Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006--2007 outburst

1.5 MB·English

by  C. M. Raiteri

Astronomy&Astrophysicsmanuscriptno.outburst (cid:13)c ESO2008
February2,2008
Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift
⋆
during the 2006–2007 outburst
C.M.Raiteri1,M.Villata1,V.M.Larionov2,3,M.F.Aller4,U.Bach5,M.Gurwell6,O.M.Kurtanidze7,
A.La¨hteenma¨ki8,K.Nilsson9,A.Volvach10,H.D.Aller4,A.A.Arkharov3,R.Bachev11,A.Berdyugin9,
M.Bo¨ttcher12,C.S.Buemi13,P.Calcidese14,E.Cozzi15,A.DiPaola16,M.Dolci17,J.H.Fan18,E.Forne´19,
L.Foschini20,A.C.Gupta21,V.A.Hagen-Thorn2,L.Hooks12,T.Hovatta8,M.Joshi12,M.Kadler22,
8 T.S.Konstantinova2,A.Kostov23,T.P.Krichbaum24,L.Lanteri1,L.V.Larionova2,C.-U.Lee25,P.Leto26,
0
E.Lindfors9,F.Montagni27,R.Nesci28,E.Nieppola8,M.G.Nikolashvili7,J.Ohlert29,A.Oksanen30,E.Ovcharov31,
0
2 P.Pa¨a¨kko¨nen32,M.Pasanen9,T.Pursimo33,J.A.Ros19,E.Semkov11,R.L.Smart1,A.Strigachev11,L.O.Takalo9,
K.Torii34,I.Torniainen8,M.Tornikoski8,C.Trigilio13,H.Tsunemi34,G.Umana13,andA.Valcheva11,31
n
a (Affiliationscanbefoundafterthereferences)
J

8

] ABSTRACT
h
p Context.The blazar AO 0235+164 was claimed to show a quasi-periodic behaviour in the radio and optical bands in the past, with the main
- outbursts repeating every 5–6 years. However, the predicted 2004 outburst did not occur, and further analysis suggested a longer time scale,
o
accordingtowhichthenexteventwouldhaveoccurredinthe2006–2007observingseason.Moreover,anextraemissioncomponentcontributing
r
t totheUV and soft X-rayfluxwasdetected, whosenature isnot yet clear. Anoptical outburst wasobserved inlate2006 – early2007, which
s
triggeredaWholeEarthBlazarTelescope(WEBT)campaignaswellastargetofopportunity(ToO)observationsbytheSwiftsatellite.
a
Aims.Inthispaper,wepresenttheradio-to-opticaldatatakenbytheWEBTtogetherwiththeUVdataacquiredbytheUltraVioletandOptical
[
Telescope(UVOT)instrumentonboardSwifttoinvestigateboththeoutburstbehaviouratdifferentwavelengthsandthenatureoftheextraemission

1 component.
v Methods.Multifrequencylightcurveshavebeenassembledwithdatafrom27observatories;opticalandUVfluxeshavebeencleanedfromthe
6 contaminationofthesouthernactivegalacticnucleus(AGN).Wehaveanalysedspectralenergydistributionsatdifferentepochs,correspondingto
3 differentbrightnessstates;extraabsorptionbytheforegroundgalaxyhasbeentakenintoaccount.
2 Results.Wefoundtheopticaloutbursttobeasstrongasthebigoutburstsofthepast:startingfromlateSeptember2006,abrightnessincreaseof
1 ∼ 5magledtotheoutburstpeakinFebruary19–21,2007.Wealsoobservedanoutburstatmmandthenatcmwavelengths,withanincreasing
. timedelaygoingtowardlowerfrequenciesduringtherisingphase.Cross-correlationanalysisindicatesthatthe1mmand37GHzfluxvariations
1
laggedbehindtheR-bandonesbyabout3weeksand2months,respectively.Theseshorttimedelayssuggestthatthecorrespondingjetemitting
0
regionsareonlyslightlyseparatedand/ormisaligned.Incontrast,duringtheoutburstdecreasingphasethefluxfadedcontemporaneouslyatallcm
8
wavelengths.Thisabruptchangeintheemissionbehaviourmaysuggestthepresenceofsome“shutdown”mechanismofintrinsicorgeometric
0
nature.ThebehaviouroftheUVfluxcloselyfollowstheopticalandnear-IRone.Byseparatingthesynchrotronandextracomponentcontributions
:
v totheUVflux,wefoundthattheycorrelate,whichsuggeststhatthetwoemissionshaveacommonorigin.
