Spectropolarimetry of the 2012 outburst of SN 2009ip: a bi-polar explosion in a dense, disk-like CSM

2.1 MB·

by  Emma Reilly

MNRAS000,1–??(2017) Preprint1February2017 CompiledusingMNRASLATEXstylefilev3.0
Spectropolarimetry of the 2012 outburst of SN 2009ip: a bi-polar
explosion in a dense, disk-like CSM (cid:63)
Emma Reilly1 , Justyn R. Maund2,3, Dietrich Baade4, J. Craig Wheeler5, Peter Höflich6,
†
Jason Spyromilio4, Ferdinando Patat4, Lifan Wang7
1AstrophysicsResearchCentre,SchoolofMathematicsandPhysics,Queen’sUniversityBelfast,BelfastBT71NN,UK.
2DepartmentofPhysicsandAstronomy,TheUniversityofSheffield,HicksBuilding,HounsfieldRoad,Sheffield,S37RH,UK
3RoyalSocietyResearchFellow
7 4ESO-EuropeanOrganisationforAstronomicalResearchintheSouthernHemisphere,Karl-Schwarzschild-Str.2,85748Garching
1 b.München,Germany
0 5DepartmentofAstronomyandMcDonaldObservatory,TheUniversityofTexas,1UniversityStationC1402,Austin,Texas78712-
2 0259,U.S.A.
6DepartmentofPhysics,FloridaStateUniversity,Tallahassee,Florida32306-4350,U.S.A.
n 7DepartmentofPhysics,TexasA&MUniversity,CollegeStation,Texas77843-4242,U.S.A.
a
J

1
3
AcceptedXXX.ReceivedYYY;inoriginalformZZZ

]
R
ABSTRACT
S
We present a sequence of eight spectropolarimetric observations monitoring the geometric
.
h evolution of the late phase of the major 2012 outburst of SN 2009ip. These were acquired
p with the FORS2 polarimeter mounted on ESO VLT. The continuum was polarised at 0.3-
-
0.8%throughouttheobservations,showingthatthephotospheredeviatedsubstantiallyfrom
o
r sphericalsymmetryby10-15%.Significantlinepolarisationisdetectedforbothhydrogenand
t heliumathighvelocities.Thesimilarityinthepolarisedsignalbetweentheseelementsindi-
s
a catesthattheyforminthesamelocationintheejecta.Thelinepolarisation(p 1-1.5%)at
[ lowvelocitiesrevealedthepresenceofahighly-asphericalhydrogenandhelium∼richcircum-

stellarmedium(CSM).MonteCarlosimulationsoftheobservedpolarimetrywereperformed
1
v inanefforttoconstraintheshapeoftheCSM.Thesimulationsimplythatthepolarimetrycan
5 be understood within the framework of a disk-like CSM inclined by 14 2◦ out of the line
±
8 ofsight,obscuringthephotosphereonlyatcertainepochs.Thevaryingtemporalevolutionof
8 polarisationathighandlowvelocitiesindicatedthatthefast-movingejectaexpandedwitha
8 preferreddirectionorthogonaltothatoftheCSM.
0
. Keywords: supernovae:general-supernovae:individual:2009ip-techniques:polarimetric
1
-circumstellarmaterial-stars:mass-loss
0
7
1
:
v
1 INTRODUCTION phasespectroscopy(Maedaetal.2008)havealsobeenutilisedto
i
X revealdeparturesfromsphericalsymmetry.Oneofthemostsub-
Thereisagrowingbodyofevidencethatsuggeststhatthesuper-
r stantiveindicatorsforasymmetryinthelastdecadehascomefrom
a novaexplosionsattheendofthelifeofmassivestarsarenotspher- spectropolarimetry of extragalactic SNe. Polarimetry has shown
ically symmetric events. The most notable evidence was the re-
thatsignificantdeparturesfromsphericalsymmetryarepresentin
solvedasymmetricstructureofSN1987A,asobservedinlatetime
alltypesofcore-collapseSNe(CCSNe)(Wang&Wheeler2008).
HSTimaging(Jansen&Jakobsen2001;Larssonetal.2013;Sin-
Theobservationsshowedthatthepolarisationoftenincreaseswith
nottetal.2013).Complexasphericalmorphologieshavealsobeen
time as the core of the explosion becomes increasingly exposed
observedinGalacticsupernovaremnants(SNRs),includingthatof
(Wangetal.1996;Leonardetal.2006;Maundetal.2007a,2009;
CassiopeiaA(Fesenetal.2001;Wheeleretal.2008;Milisavljevic
Chornock et al. 2010). This suggests that the explosion itself is
&Fesen2013;Restetal.2011b).LightechoesofSNe(Crotts&
asymmetricinnature.
Yourdon2008;Restetal.2011a;Sinnottetal.2013)andnebular
Inadditiontobeingapowerfulprobeofthe3Dstructureof
SNeejecta,spectropolarimetrycanalsodivulgeinformationonthe
(cid:63) BasedonobservationsmadewithESOTelescopesattheParanalObser- mediumsurroundingtheprogenitorstars.Thisisparticularlyrel-
vatory,underprogramme290.D-5006. evantfortheinteraction-dominatedTypeIInSNe.Thespectraof
† Email:[email protected] Type IIn SNe are hydrogen rich and characterised by narrow (∼
(cid:13)c 2017TheAuthors
2 Reillyetal.
1000kms−1)emissionlines,andareanindicatorthattheejecta totheplaneoftheCSM.Theyproposethatthesmallangularcover-
arestronglyinteractingwithdenseinterstellarorcircumstellarmat- ageoftheCSMrequiresalargeSN-likeimpactvelocitytoproduce
ter (Schlegel 1990; Filippenko 1997). They are thought to origi- theobservedluminosity.
natefromthedeathsofverymassivestarsthathavelostaconsid- In this paper we present spectropolarimetric observations of
erableamountoftheirenvelopethroughepisodicmass-loss(Gal- thedeclineofthe2012bevent.Section2containsinformationon
Yametal.2007;Taddiaetal.2013,seealsoSmith2014).Thereare, theobservationsanddatareductionandinSection3webrieflydis-
however,otherchannelsthathavebeenproposed(suchasthrough cussthespectralevolution.Thepolarimetricresultsarethenanal-
thermonuclearexplosionsinadensecircumstellarmedium;Taddia ysedinSection4.InSection5wepresentaMonteCarlosimulation
etal.2012). ofthegeometryoftheCSMandpresentourconclusionsinSection
Oneofthemostcontroversialandbeststudiedobjectsofthis 6.
classisSN2009ip.ItwasdiscoveredbyMazaetal.(2009)inthe
spiral galaxy NGC 7259 in August 2009 and designated a super-
nova before it was re-classified as the extreme brightening of a
Luminous Blue Variable (LBV) (Berger et al. 2009; Miller et al.
