The first Fermi multifrequency campaign on BL Lacertae: characterizing the low-activity state of the eponymous blazar

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by  A. A. Abdo

ACCEPTEDBYTHEASTROPHYSICALJOURNAL
PreprinttypesetusingLATEXstyleemulateapjv.11/10/09
THEFIRSTFERMIMULTIFREQUENCYCAMPAIGNONBLLACERTAE:
CHARACTERIZINGTHELOW-ACTIVITYSTATEOFTHEEPONYMOUSBLAZAR
A.A.ABDO1,2,M.ACKERMANN3,M.AJELLO3,E.ANTOLINI4,5*,L.BALDINI7,J.BALLET8,G.BARBIELLINI9,10,D.BASTIERI11,12,K.BECHTOL3,
R.BELLAZZINI7,B.BERENJI3,R.D.BLANDFORD3,E.BONAMENTE4,5,A.W.BORGLAND3,J.BREGEON7,A.BREZ7,M.BRIGIDA13,14,P.BRUEL15,
R.BUEHLER3,S.BUSON11,12,G.A.CALIANDRO16,R.A.CAMERON3,A.CANNON17,18,P.A.CARAVEO19,S.CARRIGAN12,J.M.CASANDJIAN8,
C.CECCHI4,5,O¨.C¸ELIK17,20,21,E.CHARLES3,A.CHEKHTMAN1,22,C.C.CHEUNG1,2,J.CHIANG3,S.CIPRINI5*,R.CLAUS3,J.COHEN-TANUGI23,
J.CONRAD24,25,26,L.COSTAMANTE3,S.CUTINI27,C.D.DERMER1,F.DEPALMA13,14,D.DONATO20,28*,E.DOCOUTOESILVA3,P.S.DRELL3,
R.DUBOIS3,L.ESCANDE29,C.FAVUZZI13,14,S.J.FEGAN15,J.FINKE1,2,W.B.FOCKE3,P.FORTIN15,M.FRAILIS30,31,Y.FUKAZAWA32,S.FUNK3,
1 P.FUSCO13,14,F.GARGANO14,D.GASPARRINI27,N.GEHRELS17,S.GERMANI4,5,N.GIGLIETTO13,14,F.GIORDANO13,14,M.GIROLETTI33,
1 T.GLANZMAN3,G.GODFREY3,I.A.GRENIER8,S.GUIRIEC34,D.HADASCH16,M.HAYASHIDA3,E.HAYS17,R.E.HUGHES35,R.ITOH32,
0 G.JO´HANNESSON36,A.S.JOHNSON3,W.N.JOHNSON1,T.KAMAE3,H.KATAGIRI32,J.KATAOKA37,J.KNO¨DLSEDER38,M.KUSS7,J.LANDE3,
2 S.LARSSON24,25,39,L.LATRONICO7,S.-H.LEE3,M.LLENAGARDE24,25,F.LONGO9,10,F.LOPARCO13,14,B.LOTT29,M.N.LOVELLETTE1,
P.LUBRANO4,5,A.MAKEEV1,22,M.N.MAZZIOTTA14,J.E.MCENERY17,28,J.MEHAULT23,P.F.MICHELSON3,T.MIZUNO32,C.MONTE13,14,
n M.E.MONZANI3,A.MORSELLI40,I.V.MOSKALENKO3,S.MURGIA3,T.NAKAMORI37,M.NAUMANN-GODO8,S.NISHINO32,P.L.NOLAN3,
a J.P.NORRIS41,E.NUSS23,T.OHSUGI42,A.OKUMURA43,N.OMODEI3,E.ORLANDO44,J.F.ORMES41,M.OZAKI43,D.PANEQUE3,J.H.PANETTA3,
J D.PARENT1,22,V.PELASSA23,M.PEPE4,5,M.PESCE-ROLLINS7,F.PIRON23,T.A.PORTER3,S.RAINO`13,14,R.RANDO11,12,M.RAZZANO7,
1 A.REIMER45,3,O.REIMER45,3,S.RITZ46,M.ROTH47,H.F.-W.SADROZINSKI46,D.SANCHEZ15,A.SANDER35,F.K.SCHINZEL48,C.SGRO`7,
3 E.J.SISKIND49,P.D.SMITH35,K.V.SOKOLOVSKY48,50*,G.SPANDRE7,P.SPINELLI13,14,M.S.STRICKMAN1,D.J.SUSON51,H.TAKAHASHI42,
T.TANAKA3,J.B.THAYER3,J.G.THAYER3,D.J.THOMPSON17,L.TIBALDO11,12,8,52,D.F.TORRES16,53,G.TOSTI4,5*,A.TRAMACERE3,54,55,
] T.UEHARA32,T.L.USHER3,J.VANDENBROUCKE3,V.VASILEIOU20,21,N.VILCHEZ38,V.VITALE40,56,A.P.WAITE3,E.WALLACE47,P.WANG3,
E B.L.WINER35,K.S.WOOD1,Z.YANG24,25,T.YLINEN57,58,25,M.ZIEGLER46,A.BERDYUGIN59,M.BOETTCHER60,A.CARRAMIN˜ANA61,
H L.CARRASCO61,E.DELAFUENTE62,C.DILTZ60,T.HOVATTA63,V.KADENIUS59,Y.Y.KOVALEV50,48,A.LA¨HTEENMA¨KI63,E.LINDFORS59,
A.P.MARSCHER64,K.NILSSON65,D.PEREIRA17,R.REINTHAL59,P.ROUSTAZADEH60,T.SAVOLAINEN48,A.SILLANPA¨A¨59,L.O.TAKALO59,
h. M.TORNIKOSKI63
p AcceptedbyTheAstrophysicalJournal
-
o ABSTRACT
r
Wereportonobservations ofBLLacertaeduringthefirst18monthsofFermiLATscienceoperations andpresentresultsfroma48-daymultifrequency
t
s coordinatedcampaignfrom2008August19to2008October7.Theradiotogamma-raybehaviorofBLLacisunveiledduringalowactivitystatethankstothe
a coordinatedobservationsofradio-band(Metsa¨hoviandVLBA),near-IR/optical(Tuorla,Steward,OAGHandMDM)andX-ray(RXTE andSwift)observatories.
[ Novariabilitywasresolvedingamma-raysduringthecampaign,andthebrightnesslevelwas15timeslowerthanthelevelofthe1997EGREToutburst.Moderate
anduncorrelatedvariabilityhasbeendetectedinUVandX-rays.TheX-rayspectrumisfoundtobeconcaveindicatingthetransitionregionbetweenthelowand

1 highenergycomponentofthespectralenergydistribution(SED).VLBAobservationdetectedasynchrotronspectrumself-absorptionturnoverintheinnermost
v partoftheradiojetappearingtobeelongatedandinhomogeneous, andconstrainedtheaveragemagneticfieldtheretobelessthan3G.Overthefollowing
monthsBLLacappearedvariableingamma-rays,showingflares(in2009Apriland2010January).Thereisnoevidenceforcorrelationofthegamma-rayswith
5
theopticalfluxmonitoredfromthegroundin18months. TheSEDmaybedescribedbyasinglezoneortwozonesynchrotronself-Compton(SSC)model,
0 butahybridSSCplusexternalradiationCompton(ERC)modelseemspreferredbasedontheobservedvariabilityandthefactthatitprovidesafitclosestto
9 equipartition.
5 Subjectheadings:Gamma rays: galaxies– BL Lacertae objects: individual: BL Lac – BL Lacertae objects:
.
