Discovery of VHE gamma-ray emission from the BL Lac object B3 2247+381 with the MAGIC telescopes

0.15 MB·English

by  The MAGIC Collaboration

Astronomy&Astrophysicsmanuscriptno.b32247 (cid:13)c ESO2012
March3,2012
Discovery of VHE γ-ray emission from the BL Lac object
B3 2247+381 with the MAGIC telescopes
J.Aleksic´1,E.A.Alvarez2,L.A.Antonelli3,P.Antoranz4,M.Asensio2,M.Backes5,J.A.Barrio2,D.Bastieri6,
J.BecerraGonza´lez7,8,W.Bednarek9,A.Berdyugin10,K.Berger7,8∗,E.Bernardini11,A.Biland12,O.Blanch1,
R.K.Bock13,A.Boller12,G.Bonnoli3,D.BorlaTridon13,I.Braun12,T.Bretz14,26,A.Can˜ellas15,E.Carmona13,
A.Carosi3,P.Colin13,E.Colombo7,J.L.Contreras2,J.Cortina1,L.Cossio16,S.Covino3,F.Dazzi16,27,A.De
Angelis16,G.DeCaneva11,E.DeCeadelPozo17,B.DeLotto16,C.DelgadoMendez7,28,A.DiagoOrtega7,8,
M.Doert5,A.Dom´ınguez18,D.DominisPrester19,D.Dorner12,M.Doro20,D.Elsaesser14,D.Ferenc19,
2 M.V.Fonseca2,L.Font20,C.Fruck13,R.J.Garc´ıaLo´pez7,8,M.Garczarczyk7,D.Garrido20,G.Giavitto1∗,
1 N.Godinovic´19,D.Hadasch17,D.Ha¨fner13,A.Herrero7,8,D.Hildebrand12,D.Ho¨hne-Mo¨nch14,J.Hose13,
0 D.Hrupec19,B.Huber12,T.Jogler13,H.Kellermann13,S.Klepser1,T.Kra¨henbu¨hl12,J.Krause13,A.LaBarbera3,
2
D.Lelas19,E.Leonardo4,E.Lindfors10,S.Lombardi6,M.Lo´pez2,A.Lo´pez-Oramas1,E.Lorenz12,13,M.Makariev21,

n G.Maneva21,N.Mankuzhiyil16,K.Mannheim14,L.Maraschi3,M.Mariotti6,M.Mart´ınez1,D.Mazin1,13,
a
J M.Meucci4,J.M.Miranda4,R.Mirzoyan13,H.Miyamoto13,J.Moldo´n15,A.Moralejo1,P.Munar-Adrover15,
2 D.Nieto2,K.Nilsson10,29,R.Orito13,I.Oya2,D.Paneque13,R.Paoletti4,S.Pardo2,J.M.Paredes15,S.Partini4,
1 M.Pasanen10,F.Pauss12,M.A.Perez-Torres1,M.Persic16,22,L.Peruzzo6,M.Pilia23,J.Pochon7,F.Prada18,
P.G.PradaMoroni24,E.Prandini6,I.Puljak19,I.Reichardt1,R.Reinthal10,W.Rhode5,M.Ribo´15,J.Rico25,1,

E] S.Ru¨gamer14,A.Saggion6,K.Saito13,T.Y.Saito13,M.Salvati3,K.Satalecka2,V.Scalzotto6,V.Scapin2,C.Schultz6,
H T.Schweizer13,M.Shayduk13,S.N.Shore24,A.Sillanpa¨a¨10,J.Sitarek9,I.Snidaric19,D.Sobczynska9,F.Spanier14,
S.Spiro3,V.Stamatescu1,A.Stamerra4,B.Steinke13,J.Storz14,N.Strah5,T.Suric´19,L.Takalo10∗,H.Takami13,
.
h F.Tavecchio3,P.Temnikov21,T.Terzic´19,D.Tescaro24,M.Teshima13,O.Tibolla14,D.F.Torres25,17,A.Treves23,
p M.Uellenbeck5,H.Vankov21,P.Vogler12,R.M.Wagner13,Q.Weitzel12,V.Zabalza15,F.Zandanel18,R.Zanin1
-
o V.Kadenius10,M.Weidinger14,andS.Buson30,31
r
t (Affiliationscanbefoundafterthereferences)
s
a
[ Preprintonlineversion:March3,2012

1
v ABSTRACT
4
3 Aims.Westudythenon-thermaljetemissionoftheBLLacobjectB32247+381duringahighopticalstate.
6 Methods.The MAGIC telescopes observed the source during 13 nights between September 30th and October 30th 2010, collecting atotal of
2 14.2hoursofgoodqualityveryhighenergy(VHE)γ-raydata.SimultaneousmultiwavelengthdatawasobtainedwithX-rayobservationsbythe
. SwiftsatelliteandopticalR-bandobservationsattheKVA-telescope.Wealsousehighenergyγ-ray(HE,0.1GeV–100GeV)datafromtheFermi
1
satellite.
