The remarkable outburst of the highly evolved post-period-minimum dwarf nova SSS J122221.7-311525

3.2 MB·

by  V. V. Neustroev

MNRAS000,1–21(2017) PreprintFebruary23,2017 CompiledusingMNRASLATEXstylefilev3.0
The remarkable outburst of the highly evolved
post-period-minimum dwarf nova SSS J122221.7−311525⋆
V. V. Neustroev1,2†, T. R. Marsh3, S. V. Zharikov4, C. Knigge5, E. Kuulkers6,
J. P. Osborne7, K. L. Page7, D. Steeghs3, V. F. Suleimanov8, G. Tovmassian4,
7 E. Breedt3, A. Frebel9, Ma. T. García-Díaz4, F.-J. Hambsch10,11, H. Jacobson9,
1
S. G. Parsons12, T. Ryu13,14, L. Sabin4, G. Sjoberg15,11, A. S. Miroshnichenko16,
0
2 D. E. Reichart17, J. B. Haislip17, K. M. Ivarsen17, A. P. LaCluyze17, J. P. Moore17

b
1FinnishCentreforAstronomywithESO(FINCA),UniversityofTurku,Väisäläntie20,FIN-21500Piikkiö,Finland
e
2Astronomyresearchunit,POBox3000,FIN-90014UniversityofOulu,Finland
F
3DepartmentofPhysics,UniversityofWarwick,GibbetHillRoad,CoventryCV47AL,UK

2 4InstitutodeAstronomia,UniversidadNacionalAutonomadeMexico,Apdo.Postal877,Ensenada,BajaCalifornia,22800Mexico
2 5SchoolofPhysicsandAstronomy,UniversityofSouthampton,Southampton,SO171BJ,UK
6EuropeanSpaceAstronomyCentre(ESA/ESAC),ScienceOperationsDepartment,28691VillanuevadelaCañada,Madrid,Spain
] 7DepartmentofPhysics&Astronomy,UniversityofLeicester,UniversityRd,Leicester,LE17RH,UK
R 8InstitutfürAstronomieundAstrophysik,UniversitätTübingen,Sand1,D-72076Tübingen,Germany
S 9DepartmentofPhysicsandKavliInstituteforAstrophysicsandSpaceResearch,MassachusettsInstituteofTechnology,Cambridge,MA02139,USA
. 10VerenigingvoorSterrenkunde,OudeBleken12,B-2400Mol,Belgium
h 11AmericanAssociationofVariableStarObservers,49BayStateRoad,Cambridge,MA02138,USA
p 12DepartmentodeFísicayAstronomía,UniversidaddeValparaíso,AvenidaGranBretana1111,Valparaíso,2360102,Chile
- 13SOKENDAI,TheGraduateUniversityforAdvancedStudies,2-21-1Osawa,Mitaka,Tokyo181-8588,Japan
o
14NationalAstronomicalObservatoryofJapan,2-21-1Osawa,Mitaka,Tokyo181-8588,Japan
r
t 15TheGeorge-ElmaObservatory,NewMexicoSkies,9ContentmentCrest,#182,Mayhill,NM88339,USA
s 16UniversityofNorthCarolinaatGreensboro,Greensboro,NC27402,USA
a
[ 17DepartmentofPhysicsandAstronomy,UniversityofNorthCarolina,ChapelHill,NC27599,USA

2
v
Accepted2017January11.Received2016December23;inoriginalform2016May29
4
3
1
ABSTRACT
3
Wereportextensive3-yrmultiwavelengthobservationsoftheWZSge-typedwarfnovaSSS
0
. J122221.7−311525duringitsunusualdoublesuperoutburst,thefollowingdeclineandinqui-
1
escence. The second segment of the superoutburst had a long duration of 33 d and a very
0 gentledeclinewitharateof0.02magd−1,anditdisplayedanextendedpost-outburstdecline
7
lasting at least 500 d. Simultaneously with the start of the rapid fading from the superout-
1
burst plateau, the system showed the appearance of a strong near-infrared excess resulting
:
v in very red colours, which reached extreme values(B−I≃1.4)about20 d later. The colours
i thenbecameblueragain,butittookatleast250dtoacquireastablelevel.Superhumpswere
X
clearlyvisible in the lightcurvefromourveryfirst time-resolvedobservationsuntilatleast
r
a 420daftertherapidfadingfromthesuperoutburst.Thespectroscopicandphotometricdata
revealedanorbitalperiodof109.80minanda fractionalsuperhumpperiodexcess.0.8per
cent,indicatingaverylowmassratioq.0.045.Withsuchasmallmassratiothedonormass
should be below the hydrogen-burningminimum mass limit. The observed infrared flux in
quiescence is indeed much lower than is expected from a cataclysmic variable with a near-
main-sequencedonorstar.Thisstronglysuggestsabrown-dwarf-likenatureforthedonorand
that SSS J122221.7−311525has already evolved away from the period minimum towards
longerperiods,withthedonornowextremelydim.
Key words: binaries: close – stars: evolution– stars: individual:SSS J122221.7−311525–
novae,cataclysmicvariables.
⋆ Thispaperincludes datagatheredwiththe6.5mMagellanTelescopes † E-mail:[email protected]
locatedatLasCampanasObservatory,Chile.
(cid:13)c 2017TheAuthors
2 V. V. Neustroevet al.
1 INTRODUCTION One such system, called SSSJ122221.7−3115251 (hereafter
SSS J122222), was discovered on 2013 January 1 at V=12.3 by
Intheaccretingwhitedwarf(WD)systemsknownascataclysmic theCatalinaRealTimeSurvey(CRTS;Drakeetal.2013),andwas
variable stars (CVs – for a comprehensive review, see Warner subsequently found to have been in outburst by 2012 December
1995),accretionisthedominant factorintheirdiscoverythrough 16(withunfilteredCCDmagnitude11.8;Levatoetal.2013).The
lineemissionandoutbursts.ThedonorcomponentsinCVsarelow- largeoutburst amplitude (the average quiescent magnitude of the
massmain-sequence starsor browndwarfswhichlosemattervia counterpart was found to be ≈19; Drakeetal. 2013), the appear-
the inner Lagrangian point. In the absence of a strong magnetic anceofitsoptical spectrum(Marshetal.2013) andthedetection
field,thematerialtransferredfromthedonor starformsanaccre- ofan‘S-wave’sinusoidalcomponent intheemissionlineswitha
tiondiscaroundtheWD.Dwarfnovaeareanimportantsubsetof periodofabout80to95minallowedKuulkersetal.(2013)topro-
CVswithrelativelylowmass-transferrates,thediscsofwhichcan posetheWZSge-typedwarfnovaclassificationforSSSJ122222.
