Discovery of very high energy gamma-rays from the flat spectrum radio quasar 3C 279 with the MAGIC telescope

0.22 MB·English

by  M. Errando

Discovery of very high energy gamma-rays from the flat
spectrum radio quasar 3C 279 with the MAGIC telescope
M. Errando∗, R. Bock†,∗∗, D. Kranich‡, E. Lorenz‡,†, P. Majumdar§, M. Mariotti∗∗,
D. Mazin∗, E. Prandini∗∗, F. Tavecchio¶, M. Teshima† and R. Wagner† (on behalf of
9
the MAGIC Collaboration)
0
0
2 ∗IFAE,EdificiCn.,CampusUAB,E-08193Bellaterra,Spain
†Max-Planck-InstitutfürPhysik,D-80805München,Germany
n
a ∗∗UniversitàdiPadovaandINFN,I-35131Padova,Italy
J ‡ETHZurich,CH-8093Switzerland
1 §DESYDeutschesElektr.-SynchrotronD-15738Zeuthen,Germany
2 ¶INAFNationalInstituteforAstrophysics,I-00136Rome,Italy

] Abstract. 3C279isoneofthebeststudiedflatspectrumradioquasarslocatedatacomparativelylargeredshiftofz=0.536.
E
Observationsintheveryhighenergybandofsuchdistantsourceswereimpossibleuntilrecentlyduetotheexpectedsteep
H
energy spectrumand thestronggamma-rayattenuation bytheextragalacticbackground lightphoton field, whichconspire
. to make the source visible only with a low energy threshold. Here the detection of a significant gamma-ray signal from
h 3C279atveryhighenergies(E>75GeV)duringaflareinearly2006isreported.Implicationsofitsenergyspectrumonthe
p
currentunderstandingoftheextragalacticbackgroundlightandveryhighenergygamma-rayemissionmechanismmodelsare
-
o discussed.
r Keywords: ActiveGalacticNuclei:individual;gammarays:observations;gamma-raytelescopes
t
s PACS: 95.55.Ka,95.85.Pw,98.54.Cm,98.62.Js,98.70.Vc
a
[
INTRODUCTION tivisticelectrons,whilethehigh-energypeakispresum-
1
ablyduetotheinverseComptonscatteringoflow-energy
v
The flatspectrumradioquasar(FSRQ) 3C 279,located photons by these electrons. The soft photon population
5
7 at a redshift of z=0.536, is one of the brightest extra- canbethesynchrotronphotonsthemselves(synchrotron
2 galacticobjectsinthegamma-raysky[1]anditisanex- self-ComptonorSSCmechanism[8])orambientradia-
3 ceptionally variable source at variousenergy bands, in- tion(externalComptonorECmechanism[9,10]).Other
1. cludingverystronggamma-rayflaresrecordedbysatel- scenarios (hadronic models) involve gamma-ray emis-
0 litedetectorEGRETin1991and1996[2].Itsdetection sionduetoacceleratedprotonsandions[11,12].
9 atveryhighenergy(VHE,definedhereasE >50GeV) ApartfrombeingthefirstFSRQdetectedintheVHE
0 gamma-raysby the MAGIC telescope [3, 4] is the first band, the detection of 3C 279 is important because
:
v detectionofanFSRQ inthisenergybandaswellasthe gamma-rays from distant sources are expected to be
i farthestobjectdetectedatthesewavelengths,asthehigh- stronglyattenuatedinintergalacticspacebythepossible
X
estredshiftpreviouslyobservedwasz=0.212[5]. interactionwithlow-energyphotons(g +g →e++e−).
r
TheEGRETexperimentdetected66blazars:51FSRQ These photons form the extragalactic background light
a
and 15 BL Lac objects [6]. Blazars are thought to be (EBL)[13]andhavebeenradiatedbystarsandgalaxies
poweredbyaccretionofmatterontosupermassiveblack in the course of cosmic history. The attenuation by the
holesresidinginthecentersofgalaxiesandejectingrel- EBLincreaseswithincreasingdistancetothesourceand
ativistic jets with small angles to the line of sight [7]. energyoftheemittedgamma-ray,therefore,VHEradia-
ExceptfortheFSRQ 3C279,alltheblazarsdetectedat tion from distant sources is stronglysuppressed at high
VHE upto knoware ofthe BL Lac type,beingthe dif- energies(forE≥300GeVandz=0.536morethan90%
ference between this two subclasses that FSRQs show oftheemittedgamma-raysareexpectedtobeabsorbed).
optical emission lines while in BL Lacs these are very
weak or absent. The spectral energy distribution (SED)
ofblazarsischaracterizedbytwobroadbumps,thefirst OBSERVATIONSAND ANALYSIS
onepeakingbetweentheinfraredandtheX-raybandand
thesecondoneinthegamma-raydomain.Thefirstpeak Observations of 3C 279 were performed using the
is usually attributed to synchrotronemission from rela- MAGIC telescope (see [14] for a detailed description),
which is located on the Canary Island of La Palma
and has a 17m-diameter tessellated reflector dish and s
a photomultipliercamera covering3.5◦ field of view. It unt180
o160
C
hasa triggerthresholdof 60GeV, an angularresolution 140
of approximately 0.1◦ and an energy resolution above 120
100
150GeVofabout25%.
