Radio to gamma-ray variability study of blazar S5 0716+714

2.1 MB·English

by  B. Rani

Astronomy&Astrophysicsmanuscriptno.ms (cid:13)c ESO2013
January31,2013
Radio to gamma-ray variability study of blazar S5 0716+714
B.Rani1,⋆,T.P.Krichbaum1,L.Fuhrmann1,M.Bo¨ttcher2,3,B.Lott4,H.D.Aller5,M.F.Aller5,E.Angelakis1,U.
Bach1,D.Bastieri6,7,A.D.Falcone8,Y.Fukazawa9,K.E.Gabanyi10,11,A.C.Gupta12,M.Gurwell13,R.Itoh9,K.S.
Kawabata14,M.Krips15,A.A.La¨hteenma¨ki16,X.Liu17,N.Marchili1,7,W.Max-Moerbeck18,I.Nestoras1,E.
Nieppola16,G.Quintana-Lacaci19,20,A.C.S.Readhead18,J.L.Richards21,M.Sasada14,22,A.Sievers19,K.
Sokolovsky1,23,M.Stroh8,J.Tammi16,M.Tornikoski16,M.Uemura14,H.Ungerechts19,T.Urano9,andJ.A.Zensus1
1 Max-Planck-Institutfu¨rRadioastronomie(MPIfR),AufdemHu¨gel69,D-53121Bonn,Germany
3
2 AstrophysicalInstitute,DepartmentofPhysicsandAstronomy,OhioUniversityAthens,OH45701,USA
1
3 CentreforSpaceResearch,North-WestUniversity,PotchefstroomCampus,Potchefstroom,2531,SouthAfrica
0
4 Universite´Bordeaux1,CNRS/IN2p3,Centred’EtudesNucla´iresdeBordeauxGradignan,33175Gradignan,France
2
5 AstronomyDepartment,UniversityofMichigan,AnnArbor,MI48109-1042,USA

n 6 IstitutoNazionalediFisicaNucleare,SezionediPadova,I-35131Padova,Italy
a 7 DipartimentodiFisicaeAstronomia,Universita`diPadova,I-35131Padova,Italy
J 8 Dept.ofAstronomyandAstrophysics,PennStateUniversity,UniversityPark,PA16802,USA

9 9 DepartmentofPhysicalSciences,HiroshimaUniversity,1-3-1Kagamiyama,Higashi-Hiroshima,Hiroshima739-8526
2 10 FO¨MISatelliteGeodeticObservatory,POBox585,1592Budapest,Hungary
11 KonkolyObservatory, ResearchCentreforAstronomyandEarthSciences,HungarianAcademyofSciences,H-1121Budapest,

] Hungary
E 12 AryabhattaResearchInstituteofObservationalSciences(ARIES),ManoraPeak,Nainital,263129,India
H 13 Harvard-SmithsonianCenterforAstrophysics,Cambridge,MA02138USA
14 HiroshimaAstrophysicalScienceCenter,HiroshimaUniversity,1-3-1Kagamiyama,Higashi-Hiroshima,Hiroshima739-8526
.
h 15 IRAM,300ruedelapiscine,F-38406Saint-Martind’He´res,France
p 16 AaltoUniversityMetsa¨hoviRadioObservatory,Kylma¨la¨,Finland
- 17 XinjiangAstronomicalObservatory,ChineseAcademyofSciences,150Science1-Street,Urumqi830011,P.R.China
o
18 CahillCenterforAstronomyandAstrophysics,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA
r
t 19 InstitutodeRadioastronom´ıaMilime´trica(IRAM),AvenidaDivinaPastora7,Local20,18012Granada,Spain
s 20 CAB,INTA-CSIC,Ctra.deTorrejo´naAjalvir,km4,E-28850,Torrejo´ndeArdoz,Madrid,Spain
a
21 PurdueUniversity,DepartmentofPhysics,525NorthwesternAve,WestLafayette,IN47907,USA
[
22 DepartmentofAstronomy,KyotoUniversity,Kitashirakawa-Oiwake-cho,Sakyo-ku,Kyoto606-8502
1 23 AstroSpaceCenterofLebedevPhysicalInstitute,Profsoyuznaya84/32,117997Moscow,Russia
v
Received———;accepted———-
7
8
ABSTRACT
0
7
Wepresenttheresultsofaseriesofradio,optical,X-rayandγ-rayobservationsoftheBLLacobjectS50716+714carriedoutbetween
.
1 April2007andJanuary2011.Themulti-frequencyobservationswereobtainedusingseveralgroundandspacebasedfacilities.The
0 intenseopticalmonitoringofthesourcerevealsfasterrepetitivevariationssuperimposed onalong-termvariabilitytrendatatime
3 scaleof∼ 350 days. Episodesoffast variabilityrecurontimescalesof∼ 60–70days. Theintenseandsimultaneous activityat
1 optical and γ-rayfrequencies favorstheSSC mechanism forthe production of thehigh-energy emission. Twomajor low-peaking
: radioflareswereobservedduringthishighoptical/γ-rayactivityperiod.Theradioflaresarecharacterizedbyarisingandadecaying
v
stageandareinagreementwiththeformationofashockanditsevolution.Wefoundthattheevolutionoftheradioflaresrequires
i
X ageometrical variationinadditiontointrinsicvariationsofthesource.Differentestimatesyieldarobustandself-consistentlower
limitsofδ≥20andequipartitionmagneticfieldB ≥0.36G.Causalityargumentsconstrainthesizeofemissionregionθ ≤0.004
r eq
a mas.Wefoundasignificantcorrelationbetweenfluxvariationsatradiofrequencieswiththoseatopticalandγ-rays.Theoptical/GeV
fluxvariationsleadtheradiovariabilityby∼65days.Thelongertimedelaysbetweenlow-peakingradiooutburstsandopticalflares
implythatopticalflaresaretheprecursorsofradioones.An orphanX-rayflarechallengesthesimple,one-zone emission models,
rendering them too simple. Herewe alsodescribe thespectral energy distributionmodeling of the source fromsimultaneous data
takenthroughdifferentactivityperiods.