sufferthermalinstabilities,resultinginoutburstsforwhichCVsare Katoetal.(2013)discussedsomeoftheoutburstpropertiesofSSS
named(forreviewofthedisc-instabilitymodelsee,e.g.,Cannizzo J122222,studiedthesuperhumpperiodchangesinthesystemand
1993; Osaki 1996; Lasota2001). Therecurrence timeof theout- foundthatSSSJ122222hasaverysmallmassratio,q<0.05.They
bursts is usually weeks or a few months and their amplitude is also concluded that the relatively long orbital period P (which
orb
mostlylessthan6mag.However,theoutburstsindifferentdwarf wasnotknown,butshouldbeclosetotheaveragesuperhumppe-
novaehavedifferentamplitudes,light-curveshapes,durationsand riodof∼111min)andlowqmakethisobjectaperfectcandidate
recurrencetimes.Basedonthevarietyoftheseparameters,afew foraperiodbouncer. Theconfirmationoftheperiodbouncer sta-
sub-typeshavebeenassignedtothedwarfnovaclass. tuswouldbeofgreatimportanceduetothelongorbitalperiodof
SSSJ122222,whichwouldmakethisbinarythemostevolvedCV
SUUMa-typedwarfnovaehaveshortorbitalperiods,usually
(Knigge,Baraffe,&Patterson2011).
belowthe2–3hCVperiodgap,andshowtwotypesofoutbursts:
Formorethanthreeyearswehavebeenperformingextensive
short (normal) outbursts lastingafew days, and lessfrequent su-
X-ray–ultraviolet(UV)–optical–near-infrared(NIR)photomet-
peroutbursts which have a slightly larger amplitude and a longer
ricandspectroscopicobservations of SSSJ122222. Theobserva-
duration of about two weeks. At the short period end of the CV
tionsweretakenfrom2013January5(fourdaysafterthediscov-
orbital period distribution, accretion rates arelow, and some sys-
ery) until 2016 April 17, during the superoutburst, the following
temsonlyoutburstonceeveryseveralyearsorevendecades.Such
declineandinquiescence.Thesedataprovidecrucialinsightsinto
systemscomposeanextremesubgroupoftheSUUMa-typedwarf
thepropertiesofSSSJ122222.Wefoundthatanumberofobserved
novae,calledWZSge-typestars(foramodernandcomprehensive
features are quite unusual when compared with other WZ Sge-
reviewoftheWZSge-typeobjects,seeKato2015).Theirotherpe-
typesystems.Althoughmostofthesefeaturesarenotcompletely
culiarpropertiesareverylargesuperoutburstamplitudesexceeding
unique, their union in one object makes SSS J122222 a remark-
6magandthelackorgreatrarityofnormaloutbursts.
able case of the WZ-Sge-type system. Our unprecedentedly de-
Auniquepropertyofsuperoutburstsistheappearanceofsu- tailed,multiwavelengthsobservations allowustoshedmorelight
perhumps,low-amplitudemodulationswithaperiodofafewper- on mechanisms underlying the system’s extraordinary properties.
cent longer than the orbital one. The superhump phenomenon is Inthispaper,weconcentrateonthelong-termevolutionofthebi-
well explained by the tidal instability of the accretion disc (Vogt naryasrevealedbytheUV–optical–NIRdata.
1982;Whitehurst1988;Osaki1989).Thisinstabilitygrowswhen
adiscexpandsbeyondthe3:1resonanceradiusthatcausesthedisc
tobecomequasi-ellipticalandprecess,initiatingsuperhumps.The
period of superhumps P is known to depend on the orbital pe- 2 OBSERVATIONS
sh
riodP andthemassratioq = M /M ,where M and M are
orb 2 wd 2 wd Duringourobservingcampaignweusedmanyground-andspace-
themassesofthedonorandWD,respectively.AnempiricalP –q
sh basedtelescopesequippedwithdifferentinstruments.Alldetailson
relation(see,e.g.,Pattersonetal.2005)isanimportanttooltoes-
theobservationsanddatareductioncanbefoundinAppendix A.
timateqwithouttherecoursetospectroscopy.Inparticular,itwas
Herewedescribeshortlythedatausedinouranalysis.
shownthatthedistributionofthemassratiosofthedonorandWD
Ouropticaltime-resolvedphotometricobservationsstartedon
inWZSge-type starsshows a sharp peak between 0.07 and 0.08
2013January8,∼7dafterthediscoveryannouncement byCRTS
(Kato2015).
(Drakeetal.2013).Fromthenon,theobservationswereconducted
The CVs with such small mass ratios are of a special in- almosteveryclearnightuntil2013April21.Afterashortinterrup-
terest because a significant number of ‘period bouncers’ should tion,theobservationswereresumedon2013May4andcontinued
be present among them. According to standard evolutionary the- until2013June18,theendofvisibilityofthesource.Thus,about
ory, CVs evolve from longer to shorter orbital periods until a 100nightsoftime-resolvedphotometricdataweretakenduringthis
minimum period is reached (≈80 min), when the donor star be- interval.Werestartedtheobservations on2014January13,when
comes of a substellar mass and partially degenerate, resulting in thestarbecamevisibleagain,andfinishedon2014June28.Atotal
a kind of a brown-dwarf-like object. Systems that have passed of35 nightsofdatawereobtained during2014. Inthebeginning
beyond P are evolving back towards longer periods (see Kolb of 2015 we obtained seven additional sets of time-resolved data.
min
1993; Howell,Rappaport,&Politano 1997; Gänsickeetal. 2009; InJanuary–Aprilof2016wetookseveralsingle-shotobservations.
Knigge,Baraffe,&Patterson 2011, and references therein). This Thefinalobservationwastakenon2016April17.Theobservations
haslongbeenpredictedtobethe‘graveyard’andcurrentstateof70 were taken with a 0.40-m f/6.8 Optimized Dall Kirkham (ODK)
percentofallCVs(Kolb1993),howeveronlyaboutadozenmore
orlessrobustcandidatesforsuchperiodbouncersystemshavebeen
identifieduntilnow,outofabout1000ofknownCVs(forafairly 1 AlsoknownasSSS130101:122222−311525, 1RXSJ122221.5−311545
recentcompilationseePatterson2011). andOTJ122221.6−311525.