2 OBSERVATIONSANDDATAREDUCTION
2009). Serendipitous pre-explosion Hubble Space Telescope im-
agesdatingfrom1999allowedSmithetal.(2010)andFoleyetal.
Spectropolarimetric observations of SN 2009ip were commenced
(2011)toestimateaninitialmassof50-60M .SN2009ipthenen-
(cid:12) on7November2012,35daysaftertheUVmaximumofthe2012b
duredthreemoreyearsofoutburstsculminatingintwospectacular event on 3rd October 2012 (Margutti et al. 2014). An outline of
eruptionsin2012(Pastorelloetal.2013).
the observations is given in Table 1. In total eight observations
In July-August 2012, SN 2009ip began to re-brighten once wereconductedoversixdifferentepochs.Aportionofthesedata,
again. This occurred in two phases, the first outburst beginning fourspectraoverthreeepochs,havepreviouslybeenpublishedin
on 24th July 2012 (2012a) reached a peak absolute magnitude Mauerhanetal.(2014).ThesearemarkedinTable1.Furtherob-
of MV ≈ −14 mags. A second re-brightening event began on servationsarepresentedhere,forthosedatapreviouslypublished
23rd September 2012 (2012b) and took the peak magnitude to a different approach to the interstellar polarisation is taken. The
MR = −18.5 mags, similar to some CCSNe. This outburst was spectropolarimetricdatawereacquiredusingtheEuropeanSouth-
accompaniedbyevidenceofmaterialtravellingatspeedsofupto ernObservatoryVLTAntutelescopewiththeFocalReducerand
15000 kms−1and a spectrum that was reminiscent of the early Low Dispersion Spectrograph (FORS2) instrument in the PMOS
phases of Type IIn SNe (Mauerhan et al. 2013; Pastorello et al. mode(Appenzelleretal.1998).InPMOSmode,FORS2operates
2013).ThisledmanytobelievethatSN2009iphadfinallyunder- asadual-beamopticalspectropolarimeter.FORS2measureslinear
gone a terminal explosion. The high SN-like velocities, however, polarisationwitha“superachromatic"halfwaveretarderplatethat
hadpreviouslybeenobservedin2011(Pastorelloetal.2013)and rotatestheplaneofpolarisationbeforeaWollastonprismseparates
theluminosityofthe2012aeventwassomewhatlowforaSNex- thelightintotwoorthogonally-polarisedbeams.Observationswere
plosion.Theseobservationscastdoubtonthesupernovaexplosion conductedwiththeretarderplateatfourangles:0.0◦,45.0◦,22.5◦
scenario.ThecuriouscaseofSN2009iphasnowbeenjoinedbya and67.5◦.The“striped"PMOSslitmaskwasusedwithslitwidth
handfulofsimilarobjects,includingSN2015bh(Ofeketal.2016; of1arcsec.The300Vand1200Rgrismswereusedinordertoob-
Soker & Kashi 2016; Elias-Rosa et al. 2016; Thöne et al. 2016), tainadispersionof∼3.23 pixel−1andaspectralresolutionof∼12
SNHunt248(Kankareetal.2015)andLSQ13zm(Tartagliaetal. across the spectrum with the former, whilst the latter provided a
2016). dispersionof∼0.73 pixel−1andaspectralresolutionof∼2.7 .To
The final fate of SN 2009ip has been much debated in the preventsecondordercontaminationatlongerwavelengthsanorder
literature. The proposed explanations for the 2012 outbursts fall separationfilter(GG435)wasused,yieldingwavelengthrangesof
into 3 scenarios: a terminal CCSN in either the 2012a or 2012b 4450-8650 forthe300Vand5750-7310 forthe1200Rgrism.
outbursts(Mauerhanetal.2013;Smithetal.2014;Mauerhanetal. ThedatawerereducedinIRAF1 usingthemethodoutlined
2014)oranother“impostor"eventfromtheeruptionsofanLBV inMaundetal.(2007a)andthenormalisedStokesparameters(q
star(Pastorelloetal.2013;Fraseretal.2013;Marguttietal.2014). and u) were calculated following Patat & Romaniello (2006). In
Analternativepossibilityisthatthe2012eventsweretheresultof ordertoincreasethesignal-to-noiseratio,thespectrawerebinned
closeencountersandthepossiblemergerofanLBVstarandamore to15 and5 forthe300Vand1200Rspectra,respectively.Thiswas
compactcompanion(Kashietal.2013;Soker&Kashi2013). performedpriortothecalculationoftheStokesparameters.
Despitethedetailedstudiesofthisobject,thereisstillnoclear Thedatawerecorrectedforthewavelengthdependentchro-
consensus on whether the progenitor star survived the events of maticzero-angleoffsetoftheretarderplate,andthepolarisedspec-
2012.Asofyettherehasbeennoevidenceofnucleosynthesisprod- tra were debiased following the prescription of Quinn (2012). At
uctsinlate-timespectrawhichwouldconfirmcorecollapse(Fraser eachepoch,calibratedfluxspectrawereproducedusingthespectra
etal.2013;Marguttietal.2014;Fraseretal.2015).Mostrecently observedatallfourretarderplateangles.Thecombinedspectrum
Thöneetal.(2015)havereportedthattheluminosityofSN2009ip wasfluxcalibratedusingobservationsofafluxstandardstar,ob-
has now declined below that of the possible progenitor observed served with the polarimetry optics in place and with the retarder
in1999.ThispossiblyfavourstheterminalSNexplosionscenario, plateat0◦.
orsuggeststhatthestarwasinoutburstduringthosepre-explosion
observations.