i
X Keywords.galaxies:active–galaxies:BLLacertaeobjects:general–galaxies:BLLacertaeobjects:individual:AO0235+164–galaxies:jets–
galaxies:quasars:general
r
a
1. Introduction data acquired by the XMM-Newton satellite during 3 point-
ings in January and August 2004, and January 2005, and by
AO 0235+164 at redshift z = 0.94 is one of the best-studied
optical spectroscopic observations with the 3.6 m Telescopio
BL Lac objects. The analysis of its radio and optical light
NazionaleGalileo(TNG).Theresultsofthisintenseobserving
curvesextendingover 25 years led Raiterietal. (2001) to sug- effortwerepublishedby(Raiterietal.2005,2006a,b,2007a;see
gest a quasi-periodic occurrence of the main outbursts ev-
alsoHagen-Thornetal.2007bforananalysisofcolourvariabil-
ery 5.7 ± 0.5 years. The next outburst was predicted around
ity). The predicted outburst was not observed, and time-series
February–March 2004, and a multiwavelength campaign was
analysisonthelightcurvesextendedto2005revealedapossible
organised by the Whole Earth Blazar Telescope (WEBT)1 to characteristicvariabilitytimescaleof∼ 8years.Moreover,the
followtheexpectedevent.Theradio-to-opticalobservationsby
XMM-Newtonobservationssuggestedthe presenceof an extra
the WEBT were complemented by the optical–UV and X-ray
emission component in the source spectral energy distribution
(SED),inadditiontothesynchrotronandinverse-Comptonones.
Sendoffprintrequeststo:C.M.Raiteri Theoriginofthiscomponent,peakingintheUV/softX-rayfre-
⋆ The radio-to-optical datapresented inthispaper arestored inthe quencyrange,was ascribed either to thermalemission froman
WEBTarchive;forquestionsregardingtheiravailability,pleasecontact accretiondisc,or to synchrotronemissionfroman innerjetre-
theWEBTPresidentMassimoVillata([email protected]).
gion.
1 http://www.to.astro.it/blazars/webt/
(seee.g.Villataetal.,2006,2007;Boettcheretal.,2007;Raiterietal., An increased radio activity was detected in the 2005–2006
2007b) observing season (Bachetal., 2007), and a dramatic optical
2 C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst
brighteningwasfinallyobservedinlate2006–early2007.This Table1.Ground-basedobservatoriesparticipatinginthiswork.
triggered a new WEBT multiwavelength campaign as well as
ToOobservationsbytheSwiftsatellite.Inthispaper,wepresent Observatory Tel.size Bands
the radio-to-optical observations performed from spring 2005 Radio
(i.e. the end of the period considered in Raiterietal. 2006a) Crimean(RT-22),Ukraine 22m 22,37GHz
to October 2007, and the data acquired by the UltraViolet and MaunaKea(SMA),USA 8×6ma 0.85,1mm
Optical Telescope (UVOT) instrument onboard Swift. X-ray Medicina,Italy 32m 5,8,22GHz
Metsa¨hovi,Finland 14m 37GHz
data from the Swift X-ray Telescope (XRT) and Burst Alert
Noto,Italy 32m 43GHz
Telescope(BAT)instrumentswillbepresentedinanotherpaper
UMRAO,USA 26m 5,8,14.5GHz
(Kadleretal.,inpreparation).
Near-infrared
This paper is organised as follows: the procedures we CampoImperatore,Italy 110cm J,H,K
adopted to treat the WEBT and UVOT data are outlined in Roque(NOT),Spain 256cm J,H,K
Sect. 2, with particular attention paid to the subtraction of the Optical
southernAGN contributionfrom the opticaland UV photome- Abastumani,Georgia 70cm R
try.Themultifrequencylightcurvesarepresentedanddiscussed Crimean,Ukraine 70cm B,V,R,I
in Sect. 3 while in Sect. 4 time lags among variations at dif- Greve,Italy 32cm R
Hankasalmi,Finland 40cm V,R,I
ferent frequencies are derived by means of statistical analysis.
Jakokoski,Finland 50cm R
Spectralenergydistributionswithsimultaneousdatafromnear-
KittPeak(MDM),USA 130cm B,V,R,I
IRtoUV areconstructedin Sect.5,with theaimofseparating
L’Ampolla,Spain 36cm R
the synchrotronand the extra emission componentsand of un-
MichaelAdrian,Germany 120cm R
derstanding their relationship. Finally, the results of our work NewMillennium,Italy 36cm R
arediscussedinSect.6. Roque(KVA),Spain 35cm R
Roque(NOT),Spain 256cm U,B,V,R,I
Rozhen,Bulgaria 50/70cm B,V,R,I
Sabadell,Spain 50cm R
2. Datareductionandanalysis
Sobaeksan,SouthKorea 61cm V,R,I
St.Petersburg,Russia 40cm V,R,I
2.1.Radio-to-opticalobservations
Torino,Italy 105cm R,I
Toyonaka,Japan 35cm B,V,R,I
Thenew WEBT campaignonAO 0235+164saw a wide inter-
Tuorla,Finland 103cm R
nationalparticipation.Datawerecollectedstartingfromtheend
Valled’Aosta,Italy 81cm V,R,I
oftheperiodconsideredinRaiterietal.(2006a),i.e.fromspring Yunnan,China 102cm R
2005toOctober2007.Table1showsthelistofparticipatingob- aRadiointerferometerincluding8dishesof6msize.
servatories,thesizeofthetelescopes,andthebands/frequencies
atwhichweacquiredthedata.
We collectedopticalandnear-IRdata asinstrumentalmag- 2.3.CorrectionforELISAcontribution
nitudes of the source and comparison stars in the same field.
