Multi-colour optical photometry of V404 Cygni in outburst

0.7 MB·

by  Josep Martí

Astronomy&Astrophysicsmanuscriptno.paper (cid:13)cESO2016
January11,2016
Multi-colour optical photometry of V404 Cygni in outburst
JosepMartí1,PedroL.Luque-Escamilla2,andMaríaT.García-Hernández1
1 DepartamentodeFísica,EscuelaPolitécnicaSuperiordeJaén,UniversidaddeJaén,CampusLasLagunillass/n,A3,23071Jaén,
Spain
e-mail:[email protected],[email protected]
2 DepartamentodeIngenieríaMecánicayMinera,EscuelaPolitécnicaSuperiordeJaén,UniversidaddeJaén,CampusLasLagunillas
s/n,A3,23071Jaén,Spain
e-mail:[email protected]
ReceivedXXXX,2015;acceptedXXXX,XXXX
6
1 ABSTRACT
0
2 Context.Thisobservationalpaperhasbeenpreparedinthecontextofthelargemulti-wavelengtheffortbymanyobserverswiththe
aimoffollowingupthetransientflaringeventofV404Cygnithattookplaceforseveralweeksin2015June.
n
Aims.Ourmainoriginalaimwastocontributetothestudyofthistransientsourcebyacquiringbroad-bandphotometricobservations
a
duringitsmostactiveflaringphases.Nevertheless,afteradetailedanalysisofthedata,severalinterestingresultswereobtainedthat
J
encouragedadedicatedpublication.

8 Methods. The methodology used was based on broad-band differential CCD photometry. This outburst of V404 Cygni rendered
thesourceaverybrighttargeteasilywithinreachofsmalleducationaltelescopes.Therefore,the41cmtelescopeavailableatthe

] AstronomicalObservatoryoftheUniversityofJaénwasusedinthiswork.
E Results.Wedetectedvariabilityatdifferenttimescales,bothinamplitudeandcolour.Individualopticalflaresappeareveryhalfhour
H onaverageduringour3hlongobservation,althoughlarge-amplitude(∼1mag)variationsarealsoobservedtooccuronintervalsas
. shortas10minutes.Also,colourvariationsappeartobehighlycorrelatedinacolour-colourdiagram.Anotherremarkablefindingis
h thedetectionoftimelag,fromaboutonetoafractionofaminutebetweenlightcurvesindifferentfilters(VR I ).
c c
p Conclusions.Theobservedbehaviouristentativelyinterpretedinanscenariobasedontheejectionofnon-thermalemitting,relativis-
- ticplasmons,withtheirsynchrotronspectraextendinguptoopticalwavelengths.ThiswouldrendersomeoftheV404Cygniflares
o
verysimilartothoseofthewell-knowmicroquasarGRS1915+105
r
t
s Keywords. X-rays:binaries–Stars:jets–Stars:individual:V404Cygni–Techniques:photometric
a
[

11. Introduction bymanyobserversthatwasquicklyreflectedinanintenseflow
v oftensofrelatedelectronictelegrams.Soon,itbecameevident
1V404 Cygni also known as GS 2023+338 is a low-mass X-ray that this was an extraordinary outburst event after decades of
4
binary (LMXB) originally discovered during its 1989 outburst quiescence.Optical,infrared,radio,X-rayandgamma-raytele-
9
by the Japanese GINGA satellite (Kitamoto et al. 1989). Inten- scopes have so far collected an impressive amount of data that
1
0sive photometric and spectroscopic follow-up of this historical willemergein thescientificliteratureintheupcoming months.
.event yielded an orbital period of about 6.5 d and remarkably But the extraordinary thing was that even small optical tele-
1
concluded with the presence of a massive stellar black hole in scopes could join this endeavor since V404 Cygni became a
0
thesystem(Casaresetal.1992;Wagneretal.1992).Thecom- brightandhighlyvariablesourceatvisiblewavelengths(Hynes
6
panionstarofV404Cygnihasbeenproposedtobeanobjectofa et al. 2015; Hardy et al. 2015; Wiersema 2015; Scarpaci &
1
late-typeK0(±1)III-Vspectraltype(Casaresetal.1993).Radio Maitra2015).