Mauerhanetal.(2014)havepreviouslypublishedpolarimetric
observationsofthisoutburst.Thelargecontinuumpolarisationthey 1 IRAFisdistributedbytheNationalOpticalAstronomyObservatories,
observeatthepeakofthe2012beventsuggestsahighly-aspherical which are operated by the Association of Universities for Research in
interactionzone,fromwhichtheyinferadisk-likeortoroidalcir- Astronomy,Inc.,undercooperativeagreementwiththeNationalScience
cumstellarmedium(CSM).Additionally,theejectaareorthogonal Foundation-http://iraf.noao.edu/.
MNRAS000,1–??(2017)
Spectropolarimetryofthe2012outburstofSN2009ip 3
Table1.TableofVLTFORS2observationsforSN2009ip
Object Date Grism Exposure MeanAirmass Epoch† S/N
(UT) (s) (days)
(cid:63)SN2009IP 2012117.02 300V 4×330 1.01 +35 550
(cid:63)SN2009IP 2012117.03 1200R 4×470 1.04 +35 400
LTT1020‡ 2012117.06 300V 10 1.12
LTT1020‡ 2012117.06 1200R 30 1.12
SN2009IP 20121114.07 300V 4×310 1.17 +42 375
SN2009IP 20121114.08 1200R 4×500 1.26 +42 300
LTT1020‡ 20121114.18 300V 10 1.06
LTT1020‡ 20121114.19 1200R 25 1.06
(cid:63)SN2009IP 2012126.04 300V 4×895 1.43 +64 300
LTT1020‡ 2012126.09 300V 15 1.01
(cid:63)SN2009IP 20121210.04 1200R 4×890 1.53 +68 200
LTT1020‡ 20121210.09 1200R 40 1.02
SN2009IP 20121215.04 300V 4×895 1.73 +73 150
LTT1020‡ 20121215.09 300V 15 1.05
SN2009IP 20121225.04 1200R 4×890 2.10 +83 50
LTT1020‡ 20121225.09 1200R 40 1.10
†RelativetoUVmaximumofthe2012beventwhichoccurredonthe3rdOctober2012(Marguttietal.2014)
‡Fluxstandard
(cid:63)DatathatwerepreviouslypublishedinMauerhanetal.(2014)
3 SPECTRALEVOLUTION ∼5755and∼5860 at+35days.Wenotethatthefeaturearound
5600-5900 shows a similar structure to the HeI 10830 feature
ThefluxspectrafortheobservationstakenwiththeFORS2instru-
observed by Fraser et al. (2013). This similarity leads us to pro-
mentbetweenthe7November2012(+35days)and25December
pose that the contribution of NaID to the broad line feature is
2012(+83days)arepresentedinFigure1.Lineidentificationsin-
minor. The HeI absorption is strongest at ∼5640 with velocity
dicatedinthefigurearetakenfromFraseretal.(2013)andMargutti
∼ −11800kms−1,theothertwocomponentsareobservedata
etal.(2014),wherethespectroscopicevolutionofSN2009ipfrom
velocityof−6200and−800kms−1.Theabsorptioncomponents
thisperiodcanbefound.
becomestrongerat+42daysandamorecomplexvelocitystruc-
At+35and+42daysHαdisplaysadistinctandasymmet-
tureemerges,withfurthernarrowabsorptioncomponentsappear-
ricbroademission(FWHM≈10,200kms−1)withanarrowemis- ing. The low-velocity (< 5000 kms−1) absorption components
sion(FWHM≈300kms−1)superimposed.Anarrowabsorption
increase in strength by +64 days, so that the feature appears to
component can be seen with a minimum at ∼−1500 kms−1at
be composed of two separate absorption components at ∼−4000
+35 and +42 days. The absorption decreases in strength until kms−1and ∼ −12000 kms−1. Note that the higher velocity
it has disappeared by +64 days. At this stage the shape of the
component may be Sc II λ5658 in absorption, as has been iden-
broademissionhaschanged,becomingmorereminiscentofSNein
tifiedinSN1999embyElmhamdietal.(2003),ScIIλ5526isalso
thenebularphase.AbroadabsorptioncomponentwithFWHM∼
seeninemissionat+64and+73days.At+64and+73daysthe
104 kms−1isapparentatallepochs.Superimposedonthisbroad
narrowemissionofNaIDisreplacedbyabroaderemissionresult-
absorption are further narrow absorption notches at −12,300,
ingfromthestrengtheningoftheHeIλ5876emissionline.Thisis
−9700, −7300, −6000 kms−1, which we label A, B, C and D
supportedbytheincreasedstrengthoftheHeIλλ5015,6678and
respectively.IntheearlyepochstheabsorptionnotchesA,BandC
7065 linesatthoseepochs.
arestrongest,at+64dayscomponentsCandDincreaseinstrength
The CaII infrared (IR) triplet appears as three weak narrow
beforenotchDbecomesweakeragainat+73days.Thesenarrow
emissionlineswithapotentialunderlyingbroademissionat+35
componentsaremoreclearlyvisibleinthe1200Robservations.Hβ
and+42days,bythethirdepochtheemissionlinesbecomemore
showsabroadabsorptiontroughwithasimilarmulticomponentve-
pronounced with associated blueward broad absorption troughs
locitystructureembedded,thisismostprominentat+42days.In (∼−4000kms−1).
contrast,Hβ doesnotshowthesamebroademissionthatHαdoes.
AbroadblendofHeI λ5876andNaID inabsorptionisob-
servedinthe300Vspectra.Inall1200Rspectra,theNaIDdoublet
isresolvedseparatelyasnarrowemissionlinesat5889and5895
4 ANALYSISOFTHEPOLARIMETRY
(each with FWHM≈220 kms−1). These are emitted at the rest
wavelengthinthehostgalaxy.ThereisnosignofHeI λ5876in ThepolarisationspectraofSN2009ipfrom7November2012(+35
emission. Multiple absorption components are visible at ∼5640, days)tothe25December2012(+83days)asobservedwiththe
MNRAS000,1–??(2017)
4 Reillyetal.
13
−
Hβ HeI NaID Hα [CaII] CaIINIR
A,B,C,D
⊕ ⊕
14
− +35d
+35d
15
− +42d
t
n
a +42d
t
s
n
o
+c 16
−
)
λ
F +64d
(
0
1
g
o +68d
l
17
−
+73d
18
−
+83d
19 HeI HeI HeI HeI
−
4500 5000 5500 6000 6500 7000 7500 8000 8500 9000
Wavelength (˚A)
Figure1.FluxspectraofSN2009ipfortheobservationsbetween7November2012(+35days)and25December2012(+83days)takenwiththeFORS2
instrumentwiththe300V and1200Rgrisms.LineidentificationsindicatedaretakenfromFraseretal.(2013)andMarguttietal.(2014),theverticallines
showthepositionoftherestwavelengthfortheselines.Alsoidentifiedarethehigh-velocitynarrowabsorption“notches"ofHα:A,B,CandD.Telluric
featuresareindicatedby⊕.