1 general–X-rays:galaxies–galaxies:jets–radiationmechanisms:non-thermal
0
1
1
v: *Correspondingauthors: S.Ciprini,[email protected]; E.An- Padova,Italy
i tolini, [email protected]; D. Donato, [email protected]; 13DipartimentodiFisica“M.Merlin”dell’Universita` edelPolitecnico
X K.V.Sokolovsky,[email protected];G.Tosti,[email protected]. diBari,I-70126Bari,Italy
1SpaceScienceDivision,NavalResearchLaboratory,Washington,DC 14Istituto Nazionale diFisicaNucleare, Sezione diBari, 70126Bari,
r
a 20375,USA Italy
2NationalResearchCouncilResearchAssociate,NationalAcademyof 15Laboratoire Leprince-Ringuet, E´cole polytechnique, CNRS/IN2P3,
Sciences,Washington,DC20001,USA Palaiseau,France
3W. W. Hansen Experimental Physics Laboratory, Kavli Institute 16InstitutdeCiencies del’Espai(IEEC-CSIC),CampusUAB,08193
for Particle Astrophysics and Cosmology, Department of Physics and Barcelona,Spain
SLACNationalAcceleratorLaboratory,StanfordUniversity,Stanford,CA 17NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA
94305,USA 18UniversityCollegeDublin,Belfield,Dublin4,Ireland
4IstitutoNazionalediFisicaNucleare,SezionediPerugia,I-06123Pe-
19INAF-IstitutodiAstrofisicaSpazialeeFisicaCosmica,I-20133Mi-
rugia,Italy
lano,Italy
5DipartimentodiFisica,Universita`degliStudidiPerugia,I-06123Pe-
20CenterforResearchandExplorationinSpaceScienceandTechnol-
rugia,Italy
ogy(CRESST)andNASAGoddardSpaceFlightCenter,Greenbelt,MD
7IstitutoNazionale diFisicaNucleare, SezionediPisa,I-56127Pisa,
20771,USA
Italy 21DepartmentofPhysicsandCenterforSpaceSciencesandTechnol-
8Laboratoire AIM, CEA-IRFU/CNRS/Universite´ Paris Diderot, Ser- ogy, University of Maryland Baltimore County, Baltimore, MD 21250,
viced’Astrophysique,CEASaclay,91191GifsurYvette,France USA
9IstitutoNazionalediFisicaNucleare,SezionediTrieste,I-34127Tri- 22GeorgeMasonUniversity,Fairfax,VA22030,USA
este,Italy 23Laboratoire de Physique The´orique et Astroparticules, Universite´
10DipartimentodiFisica,Universita`diTrieste,I-34127Trieste,Italy Montpellier2,CNRS/IN2P3,Montpellier,France
11Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 24DepartmentofPhysics,StockholmUniversity,AlbaNova,SE-10691
Padova,Italy Stockholm,Sweden
12Dipartimento di Fisica “G. Galilei”, Universita` di Padova, I-35131 25TheOskarKleinCentreforCosmoparticlePhysics,AlbaNova,SE-
2 Abdoetal.
1. INTRODUCTION proposed by Abdoetal. (2010a) to replace the older HBL-
IBL-LBL scheme (Padovani&Giommi 1995; Fossatietal.
BLLacertae(BLLac,S42200+42,OY401,B32200+420,
1998). BL Lac occasionally showed peculiar behavior that
1ES 2200+420, 3EG J2202+4217, 1FGL J2202.8+4216) at
hasquestionedboththisclassification andasimple interpre-
redshift z = 0.0686 (e.g., Vermeulenetal. 1995) was his-
tationofitsbroad-bandemissionintermsofsynchrotronand
torically the prototype of the class of active galactic nuclei
synchrotronself-Compton(SSC)emissionproducedbyasin-
(AGN)forwhichBLLachasbecometheeponym. Itiscate-
gle blob. In fact, BL Lac is a source that has shown quite
gorizedasanintermediatesynchrotronpeaked(ISP)BLLac
complexanddistinctX-rayspectralbehavior(Madejskietal.
object.ThisisbasedonthelatestHSP/ISP/LSPclassification
1999;Ravasioetal.2002;Bo¨ttcher&Reimer2004)anddur-
ing several epochs broad Hα and Hβ emission lines with
10691Stockholm,Sweden luminosity (∼ 1041ergs−1) comparable to those of type I
26RoyalSwedishAcademyofSciencesResearchFellow,fundedbya
Seyfert galaxies (Vermeulenetal. 1995; Corbettetal. 1996,
grantfromtheK.A.WallenbergFoundation
27AgenziaSpazialeItaliana(ASI)ScienceDataCenter,I-00044Fras- 2000). There is evidence for an increase by ∼ 50% in ten
cati(Roma),Italy yearsof thebroadHα lineand anunderluminousbroadline
28DepartmentofPhysicsandDepartmentofAstronomy,Universityof region(BLR),comparedtootherAGN.Thenarrowlinesand
Maryland,CollegePark,MD20742,USA radioluminositiesofBLLacmatchthoseofoflow-excitation
29Universite´Bordeaux1,CNRS/IN2p3,Centred’E´tudesNucle´airesde
andminiatureradiogalaxies(Capettietal.2010). Longterm
BordeauxGradignan,33175Gradignan,France
30DipartimentodiFisica,Universita`diUdineandIstitutoNazionaledi radio-opticalfluxdensitymonitoringandseveralmultiwave-
FisicaNucleare, SezionediTrieste,GruppoCollegatodiUdine,I-33100 length campaignshave been carried out on BL Lac, like the
Udine,Italy pastcampaignsoftheWholeEarthBlazarTelescope(WEBT,
31Osservatorio Astronomico di Trieste, Istituto Nazionale di As- Villataetal. 2002a,b) dedicated to this source, providing
trofisica,I-34143Trieste,Italy
very complete datasets (for example, Bloometal. 1997;
32Department of Physical Sciences, Hiroshima University, Higashi-
Sambrunaetal. 1999; Madejskietal. 1999; Ravasioetal.
Hiroshima,Hiroshima739-8526,Japan
33INAFIstitutodiRadioastronomia,40129Bologna,Italy 2003; Bo¨ttcheretal. 2003; Villataetal. 2002a, 2004a,b,
34CenterforSpacePlasmaandAeronomicResearch(CSPAR),Univer- 2009;Raiterietal.2009,2010;Marscheretal.2008). Inpar-
sityofAlabamainHuntsville,Huntsville,AL35899,USA ticular in summer 1997 BL Lac showed the largest optical
35Department of Physics, Center for Cosmology and Astro-Particle outbursteverrecordedin almost33 years(Nescietal. 1998;
Physics,TheOhioStateUniversity,Columbus,OH43210,USA Tostietal.1999;Villataetal.2004b).
36ScienceInstitute,UniversityofIceland,IS-107Reykjavik,Iceland
IntheX-raybandthetwobroadcomponentsofthespectral
37ResearchInstituteforScienceandEngineering, WasedaUniversity,
energy distribution (SED) are overlapping and the radiation
3-4-1,Okubo,Shinjuku,Tokyo,169-8555Japan
38Centred’E´tudeSpatialedesRayonnements,CNRS/UPS,BP44346, atthisbandis attimesdominatedbythe high-energyendof
F-30128ToulouseCedex4,France thesynchrotronemission,whileatotheroccasionsitisdom-
39DepartmentofAstronomy,StockholmUniversity,SE-10691Stock- inatedbythelow-frequencyportionofthehigh-energycom-
holm,Sweden
ponent(e.g.,Madejskietal. 1999; Ravasioetal. 2002). The
40IstitutoNazionalediFisicaNucleare,SezionediRoma“TorVergata”,
I-00133Roma,Italy rapidX-rayvariabilityismainlyrestrictedtothelow-energy
41DepartmentofPhysicsandAstronomy,UniversityofDenver, Den- excess portion of the X-ray spectrum, presumably produced
ver,CO80208,USA by synchrotron radiation (Ravasioetal. 2002, 2003). The
42Hiroshima Astrophysical Science Center, Hiroshima University, X-ray variability of BL Lac, except for a few major flaring
Higashi-Hiroshima,Hiroshima739-8526,Japan
events, has a log-normal distribution (Giebels&Degrange
43Institute ofSpaceandAstronautical Science, JAXA,3-1-1Yoshin-
odai,Sagamihara,Kanagawa229-8510,Japan 2009), meaningthatthe emission is a multiplicativeproduct
44Max-Planck Institut fu¨r extraterrestrische Physik, 85748 Garching, ofalargenumberofindependentrandomevents.
