Monitoring of AGNs for variability from MIRO
K.S. Baliyan,
U. C. Joshi and S. Ganesh
PhysicalResearch Laboratory, Ahmedabad 380009,India
Abstract. Asalongtermprogramme,asampleofblazarsisbeingmonitored
from MtAbuInfraredObservatory(MIRO).Blazarsare theradio-loudAGNs
with polarized, highly luminous and rapidly variable non-thermal continuum
emission ranging from radio to rays, from a relativistic jet. The emission
showsvariabilityat several time scales. Radio through-ray spectral energy
distribution (SED) of blazarsexhibits twobroadhumps- rst at UV/optical,
consideredtobeduetosynchrotronemissionfromtherelativisticelectrons,and
secondoneintheregionextendingfromX-rayto-rayswhoseoriginislessun-
derstood. Longtermmulti-wavelengthmonitoringoftheblazarsforvariability
is the keyto obtain informationon thegeometry of thejets and thephysical
processesresponsibleforthehighenergyemissions. Herewereportmonitoring
of severalblazars using1.2 m IR telescopeanddiscuss recentresultsonsome
of them. Inparticular, PKS0716+714and 3C66Ahaveshown intenseactivity
andcontinuedinbrightphaseduring2003-04,drawingworldwideattentionfor
aco-ordinatedmonitoringcampaign.
Keywords: AGN:Blazars{multiwavelength{variability{non-thermalemis-
sion{relativisticjet
1. Introduction
ActiveGalacticNuclei(AGN)areenigmaticobjectsproducingveryhighluminositiesina
verycompactvolume. BLLacobjectsandatspectrumradioquasars(FSRQs)areAGNs
commonlyuniedintheclassofblazars. Themembersofthissubclassarecharacterized
bynon-thermalcontinuumspectra,ahighdegreeoflinearpolarization,rapidvariability
in ux and polarizationat all wavelengths andradio jets oftenexhibiting superluminal
motion(BlandfordandRees1978). Manyofthesesourceshavealsobeendetectedinhigh
energy(> 100MeV)gamma-raysbytheEGRETinstrumentonboardCGRO(Mattox,
Hartmanand Reimer2001). In theframework of relativisticjet model, thelow energy
(radio-optical/UV) emission from these sources is interpreted as synchrotron emission
from nonthermal electrons in the relativistic jet. The high-frequency (X-ray - -ray)
emissioncouldeitherbeproducedviaComptonupscatteringoflowfrequencyradiationby
thesameelectronsresponsibleforthesynchrotronemission(leptonicjetmodel;Bottcher
2002),orduetohadronicprocesses(Mukeetal.,2003). Theoverallradioto-rayspectral
energy distributions (SEDs) of blazars exhibit a broadtwo hump structure (Fassati et
al., 1998). The rst peak occurs either in the IR/optical (low energy blazars) or in
the UV/X-ray region (high energy peaked blazars). These are also called red or RBL
and Blue or XBL, respectively, based upon peaking energy or detection criteria. The
secondspectralcomponentextendsfromX-rayto-ray(GeV/TeV),anditsoriginisless
understoodbut,asmentionedabove,isconsideredtobeinverseCompton(IC)scattering
of low energy photons. Origin of these photons is notveryclear- they can beinternal
(synchrotronselfCompton,SSC)orexternal(External Compton)tothejet. Adetailed
study of themultifrequency blazarux variationmayprovideconsiderable information
ontheemittingregion dynamics. Keeping thisin mind, several multiwavelength blazar
monitoring campaigns have been conducted on several sources where observatories in
dierentpartsofworldparticipate. Theseareoftenco-ordinatedwithobservationsfrom
spacebornefacilitiesatmanywavelengths. Sucheortsenablestudyofcorrelationamong
ux variationsin dierent bands which isof uttermost importance to shedlight onthe
physicalprocessesresponsiblefortheemission.
ThemonitoringofblazarsfortheirvariabilitystudyatthePhysical ResearchLabo-
ratorystartedmuchbefore thecommissioning ofthe 1.2mInfraredObservatoryat Mt.