Key words. galaxies: active – BLLacertae objects: individual: S50716+714 – Gammarays: galaxies –X-rays: galaxies– radio
continuum:galaxies
1. Introduction
⋆ MemberoftheInternationalMaxPlanckResearchSchool(IMPRS) The BL Lac objectS5 0716+714is amongthe mostextremely
for Astronomy and Astrophysics at the Universities of Bonn and variableblazars.Theopticalcontinuumofthe sourceissofea-
Cologne tureless that it is hard to estimate its redshift. Nilssonetal.
1
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
(2008) claimeda value ofz = 0.31±0.08basedon the photo- tical emissions. This source is also among the bright blazars
metricdetectionofthehostgalaxy.Veryrecently,thedetection in the Fermi/LAT (LargeArea Telescope)Bright AGN Sample
of intervening Lyα systems in the ultra-violet spectrum of the (LBAS) (Abdoetal., 2010a), whose GeV spectra are governed
sourceconfirmstheearlierestimateswitharedshiftvalue0.2315 byabrokenpowerlaw.ThecombinedGeV–TeVspectrumof
< z<0.3407(Danforthetal.,2012).Thissourcehasbeenclas- thesourcedisplaysabsorption-likefeaturesin10–100GeVen-
sified as an intermediate-peakedblazar(IBL) by Giommietal. ergyrange(Senturketal.,2011).
(1999),asthefrequencyofthefirstspectralenergydistribution The broadbandflaring behaviorof the source is even more
(SED)peakvariesbetween1014and1015Hz,andthusdoesnot complex. A literature study reveals that the broadband flar-
fallintothewavebandsspecifiedbytheusualdefinitionsoflow ing activity of the source is not simultaneous at all frequen-
andhighenergypeakedblazars(i.e.LBLsandHBLs). cies(Chenetal.,2008;Villataetal.,2008;Vittorinietal.,2009;
S5 0716+714isoneof thebrightestBL Lacsin theoptical Ostoreroetal., 2006). Also, it is hard to explain both the slow
bandsandhasanopticalintradayvariability(IDV)dutycycleof modes of variability at radio and hard X-ray bands and the
nearlyone(Wagner&Witzel,1995).Unsurprisingly,thissource rapid variability observed in the optical, soft X-ray, and γ-ray
hasbeenthesubjectofseveralopticalmonitoringcampaignson bands using a single component SSC model (see Villataetal.,
intraday(IDV)timescales(e.g.Wagneretal.,1996; Ranietal., 2008; Giommietal., 2008; Chenetal., 2008; Vittorinietal.,
2011;Montagnietal.,2006;Guptaetal.,2009,2012,andrefer- 2009). The X-ray spectrum of the source contains contribu-
encestherein).Thesourcehasshownfivemajoropticaloutbursts tions from both synchrotron and IC emission (Foschinietal.,
separatedroughlyby∼3.0±0.3years(Raiterietal.,2003).High 2006; Ferreroetal., 2006) and the simultaneous optical-GeV
opticalpolarization∼20%–29%hasalsobeenobservedinthe variationsfavoranSSCemissionmechanism(Chenetal.,2008;
source(Takaloetal.,1994;Fanetal.,1997).Guptaetal.(2009) Vittorinietal.,2009).
analyzedtheexcellentintradayopticallightcurvesofthesource Despite of several efforts to understandthe broadbandflar-
observedbyMontagnietal.(2006)andreportedgoodevidence ingactivityofthesource,westilldonothaveaclearknowledge
ofnearlyperiodicoscillationsrangingbetween25and73min- oftheemissionmechanismsresponsibleforitsorigin.Inpartic-
utesonseveraldifferentnights.Goodevidenceforthepresence ular, the location and the mechanism responsible for the high-
of∼15-minperiodicoscillationsatopticalfrequencieshasbeen energyemission and the relationbetween the variabilityat dif-
reportedbyRanietal.(2010a).Adetailedmultibandshort-term ferentwavelengthsare notyetwellunderstood.Therefore, itis
opticalfluxandcolorvariabilitystudyofthesourceisreported importanttoinvestigatewhetheracorrelationexistsbetweenop-
inRanietal.(2010b).Therewefoundthattheopticalspectraof ticalandradioemissions,whicharebothascribedtosynchrotron
thesourcetendtobebluerwithincreasingbrightness. radiationfromrelativisticelectronsin a plasmajet. Ifthe same
There is a series of papers covering flux variability studies photonsareup-scatteredtohighenergies,simultaneousvariabil-
(Quirrenbachetal., 1991; Wagneretal., 1996, and references ityfeaturescouldbeexpectedatoptical–GeVfrequencies.But
therein) and morphological/kinematicstudies at radio frequen- theobservedvariabilityoftenchallengessuchscenarios.Toex-
cies (Witzeletal., 1988; Jorstadetal., 2001; Antonuccietal., plore the broadband variability features and to understand the
1986;Bachetal.,2005;Rastorguevaetal.,2011,andreferences underlying emission mechanism, an investigation of long-term
therein).Intradayvariabilityatradiowavelengthsislikelytobe variability over several decades of frequencies is crucial. The
a mixture of intrinsic and extrinsic (due to interstellar scintil- aim of the broadbandvariabilitystudyreportedin this paperis
lation)mechanisms.Asignificantcorrelationbetweenoptical– to provide a general physical scenario which allows us to put
radiofluxvariationsatday-to-daytimescaleshasbeenreported eachobservedvariationofthesourceacrossseveraldecadesof
byWagneretal.(1996).Thefrequencydependenceofthevari- frequenciesinacoherentcontext.
ability index at radio bands is not found to be consistent with Here, we report on a broadband variability study of the
interstellar scintillation (Fuhrmannetal., 2008), which implies source spanning a time period of April 2007 to January 2011.
the presence of very small emitting regions within the source. Themulti-frequencyobservationscompriseGeVmonitoringby
However, the IDV time scale does show evidence in favor of Fermi/LAT, X-ray observationsby Swift-XRT, as well as opti-
anannualmodulation,suggestingthattheIDVofS50716+714 cal and radio monitoring by several ground based telescopes.
couldbedominatedbyinterstellarscintillation(Liuetal.,2012). More explicitly, we investigate the correlation of γ-ray activ-
EGRET on board the Compton Gamma-ray Observatory itywiththeemissionatlowerfrequencies,focusingontheindi-
(CGRO)detectedhigh-energyγ-ray(>100MeV)emissionfrom vidualflaresobservedbetweenAugust2008andJanuary2011.