Germany Gamma-rayobservationsbyEGRETresultedinseveralde-
45Institut fu¨rAstro-undTeilchenphysik andInstitut fu¨rTheoretische tections (Figure 1). Observations after 1995 resulted in an
Physik,Leopold-Franzens-Universita¨tInnsbruck,A-6020Innsbruck,Aus- average γ-ray flux above 100 MeV of (40 ± 12) × 10−8
tria
46SantaCruzInstituteforParticlePhysics,DepartmentofPhysicsand ph cm−2 s−1 (Cataneseetal. 1997) and in a flare in 1997
Department ofAstronomyandAstrophysics, University ofCalifornia at at a level (171±42)×10−8 ph cm−2 s−1 with 10.2σ sig-
SantaCruz,SantaCruz,CA95064,USA
nificance (Bloometal. 1997). Correlated γ-rays and opti-
47Department of Physics, University of Washington, Seattle, WA
98195-1560,USA cal flaring emission were observed during the EGRET era
48Max-Planck-Institutfu¨rRadioastronomie,AufdemHu¨gel69,53121 (Bloometal. 1997). Very high energy (VHE) γ-ray emis-
Bonn,Germany sionwasclaimedbytheCrimeanGT-48(Neshporetal.2001)
49NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025,
USA
50Astro Space Center of the Lebedev Physical Institute, 117810 58SchoolofPureandAppliedNaturalSciences,UniversityofKalmar,
Moscow,Russia SE-39182Kalmar,Sweden
51Department ofChemistryandPhysics, PurdueUniversityCalumet, 59TuorlaObservatory,UniversityofTurku,FI-21500Piikkio¨,Finland
Hammond,IN46323-2094,USA 60DepartmentofPhysicsandAstronomy,OhioUniversity,Athens,OH
52Partially supported by the International Doctorate on Astroparticle 45701,USA
Physics(IDAPP)program 61InstitutoNacionaldeAstrof´ısica,O´pticayElectro´nica,Tonantzintla,
53Institucio´ Catalana de Recerca i Estudis Avanc¸ats (ICREA), Puebla72840,Mexico
Barcelona,Spain 62Instituto de Astronom´ıa y Meteorolog´ıa, CUCEI, Universidad de
54Consorzio Interuniversitario per laFisica Spaziale (CIFS), I-10133 Guadalajara,44130Guadalajara,Jalisco,Mexico
Torino,Italy 63AaltoUniversityMetsa¨hoviRadioObservatory,FIN-02540Kylmala,
55INTEGRALScienceDataCentre,CH-1290Versoix,Switzerland Finland
56Dipartimento diFisica, Universita` diRoma“TorVergata”, I-00133 64InstituteforAstrophysicalResearch,BostonUniversity,Boston,MA
Roma,Italy 02215,USA
57Department of Physics, Royal Institute of Technology (KTH), Al- 65Finnish Centre for Astronomy with ESO (FINCA), University of
baNova,SE-10691Stockholm,Sweden Turku,FI-21500Piikkio¨,Finland
ThefirstFermimultifrequencycampaignonBLLac 3
section3.4andconclusionsinsection4.
A ΛCDM cosmology with values given within 1σ of the
WMAP results (Komatsuetal. 2009), namely Ω = 0.27,
m
Ω =0.73,andH =71kms−1Mpc−1isused.
Λ 0
2. GAMMA-RAYOBSERVATIONSANDRESULTSBYFERMI-LAT
2.1. LATanalysisandobservations
The Large Area Telescope (LAT), on board the
Fermi Gamma-ray Space Telescope (Atwoodetal. 2009),
is a pair-conversion γ-ray telescope, sensitive to photon
energies from about 20 MeV up to >300 GeV. It consists
of a tracker (composed of two sections, front and back,
withdifferentangularresolutions),acalorimeterandananti-
coincidencesystemtorejectthecharged-particlebackground.
The LAT has a large peak effective area (∼ 8000 cm2 for
1 GeV photons in the event class considered here), viewing
FIG.1.—Gamma-raylightcurveofBLLacobtained byEGRETinthe ≈2.4sroftheskywithangularresolution(68%containment
period1991-1997. Datapointshavesourcedetectionsignificanceabove4σ radius) better than ≈ 1◦ at E = 1 GeV. The large field of
(Nandikotkuretal.2007).
view, improvedeffective area and sensitivity and the survey
nominal mode make Fermi-LAT an optimal all-sky hunter
and HEGRA (Kranich 2003) atmospheric Cherenkov tele- forhigh-energyflaresandanunprecedentedmonitorofγ-ray
scopes. A significant(> 5σ) detectionabove200GeV was sources. Thedatasetusedinthispaperwascollectedduring
made only by the MAGIC telescope in 2005 (Albertetal. thefirst18monthsofFermi scienceobserations,from2008
2007). Gamma-rayemissionwasexplainedwiththerequire- August4to2010February4(about550days,78weeksfrom
ment of Comptonization of external-jet photons (external- MJD54682.7to55232.9,asshownintheweeklylightcurve
radiationCompton,ERC)inadditiontotheSSC(in-jet)emis- in Figure 2, upper panel). This interval includes the period
sion(Madejskietal. 1999;Bo¨ttcher&Bloom2000). Super- chosenfor thefirst Fermi MW campaignonBL Lac(2008,
luminal motion of βapp up to (10.57± 0.74) has been ob- Aug. 19 – Oct. 7, MJD 54697.8-54746,i.e. about 48 days,
served in this object (Listeretal. 2009; Dennetal. 2000). indicated by the line in Figure 2, upper panel). Fermi -LAT
Followingresultsofpastmultifrequencycampaigns,possibly data analysis was performed with the standard Fermi -LAT
distinctVLBIjetstructuresareassumedtocontribute,some- ScienceToolssoftwarepackage66usingversionv9r15p5.
times with delays, to the radio flux density lightcurves, and
Only events belonging to the “Diffuse” class in the energy
sometimesaresuggestedtoberesponsibleforopticaltoTeV
range0.1−100 GeV were used in the analysis. Instrument
γ-ray flares (for example, Bachetal. 2006; Marscheretal.
response functions (IRFs) used were P6 V3 DIFFUSE. In
2008).
order to provide protection against significant background
In this paper, we report on LAT observations of BL Lac
contamination by Earth-limb γ rays, all events with zenith
duringthefirst18monthsofFermi scienceoperations(from angles>105◦wereexcluded.