Abuin1995. ItwouldnotbeoutofcontexttonotetheeortsofProf. M.R.Deshpande
and one of us (UCJ) who took this up as one of the most challengingproblems of all
times- tounderstand theenergymechanismofAGNs. Initially theopticalpolarimeter
attheArizonaObservatorywasusedtomonitorOJ287andlateronwedesignedandfab-
ricatedourownopticalpolarimeter(Deshpande etal.,1985)whichwasusedatKavalur
Observatoryand UPSO(Nainital) to study severalBL Lacobjects. Thesestudies from
MIRO and other observatories resulted in a series of publications( Kulshrestha et al.,
1984;1987, Baliyan et al., 1996, Deshpande et al., 1997, Joshi et al., 2000, Baliyan et
al., 2001, Joshi et al., 2002). Oneof the important result was detection of 20 minute
variabilitytimescaleanditsimplications(Kulshresthaetal.,1984,Baliyanetal.,1996)
in BLLacobjectOJ 287. Inaddition tooptical polarimeter,opticalCCD (1kx1k) and
near infraredarraycamera, NICMOS-3arealso used forthemonitoringof asample of
blazars. Nowwehavedata onseveralobjects(OJ287,BLLac,Mrk421,Mrk501etc)
spawning almost a decade and are being put together to generate light curves. Here
we will discussa BLLacobject, PKS0716+714,which had beenthe subjectof intense
worldwidemultiwavelengthmonitoringcampaign recently. Weparticipatedin thiscam-
Thesourcewasconsistentlyinbrightphaseandunderwentseveraloutburstsduring the
courseofobservations.
2. PKS0716+714
BLLacobjectPKS0716+714isfromS5catalogofstrongsourcesperformedat4.9GHz
(Kuhr et al., 1981). It has a compact core-jet structure and a VLBI study suggests
dierent componentsmoving at slightly dierent velocities (Bach et al., 2003). It was
alsoconrmed asBL Lacsource,3EGJ0721+7120,inthe 3EGEGRET survey(Nolan
et al.,2003). Thepolarization atoptical wasfoundto bevariableon shorttime scales,
with possible quasi-periods of12.5, 2.5 and 0.14days(Impey et al., 2000). As ofnow,
no spectroscopic observation has detected any feature in its spectrum. The redshift of
0.44, determined using observational features, eg. starlikeappearance, absence of host
galaxy, smallangularsize etc, stillremains unconrmed. Fortherst time,Biermanet
al., (1981)reported variabilityin magnitude, polarization and polarization angle. This
variability coupled with featureless continuum designated it as a BL Lac object. A
UBVRI photopolarimetric study was carried out by Takalo et al., (1994) during two
nightswho noticed avariable , high, wavelength dependent polarization. Sagar et al.,
(1999)performed a BVRI monitoringin 1994. For the rsttime, intra day variability
(IDV)wasdetectedbyHeidtandWagner(1996)whoalsonoticeda4dayperiodicityin
thesourcevariation. Nesciet al.,(2002)dida52nightmonitoringandreportedtypical
variationsof 0.02magperhour. Ghissellini et al.,(1997)detected aspectralattening
when theux was high during rapid ares. They interpretedit asdue to the presence
of twoprocesses operating in the source. Therst process- energy injection in alarge
region, remained stable overa few monthstime scale. It caused achromatic longterm
uxvariation. Fastvariations,ontheotherhand,weresuggestedtobedueeithertothe
curved trajectory of relativisticelectronsemitting blob orveryrapid electron injection
andcoolingprocesses.
Inordertoascertainwhetherthefastradiovariationswereduetopropagationeects
(interstellarscintillation)oranintrinsicphenomenon,thissourcewasmonitoredinoptical
andradio,simultaneously(Quirrenbachetal.,1991). Somecorrelationinthestrongux
variationswerenoticedbetweentwowavebandsbutwithoutanydeniteanswer. Wagner
et al., (1996)found aclosecorrelation betweenoptical and radio and possibly between
the optical and X-ray bands. Recently, Raiteri et al., (2003) reported optical (1994-
2001)and radio(1978-2000)observations onPKS0716+714,usingvarious bands. They
mentionvariationsby 1.5and 2.0 magin opticalregion. Theyalso reported a2.3mag
increasein 9 daysin October2000, highestlevelof brightness,not onlyfor this source
butforblazarclass.