S5 0716+714severaltimesfrom1991to 1996(Hartmanetal., The evolution of radio (cm and mm) spectra is tested in the
1999; Linetal., 1995). Two strong γ-ray flares on September context of a standard shock-in-jet model. The broadband SED
and October 2007 were detected in the source with AGILE ofthesourceisinvestigatedusingaone-zonesynchrotronself-
(Chenetal., 2008). These authors have also carried out Compton(SSC)modelandalsowithahybridSSCplusexternal
SED modeling of the source with two synchrotron self- radiationComptonmodel.Inshort,thisstudyallowsustoshed
Compton(SSC)emittingcomponents,representativeofaslowly lightonthebroadbandradio-to-γ-rayfluxandspectralvariabil-
and a rapidly variable component, respectively. Observations ityduringaflaringactivityphaseofthesourceoverthisperiod.
by BeppoSAX (Tagliaferrietal., 2003) and XMM-Newton Thispaperisstructuredasfollows.Section2providesabrief
(Foschinietal., 2006) provide evidence for a concave X-ray descriptionofthemulti-frequencydataweemployed.InSection
spectrum in the 0.1 – 10 keV band, a signature of the pres- 3,wereportthestatisticalanalysisanditsresults.Ourdiscussion
ence of both the steep tail of the synchrotronemission and the isgiveninSection4andwepresentourconclusionsinSection
flat part of the Inverse Compton (IC) spectrum. Recently, the 5.
MAGICcollaborationreportedthefirstdetectionofVHEγ-rays
fromthe sourceat the 5.8σsignificancelevel(Anderhubetal.,
2. Multi-frequencyData
2009). The discovery of S5 0716+714 as a VHE γ-ray BL
Lac object was triggered by its very high optical state, sug- FromApril2007toFebruary2011,S50716+714wasobserved
gesting a possible correlation between VHE γ-ray and the op- using both ground and space based observing facilities. These
2
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
Table1. Groundbasedradioobservatories 2.2.Opticaldata
Observatory Tel.dia. Frequency(GHz) Optical V passband data of the source were obtained from the
Effelsberg,Germany 100m 2.7,4.8,6.7 observationsatthe1.5-mKanataTelescopelocatedonHigashi-
8.3,10.7,15,23 Hiroshima Observatory over a time period of February 14,
32,43 2009 to June 01, 2010 (JD’ = 877 to 1349). The Triple Range
UMRAO,USA 26m 4.8,8,14.5 ImagerandSPECtrograph(Watanabeetal.,2005)wasusedfor
NOTO,Italy 32m 5,8,22,43 the observations from JD’ = 612 to 1228. Then, the HOWPol
Urumqi,China 25m 4.8
(Hiroshima One-shot Wide-field Polarimeter, Kawabataetal.,
OVRO,USA 40m 15
2008) was used from JD’ 1434 to 2233 with the V-band filter.
Metsahovi,Finland 14m 37
Exposuretimesforanimagerangedfrom10to80s,depending
PdBI,France 6×15m 86,143,230
PicoVeleta,Spain 30m 86,143,230 onthemagnitudeoftheobjectandtheconditionofsky.Thepho-
SMA,USA 8×6m 230,345 tometryontheCCDimageswasperformedinastandardproce-
dure:afterbiassubtractionandflat-fielddivision,themagnitudes
werecalculatedusingtheaperturephotometrytechnique.
Additionaloptical V-passbanddata were obtainedfrom the
2.3 m Bok Telescope of Steward Observatory from April 28,
2009 through June 2, 2010 (JD’ = 950–1350). These data are
multi-frequency observations of the source extend over a fre-
from the public data archive that provides the results of po-
quency range between radio and γ-rays including optical and
larization and flux monitoring of bright γ-ray blazars selected
X-rays. In the following subsections, we summarize the obser-
from the Fermi/LAT-monitored blazar list3. We have also in-
vationsanddatareduction.
cluded optical V passband archival data extracted from the
American Association of Variable Star Observers (AAVSO;
see http://www.aavso.org/for more information) over a period
2.1.Radioandmmdata
September2008to January2011(JD’ = 710–1600).The com-
Wecollected2.7to230GHzradiowavelengthdataofthesource bined optical V passband flux light curve of S5 0716+714 is
overa time period of April 2007 to January 2011 (JD’1 = 200 showninFigure2(c).
to 1600) using the 9 radio telescopes listed in Table 1. The
cm/mm radio light curves of the source were observed as a
2.3.X-raydata
part of observations within the framework of F-GAMMA pro-
gram(Fermi-GSTrelatedmonitoringprogramof γ-rayblazars, The X-ray (0.3 – 10 keV) data were obtained by Swift-XRT
Fuhrmannetal., 2007; Angelakisetal., 2008). The overallfre- overa time period of September 2008 to January 2011 (JD’ =
quency range spans from 2.7 GHz to 230 GHz using the 710–1600).TheSwift-XRTdatawereprocessedusingthemost
Effelsberg100mtelescope(2.7to43GHz)andtheIRAM30m recent versions of the standard Swift tools: Swift Software ver-
TelescopeatthePicoVeleta(PV)Observatory(86to230GHz). sion3.8,FTOOLSversion6.11andXSPECversion12.7.0(see
Thesefluxmeasurementswereperformedquasi-simultaneously Kalberlaetal.,2005,andreferencestherein).