MNRAS000,1–21(2017)
Thesuperoutburstof SSSJ122221.7−311525 3
Table1.TheaveragedmagnitudesofSSSJ122222on2013February5(theplateaustageofthesuperoutburst)andon2016April7–17(quiescence).The
superoutburstUmagnitudeisgivenintheBesselfilter,whereasforthequiescentstateweshowtheSwiftumagnitude.
State uvw2 uvm2 uvw1 u/U B V Rc Ic J H Ks
Superoutburst 11.08(5) 11.91(5) 12.65(4) 12.69(3) 12.68(3) 12.77(4)
Quiescence 17.65(8) 17.53(9) 17.57(7) 17.97(7) 19.05(5) 19.00(5) 18.72(5) 18.76(5) 18.47(5) 18.31(7) 17.77(23)
telescopeoftheRemoteObservatoryAtacamaDesert(ROAD)in all light curve which includes the pre-outburst 1-d averaged data
Chile, a 0.35-m Celestron C14 robotic telescope at New Mexico from the Catalina Real-time Transient Survey (CRTS) observa-
SkiesinMayhill,NewMexico,two0.4-mroboticPROMPTtele- tions2(Drakeetal.2009)begun10yearsagofrom2005August5.
scopeslocatedinChile(Reichartetal.2005),andwiththe0.84-m Thepre-outburst datarevealahalf-magnitudevariabilitybetween
telescopeattheObservatorioAstronómicoNacionalatSanPedro ∼18.5and∼19magonfewyears’time-scale,butshownoindica-
Mártir(OANSPM)inMexico.Asignificantpartofourphotomet- tionofpreviousoutbursts.
ricdatawastakenwithJohnson–CousinsBV(RI) filters,andother The most notable feature of the superoutburst is its unusual
C
observationswereperformedwiththeV orRfilters,orunfiltered. doublestructureandlongduration(Fig.2).Unfortunately,thefirst
TableA2providesthejournaloftheseobservations. segmentofthesuperoutburstwasscarcelyobserved.Themostre-
On 2016 April 7 we conducted a near-infrared observation centobservationspriortothesuperoutburstwereobtainedon2012
using the Infrared Survey Facility (IRSF) at Sutherland, South August13(T=−147),whenthesystemwasatV∼18.3.Theearliest
Africa.TheIRSFconsistsofa1.4mtelescopeandSimultaneous observationsofSSSJ122222inoutburstwereobtainedonT=−22
Infrared Imager for Unbiased Survey (SIRIUS: Nagayamaetal. (∼11.8 mag; Levatoetal. 2013). Subsequent photometry showed
2003),whichcanobtainJ,HandKs-bandimagessimultaneously. that the object had still been near maximum light until at least
TheopticalspectroscopicobservationsofSSSJ122222were T=−5.8(V=12.3; Drakeetal. 2013), after that SSS J122222 had
performed on 16 nights between 2013 January 5 and 2013 May fadedtomagnitude∼15.5onT=−2.4and∼16onT=−1.5.Thus,
5. Additional observations were taken on 2015 May 5, and on thedurationofthesuperoutburst’sfirstsegmentwasatleast16d.
2016February9and10.Thespectrawereobtainedwiththe4.2-m When fading, the transient did not reach its quiescent level,
WilliamHerschel Telescope on LaPalma, the 2.1-m telescope at andrebrightenedagainonT=3,enteringthesecondsegmentofthe
the OAN SPM, the 6.5-m Magellan Clay Telescope at LasCam- superoutburst (Neustroev&Sjoberg 2013). We began our photo-
panasObservatorynearLaSerenainChile,andwiththeVLT-UT1 metricobservations onT=0,near theminimumlight ofthistem-
and VLT-UT2 telescopes at Paranal Observatory, Chile. Spectra porary fading stage (V=16.8).During therisetheoptical fluxin-
wereobtained inboth single-shot and time-resolvedmanner with creasedatarateof ∼2.8magd−1.Theoutburst’ssecond segment
different instrumental setups and exposure times. Table A4 pro- hadreachedthemaximumofV=12.2onT=7.Afterashort‘dip’
videsthejournaloftheopticalspectroscopicobservations. by∼0.4magexhibitedduringT=9–10,SSSJ122222reachedan-
TheSwiftX-raysatellite(Gehrelsetal.2004)startedobserv- otherlocalmaximumofV=12.3onT=13andthenmonotonically
ing SSS J122222 on 2013 January 6, 5.8 d after the discovery fadedveryslowly.OnT=37weobservedarapidfadingwitharate
announcement (Kuulkersetal. 2013). For each observation, data of 1.31±0.01 magd−1, followed by a slow decline. Thus, the du-
werecollectedusingboththeX-rayTelescope(XRT;Burrowsetal. rationofthesecondsegmentofthesuperoutburstwasabout33d.
2005)andtheUV/OpticalTelescope(UVOT;Romingetal.2005) WenotethatthoughthepeculiarpatternofthelightcurveofSSS
withtheuvm2filterinposition.Inthispaperwepresent onlythe J122222 is not unprecedented and similar double superoutbursts
UV/Optical part of the observations. The observations were ob- havebeenseeninafewother CVs(e.g.,OTJ184228.1+483742,
tainedapproximatelyevery1–3duntil2013July1.Twoadditional seefig.1inKatoetal.2013)and transientlow-massX-raybina-
observations were performed on 2014 June 26 and 2015 January ries(e.g.,XTEJ1118+480,Kuulkers2001),yetthesuperoutburst
16. A final data set was collected on 2016 April 17. This UVOT of SSS J122222 was the longest of any other known dwarf no-
observationwascarriedoutinallsixavailablefilters. vae.During33dofthesecondsegmentofthesuperoutburst SSS
InSection3.1weshowthatSSSJ122222hadreturnedtoqui- J122222fadedbyonly∼0.7mag.Theaveragefadingrateduring
escence in the beginning of 2015. In Table 1 we summarize the T=13–20wasjust0.006±0.001 magd−1,afterthatitincreasedto
averaged magnitudes of SSS J122222 in all the available filters, 0.032±0.001magd−1 (T=20–35).Suchaslow,gradual declineis
obtainedon2016April7–17(quiescence)andon2013February5 atypical for SU UMa stars, whose plateau slope is usually about
(thesuperoutburst). 0.11±0.01magd−1(Warner1995).SeeSection6.1forfurtherdis-
cussionofatypicalpropertiesofthelightcurve.