:
vdetectionsusingVeryLongBaselineInterferometry(VLBI)en- In this work, we present the optical data acquired using the
XiabledanaccurateparallaxmeasurementthatplacesV404Cygni educationalastronomicalfacilitiesavailableattheUniversityof
atadistanceof2.39±0.14kpc(Miller-Jonesetal.2009).These Jaén(UJA).Ouraimistocontributetothewealthofpublicastro-
r
aVLBIobservationsalsoconstrainedthesizeofanyquiescentjets nomicaldataaboutthistransienteventwhile,atthesametime,
tolessthan1.4AU.ThequiescentX-rayspectrumhasapower- attemptingtobetterconstrainthenatureofthisphenomenon.
lawphotonindexΓ (cid:39) 2.0seenthroughaatotalcolumndensity
ofN =(1.0±0.1)×1022cm−2(seee.g.Reynoldsetal.(2014)
H
andreferencestherein).Theunabsorbed0.3-10keVluminosity 2. Observations
approachesseveraltimes1032 ergs−1,thusmakingV404Cygni
Theobservationswerecarriedouton26June2015fromtheUJA
thebrightestblackholeLMXBinquiescence.
AstronomicalObservatory.Theobservatoryislocatedinanur-
On16June2015,V404Cygniwasreportedtobeinoutburst banareainsidetheCampusofLasLagunillas,andhostsanau-
bytheSwiftBurstAlertTelescope(Barthelmyetal.2015),and tomated41cmSchmidt-Cassgrainwithf/8focalratio.TheUJA
soonconfirmedbytheMonitorofAll-skyX-rayImageonboard telescope,heareafterUJT,operatesusingaST10-XMEcommer-
theInternationalSpaceStation(Negoroetal.2015).Afterthese cial CCD camera with 2184×1472 pixels of 6.8 µm size. The
early warnings, an intensive observational effort was deployed pixelscaleis0(cid:48).(cid:48)42pixel−1.Thecameraisequippedwithawheel
Articlenumber,page1of9
A&Aproofs:manuscriptno.paper
ofUBVRcIc Johnson-Cousinsfilters(Johnson&Morgan1953; #1 #2 #3 #6
Cousins 1974a,b) manufactured according to the Bessell filter 10,0 #4 #5?
I
prescription(Bessell1979).Theseeingaveragewastypicallyof
c
about2(cid:48)(cid:48),andtherefore2×2binningwasused.
11,0
Differential VR I photometry was performed on V404 de R
c c u
Cygniduring3hwithexposuretimesof60sineachfilter.The nit c
g
totalnumberofmeasurementsacquiredperfilterwasN =51. Ma12,0
obs
Four comparison stars in the field were used whose photomet- V
ricbehaviour,within0.01-0.02mag,wasfoundtobeverystable 13,0
V404 Cygni
in all bands. Their VR I magnitudes were retrieved from the
c c
AAVSOdatabase1,andaregiveninOnlineTable1.Photometry 14,0
57199,92 57199,94 57199,96 57199,98 57200,00 57200,02 57200,04
inU and Bbandswasnotacquiredbecauseofverylowsource
counts. For each observed photometric band, here generically
11,0
indicated by an effective wavelength λ, the magnitudes of the
variabletargetmvar werederivedwithrespecttothecomparison de Comparison stars
λ u
starsaccordingto: nit12,0
g
a
mvar =mcom+∆m +T ∆C , (1) M
λ λ λ,ins λ,Cλ λ,ins
wheremcom isthemagnitudeofthecomparisonstarbeingused, 13,0
λ
and∆m and∆C aretheinstrumentaldifferencesinmagni-
λ,ins λ,ins
57199,92 57199,94 57199,96 57199,98 57200,00 57200,02 57200,04
tudeandcolourbetweenthetargetandcomparisonstar,respec-
Modified Julian Date
tively. The factor T is the colour transformation coefficient
λ,Cλ Fig.1.Top.LightcurvesofV404Cygniinoutburstasobservedwith
that was separately determined using standards in clusters and
theUJTon26June2015intheV,RandIbands.Flaresarelabelledfor
other fields. The colour (V −R ) was used in Eq. 1 for V- and
c clarityusingverticalarrows.Bottom.Behaviourofthefourcomparison
R -bandobservations,whilethecolour(V−I )waspreferredfor
c c stars,plottedatthesamescale,whichremainedconstantinbrightness
Ic-bandobservationsinstead.Thesystemphotometricproperties within0.01-0.02mag.
