Table Of ContentAstronomy&Astrophysicsmanuscriptno.ms (cid:13)c ESO2011
December6,2011
BL Lacertae objects beyond redshift 1.3 - UV-to-NIR photometry
and photometric redshift for Fermi/LAT blazars
A.Rau,1,P.Schady,1,J.Greiner1,M.Salvato2,3,M.Ajello4,5,E.Bottacini4,N.Gehrels6,P.M.J.Afonso1,7,J.Elliott1,
R.Filgas1,D.A.Kann8,S.Klose8,T.Kru¨hler1,3,9,M.Nardini1,10,A.NicuesaGuelbenzu8,F.OlivaresE.1,A.Rossi8,
V.Sudilovsky1,A.C.Updike11,andD.H.Hartmann12
1 Max-Planck-Institutfu¨rExtraterrestrischePhysik,Giessenbachstraße1,85748Garching,Germany
e-mail:[email protected]
1 2 Max-Planck-Institutfu¨rPlasmaPhysikandExcellenceCluster,Boltzmannstrasse2,85748Garching,Germany
1 3 UniverseCluster,TechnischeUniversita¨tMu¨nchen,Boltzmannstraße2,85748Garching,Germany
0 4 W.W.HansenExperimentalPhysicsLaboratory&KavliInstituteforParticleAstrophysicsandCosmology,StanfordUniversity,
2 USA
c 5 SLACNationalAcceleratorLaboratory,StanfordUniversity,Stanford,CA94305,USA
e 6 NASA-GoddardSpaceFlightCenter,Greenbelt,Maryland20771,USA
D 7 AmericanRiverCollege,Physics&AstronomyDpt.,4700CollegeOakDrive,Sacramento,CA95841
8 Thu¨ringerLandessternwarteTautenburg,Sternwarte5,07778Tautenburg,Germany
5 9 DarkCosmologyCentre,NielsBohrInstitute,UniversityofCopenhagen,JulianeMariesVej30,2100Copenhagen,Denmark
10 Universita´deglistudidiMilano-Bicocca,PiazzadellaScienza3,20126,Milano,Italy
] 11 DepartmentofPhysicsandAstronomyDickinsonCollegeCarlisle,PA17013,USA
O 12 DepartmentofPhysicsandAstronomy,ClemsonUniversity,Clemson,SC29634,USA
C
ReceivedSeptember27,2011;acceptedNovember19,2011
.
h
p ABSTRACT
-
o Context.Observationsoftheγ-rayskywithFermiledtosignificantadvancestowardsunderstandingblazars,themostextremeclass
r ofActiveGalacticNuclei.AlargefractionofthepopulationdetectedbyFermiisformedbyBLLacertae(BLLac)objects,whose
t
s samplehasalwayssufferedfromasevereredshiftincompletenessduetothequasi-featurelessopticalspectra.
a Aims.Ourgoalistoprovideasignificantincreaseofthenumberofconfirmedhigh-redshiftBLLacobjectscontainedinthe2LAC
[ Fermi/LATcataloge.
Methods.For103Fermi/LATblazars,photometricredshiftsusingspectralenergydistributionfittinghavebeenobtained.Thepho-
2
tometryincludes13broad-bandfiltersfromthefarultraviolettothenear-IRobservedwithSwift/UVOTandthemulti-channelimager
v
GRONDattheMPG/ESO2.2mtelescope.Datahavebeentakenquasi-simultaneouslyandtheremainingsource-intrinsicvariability
5
hasbeencorrectedfor.
2
Results.WereleasetheUV-to-near-IR13-bandphotometryforall103sourcesandprovideredshiftconstraintsfor75sourceswithout
0
previouslyknownredshift.Outofthose,eighthavereliablephotometricredshiftsatz & 1.3,whilefortheother67sourceswepro-
0
videupperlimits.SixoftheformereightareBLLacobjects,whichquadruplesthesampleofconfirmedhigh-redshiftBLLac.This
.
2 includesthreesourceswithredshiftshigherthanthepreviousrecordforBLLac,includingCRATESJ0402-2615, withthebest-fit
1 solutionatz≈1.9.
1
Keywords.Techniques:photometric,(Galaxies:)BLLacertaeobjects:general,Galaxies:distancesandredshifts
1
:
v
i
X 1. Introduction hole) (e.g., Abdoetal. 2010d) to probes of the extra-galactic
background light (EBL) through attenuation of γ-ray photons
r
a Since its launch in 2008, the Fermi Space Laboratoryhas dra- (e.g.,Abdoetal.2010c).Oneofthecrucialparametersforthese
matically extended our view of the high-energy sky. The re-
applicationsisthedistancetotheobjects,whichunfortunatelyis
cently released 24-month catalog (2LAC) of Active Galactic
noteasytoobtaininmostcases.
Nuclei (AGN) detected by the Large Area Telescope (LAT;
Atwoodetal. 2009) revealed 885 high-significance sources,
the large majority of them being blazars (Ackermannetal. Twoclassesofblazarsdominatethe2LACpopulation.These
2011). The latter form the most extreme class of AGN with are the flat-spectrum radio quasars (FSRQs, 310 sources) and
their observational characteristics governed by the small an- BL Lac objects (395), named after the prototype BL Lacertae
gle between their relativistic jets and the observer’s sight (Hoffmeister1929).Whilefortheformerredshiftmeasurements
line (Blandford&Rees 1978). The resulting Doppler boosting areroutinelyperformedusingtheirstrongemissionlinesatUV-
makes blazars exceptionally bright sources at nearly all wave- optical wavelengths, the featureless, power-law optical spectra
lengthsandthereforevisibleouttohighredshift. ofBLLacobjectshaveproventobeachallenge(e.g.,Shawetal.
The scientific relevance of blazars is very broad, ranging 2009).Indeed,220ofthe395BLLacsinthe2LAC(55%)lack
fromlaboratoriesforthephysicsandstructureofrelativisticjets redshift estimates. Until this incompleteness is resolved, con-
(andthustheextractionofenergyfromthecentralmassiveblack clusionsabouttheEBL,theblazarsequence(e.g.,Fossatietal.
