Spectral Evolution of the Peculiar Ic Supernova 1998bw

8 Pages·0.35 MB·

by  R. A. Stathakis

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed1February2008 (MNLaTEXstylefilev1.4)
Spectral Evolution of the Peculiar Ic Supernova 1998bw
R. A. Stathakis,1 B. J. Boyle,1 D. H. Jones,2 M. S. Bessell,2 T. J. Galama,3
Lisa M. Germany,2 M. Hartley,1 D. M. James,1 C. Kouveliotou,4,5 I. J. Lewis,1
Q. A. Parker,6 K. S. Russell,1 E. M. Sadler,7 C. G. Tinney,1 J. van Paradijs8,9
and P. M. Vreeswijk8
1Anglo-Australian Observatory, P.O. Box 296, Epping, N.S.W. 1710, Australia
0 2Research School of Astronomy and Astrophysics, The Australian National University, Private Bag, Weston Creek, A.C.T.2611,
0 Australia
0 3Astronomy, MS105-24, Robinson Lab, Caltech, Pasadena, CA 91125, USA
2 4UniversitiesSpace Research Association
5NASA Marshall Space Flight Center,ES-84, Huntsville, AL 35812, USA
n 6Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
a 7School of PhysicsA29, Universityof Sydney, N.S.W. 2006, Australia
J 8Astronomical Institute “Anton Pannekoek”, University of Amsterdam, & CenterforHigh Energy Astrophysics, Kruislaan 403,

8 1098 SJ Amsterdam, The Netherlands
2 9PhysicsDepartment, Universityof Alabama inHuntsville, Huntsville, AL 35899, USA

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1February2008
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1 ABSTRACT
0
0 SN 1998bw holds the record for the most energetic Type Ic explosion, one of
0 the brightest radio supernovae and probably the first supernova associated with a
/ γ-ray burst. In this paper we present spectral observations of SN 1998bw observed
h
in a cooperative monitoring campaign using the AAT, UKST and the SSO 2.3-m
p
telescope. We investigate the evolution of the spectrum between 7 and 94 days after
-
o V-band maximum in comparison to well-studied examples of Type Ic SNe in order
r to quantify the unusual properties of this supernova event. Though the early spectra
t
s differ greatly from observations of classical Ic SNe, we find that the evolution from
a
the photospheric to nebular phases is slow but otherwise typical. The spectra differ
:
v predominantlyintheextensivelineblendingandblanketingwhichhasbeenattributed
i to the high velocity of the ejecta. We find that by day 19,the absorptionline minima
X
blueshifts are10%–50%higher thenother SNe andonday 94emissionlines are45%
r
broader,as expected if the progenitor had a massive envelope. However, it is difficult
a
to explain the extent of line blanketing entirely by line broadening, and we argue
that additional contribution from other species is present, indicating unusual relative
abundances or physical conditions in the envelope.
Key words: supernovae: general – stars: evolution – supernovae: general – super-
novae: individual: SN 1998bw – gamma-rays:bursts
1 INTRODUCTION characteristics of SN 1998bw. Patat & Piemonte (1998a)
have classified SN 1998bw as a Type Ic supernova, since
spectral lines dueto helium, silicon and hydrogen are weak
1.1 SN/GRB association
orabsentintheearlyspectra.However,atMB =−18.88at
The suggestion that type Ic supernova (SN) 1998bw is the maximum(Galama etal.1998), thisobject wasthreetimes
opticalcounterpartofγ-rayburst(GRB)980425 hasforced brighterthantheaverageSNIc,andearlyspectrashowex-
ustorethinkthemechanicsofbothSNeandGRBs.Theas- tremely broad lines and unusual line ratios (Lidman et al.
sociationofSN1998bwandGRB980425issupportedbythe 1998). SN 1998bw rivals the brightest radio supernovae yet
extremely low probability of a chance coincidence (Galama observed and radio observations indicate that the shock of
et al. 1998) and particularly by the peculiar observational
(cid:13)c 0000RAS
2 R. A. Stathakis et al.
Table 1.Spectralobservations ofSN1998bw.
