Exploring the Active Galactic Nuclei population with extreme X-ray to optical flux ratios (Fx/Fo >50)

1.1 MB·English

by  R. Della Ceca

Mon.Not.R.Astron.Soc.000,1–16(0000) PrintedJanuary8,2015 (MNLATEXstylefilev2.2)
Exploring the Active Galactic Nuclei population with extreme X-ray
to optical flux ratios ( f /f > 50)
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R. Della Ceca1⋆, F.J. Carrera2, A. Caccianiga1, P. Severgnini1, L. Ballo1, V. Braito1,
5
1 A. Corral3, A. Del Moro4, S. Mateos2, A. Ruiz2, and M.G. Watson5
0
1INAF-OsservatorioAstronomicodiBrera,viaBrera28,20121Milano,Italy
2
2InstitutodeFisicadeCantabria(CSIC-UC),AvenidadelosCastros,E-39005Santander,Spain

n 3NationalObservatoryofAthens(NOA),PalaiaPenteli,GR-15236Athens,Greece
a 4DepartmentofPhysics,DurhamUniversity,SouthRoad,DurhamDH13LE,UK
J 5DepartmentofPhysics&Astronomy,UniversityofLeicester,Leicester,LE17HR,UK

7

]
A
G
ABSTRACT
.
h
p Thecosmichistoryofthegrowthofsupermassiveblackholesingalacticcentersparallels
- thatofstar–formationintheUniverse.However,animportantfractionofthisgrowthoccurs
o inconspicuouslyinobscuredobjects,whereultraviolet/optical/near-infraredemissionisheav-
r
t ily obscured by dust. Since the X–ray flux is less attenuated, a high X-ray–to–optical flux
s ratio( f /f )isexpectedtobeanefficienttooltofindouttheseobscuredaccretingsources.
a x o
We explore here via optical spectroscopy, X-ray spectroscopy and infrared photometry the
[
most extreme cases of this population (those with f /f > 50, EXO50 sources hereafter),
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1 using a well defined sample of 7 X-ray sources extracted from the 2XMM catalogue. Five
v EXO50sources(∼ 70percentof thesample)in the brightfluxregimeexploredby oursur-
4 vey(f > 1.5×10−13 ergcm−2 s−1 )areassociatedwithobscuredAGN(N > 1022
4 (2−10keV) H
cm−2),spanningaredshiftrangebetween0.75and1andcharacterisedby2-10keVintrinsic
4
luminositiesin theQSO regime(e.g.wellin excessto1044 ergs−1 ).We didnotfindcom-
1
0 pellingevidenceofComptonThickAGN.OveralltheEXO50Type2QSOsdonotseemto
. bedifferentfromstandardX-rayselectedType2QSOsintermsofnuclearabsorption;avery
1
high AGN/host galaxy ratio seems to play a major role in explaining their extreme proper-
0
ties. Interestingly3 outof 5EXO50Type 2QSO objectscan beclassified as ExtremeDust
5
ObscuredGalaxies(EDOGs, f /f > 2000),suggestingthataveryhighAGN/hostratios
1 24µm R
: (along with the large amount of dust absorption) could be the natural explanation also for
v
a part of the EDOG population. The remaining 2 EXO50 sources are classified as BL Lac
i
X objects,havingratherextremeproperties,andwhicharegoodcandidatesforTeVemission.
r Key words: galaxies: active – galaxies: neclei – BL Lacertae objects: general – Quasars:
a
general–X-rays:galaxies
1 INTRODUCTION there are now increasing evidence that the formation and growth
of galaxies and their nuclear supermassive black holes (SMBHs,
The study of high-z obscured quasars (Type 2 QSOs: the high
M > 106 M ) are intimately related; feedback from actively
luminosity counterpart of Seyfert 2 galaxies) is one of the hot BH ⊙
accreting SMBH, AGN, must play a fundamental role in regu-
topics of current extragalactic astronomy. Their observed distri-
lating both star formation and accretion throughout galaxy’s his-
butions (e.g. redshift, flux and absorption) and physical proper-
tory (e.g. Silk&Rees 1998, Granatoetal. 2004, DiMatteoetal.
ties (e.g. bolometric luminosity, black hole mass, accretion rate),
2005, Crotonetal. 2006, Hopkinsetal. 2008, Mencietal. 2008,
compared to those of unobscured QSOs, are key parameters to
King 2010, Faucher-Gigue`re&Quataert 2012). Although the na-
understand and to test the orientation-based Unified Schemes
ture of this relationship is still poorly understood, there are hints
(Antonucci 1993) for Active Galacic Nuclei (AGN) and to con-
that most of the SMBHaccretion takes place during an obscured
strainthecontributionof QSOstotheX−rayBackground (XRB:
quasarphase.Theinfrared(IR),opticalandX−rayspectralanaly-
e.g. Gillietal. 2007; Treisteretal. 2009). Even more important,
sisofalargesampleofobscuredquasarsprobablyrepresentsone
of the best methods to test a possible coevolution between mas-
sivegalaxiesandAGNactivity(seee.g.Hopkinsetal.2006)and
⋆ E-mail:[email protected]
(cid:13)c 0000RAS
2 DellaCeca et al.
toinvestigateifandhowtheAGNfeedbackcanaffectthegalaxy latedsofar(intheradio,IR,optical,andX-rayenergyranges)on
evolution(seee.g.Boweretal.2006).Indeed,inthesesourcesthe the7EXO50objectsdiscussedhere,whiletheirproperties,source
propertiesofthehostgalaxycanbestudiedusingtheoptical and bysource,arediscussedinSection4.InSection5wecomparethe
near-IR data, where the absorbed AGN is supposed to contribute broadband,fromIRtoX-rays,propertiesoftheseEXO50sources
marginally, while the AGN properties can be investigated using with other similar objects found in the literature. Summary and
theX-rayandmid-IRdatawhere,conversely,theAGNemissionis conclusionsarepresentedinSection6.Throughoutthispaper,we
dominant(seeBalloetal.2014andreferencestherein).Obscured consider the cosmological model with (H ,Ω ,Ω )=(70,0.3,0.7);
o M λ
QSOs,arareclassofobjects,arethusexpectedtohavelargeval- resultsfromotherpapershavebeenrescaledtothiscosmological
uesof theX-ray-to-optical fluxratio.Furthermore, sincethedust framework.Alltheoptical/IRmagnitudesreportedhereareinthe
extinctionincreasesintheultraviolet(UV)whiletheX-rayabsorp- Vegasystem.Unlessdifferentlyquoted,X-rayluminositiesarein-
tiongoes in theopposite direction, i.e. strongly decreasing going trinsic(i.e.unabsorbed)luminositiesintherest-frame2−10keV
towardsthehighenergies, aredshift dependence isalsoexpected energy range. In this paper we use the term Type 1 and Type 2
(∼(1+z)3.6;Fioreetal.2003).However,inspiteoftheoutstanding AGN as broad or narrow line (FWHMof the permitted emission
progress,obtainedbyusingmediumanddeepX–raysurveys(see lines< 1500 kms−1 ) AGN, irrespective totheir intrinsicX-ray
e.g.Brusaetal.2010andreferencestherein),theweakness,bothin luminosity, while weusethe termType 1QSO and Type2 QSO
theX-rayandintheopticalband,oftheselectedsourceswithhigh (or obscured QSO) as broad or narrow AGN with an intrinsic 2-
valuesoftheX-ray-to-opticalfluxratiousuallypreventsfromade- 10keVluminosityinexcessto1044 ergs−1 .Finally,errorsareat
tailedanalysisoftheindividualobjects(seee.g.Koekemoeretal. 90%confidencelevelfortheX-rayspectralparametersderivedus-
2004,Civanoetal.2005). ingXSPEC(asusuallydoneinX-rayastronomy)and68%forall
To explore the most extreme examples of obscured QSOs theotherquantities.