80
Data were taken in on/off mode, which includes on 60
Observation time: 82 mins
events taken with the telescope pointing at the source 40 Excess events: 166 – 28
20 Excess significance: 6.15 s
and a similar amount of off events taken at a sky loca-
0
0 10 20 30 40 50 60 70 80 90
tion near the source with very similar operating condi-
|ALPHA| [deg]
tions.3C279wasobservedover10nightsbetweenJan-
uary and April 2006 for a total of 9.7 hours. Simulta- FIGURE 1. Histogram of the image parameter alpha ob-
neousopticalR-bandobservationswiththe1.03-mtele- tained for 3C 279 in the night of 23 February 2006 for on
scopeattheTuorlaObservatory(Finland)andthe35-cm (dots) and off (crosses and filled area) pointings. The dotted
KVAtelescopeonLaPalmarevealedthatduringMAGIC lineisasimpleparabolicfitwithoutlineartermtotheoff events
between0◦and80◦,servingtosmoothenthebackgrounddistri-
observations 3C 279 was in high optical state, a factor
butionforabetterextrapolationtowardsalpha=0.Thenum-
of 2 aboveits long-termbaseline flux (hostgalaxysub-
berofexcesseventsisobtainedwithrespecttothisline.
tracted).
The data anaysis was carried out using the standard
MAGIC analysis and reconstruction software [14]. To ·10-9
searchforagamma-raysignal,firstthestandardcalibra- -2] sm-1 0.6 MAGIC
tictineaorglinizbpaorrtafiotoectnhdeedosuifdrgaentthaawelswa.ssahTsaohpwpepeelrnrifeeoidxmrtmuassegtideenp.sgT[itn1hhc5eeln]u.adamHendapidlmtirhtouaendgiepecaocrblafeamacthnkee--- integral flux (100-500 GeV) [c00..42 22 Feb 2006 23 Feb 2006
groundsuppressionwasachievedusingtheRandomFor- 0
est (RF) method [16, 17], where for each event the so-
-0.2
called hadronness (h) parameter is computed based on
8 KVA
theimageparameters.Moreover,theRFmethodwasalso mJy]
uuslaetdedfogratmhemean-erargyysaemstpimleawtioitnhuthseinsgamaeMzoenntiethCaanrglolesdimis-- optical flux [ 6
tributionasthedatasample.Allimageparameterswere 4
checkedforconsistencybetweenonandoff data. 760 780 800Time (Julian Date8 2- 02453000)
FIGURE2. MAGIC(top)andopticalR-band(bottom)light
curvesobtainedfor3C279fromFebruarytoMarch2006.The
RESULTS
long-term baseline for the optical flux is 3mJy (host galaxy
subtracted).
Theexcesseventswereobtainedbysubtractingsuitably
normalized off from the on data. Cuts in h and alpha
(thelatterdescribesthedirectionofthemainaxisofthe TheobserveddifferentialVHEspectrum(Fig.3)was
image)wereoptimizedusingCrabNebuladatatakenat obtained with looser cuts to increase the number of
similar zenith angles. For the detection and light curve gamma-raycandidates,andcanbedescribedbyapower
thecutsh<0.12andalpha<12◦whereused,resulting lawwithspectralindexofa =4.1±0.7 ±0.2 .The
stat syst
ina combinedefficiencyof40%forgammas.Forthese measuredintegratedfluxabove100GeVon23February
cuts and a quality cut that removed small images the is(cid:0)5.15±0.82stat±1.50syst(cid:1)×10−10cm−2s−1.
resultingenergythresholdis170GeV.
3C279wasclearlydetectedonthenightof23Febru-
ary 2006 (Fig. 1) with a significance of 6.2s , which DISCUSSION
translates into 5.8s when corrected with a trial factor
correspondingto10nightsofobservation.Alesssignif- TheEBLinfluencesthepropagationofgamma-rayemit-
icant excess on the previousnightis visible in the light ted from distant sources resulting in an exponentialde-
curve(Fig.2).Asdeterminedbythec 2test,theprobalil- creasewithenergyandacutoffinthemeasuredgamma-
ity thatthe gamma-rayfluxonall10nightswas zerois ray spectrum. Several models have been proposed for
2.3×10−7, correspondingto 5.0s in a Gaussian distri- the EBL [13]. All havelimited predictivepowerfor the
bution.