are approximately stable from night to night and we typically
obtain|T | < 0.1.Finally,thedifferentmeasurementsofmvar
λ,Cλ λ 3,0
wereweightedaccordingtotheirrespectiveuncertaintyandthen V-I
V404 Cygni
averaged.ThecorrespondingresultsarepresentedinOnlineTa- c
2,5
ble2andFigs.1and2.Theerrorsquotedherearerepresentative
of the differential photometry process, and do not include the mag)
uncertainty in the absolute calibration of the comparison stars ex (2,0 V-R
t(o∼60f.0o2rlmataegr)d.iMscuaisnsioflna.ringeventsinFig.1arelabelledfrom1 or Ind1,5 c
ol
C
1,0
R -I
3. Discussion
c c
The light curves in Fig. 1 and 2 clearly show how V404 Cygni 0,5
57199,92 57199,94 57199,96 57199,98 57200,00 57200,02 57200,04
varied and flared intensively during the observation. The am-
plitude of variation was as large as one magnitude and several Comparison stars
g)1,00
tenths of a magnitude in brightness and colour, respectively. a
m
Thesevariationsoccurredontimescalesasshortas10minutes. x (
e
The significance and intrinsic origin of these flares is ensured nd0,50
given that the comparison stars remained practically flat within or I
ol
a few hundreds of a magnitude. This agrees well with the be- C0,00
haviourreportedbydifferentobserversatothertimesduringthe
intensivecoverageoftheoutburst(e.g.Hynesetal.2015).From 57199,92 57199,94 57199,96 57199,98 57200,00 57200,02 57200,04
causality arguments, our data puts a coarse upper limit of ∼ 1 Modified Julian Date
AU to the region from where the optical emission arises in the Fig.2.Top.VariabilityofcolourindicesV−R,V−IandR−IofV404
vicinityoftheaccretiondiskaroundtheV404Cygniblackhole. CygniinoutburstasobservedwiththeUJTon26June2015.Theyare
Itisalsoveryinterestingthattheobservedvariabilityappar- shownplottedinblack,red,andmagentacolours,respectively.Bottom.
ently followed a clear pattern in colour-colour diagrams. This Thesamekindofplotforthefourcomparisonstarsplottedatthesame
scale.Theircoloursremainedconstantwithin0.02-0.03mag.
is illustrated in Fig. 3 where the pattern becomes more evident
asthesourceevolvedinaveryrestrictedregionofthediagram.
A simple regression fit yields a correlation coefficient as high
Theintrinsicamplitudesoftheobservedflaresbecomemore
as 0.89, thus suggesting that a strong connection between the
obvious when de-reddening the photometric observations, and
source colours existed during the flaringevents. The modelling
expressingthemintermsoffluxdensity.Theinterstellarextinc-
ofsuchbehaviourisbeyondtheobservationalscopeofthispa-
tion law in Mathis (2000) was used for de-reddening purposes.
per,andwelimitourselvesheretostatethisobservationalfact.
This is presented in Fig. 4, where two isolated flares (#3 and
Nevertheless, we speculate that it could be intimately linked to
#4)areeasilyvisiblewiththeirpeaksseparatedbynearlyahalf
thesourcespectralevolutiondiscussedbelow.
hour.Otherflaringeventsbothatthebeginning(#1and#2)and
1 American Association of Variable Stars Observers, the end (#5 and #6) of the observation are also present within
http://www.aavso.organdreferencestherein. comparable time intervals. However, their individual evolution
Articlenumber,page2of9
JosepMartí etal.:Multi-colouropticalphotometryofV404Cygniinoutburst
derstoodinthiscontext.ThesimilaritybetweenV404Cygniand
GRS1915+105hasalreadybeennotedbyMooleyetal.(2015).
2,8 The observed optical flares appeared superimposed onto an
averagepedestalemissionlevellikelyduetohotaccretiondiskin
V404Cygni.FromFig.4,theaveragede-reddenedfluxdensities
werefoundtobe2.27±0.16,2.02±0.11,and1.10±0.04Jyinthe
V,R and I bands,respectively.Byfittingasimplepowerlaw,
2,6 c c
the average spectral energy distribution (SED) of V404 Cygni
)
ag dependsonfrequencyroughlyas∝ν2.1±0.3.Nocorrectionforthe
m
(c contributionoftheHαemissionlinetoRc-banddatahasbeenat-
-I tempted here. The resulting power law index is consistent with
V
2,4 theUJTdatasamplingtheRaleighJeansν2partoftheaccretion
diskspectrum.Extrapolationofthispowerlawtotheradioband
falls orders of magnitude below the reported radio flux densi-
ties.Therefore,anadditionalemissioncomponentmustexistto
explaintheTsubonoetal.(2015)radiodetection.Asstatedbe-
2,2 fore,thiscomponentismostnaturallyinterpretedasoriginating
inplasmaejectionevents.
