Two active states of the narrow-line gamma-ray-loud AGN GB 1310+ 487

18 Pages·2013·0.96 MB·English

Astronomy&Astrophysicsmanuscriptno.1310 cESO2014
(cid:13)
March14,2014
Two active states of the narrow-line gamma-ray-loud AGN
+
GB1310 487
K.V.Sokolovsky1,2,3,⋆,F.K.Schinzel1,4,Y.T.Tanaka5,P.K.Abolmasov3,E.Angelakis1,A.Bulgarelli6,
L.Carrasco7,S.B.Cenko8,9,C.C.Cheung10,K.I.Clubb8,F.D’Ammando11,12,13,L.Escande14,S.J.Fegan15,
A.V.Filippenko8,J.D.Finke16,L.Fuhrmann1,Y.Fukazawa17,E.Hays9,S.E.Healey18,Y.Ikejiri17,R.Itoh17,
K.S.Kawabata5,T.Komatsu17,Yu.A.Kovalev2,Y.Y.Kovalev2,1,T.P.Krichbaum1,S.Larsson19,20,21,M.L.Lister22,
B.Lott23,24,W.Max-Moerbeck25,I.Nestoras1,C.Pittori26,T.Pursimo27,A.B.Pushkarev28,29,1,A.C.S.Readhead25,
4 E.Recillas7,J.L.Richards22,D.Riquelme30,R.W.Romani18,K.Sakimoto17,M.Sasada31,R.Schmidt1,
1 M.S.Shaw18,A.Sievers30,D.J.Thompson9,M.Uemura5,H.Ungerechts30,S.Vercellone32,F.Verrecchia26,
0
M.Yamanaka33,M.Yoshida5,andJ.A.Zensus1
2

r
a (Affiliationscanbefoundafterthereferences)
M
ReceivedNovember8,2012;acceptedJanuary8,2014

3
1
ABSTRACT

]
E Context.Previouslyunremarkable,theextragalacticradiosourceGB1310+487showedaγ-rayflareon2009November18,reaching
adailyfluxof 10 6photonscm 2s 1atenergiesE >100MeVandbecameoneofthebrightestGeVsourcesforabouttwoweeks.
H ∼ − − −
Its optical spectrum shows strong forbidden-line emission while lacking broad permitted lines, which is not typical for a blazar.
. Instead,thespectrumresemblesthoseofnarrowemission-linegalaxies.
h
Aims.Weinvestigatechangesintheobject’sradio-to-GeVspectralenergydistribution(SED)duringandaftertheprominentγ-ray
p
flarewiththeaimofdeterminingthenatureoftheobjectandofconstrainingtheoriginofthevariablehigh-energyemission.
-
o Methods.ThedatacollectedbytheFermiandAGILEsatellitesatγ-rayenergies;SwiftatX-rayandultraviolet(UV);theKanata,
r NOT,andKecktelescopesatoptical;OAGHandWISEatinfrared(IR);andIRAM30m,OVRO40m,Effelsberg100m,RATAN-600,
t andVLBAatradioareanalyzedtogethertotracetheSEDevolutionontimescalesofmonths.
s
a Results.Theγ-ray/radio-loudnarrow-lineactivegalacticnucleus(AGN)islocatedatredshiftz=0.638.Itshinesthroughanunrelated
[ foregroundgalaxyatz=0.500.TheAGNlightisprobablyamplifiedbygravitationallensing.TheAGNSEDshowsatwo-humped

structuretypicalofblazarsandγ-ray-loudnarrow-lineSeyfert1galaxies,withthehigh-energy(inverse-Compton) emissiondomi-
5 natingbymorethananorderofmagnitudeoverthelow-energy(synchrotron)emissionduringγ-rayflares.Thedifferencebetween
v
thetwoSEDhumpsissmallerduringthelow-activitystate.Fermiobservationsrevealastrongcorrelationbetweentheγ-rayfluxand
1
spectralindex,withthehardestspectrumobservedduringthebrightestγ-raystate.Theγ-rayflaresoccurredbeforeandduringaslow
5
risingtrendintheradio,butnodirectassociationbetweenγ-rayandradioflarescouldbeestablished.
1
Conclusions.Iftheγ-rayfluxisamixtureofsynchrotronself-Compton(SSC)andexternalCompton(EC)emission,theobserved
2
GeVspectralvariabilitymayresultfromvaryingrelativecontributionsofthesetwoemissioncomponents.Thisexplanationfitsthe
.
1 observedchangesintheoverallIRtoγ-raySED.
0
Key words. quasars: individual: GB1310+487 – galaxies: jets – gamma rays: galaxies – radiation mechanisms: nonthermal –
4
galaxies:active
1
:
v
i
X1. Introduction cess,EC;e.g.,Sikoraetal.1994;Dermer&Schlickeiser2002).
The sources of the external seed photons for the EC process
rBlazars are active galactic nuclei (AGNs) in which relativisti-
a include the accretion disk, broad-line region (BLR) clouds,
callybeamedemissionfromthejetdominatestheradiativeout-
warm dust (dusty torus), synchrotron emission from other
put across most of the electromagnetic spectrum. The spectral
faster/slowerregionsofthejet,andthecosmicmicrowaveback-
energy distribution (SED) of a blazar has two broad compo-
ground(CMB),withtheirrelativecontributionsvaryingfordif-
nents: one peaking between far-IR and X-ray wavelengths and
ferent objects. The models based on inverse-Compton scatter-
theotherpeakingatγ-rays(Abdoetal.2010b).Thelow-energy
ing by relativistic electrons are generally referred to as lep-
emissioncomponentisbelievedtobedominatedbysynchrotron
tonicmodels(Celotti&Ghisellini2008;Ghisellini&Tavecchio
radiation of relativistic electrons/positronsin the jet. Radiation
2009;Boettcher2010,2012).Analternativeviewregardingthe
at higher energies could be due to the inverse-Compton scat-
originofblazarhigh-energyemissionisrepresentedbyhadronic
tering of synchrotron photons emitted by the electrons them-
models (Mücke&Protheroe 2001; Mückeetal. 2003; Sikora
selves(synchrotronself-Comptonprocess,SSC;e.g.,Jonesetal.
2011), where relativistic protons in the jet are the primary ac-
1974; Ghisellini&Maraschi 1989; Marscher&Travis 1996)
celeratedparticles.Weadopttheleptonicmodelsasthebasisfor
and/or photons from external sources (external Compton pro-
thefollowingdiscussion.
⋆ e-mail:[email protected]
Articlenumber,page1of18
A&Aproofs:manuscriptno.1310
Two types of radio-loud AGNs give rise to the blazar The radio source GB1310+487 [also known as
phenomenon: flat-spectrum radio quasars (FSRQs) and GB6B1310+48441, and CGRaBSJ1312+4828, listed in
BL Lacertae-type objects (BL Lacs). Flat-spectrum radio the Fermi γ-ray source catalogues as 1FGLJ1312.4+4827
quasars are characterized by high luminosities, prominent (Abdoetal. 2010d) and 2FGLJ1312.8+4828 (Nolanetal.
broad emission lines in their optical spectra, and the peak of 2012), radio VLBI position2 α = 13h12m43s.353644
J2000
±
synchrotron jet emission occurring at mid- or far-IR wave- 0.22mas,δ = +48 2830.94047 0.16mas(Beasleyetal.
J2000 ◦ ′ ′′
±
lengths.Thermalemission,probablyoriginatingintheaccretion 2002)] is a flat-spectrum radio source. It was unremarkable
disk surrounding the central black hole, may contribute a among other faint γ-ray detected blazars (the E > 100MeV
significant fraction of the optical and UV emission in some flux during the first 11 months of the Fermi mission was
FSRQs (Villataetal. 2006; Jolleyetal. 2009; Abdoetal. 3 10 8photonscm 2s 1,asreportedinthe1FGLcatalogue;
− − −
∼ ×
2010a). BL Lacertae-type objects, on the other hand, show Abdoetal.2010d)untilitappearedinthedailyFermiskywith
mostlyfeaturelessopticalspectradominatedbythenonthermal afluxof 1.0 10 6photonscm 2s 1 on2009November183
− − −
∼ ×
continuum produced by a relativistic jet. Their synchrotron (Sokolovskyetal.2009).AGILEobservationsreportedtwodays
emissionpeakislocatedbetweenfar-IRandhard-X-rayenergies laterconfirmedthehigh-fluxstateofthesource(Bulgarellietal.