in the bright flux regime, we have started a project focused on
the very “high” X-ray-to-optical flux ratio, f /f (see Eq. 1),
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population, namely those sources with f /f >50 (more than
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15 times the average values of unobscured broad line AGN, e.g. 2 THEEXO50SAMPLE
Caccianigaetal. 2004,DellaCecaetal. 2004,Civanoetal.2012)
Inobscured QSOsthenuclear UV/optical emissionissuppressed
andwith f >10−13ergcm−2s−1.Inthiswayweshouldefficiently
x bydustobscuration(thusleavingonlythegalaxycomponent vis-
selectthebestcandidatestobeobscuredQSOs;atthesametime,
ible), whilst the nuclear X-ray flux (e.g. in the 2-10 keV energy
thesourcebrightnessensuresthattheopticalspectroscopicidenti-
range), even if exhibiting signatures of photoelectric absorption,
ficationcanbeachieved1 forsourceswithreasonablygoodqual-
islessattenuated (seee.g. Fioreetal.2003).Obscured QSOsare
ityX-raydata(>fewhundredcounts)tocarryoutareliableX-ray
thereforeexpectedtodisplaylargevaluesofX-raytoopticalflux
spectralanalysis.HereafterwewillcallthesesourcesEXO50–for
ratio,definedhereas:
“extremeX-raytoopticalfluxratio”.Oneexampleofsuchextreme
obscuredobjectsisXBSJ021642.3-043553with fx/fo∼200,for fx/fo= fx/(∆λ×2.15×10−9×10−0.4×R) (1)
which the presence of a Type 2 QSO at z ∼ 2 was spectroscop-
ically confirmed with VLT/FORS (Severgninietal. 2006). Using where fxreferstotheobserved2-10keVflux(correctedforGalac-
theredshiftinformationandthespectralenergydistribution(SED), ticabsorption),RistheobservedopticalmagnitudeintheRband
weestimatedastellarmassof∼1011M forthehostgalaxy,which (λ≃6410Å)and∆λ≃1568Å(seeFukugitaetal.1995).
⊙
supportsthestronglinkbetweenhigh-redshiftmassivegalaxiesand Wedefine EXO50the sources with fx/fo > 50; these area
powerfulobscuredhigh-redshiftQSOs.Inspiteoftheimportance rareclassofX-rayemittingobjects,astypicallyAGNhave fx/fo=
of obscured QSOs in the cosmological context, only less than a 1-10 (see e.g. Caccianigaetal. 2004, DellaCecaetal. 2004,
dozen of such extreme f /f sources have been found and stud- Civanoetal.2012).Forinstance,intheXMM-NewtonBrightSur-
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iedsofarwithgoodX-rayandopticaldata(seee.g.Gandhietal. vey(XBS,DellaCecaetal. 2004,Caccianigaetal.2008),acom-
2006, Severgninietal. 2006, DelMoroetal. 2009, Campisietal. pletesampleofbright(f(0.5−4.5keV) &7×10−14 ergcm−2s−1)X-ray
2009,Brusaetal.2010Brusaetal.2015,Pernaetal.2014). selectedsourcesalmostcompletelyidentified(spectroscopiciden-
Herewediscussthefirstresultsobtainedfromthisprojecton tificationlevel∼98%),EXO50objectsrepresentsonly∼ 0.5%of
asmall,butstatisticallycompleteandrepresentative,sub-sampleof thesourcepopulation.
7EXO50objectswith f > 1.5×10−13 ergcm−2 s−1 .Itisworth ToconstructoursampleofEXO50sourcesinthebrightflux
x
mentioning that other two interesting (and rare) classes of extra- regime we used one of the largest well defined and complete X-
galacticsourcesareexpectedtoshowupinthehigh f /f domain raysourcesamplederivedsofar(discussedinMateosetal.2008),
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exploredhere,namelyBLLacobjectsandhighredshift(i.e.z>0.6 basedonthe2XMMsourcecatalog2(Watsonetal.2009).
in order to have the 4000 Å break shifted at longer wavelengths Firstwehaveconsideredthefollowingselectioncriteria:
than the R band filter) clusters of galaxies. Clusters of galaxies, a) “point-like” X-ray sources (parameters EP EXTENT and
however,arenotexpectedtoberepresentedinoursamplesincewe EP EXTENT ML inthesourcecatalogue equal to0), inorder to
haverestrictedoursearchonlytothepoint-likeX-raysources,thus minimisethepresenceofclustersofgalaxieswhichisanotherpos-
stronglyminimisingtheirpossibleselection. sibleclassofhigh fx/fosources;
This paper is organized as follows: in Section 2 we discuss b) north of -10 deg declination, in order to be accessible to
thestrategy usedto defineastatisticallyrepresentative sampleof opticalinvestigationfromtheItalianTelescopioNazionaleGalileo
sourceswith f /f >50.InSection3wepresentthedataaccumu- (TNG), from the Large Binocular Telescope (LBT) and from the
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SpanishGranTelescopioCanarias(GTC);
1 AtthechosenX-rayfluxlimitof fx∼10−13 ergcm−2s−1asourcewith
fx/fo∼50(300)hasanopticalRmagnitudeof∼23(∼25). 2 Seehttp://xmmssc-www.star.le.ac.uk/Catalogue/xcat public 2XMM.html
(cid:13)c 0000RAS,MNRAS000,1–16
Fx/Fo>50 3
c)sourcesselectedinthe2-10keVenergybandwithaf
2−10keV
>10−13 ergcm−2s−1;
d)onlyserendipitoussources(e.g.sourcesthatarenotrelated
to nearby galaxies or to the XMM-Newton pointing) have been
takenintoconsideration.Wehavealsoexcludedtheserendipitous
sourcesalreadyclassifiedasnonAGN.
Startingfromasourcelistof9431sources(seeMateosetal.
2008), thesefirstselection criteriaprovide uswithalistof about
600X-rayemittingobjects.
The second step was to select the X-ray sources with
f /f >50,using:
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i)opticalsourcearchives(e.g.APM,SloanDigitalSkySurvey
-SDSS,etc);
ii) optical imaging data from archives (STScIDigitized Sky
Survey,ESOarchive,etc..);
iii)imagingdatafromourdedicatedobservingprograms(e.g.
TNGimaging).