A major point in the analysis of the optical and UV radiation
Magnitudecalibration was performedusing preferablyStars 1,
coming from AO 0235+164 is to eliminate the contamination
2, and 3 by Smithetal. (1985) in the optical bands2, and the
fromthesouthernAGN,namedELISAbyRaiterietal.(2005).
Gonza´lez-Pe´rezetal. (2001) photometricsequencein the near-
Since it lies only 2 arcsec south of the source and it is very
IR. Optical light curves were carefully assembled and cleaned
faint(Nilssonetal., 1996), this objectis notresolvedfromAO
(seee.g.Villataetal.,2002).
0235+164inmostopticalframesaswellasintheUVOTones.
We collectedradiodataascalibratedfluxdensities;nosys- Consequently,itgivesacontributiontotheAO0235+164pho-
tematic offset among datasets provided by different observers tometry, which affects more the blue than the red part of the
was evident; data binning was needed in a few cases to re- optical–UVspectrumandisstrongerwhentheblazarisfainter.
duce the noise. We also included data from the VLA/VLBA To correct for this effect, Raiterietal. (2005) estimated the
PolarizationCalibrationDatabase3(PCD)inourlightcurves. UBVRI magnitudes of ELISA from images taken at medium-
sizetelescopesparticipatinginthe2003–2004WEBTcampaign.
More recently, Raiterietal. (2007a) analysed an ELISA opti-
2.2.UVobservationsbySwift
cal spectrum taken at the 8 m Very Large Telescope (VLT).
TheUBVRImagnitudesderivedfromthisspectrumconfirmthe
The Swift satellite observed the source from January 28 to
February26,2007.TheUVOTinstrument(Romingetal.,2005) previousphotometryresultsandlowertheuncertaintiesto0.05
mag;only in thecase ofthe V filter a slightly faintervalueap-
acquireddataintheU,UVW1,UVM2,andUVW2filters,which
were processed with the uvotmaghisttask of the HEASOFT pearsmoreappropriate.Hence,tosubtracttheELISAcontribu-
6.3 package. Following the recommendations contained in the tion from the optical fluxes, we adopted the following ELISA
magnitudes:U = 20.80,B = 21.40,V = 20.95,R = 20.50,and
release notes, we used an aperture radius of 5 arcsec in agree-
I =19.90.Zero-magnitudefluxesweretakenfromBesselletal.
mentwiththestandardphotometricaperturedefinedinthecal-
ibrationfiles(CALDBupdatedasJuly2007).Backgroundwas (1998).
extracted in an annulus centred on the source, with inner and We then estimated the ELISA contribution in the UVW1,
outerradiiof8and18arcsec,respectively. UVM2, and UVW2 filters from the HST/FOS composite
spectrum published by Burbidgeetal. (1996)4. We subtracted
2 In some cases, we also used Stars 9, 10, and 11 by 4 WenotethatthefluxintheUVW1bandisaffectedbythecontri-
Gonza´lez-Pe´rezetal.(2001). butionoftheC]emissionline,andthattheHST/FOSUVspectrum
3 http://www.vla.nrao.edu/astro/calib/polar/ nicelyreconnectsinfluxwiththeVLTopticalspectrum.
C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst 3
Fig.1.Opticalandnear-IRlightcurvesofAO
0235+164inthe2006–2007observingseason.
The optical magnitudes are corrected for the
ELISAcontribution. Maximum and minimum
brightnesslevelsarehighlightedwithhorizon-
taldottedlines.
these ELISA flux densities from the total flux densities ob- =2454077.4(December7,2006).Itwascharacterizedbya2.0
tained with uvotmaghistand converted the results into mag- magbrighteningin 4 daysin theR band,andby noticeablein-
nitudes with zero-magnitude fluxes of 3.963, 4.544, 5.400 × tranight variability (see also Hagen-Thornetal., 2007a). After
10−9ergcm−2s−1Å−1. UV magnitudes of ELISA are: 20.45, the dropof 1.5 mag in 3 days, the sourcebrightnesscontinued
20.50,and 20.65in the UVW1, UVM2, and UVW2 bands, re- torisesmoothly,withminorflaressuperposed.Theoutburstcul-
spectively,againwitha0.05maguncertainty. minatedonJD=2454151–3(February19–21,2007).Thispeak
As in the previous works, the ELISA contamination in the iswellsampledintheopticalbands,butwasmissedbythenear-
near-IRisassumedtobenegligible,andnullinthemmandcm IRobservations,whichregisteredthebrightestlevelsomedays
bands. later, when the optical flux was already in a decreasing phase.
Wenoticethatinthisobservingseasonthevariabilityamplitude
inthebest-sampledRbandwas∼5mag.