2 A.Rau,etal.:BLLacobjectsbeyondredshift1.3-UV-to-NIRphotometryandphotometricredshiftforFermi/LATblazars
1998; Ghisellinietal. 1998), and the blazar population in gen-
120
eralremaintentativeatbest.
100%
SeveralmethodshavebeenexploitedtoincreasetheBLLac s 100
redshiftsample.Atlowdistance,onecanutilizetheremarkably e
c
uniformabsolute brightnessof the giant elliptical BL Lac host ur 90%
o 80
galaxies (Sbarufattietal. 2005; Meisner&Romani 2010) and s
very-high signal-to-noise optical spectroscopy to help to iden- of
# 60
tify weak emission or absorption features in a few other cases e 50%
(e.g., Shawetal. 2009). An alternative method, applicable to ativ 40
the moredistantsources, is the photometricredshifttechnique, ul
whichconsistsoffittingspectralenergydistribution(SED)tem- m 25%
u
platestomulti-bandphotometry.Neutralhydrogenalongtheline c 20
10%
ofsighttotheblazarwillimprintaclearattenuationsignatureat
0
theLymanlimitandthusallowsanaccurateestimateofthered-
0.01 0.10 1.00 10.00 100.00
shift of the absorber. Even thoughthe absorberwill be located
∆t [days]
somewherealongthe line of sightand itsredshiftwillthusnot GROND-UVOT
necessarilycorrepondtothatoftheblazar,photometricredshifts,
Fig.1. Cumulative distribution of the (absolute) time between
z ,willprovideareliablelowerlimitontheblazarredshift.
phot the mid-points of the GROND and Swift/UVOT observations.
In this paper we explore the use of ultra-violet to near-
The median offset is 1.5d. For longer UVOT observations,
infraredquasi-simultaneousphotometrytomeasuretheredshift
whichwerespreadoverseveralsatellite orbits,themid-timeof
of 103 2LAC blazars, 86 of them without previous redshift
theuvw2-exposurewastakenasreference.
constraints. The sample selection is described in Sect. 2 while
the observations and data reduction are detailed in Sect. 3 and
Sect.4.TheresultsandtheirdiscussionarepresentedinSect.5
TheUVOTpointingshavebeenobtainedaspartoftheSwift
andSect.6,respectively.
fill-in program from January 2010 to October 2011. With few
exceptions,they comprise observationsin three optical(u,b,v)
andthreeUV(uvw2,uvm2,uvw1)lenticularfilters,coveringthe
2. SampleSelection
wavelength range between 160 nm and 600 nm (Pooleetal.
The sample is selected mainly from the clean sample 2008). The exposure times vary from source to source de-
of the 24-month catalog of AGN detected by Fermi/LAT pending on visibility and brightness of the optical counterpart.
(Ackermannetal. 2011) which includes 885 high-significance Approximatevaluesare100seachinu,b,vand200,250,400s
γ-raysourceswhicharestatistically associatedwithAGNsand in uvw1,uvm2,uvw2, respectively. For a few sources multiple
locatedathighGalacticlatitudes(|b| > 10deg).Toaccomodate consecutiveorbits,andthuslongerexposures,wereobtained.
thelocationofoneofourfollow-upinstrumentsinthesouthern WhileUVOTneedsto cyclethroughallfiltersinsequence,
hemisphere,weappliedacutindeclination(δ < +25deg). GROND observesin it’sfouropticalg′,r′,i′,z′ andthreenear-
J2000
Furthermore, the importance of the ultra-violet bands for the IRJ,H,Ks bandssimultaneously.Thiscapabilityisparticularly
z estimatesleadustodiscardallsourceswithGalacticfore- importantforstudiesoffast-varyingsourcessuchasblazarsasit
phot
ground reddening of E > 0.2mag, as derived from the allowstheconstructionofareliablespectralenergydistribution
B−V
Schlegeletal.(1998)maps.Thefinalsampleiscomposedof80 (SED) withoutthe need to correctfor source-intrinsicvariabil-
sourceswithoutknownredshiftbutwithopticalorradiocounter- ity. Unfortunately,schedulinglimitationsrarely allowed simul-
partassociationsfromBayesianstatistics(Abdoetal.2010a,b), taneousobservationswith bothinstruments,UVOTfromspace
Likelihood Ratio, or log N – log S methods (Ackermannetal. and GROND from the ground. However, in order to minimize
2011)andwasfurtherextendedbyeightsourcesthatdonotbe- theimpactofvariabilityonthecombinedSED,GRONDobser-
longtotheclean2LACsample1.Inaddition,observationsfor16 vations were executed as close as possible in time to the Swift
2LACsourceswithexistingredshiftmeasurementsareincluded pointings.Exceptinafewcaseswherevisibilityorweathercon-
for verification. We emphasize here that our sample is not sta- straintsleadtooffsetsofmorethan10d,observationsweretyp-
tisticallycomplete,butbiasedbyselection.Thefinallistof104 icallyachievedwithin1-2dofeachother(seeFig.1).Thezphot
targetsispresentedinTable1.Ofthose,82havebeenclassified uncertainties associated with these offsets will be discussed in
asBLLacobjects,threeasaFSRQ,andtheremaining19areof Sect.4.3.
unknowntype(Ackermannetal.2011). A typical GROND observation had an integration time of
2.4minin(g′,r′,i′,z′)and4.0minin(J,H,K ).
