2102.02075.pdf

Infrared Spectropolarimetric Detection of Intrinsic Polarization

from a Core-Collapse Supernova

Samaporn Tinyanont

Maxwell Millar-Blanchaer

Mansi M Kasliwal

Dimitri Mawet

3,4

Douglas C Leonard

Mattia Bulla

Kishalay De

Nemanja Jovanovic

Matthew Hankins

Gautam Vasisht

Eugene Serabyn

Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA

Department of Physics, University of California, Santa Barbara, CA 93106, USA

Division of Physics, Mathematics and Astronomy, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA

91125, USA

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, CA 91109, USA

Department of Astronomy, San Diego State University, San Diego, CA 92182, USA

Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 106 91 Stockholm, Sweden

Department of Physical Sciences, Arkansas Tech University, 1701 N. Boulder Avenue, Russellville, AR 72801, USA

Abstract

Massive stars die an explosive death as a core-collapse supernova (CCSN). The

exact physical processes that cause the collapsing star to rebound into an explosion is

not well-understood, and the key in resolving this issue may lie in the measurement of

the shape of CCSNe ejecta. Spectropolarimetry is the only way to perform this mea-

surement for CCSNe outside of the Milky Way and Magellanic Clouds. We present

an infrared (IR) spectropolarimetric detection of a CCSN, enabled by the new highly

sensitive WIRC+Pol instrument at Palomar Observatory, that can observe CCSNe

(

−

17 mags) out to 20 Mpc to

∼

0.1% polarimetric precision. IR spectropolarime-

try is less affected than optical by dust scattering in the circumstellar and interstellar

media, thereby providing a more unbiased probe of the intrinsic geometry of the SN

ejecta. SN 2018hna, a SN 1987A-like explosion, shows 2

3% continuum polariza-

tion in the

J

band oriented at

∼

160

°

on-sky at 182 d after the explosion. Assuming

prolate geometry like in SN 1987A, we infer an ejecta axis ratio of

0.48 with the axis of

symmetry pointing at 70

°

position angle. The axis ratio is similar to that of SN 1987A

suggesting that they may share intrinsic geometry and inclination angle. Our data

do not rule out oblate ejecta. We also observe one other core-collapse and two ther-

monuclear SNe in the

band. SN 2020oi, a stripped-envelope Type Ic SN in Messier

100 has

= 0

09% at peak light, indicative of either a 10% asymmetry or host

interstellar polarization. The SNe Ia, 2019ein and 2020ue have

p <

0.33% and

1.08%

near peak light, indicative of asymmetries of less than 10% and 20%, respectively.

Main

The shape of an astronomical explosion embeds crucial information about the underlying

mechanism and the surrounding environment. A massive star (

) dies in a core-

collapse supernova when its nuclear fusion fuel is exhausted and its core collapses. The

physical processes that launch a shock, disrupting the star, remain an open question.

(1,2,3)

Hydrodynamical simulations have begun to readily produce explosions only in the past few

years when asymmetric three-dimensional processes are included.

(4,5)

The measurements of

core-collapse supernova ejecta’s shape allow us to test these models. For Type Ia supernovae,

which are thermonuclear explosions of white dwarfs, the progenitor systems and detonation

arXiv:2102.02075v1 [astro-ph.SR] 3 Feb 2021

mechanisms also remain debated

(6)

, despite their use as standardizable candles to detect

the accelerated expansion of the universe.

(7,8)

The ejecta shape can also distinguish among

competing models.

(9,10)

Until the next supernova in the Galaxy or Magellanic Clouds with

spatially resolved observations, spectropolarimetry remains a unique tool to directly measure

the SN ejecta shape in the plane of the sky. We present spectropolarimetric measurements

of supernovae in the near-infrared for SNe: 2018hna (87A-like), 2019ein (Ia), 2020oi (Ic),

and 2020ue (Ia).