0
Results.TheBLLacobjectB32247+381(z=0.119)wasdetected,forthefirsttime,atVHEγ-raysatastatisticalsignificanceof5.6σ.Asoft
2
VHEspectrumwithaphotonindexof-3.2±0.6wasdetermined. Nosignificantshorttermfluxvariationswerefound.Wemodelthespectral
1
energydistributionusingaone-zoneSSC-model,whichcansuccessfullydescribeourdata.
:
v
Keywords.BLLacobjects:individual(B32247+381)–galaxies:active–gammarays
i
X
r
a
1. Introduction al.2007a),BLLacertae(partofthedatatakenasToOduetoop-
ticalhighstate,Albertetal.2007b),3C279(Albertetal.2008a,
The numberof knownextragalacticvery high energy(VHE, E Aleksic´ etal.2011a),MAGICJ0223+430(Aliuetal.2009),S5
> 100GeV) γ-ray emitters has increased from 6 to almost 50 0716+714 (Anderhub et al. 2009), B3 2247+381 (Mariotti et
(July2011)duringthepastfiveyears1.Mostoftheseobjectsare al.2010,thispaper)andmostrecently1ES1215+303(Mariotti
active galactic nuclei (AGN), especially blazars (flat spectrum et al. 2011). This suggests that high optical states are an in-
radio quasars and BL Lac objects), which are known for their dication of a higher state also in VHE γ-ray band, at least in
largevariabilityacrosstheelectromagneticspectrumfromradio somesources.AdditionallyforPG1553+113alongtermstudy
to VHE γ-rays.Eightofthese new discoverieswere madedur- (Aleksic´, J., Alvarez, E.A., Antonelli,L.A., Antoranz,P. et al.,
ing high optical states of these sources, reported by the Tuorla ApJ, in prep.) suggests a correlation between the optical and
blazarmonitoringprogram,whichhavetriggeredMAGICobser- VHEγ-rayflux,whileforPKS2155-304itseemsthat,atleast,
vations:Mrk180(Albertetal.2006),1ES1011+496(Albertet insomecasesthetwowavebandsarecorrelated(Foschinietal.
2007;Aharonianetal.2009).
1 http://www.mpp.de/rwagner/sources/
1
J.Aleksic´etal.:B32247+381
TheobjectB32247+381wasselectedfromthesamplepre- Inorderto reconstructthe showerarrivaldirectionwe used
sentedinNieppolaetal.(2006)witha reportedX-rayfluxF(> therandomforestregressionmethod(RFDISPmethod,Aleksic
1keV)> 2mJy.Donatoetal.(2001)classifyitashigh-energy- etal.2010)whichwasextendedusingstereoscopicinformation
peakedBLLacobjectandthereportedX-rayfluxofthesource suchas theheightofthe showermaximumandthe impactdis-
is F(>1keV)= 0.6mJy2. B3 2247+381(z = 0.119, Falco et al. tance of the shower on the ground.We estimated the RF DISP
1988)hasbeenpreviouslyobservedbytheMAGICtelescopeas foreachtelescopeseparatelyandobtainedtwopossiblesolutions
partof the systematic search of VHE γ-raysfrom X-raybright alongthemajoraxisoftheshowerimageineachtelescope,re-
BL Lac objects (Aleksic´ et al. 2011b)3. MAGIC observed the spectively.Finally,we searchedfor the combinationof two so-
sourcebetweenAugustandSeptember2006,whichresultedin lutions(onefromeachtelescope)thathavetheshortestsquared
an upper limit F(>140GeV) < 1.6 · 10−11cm−2s−1. The source angulardistancebetweenthem.Ifthissquaredangulardistance
hasbeenmonitoredintheR-bandbytheTuorlablazarmonitor- is greater than 0.05 degree2 the event is removed from further
ingprogrameversince. analysis. This improvesthe backgroundrejection since hadron
B3 2247+381 was also included in the list of potentially inducedshowershavealargererroronthereconstructionofthe
interesting TeV sources released to the Imaging Atmospheric arrival direction. The final arrival direction is the mean of the
Cherenkov Telescope experiments by the Fermi-LAT collabo- twosolutions(weightedbythenumberofpixelsineachshower
ration on 27th October 2009 (Fermi-LAT Collaboration 2009, image).
priv.comm.).B32247+381islistedinthefirstFermi-LATcat- Forthegamma-hadronseparationtherandomforestmethod
alogofAGNs(Abdoetal.2010)as1FGLJ2250.1+3825witha is used (Albert et al. 2008b). In the stereoscopic analysis im-
veryhardspectrum(spectralindexof-1.6±0.1).Inthesecond ageparametersofbothtelescopesareusedaswellastheshower
Fermi-LAT catalog this object is listed as 2FGL J2250+3825 impact point and the shower height maximum. A detailed de-
withaspectralindexof-1.84±0.11(Ackerman,M.,Ajello,M., scription of the stereoscopic MAGIC analysis can be found in
Allafort, A., Antolini, E. et al., ApJ, in prep.). Neronov et al. (Aleksic J., Alvarez, E.A., Antonelli, L.A., Antoranz, P. et al.,
(2010) found a hint of signal at 2.73σ in the Fermi-LAT data Astr.ParticlePhys.inprep.).
above100GeVovertheperiodAugust2008-April2010.
