seen probably due
to the poor S/N or the relatively narrow wavelength range
of those observations.
2. observations and data reduction
The spectropolarimetry was taken with the 8.2-m Sub-
aru Telescope equipped with the Faint Object Camera and
Spectrograph (FOCAS, Kashikawa et al. 2000; Yoshida
et al. 2000) from 2002 February 9 through June 9. For
February and June observations, we used a 300 grooves
mm−1grism (the central wavelength of 5500˚ A) with a
0.′′4 width slit, resulting a spectral resolution (λ/∆λ) was
∼ 1200.For March observation, we used another 300
gr mm−1grism (7500˚ A) with a 0.′′8 width slit, result-
ing in λ/∆λ ∼ 650.The nightly total exposure time
was 960 to 4000 sec. The linear polarimetric module of
FOCAS consists of a rotating superachromatic half-wave
plate and a crystal quartz Wollaston prism, and both the
ordinary and the extraordinary rays are simultaneously
recorded on two MIT/LL CCDs (2k×4k×15µm). A typ-
ical observing sequence consisted of four integrations at
the ψ = 0◦,45◦,22.◦5 and 67.◦5 positions of the half-wave
plate. Stokes Q/I and U/I were calculated as in §6.1.2
of Tinbergen (1996). For polarimetric calibration, we ob-
tained data for unpolarized and polarized standard stars,
including measurements of flatfield lamps through fully-
polarizing filters. The flux was calibrated using observa-
tions of G191B2B and BD+28◦4211 (Oke 1990), and then
was multiplied by a constant to match the VSNET14pho-
tometric data.
3. results and discussion
Figure 1 shows the observed flux and polarization spec-
tra. Several blueshifted broad absorption lines can be iden-
tified in the February flux spectrum (Mazzali et al. 2002).
For March and June flux spectra a detailed analysis has
yet to be done. However, we note that the March spectra
resemble that of SN 1997ef at day 67 (Mazzali, Iwamoto,
& Nomoto 2000), including the onset of net emission in
Ca II λλ8498, 8542, 8662 (i.e., the Ca II IR triplet). The
significant emission line at ∼ 6300˚ A in the June spectrum
is identified as [O I] λ6300, 6363 as in SN 1997ef (Mazzali
et al. 2001). The polarization is ? 0.5 % at a position an-
gle (PA) of θ = 120◦±20◦over the observed wavelengths.
Significant day-by-day variation is not seen in the polariza-
tion spectra within each month. Here we will only analyze
monthly averages.
3.1. Interstellar and Intrinsic Polarization
Interstellar polarization (ISP) varies slowly with wave-
length in the optical and is well approximated by the em-
pirical formula pISP(λ) = pmax· exp[−1.15ln2(λmax/λ)],
where pmax
(Serkowski, Mathewson, & Ford (1975), hereafter SMF).
In March the emission feature of the Ca II IR triplet line
profile shows strong net emission due to NLTE processes.
Such NLTE line flux is necessarily unpolarized on emission.
Since the line profile is still broad (absorption minimum
at a ∼ −14,000 km s−1redshift, corresponding to an en-
closed mass of 2 M⊙in model CO100/4, which has a total
mass of 2.4 M⊙(Mazzali et al. 2002)), much of the emis-
sion is probably coming from far out in the ejecta, where
the electron optical depth is low. We conclude that the
flux from this emission line is mostly unscattered by elec-
trons and unpolarized, and that it dilutes the polarized
electron scattered flux. If this were the only effect, then
the intrinsic polarization should show a distinct minimum
nearly exactly at the wavelength of the flux emission max-
imum. Since, in fact, the polarization is roughly constant
across the P Cygni emission feature (8400–9000˚ A), apart
from small variations that may be mostly noise, the line
is not only diluting the polarized flux but is also probably
strongly depolarizing it. The intrinsic polarization across
the emission feature is probably close to zero. We will
assume that the observed polarization of the emission fea-
ture is the ISP: thus pISP(8600˚ A) ≈ 0.5 %. We next adopt
the median λmax= 5370˚ A found by SMF for 30 stars with
pmax/EB−V ≥ 7.0. Then from a non-linear regression we
find pmax= 0.64 ± 0.20 % and θISP= 120◦± 10◦. (The
uncertainties are crude estimates based on the alternative
assumption that only line flux dilution, and not line depo-
larization, occurs in the region of the Ca II IR triplet emis-
sion feature.) Since Takada-Hidai, Aoki, & Zhao (2002)
derive a color excess for SN 2002ap of EB−V = 0.09 (a
sum of 0.07 within our Galaxy and 0.02 within M74) from
interstellar Na D absorptions, our assumption of the SMF
λmaxis consistent: pmax/EB−V = 0.64/0.09 ≈ 7.