Optical and near-infrared (IR) light from a supernova (SN) becomes polarized primarily

by electron scattering. The shape of the ionized ejecta determines the amount and orientation

of the polarization; more asymmetric ejecta generally yield more polarization. A review

(11)

of optical polarimetry of SNe found that core-collapse (CC) SNe are significantly polarized

∼

1% level when the inner ejecta becomes visible, indicating a global asymmetry in the ex-

plosion mechanism. SNe Ia, in contrast, have small continuum polarization with significant

silicon and calcium line polarization, indicating globally symmetric ejecta with metal-rich

clumps. These measurements need to account for the polarization induced by dust scattering

along the line of sight, both from the circumstellar medium around the SN

(12,13)

and in the

interstellar medium in the host galaxy and the Milky Way.

(14)

Measuring these intervening

effects is difficult and often leads to inaccurate measurements of the polarization intrinsic to

the SN. Near-IR spectropolarimetry is less susceptible than optical to dust-induced polariza-

tion contamination by a factor of 2–4 (

and

bands compared to

band, assuming Milky

Way dust properties .

(15)

) Furthermore, the near-IR is less contaminated by atomic lines

(16)

allowing for more accurate measurements of continuum polarization, and thus the global

geometry of the SN ejecta. Until now, Near-IR spectropolarimetry has not been possible for

most SNe because of the lack of sensitivity.

WIRC+Pol is a near-IR spectropolarimeter on the 200-inch Hale Telescope at Palomar

Observatory, starting science operation in March 2019. It offers low spectral resolution

(

∼

200; 0.006

m per spectral channel in the

band) with high throughput (

90%)

as it leverages a novel liquid crystal based polarization grating (PG

(17,18)

). WIRC+Pol can

observe sources as faint as

∼

5 to a polarimetric accuracy (1

) of

∼

0.1% per spectral

channel in less than two hours; much fainter than possible with IR spectropolarimeters

previously available.

(19,20)

WIRC+Pol operates in the

and

bands, which cover the strong

hydrogen Paschen-

(1.282

m) line for Type II SNe and a continuum region for all SNe.

Details of the instrument and the data reduction pipeline can be found in refs.

(21,22)

. Since

the beginning of science operation, we have obtained IR spectropolarimetry of all SNe with

J <

5 visible from Palomar. Four SNe satisfied this criterion between March 2019 and

March 2020, and we report their IR spectropolarimetric measurements here. Details of the

observations, data reduction, and polarimetric calibrations can be found in Method.

We observed the following SNe with WIRC+Pol. SN 2018hna, a Type II-peculiar (SN 1987A-

like)

(23)

, shows a significant

∼

2% (

) per spectral channel at 182 d post-explosion (95

d after peak light). SN 2020oi, a Type Ic stripped-envelope SN in Messier 100, has

0.9%

) per spectral channel at peak light (

= 0

09% broadband). SNe 2019ein and

2020ue, both Type Ia SNe, have

1.2% and

3.5% (3

) per spectral channel near peak

light (

33% and

08%, 3

broadband), respectively. Fig. 1 and 2 summarize our ob-

servational results while Fig. 3 compares them to optical spectropolarimetry of similar SNe

in the literature. Now we discuss each SN in detail.

SN 2018hna

(1987A-like). The IR spectropolarimetry of SN 2018hna is shown in Fig. 1

along with the flux spectrum (Method). SN 2018hna shows significant (

) polarization of

typically 2

7% per spectral channel at 1.18–1.21

m and 1.24–1.27

m that cannot be

attributed to dust polarization, either in the CSM or ISM. If dust is responsible, assuming

it behaves like Milky Way dust, the polarization in the

band would be 4.3% and the dust

reddening would be

(

−

)

∼

47 mags, inconsistent with what is observed in the optical

(Method). Spectral lines corresponding to these ranges are shown in Fig. 1. The region

between 1.21–1.24

m suffers most from sky emission, and is less than 2% polarized (3

Combining all these spectral channels together results in 2

3% continuum polarization.