UTDate Age1 Telescope Instrument λCoverage λRes. Observer
(days) (˚A) (˚AFWHM)
98/05/19.7 7 2.3m NasmythBImager 3600-10200 14 Jones,Bessell
98/05/20.6 8 UKST FLAIR 4001-7151 13 Russell,Parker
98/05/23.8 11 2.3m NasmythBImager 3600-9990 14 Jones,Bessell
98/05/23.8 11 AAT 2dF 3600-7900 12 Stathakis,Lewis
98/05/27.8 15 AAT 2dF 4035-6263 5 Lewis
98/05/29.7 17 UKST FLAIR 3950-7202 13 Hartley,Parker
98/05/30.7 18 UKST FLAIR 3950-7202 13 Hartley,Parker
98/05/31.8 19 AAT RGO 3520-9300 5 Stathakis,James
98/06/16.6 35 UKST FLAIR 5700-7560 12 Hartley,Parker
98/06/26 45 2.3m DBS 3850-7590 6 Germany,Schmidt
98/08/14 94 2.3m DBS 3800-7550 6 Germany,Schmidt
1Ageisgivenrelativetothedateofvisualmaximum,1998May12.2(Galamaetal.1998).
theexplosionwasrelativistic(Kulkarnietal.1998;Wieringa mixed with theNi56 (Woosley & Eastman 1997). Piemonte
et al. 1998). Assuming the association and the low redshift (2000) stresses the spectral and photometric variation for
(Tinney et al. 1998) the GRB was also unusual – at least both Ib and Ic SNe. These variations indicate a wide range
4 orders of magnitude fainter than otherGRBs (Galama et of ejecta mass, which would depend on the main sequence
al. 1998). mass of the progenitor and its mass loss history as well
To what extent SNe are associated with GRBs is un- assecondary parameterssuch as metallicity andconvection
clearduetothelownumbersofwell-observedIb/cSNeand (Woosleyetal.1995).Ingeneral,ejectamassisexpectedto
the large positional error of most GRBs. Wang & Wheeler besmall compared to other SN types.
(1998) argue that all Ib/c could produce GRBs, but due
to beaming we would see only a fraction. Kippen et al.
1.3 SN 1998bw
(1998) found no evidence for an association between SNe
and strong GRBs, and Bloom et al. (1998) model the radio The models for SN 1998bw presented to date tend to fall
signature and suggest that 1% of GRBs are produced by into two classes – an intrinsically energetic event or hy-
SNe. If SN 1998bw is a member of a previously unobserved pernova (Iwamoto et al. 1998; Woosley et al. 1999) with
subclassofGRBprogenitors(Iwamotoetal.1998),wehave a massive progenitor star and more normal SNe artificially
a rare opportunity to test and refine our understanding of brightened by beaming (Wang & Wheeler 1998). Radio ob-
SNeand GRBs. servationssuggestthatmaterialhasbeenejectedirregularly
byacentralengine(Li&Chevalier1999). Allmodelsagree
inrequiringsomeformofnon-symmetricgeometry,possibly
1.2 Type Ib/c supernovae produced byan asymmetric explosion.
With such diverse interpretations, it is important that
SupernovaeoftypeIb/careidentifiedbytheirearlyoptical allavailabledataareusedtoprovideobservationallimitsfor
spectra,whichlackthedeepSiIIabsorptionfeatureseenat the models. In this paper we present the results of a coop-
6150 ˚A in Ia spectra and the prominent hydrogen lines of erative spectral monitoring campaign carried out at Siding
TypeIISNe.UnlikeIaSNe,Ib/cobjectsareradioemitters Spring, Australia, on the AAT,UKSTand SSO 2.3-m tele-
and are typically fainter at maximum by MB ∼1.5 magni- scope between May and November 1998 and compare the
tudes(Filippenko1997).Theparentgalaxiesoftheseevents spectral evolution and velocity shifts of SN 1998bw with
(Sbc or later) and heterogeneity of this class suggest that otherwell-observed IcSNe.