3. Lightcurvesbehaviour
Observations by the Swift satellite were triggered in late
ThefinallightcurvesareshowninFigs.1–3.TheopticalandUV Januaryduringtheculminatingphaseoftheoutburst.TheU and
magnitudeshavebeencorrectedfortheELISAcontribution,as UVlightcurvesfromUVOTareshowninFig.2asbluecircles,
explainedintheprevioussection. and compared to the R-band light curve. In the U-band panel,
Figure1displaystheopticalandnear-IRlightcurvesofthe ground-based data from the Nordic Optical Telescope (NOT)
2006–2007observingseason.Notwithstandingthedifferencesin were plotted as red diamonds. UVOT data have been binned
timesampling,thegeneraltrendissimilaratallwavelengths,as daily,andalowerlimittotheerrorof0.1maghasbeenadopted
expected. At the beginning of the observing season the source to take into account statistical and systematic errors. The grey
wasin afaintstate, butbyin late Septemberthesourcebright- lineineachpanelrepresentsacubicsplineinterpolationthrough
ness started to increase. We observeda fast flare to peak at JD the1-daybinnedR-bandlightcurve(rescaledaccordingtoaver-
4 C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst
lar conjunction show that the mm flux density had reduced by
morethanafactorof2.Wenoticethatafirsthumpinthe1mm
lightcurve,aroundJD=2454100,maybecorrelatedtothefast
opticalflareobservedatJD∼2454080.
Inthefollowingpanels,radiolightcurvesatlowerandlower
frequenciesare displayed.We have plotted the cubic spline in-
terpolation through the 20-day binned light curve at 37 GHz,
thebest-sampledone,toallowaneasiercomparisonoftheflux
density behaviour at different wavelengths. In general, the ra-
dio spectrum appears “inverted”: the flux density is higher at
higherfrequencies.Thisisnottrueintheperiod(JD∼2453900–
2454050) leading to the minimum flux levels, where the flux
density appears higher at longer wavelengths, and during the
outburst decreasing phase, where the radio spectrum seems to
beflat.
The radio emission had already undergone a rather bright
phase in 2005–2006,a kind of “minor outburst” like those of-
tenobservedin thepast,whichsometimesareaccompaniedby
noticeable optical events and sometimes not. A flux minimum
wasreachedaroundJD = 2454000–2454100,andthentheflux
started to increase again, all these variations occurring first at
the higher and then at the lower frequencies. However, the 22
GHz flux density reacheda maximumvalue similar to that ob-
servedat37GHzalmostatthesametime,andthefluxdecrease
wasalsosimilarandcontemporaneous.Thisishighlightedinthe
fourthpanel of Fig. 3 by the overlappingof the spline through
the37GHzdatawiththatthroughthe22GHzones.Thelower-
frequencylightcurvesare moresparsely sampled,buttheytoo
suggestaprogressivelydelayedrisingphase,andacontempora-
neousdecreasingphase.
Fig.2. UVOT light curves of AO 0235+164 compared to the Whilethegeneralmultifrequencybehaviourduringtheout-
ground-basedU data from the NOT (red diamonds)and to the burst seems in agreement with the predictions of shock-in-jet
R-banddata(bottompanel).Alldatahavebeencorrectedforthe models (see e.g. Hughesetal., 1989; Valtaojaetal., 1992), the
ELISAcontribution.Thecubicsplineinterpolationthroughthe decay stage might suggest a more complex picture. Indeed, it
1-daybinnedR-bandlightcurveisshownineachpanel,properly seemsasifakindof“shutdown”occurredwhentheperturbation
rescaled(seetext). thatwasresponsibleforthefluxincreasereachedthejetregion
emittingatcmwavelengths.Theshapeofthe37GHzoutburst,
sharp and withouta plateau, may indicatethat the outburstob-
agecolourindices:U−R=2.1;W1−R=2.1;M2−R=2.3;and served at this frequencyhas already been affected by the shut-
W2−R=2.7).IthelpstorecognizethatthetrendoftheR-band down.Thesignatureoftheshutdownwouldbemainlyvisiblein
lightcurveiswellfollowedbyalltheUVOTlightcurves.Also, the22GHzlightcurve,whichischaracterizedbyanabruptflux
thevariabilityamplitudeappearsnearlythe samein theoptical decay.