In this paper we present the first detection of VHE γ-ray
2.2.Opticalobservationsanddataanalysis
emission triggered by the optical high state of the source and
thefirstopticallightcurveofB32247+381.Wealsopresentsi- B3 2247+381 has been observed by the Tuorla group4, using
multaneous X-ray data obtained by the Swift and High energy the 35cm KVA telescope, located on La Palma, since 2006
(HE)dataobtainedbyFermi-LATsatellites. (see Aleksic´ et al. 2011a for a description of the telescope).
Observationshavebeenmadein theR-band.Thebrightnessof
theobjectwasmeasuredbasedonstarsinthesameCCD-frames
2. Observationsanddataanalysis
asB32247+381.ThesestarswerecalibratedbytheTuorlagroup
2.1.MAGICobservationsanddataanalysis during the observing seasons. R-band magnitudes were con-
vertedtofluxes,using:F(Jy)=3080·10−mR/2.5.Thefluxeswere
TheVHEγ-rayobservationswerecarriedoutwiththeMAGIC
corrected for galactic absorption by R=0.398mag (Schlegel et
telescopes located on the Canary Island of La Palma (28.8◦ N,
al.1998).Duringtheyears2006-2009B32247+381wasaquite
17.8◦Wat2200m.a.s.l).Thetwo17mtelescopesusetheatmo-
faint and steady source at R ∼1.8mJy, but during late summer
spheric Cherenkov imaging technique and allow for measure-
2010 it went to a high optical state, reaching an average flux
mentsatathresholdaslowas50GeV.
levelof2.4mJy.Thesourcealsostayedatthislevelthroughout
B32247+381wasobservedwiththeMAGICtelescopesdur-
the observingseason (see Figure 3). In late Septemberan alert
ing 13 nights between September 30th and October 30th 2010
was givento MAGICand it started observationson September
collectingatotalof21.2hoursofdata,ofwhich5.3hourswere
30th. An alert is issued when the optical flux has increased by
discardedbasedontheeventrate.Theeffectivetimeofthisob-
50%fromthelongtermrunningaverage.
servation,correctingforthedeadtimeofthetriggerandreadout
systemsis14.2hours.Partofthedatawastakenundermoderate
moonlightconditions. 2.3.Swiftobservationsanddataanalysis
Allthedataweretakeninthefalse-sourcetracking(wobble)
The prime objective of the Swift Gamma-Ray Burst observa-
mode(Fominetal.1994),inwhichthetelescopewasalternated
tory,launchedin November2004(Gehrels et al. 2004),was to
every20minutesbetweentwoskypositionsat0.4◦ offsetfrom
detect and follow up on Gamma-Ray bursts, but it has turned
thesource.Thezenithanglewasbetween8◦and35◦.
into a multi-purpose observatory due to its fast slewing and
The data was analysed using the standard MAGIC analy-
response capacity and its multi-wavelength coverage. Swift is
sis framework ”MARS” as described in Moralejo et al. (2009)
equippedwiththreetelescopes,theBurstAlertTelescope(BAT;
with additional adaptations incorporating the stereoscopic ob-
Barthelmy2005),whichcoversthe15-150keVrange,theX-ray
servations. The images were cleaned using timing information
telescope (XRT; Burrows et al. 2005) covering the 0.3-10keV
as describedin Aliu et al. (2009)with absolute cleaning levels
energyband,andtheUV/OpticalTelescope(UVOT;Rominget
of6phe(so-called”corepixels”)and3phe(”boundarypixels”)
al.2005)coveringthe1800-6000Åwavelengthrange.
forthefirsttelescopeand9pheand4.5pherespectivelyforthe
SwiftTargetofOpportunityobservationswererequestedand
secondtelescope.Theimageswere parameterizedin each tele-
fromOctober,5to16,2010.Swift/XRTobservedthesourcefor
scopeseparatelyfollowingtheprescriptionofHillas(1985).