s
3. Observations
4. Data&Analysis
The observations were performed with the Ultraviolet and
4.1.Swift/UVOTandGROND
Optical Telescope (UVOT; Romingetal. 2005) onboard the
Swift satellite (Gehrelsetal. 2004) and the Gamma-ray UVOT photometry was carried out on pipeline processed sky
Optical/Near-Infrared Detector (GROND; Greineretal. 2008) images downloadedfrom the Swift data center 2, followingthe
mounted on the MPI/ESO 2.2m telescope at La Silla, Chile. standard UVOT procedure (Pooleetal. 2008). Source photo-
Each source has been observed at least once with both instru- metric measurements were extracted from the UVOT imaging
ments, although for some targets several epochs had been ac- datausingthetooluvotmaghist(v1.1)withacircularsourceex-
quired. tractionregionthatrangedfrom3′.′5-5′′ radiusto maximisethe
1 flaggedduetoanalysisissuesbytheLATteam 2 http://www.swift.ac.uk/swift portal
A.Rau,etal.:BLLacobjectsbeyondredshift1.3-UV-to-NIRphotometryandphotometricredshiftforFermi/LATblazars 3
Table1. TargetList(firstfiverows,restinJournalonly)
Name FGLNamea α b δ b offset[′′]c E [mag]d
J2000 J2000 B−V
CRATESJ0001-0746 2FGLJ0000.9-0748 00:01:18.00 -07:46:26.9 0.2 0.03
CRATESJ0009+0628 2FGLJ0009.0+0632 00:09:03.93 06:28:21.3 1.1 0.07
CRATESJ0021-2550 2FGLJ0021.6-2551 00:21:32.56 -25:50:49.1 0.8 0.02
1RXSJ002209.2-185333 2FGLJ0022.2-1853 00:22:09.26 -18:53:34.9 3.1 0.03
RXJ0035.2+1515 2FGLJ0035.2+1515 00:35:14.71 15:15:04.2 3.4 0.07
.. .. .. .. .. ..
a:Fermi/LATID.
b:GRONDcoordinatesoftheopticalcounterpartwithtypicaluncertaintiesof0′.′3inbothdirections.
c:OffsetofcounterpartlocationfromthosegiveninAckermannetal.(2011).
d:FromSchlegeletal.(1998).
signal-to-noise. In order to remain compatible with the effec-
30
tiveareacalibrations,whicharebasedon5”aperturephotome-
try(Pooleetal.2008),anaperturecorrectionwasappliedwhere
25
necessary. This correction was at maximum 5–6% of the flux,
dependingonthefilter. Forthe furtheranalysis, allmagnitudes
s 20
wereconvertedintotheABsystemandarepresentedinTab.2. e
c
Typicallyachieved3-σlimitingmagnitudesareuvw2AB ≈ 22.4 ur
andTbhAeB ≈GR19O.6N.D data (Tab. 3) were reduced and analyzed of so 15
with the standardtoolsand methodsdescribed in Kru¨hleretal. # 10
(2008). The g′,r′,i′,z′ photometry was obtained using point-
spread-function(PSF)fittingandcalibratedagainstobservations 5
of fields covered by the SDSS Data Release 8 (Aiharaetal.
0
2011). Due to the undersampled PSF in the near-infrared,
10 12 14 16 18 20 22 24
the J,H,K photometry was measured from apertures with
s r’-band Brightness [mag]
the sizes corresponding to the Full-Width at Half Maximum
(FWHM) of field stars and calibrated against selected 2MASS
Fig.2.ObservedGRONDr′-bandmagnitudedistributionofthe
stars (Skrutskieetal. 2006). This resulted in 1σ accuracies of
opticalcounterparts.
0.04mag(g′,z′),0.03mag(r′,i′),0.05mag(J,H),and0.07mag
(K )fortheabsolutecalibration.WhiletheSDSScalibrationdi-
s
rectlyprovidesmagnitudesin the ABsystem, the near-IRpho-
tometryrequiredfurthertransformationfromthe2MASS-native offsetsforthe103sourceswithidentificationaregiveninTab.1.
Vega system into AB. Typically achieved 3-σ limiting magni- Most of the identified optical sources are bright and point-like
tudesarer′ ≈23.5andK ≈19.8. with typical magnitudes in the GROND r′-band of 16–18mag
AB s,AB
CorrectionforGalacticforegroundextinctionwasperformed (Figure2).
followingtheproceduredescribedinCardellietal.(1989)with The source morphology can be used as a prior for the se-
E fromSchlegeletal.(1998).FortheUVOTbands,thecor- lection of the SED template library(see Sect. 4.4) and redshift
B−V
rection factors presented in Kataokaetal. (2008) were used. range, i.e., clearly extended sources will have a contribution
UncertaintiesinE (10%;Schlegeletal.1998)andinthered- fromthehostgalaxyemissiontotheobservedSEDandarelikely
B−V
deninglawproduceanadditionalcontributiontothephotometric located at low redshift. A simple morphological distinction in
errorbudget.Asthissystematicuncertaintyiscoupledbetween point-likeandextendedhasbeenperformedontheGRONDim-
thethephotometricbandsforagivenpositioninthesky,itsim- ages and will be used for discussing the reliability of the zphot
pact on the SED fitting is smaller than the contribution to the estimates.
photometryineachindividualfilter.Anexactcalculationisvery
complex. Thus, we adopt a conservative 5% of the reddening
4.3.Variabilitycorrection
value in each band which is added in quadrature to the photo-
metricuncertainties. The optical emission of blazars is known to vary on a wide
range of time scales. Brightness variations range from a few
tenths of a magnitude within minutes to days (e.g., Racine
4.2.Counterpartselectionandmorphology
1970; Milleretal. 1989; Carinietal. 1991; Urryetal. 1993)
Astrometricsolutionsfor the GROND opticalbandshave been to several magnitudes over weeks to years (e.g., Ciprinietal.
obtained throughcomparison with USNO or, if available, with 2003; Raiterietal. 2005). This variability contributes signifi-
SDSSmeasurements,achievingatypicalrmsof0′.′3inbothco- cantlytotheuncertaintieswhenconstructinganSEDfromnon-
ordinates.Forthelargemajorityofourtargets(91)onlyasingle simultaneousmulti-band,multi-instrumentobservations.