Ib/c SNe are powered by the same mechanism as Type II
SNe–corecollapseofamassivestar.TheprogenitorsofIb/c
SNehavebeenmodelledasWolf-Rayetstarswhichhavelost
2 OBSERVATIONS
their hydrogen envelopes either via close binary interaction
(Nomoto,Iwamoto&Suzuki1995),throughastrongstellar SpectralobservationsofSN1998bwweremadeinDirector’s
wind(Woosley,Langer&Weaver1993)oracombinationof overridetimeandservicetimeontheAnglo-AustralianTele-
thetwo mechanisms (Woosley, Langer & Weaver 1995). scope(AAT)andUKSchmidtTelescope(UKST),andwith
This class is further divided into Ib SNe with strong thecooperation ofscheduledobserversontheSidingSpring
helium lines in the early spectra, and Ic where helium lines Observatories(SSO)2.3-mtelescope.Unfortunately,thesite
are weak or absent. Opinion varies as to the relationship experienced the poorest observing statistics of the decade
between Ic and helium-rich Ib SNe and whether there is a and > 60% of allocated time was lost. We obtained useful
smooth or bimodal variation of He I strengths (Filippenko spectraat 10epochswhichspan7to94dayspastVmax (or
etal.1995;Clocchiattietal.1996).Icprogenitorsmayhave 23 to 110 days past the GRB event). Observational details
lost their helium envelopes in another stage of mass loss, are given in Table 1.
leavingabareCOstaratcorecollapse(Harknessetal.1987; Observations were made using the scheduled instru-
Nomoto et al. 1995) or the helium envelope may be poorly ments for the telescopes which included both conventional
(cid:13)c 0000RAS,MNRAS000,000–000
Spectral Evolution of SN 1998bw 3
Figure 1.ComparisonofB-band(top panel)and R-band(bot-
tom panel) spectrophotometry from the corrected spectra with
publishedphotometry. Bothbands showscatter withσ =6%.
long-slit spectrographs (DBS, RGO) and fibre-fed spectro-
graphs(2dF,FLAIR).TheNasmythBImageratthe2.3-m
was used as a long-slit spectrograph by inserting blue and
red grisms and an order-sorting filter. Long-slit data were
processed in the usual way using the FIGARO data reduc-
tionpackage(Shortridgeetal.1997).Afirstordercorrection
hasbeenmadeforbackgroundemissionfromthegalaxy,but
narrow emission from underlyingH iiregions remain in the
spectra. At least two wavelength regions were observed on
eachofthelong-slitdataepochs,andspectrawerecombined
by normalising to match the overlap regions.
Data taken on FLAIR were processed using the IRAF
Figure2.SpectralevolutionofSN1998bw.Agesareshownrela-
datareductionpackageasoutlinedinDrinkwater&Holman
tivetodateofvisualmaximum,1998May12.Spectrahavebeen
(1996). 2dF observations were extracted using the S-DIST
normalisedbytheV-bandphotometry(seetext)andoffsetinflux
and C-DIST utilities from FIGARO. Sky emission was re- forthepurposeofcomparison.
movedusingneighbouringfibres.Itwasnotpossibletocor-
rectthe2dForFLAIRdataforbackgroundgalaxyemission,
but the level of contamination was low during this period (Figure1).Errorsattheedgeofthespectraarelikelytobe
with theexception of thenarrow nebularlines. largerandingenerallinefluxesfromthisdatasetshouldbe
Telluric absorption lines have been removed from the regardedwithcaution.Forthepurposeofcomparison,spec-
datausingobservationsoflow-metallicity starsobservedon trashowninthispaperhavebeennormalisedbytheV-band
days 7, 11 and 19, scaled where necessary to match the photometry,andshiftedtorestwavelengthsusingvz =2580
strongest O2 features. Considering thepoor conditions it is kms−1asderivedfromthenarrowHiiregionemission.Both
probablynotsurprisingthattelluriclineratioswerevariable the fluxed and normalised spectra are available via anony-
and weak residuals can be seen in the spectra, particularly mous ftp at ftp.aao.gov.au/pub/local/ras/98bw.
around 7500 to8100 ˚A and at λ>9500 ˚A.