2008August04to2010February04).TheLAT’sreasonably
The 18-month light curve was built using 1-week time
uniformexposure,highsensitivity,andcontinuousskymoni-
bins (Figures 2 and 4). For each time bin the inte-
toringmakeitanexcellentinstrumentaroundwhichtoorga-
grated flux (E>100 MeV) values were computed using the
nizeasimultaneousmultifrequencycampaign. Theso-called
maximum-likelihood algorithm implemented in the science
Fermi plannedintensivecampaign(PIC)dedicatedtoBLLac
tool gtlike. For each time bin we analyzed a region of
was performed in 2008 August 19 – 2008 October 7 (MJD interest(RoI)of12◦inradius,centeredonthepositionofthe
54697.8–54746),duringroughlythefirsttwomonthsofsci-
source. All point sources listed in the 1FGL (1-year) LAT
ence operations, irrespective of the brightnessof the source. catalog(Abdoetal.2010b)within19◦fromBLLacertaeand
This was part of a series of Fermi-LAT-collaboration PICs having test statistic67 TS > 50 and fluxes above 10−8 ph
(Tosti2007;Thompson2007),involvingobservingproposals cm−2 s−1 were included in the RoI model using a power-
submitted to the RXTE (Cycle 12) and Swift (Cycle 4) X-
lawspectrum(dN/dE ∝E−Γ,whereΓisthephotonindex).
raysatellites,andorganizedinadvanceoftheFermi launch.
Theisotropicbackground(thesumoftheresidualinstrumen-
This ensured access to the facilities allowing the best multi-
tal background and extragalactic diffuse γ-ray background)
wavelength (MW) coverage. The aim of the campaign is to
was included in the RoI model using the standard model
shedlightforthefirsttime onthe broad-bandradio-to-γ-ray
fileisotropic iem v02.txt68,andtheGalacticdiffuse
SED, including the high energy(X-ray and γ-ray) behavior,
emissionwasincludedinthemodelingusingthefileandthe
duringalowactivityphaseofthesource. The18-monthLAT
standard file gll iem v02.fit. In the final light curve
lightcurveshowsthatBLLacwasvariableinγ-raysformost
computationthe photon index value was frozen to the value
ofthese18months,withtheexceptionofthefirst2months,
resultingfromthelikelihoodanalysisontheentireperiod.For
whichcorrespondstotheperiodoftheMWcampaign.Insec-
eachtimebin,iftheTSvalueforthesourcewasTS < 4or
tion2thelightcurvebyFermi-LATduringthefirst18months
thenumberofmodelpredictedphotonsN <10,thevalue
ofsurveyandsimultaneousopticallong-termmonitoringdata pred
arepresentedjoinedtotheanalysisoftheγ-rayspectrum. In
66http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicer
section3themultiwavebandradio-bandtoX-rayfluxdensity
67TheteststatisticisdefinedasTS=2∆log(Likelihood)betweenmod-
andparsec-scaleradiostructureobservationscollectedduring elswithandwithoutthesourceanditisameasureofthesourcesignificance
thecampaignperiodarereportedanddiscussed. Theassem- (Mattoxetal.1996).
bledspectralenergydistributionandmodelingarereportedin 68http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.
4 Abdoetal.
FIG.2.— Toppanel: 18-monthweeklyintegrated flux(E > 100MeV)lightcurveofBLLacobtained obtained from2008, Aug. 04to2010, Feb. 04.
Bottompanel:superposedopticalV-bandandR-bandobservedflux(notcorrectedforinterstellarabsorptionandhostgalaxycontribution)lightcurvesobtained
respectivelyattheStewardObservatory,KittPeak,Arizona,USA,andTuorlaObservatory,Turku,Finland,inthesameperiod.
ofthefluxeswerereplacedbythe2-σupperlimits. Allerrors
reportedinthefiguresorquotedinthetextare1-σstatistical 10−7 LAT spectrum campaign period
errors. The estimated relative systematic uncertainty on the Power−law
flux,accordingtoAbdoetal.(2010b)andreflectingtherela- LAT spectrum 18 months
tivesystematicuncertaintyoneffectivearea,issetto10%at −1]s10−8 LogParabola
100MeV,5%at500MeVand20%at10GeV. −2 m
c
h 10−9
2.2. Gamma-rayspectralandtemporalbehavior E [p
boTthhefoγr-trhaeyespneticrtera1l8a-nmaolynstihspoefrBioLd,Laancderftaoerwthaes4p8erdfoayrmseodf dN/d10−10
E
thecampaign. ThesespectraareshowninFigure3. Theflux
valueforeachenergybinandforthetwoperiodsarereported 10−11
in Table 1. The most energetic photon observed within the
95% point spread function containmentradius of the source
for the 18-month period had energy 70 GeV, while for the 10−12 102 103 104 105
campaignperiodthehighestenergyphotonhad10GeV.The Energy [MeV]
energy-spectrumbinningwasbuiltrequiringTS >50and/or
model-predicted source photons > 8, except for the last FIG.3.— 18-monthaveraged energy spectrum ofBLLacobtained with
abandlikelihoodovertheaccumulatedobservationsfrom2008Aug.04to
bin, which had an upper limit rather than a detection. The 2010Feb.04(MJD54682.7-55232.9).Thespectrumshowsadeparturefrom
18-month spectrum results in an integrated average flux of asimplepowerlawthatcanbefittedbyalogparabolafunction. Theblue
(19.85±0.40)×10−8 phcm−2 s−1 ,inthe0.1−100GeV (opensquare)symbolsrepresentthespectrumcorrespondingonlytothe48-
dayperiodofthemultifrequencycampaign.
range,withTS = 2375andphotonindexΓ = 2.38±0.01,
whilethevalueforthe48-dayintervalofthemultifrequency
campaign,(11.5±2.7)×10−8phcm−2s−1,andthephoton
asinAbdoetal.(2010b),
indexslightlyflatter(Γ=2.27±0.10),withaTS =111.
Inordertoquantifythedepartureofthe18-monthspectrum (F −FPL)2
fromapowerlawshape,weuseaspectralcurvatureindexC C =X i i , (1)
σ2+(frelF )2
i i i
ThefirstFermimultifrequencycampaignonBLLac 5
TABLE1
BLLACγ-RAYSPECTRUMOBTAINEDWITHLIKELIHOOD F = PwiFi , (4)
ANALYSISINEACHENERGYBAND. wt w
P i
Emin−Emax γ-rayflux Syst. andtheindexirunsoveralldatapointsexcepttheupperlimits
[MeV] [×10−8phcm−2s−1] (61 bins). Here F and σ are flux values and the relative
i i
18months statistical errors, respectively, and f is equalto 3% of the
100-200 7.62±1.03 10% flux for each interval, as suggestedrbely Abdoetal. (2010b).
200-562 5.14±0.27 5% For the 18 months, F was found to be 17.48× 10−8 ph
562-794 0.73±0.06 5% wt
794-1122 0.45±0.04 5% cm−2 s−1 and V = 193 with 60 d.o.f., which is consistent
1122-1585 0.31±0.03 10% with variability (the probabilitythe source is non-variableis
1585-2239 0.18±0.02 10% lessthan10−15). Thesamevaluescomputedjustforthefirst
2239-3162 0.11±0.02 10% 9 weeks, containing the period of the MW campaign, give
3162-6310 0.10±0.01 15% F = 10.67 × 10−8 ph cm−2 s−1 and V = 0.6 with 8
6310-35481 0.04±0.01 15% wt
d.o.f.,whichisconsistentwithanon-variabilityhypothesisat
Aug19-Sep9
> 99%confidence. Each of the LAT flux data pointsin the
100-562 7.18±2.12 5%
562-2239 1.28±0.24 10% upperpanelofFigure4hasTS >10.