The estimated ISP (EISP) is consistent with other fac-
tors. In an ISP catalog (Heiles 2000), 16 stars are within
10◦of SN 2002ap.The data for these stars suggest a
possible positive correlation between polarization and the
distance along the line of sight toward SN 2002ap. The
two most distant stars among them, HD8919 (d = 525 pc)
and HD9560 (d = 437 pc) show (p, θ) = (0.32 ± 0.10 %,
99◦± 9◦) and (0.48 ± 0.09 %, 123◦± 5◦), respectively.
On the other hand, it has been found that pmax(%) has
an empirical upper limit of 9EB−V (SMF). From the de-
rived EB−V = 0.09, an upper limit on the ISP toward
the supernova is 0.81 %. The EISP is nicely sandwiched
between the possible lower bounds of the cited stars and
the empirical upper limit. The EISP position angle is also
consistent with the position angle of the spiral arm in M74
at the position of SN 2002ap, 110◦–140◦(see DPOSS im-
ages). The June 9 polarization spectrum has polarization
consistent with the EISP at the peak of the strong emis-
sion flux of the [O I] λ6300, 6363 forbidden line (Figure
1c): one would expect that polarization to be ISP for a
strong emission line in the nebular epoch. (The June 9
data has low S/N: in the following, we analyze only the
higher S/N polarization spectra of the earlier two epochs.
Figure 2 shows the intrinsic (i.e, EISP-subtracted) po-
larization plotted on a QU diagram:
=pISP(λmax) is the peak polarization
the polarization
14http://www/kusastro.kyoto-u.ac.jp/vsnet

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Optical Spectropolarimetry of SN 2002ap3
points are connected according to their wavelength order-
ing. Given the uncertainty in the EISP, points within 0.2
% of the origin must be considered very uncertain.
If one assumes that the intrinsic supernova polarization
is produced by a single axisymmetric component in the
ejecta, then the intrinsic polarization plotted on a QU di-
agram should lie on a line passing through the origin. It
can be seen that the polarization in February has two clear
position angles, PA less than or ∼ 120◦(associated with
the O I/Ca II line trough) and PA∼ 80◦± 20◦(associ-
ated with the continuum from ∼ 5700–8200˚ A), joined by
a somewhat complicated transition. The polarization in
March has a clear position angle PA∼ 150◦associated with
the Ca II line trough and with at least some of the con-
tinuum. We conclude that there are multiple asymmetric
components, and that their contribution varies with time.
It is likely that the recession of the supernova photosphere
uncovers different asymmetries.
3.2. Possible Models
Figures 3a, b, and c show the flux and intrinsic po-
larization corrected for heliocentric redshift [vhelio
+631km s−1(Smartt & Meikle 2002)] and interstellar ex-
tinction (EB−V = 0.09). The figures show that polariza-
tion is low and barely significant (given the uncertainty
in the EISP), except in the regions ∼ 6700–8000˚ A for
February and ∼ 6700–8300˚ A for March.
The low continuum polarization blueward of ∼ 6700˚ A
in both epochs may be due to the depolarizing effect of
lines (Howell et al. 2001): in supernova spectra, lines gen-
erally become stronger further to the blue. The continuum
polarization, where it is significant (a relatively small re-
gion) is ∼ 0.4 % in both epochs. If the asymmetry is
assumed to be an axisymmetric, global prolate or oblate
asymmetry, then the continuum polarization can be ex-
plained by an axial ratio (assuming there is a main axis)
projected on the sky that is different from 1 by of order
10 %. This estimate is a crude one based on realistic,
but parameterized, calculations (H¨ oflich 1991, 1995). The
estimate is also crude because, as noted above, the asym-
metry cannot be completely axisymmetric. The estimated
asymmetry is not large, compared to those estimated for
some other supernovae (e.g., Wang et al. 2001).