Wehavelooked atandvisuallyinspectedalltheselectedX-
raysourcesforpossibleopticalcounterparts,usingasearchradius
of4arcsec(seeCaccianigaetal.2008),andderivedtheirRmag-
nitude.Forallthesourcesreportedanddiscussedhere(seeTable
1), the offset between the X-ray and the optical position, derived
a posteriori, is below 2.1 arcsec, fully consistent with the results >Fig1u.5r×e11.0H−1i3stoegrgracmmo−f2tsh−e1(f3x/10foodbijsetrcitbsu).tiWoneoaflstoheshsotawrtiansgfislalemdphleiswtoigthramfx
obtainedbyCaccianigaetal.(2008)fromtheanalysisoftheX-ray
the fx/fodistributionofthe7EXO50sourcesdiscussedinthispaper.
toopticaloffsetfortheX-raysourcesbelonging totheXBS.The
accumulated magnitudes, combined with the observed 2-10 keV
fluxes,havebeenusedtocomputethe f /f ratioandthustoselect
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EXO50sources.
reductionprocessusingIRAF2.14.Allimageswerede-biasedand
Inthispaper weconsider afirstcompletesampleofEXO50
sourceshaving f >1.5×10−13 ergcm−2s−1;thissampleiscom- correctedforflat-fieldeffects.Wecombinedthedifferentexposures
x
foreachsourceandtheresultingimageswerefluxcalibratedusing
posed by the 7 EXO50 objects reported in Table 1. In Figure 1
standard stars and the standard extinction curve for the observa-
we show the f /f distribution of the sample composed by all
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tory.Finally,wedidanastrometriccalibrationofeachimage;the
theserendipitous point-likesources inthecovered areahaving f
x
>1.5×10−13 ergcm−2s−1(i.e.thestartingsample,310objects).In astrometricaccuracyofourimagingistypicallybetterthan∼ 0.3
arcsec.TheRmagnitudeswereestimatedwithIRAFroutinesfol-
thesamefigureweshowhowextremethe7sourcesarewithrespect
lowingthestandardprocedureofaperturephotometrycenteredat
tothestartingsample;theEXO50sourcesdiscussedhererepresent
only∼2%ofthepopulationconsidered.Westressthataccording thesourceposition.
to the selection criteria discussed above and the optical material
atourdisposalthese7objectsaretheonlyEXO50sources,north
of-10degdeclination,with f > 1.5×10−13 ergcm−2 s−1 inthe
x
sky survey areadefined inMateosetal. (2008); thesky coverage 3.2 Opticalspectroscopyandidentification
investigatedtofindoutthese7objectsisabout60.4sqdeg.
The optical spectra for 6 of our sources were taken at GTC dur-
As detailed in the following sections, out of the 7 EXO50
ing 3 different observing runs (see Table 2) using the R500R
sources 2 are BL Lac objects, 3 are confirmed Type 2 QSO and
grism on the OSIRIS instrument 3. For the reduction we used
2remainspectroscopicallyunidentified,althoughtheirbroadband
thestandardIRAFlong-slitpackagefollowingthestandardsteps.
properties strongly suggest an obscured QSO nature. Using the
The resulting spectra (see Figure 2 middle panels, in arbitrary
JAVAsoftwareGALAXYCOUNT(Ellis&Bland-Hawthorn2007)
units) give us spectroscopic information (classification and red-
and considering the optical magnitudes of the proposed optical
shift)for4outofthe6GTCsources.Fortheremaining2sources
counterparts(seeTable1)weestimatelessthan0.3normalgalaxies
(2XMMJ100038.9+050955 and 2XMMJ135055.7+642857) the
bychanceinthetotalareacoveredfromthe7errorboxes,assuring
spectra are inconclusive and do not allow us to derive any rele-
usaboutthereliabilityofthespectroscopicidentificationproposed
vantinformation. Thesource2XMMJ121026.5+392908 isawell
here.
known object and its redshift and classification have been taken
from the literature. In total we have spectroscopically identified
(usingtheGTCdataaswellasdatafromtheliterature)5EXO50
objects;twosourcesremainstillspectroscopicallyunidentified,al-
3 OBSERVATIONSANDDATAREDUCTION
thoughthedataatotherwavelengthsdiscussedinsection4and5
3.1 Opticalimaging stronglysuggestanobscuredQSOnature.
InFigure2(leftpanels)wepresenttheopticalfindingchartsofthe
7 sources discussed here (1x1 arcmin) produced using data from
dedicatedTNGobservingruns(Rfilter)orfromtheSDSS(rfilter).
TNG rawimages werereduced following thestandard CCD 3 Seehttp://www.gtc.iac.es/instruments/osiris/osiris.php
(cid:13)c 0000RAS,MNRAS000,1–16
4 DellaCeca et al.
2XMM J022256.9-024258
67889"666:;$<"6=6:>
✂
-2°42'36.0" !
/4-5!✁ "$"!
Dec (J2000) 43'0408..00"" %&’()*+,-./0&1%23/3✁ !"(cid:0)#
6
12.0" ?)2+& !$:!
24.0" "$:
6 :
58.0s 57.0s 56.0s 2h22m55.0s @%-’AB/C4-5D
RA (J2000)
2XMM J100038.9+050955
24.0"
$9::;!"""#<=>?"’">’’
!
12.0" 2708!✁ $%!"(cid:0)#
Dec (J2000) 10'4080..00"" ()*+,-./0123)4(5626✁ ’$%%!!!"""(cid:0)(cid:0)(cid:0)&&#
$=’
+5°09'36.0" @,5.) !=’$
!
"=’
$ ’
40.0s 39.0s 38.0s 10h00m37.0s A(0*BC2D708E
RA (J2000)
2XMM J121026.5+392908
36.0" 67889!6!"6:$;<#=6=">
!
/4-5!✁ !
Dec (J2000) 2142..00"" %&’()*+,-./0&1%23/3✁ "!!"$""$$(cid:0)?!!#
29'00.0"
!$6
@)2+& !
+39°28'48.0"
"$>
"$; ! 6 ;
29.0s 28.0s 27.0s 26.0s 25.0s12h10m24.0s A%-’BC/D4-5E
RA (J2000)
Figure2. Left panels: Optical finding chart (1x1arcmin) forthe7EXO50objects reported inthis paper produced usingmaterial fromdedicated TNG
observingruns(Rfilter;sources:2XMM022256.9-024258,2XMM100038.9+050955,2XMM123204.9+215254,2XMM135055.7+642857)orfromSDSS
(rfilter;sources:2XMM121026.5+392908,2XMM121134.2+390054,2XMM143623.8+631726). Thecircleis4arcsecradiusaroundtheX-rayposition.
Middlepanels:TheopticalspectraoftheEXO50objectsobservedattheGTC.Thesolidblacklineinthespectroscopicallyidentifiedobjectsrepresentsthe
underlinegalaxycomponentusedtoreproducetheopticalspectrum,exceptfor2XMMJ121134.2+390054,forwhichweshowthegalaxycomponent(blue
line),theAGNcomponent(continuumplusbroademissionlines,redline)andthecombinedcomponent(galaxyplusAGN;blackline).Thespectrallinesused
toclassifytheobjectsarealsomarked.2XMMJ121026.5+392908isawellknownobjectanditsredshiftandclassificationhasbeentakenfromtheliterature.
Rightpanels:X-raydata(observerframe)andresidualsfortheEXO50objectsdiscussedhere.Alltheobjectsarewellfittedwithanabsorbedpower-law
model.Redfilledtriangles:pndata.Blackopencircles:MOSdata.