andUVOTbands. It is interesting to compare this recent behaviour of the
Theincreaseoftheopticalandnear-IRfluxeswasfollowed sourcewithitspasthistory.Figure4displaysthelightcurvesin
soonafterbyanincreaseofthemillimetricandthenofthecenti- theopticalRbandandattworadiofrequenciesinthelast∼12.5
metricfluxes,withaprogressivetimedelaygoingtowardlonger years,includingthepreviousradio-opticaloutburstobservedin
wavelengths. Figure 3 shows the radio light curves starting 1997–1998. We notice that the last outburst had a shorter du-
from the end of the observingperiod presented in Raiterietal. ration with respect to the outburst in 1997–1998.Moreover, if
(2006a),comparedtotheR-bandone(toppanel).Opticalobser- wecomparethe2005–2006and2006–2007events,weseethat
vationsinthe 2005–2006seasonwere rathersparse;theyshow thelatterwasmuch“harder”(i.e.strongerathigherfrequencies)
moderate source activity at a level similar to that observed in than the former. Similarly, the 1999–2000 post-outburst event
thepreviousseason(Raiterietal.,2006a).Thefigurealsoshows is harder than the preceding major one. Other very hard (opti-
that,afterthesolarconjunctionin2007,thesourcewasagainin calonly)eventsarevisiblealongalltheperiodshowninFig.4,
averyfaintopticalstate. starting from the 1995–1996 event (Takaloetal., 1998). As in
The second panelfrom the top displays 1 mm and 850 µm thecaseof3C454.3discussedbyVillataetal.(2007),theseoc-
datafromtheSubMillimeterArray(SMA).Acubicsplineinter- currencesmightbe due to (temporary)differentorientationsof
polationthroughthe10-daybinned1mmlightcurveisplotted the optical and radio emitting regions, harder events being ob-
to better follow the behaviour of the source flux at this wave- servedwhentheopticalemittingregionismorealignedwiththe
length. The 1 mm flux density increased by a factor ∼ 8 from lineofsightthantheradioone,andhencethecorrespondingra-
the beginning of the 2006–2007 season to the time of the ob- diation is more beamed.Alternatively,harderand softer events
served maximum, on March 6, 2007. After this date, the only mightbeintrinsicallydifferent.Inthiscasehardereventswould
datapointacquiredbeforethesolarconjunctionepochsuggests correspondtosituationswheretheperturbationvanishesearlier
thattheoutburstpeakwasalreadyover.Moreover,dataafterso- in the jet, sometimes being unable to reach the radio emitting
C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst 5
Fig.3. Comparison among the R-band magni-
tudes(toppanel),correctedfortheELISAcon-
tribution,andtheradiofluxdensities(Jy)atdif-
ferent frequencies. Cubic splineinterpolations
through the 10-day binned light curves in the
R and 1 mm bands have been drawn in the
first and second panel, respectively. Similarly,
the cubic spline interpolation through the 20-
day binned light curve at 37 GHz has been
drawnintheotherpanels,tohighlightthetime
delays between the flux variations at 37 GHz
and those at lower frequencies. In the fourth
panelthecubicsplineinterpolationthroughthe
15-day binned light curve at 22 GHz is also
shown (cyan line). Minimum (and maximum)
fluxdensitylevelsareindicatedwithhorizontal
dottedlines.
regions.Regardlessoftheirorigin,hardflaresinaverageshould of the average time delays between the flux changes observed
beshorter,sincetheycannotfullydevelop. atdifferentfrequencies,we applythe discrete correlationfunc-
Inthisframeworkwecouldalsoexplainthesharpnessofthe tion(DCF;Edelson&Krolik1988;Hufnagel&Bregman1992)
last radiooutburst,and the fastcontemporaneousflux decrease analysis.Sinceweareanalysingalimitedtimeperiodwithlong
atallwavelengths.Theymightbedueeitherto ageometricef- data gaps, we use the cubic spline interpolations through the
fect,i.e.ajetbendingintheradioemittingregion(asproposed binned light curves that had already been shown in Fig. 3. In
for 3C 454.3; see Villataetal. 2007), or to an intrinsic pertur- this way we certainly oversmooththe variations hidden by the
bationpower off in the same jet region.We speculate that, had solarconjunctionperiod,butbetterdefinethemajorsignalswe
thisshutdownoccurredinaneveninnerpartofthejet,wewould arelookingfor.
have notobserved the outburstin the radio band, i.e. the event
Cross-correlation between the R-band and 1 mm, 37 GHz,
wouldhavebeenevenharder.
and22GHzsplinesyieldstheDCFcurvesdisplayedinFig.5.In
allcasesthedistributionofpointsshowsasignificantmaximum
(∼1)atpositivetimelagsτ,whichindicatesastrongcorrelation
4. Cross-correlationanalysis withvariationsatthelowerfrequencylaggingbehindthehigher-
frequencyones.ThedelayscorrespondingtotheDCFpeaksare
Inthe previoussection wesaw thatthebrightnessincreaseand
20,60,and70daysforthe1mm,37GHz,and22GHzcurves,
consequentoutburst observed in the optical and near-IR bands
respectively.We obtainabetterestimateofthetimelagbycal-
were soon followed by similar events at mm and cm wave-
culating the centroid of the DCF (Petersonetal., 1998), which
lengths.Wecanofcourseestimatetimedelaysbycalculatingthe
is defined as τ = Pτ DCF/PDCF for all the points i with
timeseparationbetweenpeaksinthedifferentlightcurves.But c i i i
DCF > kDCF . By considering 0.7 ≤ k ≤ 0.8, we obtain
this procedure neglects the light curves behaviour as a whole, i peak
lagsbetween22and23daysfortheR-mm,56–58daysforthe
andstronglydependsonthetimesampling.Forinstance,when
R-37GHz,and70–72daysfortheR-22GHzcross-correlations.
comparingtheR-bandtothe1mmlightcurve,wefindalagof
about1 monthif we considerthe December2006opticalflare, However,wehavealreadydiscussedintheprevioussection
while the lag is reduced to a couple of weeks if we consider thatthe22GHzoutburstseemstobeaffectedbyasortofshut-
the outburst peak. Hence, in order to obtain a better estimate down causing the sudden flux decrease. In the absence of this
6 C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst
Fig.4. Comparison among the R-band magni-
tudes(toppanel),correctedfortheELISAcon-
tribution,andtheradiofluxdensities(Jy)at37
and8GHzoverthelast∼12.5years.
effect,theoutburstpeakandfollowingdecreasingphasewould
likelyhavebeendelayed.Hence,the70–72daydelayindicated
by the DCF appears as a “compromise” between the more de-
layedrisingphaseandthelessdelayeddecayingphase.Indeed,
bylookingatthefourthpanelofFig.3,weseethatthefluxden-
sityincreaseat22GHzfollowedthatat37GHz afterabout25
days, the peak was reached 1 week later, and no lag is visible
duringthefluxfading.