using the cross-scan method slewing over the source position, All of the observations were obtained in photon counting
inazimuthandelevationdirectionwithadaptivenumberofsub- (PC) mode. Circular and annular regions are used to describe
scans for reaching the desired sensitivity. Subsequently, atmo- the source and background areas, respectively, and the radii
sphericopacitycorrection,pointingoff-setcorrection,gaincor- of both regions depend on the measured count rate using the
rectionandsensitivitycorrectionhavebeenappliedtothedata. FTOOLS script xrtgrblc. Spectral fitting was done with an ab-
This source is also a part of an ongoing blazar mon- sorbedpower-lawwithNH =0.31×1021cm−2settotheGalactic
itoring program at 15 GHz at the Owens Valley Radio valuefoundbyKalberlaetal.(2005).Forthreeofthe observa-
Observatory(OVRO) 40-m radio telescope which providesthe tions,thereweremorethan100countsinthesourceregion,and
radio data sampled at 15 GHz. We have also used the com- a chi-squared statistic is used with a minimum bin size of 20
bined data from the University of Michigan Radio Astronomy cts/bin.ForthebincenteredonJD’=1214,only62countswere
Observatory(UMRAO;4.8,8and14.5GHz,Alleretal.,1985) foundinthesourceregion,andtheunbinneddataarefitted us-
and the Metsa¨hovi Radio Observatory (MRO; 22 and 37 GHz; ing C-statistics as described by Cash (1979). One sigma errors
Tera¨srantaetal.,1998,2004),whichprovideuswithradiolight inthede-absorbedfluxwerecalculatedassumingthattheyshare
curves at 5, 8, 15 and 37 GHz. Additional flux monitoring at thesamepercentageerrorsastheabsorbedfluxforthesametime
5, 8, 22 and 43 GHz radio bands is obtained using 32 m tele- andenergyrange.TheX-raylightcurveofthesourceisshown
scopeatNOTOradioobservatory.TheUrumqi25mradiotele- inFig.2(b).
scope monitors the source at 5 GHz. The 230 and 345 GHz
dataareprovidedbytheSubmillimeterArray(SMA)Observer
2.4.γ-raydata
Center2database(Gurwelletal.,2007),complementedbysome
measurementsfromPV andthePlateaudeBureInterferometer We employ γ-ray (100 MeV – 300 GeV) data collected be-
(PdBI). The radio light curves of the source are shown in Fig tween a time period August 08, 2008 to January 30, 2011
1. The mm observationsare closely coordinatedwith the more (JD’=686–1592)insurveymodebyFermi/LAT.TheLATdata
general flux density monitoring conducted by IRAM, and data areanalyzedusingthestandardScienceTools(softwareversion
frombothprogramsarealsoincluded. v9.23.1)and the instrument response function P7V64. Photons
in the Source event class are selected for this analysis. We se-
1 JD’=JD−2454000 3 http://james.as.arizona.edu/∼psmith/Fermi
2 http://sma1.sma.hawaii.edu/callist/callist.html 4 http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/overview.html
3
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
Fig.1.RadiotommwavelengthlightcurvesofS50716+714observedoverthepast∼3years.Forclarity,thelightcurvesatdifferent
frequenciesareshownwitharbitraryoffsets(indicatedbya“Frequency+xJy”label).Themajorradioflaresarelabeledas“R0”,
“R6”and“R8”(seeFig.2forthedetailsoflabeling).
lect γ-rays with zenith angles less than 100 deg to avoid con- (isotropic p7v6source.txt)withbothfluxandspectralphotonin-
tamination from γ-rays producedby cosmic ray interactionsin dexleftfreein themodelfit. Thedataanalysisisdonewith an
theupperatmosphere.ThediffuseemissionfromourGalaxyis unbinned maximum likelihood technique (Mattoxetal., 1996)
modeled using a spatial model5 (gal 2yearp7v6 v0.fits) which using the publicly available tools6. We analyzed a Region of
was derived with Fermi/LAT data taken during the first two Interest (RoI) of 10◦ in radius, centered on the position of the
years of operation. The extragalactic diffuse and residual in- γ-raysourceassociatedwithS50716+714,usingthemaximum-
strumentalbackgroundsaremodeledasanisotropiccomponent likelihood algorithm implemented in gtlike. In the RoI model,
5 http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html 6 http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/likelihood tutorial.html
4
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
AAuugguusstt 22000088 JJaannuuaarryy 22001111
((aa))
((bb))
((cc))
((dd))
JJDD [[22445544000000 ++]]
Fig.2. Light curvesof 0716+714from γ-ray to radio wavelengths(a):GeV light curveat E>100MeV, (b): X-ray light curveat
0.3−10KeV, (c):opticalV passbandlightcurveand(d):5 to 230GHz radiolightcurves.Verticaldottedlinesare markedw.r.t.
differentopticalflareslabelingthebroadbandflaresas“G”for γ-rays,“X”forX-rays,“O”foropticaland“R”forRadiofollowed
bythenumberclosetoflare.TheyellowarearepresentstheperiodforwhichweconstructthebroadbandSEDsofthesource(see
Section3.5fordetails).
we include all the 24 sources within 10◦ with their model pa- ismuchsmallerthanthestatisticalerrorsreportedforthe source
rameters fixed to their 2FGL catalog values, as none of these (Ackermannetal.,2012).