Long-slit observations were corrected for instrumental
response using a contemporaneous observation of a spec-
3 SPECTRAL EVOLUTION
trophotometricstandard.Fibreobservationsdidnotinclude
accompanying standards, so a linear interpolation between EarlyspectralevolutionofSN1998bwhasbeenpresentedin
bracketing long slit observations were used to correct these Iwamotoetal.(1998)(days–9to+11).InFigure2thespec-
data. As these observations were made in non-photometric tral evolution of SN 1998bw is shown between days 7 and
conditions and through narrow fibres or slits (∼2 arcsec), 94. During this period, spectra are dominated by a strong
correctiontoabsolutefluxwasmadebyscalingtomatchthe continuum peaking around 5400 ˚A,with a small numberof
V-bandlightcurve(Galamaetal.1998;McKenzie&Schae- broadfeatureswhichbecomeincreasinglydominantrelative
fer 1999) (theV-band was not covered on day 35 so theR- tothecontinuum.Linewidthsremainapproximatelystable.
bandwasused).Thesuccessofthismethodwascheckedby Ourobservation onday94agrees qualitativelywith thede-
comparingthephotometryintheBandRbandswithspec- scription by Patat & Piemonte (1998b) of the spectrum on
trophotometry from the corrected spectra, measured using day 123.
filterresponses from Bessell (privatecommunication). Both Thebreadthofthespectralfeaturesandtheuncertainty
BandRbandsresultedinameanratioof0.95,withσ=6% in continuum level hampers detailed analysis. In this paper
(cid:13)c 0000RAS,MNRAS000,000–000
4 R. A. Stathakis et al.
Figure3.Left:SN1998bwondays7to11anddays17to19.Right:SpectraofotherIcSNeatsimilarepochs.Allspectrahavebeen
correctedfortheredshiftoftheparentgalaxy.Positionsofmedianlinecentresshiftedby−10000kms−1 relativetorestareshownin
bothpanelsfortheFeiimultipletλ4274, Mgiiλ4481, Feiiλλ4923,5018, 5169,Naiλλ5890, 5896andSiiiλλ6347, 6371.
we present preliminary results by looking at the most no- distinct theminima are for each line. SN 1998bw blueshifts
table features of the spectrum in comparison to classic Ic are ∼30% higher than SN 1983V and ∼50% higher than
SNe SN 1983V (Clocchiatti et al. 1997), SN 1987M (Filip- SN 1994I, but are comparable with SN 1987M.
penko, Porter & Sargent 1990) and SN 1994I (Clocchiatti Theabsorptionbandbetween4100–4300˚Aisresolved
etal. 1996; Filippenkoet al.1995), andtoSN1997ef which into two minima in SN 1998bw by days 17-19. The two
hasunusuallybroadlinesandbearstheclosest resemblance minima are also seen in SN 1983V and SN 1994I. This ab-
to SN 1998bw (Iwamoto et al. 2000). sorption band is present but not resolved in SN 1997ef. In
SN1987Mthebandextendsbluewardcomparedtotheother
SNe.TheabsorptionbandisattributedtoFeiiblends,cen-
3.1 Days 7-19 tredatrestwavelengthsof4274˚Aand4555˚A(Clocchiattiet
al. 1997). An alternative identification for the second mini-
Eightofourspectrafallwithinthephotosphericperiod,dur-
mumisMgiiλ4481(Filippenko1997),whichgivesvelocities
ing which theB-band and V-bandlight curves were declin-
whichare moreinagreement with otherfeatures forall but
ing steeply prior to the radioactive tail. Spectra show little
changeovertheperiod intherange4000to7000 ˚A,asseen SN 1987M. Other species identified in this region are Ti ii
andCii(Baronetal.1996)andCrii(Iwamotoetal.1998).
in Figure 3 (left panel), where spectra have been binned in
wavelength and time to improve the signal-to-noise ratio. Fe ii is also the usual identification for the absorption
The height of the continuum is poorly defined as overlap- band between 4700 ˚A and 5200 ˚A. In SN 1987M the fea-
ping P-Cygniprofiles result in line blanketingover much of ture is resolved into three minima which match well with
the spectral range – apparent peaks are merely regions of Fe ii λλ4923, 5018, 5169 (multiplet 42). In SN 1983V and
relatively low opacity (Iwamoto et al. 1998). In the right SN1994Ionlytwominimaareresolved,identifiedas5018˚A
panel of Figure 3 spectra of other supernovae are shown and 5169 ˚A (Clocchiatti et al. 1997; Iwamoto et al. 1998;
at similar epochs. Qualitively SN 1998bw is very different Iwamoto et al. 2000). However, we find that the identifi-
from the other supernovae, with merged absorption blue- cation of the bluer line as 4923 ˚A results in better agree-
ward and redward of 5300 ˚A. SN 1998bw lacks the spec- ment with other features. In SN 1998bw the band is only
traldetailandtheoveralleffectisthatofheavysmoothing. marginally resolved onday19,with 4923˚Aunusuallydom-
However,detailedcomparisonbetweentheSNespectrashow inant.InSN1997efthelineratiosaremoretypical,andthe
thatthesamebandscanbeidentified,and most differences band is resolved into two minima (Iwamoto et al. 2000).