(Padovani&Giommi 1995; Fossatietal. 1998; Ghisellinietal. 2009). Follow-up observations in the near-IR (Carrascoetal.
1998). In GeV γ-rays, BL Lacs show a wide distribution of 2009)andoptical(Itohetal.2009)alsofoundGB1310+487in
spectral slopes, while FSRQs almost exclusively exhibit soft a high state compared to historical records. The daily average
γ-rayspectra(Abdoetal.2010c).Itisnotclearwhetherthereis γ-ray flux remained at 1.0 10 6photonscm 2s 1 for more
− − −
∼ ×
a physical distinction between BL Lacs and FSRQs, or if they thanaweek(Hays&Escande2009).
represent two extremes of a continuous distribution of AGN This paper presents multiwavelength observations of
propertiessuchasblackholemass(M ),spin,oraccretionrate GB1310+487before,during,andafteritsactiveγ-raystate,and
(Ghisellinietal. 2011). Recently, five• radio-loud narrow-line suggestspossibleinterpretationsoftheobservedSEDevolution.
Seyfert 1 galaxies (NLSy1s) have been detected in γ-rays by In Sect. 2 we describe the observingtechniquesand data anal-
Fermi/LAT, suggesting the presence of a new class of γ-ray- ysis. Sect. 3 presents an overview of the observational results.
emitting AGNs (Abdoetal. 2009d; D’Ammandoetal. 2012). InSect.4wediscusstheirimplications,andwesummarizeour
TherelationshipbetweenNLSy1andblazarsisunderdebate.It findings in Sect. 5. Throughout this paper, we adopt the fol-
hasbeensuggestedthatradio-loudNLSy1galaxiesharborrela- lowing convention:the spectral index α is defined through the
tivisticjets(Foschini2013;D’Ammandoetal.2013),butunlike energy flux density as a function of frequency F ν+α, the
ν
∝
blazarstheyarepoweredbylessmassiveblackholeshostedby photon index Γ is defined through the number of incoming
ph
spiral galaxies (Yuanetal. 2008; Komberg&Ermash 2013). photons as a function of energy dN(E)/dE E−Γph, and the
Thepresenceofa relativisticjetis supportedbyobservationof two indices are related by Γ = 1 α. We u∝se a ΛCDM cos-
ph
superluminalmotionsintheparsec-scaleradiojetoftheNLSy1 mology,withthe followingvaluesf−orthe cosmologicalparam-
SBS0846+513 (D’Ammandoetal. 2012). The observational eters: H = 71kms 1Mpc 1, Ω = 0.27,and Ω = 0.73 (see
0 − − m Λ
evidencethatradio-loudNLSy1 have M smaller than those of Komatsuetal.2009;Hogg1999),whichcorrespondstoalumi-
blazarshasrecentlybeenchallengedbyC•alderoneetal.(2013). nosity distance of D = 3800Mpc, an angular-size distance of
L
SomenearbyradiogalaxiesincludingCenA(NGC5128),PerA D = 1400Mpc,andalinearscaleof6.9pcmas 1atthesource
A −
(NGC1275, 3C84), and Vir A (M87, 3C274) are detected by redshiftz=0.638(seeSect.3.4).
Fermi/LAT (Abdoetal. 2010j). While part of their γ-ray
luminosity is attributed to inverse-Comptonscattering of CMB
2. Multiwavelengthobservations
photonsontheextended(kpc-scale)radiolobesofthegalaxies
(Cheung 2007; Abdoetal. 2010i), contribution from the core
2.1.Gamma-rayobservationswithFermi/LAT
regionis also evident(Abdoetal. 2009b, 2010h). Unlike other
radio galaxies studied by Abdoetal. (2010j), Per A exhibits Fermi Gamma-ray Space Telescope (FGST; hereafter Fermi)
episodes of rapid GeV variability (Donatoetal. 2010; Ciprini is an orbiting observatory launched on 2008 June 11 by a
2013). The core γ-ray emission in radio galaxies is probably DeltaII rocket from the Cape Canaveral Air Force Station in
produced by the same mechanisms as in blazars, but with less Florida, USA. The main instrument aboard Fermi is the Large
extremerelativisticbeaming. Area Telescope (LAT; Atwoodetal. 2009; Abdoetal. 2009a;
Sinceearlysatelliteobservationsestablishedtheassociation Ackermannetal.2012),apair-conversiontelescopedesignedto
ofsomediscreteγ-raysourceswithAGNs,itbecameclearthat cover the energy band from 20MeV to greater than 300GeV.
TheFermi/LATisprovidingauniquecombinationofhighsen-
blazarsemita considerablefractionoftheirtotalenergyoutput
sitivityandawidefieldofviewofabout60 .Fermiisoperated
above 100MeV (Swanenburgetal. 1978; Hartmanetal. 1999; ◦
Mukherjee 2002). The current generationof space-based γ-ray inanall-skysurveymodemostofthetime,whichmakesitideal
formonitoringAGNvariability.
telescopes that use solid-state (silicon) detectors is represented
The dataset reported here was collected during the first
by instruments onboard AGILE (Tavanietal. 2009, 2008) and
Fermi(Atwoodetal.2009),whichopenawindowintotheworld 33 months of Fermi science observations from 2008 August 4
to 2011 June 13 in the energy range 100MeV – 100GeV. The
ofGeVvariabilityandspectralbehaviorofγ-ray-loudAGNs.In
33monthtimeintervalisdividedintosubintervalsaccordingto
contrast to previousexpectations(Vercelloneetal. 2004), most
ofthebrightestγ-rayblazarsdetectedbyFermiandAGILEwere thelevelofitsγ-rayactivityasobservedbyFermi/LAT(seeTa-
ble1).
alreadyknownfromtheEGRETera(Tavani2011).Ontheother
hand,manyblazarspreviouslyunknownasγ-rayemitterswere
1 The correct B1950 source name, if its declination is expressed as
observedtoreachhighfluxes(> 10−6photonscm−2s−1 atener- threedigits,is1310+487,while1310+484isanunrelatednearbyradio
giesE >100MeV)foronlyashortperiodoftimeduringaflare. source.
Inthiswork,wepresentadetailedinvestigationofonesuchob- 2 seehttp://astrogeo.org/vlbi/solutions/rfc_2012b
ject. 3 UTdatesareusedthroughthetext.
Articlenumber,page2of18
K.V.Sokolovskyetal.:Twoactivestatesofthenarrow-linegamma-ray-loudAGNGB1310+487
Table1.Changesintheγ-rayspectrumbetweentimeintervalsconsideredintheanalysis.
Period UTinterval 100MeV–100GeVflux Γ TS N
ph
33months 2008-08-04–2011-06-13 (1.03 0.04) 10 7 2.18 0.02 4415 3566
−
± × ±
pre-flare 2008-08-04–2009-11-16 (0.34 0.05) 10 7 2.41 0.09 205 476
−
± × ±
Flare1 2009-11-16–2009-12-21 (6.94 0.32) 10 7 1.97 0.03 3333 947
−
± × ±
Interflare 2009-12-21–2010-04-26 (1.37 0.11) 10 7 2.15 0.06 917 592
−
± × ±
Flare2 2010-04-26–2010-07-26 (2.83 0.16) 10 7 2.14 0.04 1839 907
−
± × ±
post-flare 2010-07-26–2011-06-13 (0.44 0.06) 10 7 2.34 0.09 236 422
−
± × ±
Column designation:Col. 1, γ-rayactivity state; Col. 2, time intervalused for spectralanalysis;Col. 3, averageflux in unitsof
photonscm−2s−1;Col.4,photonindex:dN(E)/dE E−Γph;Col.5,TestStatistic(TS)definedinSect.2.1;andCol.6,numberof
∝
photonsattributedtothesource(modeldependent).