sourcesarereportedtobevariable(seeNolanetal.,2012,forde- Source variability is investigated by producing the light
tails).Thecontributionofthese24sourceswithintheRoImodel curves(E >100MeV)bylikelihoodanalysiswithtimebinsof
to the observed variability of the source is negligible as they 1,7and30days.Thebin-to-binexposuretimevariationisless
areveryfaintcomparedtoS50716+714.TheLATinstrument- than 7%. The light curvesare producedby modeling the spec-
inducedvariabilityistestedwithbrightpulsarsandis ∼2%and traovereachbinbyasimplepowerlawwhichprovideagoodfit
5
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
Table2.Variabilitytimescalesatradiowavelengths
Frequency β1∗ t (days) β2 t (days) 223300 GGHHzz
var1 var2 8866 GGHHzz
15GHz 0.95±0.03 100±5 1.32±0.04 195±5
37GHz 1.50±0.13 100±5 1.64±0.10 200±5
43GHz 0.99±0.02 90±5 1.23±0.04 180±5
86GHz 1.60±0.10 90±5 0.89±0.02 180±5
230GHz 1.04±0.03 90±5 1.31±0.03 180±5
∗:P(f)∼ fβ,βistheslopeofthepowerlawfit. 3377 GGHHzz 4433 GGHHzz
overthesesmallbinsoftime.Here,wesetboththephotonindex
andintegralfluxasfreeparameters.Wewillusetheweeklyav-
eragedlightcurvesforthemulti-frequencyanalysis,asthedaily 1100 GGHHzz 1155 GGHHzz
averagedfluxpointshavea lowTS(TestStatistics) value(<9).
In a similar way,we constructthe GeV spectrumofthe source
overdifferenttimeperiods(seeSection3.5fordetails).Wesplit
the 0.1 to 100 GeV spectra into 5 different energy bins : 0.1 –
0.3,0.3–1.0,1.0–3.0,3.0–10.0and10.0–100.0GeV.Asim- 55 GGHHzz 88 GGHHzz
ple power law with constant photon index Γ (the best fit value
obtained for the entire energy range) providesa good estimate
ofintegralfluxovereachenergybin.TheGeVspectrumofthe
sourceisinvestigatedoverfourdifferenttimeperiods,whichrep- 1100 110000
resentdifferentbrightnessstates of thesourceandwill beused
in broadband spectral modeling. The details of the broadband
spectralanalysisaregiveninSection3.5.
VV ppaassssbbaanndd
3. AnalysisandResults
3.1.LightCurveAnalysis
3.1.1. Radiofrequencies
Fig.1displaysthe2.7–230GHzradiobandlightcurvesofthe
sourceobservedoverthepast∼3years.Thesourceexhibitssig- 110000 MMeeVV -- 330000 GGeeVV
nificant variability, being more rapid and pronounced towards
higher frequencies over this period with two major outbursts.
Towardslowerfrequencies(<10GHz) the individualflaresap-
pearlesspronouncedandbroadenintime.
To quantifythestrengthof variabilityat differentradiofre-
quencies,we extractthetimescale ofvariability(t )fromthe
var
observedlightcurves.Weemployedthestructurefunction(SF)
(Simonettietal., 1985) analysis method following Ranietal. Fig.3. Top: Structure functions at radio frequencies. The solid
(2009).TheradioSFcurvesareshowninFig.3. curves represent the best fitted power laws. The dotted lines
At 15 GHz and higher radio frequencies, the SF curve fol- in each plot indicate the timescale of variability (t ). Bottom:
var
lowacontinuousrisingtrendshowingapeakattvar1,following Structurefunctionsatopticalandγ-rayfrequencies.
a plateauand againreachinga maximumat t . However,the
var2
SFcurveat10GHzandlowerfrequenciesdonotrevealasharp
break in the slope as the variability features seem to be milled differenttime lags.In fig. 4, we show theSF vsfrequencyplot
outatthesefrequencies.Wedonotconsidervariabilityfeatures atatimelagof100days.Thesourcedisplaysmorepronounced
attimelagslongerthanhalfofthelengthoftheobservationsdue and faster variability at higher radio frequencies compared
totheincreasingstatisticaluncertaintyoftheSFvaluesinthisre- to lower ones, peaking at a frequency of 43 GHz, and have
gion.Toextractthevariabilitytimescales,wefittedapowerlaw similar amplitudeat higherfrequencies.It seems thatthe radio
(P(f)∼ fβ)tothetworisingpartsoftheSFcurves.Thevariabil- variability saturates above this frequency. We notice a similar
ity timescale is given by a break in the slope of the power-law trendat50and200daytimelags. Thus,fordifferenttime lags
fits.InFig.3(top),theredcurvesrepresentthefittedpower-law thevariabilitystrengthexhibitsasimilarfrequencydependence.
torisingtrendofSFcurvesandtheverticaldottedlinesstandfor
t andt .Thebestfittedvaluesofthepower-lawslopesand
var1 var2
theestimatedtimescaleofvariabilityaregiveninTable2.Thus,
3.1.2. Opticalfrequencies
the SF curve reveals two different variability time scales, one
which reflects the short-term variability (t ) while the other Thesourceexhibitsmulti-flaringbehavioratopticalfrequencies
var1
onereferstothelong-termvariability(t ). witheachflareroughlyseparatedby60–70days(seeFig.2(c)).
var2
TheSF valueisproportionaltothesquareoftheamplitude The optical V band SF curve in Fig. 3 (bottom) shows rapid
of variability, so to compare the strength of the variability at variability with multiple cycles of rises and declines. The first
different frequencies, we produce SF vs frequency plots at peak in SF curve appears at a timescale t ∼ 30 days which
var
6
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
Fig.4.SFvsfrequencyatGHzfrequenciesforastructurefunc-
tiontime=100days. Fig.6.OpticalVpassbandlightcurveofS50716+714withdif-
ferenttime binning.The lightcurve appearsas a superposition
offastflaresonamodulatedbaselevelvaryingona(350±9)day
timescale.Theseslowervariationscanbeclearlyseenin75days
binnedlightcurve(errorbarrepresentsvariationsinfluxoverthe
binnedperiod).Thedashedlinerepresentsasplineinterpolation
throughthe75daybinnedlightcurve.Dottedlinesareobtained
byshiftingthesplineby±0.65mag.