In the 14.5, 8, and 5 GHz lightcurves,the time of the out-
burst peak cannot be defined because of the inferior sampling,
butwe nonethelessinferthattheascendingpartofthe outburst
was delayedby ∼ 50, ∼ 65, and >∼ 90 days with respectto the
rising part of the 37 GHz outburst, while, again, no significant
delayisvisibleinthedimmingphase.
The application of the DCF method to auto-correlate the
composite R-band historical light curve5 results in a reinforce-
ment of the ∼ 8.5 year signal already found by Raiterietal.
Fig.5. Discrete correlation functions between the cubic spline
(2006a).
interpolationsthroughthebinnedR-bandand1mmlightcurves
(blue dots), the R-band and 37 GHz ones (red diamonds), and
5. Spectralenergydistributions the R-band and 22 GHz ones (green triangles). Vertical strips
highlighttherangeoftimelagsτincludedbetweenthelocation
OneofthemajorissuesraisedbytheworksofJunkkarinenetal. ofthepeakandthecentroidofthedistributions.
(2004) and Raiterietal. (2005, 2006a,b) is the existence of an
extraemissioncomponentintheSEDofAO0235+164,inaddi-
tiontothesynchrotronandinverse-Comptoncomponents.This
states, and thatthe componentappearsto be variable.But they
componentis representedby a bump in the UV and soft X-ray
couldnotdiscriminatebetweenthetwomostobviousinterpreta-
energyrange,anditsnatureisnotyetclear.Inthecaseofquasar-
tions,i.e.thermalemissionfromanaccretiondiscandanaddi-
typeblazars,asimilarfeatureisclearlyvisibleonlyduringfaint
tionalsynchrotroncomponentfromaninnerjetregion.
states, and it is generallyascribedto thermalemission froman
With the aim of shedding some light on this, we se-
accretion disc. However, when these sources undergo a flaring
lected epochs in which the source was in different brightness
state,thebumptendstodisappear,sinceitisoverwhelmedbythe
statesandin whichcontemporaneousnear-IR-to-UVdatawere
synchrotronemission(seee.g.Raiterietal.,2007b,forthecase
available. Magnitudes were corrected for both Galactic and
of3C454.3).Incontrast,bycomparingSEDsofAO0235+164
foreground-galaxyabsorptionbyfollowingthe prescriptionsof
obtained at various observing epochs in which X-ray observa-
Junkkarinenetal.(2004)(seeTable5byRaiterietal.2005).We
tionswereperformed,Raiterietal.(2006a)pointedoutthatthe
thenconvertedde-reddenedmagnitudesintofluxesusingthecal-
presence of the extra componentcan also be inferred in bright
ibrationbyBesselletal.(1998)intheoptical,andthezero-mag
fluxesfortheUVOTultravioletbandsmentionedinSect.2.3.
5 This composite light curve has been obtained by considering the
R-banddatabacktoJD=2447000,andB-banddataconvertedintoR- WeshowthreeoftheseSEDsinFig.6,comparedwiththose
bandones(throughthemeancolourindexB−R=1.7)beforethatdate corresponding to the XMM-Newton pointings of January and
(Raiterietal.,2006a). August 2004, published in Raiterietal. (2006a). We built the
C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst 7
Fig.6. Near-IR-to-UV spectral energy distributions at vari-
Fig.7.Thecontributiontothefluxdensityfromtheextracom-
ous epochs: the SED corresponding to the outburst maximum
ponentversusthatfromthesynchrotroncomponentinthethree
(February19,2007)iscomparedtothefaint-stateSEDsobtained
UVOTbands(seetextfordetails).Linesrepresentlinearfitsto
during the January and August 2004 XMM-Newton pointings
thedata.
(Raiterietal., 2006a) and to two intermediate-state SEDs pre-
cedingtheoutburstpeak.
outbursttowardthe2006–2007observingseason.Anincreased
radio activity was registered in 2005–2006(Bachetal., 2007),
brightest SED with data taken on February 19, 2007, at the
and a major optical outburst was finally observed in late 2006
culmination of the outburst; since there were no simultaneous
– early 2007,about8.5 yearsafter the previousmajor outburst
JHK data, we inferred near-IR magnitudes by considering the
peak,thusconfirmingtheRaiterietal.(2006a)prediction.
