Multi-epoch time-resolved photometry of the eclipsing polar CSS081231:J071126+440405

0.81 MB·English

by  A. D. Schwope

Astron. Nachr./AN000,No.00,000–000(2014)/DOIpleasesetDOI!
Multi-epoch time-resolved photometry of the eclipsing polar
CSS081231:071126+440405
A.D. Schwope1,⋆, F. Mackebrandt1, B.D. Thinius2, C. Littlefield3,4, P. Garnavich3, A. Oksanen5, and
T.Granzer1
1 Leibniz-Institutfu¨rAstrophysikPotsdam(AIP),AnderSternwarte16,14482Potsdam,Germany
5 2 InastarsObservatory,Hermann-Struve-Str.10,14469Potsdam,Germany
1 3 DepartmentofPhysics,UniversityofNotreDame,225NieuwlandScienceHall,NotreDame,IN46556,USA
0 4 DepartmentofAstronomy,WesleyanUniversity,Middletown,CT06459,USA
2 5 HankasalmiObservatory,Jyva¨skyla¨nSiriusry,Vertaalantie419,40270Palokka,Finland

n
a
ReceivedDecember4,2014,acceptedJanuary21,2015
J

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2 Keywords stars:individual:CSS081231:071126+440405–binaries:eclipsing–stars:cataclysmicvariables
Theeclipsing polar CSS081231:071126+440405 turned bright (Vmax ∼ 14.5) inlate2008 and wassubsequently ob-
]
servedintensivelywithsmallandmedium-sizedtelescopes.Ahomogeneousanalysisofthiscomprehensivedatasetcom-
R
prising109eclipseepochs ispresentedandalinearephemeriscoveringthefiveyearsofobservations, about24000 or-
S
bital cycles, is derived. Formally this sets rather tight constraints on the mass of a hypothetical circumbinary planet,
h. Mpl ≤ 2MJup. This preliminary result needs consolidation by long-term monitoring of the source. The eclipse lasts
p 433.08±0.65s,andtheorbital inclinationisfound tobei = 79.3◦ −83.7◦.Thecentreofthebrightphasedisplays
- accretion-ratedependentazimuthalshifts.Noaccretiongeometryisfoundthatexplainsallobservationalconstraints,sug-
o gestingacomplexaccretiongeometrywithpossiblepoleswitchesandalikelynon-dipolarfieldgeometry.
r
t
s (cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
a
[

1 1 Introduction CSS081231:071126+440405 (henceforth J071126)
v was discovered as variable transient-like object in the
7 Catalina Real-Time Transient Survey (Drakeetal. 2009)
Jupiter-sized planets on Jupiter-like wide orbits were dis-
1 whenitsuddenlyturnedbrightfromV ∼ 18toV ∼ 14.5
coveredin recentyearsaroundeclipsing close binarystars
2
in the fall of the year 2008. Following vsnet-alert #10867
5 with white-dwarf primaries and main-sequence secondary
Denisenko&Korotkiy(2008,reportedinCVnetdiscussion
0 stars (WDMS binaries). Such discoveries were possible
1. via the light-travel time effect, i.e. via periodic shifts of #1208)obtainedthe first time-resolvedphotometricobser-
vations.Theyconfirmedamaximumbrightnessof14.4and
0 the eclipses of the central binary, and they require pre-
5 cise eclipse timing and long monitoring campaigns. Suit- reportedthe occurrenceof deepeclipses, whichwere used
1 todetermineanorbitalperiodof116.6m.Theshapeofthe
able target stars are relatively rare, to date less than 100
: light-curve led them to classify the object as an eclipsing
v such objects are known. Circumbinaryplanets were found
i orbiting accreting and non-accreting binaries of WDMS polar.Initshighstateitwasfoundtobesimilarlybrightas
X
thetemplateeclipsingpolarHUAquarii.
type,i.e.cataclysmicbinaries(CVs)andpre-CVs(seee.g.
r
a Beuermannetal.2010;Qianetal.2011). Polarsaremagneticcataclysmicvariablestars.Contrary
Orbital period variations in those objects may come to normalCVs the strong magnetic momentof the accret-
from differentsources, they may be caused by activity cy- ing white dwarf keeps both stars in synchronous rotation.