It is for this reason that we further inspect the significance of
this frequencywith the PowerSpectralDensity (PSD) analysis
method(Vaughan,2005) whichisa powerfultooltosearchfor
periodicsignalsintimeseries,includingthosecontaminatedby
whitenoiseand/orrednoise.Toachieveauniformsamplingin
theopticaldata,weadoptatime-averagebinningof3days.We
foundthatthe significanceoftheperiodat ∼ 60daysisonly2
σ.Wethereforecannotclaimasignificantdetectionofaquasi-
periodicvariabilityfeatureatthisfrequency.
Wealsonoticethatduringthecourseofouropticalmonitor-
ing,thepeak-to-peakamplitudeoftheshort-termvariationsre-
mainsalmostconstant,∼1.3magnitudes.Hence,thevariability
trendtracedbytheminimaisverysimilartothatbythemaxima
of the light curve during the course of ∼ 2 years of our moni-
Fig.5.LSPanalysiscurveshowingapeakataperiodof63and toring. The constant variability trend is displayed in Fig. 6. In
359days. thisfigure,thedashedlinedenotesasplinethroughthe75-days
binnedlightcurveandthe dottedlinesareobtainedby shifting
thesplineby±0.65mag.So,theobservedvariationsfallwithina
isfollowedbyadipat∼ 60days.Thispeakcorrespondstothe constantvariationarea.Aconstantvariabilityamplitudeinmag-
fastestvariabilitytimescale.ThepeaksintheopticalSFcurveat nitudesimpliesthatthefluxvariationamplitudeisproportional
t = 90, 180, 240 and 300 days representthe long-termvari- tothefluxlevelitself(followingm −m2∝log (f1/f2)).This
var 1 10
ability timescales. This indicates a possible superposition of a can be easily interpreted in terms of variations of the Doppler
short 30-40 day time scale variability with the long-term vari- boostingfactor,δ = [Γ(1−βcosθ)]−1(Raiterietal.,2003).In
abilitytrend. such a scenario, the observedflux is relativistically boostedby
Multiple cyclesin theopticalSF curverepresentthe nearly afactorofδ3 andrequiresavariationin δbyafactorof ∼ 1.2.
periodic variations at ∼60 days timescale. We used the Lomb- Suchachangeinδcanbeduetoeitheraviewingangle(θ)vari-
Scargle Periodogram (LSP) (Lomb, 1976; Ranietal., 2009) ation or a change of the bulk Lorentz factor (Γ) or may be a
method to test the presence of this harmonic. The LSP analy- combination of both. A change in δ by a factor of 1.2 can be
sisofthewholedatasetisdisplayedinFig.5.TheLSPanalysis easilyinterpretedasafewdegreevariationinθwhileitrequires
reveals two significant (>99.9999% confidence level) peaks at a noticeable change of the bulk Lorentz factor. Therefore, it is
359 and 63 days. The peak at 359 days is close to half of the more likely that the long-term flux base-level modulations are
duration of observations, so it is hard to claim this frequency dominatedbyageometricaleffectthanbyanenergeticone.
due to limited numberof cycles. The nearly annualperiodicity Hence, we consider that the optical variability amplitude
could also be effected by systematic effects due to annual ob- remains almost constant during our observations. A similar
servingcycle.A visualinspectionofthelightcurveindicatesa variabilitytrendwasalsoobservedinthissourcebyRaiterietal.
totalof7rapidflaresseparatedby60–70days.Also,theLSPis (2003).Theyalsonoticedapossibilityofnearlyperiodicoscil-
onlysensitiveforadominantwhite-noiseprocess(P (f)∝ f0). lations at a timescale of 3.0±0.3 years, but due to the limited
N
7
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
timecoveragethisremainsuncertain.Theopticallightcurveof
the source also displays fast flares with a rising timescales of
∼ 30day.However,therisingtimescaleoftheradioflaresisof
theorderof∼60days(seeTable5).Thus,theopticalvariability
featuresareveryrapidcomparedtothoseatradiowavelengths.
3.1.3. X-rayfrequencies
Fig.2(b)displaysthe0.3—10keVlightcurveofS50716+714.
Althoughthe X-raylightcurveis notas wellsampled as those
at other frequencies, the data indicate a flare at keV energies
betweenJD’=1122to1165.Duetolargegapinthedatatrain,
it is not possible to locate the exact peak time of this flare.
Still, it is interesting to note that this orphan X-ray flare is not
simultaneous with any of the GeV flares nor with the optical
flares.Itsoccurrencecoincideswiththeoptical–GeVminimum
afterthemajorflaresatthesefrequencies(O5/G5,seeFig.2).
Fig.7. Gamma-ray flux and photon index light curve of
S5 0716+714 measured with the Fermi/LAT for a time period
betweenAugust04,2008toFebruary07,2011.Thebluesym-
3.1.4. Gamma-rayfrequencies
bolsshowtheweeklyaveragedfluxwhilemonthlyaveragedval-
The GeV light curve observed by Fermi/LAT is extracted over uesareplottedin red.Differentactivitystatesofthesourceare
separatedbyverticalgreenlines.Amarginalsofteningofspec-
thetimeperiodofAugust2008toFebruary2011.Fig.7shows
trumoverthequiescentstatecanbeseenhere.