Fermi/LAT data comprise a database containing arrival denttimebins.Thetimebinwidthwaschosentobesevendays.
times, directions, and energies of individual silicon-tracker Sourceswithlessthanonephotondetectedintheindividualbin
events supplemented by informationabout the spacecraft posi- or with TS < 25 were excluded from the sky model for that
tion and attitude needed to calculate the effective exposure for bin. The lightcurves were computed by integrating the power-
a celestial region and time interval of interest. The maximum- lawmodelintheenergyrange100MeV–100GeV.
likelihoodmethodisusedtoanalyzethesedatabyconstructing For lightcurves with time bins of fixed widths, the choice
an optimal model of the sky region as a combinationof point- of bin width is a compromisebetween temporalresolution and
likeanddiffusesourceshavingaspectrumassociatedwitheach signal-to-noise ratio for the individual bins. For Fermi/LAT
one of them (Mattoxetal. 1996; Abdoetal. 2010d). The sig- an alternative method has recently been developed (Lottetal.
nificance of source detection is quantified by the Test Statis- 2012), in which the time bin widths are flexible and chosen to
tic (TS) value,determinedbytakingtwice the logarithmof the producebins with constantflux uncertainty.Flux estimates are
likelihoodratiobetweenthe modelsincludingthe targetsource stillproducedwiththestandardLATanalysistools.Inthiscase
(L )andoneincludingonlythebackgroundsources(L ):TS we used Fermi Science Tools v9r27p1 and P7CLEAN_V6
1 0
≡
2(lnL lnL ).TheratiosL andL aremaximizedwithrespect event selection and IRFs, for which the current version of the
1 0 0 1
−
to the free parametersin the models.The Monte-Carlosimula- adaptivebinningmethodhasbeenoptimized(wehavechecked
tion performed by Mattoxetal. (1996) for EGRET confirmed thatusingtheP7SOURCE_V6classyieldsverysimilarfluxes).At
theoreticalpredictions(Wilks1938)thatforaGeVtelescope,in times of high source flux, the time bins are narrower than dur-
mostcases,theTSdistributionisclosetoχ2. inglowerfluxlevels,thereforeallowingustostudymorerapid
The unbinned likelihood analysis was performed with variabilityduringtheseperiods.
the Fermi Science Tools package4 version v9r21p0. The The lower energy limit of the integral fluxes computed for
DIFFUSEclass eventsin the energyrange100MeV – 100GeV the adaptively binned lightcurve is chosen to minimize the bin
were extracted from a region of interest defined as a circle widths neededto reach the desired relative flux uncertaintyfor
of 15 radius centered at the radio position of GB1310+487. most bins. The derivation of this energy limit, called the op-
◦
A cut on zenith angle > 100 was applied to reduce timum energy, is presented by Lottetal. (2012). Because the
◦
contamination from Earth-limb γ-rays, produced by cosmic source is variable and the optimum energy value depends on
rays interacting with the upper atmosphere (Shawetal. 2003; theflux,wecomputetheoptimumenergywiththeaverageflux
Abdoetal. 2009c). Observatoryrocking angles of greater than overthefirsttwo yearsof LAToperationreportedin the2FGL
52 were also excluded. A set of instrument response func- catalogue(Nolanetal. 2012). The optimumenergyis foundto
◦
tions (IRFs) P6_V11_DIFFUSE was used in the analysis. The be E = 283MeV for this source. We produced two sets of
0
sky model contained point sources from the 2FGL catalogue adaptivelybinnedlightcurvesinthe283MeV–200GeVenergy
(Nolanetal. 2012) within 20 from the target, as well as range, one with 25% flux uncertainties and another with 15%
◦
Galactic gll_iem_v02_P6_V11_DIFFUSE.fit and isotropic uncertainties.For each of these uncertaintylevelswe created a
isotropic_iem_v02_P6_V11_DIFFUSE.txt diffuse compo- secondversionofthelightcurvebyperformingtheadaptivebin-
nents5.Allpoint-sourcespectraweremodeledwithapowerlaw; ninginthereverse-timedirection.
thephotonindexwasfixedtothecataloguevalueforallsources
except the target. The diffuse-backgroundparameters were not
2.2.Gamma-rayobservationswithAGILE/GRID
fixed. The estimated systematic uncertainty of flux measure-
ments with LAT using P6_V11 IRFs is 10% at 100MeV, 5%
The AGILE γ-ray satellite (Tavanietal. 2009, 2008) was
at500MeV,and20%at10GeVandabove6.
launched on 2007 April 23 by a PSLV rocket from the Satish
The lightcurve of the target source was constructed by ap- Dhawan Space Centre at Sriharikota, India. AGILE is a mis-
plying the above analysis technique to a number of indepen- sion of the Italian Space Agency(ASI) devoted to high-energy
astrophysics, and is currently the only space mission capa-
4 For documentation of the Science Tools, see ble of observing cosmic sources simultaneously in the en-
http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/
ergy bands 18–60keV and 30MeV – 30GeV thanks to its
5 The models are available from the Fermi Science Support Center
two scientific instruments: the hard X-ray Imager (Super-
http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html
AGILE;Ferocietal.2007)andtheGamma-RayImagingDetec-
6 For newer P7_V6 IRFs used in the adaptive lightcurve analysis
tor(GRID;Rappoldi&AGILECollaboration2009).Duringthe
described below, the systematic uncertainties are lower at high ener-
gies: 10% at 100MeV, 5% at 560MeV, 10% at 10GeV and above first two years of the mission, AGILE was mainly operated by
(Ackermannetal.2012);however,thisdifferenceisnotcriticalforthe performing2–4 week-longpointed observations, but following
presentanalysis. thereactionwheelmalfunctioninOctober2009itwasoperated
Articlenumber,page3of18
A&Aproofs:manuscriptno.1310
in a spinningobservingmode,surveyinga largefractionof the mildspectralchanges.TheSwift/XRTobservationobtaineddur-
skyeachday. ingtheFlare2andpost-flareintervals(Table1)resultedinalow
The AGILE/GRID instrument detected enhanced γ-ray numberofdetectedphotons.TheCash(1979)statisticisapplied
emission from GB1310+487 from 2009 November 20 17:00 tofitthisdatasetwiththeabsorbedpower-lawmodel.TheCash
(JD2455156.2) to 2009 November 22 17:00 (JD2455158.2) statisticisbasedonalikelihoodratiotestandiswidelyusedfor
(see Bulgarellietal. 2009, for preliminary results). Level 1 parameter estimation in photon-counting experiments. The net
AGILE/GRID data were reanalyzed using the AGILE Standard countrateinthe0.3–10keVenergyrangechangedbyafactorof
AnalysisPipeline(seePittorietal.2009;Vercelloneetal.2010, 1.6betweenthetwo observationsconductedduringFlare1 and
for a description of the AGILE data reduction). We used γ- by a factor of 3.5 over the whole 33-month period. The X-ray
ray events from the ASDCSTDe archive, filtered by means of spectralanalysisresultsarepresentedinTable2.
the FM3.119 pipeline. Counts, exposure, and Galactic back-
groundγ-raymapswerecreatedwithabinsizeof0.5 0.5,for
◦ × ◦ 2.4.Ultraviolet–opticalobservations
E 100MeV.SinceAGILEwasinitsspinningobservingmode,
≥
allmapsweregeneratedincludingalleventscollectedupto50◦ The Swift Ultraviolet-Optical Telescope (UVOT; Romingetal.
off-axis. We rejected all γ-ray events whose reconstructed di- 2005) has a diameter of 0.3m and is equipped with
rectionsformangleswiththesatellite-Earthvectorsmallerthan a microchannel-plate intensified CCD detector operated in
90◦, reducingthe γ-rayEarthlimb contaminationby excluding photon-countingmode.Swift/UVOTobservedGB1310+487si-
regionswithin 20◦fromtheEarthlimb.Weusedthelatestver- multaneously with Swift/XRT. Various filters were used at dif-
∼
sion(BUILD-20)oftheCalibrationfiles(I0023),whichwillbe ferent epochs ranging from the U to M2 bands (as detailed
publiclyavailableattheASIScienceDataCentre(ASDC)site7, in Table 2), with the best coverage achieved in the U band.
andtheγ-raydiffuseemissionmodel(Giulianietal.2004).We Sincetheobjectisveryfaint,multiplesubexposurestakenduring
subsequentlyrantheAGILEMulti-SourceMaximumLikelihood eachobservationwerestackedtogetherwiththetooluvotimsum
Analysis(ALIKE)taskusingaradiusofanalysisof10◦ inorder from the HEASoft package. A custom-made script based on
to obtain the position and the flux of the source. A power-law uvotsourcewas employedforaperturephotometry(usingthe
spectrumwithaphotonindexΓ = 2.1wasassumedintheanal- standard5 aperturediameter)andcountratetomagnitudecon-
′′
ysis. versiontakingintoaccountthecoincidenceloss(pile-up)correc-
tion(Pooleetal. 2008;Breeveldetal. 2010). TheGalactic red-
deninginthe directionofthissourceis E(B V) = 0.013mag
2.3.X-rayobservationswithSwift/XRT −
(Schlegeletal.1998).UsingtheextinctionlawofCardellietal.