300Vand1200RgrismsarepresentedinFigures2and3below,re- Thelowpolarisationiscorroboratedbythatofastandardstar,HD
spectively.Thefluxspectraarealsoshowninordertoaidtheidenti- 212146,40’fromthepositionofSN2009ip,whichhasp=0.08±
ficationofpolarisedsignalsassociatedwithparticularlinefeatures. 0.04%(Heiles2000);however,thedistancetothisstarisonly∼
40pc(vanLeeuwen2007),suchthatitonlyprovidesalowerlimit
ontheGalacticISParisingalongthelineofsight.
4.1 Interstellarpolarisation The component of the ISP from the host galaxy is also ex-
pectedtobesmall,asNGC7259isviewedface-onandSN2009ip
Photonsscatteringfromgasanddustgrainsintheinterstellarand
is positioned at the extreme edge of the optical disk at 4.3kpc
circumstellar material introduce a source of polarisation in the
(Fraser et al. 2013). The data (Figures 2 and 3) show no strong
spectrum that is not intrinsic to the transient. The removal of the
wavelength dependence in the continuum polarisation, indicating
interstellar polarisation (ISP) is therefore vital to the analysis so
thattheISPissmall.Astraightlinefitwithaleastsquaresmethod
thatwemayunderstandthegeometryoftheevent.
totheStokesq anduspectraat+35dayswitha1st orderpoly-
TheforegroundGalacticreddeningfortheMilkyWayGalaxy
nomialyieldedaweakdependenceonthewavelengthofdu/dλ=
(MW)alongthelineofsighttoSN2009ipisE(B−V)=0.019
mags(Schlegeletal.1998)2.Theinterstellarpolarisationexpected +1.5±1.9×10−7/anddq/dλ=+4.9±2.0×10−7/.
fortheMWdustcomponentis< 0.17%(Serkowskietal.1975). AvaluefortheISPwasdeterminedassumingthattheemis-
sionofHαwasintrinsicallyunpolarised,suchthatthelinepolari-
sation(q andu )atthesewavelengthsshouldreflecttheISP.
line line
2 http://ned.ipac.caltech.edu Indeed,thereisevidenceofdepolarisationassociatedwiththenar-
MNRAS000,1–??(2017)
Spectropolarimetryofthe2012outburstofSN2009ip 5
3.0 13.0
−
%) 22..05 A,B,C,D +35days −13.5+const.
( 1.5 14.0)λ
p 01..50 −14.5og(F10
− l
0.0 13.0
Wavelength(˚A) −
%) 22..05 +42days −13.5+const.
( 1.5 14.0)λ
p 01..50 −14.5og(F10
− l
0.0
Wavelength(˚A) +64days 13.6
22..05 − onst.
%) 14.4+c
( 1.5 − )λ
p 1.0 15.2g(F10
0.5 − lo
0.0
Wavelength(˚A) +73days
4.0 -14.0nst.
o
%) 3.0 +c
p( 12..00 -15.0og(F)λ10
l
-16.0
0.0
4500 5000 5500 6000 6500 7000 7500 8000 8500 9000
Wavelength(˚A)
Figure2.Spectropolarimetricdatatakenwiththe300Vgrismstartingon7November2012andendingon15December2012.Thedegreeofpolarisationasa
functionofwavelengthispresentedwiththedatabinnedto15 andcorrectedfortheISP.Thefluxspectraareingreywithunits10−14ergs−1cm−2−1and
arecorrectedfortherecessionalvelocityofthehostgalaxy,cz=1782±5kms−1(Meyeretal.2004).Thewavelengthregionsthatwereusedtocalculate
thecontinuumpolarisationareshownbytheredhorizontalbars.Notethedifferentscalingineachpanelinfluxandthedegreeofpolarisation.
rowemissionlineatallepochsandwiththebroadwingsat+35 wasmeasuredfromthebottomofthebroadHαwings.Inthiscase
and+42days. Twoestimatesof thelinepolarisation weredeter- q = q , u = u and F = F in the above
base cont base cont base cont
minedunderdifferentassumptions: equations.
ForISP ,thedepolarisationobservedinStokesq anduas-
(i) ISP assumedthatthebroademissionofHαunderthenar- B
A sociatedwiththebroadwingsofHαat+35and+42days,were
rowemissioncomponentwasunpolarised;
approximatedwithaGaussianprofile.Thelineintensitywasmea-
(ii) ISP assumed that only the narrow emission component
B sured from the base of the narrow HαP Cygni profile. To deter-
wasunpolarised.
minethelinepolarisationinthiscase,thecontinuumpolarisation
The Stokes line polarisation parameters, q and u are andthebroadHαcomponentwereaccountedforsuchthat
line line
definedasfollows;
q =q +q (3)
base cont broad
q F −q F
qline = tot totF base base (1) andsimilarlyforubaseand:
line
F =F +F (4)
base cont broad
u F −u F
uline = tot totF base base (2) Theqline anduline ofa5 bincentredonthenarrowHαemission
line inthe1200RspectraweretakenastheestimateoftheISPineach
Theseequationsassumethat,atanygivenwavelength,theob- epoch.Thehigherspectralresolution1200Rdatawerechosento
servedtotalpolarisationisthesumofthelinepolarisation(ifany) avoidcontaminationfrompotentialpolarisationinthebroadwings
andsomebasepolarisationthatneedstoberemovedfromtheob- ofHα.ThevalueadoptedastheISPwasthentakenastheinverse
servedpolarisationtodeterminethelinepolarisation.Thefluxof error weighted average of those from the first 3 of the 4 epochs
eachcomponentisdenotedbyF. of1200Rdatasetspresented.Thesignal-to-noiseratioofthepolar-
ForISP ,aflatcontinuumpolarisationwastakenasthebase isation spectrum at +83 days was considered too low to provide
A
polarisation.Thecontinuumpolarisation,q andu ,wasde- anaccuratedeterminationoftheISP.Averagingthemeasurements
cont cont
terminedastheweightedaverageofStokesq anduinthewave- acrossmultipleepochsmakesthebestuseofthedata,astheISP
length regions indicated in Figure 3. The continuum flux, F shouldnotvarywithtime.