Inthescenariooutlinedhere,onewouldexpectthepeaktime
1,20 1,30 1,40 1,50 1,60 ofanindividualflaretobedependentonthewavelengthofob-
V-R (mag) servation.AccordingtothesimplevanderLaan(1966)modelof
c
a synchrotron-emitting expanding plasmon, the light curve ob-
Fig. 3. Evolution of V404 Cygni in the colour-colour plane during a
servedatawavelengthλreachesitsmaximumafteratimegiven
fewhourson26June2015asobserverwiththeUJT.Thedottedline
by
isalinearregressionfitguidingtheeyetobetterappreciatethecolour-
colourcorrelationsuggestedinthetext.
tm,λ =tm,λ0[λ/λ0]−4pp++46, (2)
where t is the time of maximum at a reference wavelength
V m,λ0
λ , and p is the power-law index of the energy distribution of
4000 0
R relativisticelectrons.
c
Intheabsenceofcontemporaneoushightimeresolutionra-
dio photometry, where a λ-dependent delay would be easier to
y)3000
mJ measure, we searched our optical data for possible time lags
nsity ( I between the different light curves at V, Rc and Ic bands. Tak-
Flus De2000 c i5n7g1,9f9o.r9i6n5stianncFei,gtsh.e1isaonladte4d,flthaerer#is3i,nwghtiimchepferaokmsathroeupneddMesJtaDl
emission level to the flare peak in the V-band is t (cid:39) 0.01 d.
m,V
Assuming p (cid:39) 1.0, and given that the central effective wave-
1000 #1 #2
lengthsofourphotometricfilerset(λ =0.545µm,λ =0.641
#3 #4 #5? #6 µm,andλIc =0.798µm),wecanestimV atetheexpecteRdctimede-
0 laysintheUJTobservations.UsingEq.2,andtakingtheV band
57199,92 57199,94 57199,96 57199,98 57200,00 57200,02 57200,04
asreference,thepredicteddelaysare1.2minand3.0minforthe
Modified Julian Date
R and I band, respectively. Similarly, a 1.8 min delay would
Fig.4.DereddenedlightcurvesofV404Cygniinoutburstasobserved c c
beexpectedbetweenI andR data.Assumingotherreasonable
withtheUJTon26June2015intheV,R and I bandsassumingan c c
interstellarextinctionofA = 4.0mag.Thcebrighctnesslevelhasbeen valuesof pdoesnotchangesignificantlythesenumbers.Forin-
convertedfrommagnitudesVtoafluxdensityscale,inmJyunits.Flares stance,taking p = 2.5onegetsdelaysof1.0min,2.4min,and
arelabelledasinFig.1. 1.4min,respectively.
What happens with real data? To answer this question, we
performed a cross-correlation function (CCF) exercise between
cannot beso easily disentangledbecause they areclose in time the different UJT light curves. A problem arises here because
or not well sampled. By analogy with other similar flaring be- thedataarenotevenlyspacedintime.TheCCFcalculationwas
haviours, such as in the LMXB GRS 1915+105 at radio and thencarriedoutintwodifferentways.Thefirstcalculationwas
near-infraredwavelengths(Mirabeletal.1998),oneistempted speciallydesignedforsuchirregulartimeseriesandisbasedon
tointerpretthisrecursiveflarepatternasepisodesofreplenish- averagingdataproductswithsimilartimelagswithinauserde-
ment and emptying of the inner accretion disk. These kind of finedbinning(Edelson&Krolik1988).Thesecondcalculation
events come accompanied by the ejection of plasmons along simplymakesalinearinterpolationtoobtainaregularsampling
collimated jets as a result of accretion disk instabilities. These of the data from which the traditional estimator of the CCF is
plasma clouds typically have a characteristic non-thermal, syn- computed.Inourcase,thissecondapproachisjustifiedbecause
chrotronemissionmechanism.Theirspectrumspansfromradio ourobservationswerenearlyevenlyspaced.Whenapplyingthe
tomuchshorterwavelengthsdependingontheenergycut-offof first method, the resulting CCFs do not have enough resolution
relativisticelectronsandthestrengthofmagneticfield.Thefact to clearly establish a non-zero time lag although hints of delay
thatstrongradioemissionfromV404Cygniwasdetectedin1.4 betweenthelongerandshorterwavelengthfilterswereobtained.