in SN1998bw can be attributed to thelarge line widths. In Ic SNe spectra, Na i λλ5890, 5896 absorption typ-
In Table 2, the wavelengths of the observed minima in ically strengthens later than the Fe ii features, becoming
SN1998bw(day19)arecomparedwiththosemeasuredfrom dominant around day 30 and fading by day 100. The line
other SNe, and converted to relative velocities (in units of emergesmoreslowly inSN1998bw comparedtoSN1983V,
103 km s−1) for suggested line identifications (see below). SN 1987M and SN 1994I, but is comparable to SN 1997ef.
Estimates of measurement errors are shown, reflecting how The Na i blueshift is similar to Fe ii blueshifts for all four
(cid:13)c 0000RAS,MNRAS000,000–000
Spectral Evolution of SN 1998bw 5
Figure 4.Left:Comparisonofspectratakenondays19,45and94.Spectrahavebeennormalisedasinpreviousplotsandshiftedto
rest at z =0.0086. In addition, nebular lines from nearby H ii regions have been removed. Right: Spectra of other Ic SNe at similar
epochs.Medianrestwavelengths areindicatedforMgi]λ4571,Feiiλ5215,Naiλλ5890, 5896,[Oi]λλ6300, 6363andCaii]λλ7298,
7323.
SNe in Table 2. The presence of He i λ5876 discussed for
Table 3.Velocityshiftsandwidthsduringthenebularphase.
otherIcSNe(Clocchiattietal.1996;Clocchiattietal.1997)
cannoteasilybeascertainedforSN1998bwsincetheweaker Line λr 98bw 94I 87M 87M
line would be severely blended with Na i. (94d) (56d) (62d) (96d)
The Si ii λλ6347, 6371 absorption shows the largest
shift in velocity during this period, with minima velocities Mgi] 4571 −1.41 +0.9 +0.2 −1.0
of −11600 km s−1 on day 11 and −7900 km s−1 on day 11.92 8.4 8.6 7.3
19. In Table 2, the Si ii line has the lowest blueshift of the [Oi] 6315 +0.7 −1.2 −1.9
measuredlinesforallSNe.ThelineprofileoftheSiiifeature 11.4 7.8 10.0
Caii] 7308 −4.0 −1.6 −2.7 −2.6
is similar to that of SN 1983V, suggesting that there may
12.1 8.2 7.5 6.5
be contribution from a second line resolved in SN 1983V, Nai] 58933 −10.1 −11.3 −10.0 −9.9
SN1994I and SN1997ef and identified byClocchiatti et al.
11.1 9.0 5.5 4.5
(1997) as O i λ6158.
1Velocityshiftoflinecentre(103 kms−1).
3.2 Days 19-94 2Widthoffeaturesathalfheight(103 kms−1).
3Absorptioncomponent.
Late-time spectral evolution of SN 1998bw is shown in
Figure 4 (left panel). During this period the light curve
is decaying linearly via radioactive decay (McKenzie &
Schaefer 1999), and as expected we see a transition from than day 96, and is very similar to SN 1994I on day 56. In
an absorption-dominated photospheric spectrum to an order to separate the new emission from the other features
emission-dominated nebular spectrum, though on day 94 in SN 1998bw, the day 45 spectrum has been subtracted
the transition is still not complete. Early nebular spectra fromtheday94spectrum,afterscalingbytheV-bandpho-
of SN 1994I and SN 1987M are shown in comparison to tometry. Likewise, day 36 has been subtracted from day 56
SN 1998bw in Figure 4 (right panel). SN 1998bw evolves for SN 1994I. The results are compared in Figure 5.
slowly compared to the other two SNe – the day 94 spec- Velocity shifts of the main features are given in Ta-
trum bears a closer resemblance to SN 1987M on day 62 ble 3. [Oi] λλ6300, 6363 is blueshifted in SN 1987M and
(cid:13)c 0000RAS,MNRAS000,000–000
6 R. A. Stathakis et al.
Table 2.Absorptionminimameasurements.