(cid:13)c 0000RAS,MNRAS000,1–16
Fx/Fo>50 5
2XMM J121134.2+390054
24.0"
67889!6!!#:$6;#<""=:
!
5✁
12.0" /4-! "$!
Dec (J2000) 01'4080..00"" %&’()*+,-./0&1%23/3✁ "!$""(cid:0)!#
6
+39°00'36.0" >)2+& !$=!
"$=
"$= ! 6 =
36.0s 35.0s 34.0s 33.0s12h11m32.0s ?%-’@A/B4-5C
RA (J2000)
2XMM J123204.9+215254
24.0"
67889!6#6":$;<6!=6=:
!
5✁
12.0" /4-! "$"!
Dec (J2000) 53'4080..00"" A B %&’()*+,-./0&1%23/3✁ !"(cid:0)#
6
+21°52'36.0" >)2+& !$=!
"$=
! 6 =
07.0s 06.0s 05.0s 04.0s 12h32m03.0s ?%-’@A/B4-5C
RA (J2000)
2XMM J135055.7+642857
24.0"
67889!#:"::$;<=>6?:;
!
5✁
12.0" /4-!
Dec (J2000) 29'00.0" %&’()*+,-./0&1%23/3✁ "!$""(cid:0)!#
48.0"
6
+64°28'36.0" @)2+& !$:!
"$:
! 6 :
51m00.0s 58.0s 56.0s 54.0s13h50m52.0s A%-’BC/D4-5E
RA (J2000)
2XMM J143623.8+631726
7899:!#$;7$%<=;$!>7;
48.0" 6!✁
05.! "%"!
Dec (J2000) 2346..00"" &’()*+,-./01’2&3404✁ !!""(cid:0)(cid:0)#$
7
12.0" @*3,’ !%?!
"%?
+63°17'00.0"
"%? ! 7 ?
28.0s 26.0s 24.0s 22.0s 14h36m20.0s A&.(BC0D5.6E
(cid:13)c 0000RAS,MNRAS000R,A1 –(J126000)
6 DellaCeca et al.
Table1.ThecompletesampleofEXO50objectsdiscussedinthispaper
n 2XMMName fx R fx/fo ID z NaH Γa Lx fr
10−13cgs 1022cm−2 1045cgs mJy
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
1 J022256.9-024258 3.50 22.1±0.4 72 AGN2 1.004 7.5+3.2 1.80+0.4 1.9 429±14
−2.9 −0.4
2 J100038.9+050955 1.63 23.6±0.4 138 - - >0.4b 1.9c - -
3 J121026.5+392908 36.1 19.16d±0.02 50 BLLac 0.617 0.06+0.004 2.23+0.01 6.4 19.0±0.7
−0.002 −0.01
4 J121134.2+390054e 8.2 20.77d±0.05 50 BLLac 0.890 0.19+0.04 2.21+0.07 3.7 10.6±0.6
−0.04 −0.06
5 J123204.9+215254e 10.0 23.9±0.4 1118 AGN2 0.763 3.61+1.03 1.31+0.25 1.9 -
−0.92 −0.21
6 J135055.7+642857 1.5 25.0±0.8 458 - - >0.1b 2.02+0.13 - 183.5±5.5
−0.08
7 J143623.8+631726 2.53 22.2d±0.2 55 AGN2 0.893 1.46+0.39 1.69+0.15 0.84 -
−0.24 −0.11
Columnsareasfollows:1)NumberusedtomarktheobjectintheplotsshowninSection5;2)sourcename;3)X-rayfluxinthe2-10keVenergyband(MOS
normalisation)correctedforGalacticabsorptioninunitsof10−13 ergcm−2s−1;4)Rbandmagnitude;5)X-raytoopticalfluxratio;6)opticalspectroscopic
classification;7)redshift;8)intrinsicabsorptionand90%confidenceintervalinunitsof1022cm−2fromfitstoX-rayspectra;9)powerlawphotonindex,Γ,
intheX-rayenergyrange,and90%confidenceinterval;10)intrinsicX-rayluminosityinthe2-10keVrestframeenergyrange,inunitsof1045 ergs−1;11)
radiofluxat1.4GHzinmJy.Notes:a-ForallthesourcesthebestfitspectralmodelintheX-rayenergybandisasimpleabsorbedpower-lawmodel;b-The
90%lowerlimitonNHobtainedassumingz=0;c-Thisparameterhasbeenfixed;d-ForthesesourcestheRbandmagnitudehasbeenobtainedfromthe
SDSSr”model”magnitudesassumingr-R=0.27(rSDSS AB=RVega+rAB(Vega)withrAB(Vega)=0.27);e-Forthesetwosourcesweproposeherea
differentredshiftwithrespecttothatalreadyreportedintheliterature.SeeSection4fordetails.
Table2.Opticalfollow-upspectroscopicobservations
2XMMName Programme Instrument/Grism Exposure(s) SlitWidth(′′) Dates
(1) (2) (3) (4) (5) (6)
J022256.9-024258 GTC18-10B OSIRIS/R500R 6×1100 1.2 2-Sep-2010(4),19-Sep-2010(2)
J100038.9+050955 GTC44-11A OSIRIS/R500R 4×1800 1.0 10-Apr-2011
J121134.2+390054 GTC18-10B OSIRIS/R500R 2×600 1.2 27-Jan-2011
J123204.9+215254 GTC44-11A OSIRIS/R500R 2×1800 1.0 10-Apr-2011(1),11-Apr-2011(1)
J135055.7+642857 GTC44-11A OSIRIS/R500R 3×1800 1.0 10-Apr-2011
J143623.8+631726 GTCMULTIPLE2-09A OSIRIS/R500R 7×1500 1.2 17-Apr-2009(1),19-Apr-2009(3),25-May-2009(3)
Columnsareasfollows:1)Sourcename;2)programID;3)instrument/grismused4)numberofexposuresandexposuretimesinseconds;5)slitwidthin
arcsec;6)datesoftheobservations.Inparenthesiswehaveindicatedthenumberofexposures.
3.3 WISEAll-SkySurveydata and 3.9σ respectively. Two sources, 2XMMJ121026.5+392908
and2XMMJ121134.2+390054,arenotdetectedintheW3andW4
The Wide-field Infrared Survey Explorer (WISE; Wrightetal.
bands.