R− JHK colourindicescorrespondingto thebrightestnear-IR
We observed the same event simultaneously in the near-IR
levelinthelightcurves(seeFig.1).Intheothertwocases(SEDs
andUVfrequencyranges,andthenatmillimetricandcentimet-
ofJanuary31andFebruary5,2007)alldataarecontemporane-
ricwavelengths,withaprogressivetimedelaytowardlowerfre-
ous(takenwithinfewhours).Theyshowintermediatefluxlev-
quencies. We estimated that the 1 mm and 37 GHz outbursts
els,withanoticeablespectral-slopevariation:thenear-IRfluxes
laggedbehindtheR-bandonebyabout3weeksand2months,
arenearlythesame,buttowardtheUVthetwoSEDsdiverge.
respectively.Thislatterlagappearsafactor∼2longerthanpre-
We foundthattheleast-orderpolynomialthatyieldsagood
viouslyestimated(Raiterietal.,2005,2006a)whenconsidering
fit to the near-IR-to-opticalpartof the SEDs is a cubic;we as-
the historical light curves until spring 2005. This may reflect
sumethatthesefits,whicharedisplayedinFig.6,canreasonably
some real change either in the jet structure at still unresolved
represent the contribution of the synchrotron component. The
scalesorinitsenergeticswithrespecttothepast.Indeed,there
UV excess is clearly recognizable in all SEDs, and represents
aresourcesthatexhibitacharacteristicbehaviorformanyyears
theextraemissioncomponent.Weestimatedthecontributionof
and then suddenly change (see e.g. Smith, 1996; Alleretal.,
this componentin the UV bands as the difference between the
1996).Anotherpossibilityisthatlackofdataforlongtimeinter-
totalandtheextrapolatedsynchrotronfluxdensities.
vals,chieflyin theopticalbandsbecauseofsolarconjunctions,
Figure 7 shows the relationship between the extra-
affects the results of the DCF analysis. Even in the case of the
componentandthesynchrotron-componentfluxdensitiesforthe
1997–1998outburst, which was intensivelyobservedthanks to
three UV bands: a fair linear correlation can be seen (correla-
theeffortsofthejust-bornWEBT(Raiterietal.,2001),thecen-
tion coefficient r = 0.996,0.995,0.994 for W1, M2, W2, re-
tralpartoftheeventwasmissedintheoptical,andthedimming
spectively). Notwithstanding the uncertainties affecting our fit-
phasewaspoorlysampledat37GHz.Asforthe2006–2007out-
tingprocedure,wethinkthatthisresultindicatesthatthisextra
burst,notwithstandingtheexceptionalsampling,wecannotrule
component can hardly be explained in terms of thermal emis-
outthatsolarconjunctionhidafurtheropticalpeak.Thiswould
sionfromthedisc,whichisnotexpectedtodependonthesyn-
shortentheradiolags.
chrotron radiation, but more likely it is produced by the same
In any case, the delays estimated in the present paper ap-
mechanismthatisresponsibleforthesynchrotronemission.
pear rather short if compared, for example, with the approxi-
mately twice-longer lags estimated by Villataetal. (2007) for
thequasar-typeblazar3C454.3incorrespondenceofits2004–
6. Discussionandconclusions
2006 exceptional outburst, and by Villataetal. (2004) and
The claim by Raiterietal. (2001) of a possible quasi-periodic Bachetal. (2006) for BL Lacertae. The short time delays in
occurrence of the major radio and optical outbursts of AO AO0235+164hadalreadybeennoticedinthepast(Webbetal.,
0235+164every5.7±0.5yearsledtoamobilizationoftheinter- 2000;Raiterietal.,2001),andthiswasoneofthereasonswhy
nationalblazarcommunityto observethenextevent,predicted microlensingwasproposedasapossibleexplanationofthema-
topeakin early2004.A WEBT campaignwasorganised,with jorvariabilityeventsinthissource(seee.g.Raiterietal.,2007a,
a hugeinternationalparticipation(Raiterietal., 2005, 2006a,b, andreferencestherein).Incontrast,wheninterpretingthemulti-
2007a), butthe sourceremainedin a faintstate andtime series frequencyvariabilityof AO 0235+164in the frameworkof the
analysisontheupdatedlightcurvessuggestedapossiblelonger helicaljetmodelbyVillata&Raiteri(1999),theshorttimede-
periodofabout8–8.5years,delayingtheoccurrenceofthenext lays imply that the correspondingjet-emitting regionsare only
8 C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst
slightly misaligned (Ostoreroetal., 2004). If, besides the geo- ponentistheresultofinverse-Comptonscatteringofradiopho-
metrical effect, the outburst is also produced by energetic pro- tonsofftherelativisticelectronsproducingtheIR–opticalemis-
cesses insidethe jet,shorttimelagsimplythattheemittingre- sion.Indeed,asstatedabove,theclosecorrelationandshorttime
gionsarealsocloseby. delaysbetweentheopticalandradiovariationssuggestthatalot
At lower (< 37 GHz) radio frequencies, the outburst be- ofradiophotonsareavailableintheopticalemittingregion.