The incomplete depolarization in the Paschen-

line (1.28

m) with no deviation in the

polarization angle shows the lack of multiple scattering at this wavelength. The polarization

is significantly enhanced to 3.5% redward of the Paschen-

line. This is likely because in a

homologously expanding SN ejecta, Paschen-

photons are redshifted in the scatterer’s frame

and are scattered redward in the observer’s frame, enhancing the degree of polarization in

the red wing of the line

(24)

. If the polarization were caused by optically thick clumps, the

angle of polarization would, in most cases, vary across spectral lines.

(11)

However, the angle

of polarization is consistently 160

, indicating that the observed polarization is created by

the global geometry of the ejecta. Since the SN is in the optically thin phase, the polarization

is probing the inner ejecta most affected by the core collapse mechanism.

We compare the continuum polarization of SN 2018hna with models of ref.

(25)

to constrain

its ejecta shape, assuming polarization from electron scattering. The electron scattering

optical depth of SN 2018hna during our observation is

∼

1 (Method), meaning

that our observed polarization is 95% of the maximum polarization (at

= 1; Figs. 1 and 5

in ref.

(25)

) Given a time-independent asymmetry in a homologous expansion, SN 2018hna’s

polarization would have peaked at

∼

4%. If the ejecta are oblate, Fig. 4 in ref.

(25)

shows that the axis ratio required to explain the observed polarization is 0

05 where

we account for both uncertainty in the polarization and optical depth. However, if the ejecta

are prolate (like SN 1987A), the polarization is about 40% smaller than the oblate case (if

τ <

1; Fig. 1 and 5 in ref.

(25)

). To account for this reduced polarization, we determine the

axis ratio from Fig. 4 (in ref.

(25)

) using a polarization of 3.5%, and obtain the maximum axis

ratio for the prolate ejecta of 0

06. These numbers are the axis ratio of the ejecta

projected on the plane of sky; the true axis ratio is subject to the unknown inclination angle.

We now compare the ejecta geometry of SN 2018hna to that of SN 1987A. Fig. 3 (top)

compares SN 2018hna’s IR spectropolarimetry to SN 1987A’s broadband IR polarimetry and

optical spectropolarimetry. Optical spectropolarimetry of SN 1987A shows strong wavelength

dependence over spectral lines. SN 1987A’s broadband polarization peaks at 1.5% and 1% in

the

and

bands at 140 days post-explosion, close to when the ejecta became optically thin.

This corresponds to a projected axis ratio of 0.6–0.7 (prolate). The angle of polarization

remains at

∼

110

, throughout its evolution.

(26)

Hubble Space Telescope

images from 22.8

years post-explosion (Fig. 4 right, and Fig. 1 in ref.

(27)

) reveal the ejecta geometry in broad

agreement with the expectation from polarization. The axis of symmetry is at

∼

on sky,

as expected from polarization from prolate ejecta (which produce an angle of polarization

perpendicular to the axis of symmetry)

(27)

. The ejecta show more asymmetry with the

projected axis ratio of

∼

0.5, suggesting that the broadband polarimetry may be diluted

by line polarization, providing only a lower limit for continuum polarization. (There is no

spectropolarimetry of SN 1987A in the nebular phase.) It is also possible that the ejecta’s

morphology had evolved by the time of the

HST

observations. Further, the symmetry axis

angle of the ejecta is within 10

from that of the inner circumstellar ring. The apparent

common symmetry from the inner ejecta out to the CSM indicates that both are likely

shaped by the binary merger that created SN 1987A’s blue supergiant (BSG) progenitor.