2239-12589 0.17±0.07 15% Thisintervalwith no observedγ-rayvariability character-
izes the intensive coordinated campaign period. After the
campaign,startingfromabouthalfwaythrough2008Novem-
whereiistheindexforaparticularenergybin,F istheob- ber until 2010 February 4, BL Lac showed variable weekly
served flux, FPL is the flux in the ith bin predicited by the γ-rayfluxwithV =162with51d.o.f.,Fwt =19.15×10−8
i phcm−2s−1 . Inparticular,a1-weekγ-rayflareoccurredin
globalpower-lawfit, σ is theerrorinF , andfrel istherel-
i i theweekMJD54928.5-54935.5(2009Apr.7-14),withaflux
ativesystematicuncertaintyinthatbin(reportedinTable1). valueof72×10−8phcm−2s−1.Thisisabout4timesgreater
Thepower-lawfithastwoparameters: thenormalizationand
than the mean flux value (F ) computed above. Moreover
photonindex. Thecurvatureparameterisexpectedtofollow wt
thisisthehighestweeklyfluxyetdetectedfromBLLac, al-
aχ2 distributionwith 9−2 = 7 degreesoffreedom(d.o.f.)
thoughanother likely flare was detected in the week around
if the power law hypothesis is true. With 99% confidence
MJD 55221 (2010 Jan. 25, Sokolovskyetal. 2010a). After
thespectralshapeissignificantlydifferentfromapowerlaw
the 2009 April flare the source also showed slightly higher
whenC > 18.48. Fortheaccumulated18-monthdataonBL
activityandflux.
LacwefoundC = 20.88(0.1 < E < 100GeV),whichim-
The 78 points of the 18-monthlight curve were also used
pliesa curvedspectrum. Thesame procedure,whenapplied
to compute the global normalized excess variance, as de-
to the spectrum of the 48-day multifrequencycampaign, re-
finedinVaughanetal.(2003);Abdoetal.(2010d): σ2 =
sultedinC = 2.83(0.1 < E < 100GeV),implyingthereis NXS
nosignificantevidencefordeviationfromasimplepower-law (S2 −σe2¯rr)/x¯2, where S2 is the varianceof the light curve
in thisspectrum(Figures3and10). Thisdoesnotnecessar- andσ2 = σ2 +σ2 , the sumin quadratureofthestatisti-
err i sys
ilymeanthatthereisnocurvatureduringthis48-dayperiod, caluncertaintyofthefluxin thetime binandthesystematic
becauseitcouldbeduetotheloweremissionstateandaccu- errorestimate(0.03hF i). Thisquantitymeasuredanintrinsic
i
mulatedstatistics. variabilityamplitudeof0.23±0.03,ingoodagreementwith
To further explore curvature, the 18-month spectrum was the value found in Abdoetal. (2010d) reporting the weekly
fitwithalog-parabolafunctioninthe0.1−300GeVenergy lightcurveofBLLac(fluxE >300MeV)duringthefirst11
band. Thiswasperformedintworuns: firstafitwithE months. This confirmsagain thatBL Lac was variable after
break
leftfree, then anotherwith E frozenatthe valuefound thefirstcoupleofmonthsofFermiscienceoperations.
break
in the first run (300 MeV) to improve the calculation of the
3. OBSERVATIONSANDRESULTSFROMTHEMULTIFREQUENCY
otherparametervalues.Theresultofthelatterfitgives CAMPAIGN
dN/dE=(2.02±0.07)×10−10 The 48-day PIC on BL Lac involved the participation of
theRXTEandSwiftX-raysatellites,andofground-basedra-
×(E/300MeV)−((2.23±0.05)+(0.07±0.02)log(E/300MeV))d,ioandopticalobservatories. Theseincludedthe Metsa¨hovi
13.7 m radio-telescope operating at 37 GHz in Finland; the
with TS = 2384.1. The higher TS value shows that the 18
VeryLongBaselineArray(VLBA)intheUSA,whichtooka
months LAT spectrum does not follow a simple power law
multi-waveband flux-structure observation on 2008 Septem-
function,andthe log-parabolamodelcan describethisspec-
ber2;twotelescopesoftheTuorlaObservatory,Finland,and
trum.
twotelescopesoftheStewardObservatory,USA,forthelong-
Theweeklylightcurveinthe0.1-100GeVbandforthe18-
termandsingle-bandopticalmonitoringaspresentedinFig-
monthperiodispresentedinFigure2(upperpanel). Inorder
ure2;andthe2.1mopticaltelescopeoftheObservatorioAs-
to quantifythe variability, the variabilityindex as definedin
trof´ısicoGuillermoHaro(OAGH,Mexico)operatinginnear-
Abdoetal.(2010b),wascomputedfrom
infrared,andthe1.3mMcGraw-HillopticalTelescopeofthe
V =Xwi(Fi−Fwt)2, (2) MopDticMalOsnbaspesrhvoattsordyur(iAngrizthoenac,amUpSaAig),nfpoerrifoudrt(hTearbmleu2l)ti.-band
i
3.1. X-rayandUVobservationsandresults
where
The RXTE/PCA space observatory performed sub-daily
1
w = , (3) monitoring of BL Lac for 20 days, during the same epoch
i σ2+(frelF )2
i
6 Abdoetal.
1] 18
−
s 16
2
−
m 14
c
h 12
p 10
−8[ 8
0
1 6
γ Weekly Fermi−LAT (E >100 MeV)
F
4
1] 10
−
s
−2 9 XX−−RRaayy ((22−−1100 kkeeVV))
m
SSwwiifftt−−XXRRTT
c 8
g XXTTEE
r 7
e
12[ 6
−
0
1 5
X
F
4
3.5
SSwwiifftt−−UUVVOOTT
WW11(cid:9)(cid:9)
y] 3 MM22
J WW22
m
2.5
[

V
U 2
F
1.5
12
11 SSwwiifftt−−UUVVOOTT
y] 10 VV
J 9 BB
m
8 UU
[
VR 7
UB 6
F 5
4
54700 54710 54720 54730 54740
55
OOAAGGHH
] 50
y JJ
J 45 HH
m
[ 40 KK
K 35
H
FJ 30
25
20
3 54700 54710 54720 54730 54740
] 2.8
y
J 2.6
[
o
di 2.4
a
R 2.2
F Metsahovi−13.7m (37GHz)
2
1.8
54700 54710 54720 54730 54740
MJD
FIG.4.—SimultaneousmultifrequencyfluxlightcurvesofBLLacfromthe48-daysFermiintensivecampaignobtainedwithFermi-LAT,Swift-XRTand
UVOT,OAGH,andMetsa¨hoviobservations.LATweeklyfluxesreportedintheupperpanelforthisperiodareallTS>10detections.UV-to-near-IRfluxesare
correctedforabsorption.
as LAT and Swift, for a total of 80 pointings between 2008 andacheckwasperformedinthe40-100keVrangetoassure
August20andSeptember9. OnlyPCU2wasactive,forato- thatthemodelbackgroundreproducedtheobservedone.The
talexposureof157.3ks. ThePCA STANDARD2datawere average net count rate in the 3-18 keV band is 0.55±0.01
reduced and analyzed with the routines in HEASOFT V6.8 ctss−1pcu−1. The RXTE spectra were extracted and fitted
usingthefilteringcriteriarecommendedbytheRXTE Guest separatelyforeachpointing,andsummedtoobtaintheaver-
ObserverFacility. Onlythetop-layereventswereprocessed, age spectrum. Each spectrum is well-fit by a single power-
ThefirstFermimultifrequencycampaignonBLLac 7
TABLE2
1.8
BLLACCOORDINATEDMULTIFREQUENCYCAMPAIGN(PIC)
ANDLONG-TERMMONITORINGOBSERVATIONS 1.6
PIC(48days)
1.4
Instrument EnergyRange 2008Epochrange #obs.