haviourchanges:whiletherisingphaseisprogressivelydelayed,
as expected, the decaying phase is observed simultaneously at Acknowledgements. WearegratefultoPhilipHughesforusefuldiscussions.We
acknowledgetheuseofpublicdatafromtheSwiftdataarchive.Thisresearch
all wavelengths. In particular,the 22 GHz flux density reaches
hasmadeuseofdataobtainedthroughtheHighEnergyAstrophysicsScience
a maximumvalue similar to the 37GHz flux density almostat ArchiveResearchCenterOnlineService,providedbytheNASA/GoddardSpace
the same time and then suddenly decreases. Although the ob- FlightCenter.ThisworkispartlybasedonobservationsmadewiththeNordic
servedgeneralmultifrequencypropertiesoftheoutburstcanbe Optical Telescope, operated on the island of La Palma jointly by Denmark,
Finland,Iceland,Norway,andSweden,intheSpanishObservatoriodelRoque
explainedbymodelsdealingwith shockspropagatingalongan
de los Muchachos of the Instituto de Astrofisica de Canarias. This research
inhomogeneousjet(seee.g.Hughesetal.,1989;Valtaojaetal.,
has made use of data from the University of Michigan Radio Astronomy
1992),theemissionbehaviourduringthedimmingphasemight Observatory, which is supported by the National Science Foundation and by
suggest that the scenario is more complicated than what is de- fundsfromtheUniversityofMichigan.TheMetsa¨hoviteamacknowledgesthe
pictedinthesemodels.Indeed,itseemsasifakindofshutdown support fromthe Academy ofFinland. AZT-24observations aremade within
an agreement between Pulkovo, Rome and Teramo observatories. The St.-
occurredinthejetbetweenthemmemissionregionandthe37–
PetersburgteamwassupportedbytheRussianFoundationforBasicResearch,
22GHzregion.Wecanspeculatethateitherajetbendingoran grant 08-02-00245. The Torino team acknowledges financial support by the
intrinsic power off of the perturbationmay producethis effect, Italian Space Agency throughcontract ASI-INAFI/088/06/0 forthe Studyof
whichcouldalsoaccountforthedifferentflavoursofeventsob- High-Energy Astrophysics. University of Joensuu acknowledges co-operation
withlocalAmateurAstronomerAssociationSeulaset.ACGworkissupported
servedinthehistoricallightcurvesofthesource:theharderout-
byNNSFofChinagrantNo.10533050.JHF’sworkissupportedbytheNational
bursts,whicharestrongeratthehigherfrequencies,suggestthat
NaturalScienceFoundationofChina(10573005,10633010)andthe973project
ashutdown(ofanyorigin)hasoccurredbetweenthehigher-and (No.2007CB815405).
the lower-frequency emitting regions. Understanding whether
the geometric or intrinsic scenario would be more plausible is
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Turnsheketal.,1988).Afurtherpossibilityisthattheextracom- 1 INAF,OsservatorioAstronomicodiTorino,Italy
C.M.Raiterietal.:Radio-to-UVmonitoringofAO0235+164bytheWEBTandSwiftduringthe2006–2007outburst 9
2 Astron.Inst.,St.-PetersburgStateUniv.,Russia
3 PulkovoObservatory,St.Petersburg,Russia
4 DepartmentofAstronomy,UniversityofMichigan,MI,USA
5 Max-Planck-Institutfu¨rRadioastronomie,Bonn,Germany
6 Harvard-SmithsonianCenterforAstroph.,Cambridge,MA,USA
7 AbastumaniAstrophysicalObservatory,Georgia
8 Metsa¨hoviRadioObs.,HelsinkiUniv.ofTechnology,Finland
9 TuorlaObservatory,Univ.ofTurku,Piikkio¨,Finland
10 RadioAstron.Lab.ofCrimeanAstroph.Observatory,Ukraine
11 Inst.ofAstron.,BulgarianAcad.ofSciences,Sofia,Bulgaria
12 DepartmentofPhysicsandAstronomy,OhioUniv.,OH,USA
13 INAF,OsservatorioAstrofisicodiCatania,Italy
14 Oss.AstronomicodellaRegioneAutonomaValled’Aosta,Italy
15 NewMillenniumObservatory,Italy
16 INAF,OsservatorioAstronomicodiRoma,Italy
17 INAF,OsservatorioAstronomicodiColluraniaTeramo,Italy
18 CenterforAstrophysics,GuangzhouUniversity,China
19 Agrupacio´ Astrono`micadeSabadell,Spain
20 INAF,IASF-Bologna,Italy
21 YNAO,ChineseAcademyofSciences,Kunming,China
22 NASA/GoddardSpaceFlightCenter,Greenbelt,Maryland,USA
23 Inst.ofAstronomy,BAN,Bulgaria
24 Max-Planck-Institutfu¨rRadioastronomie,53121Bonn,Germany
25 KoreaAstronomyandSpaceScienceInstitute,SouthKorea
26 INAF,IstitutodiRadioastronomia,SezionediNoto,Italy
27 GreveObservatory,Italy
28 Dept.ofPhys.,“LaSapienza”Univ.,Roma,Italy
29 MichaelAdrianObservatory,Trebur,Germany
30 HankasalmiObservatory,Finland
31 SofiaUniversity,Bulgaria
32 JakokoskiObservatory,Univ.ofJoensuu,Finland
33 NordicOpticalTelescope,SantaCruzdeLaPalma,Spain
34 OsakaUniversity,Japan