cont
MNRAS000,1–??(2017)
6 Reillyetal.
2.0
+35days
1.6 A,B,C,D −13.6onst.
%) 1.2 +c
p( 00..48 −14.4g(F)λ10
15.2lo
0.0 −
Wavelength(˚A) +42days
1.6 −13.6onst.
%) 1.2 +c
p( 00..48 −1154..24og(F)λ10
− l
0.0
Wavelength(˚A) +68days 13.6
22..05 − onst.
%) 14.4+c
( 1.5 − )λ
p 01..50 −15.2og(F10
l
0.0
56..00 Wavelength(˚A) +83days -14.0nst.
o
%) 4.0 +c
p( 23..00 -15.0g(F)λ10
1.0 o
-16.0l
0.0
5800 6000 6200 6400 6600 6800 7000 7200
Wavelength(˚A)
Figure3.SameasFigure2,butforspectropolarimetricdatatakenwiththe1200Rgrismstartingon7November2012andendingon25December2012.
ISP was found to be q =0.10±0.16% and certaintyoftheStokesqanduofthecontinuumcorrespondstothe
A ISP
u =−0.224±0.019%, corresponding to p =0.18±0.13% standarddeviationinthechosenregions.Thedegreeandangleof
ISP ISP
and θ =146±22◦. This is consistent with the upper limit polarisation,pandθ,forthecontinuumwerethencalculated,with
ISP
derived from the reddening above. Under the assumption that pde-biasedasdescribedinSection2.ThesearepresentedinTable
the broad emission was polarised, ISP was determined to be 2.
B
qISP=0.30±0.09% and uISP= −0.17±0.06%, corresponding to Thecontinuumissignificantlypolarisedinthefirstepochat
pISP=0.33±0.09% and θISP=164±8◦. The two estimates for the p ∼0.7% and θ ∼ 45◦. By the second epoch of +42 days, the
ISPagreetowithintheirerrors.TheuncertaintiesonqISPanduISP degree of polarisation has slightly decreased and is accompanied
arethestandarddeviationoftheindividualmeasurementsfromthe byasmallrotationinthepolarisationangle.Thereisnosignificant
threeepochsaroundthefinalinverseerrorweightedaverage. change in the degree of polarisation between +42 days and +64
As ISPA relied on fewer assumptions, this was the one days, there is, however, a substantial change in the angle which
adopted as the ISP. The ISP lies close to the origin on the q-u undergoesarotationof∼60◦.Followingthis,boththedegreeand
plane,inthepositiveq,negativeuquadrant,asmarkedinFigure4. angleofpolarisationremainapproximatelyconstantatthe1σlevel
Asthereisnodiscerniblewavelengthdependenceintheobserved throughout the rest of the observations. The uncertainty in p and
continuumpolarisationatallepochs,weapplyasinglecorrection θ increasesinthelaterepochsduetothedecreasingsignalasthe
for the ISP to the entire wavelength range. ISPA was subtracted transientfades.
fromStokesq anduandtheuncertaintiespropagatedbyaddition
Therotationofthepolarisationanglebythethirdepochisalso
inquadrature,pandθwerethenrecalculated.
clearfromtheevolutionontheq-uplane(Figure4).Thelargerota-
tionoccursatthesametimeasadropinthebolometriclightcurve
asreportedbyMarguttietal.(2014).Thechangeintheorientation
4.2 Theintrinsiccontinuumpolarisation
ofthepolarisationispossiblyrelatedtothisevent.TheI-bandlight
UponremovalofISP ,thecontinuumpolarisationwasdetermined curveproducedbyGrahametal.(2014)isshowninFigure5and
A
bytakingtheinverseerrorweightedaverageofStokesq anduin alsoshowsthedropinfluxat∼+70days.TheI-bandlightcurve
the wavelength regions specified in Figures 2 and 3. Wavelength waschosentodisplayasthedropat∼+70daysismostobvious.
regions where the polarisation was approximately flat and where Therobustnessoftheobservedrotationwastestedbychangingthe
therewerenoobviouslinefeaturesinthecorrespondingfluxspec- determinedISP toitsupperandlower3σ boundaries.Themea-
A
trumwerechosentodeterminethecontinuumpolarisation.Theun- sured rotation was found to be partially dependent on the choice
MNRAS000,1–??(2017)
Spectropolarimetryofthe2012outburstofSN2009ip 7
+35days +42days +64days +73days
2.0
Continuum Continuum Continuum Continuum
1.0
)
%
0.0
(
u
-1.0
-2.0 ISP ISP ISP ISP
-2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0
q (%) q (%) q (%) q (%)
5500 5750 6000 6250 6500 6750 7000 7250 7500
Wavelength (˚A)
+35days +42days +68days +83days
2.0
Continuum Continuum Continuum Continuum
1.0
)
%
0.0
(
u
-1.0
-2.0 ISP ISP ISP ISP
-2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0
q (%) q (%) q (%) q (%)
6000 6200 6400 6600 6800 7000 7200
Wavelength (˚A)
Figure4.TheevolutionofSN2009ipontheq-uplane.Thetoppanelcontainsthedatafromthe1200Robservations(binnedto5 or∼230kms−1)andthe
bottompaneltheobservationstakenwiththe300Vgrism(binnedto15 or∼680kms−1).Thepositionofthecontinuumismarkedbythewhitestarwith
the1σerrorbars.Thedataarecorrectedfortheinterstellarpolarisationshownasagreystarwithblackerrorbars.