VLBA 4.6-43.2GHz Sep2 7 y]
Metsa¨hovi 37GHz Aug20-Oct6 22 mJ 1.2
OMADGMH UJBHVKRI SeOpc6t6--O1c0t6 1185 ) [Fν 1
Swift-UVOT W2M2W1UBV Aug20-Oct2 141 (0
Swift-XRT 0.4-8KeV Aug20-Sep18 24 g10.8 Aug. 20
RXTE-PCA 3-18KeV Aug20-Sep8 19 Lo Aug. 23
Fermi-LAT 100MeV-100GeV Aug19-Oct7 48days 0.6 Sept. 01
Sept. 04
18months
Sept. 06
Instrument EnergyRange Epochrange(MJD) #obs. 0.4
Tuorla R 54709.8-55191.8 162 Oct. 06
Steward V 54743.2-55213.1 89
0.2
Fermi-LAT 100MeV-100GeV 54682.7-55070 78weeks 14.2 14.4 14.6 14.8 15 15.2
Log ν [Hz]
10
FIG.5.—Simultaneousnear-IRtoUVSEDsofBLLacatdifferentepochs,
collectedwithUVOT(Aug.20,23,Sept.1,4),OAGH(Sept.6),andMDM
law model with Galactic absorption. The Galactic N was (Oct.6)observations.
H
fixed at two values: that measured by Elvisetal. (1989),
basedondedicated21-cmobservations(2.015×1021cm−2); The XRT data analysis is performed using the script
and the sum of this value with that inferredfrom millimeter xrtgrblc, available in the HEASoftsoftware package. In
observations (Lucas&Liszt 1993), which includes the con- brief, the script selects the best source and background ex-
tribution from molecular hydrogen along the line of sight tractionregionsbasedonthesourceintensity(acircleandan
(NH ≃ 3.6 × 1021 cm−2). In the RXTE band, however, annulus,respectively). InthecaseofBL Lac,thesourceex-
this difference yields negligible effects, with a difference in tractionregionhastypicallya55′′radiusandthebackground
spectralindices∆ΓX ∼0.02. regionhas a 110′′-210′′inner-outerradius. Field sourcesare
TheRXTE datashowedmodestfluxvariations,witharate excluded applying circular masks whose radius depends on
whichremainedconstantat∼ 0.6cts s−1 uptoMJD54708, the field source intensity. Using these regions, source and
andslowlydecreasingto∼ 0.4cts s−1 towardsMJD54718. backgroundcountrates,spectraandeventlistsareextracted.
Spectralvariationsweremodestaswell,rangingfrom1.95± Thenetsourcecountratemustbecorrectedforirregularities
0.09 to 2.28± 0.07. The total average spectrum is well-fit in the exposure map and the Point-Spread Function (PSF).
withasinglepower-lawmodel,withΓX = 2.18±0.06,and ThetotalcorrectionfactorisobtainedusingtheHEASofttool
anunabsorbedfluxinthe2-10keVbandofF2−10 = (7.2± xrtlccorr.
0.3)×10−12 ergcm−2s−1 and χ2 = 0.81 (15d.o.f.). This ThescriptthathandlestheUVOTanalysisisuvotgrblc
r
spectrumcanalsobefitbyaconcavelogparabolaorbroken (available within HEASoft). It determines the presence of
power-law model, with the slope hardening above 8-9 keV fieldsourcesandexcludesthemfromthebackgroundregion
from∼2.2to∼1.9(χ2 ≃0.7);however,thelowstatisticsdo with circular regions whose sizes depend on their intensity.
r
notallowonetodetermineiftheimprovementissignificant, The optimalbackgroundregion is chosen after comparing3
and the single, rather flat (Γ ≃ 2, therefore νF ∝ ν0), annular regions centered on the main source in the summed
X ν
powerlawgivesagoodfit. images. The region with the lowest backgroundis selected.
In the context of the overallSED, the low X-ray flux and The inner and outer radii are 27′′and 35′′, respectively, for
the Γ ≃ 2 spectrum (flat X-ray SED) is evidence that the all the filters except V, for which the two radii are 35′′and
X
RXTE bandmightcorrespondtothepassagebetweenthetwo 42′′. Thesourceextractionregionisalsointensitydependent:
humps of the blazars’ SED, as typical for ISPs. In the fol- The script selects the aperture size between 3′′and 5′′based
lowingweshowthattheSwift-XRTresultsprovideusfurther on the observed source count-rate and the presence of close
evidenceforthisconclusion. field sources. BL Lac is a relatively bright object and an
The Swift gamma-ray burst satellite (Gehrelsetal. 2004) extraction region of 5′′is preferred. The field of view in
performedadailymonitoringsimultaneoustoRXTE andLAT the V and B filters is full of field sources and in particular
observations for 20 days (from 2008 Aug. 20, 15:19 UT, to there is a bright star east of the blazar. To avoid contami-
2008Sep. 09, 02:37UT;MJD 54698.64- 54718.11),plus3 nation from such a source, the extraction region is reduced
moreseparatedobservationsafter18September. Datareduc- to 3′′. The script estimates the best source position using
tion and analysiswas performedrunninga processing script thetaskuvotcentroidandthephotometryusingthetask
customized for the XRT and UVOT data (Donato et al., in uvotsource.Theobtainedvaluesarealreadycorrectedfor
preparation). ThescriptreprocessestheSwift data,storedin apertureeffectswhileweusedthedustmapsofSchlegeletal.
theHEASARCarchiveusingthestandardHEASoftsoftware (1998) and the Milky Way extinction curve of Pei (1992) to
(version6.8)andthelatestcalibrationdatabase(20091130for compensate the Galactic extinction. XRT and UVOT light
XRT and 20100129for UVOT). The reduction of XRT data curvesandSEDsareshowninFigures4,5,and6.
consistsofrunningxrtpipeline,selectingonlytheeventswith As with the harder energy band with RXTE, BL Lac does
0-12 grades in photon counting mode (PC). The UVOT im- notshowanymajorsignofvariabilityintheSwift-XRTdata.
age mode data are processed followingthe steps reportedin The same Galactic (N ≃ 3.6 × 1021 cm−2) fixed value
H
theUVOTSoftwareGuide2.2. was used also for the XRT analysis. The 2-10 keV flux de-
8 Abdoetal.
BL Lacertae, RXTE total flux is 6.37+0.12 ×10−12 erg cm−2 s−1. The X-rayphoton
PCU2, power−law −0.11
indexis consistentwithin 1σ with the resultsobtainedusing
RXTE data.
0.1
UsingtheentireXRTenergybandfrom0.3to10keV(Fig-
0.05 ure 6) and profiting from the increased statistics (the spec-
V−1 trum is binned with 100 counts/bin), the spectral fit is more
nts s ke−1 0.02 cfiotn(sχtr2rai=ned0a.8n2duwnidther1li4e6stdw.oo.ef.m)isissioonbtcaoinmepdowneinthts:athberobkeesnt
ou 0.01 power-law with the energy break located at 1.71+0.15 keV
C −0.14
andfixedabsorptionattheGalacticvalue. Thesoftandhard
5×10−3
photonindicesare2.57+0.06and2.09±0.05,respectivelyand
−0.05
thereforethe breakin this XRT spectrumis significantcom-
1.4
pared to the power law. The 0.3-10 keV integrated flux is
1.2
Ratio 1 9.67+−00..1140×10−12ergcm−2s−1.