2010)hasrecentlycarriedoutanallskysurveyinthemediumin-
Using a simulated sample withrandom positions of ∼ 1600
frared regime, detecting hundreds of millions of objects. The es-
object we estimated that the probability to have a random WISE
timated 5σ point sources sensitivities (in unconfused regions) in
sourceinsideacircleof 2arcsecradiusis∼ 1%(i.e.0.07 WISE
thefourobservedchannels(W1=3.4µm,W2=4.6µm,W3=12µm
sourcesexpectedbychanceinthe7errorcirclesinvestigatedhere),
and W4=22 µm) are better than 0.08, 0.11, 1 and 6 mJy, while
implying that all the detected WISE counterparts are very likely
the angular resolution (FWHM) are 6.1, 6.4, 6.5, 12 arcsec re-
associatedwiththehigh f /f sources.
spectively; theastrometricprecisionforhighsignal-to-noise ratio x o
Fluxdensitiesat3.4µm,4.6µm,12µmand22µmhavebeen
(SNR)sourcesisbetterthan0.15arcsec.ToaddtheWISEinfor-
computed from the magnitudes reported in Table 3 by assuming
mationtoour studywehavecross-correlated our EXO50sample
themagnitude zeropointsof theVegasystemcorresponding toa
with the WISE All-Skysource catalog 4 using a positional toler-
power-lawspectrum(f ∝ν−α)withα=1(seeWrightetal.2010).
ance, from the X-ray source position, equal to 4 arcsec . All our ν
Thedifferencesinthecomputedfluxdensitiesexpectedusingflux
sources have been detected by WISE (see below) and a posteri-
correctionfactorsthatcorrespondtoa∆α=±1arelowerthan0.8
ori the offset between the WISE and the optical position of our
percent,0.6percent,6percent,and0.7percentintheW1,W2,
sourcesisbelow2arcsec.Theresultsofthiscross-correlationare
W3andW4band,respectively(Wrightetal.2010).
reported in Table 3. We found a single WISE counterpart for all
the 7 sources discussed here. The detections reported in Table 3
have a significance greater than 7σ in all bands, with the excep-
3.4 XMM-Newtonspectroscopy
tionof2XMMJ100038.9+050955,2XMMJ123204.9+215254and
2XMMJ143623.8+631726,detectedintheW4bandat6.1σ,3.6σ InTable4wereportdetailsfortheXMM-Newtondatausedforthe
X-rayspectralanalysisofeachsourcediscussedhere.TheXMM-
NewtondatawerecleanedandprocessedwiththeXMM-Newton
4 We use here the public available All-Sky Data Release ScienceAnalysisSoftware(SAS)andwereanalyzedwithstandard
that covers >99% of the sky (March 2012 release; see softwarepackages(FTOOLS;XSPEC,Arnaud 1996).Eventfiles
http://wise2.ipac.caltech.edu/docs/release/allsky/) produced from the pipeline were filtered from high-background
(cid:13)c 0000RAS,MNRAS000,1–16
Fx/Fo>50 7
Table3.Infrared(WISE)datafortheEXO50sourcesdiscussedhere.
n 2XMMName fx/fo ID(z) W1 W2 W3 W4 Log(νLν)
(12.3µm)
(1) (2) (3) (4) (5) (6) (7) (8) (9)
1 J022256.9-024258 72 AGN2(z=1.004) 16.02±0.06 14.89±0.07 11.08±0.09 8.07±0.15 45.61
2 J100038.9+050955 138 - 16.22±0.08 14.29±0.05 10.33±0.06 7.97±0.18 -
3 J121026.5+392908 50 BLLAC(z=0.617) 14.86±0.04 14.59±0.06 >12.34 >8.66 -
4 J121134.2+390054 50 BLLAC(z=0.890) 15.21±0.04 15.03±0.08 >12.50 >9.45 -
5 J123204.9+215254 1118 AGN2(z=0.763) 15.35±0.05 13.84±0.04 10.19±0.05 8.48±0.30 45.10
6 J135055.7+642857 458 - 14.90±0.03 13.85±0.03 10.46±0.04 8.47±0.15 -
7 J143623.8+631726 55 AGN2(z=0.893) 15.09±0.03 14.05±0.04 11.36±0.09 8.91±0.28 45.10
Columnsareasfollows:1)NumberusedtomarktheobjectintheplotsshowninSection5;2)sourcename;3)X-raytoopticalfluxratio;4)optical
spectroscopicclassificationandredshift;5)W1magnitude(3.4µm)and1σerror;6)W2magnitude(4.6µm)and1σerror;7)W3magnitude(12µm)and1σ
error.Themagnitudelowerlimitscorrespondtothe95%confidencelevelasreportedintheWISEcatalog;8)W4magnitude(22µm)and1σerror.The
magnitudelowerlimitscorrespondtothe95%confidencelevelasreportedintheWISEcatalog;9)Logoftherestframe12.3µmluminosities
(ν12.3µm×L12.3µmin ergs−1)computedasdescribedinSection3.3.
Table4.EPICXMM-Newtonobservationdetails.
Name OBSID NH Instr. Filter NetExp.Time NetCounts
(1) (2) (3) (4) (5) (6) (7)
J022256.9−024258 0037981601 2.34MOS1&2 thin 26.1 265
pn thin 7.9 127
J100038.9+050955 0204791101 2.41 MOS1 med 16.1 27
pn med 12.5 63
J121026.5+392908 0112830501, 2.00MOS1&2 med 161.5 119876
0112830201 pn med 68.9 175724
J121134.2+390054 0112190201 1.84MOS1&2 med 27.8 2311
pn med 8.9 2488
J123204.9+215254 0112650301 1.80 MOS1 thin 19.3 248
MOS2 med 19.5 250
pn thin 13.1 447
J135055.7+642857 0147540101 1.70MOS1&2 med 44.6 873
pn med 18.9 1064
J143623.8+631726 0204400301 1.37MOS1&2 med 46.7 862
pn med 12.7 648
Col.(1):2XMMsourcename;Col.(2):OBSIDoftheXMM-Newtonobservation;Col.(3):Galacticabsorbingcolumndensityalongthelineofsightinunits
of1020cm−2;Col.(4):EPICinstrument;Col.(5):EPICfilter;Cols(6):Exposuretimeafterremovinghigh-backgroundintervals,inunitsofksec;Col.(7):
Netcountsintheenergyrange0.3−10keV.
timeintervals and only events corresponding to patterns 0-12 for foreachsourcewerecombinedbyusingthetasksaddrmfandad-
MOSand0-4forpnwereused.Allspectrawereaccumulatedfrom darf.Forallthesourcesbut2XMMJ100038.9+050955wegrouped
acircularextractionregionwitharadiusof20-30arcsec,depend- thespectrainbinscontainingmorethan20(source+background)
ingonthesourceoff-axisdistance.Backgroundcountswereaccu- counts and used the χ2 minimization technique; in the case of
mulatedinnearbycircularsourcefreeregions,usinganareausu- 2XMMJ100038.9+050955wegroupedthespectrainbinscontain-
allyaboutafactorof4largerthantheoneusedtoextractthesource ing 10 (source+background) counts and use the Cash statistics.
counts. The X-ray spectra usually cover the 0.3-10 keV energy Inthecaseof 2XMMJ121026.5+392908 twoXMMobservations
range; the total (MOS1+MOS2+pn) net (background subtracted) wereused;pn(MOS)datafromthetwoindependentdatasetwere
counts range from ∼ 102 to ∼ 3×105 counts. The ancillary re- combinedtogetherandtheancillaryanddetectorresponsematrices
sponsematrixandthedetectorresponsematrixwerecreatedbythe were created using the same procedures quoted above. We fitted
XMM-SAStasksarfgenandrmfgenateachsourcepositioninthe pnandMOSspectrasimultaneouslyinthe0.3-10keVband,tying
EPIC detectors. To improve the statistics, the MOS1 and MOS2 togetherallpnandMOSparametersexceptforarelativenormal-
spectraobtained byusing thesame filterwerecombined aposte- ization, which accounts for the differences between pn and MOS
riori by using the FTOOLS task mathpha; in this case ancillary fluxcalibrations(seeMateosetal.2009).Inthefollowing,derived
anddetectorresponsematricesfortheMOS1andMOS2detectors fluxesandluminositiesrefertotheMOSinstrument.