The X-ray Telescope (XRT; Burrowsetal. 2005) onboard the (1989) and coefficients presented by Romingetal. (2009), the
Swiftsatellite(Gehrelsetal.2004)providessimultaneousimag- following extinction values were obtained for the individual
ing and spectroscopic capability over the 0.2–10keV energy bands:A = 0.041,A = 0.053,A = 0.065,andA = 0.122
V B U M2
range.ThesourceGB1310+487wasobservedbySwiftatseven mag. Magnitude-to-flux-density conversion was performed us-
epochs during the two γ-ray activity periods and in June 2011 ingthecalibrationofPooleetal.(2008).
during the low post-flare state. A summary of the Swift ob- A star-like object is visible in Nordic Optical Telescope
servations is presented in Table 2. Swift/XRT was operated in (NOT) images about 3 southwest of the AGN. This object
′′
photon-counting (pc) mode during all observations. The low would be blended with the AGN in UVOT images which lack
count rate of the source allows us to neglect the pile-up effect adequate angular resolution. Contribution of this object to the
whichisofconcernfortheXRTinpcmodeifthecountrate8is total flux measured by UVOT is the likely reason for the dis-
0.6counts 1. crepancybetweenUVOTandNOTU-bandmeasurementsdur-
−
≥ The xrtpipelinetask fromthe HEASoft v6.14package ingthelowstate ofGB1310+487.Thenearbygalaxy(Sect.3)
was used for the data processing with the standard filtering alsocontributestothemeasuredUVOTflux.
criteria. To increase the number of counts for spectral analy- The Nordic Optical Telescope, a 2.5m instrument located
sis, the resulting event files were combined with xselect to on La Palma, Canary Islands, conducted photometric observa-
produce average X-ray spectra for the periods of Flare1 and tionsofGB1310+487withitsALFOSCcameraon2010July7
Flare2 defined in Table 1. The spectrum for the Flare1 pe- and2011May29duringthesecondflareandthepost-flarelow
riod was binned to contain at least 25 counts per bin to utilize state,respectively.TheVaST9software(Sokolovsky&Lebedev
the χ2 minimizationtechnique.Thecombinedspectrawerean- 2005) was applied for the basic reduction (bias removal, flat-
alyzed with XSPEC v12.8.1. The simple absorbed power-law fielding) and aperture photometryof the NOT images. A fixed
model with the H I column density fixed to the Galactic value aperture 1.5 in diameter was used for the measurements.
′′
N = 0.917 1020cm 2 (obtainedfromradio21cmmeasure- The source 3UC277-116569, which served as the compari-
HI −
×
mentsbyKalberlaetal.2005)providedastatisticallyacceptable son star for the Kanata observations (see below), was satu-
fit (reduced χ2 = 1.2 for 12 degrees of freedom) to the 0.3– ratedonNOTibandimagesandsocouldnotbeused.Instead,
10keVspectrum.LeavingN freetovaryresultsinthevalues SDSSJ131240.83+482842.9 (α = 13h12m40s.84, δ =
HI J2000 J2000
N = 0.4 0.5 1022cm 2 andΓ 1.3.However,this +48 2842.9, Abazajianetal. 2009; see Fig. 1) was used as
HI − phX ray ◦ ′ ′′
modeldoes−noti×mprovethefits.Thelo−wp∼hotoncountsprevent thecomparisonstar.ItsJohnson-Cousinsmagnitudeswerecom-
amoredetailedstudy. putedfromtheSDSSphotometryusingconversionformulasof
Individual observations obtained during the periods of Jordietal. (2006): U = 19.133 0.081, B = 19.303 0.013,
± ±
Flare1andFlare2werealsoanalyzedusingthesamefixed–N V =18.822 0.012,R=18.589 0.011,andI =18.177 0.018
HI ± ± ±
model,butnoevidenceofspectralvariabilitywithintheperiods mag.
was found; however, the low photon counts could easily hide Kanata, a 1.5m telescope at the Higashi-Hiroshima Ob-
servatory, observed GB1310+487 in the R and I bands with
7 http://agile.asdc.asi.it
8 http://www.swift.ac.uk/pileup.shtml 9 http://scan.sai.msu.ru/vast
Articlenumber,page4of18
K.V.Sokolovskyetal.:Twoactivestatesofthenarrow-linegamma-ray-loudAGNGB1310+487
Table2.SwiftobservationsofGB1310+487.
ObsID Date(UT) JD Exposure UVOT XRT 0.3–10keVunabs.flux Γ
phX ray
2455... (ks) (mag) (cts/s) (10 13ergcm 2s 1) −
− − −
Flare1period
001 2009-11-27 162.77 8.7 M2=21.1(2) 0.022(2)
16.7 3.3 0.88 0.18
002 2009-11-30 165.97 8.1 U =21.2(4),B>21.3,V >20.9 0.013(1) ± ±
Flare2period
003 2010-06-25 372.66 4.5 U >21.3,B>21.5,V >20.5 0.010(1)
004 2010-07-03 380.65 4.5 U =20.9(3),B>21.5,V >20.3 0.009(1)
6.6 1.5 1.15 0.23
005 2010-07-07 384.63 4.9 U =21.4(3) 0.009(1) ± ±
006 2010-07-11 388.68 4.6 U =21.3(3) 0.010(1)
post-flareperiod
007 2011-06-03 717.12 9.5 U =21.1(2) 0.006(1) 5.5 1.8 0.93 0.34
± ±
Columndesignation:Col.1,observationnumberintheSwiftarchiveomittingtheleading00031547;Cols.2,3,dateofobservation
givenby the GregorianandJulian Date;Col. 4, exposuretime in kiloseconds;Col. 5, Swift/UVOT photometry(here and later in
the texttheerrorin parenthesescorrespondsto the lastdecimalplaceof the valuebeforethe parentheses);Col. 6, Swift/XRT net
countrateincounts/sanditsuncertainty;Col.7,0.3–10keVunabsorbedfluxderivedfromfittingSwift/XRTdatawiththepower-
law model (datasets 1–2 and 3–6 are combined to increase the photon statistics); and Col. 8, X-ray spectral index (Γ ) as
phX ray
definedinSect.1. −
Imaging Multi-Object Spectrograph (DEIMOS) (Faberetal.
2003) on 2013 April 07 with 1 seeing. Two 600s inte-
′′
∼
grations were obtained using the 600 lines per mm (7500Å
blaze11) grating,providingcoveragein the 4450–9635Å range
with a 100Å gap between the two CCDs. With the 1.0 slit,
′′
∼
the spectra have an effective resolution of 3.0Å. Conditions
∼
weregood,butnotcompletelyphotometric;thefluxscalemight
be uncertain by roughly a factor of 2. Moreover, we obtained
2 180sg-andR-bandimagesoftheobjectwiththetwocam-
×
erasontheKeckILowResolutionImagingSpectrometer(LRIS)
(Okeetal.1995)onMay10under 1 seeing,asshowninFig-
′′
∼
ure 2. The DEIMOS slit on April 07 was placed on the bright
coreofthesource,attheparallacticangle(Filippenko1982)of
PA = 143 (measuredfromNtoE),andtheextendedwingsof
Fig. 1. Nordic Optical Telescope R-band image of the GB1310+487 ◦
the host were also included. The companion was 3 off the
region obtained on 2011 May 29. The exposure time was 300s. ∼ ′′
slit.
North is up and east is to the left. The AGN and comparison star
SDSSJ131240.83+482842.9usedintheNOTdataanalysisaremarked A second KeckII/DEIMOS spectrum was obtained on
withtheletters“Q”and“C,”respectively. June 10 with a different slit position and 0.7 seeing. It has
′′
∼
a higher signal-to-noiseratio than the first DEIMOS spectrum;
however,itwasaffectedbyacosmic-rayhitthatpreventedaccu-
the HOWPol instrument (Kawabataetal. 2008) in the nonpo- ratemeasurementofHγinthez=0.638system,andconditions
larimetric mode for eight nights during the first and second werenotphotometricwhenthestandardstarwasbeingobserved.
γ-ray flares. Relative point-spread function (PSF) photometry Thetwospectraarenormalizedtoepoch1(April07)usingthe
was conductedusing 3UC277-116569(αJ2000 = 13h12m54s.09, [OII]λ3727lineatz=0.500.Thecontinuumcannotbeusedto
δJ2000 = +48◦27′58′.′2, J2000; R = 16.109, I = 15.657 cross-calibratethetwospectrabecauseofthesignificantlyvari-
mag;Zachariasetal.2010)asthecomparisonstar.Theadopted able AGN flux contribution;the second-epochcontinuumlevel
GalacticextinctionvalueswereAR =0.035andAI =0.025mag appearstohavedroppedrelativetotheemissionlinesby 1/3.