opticalsourcewasfoundwithin2′′ radiusofthelocationgiven AsGRONDobservesinallsevenbandsatthesametime,the
in the 2LAC catalog. For an additional twelve objects isolated g′toK photometrycanbeconsideredasasnapshotoftheSED
s
candidate counterparts were detected at offsets of up to 6′′. In and is thusunaffectedby variabilityon time scales longerthan
one case (CGRaBS 1407-4302),several blended sources made the exposure of an individual observation. This leaves two ar-
areliableidentificationimpossible.Counterpartcoordinatesand easwhereapropertreatmentofvariabilityhastobeperformed,
4 A.Rau,etal.:BLLacobjectsbeyondredshift1.3-UV-to-NIRphotometryandphotometricredshiftforFermi/LATblazars
Table2. Swift/UVOTphotometry(firstfiverows,restinJournalonly)
Name UTDatea ABMagnitudeb ∆m c
GR→UV
uvw2 uvm2 uvw1 u b v [mag]
CRATESJ0001... 2010/10/1410:40 18.99±0.08 18.74±0.10 18.58±0.09 18.18±0.09 17.88±0.10 17.90±0.19 0.01
CRATESJ0009... 2010/12/0708:55 20.16±0.15 19.71±0.19 19.69±0.18 19.24±0.19 18.51±0.18 18.14±0.24 -0.24
CRATESJ0021... 2010/11/1818:32 18.86±0.08 18.90±0.27 18.57±0.09 18.15±0.10 17.82±0.11 17.93±0.21 0.09
1RXSJ002209.2... 2010/11/1813:49 18.36±0.07 17.95±0.08 17.91±0.08 17.48±0.07 17.20±0.08 16.97±0.12 -0.08
RXJ0035.2... 2011/01/1401:08 18.36±0.08 17.95±0.08 17.91±0.08 17.48±0.07 17.20±0.08 16.97±0.12 -0.24
.. .. .. .. .. .. .. .. ..
a:Approximatestarttimeofuvw2exposure.
b:CorrrectedforGalacticforegroundreddening.Upperlimitsare3-σ.
c:Variability-correctionfactortobeappliedtoGRONDphotometry(seetext).
Table3.GRONDphotometry(firstfiverows,restinJournalonly)
Name UTDatea ABMagnitudeb
g′ r′ i′ z′ J H K
s
CRATESJ0001... 2010/10/1403:33 17.80±0.05 17.43±0.05 17.18±0.05 16.86±0.05 16.48±0.05 16.16±0.06 15.76±0.08
CRATESJ0009... 2010/12/1200:41 18.91±0.05 18.49±0.05 18.15±0.05 17.89±0.06 17.34±0.06 16.90±0.06 16.46±0.08
CRATESJ0021... 2010/11/2300:13 17.69±0.05 17.43±0.05 17.20±0.05 16.98±0.05 16.76±0.05 16.51±0.05 16.15±0.07
1RXSJ002209.2... 2010/11/2300:21 17.30±0.05 16.92±0.05 16.71±0.05 16.51±0.05 16.22±0.05 16.00±0.06 15.57±0.08
RXJ0035.2... 2011/01/1400:48 17.39±0.05 17.10±0.05 16.96±0.05 16.77±0.05 16.41±0.06 16.17±0.05 15.92±0.08
.. .. .. .. .. .. .. .. ..
a:Exposurestarttime.
b:CorrrectedforGalacticforegroundreddening.Notcorrectedforvariability.Upperlimitsare3-σ.
i) between the exposuresin the individualUVOTfilters and ii)
betweentheGRONDandUVOTpointings.
0.6
UVOT observes in each band separately and, for our pro-
gram, typically cycled through its filters in a specific order
(uvw1,u,b,uvw2,v,uvm2).Acompletesequenceinallsixbands
took≈ 12min,duringwhichtimethetargetmayvaryinbright- 2 0.4
w
ness.Withoutaprioriknowledgeofthespectralshape,thisvari- uv
g
abilitycanonlybeaccountedforstatistically, i.e.,byincluding a
m
an additional contribution to the systematic uncertainties. For ∆ 0.2
this purpose we analyzed the photometry for those blazars for
whichmultipleUVOTexposuresinasinglebandwereavailable.
Here, we focused on the bluest filter (uvw2) which would be
0.0
leastaffectedbyapotentialhostgalaxycontributionandshould
thus give a good representationof the maximum variability.In
0.1 1.0 10.0 100.0
Fig.3thedistributionofvariabilityasfunctionoftimebetween
∆t [days]
twouvw2exposuresisshownfor25targetswith accuratepho-
tometry(δm < 0.1mag)ineachimage.Whilevariabilityas
uvw2 Fig.3. Absolute brightness changes in the uvw2 filter as func-
largeas∆m ≈0.4magcanbeseenforsomesourcesalsoon
uvw2 tion of time between two UVOT pointings for all 25 sources
timescalesof∆t < 0.1d,themedianvariationfor∆t < 50dis
withmultipleobservations.Theblackcrossesshowthemedian
≈0.1mag,comparabletothephotometricuncertainty.
values(andmedian absolutedeviation)and the greydots mark
The shortest time sampled between two uvw2 exposuresin
all174pairsofobservations.Whilemicro-variability(∆m ≈
thissampleisonesatelliteorbit(≈90min)andthereforelonger uvw2
0.1mag) dominates on time scales shorter than 50d, signif-
than it takes to cycle throughthe six filters. However,it is rea-
icantly larger variability (∆m > 0.3mag) is observed at
sonable to assume that the median variation on a 12min time uvw2
∆t>50d.
scaleisnotmuchlargerthantheoneover96min.Thus,wecon-
servativelyadopted∆m = 0.1magasanadditionalcontribution
tothesystematicuncertaintiesforallUVOTfilters. tionoffset.Hereweusedthecalibrationwiththelargestspectral
In order to correct between GROND and UVOT we made overlap,namelytheonebetweenb,g′,andr′:
use of the spectral overlap provided by the two instruments
(Kru¨hleretal. 2011). Under the assumption that the SED re- b−g′ =0.17(g′−r′)+0.03(g′−r′)2 (1)
mainsunchangedandcanbeapproximatedbyapowerlawand
thus,thattheGRONDandb,vphotometryfollowthesamespec- whichis validfor−1 ≤ (g′−r′) ≤ 2 (Kru¨hleretal. 2011).
tral slope 3, color-terms can be used to derive the normaliza- Thesecorrections,∆magGR→UV (Tab2),werecalculatedforall
sourceswithaccuratephotometryinb,g′,andr′(δm<0.2mag).