(cid:13)c 0000RAS,MNRAS000,1–16
8 DellaCeca et al.
For the spectral modelling we considered a simple ab-
sorbed power-law model that takes into account both the Galac-
tic hydrogen column density along the line of sight (from
Dickey&Lockman 1990)andapossibleintrinsicabsorptionatthe
source redshift (abundances relative to the Solar one as reported
in Wilmsetal. 2000). In the X-ray spectral modelling we made
use,whenavailable,oftheredshiftsobtainedfromtheopticalspec-
troscopy.AlltheX-rayspectraarewellfittedbythissimplemodel;
theresultsarereportedinTable1alongwiththecorrespondingflux
(corrected for Galactic absorption) and intrinsic luminosity (i.e.
corrected for both Galactic and intrinsic absorption) in the stan-
dard2-10keVenergyband.TheX-rayspectraareshowninFigure
2(rightpanels).Inthecaseof2XMMJ100038.9+050955,thespec-
tralqualitydoesnotallowustoconstrainthepower-lawphotonin-
dexandtheintrinsicabsorptionatthesametime,sowehavefixed
the power-law photon index Γ to 1.9, a common value for unab-
sorbedAGN(Mateosetal.2010,Corraletal.2011,Lanzuisietal.
2013).
1 arcmin
4 RESULTS
Beforediscussingthepropertiesofeachsingleobjectinthissam-
ple we summarise here their main properties. From optical spec- Figure3.Radio(FIRST)contourplotssuperimposedtoaCCDRimage
troscopy, out of the 7 EXO50sources 2 are classified asBL Lac (takenattheTNG)of2XMMJ022256.9−024258; theredhatchindicates
objects,3areclassifiedasType2QSOand2remainunidentified. theobjectresponsiblefortheX-rayemission.Theradiocontoursstartfrom
The 3sources classifiedas Type 2 QSO arein the redshift range 0.2mJywithamultiplicativestepof∼1.58.Thelargebluebaratthebottom
0.7−1 and are characterised by an intrinsic X-ray absorbing col- is1′wide:attheredshiftofthesource1′correspondsto∼0.48Mpc.North
umndensity,N ,intherangebetween1.5and8×1022 cm−2.For isupwards,Eastisontheleft.
H
thetwounidentifiedobjectstheX-rayanalysisprovideslowerlim-
itstotheintrinsicN of4×1021cm−2and1021cm−2,respectively.
H
TherearenoComptonThickAGN(N inexcessto∼ 1024 cm−2)
H
within the X-ray error circle (see Figure 2). The optical spec-
amongstthe3Type2QSO,neitherissuspectedtheirpresencein
trum of this object shows several narrow (observed FWHM <
the2stillspectroscopicallyunidentifiedsources,sincetheirX-ray
1000 − 1500 kms−1 ) emission lines that we associate with
spectra are at odds withthat usually observed in Compton Thick
MgIIλ2798,[NeV]λ3346,3426,[OII]λ3728,[NeIII]λ3869,H ,H ,
AGN (e.g. the presence of a prominent Iron line at 6.4 keV, rest ǫ δ
Hγ,[OIII]λ4364andHeIλ4471atz= 1.004.Thepermittedemis-
frame,oraveryflatX-rayspectrum).The2BLLacobjectsareat
z=0.62andz=0.89and,asitwillbediscussedinSection5.7,they sionlines(HγandMgIIλ2798)havewidthssimilartotheforbidden
ones.TheHγhasaFWHMof550-600km/sthatisclosetothein-
areratherextremeintheirSED.
strumental resolution. TheMgIIλ2798 has astrongly asymmetric
For the three EXO50 sources spectroscopically identified as
profile,whichisdifficulttoanalysewiththepresentdata,beingthe
Type 2 QSOs (see below), we report in Table 3 the rest frame
lineattheborderofthesampledwavelengthrange;theFWHMof
12.3 µm luminosity obtained by interpolating the observed lumi-
thislineislikelybelow1500km/s.Itisworthnotingthepresence
nosity(i.e.notcorrectedforreddening)intheW3(observedframe,
in this object of the [NeV]λ3426 line, a reliable signature of nu-
12 µm) and W4 (observed frame, 22 µm) WISE bands and us-
clearactivitysinceitcannotbeproducedinstarburst/starforming
ing the measured spectral index between 12 µm and 22 µm; the
galaxies(seee.g.Gillietal.2010,Mignolietal.2013).TheX-ray
same spectral index has been used to evaluate the K-correction.
spectrumiswelldescribedbyanabsorbedpower-lawmodelwith
Gandhietal. (2009), studying a sample of Type 1 and Type 2
anN of∼ 7.5×1022 cm−2 ;theintrinsic,rest-frame,2-10keV
Seyferts, showed that the observed luminosity around 12.3 µm H
rest frame (ν × L ) should represent an accurate proxy luminosityis∼1.9×1045 ergs−1.BasedontheopticalandX-ray
12.3µm 12.3µm
spectral properties (optical line widths, intrinsic N and intrinsic
for the AGN intrinsic power; in powerful AGN, the contribution H
2-10 keV luminosity in excess to 1044 ergs−1 ) we classify this
expected from the host galaxy at these wavelengths is marginal
sourceasaType2QSO.2XMMJ022256.9−024258 isdetectedin
(< 10%;Balloetal.2014).Overallthederivedinfraredluminosi-
alltheWISEbands(seeTable3).
tiesareintherangetypicalofLuminousInfraredGalaxies(LIRG:
L > 1011L ) and Ultra Luminous Infrared Galaxies (ULIRG: 2XMMJ022256.9−024258 also has strong radio emission as-
IR ⊙
L >1012L ,seee.g.Sanders&Mirabel1996).Assumingabolo- sociated with the radio source 4C-02.11. The total 1.4 GHz ra-
IR ⊙
metriccorrectionfactortothe12.3µmluminosityof∼10.8,appro- diofluxfromtheFIRSTsurvey(Whiteetal.1997)is∼ 340mJy
corresponding to a radio power of ∼ 1.7×1034 erg s−1 Hz−1 at
priateforhighluminosityAGN(seeBalloetal.2014)theimplied
z=1.004. The source is also detected in the lower spatial reso-
bolometric luminosities for the three spectroscopically identified
Type2QSOareintherange1.5−4.5×1046 ergs−1. lution NVSS survey (Condonetal. 1998). The total NVSS radio
fluxdensity(power) is429±14 mJy(2.18±0.07×1034 ergs−1
• 2XMMJ022256.9−024258; f /f =72 Hz−1); the radio flux density from the NVSS is a more reliable
x o
Asinglesource(R=22.1)isevidentintheopticalfindingchart measure of the total radio flux, since the VLA-B configuration
(cid:13)c 0000RAS,MNRAS000,1–16
Fx/Fo>50 9
used for the FIRST survey could miss some of the diffuse ex- BLLacclassification(Stockeetal.1991;Caccianigaetal.1999).