the weekly and monthlyaveraged γ-ray light curvesintegrated
overthe energyrange 100 MeV to 300 GeV. There is a signif-
icant enhancement in the weekly averaged γ-ray flux over the
timeperiodofJD’=900to1110,peakingatJD’ ∼1110,with activity. The source displayed multiple flares across the whole
a peakfluxvalueof(0.57±0.05)×10−6 phcm−2s−1, whichis electromagnetic spectrum over this period, which we label as
∼6timesbrighterthantheminimumfluxand∼3timesbrighter follows.Wemarktheverticaldottedlinesw.r.t.differentoptical
thantheaveragefluxvalue.Lateritdecays,reachingaminimum flares,labelingthebroadbandflaresas“G”forγ-rays,“X”forX-
atJD’=1150,andthenitremainsinaquiescentstateuntilJD’ rays,“O”foropticaland“R”forradiofollowedbythenumber
=1200.Thequiescentstateisfollowedbyanothersequenceof adjacenttothem.Forexample,theopticalflaresshouldberead
flares.Thesourcedisplayssimilarvariabilityfeaturesinthecon- as“O1”to“O9”.
stantuncertaintylightcurveobtainedusingtheadaptivebinning Tosearchforpossibletimelagsandtoquantifythecorrela-
analysis method following Lottetal. (2012). A more detailed tion among the multi-frequency light curves of the source, we
discussionofGeVvariabilityisgiveninRanietal.(2012). computeddiscretecross-correlationfunctions(DCFs)following
Thesourceexhibitsamarginalspectralsofteningduringthe themethoddescribedby(Edelson&Krolik, 1988).Thedetails
quiescent state in the monthly averaged light curve. The γ-ray of this method are also discussed by Ranietal. (2009). In the
photonindex(Γ)changesfrom∼2.08±0.03(II)to2.16±0.02 followingsections, we will discuss in detail the correlationbe-
(III),thenagainto 2.04±0.04(IV).Differentactivitystates of tweentheobservedlightcurves.
thesourceareseparatedbyverticallinesinFig.7.Wenoticeno
clear spectral variation in the weekly averaged light curve due
3.2.1. Radiocorrelation
tolargeuncertaintyandscatterofindividualdatapoints.
At radio wavelengths, the source exhibits significant flux vari-
TheSF curveofthe sourceatGeV frequenciesisshownin ability, being more rapid and pronounced towards higher fre-
Fig. 3 (bottom). The variability timescales are extracted using quencies. We label the two major outbursts as “R6” and “R8”
thepower-lawfittingmethodasdescribedabove,whichgivesa (seeFig.2).Apparently,themmflaresareobservedafewdays
breakat ∼ 30 and 180days. We notice thatthe variability fea- earlierthanthecmflares. Ourdensefrequencycoveragefacili-
turesatt ∼ 180daysare also observedatradioandoptical tatesacross-correlationanalysisbetweenthedifferentobserving
var
frequencies.However,thefastervariability(t ∼ 30days)ob- bands. Owing to its better time sampling, we have chosen the
var
servedatopticalandγ-rayfrequenciesdoesnotextendtoradio 15GHzlightcurveasareference.Fig.8showstheDCFcurves
wavelengths.Asimilarlong-termvariabilitytimescaleatγ-ray, adoptingabinningof10days.
optical and radio frequencies provides a possible hint of a co- WeusedaGaussianprofilefittingtechniquetoestimatethe
spatialorigin.Inthefollowingsections,wewillsearchforsuch timelagandrespectivecross-correlationcoefficientvalueforthe
possiblecorrelations. DCF curves. Around the peak, the DCF curve as a functionof
time lag t can be reasonably well described by a Gaussian of
theform:DCF(t) = a× exp[−(t−b)2],whereaisthepeakvalue
3.2.CorrelationAnalysis 2c2
of the DCF, b is the time lag at which the DCF peaks and c
Themulti-frequencylightcurvesofS50716+714arepresented characterizesthewidthoftheGaussianfunction.Thecalculated
in Figure 2. The analysis presented in this section is focused parameter (a, b and c) values for each frequency are listed in
ona timeperiodbetweenJD’ = 800to 1400,whichcoversthe Table3.ThesolidcurverepresentsthefittedGaussianfunction
twomajorradioflaresandtherespectiveoptical-to–γ-rayflaring inFig.8.
8
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
Fig.8.TheDCFcurvesamongthedifferentradiofrequencylightcurves.Thesolidcurvesrepresentthebest-fitGaussianfunction.
Table3. Correlationanalysisresultsamongradiofrequencies belesscorrelatedwiththoseat15GHz,whichisobviousasthe
flaringbehaviorisnotclearlyvisiblebelow15GHz.
Frq.(GHz) a b(days) c(days)
The long term radio light curve shows three major radio
230vs15 1.56±0.15 7.96±2.23 19.81±2.24
flares, labeled as R0, R6 and R8 in Fig. 1. In the correlation
86vs15 0.94±0.13 6.65±3.28 25.80±4.28
analysis of the entire light curves, these flares are blended and
43vs15 1.04±0.11 5.95±2.08 23.86±3.09
37vs15 1.13±0.09 4.95±2.21 29.39±2.81 folded into a single DCF. In order to separate the flares from
23vs15 1.17±0.10 3.74±1.50 25.00±2.50 each other, we performeda correlation analysis overthree dif-
10vs15 0.89±0.09 -1.01±1.09 35.07±4.49 ferent time bins which cover the time ranges of the individual
8vs15 0.84±0.08 -1.09±1.01 35.96±4.10 radio flares: JD’ = 500 to 750 (R0 flare), JD’ = 1000 to 1210
5vs15 0.84±0.10 -1.23±1.25 33.13±4.15 (R6 flare) and JD’ = 1210to 1400(R8 flare). Thetime lagsof
2.72vs15 0.59±0.12 -78.75±12.39 53.54±13.86 these flaresrelativeto the15 GHz dataare estimatedas above.
a:peakvalueoftheDCF, Howeverduetosparsedatasampling,itwasnotpossibletoes-
b:timelagatwhichtheDCFpeaks,and
timatethetimelagsforR8directlyusingtheDCFmethod.So,
c:widthoftheGaussianfunction(seetextfordetails)
forthisflare,wefirstinterpolatethedatathroughasplinefunc-
tionand thenperformthe DCF analysis,exceptat23 GHz and
33GHz(duetolongdatagaps).Thecalculatedtimelagsofeach
flarearegiveninTable4.