The shoulder in the beginning of the second segment of the
superoutburst resembles a so-called precursor outburst, a normal
3 OPTICALANDUVPHOTOMETRY outburst which isoften observed shortly before thesuperoutburst
(Marino&Walker1979),andwhichisthoughttotriggerit(Osaki
3.1 Long-termlightcurveandthecolourevolution
1989; Osaki&Kato 2013). We note, however, that in contrast to
Throughout thepaper, wewilldenote timeinunits of days since SSSJ122222,themaximumlightofprecursorsisusuallylowerby
2013January8,whenwebeganourtime-resolvedphotometricob- 0.5–1 mag than that of the following superoutbursts (see e.g. the
servations.Thus,T=0correspondstoHJD2456300.0. verydetailedhigh-fidelityKeplerlightcurvesofseveralsuperout-
Fig. 1shows thelight curves of SSSJ122222 containing all
our observations in the optical and UV filters taken over about
three years. In the inset of this figure we also show the over- 2 http://nesssi.cacr.caltech.edu/DataRelease/
MNRAS000,1–21(2017)
4 V. V. Neustroevet al.
12 uvm2 (+1.8 mag) 12
B
13 13
V
14
R
14
e I 15
d
nitu15 16
Mag16 17
18
17
19
18 -2800 -2300 -1800 -1300 -800 -300 200 700 1200
19
20
0 50 100 150 200 400 450 500 900 1100
T (days)
Figure1.ThelightcurveofthesuperoutburstofSSSJ122222inalltheavailableopticalandUVfilters.Eachpointisthe1-daverageofobservations in
thecorrespondingfilter.Shortverticallinesmarkepochsofopticalspectra.Intheinsettheoveralllightcurveisshownwhichincludesthepre-outburst1-d
averageddatafromtheCRTSobservations(theopensquares).T isthenumberofdayselapsedsinceHJD2456300.0.
11 1st segment Temporary fading Plateau Rapid fading Post-superoutburst
12
13
e
d
u14
nit
g
a15
M
16
17
18 V
Rise & Shoulder
uvm2 + 1.8 mag
the quiescent level
19
-20 -10 0 10 20 30 40 50 60 70 80
T (days)
Figure2.AsuperoutburstsegmentofthelightcurveintheV anduvm2filters.Blackpointsaretheindividual observations intheV filter.Thetriangles
representtheopticaldatatakenfromtheliteratureandothersources.Blue/yellowsymbolsaretheobservationsintheuvm2filter,shiftedby1.8magforclarity.
Shortverticallinesmarkepochsofopticalspectra.Thedifferentstagesofthesuperoutburstareindicated.Theredverticalribbonmarksthedaysofthereddest
opticalspectra.Intheinsettheriseandtheshoulderinthebeginningofthesecondsegmentoftheoutburstareshown.
-0.4 -0.4
12
-0.2 -0.2
0.0 0.0 13
0.2 0.2
g) g) 14
ma 0.4 ma 0.4 g)
olour ( 0.6 olour ( 0.6 15V (ma
C 0.8 C 0.8 16
1.0 1.0
17
1.2 1.2 B-I
B-I (uvm2-V)+1.5
1.4 (uvm2-V)+1.5 1.4 V (right axis) 18
0 100 200 300 400 500 700 1200 -10 0 10 20 30 40 50 60 70 80 90
T (days) T (days)
Figure3.VariationsofopticalandUVcolourindices.Left:theentireperiodofourobservations.Right:asuperoutburstsegmentisshowntogetherwiththe
V-bandlightcurve.
MNRAS000,1–21(2017)
Thesuperoutburstof SSSJ122221.7−311525 5
Table2.Thepower-law indices (columns2and3)ofthebrokenpower-
lawdeclineofSSSJ122222intheB,V,RandIbands(influxunits),and
thenumberofdays(column4)elapsedbetweenthebeginningoftherapid
16 fadingphaseatT=36.4andtheslope-break.
I
e Band Beforethebreak Afterthebreak BreaktimeafterT=36.4
d17
u
nit B −0.58±0.03 −0.33±0.01 ∼23
ag V −0.64±0.01 −0.43±0.01 ∼18
M V R −0.54±0.02 −0.43±0.02 ∼24
18 I −0.38±0.03 −0.70±0.01 ∼24
Quiescence
19 power-lawdeclineamongCVsandLMXBs,becausewefoundno
papers,eitherobservationalortheoretical,reportingonit.
FromFig.4itisalsoclearlyseenthatSSSJ122222 hasnot
1 2 3 6 10 20 30 60 100 200300 6001000 2000
fullyrecoveredfromtheoutbursteven500daftertherapidfading
Days after the beginning of the rapid fading phase
phasebecauseitwasstillfadingwiththesamerate.Onlyourobser-
vationsobtainedonandafterT=736revealedthatthedatadonot
followthedeclinetrendanymore,confirmingthatthetransienthad
Figure4.TheVandIphotometryduringthedeclinestageofthesuperout-
burst,plottedwithalogarithmictime-scale.Thedaysarecountedfromthe finallyreturnedtoquiescence.Thefollowingobservationsshowed
beginningoftherapidfadingphaseatT=36.4. nearly constant magnitudes. Thus, the total amplitude of the out-
burstwas∼7maganditspost-outburstdeclinelastednolessthan
500d.