tended emission. For this source we also found in the literature The measured α (< 0.8) suggests a high-frequency-peaked BL
RX
fluxes at 178 MHz (f = 2.6±0.65 Jy; Goweretal. 1967) Lac object (HBL) in which the low-energy component of their
178MHz
and 4.85 GHz (f = 0.116±0.012 Jy; Griffithetal. 1995); usual,double-peak, SEDpeaksbetweentheUVbandandX-rays
4.85GHz
assuming a power-law model the derived radio spectral index is (Padovani&Giommi1995).TheobservedX-rayspectrumiswell
∼0.94,consistentwiththetypicalvaluesoflobe-dominatedAGN describedbyafeaturelesspower-lawmodel(Γ=2.23+0.01)within-
−0.01
(Kellermann&Owen1988). trinsicN =6×1020 cm−2andintrinsic2-10keVluminosityof∼
H
InFigure3weshowtheradiointensitycontoursfromtheFIRST 6.4×1045 ergs−1.IntheWISEsurvey2XMMJ121026.5+392908
radio survey overlaid on the optical image; the radio counterpart isdetected only at 3.4 and 4.6 µm (WISEband W1 and W2, re-
of 2XMMJ022256.9−024258 isadouble radiosource havingthe spectively).
typical Fanaroff-Riley Type II morphology (i.e. sharp edge lobes • 2XMMJ121134.2+390054; f /f =50
x o
andbrighthotspots);theabsoluteopticalmagnitude(M ≃ −24, The optical object clearly visible at the centre of the
R
assumingaK-correctionof∼2mag,typicalofalatetypegalaxy, error circle (see Figure 2) has a magnitude R=20.77.
see Fukugitaetal. 1995) and the total radio power (∼ 2.2×1034 2XMMJ121134.2+390054 is a well known object since the
ergs−1 Hz−1)areconsistentwiththeFRIIclassificationaccording epoch of the Einstein Extended Medium Sensitivity Survey
tothedividinglineintheM −L planebetweenFRIandFRII (Stockeetal. 1991) and it is classified as a BL Lac object
R radio
radiogalaxies(seeGhisellini&Celotti2001). (MS1209+3917; Rectoretal. 2000). The observed optical mag-
Theprojectedseparationofthetwobrighthotspotsis∼41arc- nitude,radio(10.6±0.6mJyat1.4GHzfromNVSS)andX-ray
seconthesky,correspondingtoaphysicalprojectedsizeof∼0.33 (∼ 8.2 × 10−13 ergcm−2 s−1 ) fluxes imply multi-wavelength
Mpcattheredshiftofthesource.Giventhediscussedradioprop- spectralindices(α =0.58;α =0.62;α =0.56)fullyconsistent
RX OX RO
erties,theobjectisclearlyaradio-loudandlobe-dominatedAGN. withtheBLLacclassification(Stockeetal.1991;Caccianigaetal.
Inthisrespect,wenotethatthemeasuredsizeandtotalradiolumi- 1999). As for 2XMMJ121026.5+392908 discussed above, also
nosityarefullyconsistentwiththatobservedinotherradioquasars 2XMMJ121134.2+390054canbeclassifiedasaHBLobject.
(seeFigure4inKuz´micz&Jamrozy2012). FromtheanalysisoftheopticalspectrumshowninFigure2we
Considering the broad band properties discussed above, reachtheconclusionthattheonlyclearfeatureisthatat∼7048Å,
2XMMJ022256.9−024258 can be considered another example mostlikelyassociated with[OII]λ3727Åatz=0.89; weestimate
of the rare class of Radio Loud Type 2 QSO, with overall anobservedequivalentwidth(EW)of∼12−15Å,thatrescaledto
properties very similar to those shown by e.g. AXJ0843+2942 z=0implyanEW∼6−8Å,veryclosetothelimitusedtoclassify
(DellaCecaetal. 2003), 6C0905+39 (Erlundetal. 2008) or asourceasaBLLacobject(EW<5Å,Stockeetal.1991).
4C+39.29 (Gandhietal. 2006). Indeed according to the data re- While our spectroscospic classification as a BL Lac object
portedinErlundetal.(2008,seetheirFigure3)thisobjectisone agrees with previous results, the redshift proposed here (z=0.89)
of the most powerful sources currently know; its intrinsic X-ray issignificantlydifferentfromthatproposedbyRectoretal.(2000)
luminosity(∼2×1045 ergs−1),ifcomparedwithits178MHzlu- (z=0.602).Theselatterauthorsdidnotfindanyevidenceofemis-
minosity(∼1.3×1035ergs−1Hz−1),ismorethanafactor10above sionline(s)intheirspectrum,andbasedtheirtentativeredshiftde-
thelocussampledbytheNarrowLineRadioGalaxiesatz<1.0in terminationonlowSNRabsorptionfeatures.Weareconfidentthat
the3CRRcatalogandsimilartothatobservedinBroadLineRadio thelineobservedat∼7048Åisrealandthislinedoesnothaveany
GalaxiesandQSOs(seeFigure3inHardcastleetal.2009). reliableidentificationiftheobject isatz=0.602. Moreover at the
• 2XMMJ100038.9+050955; f /f =138 proposed redshift (z=0.89) we were able to reproduce quite well
x o
The optical magnitude of the single object in the error circle alsotheshapeoftheunderlyingopticalcontinuum(seeFigure2).
(see Figure 2) is R=23.6. The gathered optical spectrum, unfor- Finallywenotethatthissourceisstronglyvariable:atleastafac-
tunately characterised by a very low SNR, seems to be rather torof5.5intheX-raydomain(bycomparingthemeasured0.1-2.4
flat and featureless, thus we have no redshift information for keVfluxwiththefluxintheROSATAllSkySurvey)andafactor
2XMMJ100038.9+050955.ThesourceisdetectedinalltheWISE of3intheopticaldomain(bycomparingourmagnitudewiththe
bandswhilethereisnoradiodetectionatthesourceposition.As- V =20 reported in Rectoretal. 2000), so this variability could
mag
suming that 2XMMJ100038.9+050955 is an absorbed QSO and explainthecleardifferent shapefromtheopticalspectrareported
usingtherelationbetween f /f andtheintrinsic2-10keVlumi- inRectoretal.(2000)andthatreportedhere(seeFigure2).
x o
nosity(Brusaetal.2010,seeSection5)wecanestimatearedshift At z=0.89 theobserved X-rayspectrum iswell described by a
z∼1.0(intherange0.6−1.6takingintoaccountascatterof0.5dex featureless power-law model (Γ = 2.21+0.07) withintrinsic N =
−0.06 H
intherelation,seeFigure4,leftpanel).Atz≃1.0theintrinsic2-10 1.9×1021 cm−2;theintrinsic2-10keVluminosityis∼3.7×1045
keV luminosity would be ∼ 9×1044 ergs−1 while the intrinsic ergs−1 . The intrinsic absorption is significantly in excess to the
N ∼5×1022cm−2. Galacticvalueandthisisatoddswithwhatisusuallyobservedin
H
• 2XMMJ121026.5+392908; f /f =50 BLLacsand,inparticular,inHBLobjects(seee.g.Massaroetal.
x o
ThissourcewastheonlyonenotobservedduringourGTCrun;it 2011andreferencestherein).OntheotherendtheX-rayspectrum
isawellknownBLLacobjectatz=0.617(Caccianigaetal.2002; could be intrinsically curved due to the bump expected in HBL,
Plotkinetal. 2010). The observed optical magnitude (R=19.16), peakingbetweentheUVandX-raybands.Wethereforefittedthe
the radio (19±0.7 mJy at 1.4 GHz from the NVSS) and the X- X-raydatawithabrokenpower-lawmodelfilteredbytheGalactic
ray (∼ 3.6 × 10−12 ergcm−2 s−1 in the 2-10 keV band) fluxes absorption; wefindagood fit(χ2 =1.1for194d.o.f.)withbest-
ν
allow us to compute multi-wavelength spectral indices 5 (α = fittingparametersΓ =1.57+0.1,Γ =2.30+0.10 andE =1.23+0.2 ,
RX 1 −0.2 2 −0.13 c −0.28
0.54; α =0.61; α =0.50) which are fully consistent with the
OX RO
−log(f5G5H.z3/8f2500Å) where f5GHz, f2keV and f2500ÅaretheK-correctedfluxes
5 αRX = −log(f5G7H.6z8/f2keV); αOX = −log(f2k2e.V60/5f2500Å); αRO = at5GHz,2keVand2500Å,respectively.