The three distinct line polarization profiles seen in Fig-
ure 3 (at the O I/Ca II flux absorption in February and the
O I and Ca II flux absorptions in March) can partially be
accounted for by the inverted P Cygni profile (see § 1): the
polarization maxima associated with line trough features
are clear. Without detailed modeling more information
probably cannot be extracted from these profiles.
For the February continuum polarization, we suggest
a radically different origin from the bulk asymmetry as-
sumed in prolate/oblate models or element inhomogeneity
models (see below). In Figure 3d we show the intrinsic po-
larized flux (p × F) compared to the observed flux scaled
down by a factor of 0.0018 and non-relativistically red-
shifted by a velocity of 0.23c: i.e., λredshifted = λ/0.77.
There is fair agreement over the range ∼ 5000–8000˚ A. In
response to this comparison, Leonard et al. (2002) made a
similar comparison for their SN 2002ap spectropolarime-
try and also obtained a similarly good agreement. The
agreement suggests (but does not prove) that a large com-
=
ponent of polarized flux comes from electron scattering in
an ionized clump (i.e., a jet) thrown out of the supernova
explosion. (We assume a single jet or clump for simplicity
here, although a pair of bipolar jets are a physical possi-
bility (e.g., Wheeler, Meier, & Wilson 2002).) In a simple
non-relativistic picture, the scattered light is redshifted by
vred∼ vjet(1 + cosi), where, vred= 0.23c, vjetis the char-
acteristic velocity of scattering relative to the supernova
center, and i is the inclination angle of jet to the line of
sight measured from the far side of the supernova. The jet
polarization component is calculated from
pjet(λ) = f ·F[λ(1 − vred/c)]
F(λ)
,
where F(λ) is the corrected flux, 1−vred/c is the blueshift
back to the origin of the scattered flux observed at λ,
and the scale factor f = 0.0018: f accounts both for
the polarization of the jet scattered flux and the frac-
tion scattered. The polarization is wavelength dependent
even though electron scattering is wavelength-independent
since the scattered flux comes from a bluer part of the
spectrum than the part it is added to. Electron scattering
depends on scattering direction: e.g., maximum polariza-
tion occurs for i = 90◦; half as much for i = 45◦or 135◦;
zero for i = 0◦or 180◦. The jet velocities corresponding to
i = 90◦, 45◦, and 0◦are 0.23c, 0.135c, and 0.115c, respec-
tively. Thus 0.115c is a lower bound on the jet velocity
and i ? 90◦would require a somewhat relativistic jet.
High velocity jet-like clumps have been proposed in
some hydrodynamic explosion models for SNe and GRB’s
(e.g., Nagataki et al. 1997; MacFadyen & Woosley 1999;
Maeda et al. 2002; Wheeler, Meier, & Wilson 2002). If
a jet is thrown out of an exploding supernova core, then
it is plausible that it carries some radioactive56Ni. The
gamma-rays from decay would keep the jet ionized to some
degree just as they keep the nebular phase bulk ejecta ion-
ized.
If the jet picture is correct, then the position angle of
the jet on the sky is ∼ 170◦(or ∼ 350◦) since the jet po-
larization component has position angle ∼ 80◦and elec-
tron scattering polarizes perpendicularly to the scattering
plane. The O I/Ca II line polarization maximum in the
February data cannot easily be associated with the jet.
As Figure 2 shows, the observed position angle makes an
excursion from ∼ 80◦up to 120◦across the polarization
maximum. Some of the line polarization may arise in the
bulk asymmetry of the supernova. It is possible that the
position angle of ∼ 120◦is the net result of a jet polarizing
at ∼ 80◦and a bulk asymmetry polarizing at ∼ 150◦(i.e.,
at the position angle observed in the March data). To test
this model we have eliminated the jet polarization compo-
nent from the February intrinsic polarization by subtract-
ing jet polarization Stokes parameters from the intrinsic
polarization Stokes parameters. The residual polarization
and position angle spectra are plotted in Figures 3e,f. The
position angle of the residual polarization for February
from the region of significant polarization (i.e., ∼ 6700–
8000˚ A) is now approximately centered on 150◦and devi-
ates by more than 30◦only in a few isolated points. The
results in Figure 3f are thus consistent with the jet model.