TheUVOToptical-UVlightcurvesshowedsomevariabil-
0.8
ityduringthecampaignperiod(Figure4),butthisisnotwell
5 10 20
correlatedwiththeX-rayorγ-rayemission(whichisnotvari-
Energy (keV)
able).Thedailysimultaneousnear-IR,optical,andUVSEDs,
reconstructedthanksto UVOT,andOAGH observations(re-
portedinFigure5),showednosignificantspectralvariability
Swift XRT 0.3−10 keV andatrendconsistentwithasinglepowerlawexceptforthe
2higherfrequencyUVbands.Thismaybeinagreementwith
0.1 thehypothesisofUVexcessduetothermalemissionfroman
V−1 accretiondisk(Raiterietal.2009,2010).
e
k
s s −1 3.2. Radiobandflux-structureobservationsandresults
nt
ou As part of an ongoing blazar monitoring program, the
c 0.01
d 37 GHz observations were made with the 13.7 m diame-
e
aliz ter Metsa¨hovi radio telescope, which is a radome enclosed
orm paraboloid antenna situated in Finland. The measurements
n weremadewitha1GHz-banddualbeamreceivercenteredat
36.8GHz.Thehighelectronmobilitypseudomorphictransis-
1.2 torfrontendoperatesatroomtemperature. Theobservations
o 1.1 are ON–ON, alternatingthe source and the sky in each feed
rati 1 horn.Atypicalintegrationtimetoobtainonefluxdensitydata
0.9
pointis1200–1400s. Thedetectionlimitofthetelescopeat
0.8
0.5 1 2 5 37 GHz is on the order of 0.2 Jy under optimal conditions.
Energy (keV) Data points with a signal-to-noise ratio < 4 are handled as
non-detections. Thefluxdensityscale is setbyobservations
ofDR21. Sources3C84and3C274areusedassecondary
FIG.6.—Toppanel: RXTE combined3-18keVspectrumofBLLacex- calibrators. A detaileddescriptionon thedata reductionand
tractedcumulatingthe20daysofobservationsofthisX-raysatelliteduring
analysis is given in Tera¨srantaetal. (1998). The error esti-
the campaign. Bottom panel: Swift -XRTcombined 0.4-8keV spectra of
BLLacextractedcumulatingthe24daysofobservationsofthisX-raysatel- mate in the flux density includes the contribution from the
liteduringthecampaign. measurementrmsandtheuncertaintyoftheabsolutecalibra-
tion.
rivedfromSwift observationslies between6 and 8 ×10−12 BLLacwasalsoobservedwiththeVeryLongBaselineAr-
erg cm−2 s−1with a hint of a lower intensity at the end ray(VLBA)atsevenfrequencies(4.6,5.0,8.1,8.4,15.4,23.8
of the campaign when the flux was below 5 × 10−12 erg and43.2 GHz) in the frameworkof a surveyof parsec-scale
cm−2 s−1. We found that in the day-by-day spectral anal- radiospectraoftwentyγ-raybrightblazars(Sokolovskyetal.
ysis the photon index does not change significantly either. 2010b). The multifrequencyVLBA observationwas carried
Using a single power-lawmodelwith fixed Galactic absorp- outon2008September2. Thedatareductionwasconducted
tion, Leiden/Argentine/Bonn (LAB) Survey of Galactic HI in the standard manner using the AIPS package (Greisen
(Kalberlaetal.2005)weightedaverageequalto1.73×1021 1990). The finalamplitudecalibrationaccuracyis estimated
cm−2),theslopevariesbetween1.75and2.06.Sincethetypi- tobe ∼ 5%at 4.6-15.4GHz rangeand∼ 10%at23.8and
calerroronthisparameterisoftheorderof0.1,thevariability 43.2GHz. The Difmap software (Shepherdetal. 1994) was
iswithin2sigma. usedforimagingandmodelingofthevisibilitydata.
Theabsenceofspectralchangesallowsustoextractanac- The VLBA images (Fig. 7, and 8) reveal a wide, rather
cumulatedspectrum.Asafirststep,acomparisonoftheXRT smooth,curvedjetextending∼50mas(at5GHz)south-east
andRXTE spectraaccumulatedoverthesameperiodandus- from the bright compact core. A few distinct, bright emis-
ingthesamelowerenergythresholdof3keVresultsinvery sionfeaturesalignedalongthesouth-southwestdirectioncan
goodagreementwith the power-lawphotonindexΓX found beseenintheinnerjetathigherfrequencies. We havemod-
byRXTE.Thebestfitinthe3-8keVrangeisobtainedwitha elled the observedbrightnessdistributionwith a small num-
power-lawwhoseslopeis2.19+0.13. The2-10keVintegrated ber of model componentshaving two-dimensionalGaussian
−0.11
ThefirstFermimultifrequencycampaignonBLLac 9
FIG.7.—InnerjetofBLLacertaeasobservedbytheVLBAonSeptember02,2008. Imagesatdifferentfrequenciesareshiftedby8masinrelativeRight
Ascension.Forthe15.4,23.8and43.2GHzimagemappeaksare1.69,1.52,1.32Jy/beam,firstcontoursare1.70,3.00,5.00mJy/beamrespectively.Thecontour
levelsareincreasedbyafactorof3.Beamsize(naturalweighting)foreachfrequencyisindicatedbythecrosstotheleftofthecorrespondingimage.
containtwo(ormore)distinctemissionfeaturesoritmaybe
a continuous emission region. The angular resolution even
at43GHz isnotsufficientto distinguishbetweenthese pos-
sibilities. However, it is evident that the radio spectrum is
changing along this region and it cannot be described by a
single, uniform, self-absorbed,synchrotronemitting compo-
nent. A turnover caused by the synchrotron self-absorption
is detected in the averaged core spectrum at a frequency of
∼ 12 GHz (an average over the whole area with the nega-
tivespectralindexat Fig. 8). Due to the inhomogeneity,the
innermostcomponentatmm-wavelengthsmostlikelyhasan
evenhigherturnoverfrequency,&40GHz.Usingthemethod
describedinSokolovskyetal.(2010b),themagneticfield,B,
of the core, in the frame of the relativistic jet, can be con-
strained, giventhe Doppler factor, D = (Γ[1−βcosθ])−1,
where Γ = (1 − β2)−1/2 and θ here are the bulk Lorentz
factorandjetangletothelineofsight,respectively. Assum-
ing a Doppler factor D = 7.3 (Hovattaetal. 2009), an up-
per limit can be placed on the magnetic field strength in the
core: B < 3G. We notethatthisupperlimitcorrespondsto
atypicalvalueacrossanextendedandinhomogeneousregion
FIG.8.—Spectralindexmap(Fν ∼ν−α,αisshownincolor)ofBLLac- contributing the bulk of the radio emission in 4.6 - 43 GHz
ertaeconstructedusingVLBAobservationsat4.6,5.0,8.1and8.4GHz.The
range.Themagneticfieldstrengthcanexceedtheabovelimit
overlaidcontoursrepresenttotalintensityat8.4GHz(thepeakintensityis
1.75Jy/beam,thefirstcontouris0.70mJy/beam,thebeamsizeis1.57×1.22 locallyorinamorecompactregionshiddenfromsightinthe
masatPA10.4◦.). Thespectralindexmapwassmoothedbyamedianfilter observedfrequencyrangebysynchrotronopacity.
withradiusequaltotheindicatedbeamsize.Themapshowsanopticallythin
jetwithα∼0.7andtheself-absorbedcoreregion(α<0.0). A2Dcross- 3.3. OpticalandnearIRobservationsandresults
correlationtechniqueusingtheopticallythinpartofthejetwasemployedto
alignimagesatdifferentfrequenciesallowingreliableextractionofthespec- Duringthe campaignseveralmulti-bandnear-infraredand
tralinformation.Thespectralsteepeningtowardsthejetedgesvisibleonthe optical observations of BL Lac were also performed from
spectralindexmapoccursontheangularscalecomparabletothebeamsize the ground by the 2.1 m telescope of the OAGH observa-
andislikelyaneffectoftheunevenuv-coverageatdifferentfrequencies.