(Schlegeletal. 1998). The calibration by Besselletal. (1998) Thetwospectrawereaveragedforfurtheranalysis. ∼
wasemployedforthemagnitude-to-fluxconversion.
The source GB1310+487was assigned a redshift of 0.501
2.5.Infraredphotometry
based on a 2007 March 21 1200s Hobby-Eberly Telescope
LowResolutionSpectrograph(HET/LRS)observation10,which
Observationsinthenear-IRwerecarriedoutwiththe2.1mtele-
showedstrong[OII]λ3727at5592Åandweakevidenceofhost scopeoftheGuillermoHaroObservatory,INAOE,Mexico.The
absorptionfeatures(Healeyetal. 2008; Shawetal. 2012).This telescopeisequippedwiththeCANICAcameratogetherwithJ,
spectrumhad insufficientsignal-to-noiseratio (S/N) to exclude H,andK filters.Wecarriedoutdifferentialphotometrybetween
s
weak broadlines, or to cleanlymeasurethe opticalcontinuum, the object of interest and other objectsin the 5 5 field. The
′ ′
leavingthenatureofthesourceuncertain. observations showed an increase of about one m×agnitude dur-
Thus, we reobserved the source with the Keck 10m tele- ing the Flare2 period with respect to the Flare1 and post-flare
scopes. Long-slit spectra were obtained with the KeckII DEep periods(resultsaresummarizedinTable3).Magnitudesarere-
10 Falcoetal.(1998)previouslyreportedz=0.313,butitwasindicated 11 The“blaze wavelength” isthe wavelength for which the grating is
asa“marginalmeasurement.” themostefficient.
Articlenumber,page5of18
A&Aproofs:manuscriptno.1310
Table3.Ground-basedphotometryofGB1310+487.
LRIS g LRIS R
0 0 Date JD(UTC) Filter mag Instrument
33. 33. 2455...
Flare1period
0 0 2009-11-28 164.33668 R 20.58(2) Kanata
30. 30. 2009-11-29 165.33620 R 20.86(9) Kanata
8: 8:
2 2 2009-12-05 171.31613 R 20.61(8) Kanata
8: 8:
4 4 2009-11-28 164.34442 I 19.41(2) Kanata
2009-11-29 165.35134 I 19.66(6) Kanata
2009-12-05 171.31613 I >18.91 Kanata
43.7 13:12:43.4 43.2 43.7 13:12:43.4 43.2
2009-12-13 179.33910 I 19.52(1) Kanata
2009-11-22 158.04390 H 15.87(6) OAGH
interflareperiod
2010-03-17 272.95922 H 15.97(5) OAGH
Flare2period
2010-07-07 385.44392 U 21.91(6) NOT
2010-07-07 385.47325 B 22.6(1) NOT
2010-07-07 385.43551 V 21.62(5) NOT
2010-06-03 351.03843 R >20.78 Kanata
2010-06-04 352.05203 R >20.52 Kanata
2010-06-05 353.02920 R >20.64 Kanata
2010-07-07 385.47773 R 20.85(2) NOT
2010-07-17 395.07192 R >19.90 Kanata
Fig.2. KeckI/LRISimagesofGB1310+487obtainedon2013May10.
2010-07-19 396.99981 R >20.48 Kanata
Top panel: the g- and R-band images (8 8 field of view; north is
′′ ′′
× 2010-06-03 351.04981 I 19.98(7) Kanata
up and east is to the left). Bottom panel: g- and R-band images after
subtraction of a scaled point source from the offset core. The boxes 2010-06-05 353.04057 I 19.80(2) Kanata
showthelocationoftheDEIMOSslitinthe2013April07observations. 2010-07-07 385.46087 I 19.79(2) NOT
Thecirclesindicatethelocationoftheradio-loudAGN.Theseresidual 2010-07-17 395.08708 I >19.27 Kanata
imagesrevealarelativelyundisturbedforegroundgalaxy. 2010-06-15 362.70970 J 16.4(1) OAGH
2010-06-16 363.75472 J 16.6(1) OAGH
2010-06-18 365.69752 J 16.3(1) OAGH
ferredtothe2MASS12surveypublishedphotometry.Thesource
2010-06-19 366.71342 J 15.7(1) OAGH
GB1310+487itselfwasnotdetectedinthe2MASSsurvey.The
2010-05-17 333.79362 H 14.59(1) OAGH
surveydetectionlimitisJ =15.8,H =15.1,andK =14.3mag
s 2010-05-20 336.83470 H 15.11(5) OAGH
(Skrutskieetal. 2006). These values may be considered upper
2010-06-15 362.70186 H 14.61(7) OAGH
limitsontheobjectbrightnessatthe2MASSobservationepoch
2010-06-16 363.74752 H 14.68(3) OAGH
ofJD2451248.8408(1999March11).
2010-06-19 366.69745 H 14.84(5) OAGH
The source GB1310+487 is listed in the Wide-field In-
2010-06-15 362.72145 K 13.8(1) OAGH
frared Survey Explorer (WISE; Wrightetal. 2010) catalogue s
2010-06-16 363.76233 K 13.78(8) OAGH
(Cutrietal. 2012) with the following magnitudes in the four s
2010-06-19 366.72800 K 13.7(1) OAGH
WISE bands: 3.4µm W1 = 12.302 0.024, 4.6µm W2 = s
± post-flareperiod
11.254 0.021, 12µm W3 = 8.596 0.021, and 22µm W4
± ± 2011-05-29 711.46544 V 21.56(4) NOT
=6.368 0.044.TheIRcolors(W1–W2=1.048 0.032,W2–
± ± 2011-05-29 711.46135 R 20.79(2) NOT
W3 = 2.658 0.030 mag) are at the blue edge of the area in
± 2011-05-29 711.46922 I 19.87(3) NOT
thecolor–colordiagramoccupiedbyblazarsandSeyfertgalax-
2011-07-31 773.69450 H 15.83(7) OAGH
ies(seeFig.12inWrightetal.2010,Fig.1inD’Abruscoetal.
Columndesignation:Cols.1,2,theGregorianandJulianDate
2012,andFig.1inMassaroetal.2011),indicatingthattheAGN
ofobservation,respectively;Col.3,filter;Col.4,magnitudeand
and not the host galaxy’s stars or warm dust is responsible for
itsuncertainty;andCol.5,telescopename.
most of the IR flux in these bands. WISE observations of this
areawereconductedon2010June3–8duringFlare2(Table1).
vationsareperformedwithadual-beam(each2.5 fullwidthat
′
2.6.Radioobservations half-maximumintensity,FWHM)Dicke-switchedsystemusing
cold sky in the off-source beam as the reference. Additionally,
As part of an ongoing blazar monitoring program, the Owens
the source is switched between beams to reduce atmospheric
Valley Radio Observatory (OVRO) 40m radio telescope has
variations.Theabsoluteflux-densityscaleiscalibratedusingob-
observed GB1310+487 at 15GHz regularly since the end of
servations of 3C286, adopting the flux density (3.44Jy) from
2007 (Richardsetal. 2011). This monitoring program studies
Baarsetal.(1977).Thisresultsina 5%absoluteflux-density-
over 1500 known and likely γ-ray-loud blazars, including all ∼
scaleuncertainty,whichisnotreflectedintheplottederrors.
CGRaBS(Healeyetal.2008)sourcesnorthofdeclination 20 .
− ◦ Multifrequency radio observations of GB1310+487 were
Theobjectsinthisprogramareobservedintotalintensitytwice
performedwith the 100m Effelsberg telescope operatedby the
per week. The minimum measurement uncertainty is 4mJy
MPIfR13. Observations were conducted on 2009 December 1
whilethetypicaluncertaintyis3%ofthemeasuredflux.Obser-
12 http://www.ipac.caltech.edu/2mass/ 13 Max-Planck-InstitutfürRadioastronomie
Articlenumber,page6of18
K.V.Sokolovskyetal.:Twoactivestatesofthenarrow-linegamma-ray-loudAGNGB1310+487
following the reported γ-ray flare, on 2010 June 28 during the
1e-09
second γ-ray active phase, and on 2011 June 5. Secondary fo- Pre-flare
Flare 1
cus heterodyne receivers operating at 2.64, 4.85, 8.35, 10.45, Inter-flare
and 14.60GHz were used. The observations were conducted Flare 2
with cross-scans (i.e., the telescope’s response was measured -1s) Post-flare
while slewing over the source position in azimuth and eleva- -2m 1e-10
tion). The measurements were corrected for (a) pointing off- g c
sets, (b) atmospheric opacity, and (c) elevation-dependentgain er
(seeFuhrmannetal.2008;Angelakisetal.2009).Themultifre- x (
u
quencyobservationswerecompletedwithin1hrforeachobserv- y fl
g 1e-11
ing session. The absolute flux-density calibration was done by er
n
observing standard calibrators such as, 3C48, 3C161, 3C286, E
3C295,andNGC7027(Baarsetal.1977;Ottetal.1994).