GHzobservationscarriedoutjustsixhoursbeforeours,peaking Incontrast,theinterpolationmethodprovidedwell-definedCCF
at0.364±0.030Jy(Tsubonoetal.2015),wouldbenaturallyun- maximashiftedfromzero(OnlineFigs.5and6).Here,thekey
Articlenumber,page3of9
A&Aproofs:manuscriptno.paper
factisthatallmaximaoccurwithaclearlynegativelag,i.e.con- predict S (cid:39) 1260 mJy and S (cid:39) 1010 mJy. Our R and
m,Rc m,Ic c
sistentwiththelongerwavelengthlightcurvesthataredelayed I lightcurvesinFig.4werenotwellsampledtoappropriately
c
with respect to the shorter wavelength curves, as expected. In- catchtheirmaxima.Withthepedestalfluxdensityremovedthey
terestingly,similarnegativetimelagshavealsobeenobservedin provide S ∼> 1020 mJy and S ∼> 650 mJy, respectively.
someoftheV404CygniflaresseenbytheINTEGRALsatellite Whilethemex,Rpcectedvaluesarenotemx,aVctlyreproduced,theresults
(Rodriguezetal.2015),wheretheopticalV-bandemissionwas arestillcompatible.
delayedwithrespecttohardX-raysandsoftγ-raysfrom1.5to Alltogether,weconsiderthatthisfindingissupportiveofour
20-30min.However,theselagscorrespondtoextremelydiffer- tentativeinterpretationoftheopticalflaresasduetonon-thermal
ent energy bands in contrast to our results here reported within plasmonejectionevents.Similarflaresintheprototypicalcaseof
theopticaldomain. GRS1915+105couldnotbeobservedintheopticalbecauseof
Themeasuredoffsetofthemaximawithrespecttotheorigin thehigherinterstellarextinction.
(inOnlineFigs.5and6)indicatesthattheIc bandlightcurveis In addition, the CCFs in Online Fig. 5 display clear sec-
delayedwithrespecttoV bandby108±1s,whiletheRc band ondary maxima corresponding to time lags of ∼ 1500 s. This
lagsitby34±1s.Concerningthetworeddestfilters,Icbandalso agrees with the idea, already expressed before, that the optical
lagstheRcbandby34±1s.Theuncertaintiesintheestimatesof light curve of V404 Cygni in outburst during our observation
theCCFpeakpositionswerederivedusingtheWhite&Peterson consistsofthesuperpositionofflaresthatappeareveryhalfhour
(1994)formula on average. Flaring behaviour with a similar timescale is also
0.38W present in optical light curves reported by other observers just
∆τ= 1+r (N CC−F 2)1/2, (3) beforeandafterourUJTobservations(Scarpaci&Maitra2015;
max total Scarpacietal.2015).
whereWCCF isthefullwidthathalfmaximumoftheCCFpeak The energy involved in each ejection event can also be de-
withrmax amplitude,andNtotal =(Nobs−1)N thefullnumberof rivedbasedonsimpleequipartitionargumentsusingthePachol-
available points, where N is the amount of sampling points. In czyk(1970)formulation.LookingatFig.4,theflareincremen-
our case, WCCF (cid:39) 1000 s, rmax (cid:39) 1, and we interpolated up to talfluxdensitywithrespecttothepedestalvalueisintherange
N =104points. ∼0.5-1.0Jy,andthisoccurstypically∼103saftertheestimated
Althoughthesenumbersarenotidenticaltothevaluesanti- onsetoftheflares.Suchfluxdensityincrementsarecomparable
cipatedusingEq.2,allofthemappeartobeintheexpectedsense tothe1.4GHzemissionlevelsreportednearthetimeofourob-
and with the expected order of magnitude. Such agreement is servationswithinafactorofafew.Thisrendersthepossibilityof
remarkablegiventhesimplicityoftheVanderLaanmodeland anearlyflatsynchrotroncomponentfromradiotoopticalwave-
thelimitationofourdata. lengthsconceivable.Assuminganexpansionvelocityof∼0.5c,
Inordertoassesstherobustnessofourfinding,weperformed theplasmonsizeatthetimeofmaximumwouldbe∼1.5×1013
asensitivityanalysisofhowinterpolatedsamplingaffectsthere- cm.Thisisequivalentto0.4milli-arcsecattheV404Cygnidis-
sults. As seen in Online Fig. 7, the derived lags always reach a tanceandconsistentwithprevioussizeupperlimits.Usingthis
constant level after a sufficiently large number of interpolated dimension,theradio/opticalfluxdensitiesquotedbefore,andas-