cles of the secondary stars or by circumbinary planets. In The magnetic field suppresses the formation of an accre-
addition, magnetic braking of the binary and gravitational tiondisk.Insteadtheaccretedmatterisguidedtowardsthe
radiationwilloccurbuttheirstrengthisnotenoughtomake magneticpole(s)wheretheaccretionluminosityeventually
an observable impact on the time-scale of a scientific ca- is released, part of it as strongly polarized cyclotron radi-
reer.Someobjectsshowstrong,quasi-regularvariabilityin ation. Polars are sources of soft and hard X-rays that are
their eclipse arrivaltimes thatcannotbe explainedby cur- originating from regions close to the magnetic poles. The
rent models(see the claimed discoveryof a planetarysys- absenceofanaccretiondiskresultsinshortresponsetimes
temaroundHUAqr; Boursetal.2014;Goz´dziewskietal. tochangesofthemasstransferratethatmayoccurontime
2012; Horneretal. 2011; Qianetal. 2011; Schwarzetal. scales from seconds to years. During low states, when the
2009;Schwope&Thinius2014;Wittenmyeretal.2012). accretionstream is reducedto a trickle, the opticallightis
dominated by the photospheres of the stellar constituents
⋆ Correspondingauthor:e-mail:[email protected] plusperhapssomecyclotronradiationfromresidualaccre-
tion. During high accretion states the spectral energy dis-
(cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
Astron.Nachr./AN(2014) 1
Table1 Facilitiesusedinthisstudyandtheirobservationalset-up
Observatory Aperture Detector Filter T #frames latency syst.unc.
exp
(cm) (s) (s) (s)
HO 40 SBIGSTL-1001E clear 30 6730 0.0 3.00
Nonndorf 25.4 Videocam.WAT-120N – 2 10218 0.0 2.56
VATT2009 180 VATT4K B/V 20,30/30 426,321/608 0.0 0.0
VATT2010 180 VATT4K U/B 30/30 526/517 0.0 0.0
VATT2010 180 VATT4K V 10,15,30 112,549,206 0.0 0.0
VATT2013 180 VATT4K R/vilS∗ 10/20 306/222 0.0 0.0
STELLA2012 120 CCD g 15 543 3.0 0.1
STELLA2013 120 CCD g 60 724 3.0 0.1
IOP 36 SBIGST-83 AstrometrikCLS 3 6845 0.0 0.0
SCT28 28 SBIGST-83 clear 8,10,15,30 6304,1353,64,438 1.5 0.0
SLKT 80 SBIGSTL-10013 Luminance 2 222 2.6 0.0
∗theVilniusSfilterhasmaximumtransparencyatHαandaFWHMof180A˚.
tribution of the binary is dominated by accretion-induced
radiationwhichoutshinesthephotosphericradiation.
BrighteclipsingobjectslikeHUAqrandnowJ071126
are prime targets for detailed investigationsaddressingthe
accretionphysicsandarequalifiedforhigh-timeresolution
observationstodeterminepreciseeclipsetimes.
The first printed publications on J071126 are by
Thorneetal. (2010) and Katysheva&Shugarov (2012)
whowererefiningtheeclipseephemerisanddiscussingthe
shapeofthelight-curveinlow,intermediateandhighstates
of accretion. The object occasionally shows a pronounced
pre-eclipse dip due to the intervening accretion stream or
accretioncurtain,whoselocationinphaseinprinciplehelps
Fig.1 Long-term light curve of
touncovertheaccretiongeometry.
CSS081231:071126+440405 derived from the CSS-
database. Vertical lines indicate the epochs used here for
At its maximum brightness the variable star is within
the determination of the binary period. Color indicates
thereachofsmalltelescopesandwastargetedbyamateurs
the equipment used (green – HO, blue - Dangl, red –
at several sites. Amateur equipment may allow short inte-
VATT,cyan–STELLA,magenta–NotreDame(SCT28&
grationtimesandtheirresultscanbeusedtostudytheLTT-
SLKT),yellow–IOP)
effect.Lowersignal-to-noiseperimagecomparedtoprofes-
sionaltelescopescanbecompensatedby’infinite’observa-
tiontimeavailableatprivateobservatories. 2 Observationsand data reduction
Motivatedbyrecentreportsofcircumbinaryplanets,by In this paper new and archival observations of J071126
thelargebrightnessofJ071126,andbytheavailabilityofa are reported. New observations were obtained with the
largebodyofarchivaldatamadeavailableviatheinternet, 1.2m STELLA/WiFSIP telescope, from the Inastars Ob-
wedecidedtoobtainfurthertime-resolvedphotometricob- servatoryPotsdam(IOP),withthe1.8mVaticanAdvanced
servationsofJ071126andanalyzethearchivalandnewown TechnologyTelescope(VATT),withthenewlyinaugurated
data in a homogeneousway to derive a reliable ephemeris 80cm Sarah L. Krizmanich telescope (SLKT) and with a
forthecentreoftheeclipseandtosearchforpossiblesys- 28cm Schmidt-Cassegrain telescope (shorthand SCT28),
tematicdeviationsthatcouldgiveahintfortheoccurrence both owned by the University of Notre Dame (Indiana).