burstsoftwoSUUMastars;Cannizzoetal.2012),thustheorigin During the superoutburst the colour indices4 demonstrated
oftheshoulderinSSSJ122222mightbedifferentfromthatofthe dramaticchanges withtime,and thecolour curve hasanextraor-
precursorsdiscussedabove. dinary shape (Fig. 3), never observed before in such detail. At
TheshapesoftheopticalandUVlightcurvesarequalitatively thesuperoutburst plateau the optical colour indices varied slowly
similar.Inparticular,thedatadonotshowanyapparentsignsofa around 0, their average values were quite usual for a dwarf nova
UV-delay in the early rising phase of the second segment of the inoutburst(Warner1995):B–V=−0.04±0.01,V–R=0.00±0.01,R–
superoutburst3. Note, however, that at the very beginning of the I=−0.09±0.01.Withthestartofthedecline,theobjecthadbegun
optical rise the UV flux showed a drop by ∼0.25 mag (see the reddening.Thecolourindicesreachedtheextremevaluessome20
inset in Fig. 2). We do not have simultaneous optical photome- d later, around the time of the slope-break of the broken power-
try which would enable us to verify such a behaviour, although law decline, at T≃54–58: B–V=0.32±0.05, V–R=0.48±0.03, R–
thedetailedKeplerobservationsofmanyCVsuperoutbursts(e.g., I=0.55±0.03. These colours are redder than in most dwarf no-
Cannizzoetal. 2012) do not seem to show such drops at optical vae in quiescence (Echevarria 1984; Echevarria&Jones 1984;
wavelengths.Ontheotherhand,thedropsoftheUVfluxinthebe- Kato,Maehara,&Uemura 2012). The colours then turned back
ginningof outburstswerereportedfor severalDNe(Wu&Panek to becoming bluer. This trend continued for several hundred
1983; Verbunt 1987). They, however, were different in the sense days. Only the data obtained after T∼472 revealed nearly sta-
that they occurred after an initial rise, and after the drop the rise ble,relativelybluecolours: B–V=0.06±0.05,V–R=0.30±0.03,R–
resumed.InSSSJ122222thisUVdropoccurredbeforetherise. I=−0.07±0.10. Although they are not so atypical for WZ Sge-
Fig.1displaysanapparentdifferenceinthepost-outburstbe- type dwarf novae (Kato,Maehara,&Uemura 2012), the colours
haviourofthetransientindifferentwavelengths.Themagnitudeof aresignificantly bluer than inmany of them inquiescence (com-
thedeclineduringtherapidfadingphasedecreasesgreatlytowards paree.g. B–I=0.43in SSSJ122222 and B–I=0.75in V455 And;
longer wavelengths, resulting in strong colour evolution (Fig. 3). Neustroevetal.2013).
Fig.4showstheopticalphotometryduringandaftertherapidfad- Thereddeningofopticallightafterthesuperoutbursthasbeen
ing phase, plotted with a logarithmic time-scale. For clarity, we previouslydetectedinseveralWZSge-typestars(Kato2015).For
showonlytheVandIlightcurves,buttheBandRlooksimilar.We example, a similarly strong cool continuum source was reported
foundthatallthelightcurvesarewelldescribedbyabrokenpower forEGCncbyPattersonetal.(1998),whichalsoshowedthered
lawasafunctionoftime,withthetimeoriginbeingatT=36.4and colours(B–I≃1.4).However,multicolourobservationsaftertheac-
aslope-breaksome20dlater.Thepower-lawindicesofthedecline tivestageofsuperoutburstsarerareandsporadic,andnodetailed
aregiveninTable2.Itisunclearhowcommonissucha(broken) studyoftheabovephenomenonhaseverbeenpresented.
In contrast to the optical wavelengths, the UV data showed
redder colours at the superoutburst plateau, and bluer during the
post-outburst stage (Fig. 3). However, the data obtained after the
3 A UV-delay, a lag as long as one day between the rise to outburst in
longgapinourobservations(afterT∼535)showthatinquiescence
theopticalfluxandtheriseinUV,isobservedinsomedwarfnovae(see
Verbunt1987andWarner1995,andreferencestherein),thecauseofwhich theuvm2-V indexdroppedagaintothelevelevenlowerthandur-
isstillnotfullyunderstood.Wenote,however,thattheobservedriseinSSS ingtheplateau,reachinganearlystablevalueof−1.44±0.10mag.
J122222hadoccurredafterthetemporaryfadingstage,fromthelevelof
about2magbrighterthanthatinquiescence.Thus,itisnotclearifthedis-
cussionoftheexistenceornon-existenceoftheUV-delayinsuchconditions 4 Inordertocalculate thevalues oftheuvm2-V indices, weinterpolated
isrelevant. theVmagnitudestothetimesoftheUVobservations.
MNRAS000,1–21(2017)
6 V. V. Neustroevet al.
Rise Plateau Rapid Fading Post-superoutburst
-0.3
-0.2
V)
e (-0.1
d
u
nit 0.0
g
a
M
0.1
0.2
3.7 3.8 10.8 10.9 11.0 14.7 14.8 14.9 15.0 24.7 24.8 24.9 25.0 36.7 36.8 36.9 37.0 40.7 40.8 40.9 41.0
T (days)
Figure5.Sixlight-curvesamplesfromdifferentstagesofthesuperoutburstofSSSJ122222(withthemeans,andlineartrendssubtractedforeachnight).

Wealsonotethreeredjumpsintheuvm2-V curve(seetheright- 0.4
handpanelofFig.3).Besidestheoneinthebeginningoftherise, )
g
mentioned above, another reddening episode had begun immedi- ma0.3
anteeoluysalyftewritthhethbeegoinpntiicnaglocofltohuersra(pbiudtfpaodsinsigblpyhaasfete,raalmsohsotrtsidmeulalyta)-. ude (0.2
However, uvm2-V reached themaximumvalueinlessthanaday plit0.1
m
andreturnedtothesuperoutburstlevelinaweek.OnT≃52–55,the
A
days of thereddest optical spectrum, theUV fluxdropped again, 0.0
producinganotherredjumpintheuvm2-Vcurve.
2
1
3.2 Developmentofsuperhumps d) Stage A
o
ri
3.2.1 Superoutburstsuperhumps e 0
p
(
C
Photometric modulations, known as superhumps, are clearly vis- O- -1 Stage B Post-superoutburst
ible in the light curve of SSS J122222 from our very first time-
resolvedobservations.Fig.5showssixlight-curvesampleswhich
-2
illustratethedevelopmentofsuperhumpsduringthesuperoutburst
stage. It isknown that the periodof superhumps P inSU UMa 12
sh
stars is not stable, but changes during a superoutburst in a com- 13
plexway.AnextensivestudyofSUUMasuperhumpsshowedthat ) 14
the evolution of their periods is often composed of three distinct mag 15
stages (see Katoetal. 2009, and their later papers). These com- (
V 16
plexperiodchangescanbeexplainedbyvariationoftheaccretion
discradiusduringasuperoutburst(Hirose&Osaki1990),whichis 17
possibly accompanied byvariable gaspressure effectsinthedisc 18
(Osaki&Kato 2013). The evolution of the superhump period in
0 10 20 30 40 50 60 70 80
SSSJ122222hasbeeninvestigatedbyKatoetal.(2013).Herewe
T (days)
extend their analysis. In order to examine theevolution of super-
humps, wedetermined the timesof superhump minima and their
amplitudesbyfittingasinewavetonightlylightcurves.Theresul- Figure6.Top:theamplitudesofsuperhumpsinSSSJ122222.Middle:the
tantO−Cdiagramandthesuperhumpamplitudesareshowninthe O−C diagramofsuperhumps.Aperiodof110.09minwasusedforthe
middleandtoppanelsofFig.6,respectively. calculations.Bottom:theV lightcurve.Theredverticalribbonmarksthe
Although the superhump period in SSS J122222 evolved daysofthereddestopticalspectra.Thestagesofthesuperhumpevolution
rather differently to other SU UMa dwarf novae (see Katoetal. aremarkedfollowingKatoetal.(2013).