tory,Sonora,Mexico,andthe1.3mMcGraw-HillTelescope
oftheMDMObservatory,Arizona,USA.Inadditionalong-
profiles. Thecoreregion(i.e., the brightfeatureatthe north termmonitoringinopticalRandV bandsduringthefirst18
end of the jet) is elongatedin the 43GHz image and can be monthsof Fermi observations(Figure 2), was performedby
modelledbytwoGaussiancomponentsseparatedby0.25mas theobservingmonitoringprogramsattheTuorlaObservatory
(0.32pcprojecteddistance).Atadistancesof1.5masand3.4 (1mTuorlaand0.35mKVAtelescopes),Turku,Finland,and
masfromthecore,therearetwootherdistinctemissionfea- attheStewardObservatory(2.3mBokand1.5mKuipertele-
turesin the jet. A table listing the parametersof the distinct scopes),USArespectively.
features (“components”) can be found in Sokolovskyetal. Optical (UBVRI) data were taken with the 1.3 m
(2010b). McGraw-HillTelescope(MDMobservatory).Exposuretimes
The inner 0.25 mas part of the jet (i.e., the “core”) may rangedfrom40s(Rband)to 120s(U band). Therawdata
10 Abdoetal.

were bias and flat-field corrected using IRAF. Instrumental
0.4
magnitudesof BL Lac andcomparisonstars B, C, H, andK
fosH25Eftpihseruf.0hlaa1rrbeIoeerorSnNcseqomtnuimetodAugrtgqmi(oi´htiOahOudpetntleheelpsEAietpontcerepesGhpaytdtcecl)ot.(yHoratiIwtocolpTnso).carpeiesmholt(lemtwheo1ionreptcae9vfoutaChratse8rtteyototakhrR5ees.ttnie)redoontaddhNmOcgwnieekneAateeadwoacOtrtGitriecehyNopb7Halenhsel5teaeexayMah1nrdKltsrav0rdieatsac2Ioxdacnta4asonietlftpcehrrcd×amediaeuoncArsrase1uttrArstgasd0esuottnnisnr2adsnCtodit4tcrytgefaaouao´ırompfldspDodl´ıifieoysfcexAγsiJcarecSOaaa-,lt,oro,hntPaiHHnrdGCoyOHo´oanAuuspwriOaniooagNtlnaTi.fuflhcderiIroiaaCrTcadmrvKienAihenefsdyode--r,, --0000....4202
s
bands. The plate scale in CANICA is 0.32 arcsec per pixel. -0.6
-40 -20 0 20 40
Observationsare usually carried out in series of 10 dithered
LAG (Days)
frames in each filter. Data are co-added after bias and flat- FIG.9.—Discrete crosscorrelation function (DCCF)between theγ-ray
fieldcorrections.CANICAobservationsofBLLacpresented andopticallightcurvesofFig.2.
inthisworkweremadeonsevenepochsbetweenMJD54715
and54747.ThedataareshowninFigures4and5,anddonot lag-20to+20days. Theresultwasacorrelationintensityof
showsignsofvariability. 0.17±0.09 (corrected for the effect of measurementnoise)
Thephotometricandpolarimetrymonitoringprogramatthe and a lag of −5±5 days, where negative lag means γ-rays
TuorlaObservatory,a divisionof the Departmentof Physics lagging the R band. The choice of lag range for the fit,
and Astronomy at the University of Turku, Finland, is per- from -15, +15 to -30, +30, as well as bin size, 3 or 4 days,
formed throughthe 1.03 m telescope at Tuorla Observatory, onlyhadasmalleffectontheestimatedtimelagofthepeak
andthe35cmtelescopeattheKVAobservatoryonLaPalma, and on the correlation strength, with the latter ranging from
Canaryislands,Spain. MostofthemonitoredsourcesareBL 0.14to0.17. Theratherweaklong-termcorrelationbetween
Lac objects listed in Costamante&Ghisellini(2002), as po- gamma-rays and optical tends to disfavor a one-zone syn-
tential TeV γ-ray sources69. All photometric measures are chrotronplus synchrotronself-Compton(SSC) model, since
takenusingtheR-filter(otherdetailsin Takaloetal.2008). inthismodelonewouldexpectelectronswithapproximately
The optical spectropolarimetry and spectrophotometry of the same energies to make both the optical and γ-ray emis-
blazarsattheUniversityofArizona,isperformedonaregular sion, and might favor an external radiation Compton (ERC)
basis since the launch of Fermi using the Steward Observa- or multi-zonesynchrotron/SSCmodel(e.g.,Aharonianetal.
tory2.3mBoktelescopeonKittPeak, Arizona,and1.54m 2009; Bo¨ttcheretal. 2009). The optical-UV variability ob-
Kuiper telescope on Mt. Bigelow, Arizona, USA. Depend- served during the 48 days of the multiwavelength campaign
ing on weather conditions, BL Lac is observed nightly dur- is not correlated with the weaker X-ray variability seen by
ing each of the monthly monitoring campaigns, which typi- Swift-XRTandRXTE norwiththenon-variableγ-rayemis-
cally last a week. All of the measurementshave been made sionobservedinthesameperiodbytheLAT(Figure4). The
withtheSPOLspectropolarimeter(Schmidtetal.1992). The lackofcorrelationmightsupportthescenariowherebothsyn-
Steward Observatory blazar monitoring program associated chrotron and IC photons contribute to the X-ray emission.
withtheFermimissionisdescribedbySchmidtetal.(2009) Variability produced by the highest energy electrons would
andthe data are publiclyavailable70. Detailsconcerningthe be smeared out by the IC component, and the mix of pho-
tons from the low and high energy component would dilute
reductionandcalibrationofboththepolarimetryandphotom-
thecorrelationwiththesynchrotron,orsynchrotronplusther-
etrycanalsobefoundinSchmidtetal.(2009)andreferences
mal,optical-UVemission.Thiswouldexplainwhyvariability
therein. The data products include high signal-to-noise flux
in the optical/UV is greater than in X-rays. Moreoverif the
andlinearpolarizationspectraspanningλ = 4000−7550A˚
synchrotron and IC emission components come from phys-
anddifferentialV-bandphotometry.
ically distinct regions, they would also weaken the correla-
tion. Noevidencefora contributionbyaccretiondiskradia-
3.4. MultifrequencycorrelationandSED
tionduringthislowactivitystateofthecampaignwasfound,
TheopticalR-andV-bandlightcurveinthebottompanel althoughthismightcontributetothelackofclearcorrelation
of Figure 2 shows more rapid variability due to the in- betweentheX-rayandoptical-UVemission. Therecentdis-
creased sampling with respect to the weekly-averaged LAT coveryofaluminousHαemissionlineintheopticalspectra
light curve. Overlapping time intervals of the 18-month ofBL Lac(Vermeulenetal.1995;Corbettetal. 1996,2000;
light curves in γ-rays and in the R band were used to cal- Capettietal. 2010), as likely a productof the broad line re-
culate the discrete cross correlation function (DCCF), fol- gion photoionizedby the disk radiation, can suggest a more
lowing Edelson&Krolik (1988). In Figure 9 the DCCF is relevantroleplayedbythediskduringmajoroutburstwitha
plotted with error bars estimated by a Monte Carlo method, highγ-rayComptondominance(asseenintheEGRET1997
taking measurement errors and data sampling into account outburst).
(Petersonetal.1998). Thecorrelationstrengthandlag were TheaveragedSEDfromtheMWcampaign(2008Aug. 20
computedbyfitting a gaussianprofileto the DCCF between –Sep.9)isreportedinFigure10. Forthefirsttimethemulti-
frequencyemissionfortheeponymousblazarsourceBLLac
69http://users.utu.fi/kani/1m/ is mapped from radio to GeV γ-rays during a low-activity
70http://james.as.arizona.edu/˜psmith/Fermi state. The relevantarchivalobservationsof BL Lac are also