TheIRAM30mPicoVeletatelescopeobservationsat86.24
and142.33GHz tookplaceon2009December7.Theobserva- 1e-12
0.1 1 10
tionsanddata-reductionstrategyweresimilartothosewithEf-
Photon energy (GeV)
felsberg;adetaileddescriptionisgivenbyNestorasetal.(2014).
Both the Effelsberg 100m and IRAM 30m telescope observa- Fig. 3. Fermi/LAT γ-ray SEDs compared to the power-law models
tionswereconductedinthe frameworkofthe F-GAMMApro- (shown as lines in this logarithmic plot) derived from unbinned like-
gram(Fuhrmannetal.2007;Angelakisetal.2008,2010,2012; lihoodanalysis.Westressthatthepower-lawmodelsarenotfitstothe
Fuhrmannetal.2014). binned energy flux values; rather, these are two independent ways to
For comparison with the latest Effelsberg, IRAM, and representFermi/LATphotondata.ThetimeintervalsdefinedinTable1
OVRO results, we use data from the RATAN-600 576m arecolorlabeled.
ring radio telescope of the Special Astrophysical Observatory
(Russian Academy of Sciences); RATAN-600 observations of
sourcewiththelower-frequencycounterpart.SeeTables1,2,3,
GB1310+487 were performed in transit mode at the southern
andthediscussionbelowfordetails.
sector with the flat reflector quasi-simultaneously at 3.9, 7.7,
Theγ-rayspectraofGB1310+487atvariousactivitystates
11.1, and 21.7GHz in June 2003 within the framework of the
listed inTable 1 arepresentedin Figure3. The plottedspectral
spectralsurveyconductedbyKovalevetal.(1999b,2002).The
binssatisfythefollowingrequirements:TS > 50and/ormodel-
flux-density scale is set using calibrators listed by Baarsetal.
predicted number of source photons N > 8. The flux value at
(1977); Ottetal. (1994). Details on the RATAN-600 obser-
eachbinwascomputedbyfittingthemodelwithsourceposition
vations and data processing are discussed by Kovalevetal.
and power-lawphotonindex fixed to the values estimated over
(1999a).
theentireperiodinthe0.1–100GeVenergyrange.TheGalactic
The National Radio Astronomy Observatory’s Very Long
component parameters were fixed, as were all other nontarget
Baseline Array (VLBA, Napier 1994, 1995) is a system of ten
sourcecomponentsofthemodel.
25m radio telescopes dedicated to very long baseline interfer-
ometry (VLBI) observations for astrophysics, astrometry, and To test if the power law (PL) is an adequate approxima-
geodesy.AfterpublicationofthereportontheNovember2009 tion of the observed γ-ray spectrum, the PL fit (represented
γ-ray flare (Sokolovskyetal. 2009), GB1310+487 was added by a straight line if plotted on a logarithmic scale) was com-
totheMOJAVE14program(Listeretal.2009a).Threeepochsof pared to the fit with a log-parabola (LP) function defined as
VLBAobservationsat15GHzwereobtainedbetween2009and dN/dE = N0(E/E0)−(α+βln(E/E0)),whereNisthenumberofpho-
2010. tons with energy E, N0 is the normalization coefficient, E0 is
a reference energy, α is the spectral slope at energy E , and β
0
is the curvature parameter around the peak. For the combined
3. Results
33 month Fermi/LAT dataset, the fit with the assumption of a
3.1.γ-rayanalysis PL spectrum for the target source provides its detection with
the Test Statistic TS = 4415, while the LP spectrum leads
PL
The γ-ray counterpart of GB1310+487 was localized, inte- to TS = 4423. These values may be compared by defining,
LP
grating 33 months of Fermi/LAT monitoring data, to αJ2000 = following Nolanetal. (2012) and in analogy with the source
198◦.187, δJ2000 = 48◦.472, with a 68% uncertainty of 0◦.014 = detectionTS described in Sect. 2.1, the curvatureTest Statistic
50′′. This is a factor of five larger than the spacecraft align- TScurve 2(lnLLP lnLPL)=TSLP TSPL.Theobtainedvalueof
ment accuracy of 10′′ = 0◦.003 (Nolanetal. 2012). The γ-ray TScurve =≡8corresp−ondstoa2.8σd−ifference,whichislowerthan
position is only 0◦.005 = 18′′ away from the radio position the TScurve > 16 (4σ) thresholdappliedby Nolanetal. (2012).
of GB1310+487. Within the Fermi/LAT error circle no other We conclude that while there is a hint of spectral curvature, it
radio sources are seen with the VLA FIRST 1.4GHz survey cannotbeconsideredsignificant.
(Whiteetal.1997),whichprovidesthebestcombinationofsen-
The broken power-law (BPL) model was also tested, but it
sitivity and angular resolution for that region of the radio sky
did not providea statistically significantimprovementover the
todate.Therefore,thepositionalassociationoftheγ-raysource PLortheLPmodels(TS =4423,forthebest-fitbreakenergy
BPL
withtheradiosourceGB1310+487isfirmlyestablished.TheX-
E = 3GeV, the photon indexes Γ = 2.30 0.04, Γ =
b ph1 ph1
raybrighteningobservedduringthefirstand,toalesserextent, ±
0.05 0.02aboveandbelowthebreak,respectively).Therefore,
thesecondγ-rayflarestogetherwiththenear-IRbrighteningdur- ±
weadoptthesimplerPLmodelforthefollowinganalysis.
ingthesecondγ-rayflaresupporttheidentificationoftheγ-ray
The 33-monthγ-ray lightcurveof the source obtained with
14 Monitoring Of Jets in Active galactic nuclei with VLBA Experi- theseven-daybinningispresentedinFigure4.Two majorflar-
ments,http://www.physics.purdue.edu/astro/MOJAVE/ ing periods are clearly visible. The first, brighter flare peaked
Articlenumber,page7of18
A&Aproofs:manuscriptno.1310
around 2009 November 27 (JD2455163) with the weekly av- 2009 2009.5 2010 2010.5 2011
eraged flux of (1.4 0.1) 10−6photonscm−2s−1. The peak Fermi 5<TS<25
flisu(x1.a9vera0g.2e)d ov1e0r−6t±hpehottwoon-×sdcamy−i2nst−e1rv.aTlhceesnoteurrecdecoonntthinauteddattoe 2-1 s) 2 Fermi 2FAσeG rumIpLpiE Te 4rS -l>dim2a5yit
be obse±rved a×t a daily flux of 0.5 10 6photonscm 2s 1 -m
for another two weeks. The sec∼ond fla×re pe−aked around−201−0 h c 1.5
p
June 17 (JD2455365) at the seven-day integrated flux of -60
(0a0r.5o.5u×4n1d±0t−h06i.sp0h7do)atto×en.s1Tc0hm−e6−tp2whsoo−1toflwnasarescsmodb−es2emsr−vo1en.dstTfroahtreeabrdeoamuiltyatrhkflraeubexlywocefoekn∼s- (1100 GeV 1
1-
trasting flux evolution: the first is characterized by a fast rise 0.
and slower decay, while the second flare shows a gradual flux y flux 0.5
risefollowedbyasharpdecay.FollowingBurbidgeetal.(1974), a
d
Valtaojaetal.(1999),andGorshkovetal.(2008),wedefinethe 7-
0
flux-variability timescale as t ∆t/∆lnS, where ∆lnS is
var
the difference in logarithm of th≡e photon flux at two epochs 4700 5000 5300 5600
separatedby the time interval∆t. The observedflux-variability JD-2450000.0
timescale during the onset of Flare1, as estimated from the Fig.4.WeeklybinnedFermi/LATlightcurve.Bluefilledcirclesareval-
seven-day binned lightcurve (Fig. 4), is tvar 3 days. The ueswithTS>25,grayfilledcirclesarevalueswith5<TS<25,and
≈
timescaleoffluxdecayafterFlare2ist 5days. grayarrowsindicate2σupper limitsfortimebinswithnosignificant
var
The Fermi/LAT lightcurve constructed≈with the alternative detections(TS < 5).Afour-dayintegratedAGILEdatapointisadded
analysis method, the adaptive binning (with 25% flux uncer- asanopenboxforcomparison.