Though the low energy peak in the SEDof PKS0716+714falls almost in thenear
IRregion,therearealmostnoobservationsinthiswindow. Wherethethermalradiation
emissionthroughIRwavebandsinblazarsismainlynonthermal. Sincethissourceemits
signicant energy in this waveband, we decided to monitor the source from Mt Abu
using1.2mtelescopeandNICMOS-3IRarray. Laterwealsojoinedtheworldwideeorts
throughWEBTcampaignonthissource. Whatisofmoreimportancetounderstandthe
energetics of the blazars is the simultaneous multiwavelength studies of the variability
withshortestpossiblesamplingtimes.
3. Observations and data analysis
PKS0716+714isbeingmonitorednowformorethanthreeyears,mostlyinnearinfrared.
HerewereportobservationsfromOctober2003toApril2004whenthesourcewasinan
unusuallybrightphase. Theobservationsweremadeatthef/131.2mtelescopeequipped
with NICMOS-3IRarraycameraat theMt Abu InfraredObservatory(MIRO).It isa
HgCdTe detector with 256x256 pixels and a plate scale of 0.98" perpixel. The lters
used are J(1.2), H(1.65)and K'(2.12) and most of the observations are made with
4'x4' FOV in order to accommodate several comparison stars in the same frame for
better calibration. Care is taken that the source remains in the same quadrant of the
array detector to avoid quadrant to quadrant variation in the detector characteristics
to aect theresults. Apart from the source observations, darkframes are taken every
night. Observationsarecarriedoutinsuchawayastoimprovesignaltonoiseratiowhile
skirtingthesaturationproblem.
Datawasreducedandanalysedusingstandardtechniques-IRAFandhomedeveloped
scripts. Images at one location were combined to improve S/N and photometry was
performed using aperture photometry. Same aperture was kept for the PKS0716+714
andcomparisonstarsandinstrumentalmagnitudeswereobtainedforallobjectspresent
intheeld. Thesourcemagnitudeisobtainedastheaverageofthosederivedwithrespect
to all the comparison stars in the individual frame. Typical photometric errorsin the
respectivebandsarelessthan0:03(J);0:03(H)and0:05(K 0
). Thecalibrationvaluesfor
thestarsaretakenfrom2MASS.Itshouldbenotedthat2MASSphotometricvaluesare
forKs(2.16m)whilewehaveusedK'(2.12m)bandforobservations. Weestimatean
errorofnotmorethan0.04mag,lessthanthetypicalphotometricerrorin K'band,due
tothisfactorwhich wehavenottriedtocorrect.
4. Results and discussion
OurlightcurvesforthePKS0716+714areoneofthelongestdurationinthenearinfrared
bands and arepresentedin Fig.1. Majoroutbursts occurred around1November 2003,
28 December2003, 26 January2004 and March 2004 (WEBT-privatecommunication).
However,ourdatadonotprovidefullcoverageoftheseoutbursts. Wenoticeanincrease
ofabout1.6magbetweenDecember10,2003andJanuary26,2004inJ,HandK'bands.
dates. Since we do not have observations in this duration, their behaviorin NIR can
not be described. The light curve appears asthe superposition of fast ares lasting a
few dayson amodulated base level in all three bands.The gure also shows fading of
thesource during December10 -12, 2003 byabout0.5 magwith afurther decrease in
magnitudeby 0.3magon December18, 2003. We nicelycapture thedecaypartof the
January 26, 2004 are, which decays through February 2004, slowlyending in ashort
are. AlsoplottedinFig. 2arelightcurvesinthreebandsduringthenightofDecember
12, 2003. We havestudied nightly variations of thesource on several nightsbut all of
themarenotdiscussedhere. Wedonotnoticeanysignicantintradayvariation(IDV)
on any nightduring the whole monitoring campaign. However,wehasten to add that
nodenite conclusionshould bedrawnbased onthe presentdata. May be ournightly
monitoringperiodwasnotlongenoughandwemightalsohavemissedthenightssource
variedovernight. Inourobservations,allthethreebands,J,HandK',showalmostsame
pattern. WedonothavecompleteobservationsduringMarch2004outburstbutcatchit's
decayingpartinAprilobservationswheresourceisstillaboveaveragebrightnesslevel.