3 This isreasonable for redshift z < 3.5and for sources where the Theywerecomputedforeachobjectindividuallyandappliedto
hostgalaxycontributiontotheemissionisnegligible.SeeSect.4.4for theGRONDphotometry,shiftingitinlinewithUVOT.Typically
furtherdiscussion. values are of the order of ±0.1mag (Fig. 4). In cases where
A.Rau,etal.:BLLacobjectsbeyondredshift1.3-UV-to-NIRphotometryandphotometricredshiftforFermi/LATblazars 5
models intrinsic extinction was again neglected, a different lu-
1.0 minositypriorof−8 < Mg′ < −30waschosentoallowforthe
expectedfainterhostmagnitudes.Thethirdlibraryconsistsofa
OT 0.8 widerangeofstellartemplates(Pickles1998;Bohlinetal.1995;
V Chabrieretal.2000),includedtotestagainstpotentialfalseas-
U
→ sociations.Foreachsourceinoursample,allthreelibrarieswere
D 0.6
ON fitindependently.
R
G The most prominent spectral feature used to measure z
g 0.4 phot
a forblazarsiscausedbytheabsorptionthroughneutralhydrogen
m
∆ along the line of sight. At z ≈ 0.8 the Lyman-limitmovesinto
0.2 thewavelengthcoveredbytheUVOTandstartstosuppressthe
fluxintheuvw2filter.Whilethisisthetheoreticallowerlimiton
0.0 z forthecombinedGRONDandUVOTcoverage,uncertain-
phot
tiesassociatedwiththephotometryandmodelingwillshiftitto
0.1 1.0 10.0 100.0
a higherredshift. Usually one would use a spectroscopic train-
∆t [days]
ingsampletoevaluatetheredshiftrangeandaccuracywhichis
accessible with our SEDs. As the majority of the sources with
Fig.4.Absolutebrightnessoffsetasfunctionoftimebetweenthe
known redshift in our sample are located at z < 1, they alone
GRONDandSwift/UVOTobservations.Theblackcrossesshow
do not suffice to provide a statistically meaningful test of the
themedianvalues(andmedianabsolutedeviation)andthegrey
method.Insteadweassessthereliabilityinourphotometricred-
dots mark all 90 sources for which the offset was determined
shiftcomputationwithaMonte-Carloapproach.
(excludingelevensourceswithlargephotometricuncertainties,
Here,wesimulated27,000testpower-lawSEDswithspec-
missing b, g′, or r′ photometry,or strong hostgalaxycontribu-
tral slopes ranging from 0.5 to 2, redshifts from 0 to 4, and at
tion). threeapparentmagnitudes,r′ =17,18,19mag.Foreachsource,
thephotometryintheindividualbandswasscatteredaroundthe
model magnitudes with a brightness-dependentstatistical con-
multiple epochs were available, we used the closest pair of tributionandwithacomponentrepresentingthesystematicun-
GRONDandUVOTobservations.ThefinalSEDforeachsource certaintiesfromthephotometriccalibrationandvariabilitycor-
corresponds to the best possible reconstruction between 160– rection.TheresultingSEDswerefedbackintoLePhareandthe
2200nm at the time of the UVOT observation. No correction outputz wascomparedtotheinputvalue.
phot
was performed for sources where the photometric uncertainty AscanbeseeninFig.5,thereisagoodcorrespondencebe-
(δm ≥ 0.2mag in b,g′, and/or r′) was larger than the typical tweentheinputandtherecoveredredshiftforz >1.2.Inpar-
sim
∆magGR→UV. ticularforbrightsources(r′ <18mag)thenumberofoutliersη,
For a few sources, the emission from the underlying definedasthenumberofsourceswith|∆z(1+z )|>0.15,drops
sim
host galaxy contributes significantly to the overall SED (see steeplyabovethisredshift.Forfaintersources(r′ ≈19mag),the
Sect.4.4).Whiletheblazaremissioncanstillvary,thehostcon- increasinglyless constrainedUV photometrycausesthe outlier
tributionnaturallyremainsunchanged,violatingtheassumption fractiontoremainhigh(≈ 20%)outtoz ≈ 2.Amorequan-
sim
ofaconstantSEDslope.Thus,novariabilitycorrectionwasper- titativeselectioncanbemadewhenusingP ,theintegralofthe
z
formedinthosecaseswheretheUVtoopticalSEDclearlyde- probabilitydistributionfunctionR f(z)dzatzphot±0.1(1+zphot),
viatesfromapowerlaw. which describes the probability that the redshift of a source is
within 0.1(1+z) of the best fit value. When choosing a cut at
P > 90%,themajorityoftheoutliersatz > 1.2,aswellas
4.4.SEDfitting z phot
nearlyallthesolutionswithz < 1.2disappear(insetFig.5).
phot
The photometric redshift computation was performed with the Thisconfimsthatforpower-lawsourceswithz < 1.2theavail-
publically available LePhare code v.2.2 (Arnoutsetal. 1999; ablephotometrycanonlyplaceanupperlimit,andsuggeststhat
Ilbertetal.2006).Theprogramusesthesimpleχ2fittingmethod P > 90%canbeusedasa criteriontoidentifyreliablephoto-
z
to differentiate between a theoretical and an observed photo- metric redshift solutions. Selecting a higher P threshold, e.g.,
z
metric catalogue. We used three custom template libraries for 99%, can further reduce the outlier fraction but will also shift
comparisonwithourGRONDandUVOTphotometry.Thefirst thelowerconstrainableredshiftboundtohighervalues.Inorder
was composed of 40 power law SEDs of the form F ∝ λ−β toreachsuchaverytightlypeakedredshiftprobabilitydistribu-
λ
with β ranging from 0 to 2 in steps of 0.05 and describes pri- tionfunctionmorethanonephotometricbandhastobeaffected
marily the central-engine dominated blazars. Here, the source- bytheLyman-limit,limitingtheredshiftrangetoz&1.5.
intrinsic extinction was assumed to be zero and the luminos-
ity prior was choosen to cover the expected range of absolute
5. Results
magnitudes (−20 < M < −30; Ve´ron-Cetty&Ve´ron 2010).