tainty at each bin), is presented in Figure 5. It confirms all the
featuresvisibleintheconstantbin-widthlightcurve,butalsoal-
2009 2009.5 2010 2010.5 2011
lows us to investigate fast variability during high-flux states in
greater detail. The first flare episode, Flare1, consists of four 2.5
2.0
prominent subflares, each with a time width of a day or less. 1)
The subflares show no obvious asymmetry and the variability 2- s 2.0
timescale t for the three point rise of the second and third -m
subflares (avtarJD2455157.5 and 2455161.5) was estimated as ph c 1.5 1.5
0.36±0.20 days and 0.45±0.23 days, respectively. A second -610 1.0
adaptively binned lightcurve was produced in the reverse-time (V 1.0
direction,whichgivesasimilar,butnotidentical,timebinning. 0 Ge 0.5
The result of the timescale estimates forthis second versionof 20
3-
the lightcurve was found to be consistent with the first analy- 28 0.5 0.0
0.
sis.Asimilaranalysisforthelightcurveswith15%uncertainties ux 5152 5159 5166
givetimescaleestimatesofabout1dayforthemostrapidvari- Fl
ability.Weconcludethattheadaptivelybinnedlightcurvesshow 0.0
evidenceofavariabilitytimescaleofhalfadaywithaconserva- 4700 5000 5300 5600
tiveupperlimitof1day.Forthesecondandfainterflareepoch JD-2450000.0
the timescales seen in the adaptive binning are consistent with
Fig. 5. Fermi/LAT lightcurve constructed with the adaptive binning
theestimatefromthefixed-binnedlightcurvedescribedabove.
method(Lottetal.2012).ThemagnifiedplotofFlare1isshowninthe
Table1presentsspectralanalysisresultsforthedifferentγ-
insert.Theenergyrange for thislightcurve ischosen tominimizethe
rayactivitystatesofthesource:“pre-flare”and“post-flare”peri-
uncertaintiesintimeandflux,whilethelightcurvewasFig.4isgiven
odsrepresentthelow-activitylevel,“Flare1”and“Flare2”rep- inthecommonlyusedE>100MeVenergyrange.
resentthehigh-activitystate,whileduringthe“interflare”inter-
valthesourceshowedanintermediateγ-rayfluxlevel.Figure3
presents the observedFermi/LAT spectrumduring these states.
12:00 UT, JD2454587.0–2454597.0) we obtained a 2σ upper
Significantevolutionoftheγ-rayphotonindex,Γ ,isdetected
ph limitofF 0.31 10 6photonscm 2s 1.
betweenthedifferentfluxstates(Table1).Figure6presentsΓph E>100MeV ≤ × − − −
asafunctionof(E > 100MeV)flux.Theharder-when-brighter
trendisclearlyvisible.
3.2.X-raytoinfraredspectrum
Integrating the AGILE observations from 2009 Novem-
ber 18 12:00 (JD2455154.0) to 2009 November 22 12:00 UT Results of the X-rayspectral analysis are presented in Table 2.
(JD2455158.0), we obtain a γ-ray flux FE>100MeV = (1.65 ± The obtained values of the X-ray photon index ΓphX ray are
0.48) 10 6photonscm 2s 1, ata significance of √TS = 6.1. among the hardest reported for blazars (Giommietal−. 2002;
− − −
×
This result is in good agreement with the flux value derived Donatoetal.2005;Sikoraetal.2009).Radio-loudNLSy1have
from the preliminary analysis by Bulgarellietal. (2009). Prior Γ similartotheonesfoundinblazars(Paliyaetal.2013;
phX ray
totheFermilaunch,AGILEobservedGB1310+487(inpointing Abdo−etal. 2009d). However,itcannotbe excludedthatthe X-
mode) during two other periods, but did not detect the source. ray spectrum with an intrinsic value of Γ is artificially
phX ray
Duringthefirstperiod(from2007October2412:00UTto2007 hardenedbyadditionalabsorbingmaterialalon−gthelineofsight
November112:00UT,JD2454398.0–2454406.0),the2σupper (seethediscussionofNOTimagingresultsbelow).Futurehigh-
limitwasF MeV 0.28 10 6photonscm 2s 1,whilein quality X-ray observations are necessary for investigating this
E>100 − − −
≤ ×
thesecondperiod(from2008April3012:00UTto2008May10 possibility.
Articlenumber,page8of18
K.V.Sokolovskyetal.:Twoactivestatesofthenarrow-linegamma-ray-loudAGNGB1310+487
2.6
2.5
2.4 Pre-flare
Post-flare
2.3
h
p
Γ
2.2
Inter-Flare Flare 2
2.1
2
Flare 1
1.9
0.1 1 10
Photon flux (E>100 MeV) 10-7 cm-2 s-1
Fig.6.Theγ-rayphotonindex,Γ ,asafunctionoffluxforthetime
ph
periodsdefinedinTable1.
The UV and blue parts of the optical spectrum are flat, in
Fig.7. KeckII/DEIMOSspectrumofGB1310+487.Thetwonarrow-
contrasttothesteepspectrumseeninthenear-IR(iJHKbands).
line systems are indicated. The region 6900–7000Å is lost to a gap
Also, the observed variability amplitude is decreasing toward
betweenthetwoCCDs.TheslitpositionisshowninFigure2.
bluerwavelengths,withtheU-bandbrightnessbeingessentially
constant.
Table4.GB1310+487emission-linestrengths.
TheH-bandfluxshowedanincreaseofaboutonemagnitude
during the Flare2 period with respect to the Flare1 and post- Species λRest Fluxa0.500 EWb0.500 Fluxa0.638 EWb0.638
flareperiods,incontrasttothebehaviorseeninotherbands.The [OII] 3726+37294.24 0.07 32 6 0.97 0.06 4.2 0.5
± ± ± ±
resultsofground-basedphotometricmeasurementsaresumma- Hγ 4340 0.65 0.053.1 0.7 cosmic-rayhit
± ±
rizedinTable3. Hβ 4861 1.69 0.066.6 0.8 0.24 0.06 4.1 0.7
± ± ± ±
[OIII] 4959 notdetected 1.00 0.06 3.6 0.8
± ±
[OIII] 5007 1.11 0.056.5 0.6 2.61 0.0512.0 2.2
± ± ± ±
3.3.ImagingwithNOT a Fluxes are in units of 10 17ergcm 2s 1, with 1σ statistical
− − −
errors.Theoverallfluxscaleisuncertainbyuptoafactorof2,
The Nordic Optical Telescope images (Fig. 1) show a fuzzy buttherelativefluxesaremuchmoreaccurate.b Theequivalent
extended object, probably a galaxy, with a point source offset width(EW)inangstroms.
0.6 from its center. Considering that there are many galaxies
′′
of comparable brightness visible in the field, this picture may
be interpreted as the AGN (correspondingto the point source) 3.4.Keckimagingandspectroscopy
shining through an unrelated foreground galaxy. This may be
Standard reductions, extractions, and calibrations of the
the source of confusion in the AGN’s redshift determination
DEIMOSdataproducedthespectrumshowninFigure7;itisthe
(Sokolovskyetal. 2009; Healeyetal. 2008; Falcoetal. 1998),
averageoftwoobservationsconductedonApril07andJune10,
anditalsoexplainsthesteepnessoftheoptical-IRSED(theob-
2013. The strongest line, [O II] λ3727, confirms the HET red-
served SED was corrected for Milky Way absorption, but ab-
shift identification at z = 0.500, and we also see [O III] and
sorption in the interveninggalaxy may also be significant). On
narrowBalmeremissionforthissystem.However,therearead-
the other hand, it is not uncommon for AGN host galaxies to
ditional lines, mostly in the red half. These representa second
havedisturbedmorphologies,makingitappearthattheAGNis
systemwithnarrowforbiddenandBalmeremission,thistimeat
off-center.