(28)

Fig. 4 shows a schematic of SN 2018hna’s ejecta informed by our observations, compared

to the

HST

image of SN 1987A. Assuming that both ejecta are prolate, the observed axis

ratios are similar between the two SNe, suggesting that their ejecta share similar underlying

geometry observed at similar inclination angle. We show a 1987A-like circumstellar ring for

scale (our observations do not probe the existence of such a ring.) The ejecta’s angular size at

this epoch is 50

as, which is impossible to resolve even by interferometry; polarimetry is the

only way to constrain the geometry in the plane of the sky. Our polarimetric measurements

indicate that the apparent ejecta of SN 2018hna, if prolate, has an axis ratio of

∼

0.48 oriented

∼

on sky, possibly with the similar underlying geometry as that of SN 1987A.

SN 2020oi

is a hydrogen- and helium-poor CCSN (Type Ic), an explosion of a highly

stripped progenitor star. The polarization of stripped-envelope SNe is typically at

∼

soon after peak light because the asymmetric inner ejecta are revealed right away.

(11)

Fig. 3

(middle) shows optical spectropolarimetry of three SESNe around peak from the literature,

all exhibiting 0.5–1% polarization. We note that the high polarization may be an observa-

tional bias because lower polarization is difficult to detect and tends to be under-reported.

For SN 2020oi, we do not detect a significant near-IR polarization in the spectropolarime-

try mode, with the typical upper limit (3

) of 0.9% per spectral channel (Fig. 2 middle).

Combining all the spectral channels, the broadband polarization is significant: 0

09%.

However, the low level of polarization combined with the host galaxy reddening (Method)

indicate that this polarization is more likely interstellar in origin.

SNe 2019ein and 2020ue (Ia)

are both unpolarized at epochs close to the peak light

with upper limits of 1.2% and 2.9% per spectral channel (3

), respectively (Fig. 2). The

broadband upper limits are 0.33% and 1.08%, are indicative of a global asymmetry of less

than 10% and 20%, respectively.

(25)

Fig. 3 (bottom) shows examples of SNe Ia spectropo-

larimetry. The low continuum polarization of SNe Ia indicates spherically symmetric ejecta.

Before and at peak luminosity, they show silicon and calcium line polarization originat-

ing from asymmetric metal-rich, high-velocity outflows. The degree of line polarization is

highly variable; e.g. SN 2004dt reaches

p >

2% in the Si

II

and the Ca triplet lines while

SN 2005df shows

= 0

3% in the same lines. The line polarization typically weakens after

peak as the symmetric ejecta become optically dominant.

(11)

The polarimetric non-detections

of SNe 2019ein and 2020ue at 2 d pre-maximum and 9 days post-maximum are consistent

with the low continuum polarization expected for SNe Ia.

In summary, we present near-IR spectropolarimetry of four nearby SNe. SN 2018hna

shows

∼

2% polarization with

significance per spectral channel (8

broadband) across

the

band with the angle of polarization around 160

at 182 d post-explosion. The result

indicates that the ejecta of SN 2018hna have an axis ratio of

48, assuming prolate

geometry, and is oriented at

∼

on sky. This inferred axis ratio, assuming prolate ejecta,

suggests that the ejecta of SNe 1987A and 2018hna share similar geometry and are observed

at similar inclination angles. Other SNe show no significant intrinsic polarization, with upper

limits (3

per spectral channel) of

∼

1% for SNe 2019ein and 2020oi, and 2.8% for SN 2020ue.

These measurements are enabled by the highly sensitive instrument WIRC+Pol on the

historic 200-inch Hale telescope at Palomar Observatory. The near-IR measurements are less

contaminated by dust, and the spectral information allows us to distinguish polarimetric

features across spectral lines, and accurately measure continuum polarization. They will

complement and build upon decades of effort in optical spectropolarimetry of SNe. IR

spectropolarimetry of SNe will provide an accurate tool to probe the shape of the SN ejecta

imprinted by the explosion mechanism, in pursuit of answering decades-old questions on the

progenitor system of SNe Ia and the explosion mechanism of CCSNe.