g′
SuchasimplemodelisagoodapproximationfortheUV-near- The fit results for all 103 sources with identified counterparts
IRSEDsofBLLacs,whichformthebulkofoursample.Also arepresentedinTab.4. Herewe givethephotometricredshifts
FSRQs, if their emission lines are not dominant, can be mod- (and their 90% confidence errors), the P values, and the best
z
eled with a power law to first order. The second library con- fit models for the power law and galaxy templates. No source
tains templates of normal (inactive) galaxies and galaxy/AGN requiresastellartemplate.
hybrids(Salvatoetal.2009,2011)andhasbeenimplementedin Asdiscussedintheprevioussection,P >90%canbeused
z
order to model host galaxydominated objects. If the host con- asa goodreliabilitycriterionforthe photometricredshiftsolu-
tributionissignificant,the4000Åbreakcanemergeandpoten- tion. Applying this cut to the power law model fits results in
tially be misinterpretedasa Lyman-limit.While forthe galaxy a sample of 15 sources with χ2 < 30. Next, we also require
6 A.Rau,etal.:BLLacobjectsbeyondredshift1.3-UV-to-NIRphotometryandphotometricredshiftforFermi/LATblazars
Table4.Excerptoftheresultstable.Onlyentriesforthosesourceswithreliableredshiftsofz > 1.2areshown.(restinJournal
phot
only)
Name z a z b powerlaw galaxy
phot,best spec,img
z c χ2 P d βe z c χ2 P c model
phot z phot z
RXJ0035.2+1515 1.28+0.14 – 1.28+0.14 2.6 90.1 0.90 0.79+0.14 8.7 75.5 I22491 60 TQSO1 40
−0.17 −0.17 −0.13
PKS0047+023 1.44+0.16 – 1.44+0.16 10.8 94.9 1.60 0.41+0.06 13.6 80.5 pl QSO DR2 029 t0
−0.19 −0.19 −0.08
PKS0055-328 1.37+0.14 – 1.37+0.14 17.7 95.2 1.25 0.47+0.23 11.2 38.4 I22491 50 TQSO1 50
−0.17 −0.17 −0.47
PKS0332-403 1.47+0.11 1.426 1.47+0.11 6.0 99.8 1.35 1.11+0.10 13.0 62.3 I22491 60 TQSO1 40
−0.12 −0.12 −0.33
CRATESJ0402-2615 1.92+0.12 – 1.92+0.12 8.6 100.0 1.25 1.28+0.22 9.5 48.9 I22491 60 TQSO1 40
−0.09 −0.09 −0.23
SUMSSJ053748571828 1.55+0.09 – 1.55+0.09 20.5 99.9 0.80 1.58+0.08 21.5 99.6 pl I22491 30 TQSO1 70
−0.13 −0.13 −0.07
PKS0600-749 1.54+0.14 – 1.54+0.14 6.9 98.1 1.20 0.46+0.24 7.5 58.1 I22491 60 TQSO1 40
−0.19 −0.19 −0.05
CRATESJ0630-2406 1.60+0.10 – 1.60+0.10 9.3 100.0 0.85 1.19+0.50 24.6 83.7 I22491 60 TQSO1 40
−0.05 −0.05 −0.12
OM235 1.72+0.13 1.549 1.72+0.13 18.2 99.9 1.10 1.54+0.10 11.9 94.9 pl I22491 30 TQSO1 70
−0.13 −0.13 −0.09
CRATESJ1312-2156 1.77+0.09 1.491 1.77+0.09 7.1 100.0 0.95 1.60+0.14 18.4 100.0 I22491 60 TQSO1 40
−0.11 −0.11 −0.09
CLASSJ2352+1749 1.45+0.21 – 1.45+0.21 7.5 91.8 1.15 0.55+0.16 7.1 64.2 pl I22491 20 TQSO1 80
−0.18 −0.18 −0.14
a:Bestphotometricredshift,seetext.
b:Spectroscopicorimagingredshift(ifknown)fromAckermannetal.(2011).
c:Photometricredshiftwith90%confidenceuncertainties.
d:Redshiftprobabilitydensityatz ±0.1(1+z ).
phot phot
e:Spectralslopeforpowerlawmodeloftheformλ−β.
f: Starburst/QSO hybrid templates with varying contributions of the two components. Template names reflect the relative contribution of the
starburst(I22491 NN)andAGN(TQSO1 NN)modelstothehybrid.“pl”marksanadditionalpower-lawcomponent atshortwavelengths.See
Salvatoetal.(2011)fordetailsonthetemplates.
thatasourcecannotbefitwellwithagalaxy/AGN-hybridtem- viously been classified as BL Lac objects and are therefore
plate at low redshift (z < 1.2). This step is required in order unlikely to show strong emission lines in their optical spec-
to identify those sources for which a degeneracy in the pho- tra. Given these considerations, we do not regard their galaxy-
tometric redshift solution does not allow a distinction between templatephotometricredshiftstobereliable.
a low-redshift galaxy/AGN-hybrid and a blazar at higher red- There are 79 sources which are well-fit by power-law tem-
shift.Inanalogywithourselectionforagoodredshiftconstraint plates (χ2 < 30) but have a low P . In other words, the spec-
z
(Pz > 90%), we consider Pz,gal < 90% as a reliable criterion tral slope is well defined but the photometric redshift cannot
that no adequatelow-redshiftgalaxy/AGN-hybridsolution was be unambiguously determined. As expected, the best solutions
found.Thisremovesfurthersevensourcesfromthesampleand are predominantly at low redshift (z < 1.2) with typical red-
leaves eight candidates, all unresolved in the GROND images shift probability distributions that are flat down to z = 0. For
andwithzphot,pl >1.2. thosewegivethe90%upperlimitofthephotometricredshift..