ofanunseencompanion. Archival observations were obtained with the VATT in
2009, by the Hankasalmi observatory (HO, Finland) and
In Sect. 2 we describe both new and archival observa- by Gerhard Dangl via his website1. The VATT data from
tions and how the data were treated to determine eclipse 2009wereoriginallypublishedbyThorneetal.(2010)but
times for the individual epochs. In Sect. 3.1 we derive an re-analyzed here in the same manner as all other data for
updatedeclipseephemeris,in Sect.3.2weanalyseproper- thesakeofhomogeneity.TheHO-datawereretrievedfrom
tiesoftheopticallightcurvesandderivebinaryparameters
anddiscussthepossibleaccretiongeometryinSect.4. 1 http://www.dangl.at
www.an-journal.org (cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
2 A.D.Schwopeetal.:Multi-epochtime-resolvedphotometryoftheeclipsingpolarCSS081231:071126+440405
Fig.2 Observedminuscalculatedtimesofeclipsecentres
Fig.3 CRTSdatafoldedoverbinaryphase
ofJ071126.ColorsarethesameasinFig.1
The robotic telescope STELLA-1 (aperture 1.2 m,
the webpageofthe observatory2. The wholedataset com-
Strassmeieretal. 2004) is equipped with the wide-field
prisestime-resolvedphotometricobservationsobtaineddur-
imaging photometer WiFSIP and was used during two
ingmorethan50nightsatthevariouslocations.Insteadof
nights in November 2012 and during five nights in
listingeverysinglenightofobservationthatwasusedinthis
April 2013 for time-resolved photometric observations of
studyweindicateeachnightforwhichwecouldderivean
J071126.AllCCD-framesweretakenthroughanSDSSg-
eclipseepochintheCSSlong-termlightcurvedisplayedin
filter.
Fig. 1.Nevertheless,anoverviewofthefacilitiesusedand
The raw CCD-frames obtained from the various sites
themainobservationalparametersisgiveninTable1.
were bias-corrected, dark-subtracted and flat-fielded using
J071126wasputontheobservingprogramoftheHan-
calibrationdataobtainedforthispurpose.Inthecontextof
kasalmiObservatoryafterits transientnaturewas reported
thecurrentstudythetimingofindividualCCD-framesisof
andwasintensivelyobservedduring2009andoccasionally
utmostimportance.Theanticipatedstartofindividualexpo-
in2010and2011.Allthosedatawereobtainedwithanin-
suresisstoredintheFITS-headersintheDATE-OBSkey-
tegrationtimeof30s.
word.However,CCDpre-flashesandotherdelaysduetothe
The observations by Dangl from Nonndorf (Austria)
operatingsystemmaygiverisetoadelayedshutteropening.
wererecordedwithavideosystem(fordetailsseehisweb-
Wheneverpossible,thisshutterlatencywasdeterminedand
page) and binned afterwardsto a time resolution of 2 sec-
addedto themeasuredtimesof eclipse ingressandegress.
onds. A GPS-clock was directly connected to the video-
Whenthiswasnotpossible,asystematicerrorofanytiming
grabber. The timing accuracy per frame is 2.56s (Dangl,
valuewastakenintoaccount.Withoutmeasuringtheshut-
privatecommunication).
ter latencythe widely used MaximDLCCD-software does
DetailsoftheobservationswiththeVATTin2009were
notwritedecimalsecondstotheFITS-headersbutcutstime
described by Thorneetal. (2010) and the description will
to the integer second. When integer seconds were written
not be repeated here. Additional data in UBV bands were
bytheCCDsoftware,halfasecondwasaddedtothemea-
obtainedduringthreenightsin 2010and in the Rand Vil-
suredtimesandasystematicuncertaintyof0.5secondswas
niusSbandsin2013.
addedtotheerrorbudget.Themeasuredorestimatedshut-
Observations with an 28cm Schmidt-Casegrain tele-
terlatenciestogetherwiththesystematictiminguncertain-
scope (SCT28) owned by the University of Notre Dame
tiesarelistedincolumns(7)and(8)ofTab.1.
wereperformedduringeightnightsin2012andfournights
Afterwards,differentialphotometryusingESO-MIDAS
in 2014,mostly in white lightbutoccasionallyduringtwo
routines was performed with respect to comparison stars
nightsin2012throughBandRfilters,respectively.
#139 or #142on the AAVSO star chart (#139:RA(J2000)
The80cmSLKT,alsoownedbytheUniversityofNotre
= 7:11:13.17,DE(J2000)= 44:03:59.7,B = 14.696,V =
Dame, was used during two commissioning nights of the
13.918; #142: RA(J2000) = 7:11:35.50, DE(J2000) =
telescopeforun-filteredobservationswithintegrationtimes
44:04:03.3,B = 15.167,V = 14.158).Thedefaultchoice
asshortas2seconds.
wastousestar#139.Thefluxratiosanddifferentialmagni-
J071126was observedfrom InastarsobservatoryPots-
tudesshowninFigs.6and7refertothatstar.
damduringfournightsinMarchandApril2014.Theinstru-
Thecentroidsofthestellarimagesofboththetargetand
mentandtheobservationalsetupwasthesameasdescribed
the comparison star were determined via two-dimensional
inSchwope&Thinius(2014).