2013foradiscussionofpossiblecauses),neverthelessour analy-
sisconfirmedthatthesuperhumpsexperiencedthreestagesoftheir
evolutionasseenfromtheO−C diagram.Thelattershowsthree T=4–14and15–21,wedeterminedtheperiodsofsuperhumpsfor
distinct segments, which we identified and marked in Fig. 6 as these dates to be 111.26(4) and 110.64(6) min, respectively. The
stages A, B and post-superoutburst, following Katoetal. (2013). periodsandtheirerrorswerecomputedusingalinearfittothecor-
Fig. 7 shows the Lomb–Scargle periodograms calculated for dif- responding segment of the O−C diagram. During the following
ferenttimeintervals.ThelinearriseofthestageAsegmentofthe ∼18d(21.T.38)ofstageBthesuperhumpsalsohadanearlysta-
O−C diagram (until T∼20)indicates an approximately constant ble,butshorterperiodof110.10(2)mindeterminedusingthedata
period.Thesuperhumpamplitudesgrewfrom∼0.04magatthebe- forT=23–34.Totheendofthisstagetheamplitudesdecreasedto
ginning of the stage to ∼0.07 mag at its end. Using the data for ∼0.04mag,butstartedgrowingagainafterthetransienthadfaded.
MNRAS000,1–21(2017)
Thesuperoutburstof SSSJ122221.7−311525 7
200 T= 4 - 14 T=15 - 21
100
150
er er
w w
o 12.4 12.6 12.8 13.0 13.2 13.4 13.6 o 12.4 12.6 12.8 13.0 13.2 13.4 13.6
P 100 Frequency P Frequency
50
50
0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
-1 -1
Frequency (cycles day ) Frequency (cycles day )
150
T=23 - 34 T=39 - 51
500
400
100
er er
w w
o 300 12.4 12.6 12.8 13.0 13.2 13.4 13.6 o 12.4 12.6 12.8 13.0 13.2 13.4 13.6
P P
Frequency Frequency
200
50
100
0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
-1 -1
Frequency (cycles day ) Frequency (cycles day )
Figure7.Lomb–ScargleperiodogramsoftheV datacombinedamongT=4–14,15–21,23–34and39–51.Theinsetshowstheenlargedregionaroundthe
frequency13.08cycled−1–theaveragedfrequencyofsuperhumpsduringthesuperoutburststageB,markedbytheverticaldashedline.
The temporary fading and rising stages The rising stage and Stage A Stage B -0.20 Rapid fading and post-superoutburst
-0.20 -0.05 -0.05
0.00 T=36
-0.10 0.00 T=3-4 0.00 T=19-20 0.20
T=37
nitudes00..1000 T=0 nitudes00..1005 T=5 nitudes00..1005 T=21 nitudes00..6400 T=38
Relative Mag000...432000 T=1 Relative Mag00..2105 TT==61-51-418 Relative Mag00..2105 TT==2237--2360 Relative Mag110...208000 TT==3494-43
0.25 0.25
1.40
0.50 T=3-4
0.30 0.30 T=31-34 1.60 T=45-47
0.60 P=111.21 min 0.35 P=111.21 min 0.35 P=110.09 min 1.80 P=110.09 min
-1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0
Phase Phase Phase Phase
Figure8.VariationofsuperhumpprofilesduringthesuperoutburstofSSSJ122222.
The third segment of the O−C diagram (T&38) has an uneven, modulations had a nearly symmetric, sinusoidal profile, although
steppedstructure,indicatingsuddenchangesoftheperiod.Thedu- there were several occasions when the superhump profiles trans-
rationofsegmentswithastableperiodisabout4d.Thesuperhump formedfromsingle-humpedtodouble-humpedandback.
amplitudereached∼0.3mag,butshowedshort-durationdecreases Ourmulticolourphotometryshowsthatthesuperhumpswere
atthetimeoftheperiodchanges.UsingthedataforT=39–51,we redder at their light maximum (Fig. 9). Such a behaviour is in
determined the mean period of superhumps to be 110.40(7) min. generalagreementwithpreviousreports(Schoembs&Vogt1980;
The periods of superhumps we find are consistent with those re- Hassall1985;Isogaietal.2015).Thiscanbeexplainedbyassum-
ported by Katoetal. (2013).The variation of superhump profiles ing that the superhump light source (SLS) is an expanded low-
during thesuperoutburst isshown inFig.8.Most of thetimethe temperature area in outer regions of the accretion disc (see, e.g.,
MNRAS000,1–21(2017)
8 V. V. Neustroevet al.
-0.08 T= 9-15 (Stage A) V -0.08 T= 23-30 (Stage B) V
B-I B-I
-0.06 -0.06
-0.04 -0.04
e e
d-0.02 d-0.02
u u
nit nit
ag 0.00 ag 0.00
M M
0.02 0.02
0.04 0.04
0.06 0.06
0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0
Superhump Phase Superhump Phase
-0.08 T= 31-34 (Stage B) V -0.08 T= 57-148 (Post-superoutburst) V
B-I B-I
-0.06 -0.06
-0.04 -0.04
e e
d-0.02 d-0.02
u u
nit nit
ag 0.00 ag 0.00
M M
0.02 0.02
0.04 0.04
0.06 0.06
0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0
Superhump Phase Superhump Phase
Figure9.Phase-averagedVlightcurvesandB–IcolourvariationsofsuperhumpsatdifferentstagesofthesuperoutburstofSSSJ122222.