∼5kseverynight,inPhotoncounting(PC)mode.Wealsoanal-
2 Note,thatVeron-Cetty&Veron(2006)listthesourceasaBLLac ysedSwiftarchivaldatafromAugust10th,2009,February18th,
candidateandthattheclassificationhasnotbeenconfirmed. 2010andApril18th,2010in orderto comparethe levelof the
3 ThestackeddatasetofallX-rayselectedblazarsresultedinasig- X-rayemissiontopreviousobservations.
nificantγ-rayexcess,whichhintstowardsthefactthatthesourceswere
emittingataverylowlevel. 4 http://users.utu.fi/kani/
2
J.Aleksic´etal.:B32247+381
Thedatawereprocessedwithstandardproceduresusingthe θ2 distribution
FTOOLS task XRTPIPELINE (version 0.12.6) distributed by nts 350
HEASARC within the HEASoft package (v.6.10).Events with Neve 300 TNiomn e= =4 8184;. 2N3o ffh = 329.0 ± 18.1
grades 0-12 were selected for the PC data (see Burrows et al. Nex = 159.0
2005)andtheresponsematricesavailablein theSwift CALDB 250 Significance (Li&Ma) = 5.58σ
(v.20100802)wereused.Forthespectralanalysis,weextracted
thePCsourceeventsinthe0.3-10keVrangewithinacirclewith 200
a radius of 20 pixels (∼ 47 arcsec). The background was ex-
150
tractedfromanoff-sourcecircularregionof40-pixelsradius.
The spectra were extracted from the corresponding event
100
files and binned using GRPPHA to ensure a minimum of
30 counts per bin in a manner so that the χ2 statistic could 50
reliably be used. Spectral analyses were performed using
XSPEC version 12.6.0. We adopted both a simple power law 0
0 0.1 0.2 0.3 0.4
model and a log-parabolic model as in Massaro et al. (2004) θ2 [degrees2]
with an absorption hydrogen-equivalent column density fixed
to the Galactic value in the direction of the source, namely Fig.1. Distribution of the squared angulardistance (θ2) for the
1.2×1021cm−2.Thetwomodelsprovidesimilarresultsinterms on-sourcecountsinthedirectionofB32247+381(blackpoints
ofgoodnessoffitabove∼ 0.7keV.Howeverbelowthisenergy with errorbars)and the normalizedoff-sourceevents(gray his-
thedifferencesareingeneralnegligibleduetolowstatistics. togram).
Swift/UVOT observed the source in the ”filter of the day”
mode,thatisadifferentfilterwasusedfordifferentobservations. included in the model of the region. The initial parameters in
This doesnotallow to comparethe UV fluxesamongdifferent theXMLfileweresettothoseofthe2FGLcatalog,leavingthe
days.UVOTsourcecountswereextractedfromacircularregion normalizationparametersfreeinthefittingprocedure.Thesys-
5 arcsec-sizedcentered on the source position, while the back- tematicuncertaintyinthefluxisestimatedas10%at100MeV,
ground was extracted from a larger circular nearby sourcefree 5%at560MeVand20%at10GeVandabove6.
region.These data were processedwith the uvotmaghisttask FortheperiodoftheMAGICobservations(30daysbetween
oftheHEASOFTpackage.Theobservedmagnitudeshavebeen September 30th and October 30th 2010),the source is not sig-
correctedforGalacticextinctionE =0.149mag(Schlegelet nificantlyresolved,andhenceonly95%confidencelevelupper
B−V
al.1998). limits were obtained.In the light curvepresentedin Fig. 3, for
eachtimebin,iftheTSvalueforthesourcewasTS< 4orthe
numberofpredictedphotonsN pred<3,thevaluesofthefluxes
2.4.Fermidataanalysis werereplacedbythe95%confidencelevelupperlimits.The2-
sigma upper limits were computed using the Bayesian method
The Fermi-LAT is a pair conversion telescope designed to
(Helene 1983),where the likelihood is integrated from 0 up to
cover the energy band from 20MeV to greater than 300GeV
thefluxthatencompasses95%oftheposteriorprobability.
(Atwood et al. 2009) which operates in survey mode, scan-
ning the entire sky every three hours. The data used in this
paper encompasses the time interval from August 5th, 2008
3. Results
to April 7th, 2011 (MJD 54683 - 55658), and were analyzed
with the Fermi Science Tools package version v9r23p0,which IntheMAGICdatathedistributionofthesquareoftheangleθ
are available from the Fermi Science Support Center (FSSC). betweenthereconstructeddirectionoftheeventsaftercutsand
Only events with the highest probability of being photons, thepositionofB32247+381(RA:22.83472◦,DEC:38.41028◦,
those in the diffuse class, located within 12◦ of B3 2247+304 J2000 as in Ficarra et al. 1985) shows an excess of 159 ± 28
were used in this analysis. In addition, a cut on the maxi- eventsabovea thresholdof200GeV.The cutswerepreviously
mum zenith angle (< 100◦) was applied to reduce the con- optimizedonatrialsampleofCrabNebuladata.Thethreshold
tamination from the Earth-limb gamma-rays, which is pro- wascalculatedfromMonteCarlosimulateddatabyfindingthe
duced by cosmic-rays interacting with the upper atmosphere. peakofthedifferentialratevs.energydistributionaftercutsand
Thebackgroundmodelusedtoextracttheγ-raysignalincludes spectral re-weighting. The expected background level with the
a Galactic diffuse emission component and an isotropic com- same cuts is 329 events, calculated from the θ2 distribution of
ponent (including residual instrument background), modelled thereconstructeddirectionoftheeventswithrespecttotheanti-
with the files gll iem v02 P6 V11 DIFFUSE.fit and isotropic sourceposition,located180◦fromtherealsourcepositioninthe
iem v02 P6 V11 DIFFUSE.txt, which are publicly available5. cameraplane(Figure1).Themeasuredexcesscorrespondstoa
The normalizations of the components comprising the total post-trial significance of 5.6σ calculated using eq. 17 from Li
backgroundmodelwereallowedtovaryfreelyduringthespec- andMa(1983).