Forthreeadditionalsourcesthepowerlawandgalaxymod- Thissub-samplealsoincludeselevensourceswithapreviously
els fit similarly well (P ∼ P > 90%) and give compara- knownredshiftfromspectroscopyorimaging,allin agreement
z,pl z,gal
blehigh-redshiftsolutions,indicatingthattheLyman-limitwas withourlimits.
fit by power law and galaxy templates alike. Including those, We also provide the 90% upper limit for four sources for
weendupwithasampleofelevensourceswithreliablephoto- which the power-law and galaxy libraries give similarly good
metricredshiftcomputation,all,withz rangingfromz ≈ constraints (P ≈P > 90%) but with very different red-
phot phot z,pl z,gal
1.28 to z ≈ 1.92. This encompasses eight sources without shift solutions. This degeneracy can occur, e.g., when a break
phot
previous redshift measurements and three blazars with known in the SED issimilarly wellfitby theLymanlimitof a power-
redshift. For the latter, the photometric redshifts are within 3- lawmodelandwiththe4000Å breakofagalaxytemplate.No
σ of the known value (see Fig. 5), indicating an accuracy of z is given for ten sources for which no satisfactory fit
phot,best
∆z/(1+zspec)<0.12forourzphotmethoddescribedinSect.4.4. (χ2 >30)wasobtained.
ThefitresultsandSEDsfortheelevensourcesarepresentedin
Tab.4andFig.6,respectively.
For21sources,thegalaxylibraryprovidesthebetterfitwith
6. Discussion&Conclusion
P > 90%, P < 90%, and χ2 < 30. All of these are
z,gal z,pl gal
best described by starburst/QSO hybrid templates at z < 1.3 In this paper we presented redshift constraints for 103 blazars
andnonerequireanellipticalgalaxy,i.e.,astrong4000Åbreak. from the 2LAC catalog using UV-to-near-IR multi-band pho-
The most significant difference between the starburst/QSO hy- tometry obtained quasi-simultaneously with Swift/UVOT and
bridtemplatesandthe power-lawmodelsisthattheformerex- GROND. We provided the first reliable redshift measurements
hibitstrongemission linesoriginatingfromstar-formation(see foreightsourcesandnewupperlimitsforanadditional66tar-
Salvatoetal. 2009, for more details on the templates). These gets. Of the eight sources with reliable redshift, seven are lo-
lines can be fit to small deviations in the photometry from a catedatz > 1.3.Six of thosebelongto the BL Lac popula-
phot
power-lawandthusprovidelowχ2 valuesandapparentlywell- tion.Forcomparison,outofthe total395BL Lac in the2LAC
constrained z solutions. However, as described in Sect. 4.3, sample only two sources have previously been known to lie at
phot
the variability correction is not reliable when applied to non- z > 1.3 (Fig. 7). Redshifts for these two, PKS 0332-403 and
power-law sources, as the color terms, and thus the correction CRATES J1312-2156,have also been confirmedwith our pho-
factors, will be erroneous. Also, 18 of these sources have pre- tometricobservations.
A.Rau,etal.:BLLacobjectsbeyondredshift1.3-UV-to-NIRphotometryandphotometricredshiftforFermi/LATblazars 7
4
r’=17mag
3 rr’’==1198mmaagg 8 z<1.2
z=1.28
hot 2
zp 4 6 z=1.37
P > 90%
Z
3
y)
1 zphot2 µJ z=1.44
x (
1 g Flu µ(arbitrary) log Flux (Jy)4 z=1.45
0 00 1 2 3 4 o z=1.47
00..440 1 2 3 4 y) l
z)z)simsim 00..22 bitrar z=1.54
++ 00..00 ar z=1.55
(1(1 ( 2
∆∆z/z/ --00..22 z=1.60
--00..44 z=1.72
00 11 22 33 44
z=1.77
%) 0
10 z=1.92
(
η
0.1 1.0
Wavelength (µm)
1
0 1 2 3 4
z Fig.6.SEDs forthe elevensourcesforwhichphotometricred-
sim
shifts larger than 1.2 were estimated (see Tab. 4) and one ex-
ample for z < 1.2. From bottom to top: CRATES J0402-
Fig.5.Recoveredbest-fitphotometricvsinputredshift(top),ac- phot
2615, CRATES J1312-2156, OM 235, CRATES J0630-
curacy (middle), and outlier fraction (bottom) for 27,000 sim-
ulated sources with spectral slope of 0.05 ≤ β ≤ 2 and op- 2406, SUMSS J053748-571828, PKS 0600-749, PKS 0332-
tical brightness of r′ = 17,18,19mag. The pink circles indi- 403, CLASS J2352+1749, PKS 0047+023, PKS 0055-328,
RXJ0035.2+1515,andBZBJ0543-5532.
cate the location of three blazars in our sample with known
redshift, PKS 0332-403 (z = 1.351, Bergeronetal. 2011),
spec
OM 235 (z = 1.549), and CRATES J1312-2156 (z =
spec spec
troscopyreliesonthedetectionofveryfaintemissionfeatures,
1.491, Ackermannetal. 2011), as well as that of PKS 0537-
286 (z = 3.104, Wrightetal. 1978), a LAT-non-detected mainly from the underlying host galaxy. It therefore requires
spec
very high signal-to-noise, often at the cost of long exposure
blazarwithavailableGRONDandUVOTphotometry.Thegreen
times. On the other hand, the photometric redshift method for
areainthemiddlepanelcorrespondstotheforbiddenparameter
space where z > 0. Systematics are visible at z < 0.8.
phot phot
The large majority of the fits have χ2 < 30 (for typically 10
d.o.f.)with the exceptionsshownin lightgrey.The insetin the
1144
top paneldisplays the results if only solutions with P > 90%
z
areselected. 1122 2LAC redshift sample
1100
c
a
The six new BL Lac redshifts at z > 1.3 represent a dra- L
L 88
matic(fromtwotoeight)increaseoftheconfirmedhigh-zFermi B
sampleandthusdemonstratetheopportunitythattheSEDtem- of 66 this paper
platefittingtechniqueholdsforobtainingphotometricredshifts #
44
for BL Lac sources. For our sample, this was possible due to
thedensely-coveredwidespectralrange(160–2200nm),neces-
22
saryfora reliableconstraintof thespectralslope,the excellent
ultra-violetcoveragetomeasuretheLyman-limit,andthequasi- 00
simultaneity of the observations, important for minimizing the 00..55 11..00 11..55 22..00
impactofsource-intrinsicvariability.Themethodappliedinthis z
workovercomessignificantchallengesinherentinotherredshift
techniques,namelythesimplicityoftheopticalemissionofBL Fig.7.RedshiftdistributionforthenineBLLacobjectswithre-
Lacs manifested as the power-law-shapedsynchrotron spectral liablephotometricredshifts(bluefilledhistogram)togetherwith
component. This complicates spectroscopic redshift measure- thedistributionfortheBLLacinthe2LACcatalog(emptyhis-
ments,whichareinmostcaseslimitedtotheopticalwavelength togram;Ackermannetal.2011).Sourceswithz<0.5havebeen
regime and thus insensitive to the Lyman-limit. Instead, spec- omittedforclarity.