Line λr 98bw 83V 87M 94I
(˚A) 19d 13d 11d 6d
CaII 3950 3750±201 3760±20 3760±10
−15.2±1.52 −14.4±1.6 −14.4±0.8
FeII 4274 4130±30 4150±20 4060±30 4170±30
−10±2 −8.7±1.6 −15±2 −7±2
MgII 4481 4330±30 4370±20 4360±20 4380±20
−10±2 −7.4±1.4 −8.1±1.4 −6.8±1.4
FeII 4555 4330±30 4370±20 4360±20 4380±20
−15±2 −12.2±1.4 −12.8±1.4 −11.5±1.4
FeII 4923 4700±20 4780±10 4750±10 4800±50
−13.6±1.3 −8.7±0.6 −10.5±0.6 −8±3
FeII 5018 4820±30 4780±10 4820±10 4800±50
−12±2 −14.2±0.6 −11.8±0.6 −13±3
FeII 5169 4960±30 5000±20 4980±10 5020±10
−12±2 −9.8±1.2 −11.0±0.6 −8.6±0.6
NaI 5893 5660±30 5710±10 5677±5 5710±7
−11.9±1.5 −9.3±0.5 −11.0±0.3 −9.3±0.4
SiII 6357 6190±30 6240±20 6152±5 6280±20
−7.9±1.4 −5.6±1 −9.7±0.3 −3.7±1.0
OI 7774 7490±10 7560±20
−11.0±0.4 −11.4±1.1
1Positionofabsorptionminimainwavelength (˚A).
2Relativevelocity(103 kms−1).
SN 1994I. In SN 1998bw on day 94 the line profile is sym- whichisonlyhalfthewidthinSN1987M,andconsiderably
metric, butin theresidual emission it is also blueshifted by narrower than the emission lines.
∼−500 km s−1.Ca ii]λλ7291, 7323 hasa similar blueshift The peak at 5200 ˚A has been tentatively identified as
in SN 1987M and SN 1994I. In SN 1998bw the blueshift is Fe ii λ5215 (Patat & Piemonte 1998b). This transition is
significantlygreater,butintheresidualspectrumtheprofile typicallyseen inolderIaSNe,butnot inIcSNe,and Patat
is similar to SN 1994I, so apparent differences are probably and Piemonte note that its presence would indicate that
due to contribution from the persisting photospheric fea- SN 1998bw was a typeIac SN. However, the feature is also
tures. The Ca ii] to [Oi] ratio is greater in SN 1987M than present in SN 1987M and more weakly in SN 1994I during
in SN 1994I and SN 1998bw, which has been attributed to the early nebular phase. It fades relative to Mg i], [O i]
a difference in the relative abundances of calcium and oxy- and Ca ii] at later times. It therefore seems reasonable to
gen(Filippenkoetal.1995).WhileMgi]λ4571istheusual assumethatthepresenceofFeiiemission inIcSNespectra
identification for the emission peak at 4500 ˚A (Filippenko is typical during the transition from the photospheric to
1997), an alternative identification is given by Patat et al. the nebular phase. In the residual spectrum (Figure 5) the
(1998b) as Fe ii λ4555, and both transitions give an ade- feature is weaker relative to the 4500 ˚A peak and has a
quatefittothelinewith asymmetricprofile.Weadopt the markedly different profile. This supports the identification
Mg i] identification for this paper. of the 4500 and 5200 ˚A peaks as transitions of different
species,andindicatesthatthe5200˚Apeakisfadingand/or
Approximate line widths have been measured using isartificially enhancedbyarelatively highcontinuumlevel.
ABLINE in FIGARO (Table 3). Line widths are similar
for the Mg i], [O i] and Ca ii] emission for each supernova.