Gaussian fits ona numberofimageswith clean detections
2 http://murtoinen.dynds.org ofbothstars.Thepositionswereusedtodeterminetheposi-
(cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2014) 3
Fig.4 Time-resolvedphotometryobtained on March 26, 2009, from Hankasalmi observatory.Shown is the flux ratio
betweenthetargetandcomparison#139fromtheAAVSOchartforthetarget.
tionaloffsetinpixelunitsbetweenthetwostars.Thenaper-
ture photometrywas performedon each CCD frame using
concentric apertures for the object (plus sky background)
and the sky background,centered initially on the compar-
ison star, then at the offset position and always using the
same apertures for target and comparison star. With this
procedureabrightnessmeasurementwaspossibleevendur-
ingeclipsesofJ071126,whenthe sourcebecametoofaint
for centering. Errors of individualmeasurements were de-
termined taken into account the sky brightness, the object
brightness and the read-noise properties of the CCD. The
flux ratio between the target and the comparison star was
furtherinvestigatedfor timingof the eclipse and fora dis-
cussionoftheshapeofthelightcurve(s).
Fig.5 Phase-averagedlightcurvesofJ071126obtainedin
3 Analysisand results 2009fromHankasalmiobservatorythroughhigh,interme-
diate, low and verylow states, respectively.The data were
3.1 Apreciselong-termephemeris averagedinto200,200,100,and50phasebinsfromtopto
bottomusingtheephemerisofEq.2(highstate:March26,
ThetimesofallCCDframeswerecorrectedtorefertomid- 27,andApril1,8,14,15,22−25;intermediatestate:Jan-
exposure,correctedforarrivalatthesolarsystembarycenter uary1,2,4,andMarch19;lowstate:January7,March21,
andconvertedtobarycentricdynamicaltime (TDB)which 24,and25;verylowstate:January16,September19,20).
equalsterrestrialtime(TT)atourprecision.
The times of individual ingress and egress were mea-
sured by averaging a few data points before and after ortheegress,couldbedetermined.Inordertoincludethose
ingress/egress,computingthehalf-lightintensityandread- eclipses in the analysis as well a two-stage process was
ingthe timeswitha cursorfroma graphofthelightcurve chosen to derive timings of the eclipse centre. Firstly the
(see Schwope&Thinius2014, foragraphicalrepresenta- meaneclipselengthwasdeterminedusingallfullycovered
tion).Uncertaintiesofindividualmeasurementsweresetto eclipses. A weighted average of 102 individual measure-
half the integrationtime of individualCCD frames,unless mentsgave∆t = 433.08±0.65s,whichcorrespondsto
ecl
adatagapdegradesthetimingaccuracyfurther.Therewere ∆φ = 0.061596±0.000092phaseunits.Thentheeclipse
occasionswhenonlyoneeclipsefeature,eithertheingress centers were determined from the timings of the ingress
www.an-journal.org (cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
4 A.D.Schwopeetal.:Multi-epochtime-resolvedphotometryoftheeclipsingpolarCSS081231:071126+440405
Fig.7 Shapeoftheeclipseinhighandlowstates.
Fig.6 Phase-averaged light curves of J071126 obtained
between 2010 and 2014 from various sites. All data
are shown twice for clarity and were phased using the
ephemeris of Eq. 2 and averaged into 100 phase bins (66
phasebinsforthe2012observationswithSTELLA).Note
thedifferentscaleusedinthefirstpanel.
and/oregresscorrectedforhalftheeclipselength.Thispro-
Fig.8 CRTSdataofCSS081231:071126+440405folded
cedureresultedinone(ingressoregress)ortwo(ingressand
over orbital phase.The diagram gives eclipse details, the
egress)independentmeasurementsoftheeclipsecentre.In
lengthofthehorizontalbarsindicatetheintegrationtimeper
the latter case the finally accepted value was the weighted
measurement. The dashed vertical lines indicate the mea-
average of the two measurements. All eclipse timings are
suredeclipselength.
listedinTab.2.