Warner1995;Uemuraetal.2012,andreferencestherein).Wenote, panel).Foldingshort subsetsattheperiod110.547 minresultsin
however,thatnotonlytheperiod,amplitudeandprofilesofsuper- modulationsofsimilaramplitudewhichareallinphasewitheach
humps had changed over the course of the superoutburst. Fig. 9 other(Fig.10,right-handpanel).Theshapeoftheaverageprofile
displaysanotabledifferenceinthecolour profilesofsuperhumps verymuchresemblesatypicalsuperhumpprofile(seee.g.fig.1in
atdifferentstagesofsuperhumpevolution.DuringthestageAthe Katoetal.2009).Thecolourvariabilityofthesepost-superoutburst
superhump colour variabilitywasrather weak,butitsignificantly superhumps had changed significantly when comparing with the
increasedafterthetransitiontotheoutburststageB.Thisindicates superhumpsobservedduringtheoutburststage(Fig.9).Theperiod
that the accretion disc at the superoutburst plateau stage experi- ofthesepost-superoutburstsuperhumpsisabout0.4%longerthan
encesmorecomplexevolutionthanjustthevariationofitsradius, thatofthestageBsuperhumps.
suggestingsubstantialexpandingoftheSLSareaor/andlowering
Themodulations werestillstrongin2014 (T=399–455), al-
ofitstemperature.
thoughduringthisseasontheyweresuperposed onalonger-term
variabilitywithatime-scaleof8–12h,whichresemblesso-called
‘brightenings’ observed inafew otherWZSge-typesystems, i.e.
3.2.2 Post-superoutburstmodulations
inV406Vir(Zharikovetal.2006;Avilesetal.2010).Thislonger-
A visual inspection of the data revealed the presence of persis- termvariabilityproducesinthepowerspectrumanexcessofpower
tentmodulationsinthelightcurvelongafterthesuperoutburst.We atlowfrequenciesof2–3cycled−1(Fig.11,left-handpanel,thered
showbelowthataperiodofthesemodulationsislongerthanthat spectrum),makingitdifficulttostudythesuperhumpmodulations.
of the stage B superhumps, thus also implying their superhump For this reason, we first detrended each night’s light curve by a
nature. We traced them until the end of our time series observa- secondorderpolynomial.TheLomb–Scargleperiodogramofthis
tionsin2015.Inordertoexplorethesemodulations,wefirststud- detrendeddatasetisdominatedbytwosharppeakswithfrequen-
iedtheobservationsobtainedin2013aftertherapidfadingphase. ciesof 12.78cycled−1 (112.6 min)anditsfirstharmonic, andof
We divided this light curve into short pieces of a 3–6 d duration 18.72cycled−1 (76.9min,Fig.11,left-handpanel,thebluespec-
and analysed them separately. Wefound that since at least T=53 trum). The period of the 112.6 min modulation is ∼1.8% longer
themodulationhadaverystableperiod.TheLomb–Scargleperi- thantheperiodofthepost-superoutburst superhumpsobservedin
odogramoftheV datacombinedbetweenT=53and148showsa 2013. Themodulations seemedverystable.Foldingshortsubsets
verynarrowandstrongpeakat13.0261cycled−1(Fig.10,left-hand ofthe2014lightcurveatthe112.6minperiodresultsinprofilesof
MNRAS000,1–21(2017)
Thesuperoutburstof SSSJ122221.7−311525 9
250
T=53 - 148
T= 53 - 58
-0.10
200 T= 60 - 64
T= 77 - 80
T= 83 - 89
T=101 - 103
-0.05 T=116 - 148
150
e
Power 100 12.8 12.9 F1re3.q0uenc1y3.1 13.2 Magnitud 0.00
50 0.05
0 0.10
0 5 10 15 20 25 30 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Frequency (cycles day-1) Phase
Figure10.Left:Lomb–ScargleperiodogramoftheVdatacombinedbetweenT=53and148.Theinsetshowstheenlargedregionaroundthefrequency13.026
cycled−1–thefrequencyofpost-superoutburstmodulationsobservedin2013.Theverticaldashedlinesmarktheorbital(blue)andstageBsuperhump(red)
frequenciesof13.11and13.08cycled−1,respectively.Right:theentireset(largestars)andshortsubsets(smallsymbols)oftheVdatabetweenT=53and
148,foldedaccordingtoephemerisHJDmax=2456300.0610(3)+0.07676887(5)·E.
-0.05
e
Power 12.6 12.8Fre1q3u.0ency13.2 13.4 agnitud 0.00
M
2014
T=399 - 455
0.05 T=399 - 409
T=414 - 421
T=426 - 430
2015
T=446 - 455
0 5 10 15 20 25 30 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Frequency (cycles day-1) Phase
Figure11.Left:Lomb–ScarglepowerspectraforopticaldataofSSSJ122222obtainedin2014(T=399–455,twoupperspectra)and2015(T=740–763,a
bottomspectrum).Theredlinerepresentsthespectrumfortheoriginal2014photometry,whereastheblueandblacklinesareforthedetrendeddata.Theinset
showstheenlargedregionaroundtheorbitalfrequencymarkedbytheverticaldashedline.Right:theentireset(largestars)andshortsubsets(smallsymbols)
ofthedetrendeddata,obtainedin2014andfoldedwiththeperiodof112.6min.
similaramplitudewhichareallinphasewitheachother(Fig.11, Thus, our observations showed that long after the super-
right-hand panel). The origin of the 18.72 cycle d−1 peak in the outburst SSS J122222 was still exhibiting very stable super-
powerspectrumisunclear. humps, with a period longer than that of the stage B super-
humps. Post-superoutburst superhumps, also called ‘late super-
Themostrecenttime-resolvedobservations,obtainedin2015
humps’(Vogt1983),wereobservedinseveralotherWZSge-type
(T=740–763), basicallyconfirmed thephotometricbehaviour ob-
objects(seeKato,Maehara,&Monard2008;Kato2015,andrefer-
servedin2014.However,thetimeseriesanalysismaysuggestthat
encestherein).InSSSJ122222,however,theyweredetectablefor
superhumps were finally replaced by orbital variability. There is
significantlylonger timethaninanyother system. InSection6.2
a peak in the power spectrum (see the inset in Fig. 11, left-hand
wediscusstheselatesuperhumpsinmoredetail.
panel) coinciding withthe orbital frequency of 13.112 cycle d−1,
derivedfromspectroscopicdatainSection4.2.Unfortunately,most
ofthe2015dataarequitenoisyandofashortduration.Thealias-
ing problem prevents us from claiming a detection of the orbital
signalintheopticallightcurve.
MNRAS000,1–21(2017)
10 V. V. Neustroevetal.
plateau the spectra changed very little, hence we show only the
T= 4 - 14 averagedoutburstspectrum(T=20–34).