3000 3050 3100
13 12 11 10
J
3000 3050 3100
12 11 10 9
H
3000 3050 3100
11 10 9 8
K PKS0716 (Dec2003, Jan,Feb&Apr 2004)
JD - 2450000
Figure1. VariationsinthenearinfraredJ,HandK'bandsduringDecember4,2003toApril
20,2004. BrighteningofPKS0716+714bymorethan1.5magisnoticeableduringDec.03-Jan.04.
We notice that colour index (J-H) decreases, albeit slightly, when sourcebrightens
fromitslevelonDec.18,2003toJan. 16,2004. Thebluerwhenbrightertrendcontinues
extinction. Thestatisticalanalysis oflightcurvesinthree bandsdonotreveal presence
ofanytimelag.
The constant variability amplitude (of about 1.0 mag) in magnitudes implies ux
variation amplitude to be proportional to ux level and can be explained as due to
Dopplerbeamingfactor,Æ=[(1 Cos)]
1
,where istheLorentzfactorofthebulk
motionofemittingelectronsinthejetwith astheviewingangle. Theintrinsicux is
relativisticallyenhancedbyafactorof Æ 3
. Thischangecouldbedueeitherto energetics
orgeometricalreasonsoracombinationofboth. Longerdurationdataisrequiredtopin
pointtheprocess.
2986.3 2986.32 2986.34 2986.36 2986.38 2986.4 13
12 11 10
J 2986.3 2986.32 2986.34 2986.36 2986.38 2986.4 12
11 10 9
H 2986.3 2986.32 2986.34 2986.36 2986.38 2986.4 11
10 9 8
K PKS0716 (12 Dec. 2003)
JD - 2450000
Figure 2. Light curvefor PKS0716+714onDecember12, 2003 innearinfrared,J,H andK'
bands.
5. Conclusions
Here we have presented one of the longest duration light curves of the BL Lacobject
PKS0716+714in nearIR fromDec. 2003to April2004. Thesourcewasconsistentlyin
brightphaseduringthemonitoring.Onthelongtermbrightenedbaselevel,variationson
shorttimescalesaresuperposed. Thereisanincreaseofmorethan1.5magbetweenDec.
19,2003and Jan. 26,2004, which isdue totheDec. 2003/Jan2004outbursts. Several
scalewithout any clearperiodicity. Studying short time scalebehaviorand looking for
spectralchangeswhilefollowingcompleteoutburstmaybethekeytopindownthebasic
emissionmechanism. Fromlighttraveltimearguments,Rc:
t
,where
t
isvariability
timescaleinsourceframe,emissionregionsappeartobeoftheorderofsolarsystem. No
short termtimescale variationsduringasinglenightof observation aredetectedabove
1- errorlevel. All threenear infraredbands showsimilar behavior,exceptthat dueto
poorS/Nratio,K'band lightcurveshowedlargespread.
Itispossiblethatthevariationsarepartlyintrinsicandpartlyduetorotatinghelical
jet. Here jetinhomogeneitycausestimelagsintheuxvariationatdiversewavelengths,
sincedierent frequencyemittingportions of thejetacquire the sameviewing angleat
dierenttimes. We,however,note that allthethree near IR bands, J,H andK', show
similar pattern in theirlight curves. Because of the limited time coverage in NIR, the
presentdataarenotenoughtoperformanyusefulstatisticaloperationtodeterminetime
scales. A longerdurationmonitoring in this window is underway. However, webelieve
thatthedatareportedherewillbeveryusefulforamultiwavebandstudy.
Acknowledgements
The authors would like to thank Ms C.R. Shah for the photometry and MIRO sta
for assistance in observations. This work is supported by the Department of Space,
GovernmentofIndia.
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