of ISP, with the rotation decreasing from ∼60◦ to ∼20-40◦. We
Table2.TheISP-correctedcontinuumpolarisationofSN2009ip.
consideritrobustthatthecontinuumpolarisationrotates,however
theerrorintroducedbythechoiceofISPimpliesthatthedegreeto Phase(days)† p(%) θ(◦)
whichitrotatesshouldbeapproachedwithcaution. 300V
+35 0.76±0.21 47±8
+42 0.37±0.30 57±21
4.3 Intrinsiclinepolarisation +64 0.29±0.24 100±15
+73 0.66±0.55 89±21
ThefluxspectraofSN2009ipexhibitcomplexlinefeatures,with
narrowandbroadPCygniprofilesandabsorptionnotchesembed- 1200R
dedatbothlowandhighvelocity.Similarly,thepolarisationspectra
+35 0.69±0.20 41±8
alsoshowacomplexstructure,withsomeofthefeaturescommon
+42 0.33±0.26 63±20
tomultiplespecies.Inthefollowingsubsectionsweseparatethese
+68 0.48±0.39 119±22
featuresaccordingtothebreadthoftheprofile(narrowandbroad
+83 0.78±1.07 149±39
features)andthevelocityatwhichtheyappear,withrespecttothe
restwavelengthofthelinewithwhichtheyareassociated.Theve- †RelativetoUV-bandmaximumofthe2012bre-brighteningonthe3rd
locities, degree and angle of polarisation of all the features sum-
October2012
marisedinthefollowingsubsectionscanbefoundinTableAinthe
Appendix.
MNRAS000,1–??(2017)
8 Reillyetal.
r3dOct t7hNov1t4hNov t6hDec1t0hDec1t5hDec2t5hDec 2.0 +35d
14 1.5
e
d
nitu15 %) 1.0
Mag16 u(
nd17 0.5
a
b
I-18
0.0
160 0.8
120 0.4
◦ %)
θ 80 (
q 0.0
40
0.4
0 −
2.4 0.8
−
2.0 5800 6000 6200 6400 6600 6800 7000 7200
%)1.6 Wavelength(˚A)
(1.2
p
0.8
0.4 Figure6.ThefittothebroadHαdepolarisationinqanduat+35daysas
0.0 observedwiththe1200Rgrism.Thetopandbottompanelsshowqandu
20 0 20 40 60 80 100 respectivelyalongwiththeirGaussianfittothedepolarisation(the±1σfits
−
Timesincethe2012bpeak(days) areshownbythedashedredlines),usedtoisolatethepolarisationofthe
low-velocitynarrowHαline.
Figure5.Theevolutionofthecontinuumpolarisationoverthecourseof
the2012boutburst.ThetoppanelshowstheI-bandlightcurveusingdata
presentedbyGrahametal.(2014)withthedatesofthepolarimetryobser- of the fit of p to the observed p are also evenly scattered for
obs
vationsoverlaidwiththedashedgreenlinesforthe300Vdataandsolid p (cid:54)0.3%.Thelinepolarisationintheouterwings,however,is
line
magentalinesforthe1200Rdata.Thepolarisationangleasafunctionof notaswellconstrained.Theχ2 isminimisedtoavalueof1.38by
ν
timeisshowninthemiddlepanel,followedbythepolarisationdegreeinthe
p =0.5±0.1%.Thiscorrespondstoonlya0.043improvementin
bottompanel.ThesmallerblackmarkersrepresenttheV-bandpolarisation line
thereducedchisquaredovereitherp =0.0%orp =0.9%.The
measurementsaspresentedbyMauerhanetal.(2014). line line
datainthisregionappeartobeequallywellrepresentedbyeitherof
thesevalues.Inthiscaseitisdifficulttodiscerniftheouterwings
4.3.1 Broadpolarisedfeatures:Hα aremorepolarisedthanthecoreofthebroademission.
At +68 days, χ2 is minimised for p = 0.3±0.1% for
Inthefluxspectrum,Hαisobservedasacombinationofanarrow ν line
both wavelength regions. This represents a 0.25 improvement in
PCygniprofilesuperimposedonabroadPCygniprofile.Thereis
χ2 inthecoreoverp =0.0%butonlya0.06improvementfor
obviousdepolarisationassociatedwiththebroademission(∼ 104 thνeouterwings.Theldinaetaappeartobewellrepresentedbyp (cid:54)
kms−1) at +35 and +42 days, as can be seen in Figures 2 and line
0.5%,withtheincreasedscatterinthepolarisationmeaningthatit
3.Nootherspeciesinthespectrashowsthesamebroademission
canbeconstrainednofurther.
ordepolarisationattheseepochs,whichcouldbeaconsequenceof
Thepolarisationbluewardofthenarrowemissionisnotwell
thestrengthoftheline.
representedbythefitsappliedtotheredwardbroademission.This
Fordatatakenat+35and+68days,theobservedpolarisa-
isalsoclearinFigure6,wheretheStokesparametersacrossHαat
tion,p ,ofthebroadHαemissionin1200Rwasmodelledwith
obs +35daysarepresented.Thedatashownarrow(mostlyinu)de-
theintrinsiclinepolarisation,p ,asavariable.Themodeltook
line parturesfromthefitat∼6370 (v ∼ −8820kms−1)and∼6540
theformof:
(v∼−1000kms−1),thesewillbeanalysedinthefollowingsub-
p = plineFline+pcontFcont (5) sections.Abroader(∼5500kms−1)deviationfromthefitisob-
obs Fobs servedmostlyinStokesq∼0.2−0.6%peakingat∼6460 .Thisis
wheretheintensityofthecontinuum,F wasfitwithasecond indicativeofpolarisationintrinsictothebroadPCygniabsorption.
cont
order polynomial. Two wavelength regions were chosen in order The polarisation angle of the broad component blueward of
todeterminewhethertheintrinsicpolarisation,p ,ofthebroad Hαwasfoundusingthe1200Rspectraat+35,+42and+68days.