Aweightedlinearregressiontoall109datapointsyields
thelinearephemerisfortheeclipsecentre even prior to this. An initial low state at V ∼ 18 was fol-
BJD(TDB)=2454833.207868(14)+ (1) lowed by a state of intermediate brightness between MJD
E×0.0813768094(6) 54000 and 54200 at V ∼ 16 (September 2006 till April
2007).The objectthenreturnedto a low state beforeturn-
(numbers in parenthesis give formal 1σ uncertainties, re- ing bright with V = 14.2 in late 2008. This was fol-
max
ducedχ2ν =0.92for107d.o.f.).Theresidualswithrespect lowedbyanotherlowstateinthefallof2009.J071126was
to this linear fit are shown in Fig. 2. The diagram and the observedinintermediatetohighstatessincethen.Overall,
fittingstatisticsshowthatalldataareinagreementwiththe the CSS-data obtainedin white light show variability with
linearfit. anamplitudeofabout5mag,themaximumbrightnessisat
14.2,theminimumat19.3.AconsiderablenumberofCSS-
3.2 ThephotometricbehaviorofJ071126through measurements revealed the object at 19th mag, i.e. about
highandlowstates onemagnitudebelowtheaveragelow-statebrightness.
Phase-folding of all CSS data (385 data points) gives
3.2.1 CRTSphotometry
further insight (see Figs. 3 and 8). It firstly shows that
The CSS long-termlight curveshown in Fig. 1 coversthe all the CSS measurements at 19th magnitude fall into the
period between November 2005 and October 2013. The eclipse. The eclipse light curve is flat-bottom at m ≃
CSS
transient behavior was recognized in late 2008 but pro- 19. This is about 1.9 mag brighter than the low-state V-
nouncedphotometricvariability is evidentin archivaldata bandeclipsebrightnessmeasuredbyThorneetal.(2010)at
(cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2014) 5
Changes of the overall brightness are accompanied by
changesofthestart,end,andlengthofthebrightphaseand
theoccurrenceofapre-eclipsedip.Theeclipselightcurves
arealsovariableandwillbediscussedinthenextsection.
The length of the bright phase changes from ∆φ =
φ − φ = 1.30− 0.75 = 0.55 in the low state to
end start
1.26−0.63=0.63phaseunitsinthehighstate(seeFigs.5
and 6). The center of the bright phase thus moves from
1.03±0.02inthelowstateto0.98±0.02intheintermediate
andfinallyto0.95±0.02inthehighstate,hencechanging
fromatrailingspotatlongitudeψ ∼−10◦toaleadingspot
at ψ ∼ 18◦. Some care shouldbe taken when interpreting
these numberspurelyin geometricalterms,since the mea-
suredspotlocationmay dependonthefilter used andthus
not necessarily indicate a change of the accretion geome-
Fig.9 UBV-photometryobtained with the VATT in Oc-
try.ThisbecomesclearfromtheVATTUBVphotometryin
tober 2010.Shown are phase-averageddata (binsize 0.005
December2010(Fig.9).Thosedatashowacolor-dependant
phase units, ∼35 seconds). Differential magnitudes were
startofthebrightphase,whereastheendofthebrightphase
correctedtothesamemeanvaluearoundphase0.4.
isnotdependentonthefilter.Theaccretionstateisassumed
tobethesame.
V =20m. 86±0m. 05.Duringtheeclipseinthelowstatethe Mostofthehigh-statelightcurvesshowamoregradual
remaininglightoriginatesmainlyonthecompanionstar.At increase of brightnessat the beginningof the brightphase
aperiodof117minoneexpectsV −I =2.54ifthedonor andastepdeclineintotheself-eclipseoftheaccretionarea.
follows the CV sequence described by (Knigge 2007). No DuringtheSTELLAobservationin2013,however,theob-
color transformation into the CSS-system is available for ject displays the opposite behavior. At this epoch the oth-
such red stars, but given the red sensitivity of the CSS- erwiseprominentpre-eclipsediphasalsovanished.Hence,
system it appears reasonable that both eclipse brightness in April 2013 the accretion geometry seemed to be rather
measurementsarecompatiblewitheachother. differentthanattheotheroccasionsinthehighstate.
IntheCRTStheobjectdisplaysout-of-eclipsevariabil- The occurrence and the phase of the pre-eclipse dip is
itybetweenbrightandfaintphaseswithaminimumampli- also dependent on the accretion state. The dip is typically
tudeofabout0.2mag. present in the high state (but not during the STELLA ob-
servationin2013)andhasatypicaldepthof1.4mag.Itis
centeredonphase0.86,butthe phaseinterval0.82−0.90
3.2.2 Multi-sitephase-resolvedphotometry is affected by absorption. In the intermediate state the dip
is less pronouncedandit seemsto showa largerphasejit-
The orbital variability pattern in high and low states can- ter.Theaveragedepthis0.3magbutmaybeaslargeas1.5
not be read from the CRTS data but only from phase- mag.Thephaseinterval0.84−0.92isaffectedbyabsorp-
resolved observations. Time-resolved and phase-averaged tion,theaveragedipoccursatalaterphasethanduringthe
light curvesobtainedby usare shownin Figs. 4, 5, and6. high state, φ = 0.89. No dip is detected during low and
c
Figures 4 and 5 contain only data from 2009 obtained at verylowstates.