The mean width of these lines in SN 1998bw on day 94 is
3.3 7000 – 9000 ˚A
11600±400 kms−1 whichis∼45%broaderthanSN1987M
and SN 1994I. Na i emerges as a P-Cygni profile between The red spectral region of SN 1998bw is shown in Fig-
days 19 and 94, evolving more slowly in equivalent width ure 6 for days 7 and 19. It is this region which differs most
and in emission to absorption ratio than SN 1987M and markedlyfrom typicalIcSNesuch asSN1987M (shown on
SN 1994I. A noticable difference between SN 1998bw and day7).IntheseSNe,strongabsorption fromOiλ7774 and
SN1987MisthewidthoftheabsorptioncomponentofNai, Caiiλλ8498,8542,8662arewellseparatedbyanabsorption
(cid:13)c 0000RAS,MNRAS000,000–000
Spectral Evolution of SN 1998bw 7
Figure 5. Top: Difference between spectra of SN 1998bw ob-
servedonday45and94,afterscalingbytheV-bandphotometry. Figure 6. The 7000 to 9000 ˚A region of Ic SNe spectra is
NarrowlinesfromnearbyHiiregionshavebeenremoved.Excess dominated by the O i λ7774 multiplet and Ca ii λλ8498, 8542,
emissionfrombroadnebularlinesarevisibleasSN1998bwenters 8662 (rest wavelengths are shown). These lines are blended in
the supernebular phase. Bottom: Differencebetween day 36 and SN1998bw onday11(a)andday19(b),butarewellseparated
56spectraofSN1994I. inSN1987Monday7(c). Notethatweakresidualtelluriclines
arepresentintheSN1998bw spectra,especiallyaround7600 ˚A.
freeregionmissinginSN1998bw.Iwamotoetal(1998)have
successfully modelled this region for day −9 with a photo- with lines of rest wavelength 8000 – 8350 ˚A, and candidate
sphericvelocityof 28000 kms−1.However,byday−1their species includeCii,CiiiandNi.Arelatively weak contri-
modelpredictsthatthetwofeaturesshouldberesolved,and butionfromanyofthesespecieswouldresultintheblended
thatbyday7OiandCaiiareunblended.Similarevolution spectrumweobserve.Modellingisrequiredtofurtherinves-
ispredictedbythedirectanalysis modelsofBranch(2000). tigate this region, and to determinewhether unusualabun-
InSN1997efthisbehaviourisseen,withblendedabsorption dances,densityortemperaturedistribution can explain the
onday3andwellseparatedfeaturesonday30(Iwamotoet observations.
al. 2000). InSN1998bw, however,thereisnosign ofsignif-
icant separation as late as day 19, our last epoch covering
this region.
4 CONCLUSION
While it is presumably possible to fit this region by
increasingthemassoftheprogenitor,andthereforetheline Duringtheperiodbetween7and94daysafterV-bandmax-
widthsoftheOiandCaiiprofiles,therearetwolimitations imum,wehaveseenthatSN1998bwresemblesotherIcSNe
which need to be considered. The first is that we do not sufficiently to support this classification, but has unusually
expecttoseeredshiftedabsorptionfromasimpleexpanding slowspectralevolution.Onday94weseetheemergenceofa
envelope. Under this assumption, O i cannot contribute to nebularspectrum,whichretainsmanyofthecharacteristics
the absorption band redward of 7774 ˚A, irrespective of the of thephotospheric period. The late onset of the superneb-
linewidth.Thesecondlimitationisthatwedetectashallow ular phase, compared to SN 1987M (62 d) and SN 1994I
dip at ∼8100 ˚A which we identify as the minimum of the (56 d), is consistent with the ejection of an unusually large
Ca ii absorption. This feature aligns well with the Ca ii mass, as predicted by lightcurve models (Iwamoto et al.