1.175

1.200

1.225

1.250

1.275

1.300

Wavelength ( m)

q (%)

WIRC+Pol Normalized Flux

NIRES Calibrated Flux

[Fe II]

Ca II

q (%)

u (%)

1.175

1.200

1.225

1.250

1.275

1.300

Wavelength ( m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

p (%)

WIRC+Pol Normalized Flux

NIRES Calibrated Flux

[Fe II]

Ca II

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

130

140

150

160

170

180

190

( )

SN2018hna (87A-like)

Figure 1:

-band IR spectropolarimetry of SN 2018hna (87A-like). The top panel shows

(red) and

(blue) spectra. The normalized, uncalibrated WIRC+Pol flux spectrum along

with a calibrated NIRES spectrum are plotted to show spectral features. Line identifications

are provided. The middle panel is the

plane color-coded by wavelengths, indicating

a significant departure from null. The bottom panel shows the debiased degree (

; green

square) and angle (

; purple circle) of polarization. For wavelengths with no significant

detection (either

q <

u <

), we plot 3

upper limits (triangle), and we do not

plot

. Error bars represent 1-

uncertainty.

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

q (%)

WIRC+Pol Normalized Flux

NIRES Calibrated Flux

[Fe II]

Ca II

q (%)

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

p (%)

WIRC+Pol Normalized Flux

NIRES Calibrated Flux

[Fe II]

Ca II

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

100

110

120

130

140

150

160

170

180

( )

SN2018hna (87A-like)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

q (%)

WIRC+Pol Normalized Flux

NIRES Calibrated Flux

q (%)

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

p (%)

WIRC+Pol Normalized Flux

NIRES Calibrated Flux

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

100

110

120

130

140

150

160

170

( )

SN2019ein (Ia)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

q (%)

WIRC+Pol Normalized Flux

q (%)

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

p (%)

WIRC+Pol Normalized Flux

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

100

110

120

130

140

150

160

170

( )

SN2020oi (Ic)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

q (%)

WIRC+Pol Normalized Flux

q (%)

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

1.30

Wavelength ( m)

p (%)

WIRC+Pol Normalized Flux

u (%)

1.18

1.20

1.22

1.24

1.26

1.28

Wavelength ( m)

100

110

120

130

140

150

160

170

( )

SN2020ue (Ia)

Figure 2:

-band IR spectropolarimetry of SNe 2019ein (Ia), 2020oi (Ic), and 2020ue (Ia),

from top to bottom. The left panels show

(red) and

(blue) spectra. The normalized,

uncalibrated flux spectrum is plotted (yellow) to show spectral features. Calibrated NIRES

spectrum is plotted for SN 2019ein. The middle panels are the

plane color-coded by

wavelengths. The right panels show 3

upper limits for the debiased degree of polarization

(

; green triangles). Error bars represent 1-

uncertainty.

p (%)

1987A, 262 d

1987A, 26 d

1987A, 2.1 d

1987A, 130-135 d

2018hna, 182 d

Type II-pec (87A-like)

p (%)

2009mi, +26.5 d

2007gr, +21 d

2005bf, +8 d

2020oi, 0 d

Type Ibc

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Wavelength ( m)

p (%)

2005df, -10.8d

2005df, +0.2d

2004dt, -7d

2004dt, +4d

2019ein, -2 d

2020ue, +9 d

Type Ia

Figure 3: Spectropolarimetry of different types of SNe at different phases. From top to

bottom, the fractional polarization as a function of wavelength of SNe of Types II-pec 87A-

like, Ibc, and Ia are shown. Colors indicate the phase of the observation: red, yellow, and

blue tones indicate pre-, near-, and post-peak respectively. Epochs for SNe Ia and Ibc are

relative to peak while those for 1987A-like are relative to explosion. Different line styles

indicate different SNe. The median error bar size for each observation is indicated in front

of the SN name/epoch label to avoid confusion in the plot. Data for SN 1987A and the -7

d epoch of SN 2004dt do not have uncertainties published. Our IR spectropolarimetry are

plotted in black; data shown in color are from the literature. Filled circles are above 3

; triangles are 3

upper limits. We caution that

is positively biased and does not

follow Gaussian distribution. Plotted values are debiased as described in . Literature data

include SNe 1987A

(26)

, 2004dt

(29,30)

, 2005bf

(31)

, 2005df

(9)

, 2007gr

(32)

, and 2009mi

(33)

. Where

applicable, error bars represent 1-

uncertainty.