points. Computing the rms dispersion of the latest 1000 points sumingrelativisticelectronswithLorentzfactorγ≤104,theto-
providesvaluesof∼1scomparabletotheuncertaintyestimates talenergycontentisestimatedtobe2×1040erg,withamagnetic
derived with Eq. 3. Online Fig. 7 also shows that, even with field of 12 G. The source brightness temperature does not ex-
coarseinterpolation,theV vsIctimelagstillremainssignificant ceedafew1012 K.Underallsuchassumptions,thecorrespond-
althoughatalowerlevel.TheVvsIctimelagamountsto86±16 ing non-thermal luminosity, from the radio to optical domain,
sforN = 10interpolatedsamplingpoints.Theotherfiltercom- amountsto2.2×1036 ergs−1,whichimpliesasynchrotronlife-
binations,V vsRc andRvs Ic,whicharecloserinwavelength, timeofabout2hforindividualflaringevents.Thisisonlyfour
remainconsistentwithzerobothamountingto22±16s. timestheirobservedrecurrenceinterval,thusflaresuperposition
Moreover, to further ensure the confidence on the lag esti- shouldbecomenegligibleafterafewflaringevents.Finally,the
mates with interpolated light curves, we conducted a series of plasmonmassinvolvedintheejectionisfoundtobe∼2×1020g
simulations by cross-correlating the observed light curves with providedthatthereisoneprotonperrelativisticelectron.These
themselves after introducing different artificial time lags (see numbersdonotdifferbymorethanoneorderofmagnitudewhen
OnlineTable3).Ourprocedurewasabletoconfidentlyrecover comparedwiththereferencecaseofGRS1915+105.
thesetimelags,withinafewseconds,butalwayswithasystem-
aticnegativebiasthatresultsasanintrinsiceffectoftheinterpo-
4. Conclusions
lationmethodused.Inourcasethisisnotexpectedtointroduce
asevereeffectbecauseofthelackofskewnessinthetimesam- We have reported new optical photometric data for the LMXB
plingdistribution(Rehfeldetal.2011).Therefore,themeasured V404Cygniduringits2015Juneoutburstobtainedwithasmall
lagvaluesquotedaboveshouldbeactuallyinterpretedaslower educational telescope. The time interval covered by our obser-
limits.Thesetwoconsistencychecksreinforceourconfidencein vations complements and contributes to the multi-wavelength
the lag detection provided that interpolation is actually a good campaigncarriedoutasaworld-wideeffortbymanyobservers.
approximation. From the analysis of our data alone, some interesting findings
Another test that could be performed is the dependence of canalreadybeadvancedandwesummarizethemasfollows.
the maximum flux densities Sm,λ as a function of wavelength. The UJT optical light curves consisted of consecutive and
FollowingvanderLaan(1966),theexpecteddependenceis: partially overlapping flaring events, appearing about every half
Sm,λ =Sm,λ0[λ/λ0]−74pp++36, (4) hvaoruirabainlidtywaisthshvoerrtyalsa1rg0emaminpultietsudcoesul(d∼be1cmleaagr)l.yTdiemteecstceadlebsoothf
Consideringthesameisolatedflareasabove,andremovingthe in brightness and colour. The optical variability observed ap-
pedestalfluxdensitywithasimplelinearfit,weestimatetheV- pearstodisplayacorrelationpatterninthecolour-colourplane,
bandmaximumasS (cid:39)1480mJy.For p=1,onewouldthen whoseinterpretationwilldeservefuturetheoreticalwork.
m,V
Articlenumber,page4of9
JosepMartí etal.:Multi-colouropticalphotometryofV404Cygniinoutburst
Thede-reddenedopticalcontinuumhadanaveragepositive
spectralindexclosethe+2value.Thus,formostofthetimethe
observed photometric bands (VR I ) were mainly sampling the
c c
Rayleigh Jeans part of the accretion disk spectrum. The V404
Cygni flares, superposed on this continuum, are tentatively in-
terpretedasnon-thermalflaringevents.Withallcaution,wesug-
gestthattheflaresareduetorelativisticplasmonsejectedfollow-
ing successive replenishing and emptying episodes of the inner
accretiondisk.Thefactthatnon-thermal,synchrotronemission
appearstocontributetoopticalwavelengthsrendersV404Cygni
averyinterestingsystemtobetterstudytheenergeticsofblack
holeLMXBs.