line
emissionwasfixedorvariableasafunctionofwavelength.These Aninverseerrorweightedaverageofq andu wascalculated
line line
were 6600 - 6700 (corresponding to ∼1700 - 6300 kms−1) for inregionsavoidingthenarrowpolarisationfeaturesassociatedwith
thecoreand6700-6800 (∼6300-10000kms−1)fortheouter theabsorptionnotchesA,B,CandD.Thefeaturebetween6300-
redwingofthelineprofile. 6500 (∼ −12000-−3000kms−1)wasfoundtobepolarisedto
At+35days,theobservedpolarisationappearstobebestfit thep ∼0.3-0.65%levelfrom+35to+68days.Thepolarisation
line
withp (cid:54) 0.3%inthecore.Theχ2 wasminimised(toavalue anglewasconsistentatθ ∼125◦ inthefirsttwoepochsbefore
line ν line
of1.68)forp =0.2±0.1%.Theimprovementinχ2 forp rotatingto85±42◦,thelargeuncertaintyonthelattervaluemeans
line ν line
=0.2%overp =0.0%isonlyfractionalat0.12.Theresiduals thatitisalsoconsistentwiththepreviousvaluestowithin1σ.
line
MNRAS000,1–??(2017)
Spectropolarimetryofthe2012outburstofSN2009ip 9
3.0
Hβ
HeI
4.8
HeI
Hα
2.5
Hα
4.0
2.0
3.2
(%) %) 1.5
p 2.4 p(
1.0
1.6
0.8 0.5
0.0
0.0
20000 15000 10000 5000 0 5000 10000 15000 20000
− − − −Velocity(103kms−1) −20000−15000−10000 −V5el0o0c0ity(1003km5s0−010) 10000 15000 20000
Figure7.Lineprofilesand300VpolarisationspectraforHα,Hβ andHeI Figure8.SameasFigure7butforHαandHeIλ5876asobservedwith
λ5876invelocityspaceat+35days.Thevelocityoftheabsorptioncom-
the1200Rgrismat+35days.ThepositionoftheNaIDemissionlinesare
ponent(atabsorptionminimum)ofthenarrowPCygniprofileofHαisin-
markedbythegreydashedlines.Againthevelocityatabsorptionminimum
dicatedbytheblackdashed-dottedverticalline.Thegreydash-dottedline
forthenarrowPCygniprofileofHαisindicatedbytheblackdash-dot
indicatesthepositionofsignificantpolarisationobservedforHβ andHeI verticalline.Apeakinthepolarisationisobservedat∼−1000kms−1in
λ5876at∼ −14000kms−1.Peaksinthepolarisationcanalsobeseen
bothhydrogenandhelium,implyingasharedstructureatlowvelocitiesfor
atlowvelocities(∼−1000kms−1)forHαandHβ.Notethatthepolar-
theseline-formingregions.TheHeIprofileistruncatedasitfellattheblue
isationspectrumacrossHα(inred)hasbeenoffsetby1%forclarity.
edgeofthewavelengthrangeofthe1200Robservations.
4.3.2 Low-velocitynarrowpolarisation
torsubtractedfromtheobserveddata.Thefittothe1200RStokes
It can be seen from Figure 7 that Hα, Hβ and HeI λ5876 have q and u at +35 days is shown in Figure 6. This was fit for both
complicated line profiles with multiple absorption components at the 1200R and 300V spectra at +35 and +42 days. For the later
bothhighandlowvelocity.Inparticular,theyeachshowanarrow epochs, when there is no discernible depolarisation, only the flat
absorption component at ∼ −1500 kms−1. In the polarisation continuum polarisation, as determined in Section 4.2, was sub-
spectraof+35days,presentedinFigure7,alargepeakisobserved tracted.
associatedwiththiscomponentforHαandHβ.Thisfeatureisalso Inthe1200Rspectrumof+35days,thedegreeandangleof
observedinthe1200RdataforHαandHeIλ5876,asseeninFig- polarisationofthelow-velocityfeatureforHαandHeIλ5876are
ure8.ForHα,thefeatureisevidentontheq-uplaneinFigure9 consistent at p ∼1% and corresponding angle of θ ∼33◦.
line line
at+42days.Itcanbeseenasasmallnumberofdatapointsinthe Thesignalsdisplayastrikingsimilaritywhencomparedinveloc-
upperleftatlowvelocities(∼−1000kms−1)thatprotrudeaway ity,asseeninFigure8.ForHβ thedegreeofpolarisation,asre-
fromwherethecontinuumisindicated.TheHαpeakisvisibleun- vealedinthe300VspectruminFigure7,atthisepochisobserved
til+68daysinthe1200Rdatabutisnotpolarisedbymorethan3σ tobeloweratp =0.4±0.3%andθ =44±24◦.Thepolarisation
line line
after+35daysinthe300Vdata.Thedifferenceinthestrengthof of the same low-velocity Hαfeature is also weaker in the 300V
thefeaturebetweenthetwodatasetsislargestat+42days.Smooth- dataatp =0.30±0.23%andtheangleslightlyrotatedandwith
line
ing the 1200R data to the resolution of the 300V grism and us- alargeruncertainty,θline=50±15◦.Similarly,theHeI λ5876po-
ingthesamebinsizeresolvedthisdiscrepancy.Thishighlightsthe larisationisnotobservedinthe300Vspectrumtakenonthesame
needforhigh-resolutionspectropolarimetricobservationsfornar- night(thisisevidentwhenFigures7and8arecompared),demon-
rowlineSNe.Thedegreeofpolarisationdecreasesovertimeasthe stratingthelossofinformationwhenthespectraarebinnedto15
strengthoftheabsorptioninthefluxspectrumdecreases. .
As the spectra contain a number of narrow features that are At +35 days, the low-velocity polarised peaks of Hαand
marginallysignificantwehaveemployedatwo-tailedz-testacross Hβ areredshiftedwithrespecttotheabsorptionlineminimumob-
ourdata(Sprinthall2011).Weonlyanalysethosefeatureswhere servedinthefluxspectrum(seeFigure7).Inthe1200Rspectrum,
thetwo-tailedz-testgivesaprobabilityofmorethan∼ 3σ (hav- thisisby350kms−1forHα,whereforHeIλ5876thepolarised
ing a probability less than 0.27%) that the feature is not consis- peak is blueshifted from the absorption minimum. It is possible
tent with the continuum. The depolarisation associated with the that blending with NaID has smeared out the total flux line pro-
broad Hαcomponent was accounted for when calculating the z- file.Giventhesimilarityinthepolarisedsignalsinvelocityspace,
test statistic for the low-velocity narrow Hαsignal and the high- itwouldappearthatNaIDdoesnotsignificantlycontributetothe
velocitynarrow“notches". polarisedlineprofile.