50 micro arcsecond

North

b/a ~ 0.48

a ~ 10

SN ejecta

(180 d)

Possible

Circumstellar Ring

ring

~ 6

⨉

SN 2018hna

)

SN 1987A

Circumstellar Ring

SN ejecta

(8338 d)

ring

⨉

0.8”

(b)

(c)

Figure 4: (a) Schematic of SN 2018hna’s ejecta assuming prolate geometry. The circumstellar

ring shown in the schematic is assuming that SN 2018hna has a ring with 6

cm (0.2

pc) radius, similar to that observed around SN 1987A

(34)

. The provided inset shows that

the ejecta are distributed in a prolate spheroid with an axis ratio of 0.48 inferred from our

continuum polarization measurement (2

3%). The symmetric angle of the ejecta is

from north based on our angle of polarization measurement of 160

. This is because

a prolate spheroid produces polarization perpendicular to its symmetry axis. Assuming a

typical expansion speed of 6,000 km s

−

, the size of the ejecta is about 10

cm at 182 d post-

explosion, as indicated in the figure. The angular size on sky at the distance of SN 2018hna

(13 Mpc) is 50

as, which is several orders of magnitude below the spatial resolution limit

of optical-IR interferometers. (b) Same schematic but for SN 1987A, showing prolate ejecta

at 8338 d post-explosion. The physical size of the circumstellar ring is the same between the

two figures. (c) The spatially resolved, color composite image of SN 1987A at this epoch from

HST

. Red, green, and blue in the image represent the F814W, F438W, and F225W filters,

respectively. The image was obtained from the Hubble Legacy Archive and the data were

from PID 11653, PI Kirshner. The axis ratio of SN 1987A is roughly measured from the image

to be about 0.5, and the SN is observed at 45

inclination angle. The similarity between the

axis ratio between SNe 1987A and 2018hna (assuming prolate geometry) suggests the same

underlying geometry observed from similar inclination angles.

Method

Infrared Spectropolarimetry Observations

All observations are conducted with the WIRC+Pol instrument at Palomar Observatory.

The instrument has all transmissive optics with a half wave plate (HWP) in front of the

cryostat, a focal plane mask at the telescope focal point inside the cryostat, and the PG in

one of the filter wheels situated in the collimated beam inside the instrument. The detector

on the focal plane is a HAWAII-2 detector. See refs.

(21,22)

for more information about the

instrument. We observed all SNe in two positions (“AB”) inside the slit for background

subtraction; each position at HWP angles of 0

, 45

, 22.5

, and 67.5

. The exposure time is

chosen to minimize sky background change between “A” and “B” observations, typically 60

s. We repeat the sequence for one hour of total exposure time to achieve

q,u

∼

1% for a

= 14 source; our observations yield a factor of a few worse in uncertainty. We observed

four nearby SNe with WIRC+Pol: SNe 2018hna (87A-like), 2019ein (Ia), 2020oi (Ic), and

2020ue (Ia). Supplementary Table 1 summarizes the sample, including their host, distance,

Galactic extinction, observed epoch, exposure times, observing conditions, and references.

Supplemental Figure 4 shows optical photometry of our four SNe from the public data stream

of the Zwicky Transient Facility (ZTF; ref.

(35)

) in the

and

bands, and near-IR

-band

photometry from the Gattini-IR telescope

(36)

. These light curves are used to determine the

phase of our spectropolarimetric observations.