z=0.638.ThelinestrengthsaregiveninTable4.The[OII]dou-
The galaxy contributes a large fraction of the total optical bletsarebarelyresolved,buttheoxygenandBalmerlinewidths
flux,whenthepointsourceisinthelowstate.Ifthehostgalaxy areconsistentwiththeinstrumentalresolution.The[OIII]emis-
of GB1310+487 is similar to the giant ellipticals studied by sion at z = 0.638 appears resolved with a deconvolved width
Satbzar=ufa0t.t5i0e0taslh.o(u2l0d0b5e),RhMRi20=(o−r202..69m±a0g.5faminatge,riatstzm=ag0n.i6tu3d8e; doofe∼s n2o0t0iknmclusd−e1.thUenfHoαrt/u[NnatIeIl]yl,inthees rfeodr eliimthietrosfysttheems.pFeocrtruthme
Sect. 3.4). A typical NL≈Sy1 from the Véron-Cetty&Véron z = 0.500system, we cover[O I]λ6300,whichis weakor ab-
(2010)cataloguehaving M = 21.4magwouldappear1mag sent.Forthez = 0.638system,wecoverMgIIλ2800,andcan
V
h i −
fainter than a giant elliptical in the R band assuming V R = place a 3σrest equivalentwidth limit of 1.0Å on any broad
− ∼
0.5mag (Xanthopoulos 1996). Therefore, the observed galaxy emission; the Hβ line is marginally detected for the z = 0.638
could be the host of GB1310+487.The visible offset between system at 4σ level. The ratio of Hβ to [O III] (z = 0.638) is
the point source and the center of extended emission could re- small,evenifthe Hβfluxistreatedasan upperlimit.Together
sultfromthedisturbedmorphologyofthehost,asnotedabove. withtheresolved[OIII]thisindicatesnuclearexcitation.
Articlenumber,page9of18
A&Aproofs:manuscriptno.1310
Thus, we clearly have two superimposedsystems and wish Table5.MultifrequencyradioobservationsofGB1310+487.
toidentifywhichsystemhoststheradio-loudcore(and,byinfer-
ence,theγ-raysource).TheKeckILRISimagesconfirmtheba- ν Fν σ ν Fν σ
sicstructureseenintheNOTimages;thesourceisextendedwith (GHz) (Jy) (Jy) (GHz) (Jy) (Jy)
a brightercoredisplaced 0.6to the west. Figure2shows8 RATAN-600 2003-06 Effelsberg100m 2010-06-28
′′ ′′
regionsaroundtheAGN.T∼heDEIMOSslitpositiononApril07 21.74 0.401 0.104 2.64 0.135 0.002
ismarkedonthegframes(left).Atthebottomweshowtheim- 11.11 0.222 0.012 4.85 0.107 0.001
agesafterremovalofapoint-sourcePSF(g = 23.89,R = 22.45 7.69 0.157 0.019 8.35 0.111 0.002
mag)fromtheoffsetcore.Theresidualsshowarelativelyregu- 3.95 0.221 0.057 10.45 0.112 0.005
largalaxyhavingFWHM=1.8,withg = 21.95andR = 20.59 Effelsberg100m 2009-12-01 Effelsberg100m 2011-06-05
′′
mag. The coordinatesystem was referencedthroughthe SDSS 2.64 0.161 0.001 2.64 0.201 0.006
imageof the field, with an estimateduncertaintyrelative to the 4.85 0.133 0.001 4.85 0.189 0.003
radio frame of 0.2; the circles show the position of the VLBI 8.35 0.130 0.002 8.35 0.207 0.005
′′
source(Sect.1)andhaveradiitwicethisuncertainty.Hence,the 10.45 0.130 0.003 10.45 0.213 0.010
radio source is coincidentwith the point-like peak of the com- 14.60 0.121 0.006
binedsource.Wealsofindthatthez = 0.500emissionlinesare IRAM30m 2009-12-07
offset0.21 0.06SEalongtheslitfromthez = 0.638system, 86.24 0.282 0.070
′′ ′′
towardthec±ontinuumtailrepresentingtheextendedgalaxy.The 142.33 0.206 0.065
deprojectedoffset is 0.35 E of the AGN core. We thus con-
′′ Column designation: Col. 1, the central observing frequency;
∼
cludethatthetrueAGNredshiftisz = 0.638,andweareview-
andCols.2,3,theobservedfluxdensityanditsuncertainty,re-
ingit throughan approximatelyface-ongalaxyshowingstrong
spectively.
narrow-lineemission.
Our extracted spectrum is weighted toward the AGN core,
0.35
althoughit also containsappreciablelight from the foreground OVRO
galaxy. Both spectra are dominated by narrow forbiddenlines, Effelsberg
VLBA core
yetthereisappreciablecontinuumassociatedwithbothcompo- 0.30
nents as well. The foregroundgalaxy is probablynot an AGN,
but we cannot be certain; without the [N II]/Hα line ratio, we 0.25
are unable to fully distinguish “LINER” (Low Ionization Nu-
clearEmission-lineRegion)emissionfromanHIIregion(e.g., y)
Hoetal.1997).However,thestrong[OII]λ3727andlackofob- (Jν 0.20
F
vious[OI]λ6300argueagainstapower-lawionizingspectrum,
suggestingthatthez=0.500emissionrepresentsstarformation 0.15
intheforegroundgalaxylackingAGNactivity.
Thepositionalaccuracyoftheavailableobservationsofmul- 0.10
tiwavelengthvariability(Sect. 2) is notsufficientto distinguish
betweentheforegroundandbackgroundobjectsdiscussedhere 0.05
andinSect.3.3asthesourceofhigh-energyemission.Thepro- 4400 4600 4800 5000 5200 5400 5600 5800 6000 6200
posedinterpretationthatthebackgroundAGNisthehigh-energy JD - 2450000.0
source rests on the consideration that the observed fast γ-ray
Fig. 8. Radio lightcurve at 15GHz obtained with the OVRO 40m
variability(Sect.3.1) istypicalof radio-loudAGNs(whichthe
telescope(points)supplementedwithtwo14.6GHzmeasurementsob-
background source is), while there are no firm indications of tained with the Effelsberg 100m telescope (square). VLBA measure-
AGNactivityintheforegroundgalaxy. mentsofthecore(componentC0,Table6)areindicatedasdiamonds.
Thetwoarrowsmarkthepeaksoftheγ-rayflaresobservedbyFermi.
3.5.Resultsofradioobservations
TheSWcomponentincreaseditsbrightnessduringthethree
The radio spectrum of GB1310+487 is generally flat, with a epochs.Comparisonwith the lightcurvein Figure 8 showsthat
wide peak locatedbetween 22GHz and 86GHz (Table 5). The thiscomponentisresponsibleformostofthefluxobservedwith
variabilityamplitudeat2.64GHzisslightlylowercomparedto single-dish instruments. The fainter component located to the
higherfrequencies.The15GHzlightcurveofGB1310+487ob- NEisgraduallyfading.IftheSWcomponentisthe15GHzcore,
tained with the OVRO 40m telescope and complemented by thepositionofthesecondcomponentalignsnicelywiththeori-
measurementswith the Effelsberg100m andthe VLBA ispre- entation of the kiloparsec-scale jet observed with the VLA at
sented in Figure 8. It shows a period of high activity with two 1.4GHz byMachalski&Condon(1983).Nosignificantproper
separatepeaksthatstartedinmid-2010andisstillongoing. motion could be detected between the three 15GHz MOJAVE
The 15GHz VLBA images (Fig. 9) show two emission re- epochs.The3σupperlimitwhichcanbeplacedonpropermo-
gions separated by 0.4mas. To quantifytheir parameterswe tion is µ < 0.3masyr 1, correspondingto β < 11 (β is in
− app app
∼
fittheobservedvisibilitieswithamodelconsistingoftwocircu- unitsofthespeedoflight)atthesourceredshift,whichiswithin
larGaussiancomponentsusingtheDifmapsoftware(Shepherd therangeofapparentjetspeedsoccupiedbyγ-ray-brightblazars
1997).ThemodelingresultsarepresentedinTable6.Theuncer- (Listeretal.2009b;Savolainenetal.2010).Theprojectedlinear
taintiesinparametersofthemodelcomponentswereestimated sizeofthedoublestructureresolvedwiththeVLBAis 2.7pc
followingLeeetal.(2008),andtheresolutionlimitachievedfor = 8 1018cm. The overall 15GHz VLBI polarization∼of the
×
each componentwas computed following Lobanov(2005) and sourcemeasuredbyMOJAVEis2.8–6.5%whichisindicativeof
Kovalevetal.(2005). beamedblazaremission.Theweaklybeamed,highviewingan-
Articlenumber,page10of18