From a CCF analysis, we have found a systematic time lag
betweentheUJTopticallightcurvesacquiredwiththedifferent
filters during our observations. These lags are such that emis-
sion at shorter wavelengths precedes emission at longer wave-
lengths by ∼ 1 min. This finding was most evident when light
curves were resampled and evenly interpolated, which we be-
lieveisareasonableapproachwhendealingwithnearlyevenly
spaceddata.Thisdelayisinqualitativeagreementwithasimple
Van der Laan model for the expansion of a synchrotron emit-
tingplasmon,andtherelativeamplitudeofflaremaximaisalso
consistent. An estimate of the plasmon physical parameters re-
quiredforthenon-thermalspectrumtoextendfromradiotoop-
ticalwavelengthsappearstobesimilartootherwell-knownsys-
tems,suchasGRS1915+105.
Acknowledgements. ThisworkwassupportedbygrantAYA2013-47447-C3-3-
PfromtheSpanishMinisteriodeEconomíayCompetitividad(MINECO),and
bytheConsejeríadeEconomía,Innovación,CienciayEmpleoofJuntadeAn-
dalucíaunderresearchgroupFQM-322,aswellasFEDERfunds.
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Articlenumber,page5of9
A&A–paper,OnlineMaterialp6
40
V vs R
1 V & I
c
30
0,8
20
10 100 1000 10000
F (normalized) 00,,46 R & I V & R Time lag (s)19000 V vs I
CC 0,2 c c c 8010 100 1000 10000
40
R vs I
0
30
-0,2
20
-3000 -2000 -1000 0 1000 2000 3000 10 100 1000 10000
Time lag (s) Number of sampling points
Fig.5. CCFsoftheopticallightcurvesV vsR (black),V vsI (blue) Fig.7. Dependenceofthederivedtimelagsusingthecross-correlation
c c
andR vsI (red).Theverticaldashedlinecorrespondstozerotimelag. techniqueontheamountofinterpolatedsamplingofthedata.Colours
c c
TheV vsR CCFisnormalizedtounitywhiletheothertwohavebeen areasinFigs.5and6.
c
slightlyscaledaboveandbelowforeasierdisplay.
V & I
c
1
d) V & R
malize0,8 Rc & Ic c
or
n
F (
C
C
0,6
0,4
-600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600
Time lag (s)
Fig.6. ZoomedviewofthecentralmaximaoftheCCFswhereaclear
asymmetryisvisible.ColoursandnormalizationareasinOnlineFig.
5.
A&A–paper,OnlineMaterialp7
Table1.AAVSOcomparisonstarsusedinthiswork.
AAVSO R.A.(J2000.0) DEC.(J2000.0) B V R I
c c
Id. (hms) (dms) (mag) (mag) (mag) (mag)
125 20:23:56.47 +33:48:16.9 12.9760.019 12.4970.014 12.2040.018 11.9260.018
128 20:24:07.24 +33:50:52.2 13.4990.017 12.8150.014 12.3840.018 11.9670.018
132 20:24:08.89 +33:54:38.6 13.8790.018 13.1640.012 12.7120.016 12.2840.016
134 20:23:53.43 +33:52:24.6 14.4670.017 13.3610.015 12.7370.019 12.2030.019
A&A–paper,OnlineMaterialp8
Table2.OpticalphotometryofV404CygniwihttheUJTtelescope.
MJD∗ V-band MJD∗ R -band MJD∗ I -band
c c
(mag) (mag) (mag)
57199.91379 12.842±0.012 57199.91456 11.488±0.007 57199.91532 10.468±0.009
57199.91610 12.825±0.013 57199.91686 11.476±0.008 57199.91763 10.299±0.010
57199.91840 12.547±0.011 57199.91916 11.258±0.008 57199.91993 10.231±0.012
57199.92070 12.613±0.013 57199.92146 11.325±0.007 57199.92223 10.222±0.009
57199.92478 12.797±0.014 57199.92554 11.476±0.006 57199.92631 10.401±0.009
57199.92708 12.731±0.015 57199.92785 11.364±0.006 57199.92861 10.309±0.010
57199.92938 12.707±0.013 57199.93015 11.354±0.006 57199.93091 10.280±0.009
57199.93169 12.688±0.012 57199.93245 11.402±0.010 57199.93322 10.438±0.010
57199.93399 12.912±0.012 57199.93476 11.589±0.008 57199.93552 10.568±0.012
57199.93629 12.948±0.016 57199.93706 11.640±0.006 57199.93782 10.598±0.011
57199.93860 13.013±0.018 57199.93936 11.655±0.007 57199.94013 10.639±0.010