SN 2018hna, Type II (SN 1987A-like),

was discovered on 2018 Oct 22 UT used

throughout;

(37)

and classified as SN II

(38)

. The photometric and spectroscopic evolution

showed similarities to those of SN 1987A, indicating that SN 2018hna was a similar explosion

of a BSG

(23)

. The Galactic extinction for this SN was

= 0

009 and the reddening

was

(

−

)

= 0

01 mags

(39)

. The expected Galactic ISP in the optical is

ISP

(

−

)

0.1%, much smaller than our detection and the IR value would be even smaller.

There may be additional ISP from the host galaxy with different wavelength dependence

from that due to Milky Way dust. Ref.

(23)

reported optical photometry and spectroscopy and

constrained the explosion date and the

-band maximum light to 2018 Oct 19.8 and 2019 Jan

15.3 respectively. They detected shock-cooling emission from the early light curve, directly

constraining the progenitor to be a

∼

BSG. They also reported optical spectroscopy

showing no Na

I

D absorption, confirming the minimal host/Galactic extinction. From their

Fig. 5, SN 2018hna became optically thin at 118 d post-explosion, while our observation

was at 182 d post-explosion, 64 d into the nebular phase. We obtained a plot of electron-

scattering optical depth as a function of time in the model of an 87A-like explosion with

the kinetic energy of 1

erg (“a4”) in Ref.

(40)

from private communication with the

author. The optical depth of an 87A-like SN is 2.2 at the beginning of the radioactive tail

phase; and 0.82 at the phase of our observation. We use this number to convert the observed

polarization of this SN to its ejecta geometry. The axis ratio is highly dependent on the

optical depth at this epoch, and we incorporate a conservative uncertainty of

1 into our

error calculation.

SN 1987A remains one of the best polarimetrically observed SNe to date. Ref.

(26)

summa-

rized all spectropolarimetric data on SN 1987A with a homogeneous ISP subtraction. It was

also the only SN with near-IR polarimetry, albeit broadband

(41,42,43)

. These measurements

provide a direct comparison to our data.

We observed SN 2018hna on 2019 Apr 20, 95 days from maximum brightness, in median

seeing conditions. Unlike other SNe, the individual exposure time was 120 s, and the A and

B frames in the dither were taken almost 10 min apart. As a result, this data set required

additional background subtraction step as simple AB subtractions left significant background

residual. We will discuss this in

and . In addition to the WIRC+Pol observation, we

also obtained IR spectrum of SN 2018hna using the Near-InfraRed Echellette Spectrograph

(NIRES) on the 10-m Keck telescope on 2019 May 24.

SN 2019ein, Type Ia,

was discovered on 2019 May 01 by the Asteroid Terrestrial-

impact Last Alert System (ATLAS) SN survey

(44)

. Early spectroscopy taken on 2019 May 03

showed a high velocity silicon feature at

∼

30000 km s

−

1 (45)

. Such a feature was suggestive of

an asymmetric metal-rich outflow, triggering our spectropolarimetric follow-up. The Galactic

extinction for this SN was

= 0

009 and the reddening was

(

−

)

= 0

011 mags

(39)

The SN was observed with WIRC+Pol on 2019 May 14, 13 days after the first detection.

We also obtained near-IR spectrum of SN 2019ein with Keck/NIRES on 2019 May 24. The

high velocity metal features had disappeared by that epoch, indicating that the photosphere

might have overrun the metal-rich clump responsible for the feature.

(46)

SN 2020oi, Type Ic,

was discovered by ZTF through the event broker Automatic Learn-

ing for the Rapid Classification of Events (ALeRCE;

http://alerce.science/

) on 2020 Jan

(47)

using the public data stream of ZTF (

https://ztf.uw.edu/alerts/public/

). It was

classified as SN Ic on 2020 Jan 09

(48)

. The Galactic extinction for this SN was

= 0

019

and the reddening was

(

−

)

= 0

023 mags

(39)

. Despite its low Galactic extinction,

the SN was close to the core of the galaxy and may have significant host extinction. A study

of this SN based on ZTF optical light curve and spectroscopy will be presented in ref.