tralpointfitting.Thespectralfluxeswerederivedwiththepost- The source position and extension, determined by a 2D
launchinstrumentresponsefunctions(IRF)P6 V11 DIFFUSE, Gaussian fit of the sky map produced with the cuts above, are
and applying an unbinned maximum likelihood technique consistentwith a point-likesourceplaced atthe position of B3
(Mattoxetal.1996)toeventsintheenergyrangespanningfrom 2247+381within 0.015◦, well within the statistical uncertainty
300MeV to 300GeV. All the sources from the 2FGL catalog andthesystematicpointinguncertaintiesofMAGIC.
(Abdo, A.A., Ackermann, M., Ajello, M., Allafort, A. et al., Theintegralfluxofthesourceabove200GeVwasestimated
ApJS, in prep.) located within 7◦ radius of B3 2247+38 were to be (5.0 ± 0.6 ± 1.1 ) · 10−12 ph cm−2s−1. The differen-
stat sys
5 http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html 6 http://fermi.gsfc.nasa.gov/ssc/data/analysis/LAT caveats.html
3
J.Aleksic´etal.:B32247+381
Flux B32247+381 Time[date]
1)
-V 22 Sep 06 04 Feb 08 18 Jun 09 31 Oct 10
e Flux
1 T10-10 Deabsorbed Flux s] 2
-s Error (stat.) 2/ MAGIC(>200GeV)
-1m Error (stat.+sys.) cm 1
dF (c10-11 -110/ 0
dN/ F [1
10-12
10-13 2/s] 8 Fermi(>300MeV)
m 6
9/c 4
10-14 -10 2
103 F [ 0
Energy (GeV)
Fig.2. The unfolded differential energy spectrum of B3 2/s] 12
2247+381observedbyMAGIC. The black data pointsrefer to m Swift/XRT (2-10 keV)
c
themeasuredspectrum,whilethegreydashedpointshavebeen g/ 8
corrected for the attenuation of the EBL. The thick black and er
dashedgreylinesarepower-lawfitstotherespectivedatapoints 2 4
1
(fit results are given in the text). The dashed band corresponds -0
1 0
tothestatisticalerrorofthefittothemeasuredspectrum,while F [
the white band surrounding it is the sum of the statistical and
systematicerrorsofthefit. 2.6
R-band
y] 2.2
J
tial energy spectrum is well described by a simple power-law: m
dN/dE = f0 (E/300GeV)γ. The photon index γ was found F [ 1.8
to be −3.2 ± 0.5 ± 0.5 , and a flux normalization f at
stat sys 0
300GeV of (1.4±0.3 ±0.2 )·10−11 ph cm−2s−1TeV−1. In 1.4
stat sys 54000 54500 55000 55500
ordertocorrecttheeffectsinthespectrumdeterminationintro-
duced by the limited energy resolution, different unfolding al- MJD
gorithms(Forward,Tikhonov,Schmelling1&2,Bertero;allde-
scribedinAlbertetal.(2007c))wereused,andallagreedwithin Fig.3.LongtermlightcurvesofB32247+381inVHEγ-rays,
errors. Taking into account the attenuation due to pair produc- Fermi-LATHEγ-rays(twomonthstimeintervals),SwiftX-rays
tion with the extragalactic background light (EBL), the spec- andopticalKVAR-band.
trum was found to be compatible with a power law with pho-
ton index γ = −2.7±0.5 ±0.5 and flux at 300GeV f =
stat sys 0
(2.0±0.3 ±0.3 )·10−11 phcm−2s−1TeV−1 (Figure2).Two the X-ray flux is significantly higher (almostfactor 2) than the
stat sys otherX-raypoints,butunfortunatelywedonothavesimultane-
different EBL models were used (Dom´ınguez et al. 2011 and
ousopticalorMAGICdataforthatnight.
Kneiske&Dole2010),andtheywerefoundtobeingoodagree-
mentwitheachother,wellwithinthestatisticaluncertainties.