8 A.Rau,etal.:BLLacobjectsbeyondredshift1.3-UV-to-NIRphotometryandphotometricredshiftforFermi/LATblazars
BLLacdoesnotrequiresignificanttelescopetime,inparticular Fossati, G., Maraschi, L., Celotti, A., Comastri, A., & Ghisellini, G. 1998,
whendatafromefficientmulti-channelinstrumentslikeGROND MNRAS,299,433
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Ghisellini, G., Celotti, A., Fossati, G., Maraschi, L., & Comastri, A. 1998,
on the detection of a particular spectral feature, which, albeit
MNRAS,301,451
strong,islocatedintherest-framefar-ultra-violet.Forz.3,this Giommi, P., Padovani, P., Polenta, G., et al. 2011, MNRAS, in press,
is only accessible with space-based ultra-violettelescopes, and arXiv:1110.4706
even then usually limited to redshifts above z ≈ 1.2, as shown Greiner,J.,Bornemann,W.,Clemens,C.,etal.2008,PASP,120,405
Hoffmeister,C.1929,AstronomischeNachrichten,236,233
above.
Ilbert,O.,Arnouts,S.,McCracken,H.J.,etal.2006,A&A,457,841
The sensitivity of this method to high-redshiftsources also Kataoka,J.,Madejski,G.,Sikora,M.,etal.2008,ApJ,672,787
allowstheplacementofupperlimitsforthoseSEDsthatdonot Kru¨hler,T.,Ku¨pcu¨Yoldas¸,A.,Greiner,J.,etal.2008,ApJ,685,376
show the imprint of the Lyman-limit. Only for three targets is Kru¨hler,T.,Schady,P.,Greiner,J.,etal.2011,A&A,526,A153
thisboundaryabovez = 2.In onecase, CRATESJ0250+1708 Meisner,A.M.&Romani,R.W.2010,ApJ,712,14
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(z < 3.1), the optical counterpart is too faint (r′ ≈ 20.7mag)
Pickles,A.J.1998,PASP,110,863
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AB
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120,95
0625 (z < 2.46) shows a significant break at z ∼ 1.9 ± 0.5. Salvato,M.,Hasinger,G.,Ilbert,O.,etal.2009,ApJ,690,1250
However, as the redshift probability distribution is broad, and Salvato,M.,Ilbert,O.,Hasinger,G.,etal.2011,arXiv:1108:6061
the χ2 of the powerlaw fit comparableto thatof a z ≈ 0.4 so- Sbarufatti,B.,Treves,A.,&Falomo,R.2005,ApJ,635,173
Schlegel,D.J.,Finkbeiner,D.P.,&Davis,M.1998,ApJ,500,525
lutionforagalaxytemplate,thephotometricredshiftisconsid-
Shaw,M.S.,Romani,R.W.,Healey,S.E.,etal.2009,ApJ,704,477
eredtobeunreliable.Exceptforthosethree,noothersourcein
Skrutskie,M.F.,Cutri,R.M.,Stiening,R.,etal.2006,AJ,131,1163
our sample has a best-fit photometric redshift at z > 2. Thus, Urry,C.M.,Maraschi,L.,Edelson,R.,etal.1993,ApJ,411,614
CRATES J0402-2615 can now be considered the most distant Ve´ron-Cetty,M.-P.&Ve´ron,P.2010,A&A,518,A10
knownBLLacwithameasuredredshiftofz≈1.92. Wright,A.E.,Peterson,B.A.,Jauncey,D.L.,&Condon,J.J.1978,ApJ,226,
L61
Fig.7indicatesthatourhigh-redshiftfindingsarethenatural
extensionoftheexisting2LACredshiftsample.However,dueto
theincompletenessesofbothsamples,werefrainfromdrawing
anyquantitativeconclusionsatthisstage.Wenote,however,the
our result is generalagreementwith the theoreticalpredictions
fromGiommietal.(2011).Amoredetailedphysicalinterpreta-
tion of the establishmentof an increased fractionof high-zBL
Lacsinthe2LACsamplewillbereportedseparately.
Acknowledgements. Wethank the referee for the valuable comments. Part of
thefundingforGROND(bothhardwareaswellaspersonnel)wasgenerously
grantedfromtheLeibniz-PrizetoProf.G.Hasinger(DFGgrantHA1850/28-1).
MSacknowledges supportbytheGermanDeutsche Forschungsgemeinschaft,
DFGLeibnizPrize(FKZHA1850/28-1).TKacknowledgessupportbytheDFG
cluster of excellence Origin and Structure of the Universe, by the European
Commission under the Marie Curie Intra-European Fellowship Programme),
as well as the DARK: The Dark Cosmology Centre, funded by the Danish
NationalResearchFoundation.FOEacknowledgesfundingofhisPh.D.through
theDeutscher AkademischerAustausch-Dienst (DAAD).SK,DAKandANG
acknowledgesupportbyDFGgrantKl766/16-1.ARossiacknowledgessupport
fromtheBLANCEFLORBoncompagni-Ludovisi,ne´Bildtfoundation.MNac-
knowledgessupportbyDFGgrantSA2001/2-1.PSacknowledges supportby
DFGgrantSA2001/1-1.ACU,ANG,DAKandARossiaregratefulfortravel
fundingsupportthroughMPE.Wealsoacknowledge theuseoftheTOPCAT
tool(Taylor2005).
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