In order to isolate the line polarisation of the low-velocity TheHαsignalremainspolarisedthroughto+68dayswhere
Hαfeature,thedepolarisationofthebroadHαemissioncompo- itisdetectedat2.8σ(accordingtothez-test)andexhibitsnosignif-
nentwasapproximatedwithaGaussianprofile.Thiswasthenvec- icantchangeinthepolarisationangle.Incontrast,thelow-velocity
MNRAS000,1–??(2017)
10 Reillyetal.
+35days +42days +68days +83days
Continuum Continuum Continuum Continuum
1.0
)
%
0.0
(
u
-1.0 ISP ISP ISP ISP
-1.0 0.0 1.0 -1.0 0.0 1.0 -1.0 0.0 1.0 -1.0 0.0 1.0
q (%) q (%) q (%) q (%)
4000 3000 2000 1000 0 1000 2000 3000 4000
� � � �
Velocity (km/s)
Figure9.TheevolutionofHαontheq−uplaneasobservedwiththe1200Rgrisminplaceusing5 (∼230kms−1)binning.Thepositionofthecontinuum
ismarkedbythewhitestarwiththe1σerrorbars.ThepositionoftheISPisindicatedbythegreystar.StokesqanduhavebeencorrectedfortheISPinthese
plots.
HeIλ5876featureisnotobservedafter+35days.TheHβ signal, couldbeinterpretedasarisingfromthesameline-formingregion
conversely,increasesindegreeofpolarisationtop =1.1±0.3% asthisfeatureatthisepoch.
line
andundergoesarotationto98±9◦beforedissipatingby+64days.
ThesimilarityinthepolarisationofHα,Hβ andHeIλ5876atlow
velocitiesindicatesthathydrogenandheliumshareanasymmetric Inthelaterepochsof+64and+73daysthereareseveralpo-
slowlymoving(−1100kms−1)line-formingregion. larisedpeaksintherangeofpline ∼0.6-1.4%forHα,Hβ andHeI
λ5876thathavenoobviouscorrespondingfeatureassociatedwith
otherlines.Thepolarisationanglesofthesefeaturesmostlycluster
aroundthesameregionoftheskyat∼120◦.At+64days,3fea-
4.3.3 Polarisationatintermediateandhighvelocities turesaredetectedinthe300VdataassociatedwithHαat−7000
-−12150kms−1withpolarisationatthe0.6-0.8%level.Incon-
Multiplespeciesshowpeaksinthepolarisationatintermediateand trast,nosignificantpolarisationisdetectedinthisvelocityrangein
highvelocityassociatedwiththeobservedabsorptioncomponents the1200Rspectrumoffourdayslaterat+68days.Itispossible
(seeFigure7).Similarlytothelow-velocityfeaturediscussedinthe thatthesefeatureshavedissipatedinthattime,howeverFigure3
previous subsection, some polarised features have corresponding shows that there are some rises in the polarisation in this range.
signalsassociatedwithanotherlineorspecies,indicatingthatthey The uncertainty on the continuum polarisation is higher at +68
mayariseinsimilarline-formingregions.This,however,isnotthe daysthaninthe300Vspectrumof+64days;thelargeruncertainty
case for all detected line polarisation. Further information of the couldmeanthatthesefeaturesarepresentbutnotdetectedasbe-
polarisationoffeaturessummarisedherecanbefoundinTableA ingsignificantlydifferentfromthecontinuumpolarisationat+68
intheAppendix. days.
At+35days,HeIλ5876andHβ displaypeaksat∼−14000
kms−1with p ∼0.5-0.7% and consistent polarisation an-
line
gle of θline ∼55◦. Figure 7 shows that the profiles are also TheCaIIIRtripletonlybecomespolarisedat+64daysinthe
similarly shaped in velocity space. Both polarisation peaks are 300Vdata(seeFigure2),associatedwiththestrengtheningofthe
also blueshifted from the nearest absorption minimum by ∼2000 emissionlinesatthisepoch(Figure1).Stronglinepolarisationis
kms−1.Similarly,at+64days,HαandHeIλ5876shareapeak observedinthetwodistinctabsorptiontroughs.Thevelocitiesof
of strength p =0.7±0.3% and angle of θ ∼120◦ at −9800 the polarised features were determined by using the nearest red-
line line
kms−1.At+73days,Hβ andHeIλ5876againshowcompara- wardemissionastherestwavelength.Notethatthesemaybeinac-
blepolarisationat−6350kms−1atasimilarpositioninthesky. curateduetoambiguityinwhichofthecomponentsofthetriplet
Influx,thevelocitystructureofHα,Hβ andHeIλ5876aresimi- isresponsibleforthefeature.At+64daysthemaximumobserved
larasshowninFigure7;thestructureinpolarisationcorroborates polarisationassociatedwiththeCaIIIRtripletispline=1.1±0.4%,
this.Thedataimplythatathighvelocitiesaswellaslow,hydrogen thecomponentsinbothabsorptiontroughshavesimilarconsistent
andheliumsharethesameasymmetricalline-formingregions. polarisationanglesof∼100◦.At+73daysbothofthesefeatures
At+42daysthereissignificantpolarisationattheabsorption strengthen.Thefeatureinthetroughofthe8542 line(atλ=8375
minimum of the Hαfeature labelled “notch C” (v = −7300 )ismoststronglypolarisedatp =3.4±0.9%,thepolarisationof
line line
kms−1) in the 1200R data. Its line polarisation was determined the8662 line,ontheotherhand,increasesto1.7±0.9%.Itisprob-
inthesamewayasthatofthelow-velocityHαfeaturediscussed ablethattheblendednatureoftheemissionhasdilutedthepolari-
above. It is observed with p =1.0±0.4% and θ =141±12◦. sationfromthe8662 line,suchthatitisnotashighlypolarisedas
line line
Thereisnosignificantpolarisationassociatedwithotherlinesthat the8542 line.
MNRAS000,1–??(2017)