Long term light curves of B3 2247+381 in VHE γ-rays
4. Discussion
(MAGIC), HE γ-rays (Fermi-LAT), X-rays (Swift) and optical
(Tuorla Observatory) are shown in Figure 3. Our detection in In this paper we report the discovery of VHE γ-rays from
VHE γ-rays is compatible with the previous upper limit from B32247+381byMAGIC.TheMAGICobservationsweretrig-
2006 and thus no variability can be established in this energy gered by an optical high state of the source, like several other
band.However,in X-raysand the opticalbanda clear increase discoveries.However,theobservedVHEγ-rayfluxisconsistent
of the flux in fall 2010 is evident, while the Fermi-LAT light with the upper limit from previous observations and we there-
curveisconsistentwithaconstantflux.Afitwithaconstantto fore cannot conclude if the source was in a higher VHE γ-ray
theelevenFermifluxpointswherethesourceissignificantlyde- stateduringtheobservations.
tected,gaveafluxvalueof(3.7±0.5)·10−9phcm−2s−1,witha In Figure 5 we show the spectralenergydistribution of the
reducedχ2 of0.7withtendegreesoffreedom.TheFermi-LAT source duringthe MAGIC observations,togetherwith simulta-
isnotsensitiveenoughfordetectingshorttermvariationsatthis neous Swift and optical data, and other non-simultaneousdata.
fluxlevel.ThetemporalevolutionoftheVHEγ-ray,X-rayand TheSwiftobservationsshowthatthesourcewasinahighstate
opticalemissionfromB32247+381duringSeptember-October also in X-rays. In the Fermi energy range the source is very
2010 observationshows no strong variability on time scales of weak, which limits the capability of detecting statistically sig-
a night (Figure 4). In particular, the MAGIC light curve above nificantflux-variabilityontimescalesofafewmonths.Thesyn-
200GeV is consistent with a non-variablesource, having a re- chrotroncomponentoftheSEDisshowingasignificantlylarger
ducedχ2 of 0.6with ten degreesof freedom.Duringone night emission in the high state, while the inverse Compton compo-
4
J.Aleksic´etal.:B32247+381
Time[date]
01 Oct 10 11 Oct 10 21 Oct 10 31 Oct 10
2/s] 2 MAGIC(>200GeV)
m 1.5
c
1/ 1
1
-0 0.5
1
x [ 0
u
Fl
s]
2/
m
12 Swift/XRT (2-10 keV)
c
g/
r 10
e
2
1 8
-0
x [1 6
u
Fl
Fig.5. Spectral energydistribution of B3 2247+381(red: EBL
correctedMAGICspectralpoints).Thegreencrossesare1FGL
2.6 Fermidatapoints(Abdoetal.2010),whilethepinkpointsrep-
resent the Fermi analysis from this work (2.5 years of data).
y] 2.5 R-band Blue arrows show the 95% confidence upper limits computed
J
m from Fermi-LAT data for the time interval of the MAGIC ob-
x [ 2.4 servation.Low(high)state SwiftdataweretakenonApril18th
u
Fl 2.3 2010 (October 5-16, 2010). The host galaxy contribution has
been subtractedfrom the KVA R-banddata (red and lightblue
2.2 squares), following Nilsson et al. (2007). The data have been
55470 55480 55490 55500 corrected for galactic absorption. Green and light blue points
MJD representnon-simultaneouslowstatedata.Thesolidlineisour
SSC-modelfittothehighstateobservations;thedottedlineisa
Fig.4.SameasFig.3,butzoomedintothetime intervalofthe fittothelowstateobservations.
MAGICobservationsinSeptember-October2010.
δ(themagneticfieldintensityB)withrespectto“standard”val-
nent is consistent with only minor changes. We must however uesismainlydueto therelativelylargeseparationbetweenthe
notethattheweakdetectionintheHEandVHEγ-raybandsig- synchrotronandICpeaks.
nificantlylimitsthedeterminationoftheinverseComptoncom- Additionally,wemodeltheSED,duringthehighstate,with
ponent. the one-zone SSC code from Weidinger et al. (2010) which is
We reproduce the SED with a one-zone synchrotron-self- shown in Figure 6. In contrast to the model from Tavecchio et
Compton(SSC)model(seeTavecchioetal.2001foradescrip- al. (2001)all parametersare basic physicalparametersand the
tion). In brief, the emission region is assumed to be spherical, electronandphotonspectrumandtheirbreaksarederivedself-
witharadiusR,filledwithatangledmagneticfieldofintensity consistentlywithacontinuousinjectionofmonochromaticelec-
B. The relativistic electrons follow a smoothed broken power- trons at the energy γ = 104 and injection rate K = 8.4·104
0
lawenergydistributionspecifiedbythelimitsγ ,γ andthe cm−3 s−1. The spectrum is the resulting steady-state solution.