57199.94090 13.095±0.013 57199.94167 11.739±0.009 57199.94243 10.808±0.009
57199.94320 13.241±0.016 57199.94397 11.814±0.008 57199.94473 10.779±0.009
57199.94551 13.241±0.017 57199.94627 11.881±0.008 57199.94704 10.847±0.011
57199.94781 13.455±0.014 57199.94858 12.172±0.010 57199.94934 11.194±0.010
57199.95012 13.539±0.018 57199.95088 11.944±0.008 57199.95165 10.901±0.012
57199.95252 13.401±0.018 57199.95328 12.031±0.007 57199.95405 11.060±0.012
57199.95482 13.480±0.018 57199.95559 11.980±0.009 57199.95635 10.966±0.009
57199.95712 13.391±0.013 57199.95789 11.900±0.008 57199.95866 10.841±0.010
57199.95943 13.194±0.014 57199.96019 11.794±0.007 57199.96096 10.708±0.011
57199.96173 12.977±0.011 57199.96250 11.584±0.008 57199.96326 10.517±0.010
57199.96404 12.920±0.011 57199.96480 11.724±0.007 57199.96557 10.796±0.010
57199.96634 13.333±0.013 57199.96711 12.024±0.006 57199.96787 11.065±0.009
57199.96865 13.631±0.016 57199.96941 12.102±0.006 57199.97018 11.086±0.011
57199.97095 13.608±0.020 57199.97172 12.271±0.007 57199.97248 11.285±0.015
57199.97326 13.677±0.021 57199.97402 12.126±0.007 57199.97479 11.068±0.012
57199.97556 13.444±0.014 57199.97632 12.157±0.007 57199.97709 11.163±0.011
57199.97786 13.500±0.016 57199.97863 12.050±0.006 57199.97939 10.966±0.009
57199.98017 13.354±0.013 57199.98093 11.995±0.007 57199.98170 10.952±0.012
57199.98247 13.166±0.013 57199.98324 11.801±0.007 57199.98400 10.695±0.011
57199.98478 13.059±0.014 57199.98554 11.872±0.007 57199.98631 10.818±0.009
57199.98708 13.235±0.011 57199.98785 11.862±0.007 57199.98861 10.838±0.010
57199.98938 13.383±0.018 57199.99015 12.152±0.006 57199.99091 11.224±0.010
57199.99169 13.720±0.014 57199.99245 12.235±0.007 57199.99322 11.233±0.013
57199.99399 13.696±0.014 57199.99476 12.250±0.007 57199.99552 11.074±0.014
57199.99629 13.354±0.012 57199.99706 11.845±0.007 57199.99782 10.727±0.011
57200.00593 13.287±0.018 57200.00669 12.022±0.007 57200.00746 11.032±0.012
57200.00823 13.317±0.012 57200.00900 11.992±0.006 57200.00976 10.870±0.011
57200.01054 13.285±0.011 57200.01130 11.961±0.006 57200.01207 10.955±0.012
57200.01284 13.297±0.013 57200.01361 11.931±0.008 57200.01437 10.941±0.012
57200.01514 13.457±0.012 57200.01591 12.160±0.007 57200.01667 10.994±0.011
57200.01745 13.291±0.013 57200.01821 11.941±0.006 57200.01898 10.945±0.013
57200.01975 13.588±0.016 57200.02052 12.265±0.007 57200.02128 11.256±0.013
57200.02205 13.550±0.021 57200.02282 11.984±0.007 57200.02358 10.745±0.014
57200.02435 12.914±0.014 57200.02512 11.559±0.008 57200.02588 10.425±0.012
57200.02665 12.755±0.009 57200.02742 11.457±0.007 57200.02818 10.406±0.013
57200.02896 12.827±0.014 57200.02972 11.436±0.006 57200.03049 10.337±0.014
57200.03126 12.646±0.016 57200.03203 11.378±0.006 57200.03279 10.406±0.016
57200.03357 12.986±0.011 57200.03433 11.741±0.010 57200.03510 10.787±0.016
57200.03587 13.294±0.015 57200.03663 11.853±0.008 57200.03740 10.811±0.016
57200.03817 13.249±0.015 57200.03894 11.916±0.008 57200.03970 10.951±0.021
(∗)TheModifiedJulianDate(MJD)givenisheliocentricandcorrespondstothemid-exposuretime.
A&A–paper,OnlineMaterialp9
Table3.RecoveredartificialtimelagswithinterpolatedsamplingCCFforV-banddata.
Artificialtimelag Recoveredtimelag Artificialtimelag Recoveredtimelag
(s) (s)
0 −0.02 60 52.92
1 −0.02 70 62.85
5 −0.02 80 72.80
10 3.22 90 82.73
20 13.15 100 92.66
30 23.08 110 102.59
40 33.06 120 112.52
50 42.99 1000 971.77