The jet may not be a completely separated amount of
ejecta, but rather a blob rich in56Ni moving at ? 0.115c.

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4 Kawabata et al.
A high-velocity,56Ni-rich region is required both in the hy-
pernova explosion models of SN 1998bw (Nakamura et al.
2001; Maeda et al. 2002), and in SN 2002ap (Mazzali et al.
2002) in order to reproduce the light curves. Spectral syn-
thesis for SN 2002ap (Mazzali et al. 2002) suggests some
material at velocities higher than the photospheric veloci-
ties of ∼ 0.1c–0.117c (Mazzali et al. 2002; Kinugasa et al.
2002) up to 0.22c. The ionization of the blob would likely
be increased by radioactive56Ni and this would likely make
the blob more polarizing than other parts of the ejecta at
the same velocity leading to net polarization. Other chem-
ical inhomogeneities in the ejecta at varying velocities are
also possible (Maeda et al. 2002) and would affect polar-
ization in complicated ways.
4. concluding remarks
Our results are summarized in the Abstract. More re-
alistic modeling is necessary for a more definitive under-
standing of the polarization. The degree of the bulk asym-
metry suggested in this paper may be tested with the line
widths and their ratios in the nebular spectrum (Maeda
et al. 2002).
We are grateful to the staff members at the Subaru
Telescope for their kind help and their rearrangement of
the telescope maintenance schedule for our observation on
March 8. This work has been supported in part by the
grant-in-Aid for Scientific Research (12640233, 14047206,
14540223) and COE research (07CE2002) of the Ministry
of Education, Science, Culture, Sports, and Technology in
Japan.
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Optical Spectropolarimetry of SN 2002ap5
Fig. 1.— Flux and polarization spectra of SN 2002ap. Heliocentric redshift, interstellar extinction and polarization have not been corrected
for. From top to bottom, we plot (a) total flux, (b, c) polarization level p and position angle θ on each observation day as indicated. The
polarimetric data are binned to a constant photon noise of 0.05 % which is shown by the error bars of polarization points. The EISP component
is shown by a dashed curve in (b,c).
Fig. 2.— QU-diagram of the monthly-averaged intrinsic (i.e., ISP-subtracted) polarization for February and March epochs. The data are
binned to a constant photon noise of 0.04 %. It can be seen that the polarization in February has, at least, two preferred axes: PA∼ 120◦
(associated with the O I/Ca II line trough) and PA∼ 80◦(associated with the continuum). The polarization in March has a clear position
angle PA∼ 150◦associated with the Ca II line trough and the significantly polarized continuum. These position angles are indicated by thick
arrows. Note that the position angle on the sky is half the angular location on a QU diagram.
Fig. 3.— Intrinsic polarization spectra corrected for heliocentric redshift and interstellar extinction. From top to bottom, we plot (a) total
flux in erg s−1cm−2˚ A−1, (b) polarization level p, (c) position angle θ, (d) polarized flux, and (e,f) p and θ of the residual polarization after
the jet polarization component has been subtracted from the February data. The February flux is the mean of Feb 9 and 11, and the March
flux is the mean of Mar 8 and 10. We adopt the normal interstellar extinction curve (Cardelli, Clayton, & Mathis 1989). Deep absorption
bands due to the terrestrial atmosphere and the interstellar medium have been removed by interpolation using nearby continuum levels. The
polarimetric data are binned in the same manner as in Fig. 2. The solid curve in (d) is the February flux multiplied by 0.0018 and redshifted
by +0.23c (see §3.2).
(a)
9.2 Feb
10.3 Feb
0
2×10–14
4×10–14
6×10–14
8×10–14
Flux (erg/sec/cm2/Å)
FeII
NaI
SiII
OI
CaII
atm.
atm.
[OI]
[CaII]
11.3 Feb
12.3 Feb 8.2 Mar (×3)
10.2 Mar (×3)
9.6 Jun (×3)
0
1
2
%Pol.
(b)
9.2 Feb
10.3 Feb
11.3 Feb
8.2 Mar
10.2 Mar
9.6 Jun
400050006000700080009000
Observed wavelength (Å)
100
120
140
PA (deg.)
(c)
Estimated ISP component (λmax=5370Å)