T=15 - 21
Atthetemporaryfadingstage(T=−1and−2),thespectraex-
-0.05
hibitstrong,verybroadanddouble-peakedBalmeremissionlines
andweakemissionlinesofneutralhelium(4471,5876, 6678Å).
Thereisalsoahintofabroadfeaturearound4700Åthatcanbe
e
ud associatedwiththeBowenblend,Heiiλ4686andHeiλ4713.
gnit 0.00 Duringtheplateaustage(T=20–34)mostoftheemissionlines
a
M were replaced by the broad absorption troughs with weak emis-
sion cores. Among the Balmer series lines, only Hα was still in
emission, even though the broad absorption wings also showed
up around the line. The full width at zero intensity (FWZI) of
0.05 theabsorptiontroughscorrespondstoavelocityof∼6800kms−1,
whichisveryclosetothefairlyextremewidthoftheemissionlines
(FWZI∼7000kms−1)observedduringthetemporaryfading.Itcan
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
be explained by the broadening effect of rapid rotation of the in-
Phase
nermost partsof theopticallythickaccretion disc. Aninteresting
featureisthatthose absorptioncomponents areclearlyredshifted
relativetotheemissioncore,whichareseeminglyalwayscentred
Figure12. TheV lightcurves fromthetimeintervals T=4–14and15–
close to the rest wavelength (Fig. 14, left-hand panel). By fitting
21folded withthe period of180.48minandaveraged in20phasebins.
Thesmallandlargesymbolsrepresenttheindividualobservationsandthe aGaussiantotheabsorptioncomponent ofHβandHγinnightly
dataaveragedin20phasebins,respectively.Alldataareplottedtwicefor averaged outburst spectra, we found a consistent and steady off-
continuity. set of ∼180–200 kms−1 from the rest wavelength. At the super-
outburst plateau the Ciii/Niii–Heii complex and Hei λ5876 and
λ6678 lines were still in emission. Also, an emission bump cen-
3.3 An∼180-minvariability tredat∼5830Åappeared. Thisbumpisprobably ablendofCiii
λ5827 and the Civ lines at λ5801 and 5812 Å, and possibly of
The periodograms of most segments of the light curve of SSS other highlyexcitedspecies ontheshortwardsideof Heiλ5876.
J122222 show an excess of power around frequencies ∼6–9 cy- Inaddition,thespectrashowtheappearanceofthesodiumDdou-
cled−1 withthestrongestpeakat∼8.0cycled−1 (180min,Figs7 blet(NaDλ5890/5896)inabsorption.Theselinesareveryuncom-
and10).ThissignalwasmostevidentduringtheoutburststageA. mon for CV spectra. Although the sodium doublet is one of the
Itthenweakened, butwasstilldetectablelongafterthesuperout- strongestinterstellarlines,itisnotexpectedtobeseeninthespec-
burst. We note that the global power spectrum in this frequency traofCVsbecauseoftheirrelativelycloseproximity.Nonetheless,
rangeiscomplex,suggestingalowerdegreeofcoherence,though NaDinabsorptionwasnoticedinspectraofseveralWZSge-type
betweenT=4and21thesignalwasratherstable.Fig.12showsthe systemsatdifferentstagesoftheirsuperoutbursts(Pattersonetal.
V light curves fromthe timeintervals T=4–14and 15–21 folded 1998;Nogami&Iijima2004;Nogamietal.2009),butitwasnot
withtheperiodof180.48minandaveragedin20phasebins. seeninotherdwarfnovaeshowinglargeamplitudeoutbursts(e.g.
A period of 180 min is not directly related to superhump BZUMa;Neustroev,Zharikov,&Michel2006).
or orbital periods of SSS J122222 (Porb=109.80 min; see Sec- After the rapid fading the emission bump at ∼5830 Å dis-
tion 4.2), which are significantly shorter. We note that there
appeared, but all the absorption lines transitioned from absorp-
are known a few other short-period CVs which exhibit vari-
tion to emission. NaD also appeared in emission, distorting the
abilities with a period of a few hours, longer than the orbital redwingofthedouble-peakedHeiλ5876lineinallpost-outburst
one (e.g., GW Lib – Vicanetal. 2011; FS Aur – Neustroev spectra, even three years after the superoutburst (see Fig. 14,
2002; V455 And – Araujo-Betancoretal. 2005). Among those right-hand panel). We point out that NaD was also observed in
systems only in FS Aur is such a period very coherent over emission in GW Lib some 40 d after the rapid fading phase
many years (Neustroevetal. 2013). It was proposed that the (vanSpaandonketal. 2010). We also note the presence of a nar-
precession of a fast-rotating magnetically accreting WD can row,unidentifiedlineat∼5903Å.Thisfeatureseemstoberealand
successfully explain this phenomenon (Tovmassianetal. 2003; notnoiseoranartefactofthedatareductionbecauseitispresent
Tovmassian,Zharikov,&Neustroev 2007). However, inother ob- indifferentspectraandalsoexhibitsatransitionfromabsorptionto
jectslongperiodmodulationsshowasignificantlylowerdegreeof emission,similarlytoNaD.
coherence (Vicanetal. 2011; Araujo-Betancoretal. 2005), simi-
As the system flux continued to decline, the emission lines
larly to SSSJ122222. There is currently no physical explanation
gotprogressivelystronger.Inthespectrumtakenduringthedecay
forthelatterphenomena. phaseonT=72,theHαhasamoderatestrength(EW=129Å)and
theBalmerdecrementisrelativelyflat:Hα:Hβ:Hγ=2.01:1.00:0.56.
However,thefinalspectratakeninquiescenceonT=848and1127
showtheextremelystrongHαlinewhichpeaksat8and11times
4 OPTICALSPECTROSCOPY the continuum and has EW=277 and 384 Å, respectively. To the
bestofourknowledge,thisisthestrongestemissionlineeverob-
4.1 Spectralevolutionthroughoutthesuperoutburst
servedinaCV.TheBalmerdecrementisalsoverysteepinthese
Fig.13displaysthespectralevolutionofSSSJ122222throughall spectra: Hα:Hβ:Hγ=3.20:1.00:0.44 (T=848) and 3.42:1.00:0.56
theoutburststagesandthefollowingdecay(theepochsofoptical (T=1127).Thischange intheBalmerdecrement fromshallow to
spectra are shown in Fig. 1). Note that during the superoutburst very steep indicates a transition from optically thick to optically
MNRAS000,1–21(2017)