3c279, measured Kneiske et al. 2002 (modified) Primack et al. (2005)
error band from 3 analyses Energy Threshold of MAGIC Stecker et al. (2006)
-1-1-2] s cmdifferential flux, dN/dE [TeV111111000000--11----098761 fdNitN0 t/=od E(m5 .=e2 a N–s 0u1 .r72e)0d·0 1sEG0p-ee1V0c [tTr-ueGmV-:1EE cBBmLL---cc2 oosrrrr-ee1]cctteedd,, SPtreimckaecrk (,f aGs*t )=, G2*.9 =4 0–. 409.9 –1 1.19 Gamma-Ray Energy [TeV]11001-1 Mkn 501H 114E2S 610+E24S1 22E198S1+ 0201103-E042S73 -21102111+4O96pacity Foτr >G a1m3C m27a9 Rayτττ ==s= 125
10-12 G = 4.11 – 0.68
7080 100 200 300 400 500
Energy, E [GeV]
10-20 0.1 0.2 0.3 0.4 0.5 0.6 0.7
FIGURE 3. Differential energy spectrum of 3C 279 mea- Redshift z
sured by MAGIC. The grey area includes the combined sta-
tistical(1s )andsystematicerrors,andunderlinesthemarginal FIGURE 5. The gamma-ray horizon. The redshift region
significanceofdetectionsathighenergy.Thedottedlineshows overwhichthegamma-rayhorizoncanbeconstrainedbyob-
compatibility of the measured spectrum with a power law of servations has been extended upto z=0.536. Theprediction
photon index a =4.1. The triangles are measurements cor- range of EBL models is illustrated by [18] (thick solid line)
rectedonthebasisofthetwomodelsforEBLdensitydiscussed and[21](dashed-dottedline).Thetunedmodelof[22](dashed
inthetext. line)representsanupperEBLlimitbasedonour3C279data,
obtained on the assumption that the intrinsic photon index is
≥1.5 (arrow corresponding to 3C 279). Limits obtained for
-2W m-1) sr 102 SAPCMtlroabeimszecmikranteci ce&rk t M e eRatit lac .aa ru2lol.e. 0w 2 2020a0800v0,06e 5u7 'Bp,f apausecptrk p egleivmrroo liluitumntidiotn' ovreetghriyeorncslooaulslerocwetosedathrbeeetsmwhooedewnenlth[b2iys2]bm.laoTcdhkeelanarrnaordwroasw,mmfiaolxlseitmdoubfmawnpdhoicissshibtlhlieee
(n Iνν Aharonian et al. 2006, upper limit tnreaanrslpyacreoninccyid(ie.en.t,wmiitnhimgaulmaxEyBcLoulnevtse.l)Tghievegnrabyya[1re8a],iwndhiiccahteiss
anopticaldeptht >1,wherethefluxofgammaraysisstrongly
suppressed.Toillustratethestrengthoftheattenuationinthis
10 areaenergiesfort =2andt =5(thinblacklines)areshown,
againwith[18]asmodel.
1 VHE spectrum difficult to reconcile with an extrapola-
10-1 1 10 102 103
λ (µm) tion of the EGRET data and with generalconstraintsin
FIGURE4. EBLmodelsat z=0and direct measurements the spectral energy distribution. On the other hand, the
oftheEBLphotondensityatvariouswavelengths.Theupper lowmodelapparentlygivesanacceptableresult.Thedis-
limitderivedfromthedetectionof3C279(blackline)isbased tance at which the flux of photons of a given energy is
on[22]withparametersadapted totheresultsfromthelatest attenuatedbyafactore(thepathcorrespondingtoanop-
galaxycounts(seeSupportingMaterialin[4]fordetails).The ticaldeptht =1) is called thegamma-rayhorizon[24]
dotted line is an upper limit from [23]. The shaded region
andisusuallyexpressedasafunctionoftheredshift.This
corresponds to the range of frequencies where the MAGIC
energy-redshiftrelationisshowninFigure5.
measuredspectrumissensitive.
Assumingahardestintrinsicphotonindexa ⋆=1.5a
modelbased on Kneiske et al. [22, 25] can be tuned to
giveanupperlimittotheEBLdensityinthe0.2−2m m
EBLdensity,particularlyasafunctionoftheredshift,as
rangeasshowninFig.4.Thisresultsupportstheconclu-
manydetailsaboutstarandgalaxyevolutionremainun-
sion drawn from VHE measurementsat lower redshifts
certain. To illustrate this uncertainty two extreme mod-
[26]thattheEBLdensityisclosetothelowerlimitsset
elsareusedtocalculatethesourceintrinsicspectrum:a
by galaxy counts. It is important to point out that this
modelbyPrimacketal.[18]closetothelowerlimitsset
limit is only valid under the assumption that the intrin-
bygalaxycounts[19,20]andthe‘fast-evolution’model
sicphotonindexcannotbeharderthan1.5,whichisthe
bySteckeretal.[21]correspondingtothehighestpossi-
hardest value given for EGRET sources and is also the
bleattenuation.Thesemodelswillbereferredaslowand
hardestthatcanbeobtainedwithclassicalleptonicemis-
highrespectivelyandareshowninFigure4.
sion mechanisms.Alternativescenarioscanproducein-
A power-law fit to the EBL-correctedspectral points
results in an intrinsic photonindexa ⋆ =2.9±0.9 ± trinsicspectrawitha ⋆<1.5.SSCmodelswithanarrow
0.5 (low absorption) and a ⋆ =0.5±1.2 ±0s.5tat electrondistributioncan producespectra with a ⋆ ≈0.7
syst stat syst
[27], and internal absorption by soft photons can also
(high absorption). The high model leads to an intrinsic
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