(49)

We determined the host extinction from measuring the equivalent width of the Na I D

absorption from the optical spectrum of SN 2020oi (described below). The equivalent width

was 0.3 and 0.55

A in the two doublets, which gives

(

−

)

host

= 0

136 mags. This gives

an ISP upper limit,

ISP

max

(

−

) of 1.2% in the

band

(14)

The ISP upper

limit in the

band is 0.5% using the modified Serkowski law

(15)

. We assume here that

the Milky Way dust polarization property applies to the host galaxy dust. We observed

SN 2020oi with WIRC+Pol on 2020 Jan 19, 12 d post-discovery and at peak. The seeing

was

∼

′′

, above average at Palomar.

SN 2020ue, Type Ia,

was discovered on 2020 Jan 12

(37)

and classified as a normal SN

(50)

. We observed SN 2020ue with WIRC+Pol on 2020 Feb 4, 23 d post-discovery and

approximately 9 d post-maximum. The Galactic extinction for this SN was

= 0

020 and

the reddening was

(

−

)

= 0

025 mags.

(39)

The observing conditions were poor with

∼

′′

seeing, rendering the SNR inadequate even after 56 min on a

∼

5 source.

Data Reduction

Calibrations and Background Subtraction

We use the WIRC+Pol Data Reduction Pipeline (DRP;

https://github.com/WIRC-Pol/

wirc_drp

) to reduce our data. The detailed data reduction steps for WIRC+Pol data with

HWP are in ref.

(22)

. We first perform dark subtraction and flat fielding using appropriate

calibration images taken on the same night. We then perform background subtraction, which

is crucial because imperfect subtraction can bias polarization measurements. We observe all

SNe at two dither positions along the slit, allowing us to use the “B” position image to

subtract background off of the corresponding “A” image. We scale the background frame

to match its median to that of the science frame to account for the constantly changing IR

sky background. Observations of SN 2018hna have the single frame exposure time of 120

s, instead of 60 s used in later observations. As a result, the “A” and “B” observations

at the same HWP angle are more than 8 minutes apart and the sky line emissions evolve

noticeably between the two frames. In this case, we remove the background by fitting

the profile across the slit with a Gaussian-smoothed piece-wise function that describes the

slit transmission. In

, we show measurements using synthetic images to verify that the

two background subtraction methods yield similar results and that they do not introduce

polarimetric biases.

Spectral Extraction

There are four spectral traces per source in WIRC+Pol data, tracing the polarization angles

of 0

, 45

, 90

, and 135

. A HWP rotation of

introduces a 2

rotation in the polarization

angle probed by these traces; this modulation allows us to measure and remove instrumental

polarization. The DRP rotates the spectral traces to align with the pixel grid using

OpenCV

bicubic interpolation. The spectra are extracted using the optimal extraction algorithm

(51)

The extraction range is set to

of the spatial profile of the spectrum. We bin the spectra

using a 5-pixel window to match the seeing limit; this results in 0.01

m spectral resolution

and

λ/

∆

= 120 resolving power in the

band.

Polarization Calculation and Calibration

From the extracted flux spectra, we compute the normalized Stokes parameters

and

. We

use the flux ratio method as it is the most robust against the non-common path systematics

and the atmospheric changes (see

2.1 in

). Consider two observations at the HWP angles

and 45

, the flux in the upper left, lower right, upper right, lower left traces are noted as

. We compute

and

−

+ 1

(1)

The uncertainties of this quantity is a quadrature sum:

√

(

)

(

)

(

)

(

)

(2)

Then,

(3)

The polarimetric uncertainty is

(

+ 1)

(4)