min max
breakatγ as wellas the slopesn and n beforeandafter the The environmentis defined by the magnetic field B = 0.07G,
b 1 2
break,respectively.Relativisticeffectsaretakenintoaccountby theaccelerationefficiencyt /t =1.09(i.e.theparticlespec-
acc esc
theDopplerfactorδ.Theusedinputmodelparametersareshown tral index is s = 2.09 and the resulting γ = 4.8·105), and
max
inTable1. theblobradiusR = 1.3·1016cm.Thebreakinthee− spectrum
ToreproducethechangeoftheComptonandsynchrotronlu- of 1 at γ = 2.9·104 arises self-consistentlyfrom IC andsyn-
b
minosityratiobetweenthelowandthehighstatewemainlyact chrotroncooling.Thecommonparametersofbothmodelsagree
ontheelectronnormalization,sourceradiusandDopplerfactor verywell.
(with slight changes also to the other parameters). The steeper TheopticalmonitoringofcandidateVHEγ-rayblazarshas
X-rayslopeinthelowstateimpliesalargervalueofn . proventobea successfultoolfortheirdiscovery.However,for
2
ThecomparisonwithparametersderivedforBLLacobjects B3 2247+381 (like for Mrk 180 and 1ES 1011+496) we can-
(e.g. Tavecchio et al. 2010) reveals that the parameters used notestablishaconnectionbetweentheopticalhighstateandthe
for B3 2247+381 are close to the typical values. As for other VHEγ-rayemissionsincetheupperlimitfrompreviousobser-
sources,thesomewhatlarger(lower)valueoftheDopplerfactor vations (during a low optical state) is higher than the detected
5
J.Aleksic´etal.:B32247+381
Table 1. Input parameters for the high and low states of the SSC-model shown as solid and dashed lines in Fig. 5. For more
explanationsseetext.
FluxState γ γ γ n n B K δ R
min b max 1 2
G cm−3 cm
High 3·103 7.1·104 6·105 2.0 4.35 0.06 2.5·103 35 8·1015
Low 3·103 6.8·104 5·105 2.0 5.35 0.08 1.15·104 30 4·1015
Aleksic´,J.etal.2011b,ApJ,729,115
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Anderhub,H.etal.2009,ApJL,704,L129
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Barthelmy,S.D.2005,SpaceScienceReviews,120,143
Burrows,D.Netal.2005,SpaceScienceReviews,120,165
10-11 Dom´ınguez,A.etal.2011,MNRAS,410,2556
Donato,D.,Ghisellini,G.,Tagliaferri,G.,Fossati,G.2001,A&A,375,739
Falco,E.E.,Kochanek,C.S.,andMunoz,J.A.1988,ApJ,494,47
-2-1F (erg cms)ν1100--1123 FFFGooiecmshacrrihernial,ns,,ViA,N.L.P.e..eteettatalaa.ll.l.1.2921080905094,74,,A,AAA&pppAJJJ,,,S662,18,51719,,3,1872015055
ν
KVA Helene,O.1983,NuclearInstrumentsandMethodsinPhysicsResearch,212,
Swift XRT
10-14 MFeArmGiI LCA (dTe-absorbed) Hilla3s,19A.M.1985,Proc.ofthe19thICRC,3,445
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Kneiske,T.M.&Dole,H.2010,A&A,515,19
10-15 Li,T.-P.&Ma,Y.-Q.1983,ApJ,272,317
Mariotti,M.etal.2010,Atel#2910
1012 1014 1016 1018ν (Hz)1020 1022 1024 1026 Mariotti,M.etal.2011,Atel#3100
Massaro,E.etal.2004,A&A,413,489
Mattox,J.R.etal.1996,ApJ,461,396
Moralejo,A.etal.2009,inProc.31stICRC(Lodz)(arXiv:0907.0943)
Fig.6.ThesolidlineshowstheSEDmodelusingWeidingeret
Neronov,A.,SemikozD.V.&Vovk,I.2010(arXiv:1004.3767)
al.(2010).Thedatapointsaredescribedintheinlayofthefigure. Nieppola,E.,Tornikoski,M.,Valtaoja,E.2006,A&A,445,441
Thefitparameterscanbefoundinthetext. Nilsson,K.etal.2007,A&A,475,199
Reinthal, R. et al. 2011, Proc. Beamed and Unbeamed Gamma-rays from
Galaxies, OpenAccess Journal ofPhysics: Conference Series (JCPS), in
press
VHEγ-rayfluxduringthediscovery.Thereforefurtherobserva-
Roming,P.W.A.etal.2005,SpaceScienceReviews,120,95
tionsarestillneededtostudytheconnectionbetweenthesetwo Schlegel,D.J.,Finkbeiner,D.P.,&Davis,M.1998,ApJ,500,525
wavebands(seealsoReinthaletal.2011). Tavecchio,F.,etal.2001,ApJ,554,725
Tavecchio,F.,Ghisellini,G.,Ghirlanda,G.,Foschini,L.,&Maraschi,L.2010,
MNRAS,401,1570
Acknowledgements. We would like to thank the Instituto de Astrof´ısica de
Veron-Cetty,M.P.,&Veron,P.2006,A&A,455,773
Canarias for the excellent working conditions at the Observatorio del Roque
Weidinger,M.,Ru¨ger,M.,Spanier,F.2010,ASTRA,6,1
delosMuchachosinLaPalma.ThesupportoftheGermanBMBFandMPG,
theItalian INFN,the SwissNational FundSNF,andthe SpanishMICINN is
gratefullyacknowledged.ThisworkwasalsosupportedbytheMarieCuriepro-
gram,bytheCPANCSD2007-00042andMultiDarkCSD2009-00064projects
oftheSpanishConsolider-Ingenio2010programme,bygrantDO02-353ofthe
Bulgarian NSF, by grant 127740 of the Academy of Finland, by the YIP of
the Helmholtz Gemeinschaft, by the DFG Cluster of Excellence “Origin and
StructureoftheUniverse”,andbythePolishMNiSzWGrantNN203390834.