Our analysis confirms the existence of a significant corre-
lation across all observed radio-band light curves till 15 GHz In Fig. 9, we report the calculated time lags as a function
withformaldelayslistedinTable3(parameterb).However,no of frequency with 15 GHz as the reference frequency. The
pronounceddelayedfluxvariationsareobservedat10GHzand estimated time lag using the entire light curves are shown
lowerfrequencies.Also,thefluxvariationsat2.7GHzseemto with blue circles. As we see in Fig. 9, the time lag increases
9
B.Ranietal.:Radiotogamma-rayvariabilitystudyofblazarS50716+714
Table4. Radiocorrelationanalysisresultsforindividualflares
Frequency Timelag∗(days)
(GHz) R0Flare R6Flare R8Flare
23 2.9±1.4 4.06±1.6 —
33 2.1±1.0 5.88±2.1 —
37 4.2±2.6 3.15±2.1 4.0±1.0
86 4.2±1.0 6.15±2.6 5.0±1.8
143 5.8±1.8 6.82±2.5 7.0±1.5
230 6.0±2.4 8.54±1.9 9.0±2.0
∗:relativetotherespectiveflaresat15GHz
Fig.10. The modeled radio flare, R6. The blue points are the
observeddatawhiletheredcurverepresentthefit.
Table5. FittedmodelparametersforR6flare
Frq. f∗0 fmax tr td t0
GHz JD’[JD–2454000]
15 0.02±0.07 4.15±0.14 61.4±6.2 37.9±3.5 1191.4±0.9
23 0.71±0.23 5.30±0.15 32.3±4.8 17.3±2.1 1190.2±0.1
37 0.58±0.11 8.20±0.59 55.5±11.0 18.6±3.2 1189.1±0.7
43 0.45±0.35 9.50±0.62 60.5±9.4 20.1±2.9 1188.0±0.8
86 0.64±0.18 10.6±2.48 60.9±28.8 25.1±25.6 1186.0±0.5
230 0.72±0.11 12.64±0.29 50.3±2.4 9.9±0.6 1184.2±0.4
∗:seetextfortheextensionoflabels.
vstimelagsresultsobtainedbytheDCFmethod.Astheflaring
Fig.9. Theplotof timelag vsfrequency,using15GHz asthe
behaviorisnotclearlyvisiblebelow15GHz,sowerestrictthis
referencefrequency.Timelagvsfrequencycurvesforindividual
analysistofrequenciesabove15GHz.Asthereisnoobservation
flaresareshownwithdifferentcolors.Thesolidlinesrepresent
availableduringtheflaringepochat23and86GHz,wefixt and
r
thebestfittedpowerlawineachcase.
t using the fit parameters from the adjacent frequencies. The
d
best fit of the functionf(t) for the R6 flare is shown in Fig. 10
andtheparametersaregiveninTable5.Theestimatedtimeshift
with an increase in frequency and seems to follow a power
aroundtheR6flareateachfrequencyw.r.t.15GHzareshownin
law. Consequently, we fitted a power law, P(f) = Nfα to
Fig.9(redsymbols)andthefittedpowerlawparametersareN =
the time lag vs frequency curve. The best fitted parameters
1.17±0.13,α=−0.31±0.03.Thus,thisalternativeestimateof
are N = 1.71 ± 0.43, α = 0.30 ± 0.08. For the individual
thepowerlawslopeusingthemodelfittingtechniqueconfirms
flares, we find: N = 1.07±0.06, α = 0.32±0.01 for the R0,
theresultsobtainedbytheDCFanalysis.
N = 1.45±0.61,α = 0.32±0.08forR6,andN = 1.33±0.01,
α=0.29±0.03forR8.Weconcludethatacommontrendinthe
timelag(with15GHzasthereferencefrequency)vs.frequency 3.2.2. RadiovsOpticalcorrelation
curveisseenforallthethreeradioflares(R0,R6andR8)with
S50716+714exhibitsmultipleflaresatopticalfrequencies.The
anaverageslopeof0.30.
flaresareroughlyseparatedby60–70days.Welabeledthedif-
ferentopticalflaresasO1–O9asshowninFig.2.Duringthis
Wealsofollowedanalternativeapproachtoestimatethetime
multi-flaringactivityperiodtwomajorflaresareobservedatra-
shift of the radio flares at each frequency w.r.t. 15 GHz. The
diowavelengths.TheradioflareR6apparentlycoincidesintime
flaresateachfrequencycanbemodeledwithanexponentialrise
withO6andR8withO8.Toinvestigatethepossiblecorrelation
anddecayoftheform:
among the flux variations at optical and radio frequencies, we
f(t)= f + f exp[(t−t )/t ], fort<t ,and (1) performaDCFanalysisusingthe2-year-longsimultaneousop-
0 max 0 r 0
ticalandradiodatatrainsbetweenJD’ = 680to 1600(see Fig.
= f + f exp[−(t−t )/t ] fort>t 2). Note that the strength of flux variability increases towards
0 max 0 d 0
higherfrequencies,peaking at 43 GHz (see Fig. 3). Therefore,
wheref isthebackgroundfluxlevelthatstaysconstantoverthe wechoosetworadiofrequencies,37GHzand230GHz,inorder
0
correspondinginterval,f istheamplitudeoftheflare,t isthe tocomparethestrengthofradio–opticalcorrelationaboveand
max 0
epochofthepeak,andt andt aretheriseanddecaytimescales, below the saturation frequency(43 GHz). The optical vs radio
r d
respectively.SinceR6isthemostpronouncedandbestsampled DCFanalysiscurvesareshowninFig.11(a).Multiplepeaksin
flare, we modelthis flare in orderto cross-checkthe frequency theDCFmayreflectaQPObehavioratopticalfrequencies.As
10