Hankasalmi.Phase-foldedlightcurvesfromotherobserva-
Pre-eclipse dips are observed in several polars, a very
toriesobtainedbetween2010and2014areshowninFig.6.
prominent example being HU Aqr (Schwopeetal. 2001).
Plotted in Figures4and 6 isthe flux ratioas a functionof
Theyarecausedbyabsorptionofradiationoriginatingfrom
theorbitalphase.Thedatashowninthetoppanelwereob-
theaccretioncolumninthemagneticallyguidedpartofac-
tainedin a lowstate (Thorneetal. 2010).Allotherdatain
cretionstream(oraccretioncurtain).Theorbitalphaseand
thisFigurerepresenthighaccretionstates.
the extent of this feature gives clues to the location of the
Thelightcurvesshowcommonalitiesandsomemarked coupling region and to the distribution of absorbing mat-
differences. All light curves, irrespective of the accretion ter in the magnetosphere of the white dwarf. The absence
state,displaythealternatingbright-/faint-phasepatterndue of this feature at some occasions while previously present
totheself-eclipseoftheaccretionregion(orregions). might thus indicate that the matter is accreted then by an-
The data obtained at Hankasalmi in the year 2009 can otherregionbelowtheorbitalplane.
besortedintofourdistinctaccretionstates. Transitionsbe- The VATT V-band light curve obtained in 2009 (top
tween different states are observed to occur on timescales panelinFig.6)ismarkedlydifferentthanthehighstatelight
ofdaysorevenafewbinarycycles.Thetransitionfroman curves.Therisetoandthefallfromthebrightphaseshow
intermediatetoahighstatehappenedwithintwobinarycy- asteepgradient.Also,thereisabroadfluxdepressioncen-
cles,seeFig.4. tered on phase 0.1. Those propertiesof the light curve are
www.an-journal.org (cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
6 A.D.Schwopeetal.:Multi-epochtime-resolvedphotometryoftheeclipsingpolarCSS081231:071126+440405
reminiscentof WW Horor DPLeo wherethe lightcurves
wereshapedbystrongcyclotronbeaming(Beuermannetal.
1990;Schwope&Mengel1997).
Occasionally a secondary maximum is observed dur-
ingthefaintphase(seee.g.theVATTV-andU-bandlight
curvesfrom2010inFig.9,theSTELLAg-bandlightcurve
from2013inFig.6andthewhite-lightdataobtained2009,
March 26, in Fig. 4). At other occasions the faint-phase
brightnessismoreorlessconstantorjustgraduallyincreas-
ing.Thissecondaryhumphasamaximumamplitudeof0.3
magandislocatedatphase0.50(April2013),0.54(October
2010,V),and0.59(October2010,U),respectively.Various
radiationcomponentsmaycontributeto the observedflux:
a second accretion region, the accretion stream, the donor
star, and the white dwarf. Photospheric emission from the Fig.10 Eclipselengthofpointsalongthecentralmeridian
starsappearsunlikelytousbecausethoseradiationcompo- foran0.8M⊙whitedwarfasafunctionoftheco-latitudeβ
nentsshouldbemoreprominentduringlowaccretionstates. and theorbitalinclination.The measuredeclipse lengthof
433.08sisillustratedbythedashedhorizontalline.
3.3 Eclipselightcurvesandbinaryparameters
In order to estimate the size of this effect, we have cal-
Figures 7 and 8 show eclipse details from phase-resolved
culated eclipse light curves for certain points on a sphere.
observationspresentedhereorfromtheCSSdatabase.The
A white dwarf with 0.8M was assumed and the points
⊙
same phase-resolved data were used as in Fig. 6 but with
were chosen along the central meridian (meridian on the
finer binning (0.004 phase units). During the low state
whitedwarfthroughtherotationaxisandthelineconnect-
the light curve in the eclipse is flat. It is thus reason-
ing both stars). Eclipse light curves were computed as a
able to assume that the residual emission is due to the
functionof the stellar co-latitudeβ, i.e. the angle between
donor star (Thorneetal. 2010). During intermediate and
therotationaxisandthepointonthemeridian.Resultsare
high states rather intense emission is remaining through- showninFig.10fori=81.7◦,±0.5◦,±1.0◦,respectively.
outtheeclipseofthewhitedwarf.Thisisduetotheaccre-
The measured eclipse length is the same for an accretion
tion stream/curtain outside the orbital plane. From its ini- spot at β = 68◦ and at the center of mass of the assumed
tialbrightnessaftereclipseingressitfadestypicallybytwo
0.8M whitedwarf.Thecomputationsshowthatoneshould
⊙
magnitudes but seems to stay above the brightness of the consideranother∼0.5◦ systematicuncertaintyoftheincli-
donorstar,hencesomepartsofthestreamarealwaysvisi-
nationderivedstraightfromtheobservedeclipsewidthvia
ble.