TheFermi-LATCollaborationacknowledgessupportfromanumberofagencies
andinstitutesforbothdevelopmentandtheoperationoftheLATaswellasscien-
tificdataanalysis.TheseincludeNASAandDOEintheUnitedStates,CEA/Irfu
andIN2P3/CNRSinFrance,ASIandINFNinItaly,MEXT,KEK,andJAXAin
Japan,andtheK.A.WallenbergFoundation,theSwedishResearchCounciland
theNationalSpaceBoardinSweden.AdditionalsupportfromINAFinItalyand
CNESinFranceforscienceanalysisduringtheoperationsphaseisalsograte-
fullyacknowledged. WegratefullyacknowledgeN.Gehrelsforapprovingthis
setofToOsandtheentireSwiftteam,thedutyscientistsandscienceplanners
forthededicatedsupport,makingtheseobservationspossible.
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J.Aleksic´etal.:B32247+381
1 IFAE,EdificiCn.,CampusUAB,E-08193Bellaterra,Spain
2 UniversidadComplutense,E-28040Madrid,Spain
3 INAFNationalInstituteforAstrophysics,I-00136Rome,Italy
4 Universita`diSiena,andINFNPisa,I-53100Siena,Italy
5 TechnischeUniversita¨tDortmund,D-44221Dortmund,Germany
6 Universita`diPadovaandINFN,I-35131Padova,Italy
7 Inst. de Astrof´ısica de Canarias, E-38200 La Laguna, Tenerife,
Spain
8 Depto. de Astrof´ısica, Universidad de La Laguna, E-38206 La
Laguna,Spain
9 UniversityofŁo´dz´,PL-90236Lodz,Poland
10 TuorlaObservatory,UniversityofTurku,FI-21500Piikkio¨,Finland
11 Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen,
Germany
12 ETHZurich,CH-8093Zurich,Switzerland
13 Max-Planck-Institutfu¨rPhysik,D-80805Mu¨nchen,Germany
14 Universita¨tWu¨rzburg,D-97074Wu¨rzburg,Germany
15 UniversitatdeBarcelona(ICC/IEEC),E-08028Barcelona,Spain
16 Universita`diUdine,andINFNTrieste,I-33100Udine,Italy
17 Institut de Cie`ncies de l’Espai (IEEC-CSIC), E-08193 Bellaterra,
Spain
18 Inst.deAstrof´ısicadeAndaluc´ıa(CSIC),E-18080Granada,Spain
19 Croatian MAGIC Consortium, Rudjer Boskovic Institute,
University of Rijeka and University of Split, HR-10000 Zagreb,
Croatia
20 UniversitatAuto`nomadeBarcelona,E-08193Bellaterra,Spain
21 Inst.forNucl.ResearchandNucl.Energy,BG-1784Sofia,Bulgaria
22 INAF/OsservatorioAstronomicoandINFN,I-34143Trieste,Italy
23 Universita`dell’Insubria,Como,I-22100Como,Italy
24 Universita`diPisa,andINFNPisa,I-56126Pisa,Italy
25 ICREA,E-08010Barcelona,Spain
26 now at Ecole polytechnique fe´de´rale de Lausanne (EPFL),
Lausanne,Switzerland
27 supportedbyINFNPadova
28 nowat:CentrodeInvestigacionesEnerge´ticas,Medioambientalesy
Tecnolo´gicas(CIEMAT),Madrid,Spain
29 now at: Finnish Centre for Astronomy with ESO (FINCA),
UniversityofTurku,Finland
30 Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131
Padova,Italy
31 Dipartimento di Fisica G. Galilei, Universita‘ di Padova, I-35131
Padova,Italy
32 * corresponding authors: K. Berger, email:[email protected]
wuerzburg.de, G. Giavitto, [email protected], L. Takalo,
email:takalo@utu.fi
7