λλ3933,3968lineprofile,andthevelocityof−15300kms−1 1998; Woosley et al. 1999).
isalreadyhigherthanblueshiftsofotherlinesatthisepoch By day 19, SN 1998bw blueshifts are up to 50%
(Table 3). larger than other Ic SNe. However, increased blueshifts
Inordertoreproducethespectrumonday19wewould aloneseeminsufficienttoexplaintheunusuallysmooth and
requireahighlyunusualgeometrytoproduceredshiftedab- blendedspectrumwhichpersiststolatetimes–forinstance
sorption from O i, or to produce Ca ii absorption with a SN 1998bw on day 19 has similar blueshifts to SN 1987M
second minimum at around −24000 kms−1. Analternative on day 11, but SN 1987M has well defined spectral fea-
explanation is thepresenceof athird component absorbing tures.Emission linewidthsonday94are45%broaderthan
at around 7700 – 8000 ˚A. Adequate fits can be produced SN1994I andSN1987M andNaiabsorption isfarbroader
(cid:13)c 0000RAS,MNRAS000,000–000
8 R. A. Stathakis et al.
inSN1998bwonday94thaninSN1987Matsimilarepochs. KulkarniS.R.etal.,1998,Nature,395,663,astro-ph/9807001
The line profiles of SN 1998bw may have disproportionally LiZ.-Y.,ChevalierR.A.,1999,ApJ,526,716,astro-ph/9903483
strong absorption wings – we lack an example of an un- LidmanC.,DoublierV.,GonzalezJ.-F.,AugusteijnT.,Harnaut
blended feature at earlier times for confirmation. Using the O. R., Boehnhardt H, Patat F., Leibundgut B., 1998, IAUC
6895
standardmodelforahomologouslyexpandingenvelope,ab-
McKenzie E. H., Schaefer B. E., 1999, PASP, 111, 964, astro-
sorptionlineswhicharebroaderbutofsimilarblueshift im-
ph/9904397
ply that theabsorption region in SN1998bw spans alarger
Nomoto K., Iwamoto K., Suzuki T., 1995, Physics Reports 256,
range of velocity space, at both higher and lower velocities
173
than other Ic SNe, as expected for a massive envelope. A PatatF.,PiemonteA.,1998a,IAUC6918
closer inspection of the7000 –9000 ˚Aregion of SN1998bw PatatF.,PiemonteA.,1998b,IAUC7017
suggeststhatwearealsoseeingcontributionfromenhanced PiemonteA.,2000,inPhillipsM.,ed,SN1987A:TenYearsLater,
linespecies.UnusuallystronglinesfromspeciessuchasNi, inpress
Cii,Ciii,TiiiandCriimayhelptoproducetheextensive Shortridge K., Meyerdierks H., Currie M., Clayton M., 1997,
lineblending.Ifso,thiscould indicateanoverabundanceof PPARCStarlinkUserNote,86.13
Tinney C., Stathakis R., Cannon R., Galama T., 1998, IAUC
theseelements,orunusualphysicalproperties oftheejecta.
6896
Moreworkisrequiredinestablishinglineidentifications
WangL.,Wheeler J.C.,1998,ApJ,504,L87,astro-ph/9806212
and spectral characteristics, best done using spectral mod-
WieringaM.,FrailD.A.,KulkarniS.R.,HigdonJ.L.,WarkR.,
els. Whether or not SN 1998bw was associated with GRB
BloomJ.S.,BeppoSAXGRBTeam,1998, IAUC6896
980425, it is of great interest as an extreme example of Ic WoosleyS.E.,EastmanR.G.,1997,inRuiz-LapuenteR.,Canal
SNe. SN 1997ef bears some resemblance to SN 1998bw and R., Isern I., eds, Thermonuclear Supernovae, NATA ASI se-
maybeanintermediateobjectbetweenSN1998bwandclas- ries,821
sical SNe, though it is too early to say whether we are see- Woosley S. E., Eastman R. G., Schmidt B. P., 1999, ApJ, 516,
ingabimodalorcontinuousvariationinproperties.Thanks 788,astro-ph/9806299
to the interest inspired by the γ-ray burst, SN 1998bw has WoosleyS.E.,Langer N.,Weaver T.A.,1993,ApJ,411,823
WoosleyS.E.,Langer N.,Weaver T.A.,1995,ApJ,448,315
been observed extensively and successful modelling of this
object is likely to enhance our understanding of this rela-
tively poorly observed class of supernova.
ACKNOWLEDGEMENTS
We thank the observers at the AAT, UKST and MSO 2.3-
m telescope who kindly donated their time to our project.
WethankAlejandroClocchiatti,CraigWheeler,JodieMar-
tin and Paolo Mazzali for providing digital supernova data
to compare with ourobservations, and PeterMeikle, David
Branch and Brian Schmidt for helpful discussions.
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