Roche-lobegeometry.Theinclinationisthusexpectedtobe
Most of the CRTS data that were obtained the white- intherangei=79.3◦−83.7◦.
dwarf eclipse show the source at V = 19 but a few
CSS
datapointsshowedtheobjectconsiderablybrighter,17.0−
17.7mag.Thesedatapointswereobtainedduringveryhigh 4 Discussionand conclusion
accretion states and the high brightness is also due to the
brightaccretionstream. Thepresentstudypresentsananalysisofmulti-epochtime-
The eclipse width ∆φ , the orbital inclination i and resolved photometric observations of the bright eclipsing
ecl
the mass ratio Q are related to each other via the Roche polar CSS081231:071126+440405.We haveestablished a
geometry(Chananetal.1976).Takingtheobservedeclipse precise long-term ephemeris based on 109 individual tim-
widthatface value,i.e.asa proxyfortheeclipsewidthof ings of the eclipse obtained between 2009 and 2014. We
the white dwarf, a relation between between i and Q can alsodeterminedapreciseeclipselengthof433.08±0.65s.
be derived. If one further assumes that the donor star fol- Aneclipselengthof408±7sreportedbyThorneetal.
lows Knigge’s sequence, M2 = 0.16M⊙ (Knigge 2007), (2010) can definitely be ruled out already on the basis of
andassumingM1 = 0.8M⊙, a typicalvalueforthewhite a few light curves that were obtained with sufficient time
dwarfs in CVs, the orbital inclination is i = 81.7◦. For a resolution. The measured eclipse length was used to con-
standard single white dwarf, M1 = 0.6M⊙, the inclina- strain the orbital inclination to lie within the range i =
tion would be 79.8◦ and for a more massive white dwarf 79.3◦−83.7◦.Theuncertaintyisduetotheunknown,hence
with M1 = 1.0M⊙ one gets i = 83.2◦. A further un- assumedmassofthewhitedwarfandtheunknownlocation
certainty derives from the fact, that the observed eclipse of the accretion spot. Photometric observations with high
width does not exactly correspond to the eclipse of the time resolutionduringa lowstate wouldbemostusefulto
white dwarf centre of mass. It belongs mainly to the ac- directlymeasurethesizeofthewhitedwarfandhigh-speed
cretion spot somewhereon the surface of the white dwarf. photometric observations during a high state would allow
(cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2014) 7
tomeasurethesizeandlocation(azimuth)oftheaccretion the low state prevents us from constraining the stagnation
spotdirectly. regionwhentheaccretionrateisfurtherreduced.
Wefindalinearrelationbetweencyclenumberandmid- Firstlywearediscussingapossiblehigh-stateaccretion
eclipsetimewhichisstableoveratimeintervalof5.3years geometry.Thepresenceofapre-eclipsediprequiresanac-
or almost 24000 orbital cycles of the binary. We estimate cretion region above the orbital plane. An only partial oc-
themaximummassofanunseencompanionbyaddingone cultationoftheaccretionstreamrequiresasufficientlylow
higherpolynomialorderandexploretheχ2 spaceuntilthe orbital inclination and a sufficiently high elevation of the
fit deteriorates. We request a 95% confidence to reject the stream.Thelatterbecomespossibleiftheinclinationofthe
hypothesesof a quadratic term in the ephemeris.The bow magneticaxisissufficientlysmall.Alocationoftheaccre-
of the parabolais used asa proxyfor the amplitudeof the tionregionclosertothelineconnectingbothstarsthanthe
sine curve due to the LTT effect. A period5.3 yearsis as- stagnation region means that the magnetically guided part
sumedfortheputativesubstellarcompanion.Itsamplitude ofthestreamhastobestronglyre-directedintheframeof
then is about 3 sec (6 s peak-to-peak) which corresponds thebinary.
toM ≃ 2M foranegligibleeccentricity,foranedge-
pl Jup
We use ourownvisualizationsoftware to test different
on planetary system and for a total mass of the system of
accretionscenarios.Theoriginalfree-fallingstreamiseven-
1M . Care hasto be taken to notoverinterpretthisresult.
⊙
tually guided by magnetic field lines towards an accretion
It may well be that the system is on an apparently linear
partoftheO−C diagramoverthepast5yearswhichover region on the white dwarf. The coupling region is found
by balancing the ram and the magnetic pressure along the
alongertimebasemightbecomesaw-toothshaped(foran
stream.Inthecouplingregionthestreamisassumedbein-
examplesee NN Ser, Beuermannetal. 2010). Monitoring
stantaneously re-directed and traveling along dipolar field
ofthesystemforseveralyearstocomeisneededtoconfirm
lines. We find a possible accretion geometry for the high
ordisprovethelinearephemeris.Thecurrentworkdefines
stateassumingaco-latitudeandanazimuthofthemagnetic
thereferenceforsuchfuturestudies.
axisofβ =18◦,andψ =−3◦,andaninclinationof79.3◦.
Monitoring over long times is also needed to measure
Forthechosenorientationofthemagneticaxistheaccretion
thecenterofthebrightphaseandprobethecouplingmech-
stream would be completely occulted at phase zero if the
anismandstabilityofthewhitedwarforientationinthesys-
orbitalinclinationwerehigher.A higherinclinationwould
tem.Withamonitoringcampaignlastingmorethan30years
bepossibleifthemagneticaxisweremorecloselyaligned
Beuermannetal. (2014) have uncoveredthe likely oscilla-
to the rotation axis. In such a geometry the magnetically
tionofthemagneticaxisaroundanequilibriumpositionof
guided part cannot be re-directed to feed an accretion re-
thewhitedwarfintheeclipsingpolarDPLeo.Thespotlon-
gionclosetothelineconnectingbothstars.Asketchofthe
gitudeof DPLeo was observedat −10◦ to −15◦ some 30
accretion geometryis given in Fig. 11 where the binary is
yearsago,similartowhatisobservedinJ071126.Contrary
seen at orbital phases 0.9, i.e.at the end of the pre-eclipse
toDPLeothelongitudeoftheaccretionspotinJ071126is
dip,andatphase0.0.
observedtovaryasafunctionofthemassaccretionrate.In
the low state the spotliesat −10◦, and in thehighstate at Ataloweraccretionratecouplingoccursearlierandthe
+10◦. The accretion-rate dependent location of the accre- magneticallyguidedpartofthestreamisliftedfurtherabove
tion spot complicatesthe proposedinvestigation.The high the plane. Hence, at lower accretion rate the magnetically
brightnessofJ071126ontheotherhandispromisingtoes- guidedpartisnotexpectedtobecompletelycoveredbythe
tablish a large database and to separate oscillations of the donorduringeclipsephase.Thepre-eclipsedipoccurslater
magneticaxisandspotmigrations. inphase,i.e.closertotheeclipse,andconsequentlytheac-
cretionregionliesclosertothemeridian.Allthoseexpecta-
Thefinalpartofthepaperisdevotedtoadiscussionof
tionsaremetintheintermediatestate.Hence,thehighand
thepossibleaccretiongeometry.Anaccretionarcisthought
intermediatestatescanbeexplainedwiththesameaccretion
to arise at the foot points of dipolar field lines connecting
geometry.
theballisticaccretionstreamwiththewhitedwarf.Possible
solutionsaresoughtwiththefollowingconstraints:–theac- Thesameargumentsapplyiftheaccretionrateisfurther
cretionspotiscenteredonphase0.95inthehighstateand reduced,i.e.ifonewantstounderstandthegeometryinthe
1.03inthelowstate.Thusthespotlongitudeisψ = +18◦ lowstate.Onemightexpecttoalwaysseesomeremaining
inthehighstateandψ = −10◦ inthelowstate(measured radiation from the magnetic stream lurking at high eleva-
with respect to the line connecting both stars in a coordi- tionsabovetheorbitalplane.Contrarytothisaflateclipse
natesystemcenteredonthewhitedwarf);–theinclination light curve is observed suggesting that matter in the mag-
is between i = 79.3◦ and 83.7◦; – in the high state parts netosphereis completely coveredby the donorstar during
of the accretion stream are never completely obscured by eclipse in the low state. Also, the observedlocation of the
the donor star; – the stagnation region is extending from accretionspot,ψ ≃−10◦isincompatiblewiththecho-
spot
about 35◦ to about 65◦ in azimuth in the high state, and sen orientation of the magnetic axis, ψ = −3◦. If a more
from ∼30◦ to ∼60◦ in the intermediate state (phase inter- extremevalue of ψ is chosen,ψ < 10◦, the high-statege-
val of pre-eclipse dip). The absence of pre-eclipse dips in ometrycannotbeexplained.
www.an-journal.org (cid:13)c 2014WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
8 A.D.Schwopeetal.:Multi-epochtime-resolvedphotometryoftheeclipsingpolarCSS081231:071126+440405
Fig.11 Possible accretiongeometryofCSS081231:071126+440405in a highstate seen atbinaryphases0.9and1.0.
Thegreenlinesillustrateaccretingfieldlines,theyellowlinesaredipolarfieldlinesthroughthemagneticmeridianthus
illustratingtheorientationofthemagneticfield.Theorbitalinclinationis79.3◦,themassratioQ=5andthemassofthe
whitedwarfMWD =0.8M⊙.
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