(cid:13)c 0000RAS,MNRAS000,1–16
10 DellaCeca et al.
which are consistent with the results reported in Massaroetal. non-transient spectroscopically identified source with the highest
(2011)onasampleofHBLsources.Theintrinsic2-10keVlumi- f /f discoveredsofar.
x o
nosityobtained withthebroken power-lawmodelisequaltothat • 2XMMJ135055.7+642857; f /f =458
x o
previouslyreportedinTable1.Inanalogywiththeotherconfirmed A very faint object (R=25.0) is visible at the centre of the er-
BL Lac object in the sample (2XMMJ121026.5+392908), in the rorcircle.Unfortunatelytheopticalspectrumisverynoisyandno
WISEsurveythissourceisdetectedonlyat3.4and4.6µm. featuresareclearlydetected; wehavenoredshift information for
thisobject.TheX-rayspectrumisdescribedbyapower-lawmodel
• 2XMMJ123204.9+215254; f /f =1118 with photon index Γ ∼ 2 and absorbing column density greater
x o
Twoopticalsources(labeledwithAandBinthefindingchart than∼2×1021 cm−2(obtainedassumingz=0).Anemissionline
reportedinFigure2)areclearlypresentinsidetheX-rayerrorcir- ispossiblydetectedintheEPIC-MOSspectrum(seetheresiduals
cle;theyhaveanRmagnitudeofR=23.94±0.41(sourceA)and inFigure2,wherethemodeldoesnotincludetheline).However
R=24.73±0.64(sourceB).AnopticalspectrumofsourceA,ob- nocompellingevidenceofthepresenceofthislineintheEPIC-pn
tainedsumminguptwoindependentexposures,isshowninFigure isfound,althoughdifferentspectralbinningsweretried.
2; the spectrum has a clear red continuum and a significant line Ifthislineisreal,andassociatedwiththeFeKαemissionline
at ∼ 6569 Å. Assuming this lineto be [OII]λ3727 Å at z=0.763 (themostprominentfeatureintheX-rayspectrumofanAGN),the
another observed feature is in very good agreement with being impliedredshift would bez∼ 0.65, theabsorbing column density
[OIII]λ5007Å;thesetwofeaturesareclearlyseeninboththesin- ∼4×1021 cm−2andtheintrinsic2-10keVluminosity∼2.5×1044
gleexposuresandarenotassociatedwithstrongskylines.Wehave ergs−1 ; the line has an observed equivalent width of EW ∼ 400
alsotriedtotakeanopticalspectrumofthesourceB,butthesource eV. Interestingly the source is also a strong (and compact) radio
isveryfaintandnousefulinformationcouldbeextractedfromthe source,detectedat15GHz(236±1mJy,Richardsetal.2011),8.4
verynoisyspectrum. GHz(∼376mJy,Healeyetal.2007)andat1.4GHz(183.5±5.5
2XMMJ123204.9+215254isdetectedinalltheWISEbandsdis- mJy,NVSSsurvey).Thebroadbandspectralindices(αRX =0.88;
cussed here. The WISE position is coincident with a source de- αOX=0.31; αRO=1.15), computed with the fluxes reported above,
tectedintheKband(K=18.07±0.05,seeDelMoroetal.2009)and and the possible presence of a Fe Kα linein the X-ray spectrum
botharesignificantlyclosertothebrightestopticalsource(source arenotconsistentwithaBLLacclassification.Weremindthatthe
A), strongly suggesting that the most probable optical counter- X-raysourceispoint-likeanddetectedalmostinthecenterofthe
part of the X-ray source 2XMMJ123204.9+215254 is the source EPICfields,soahighzclusterofgalaxiesisveryunlikely.
A. In the following of this paper we therefore assume that the The radio spectrum is very flat (indeed almost inverted, with
2XMMJ123204.9+215254 is spectroscopically identified with a a maximum at ∼ 8 GHz) suggesting that this source could be
Type2QSOatz=0.763. classified as a Giga-Hertz Peaked Spectrum radio source, a class
2XMMJ123204.9+215254 was previously discussed in of sources supposed to be young radio galaxies which are often
DelMoroetal. (2009) who report an infrared spectrum taken characterisedbyhighintrinsicabsorption(seee.g.Guainazzietal.
at the Subaru telescope (with the MOIRCS instrument) clearly (2006)).
revealingthepresenceofalineattheobservedframeof1.8837µm. Assuming that this source is an absorbed QSO and using the
Lackingoptical spectroscopy andadeep optical image,theseau- relationbetween fx/fo andtheintrinsic2-10keVluminosity(see
thorsdiscussedseveralpossibleidentificationsfortheinfraredline Section5)wecanestimatearedshiftz∼1.7(zintherange1.1−2.7,
and suggested the Hα line at z=1.87 as itsmost probable origin. inconsistent withtheredshift estimatedbytheputativeironline),
AsdiscussedaboveouropticalspectroscopyattheGTCsuggestsa withan intrinsic2-10 keV luminosity ∼ 3×1045 ergs−1 and an
lowerredshiftofz=0.763.InthiscasetheNIRfeaturereportedin intrinsicNH ∼ 1.2×1022 cm−2.Thissourceisdetectedinallthe
DelMoroetal.(2009,observedat1.8837µm)couldbeassociated WISEbands.
with the line complex HeIλ10830+Paγ10941, a strong and quite • 2XMMJ143623.8+631726; fx/fo=55
common feature in AGN (see Glikmanetal. 2006), expected to AfaintobjectwithopticalmagnitudeR=22.16ispresentinthe
be at ∼ 1.92 µm (at z=0.763), slightly higher than the observed centreoftheerrorcircle(seeFigure2).Theopticalspectrumofthis
feature.HowevertheobservedNIRlinefallsinawavelengthrange objectisshowninFigure2;thetwomarkedlinescanbeassociated
dominatedbytheeffectofatmosphericabsorption,soitisdifficult with[OII]λ3728 and Hγ+HeI (observed FWHM< 1100 kms−1
todeterminetheintrinsiclinecentroid; allinall,weconsider the )atz=0.893.TheobservedX-rayspectrumisdescribedbyanab-
association of the infrared line with the HeIλ10830+Paγ10941 sorbedpower-lawmodelwithintrinsicNH =1.46×1022 cm−2;the
complex as highly plausible. On the contrary none of the faint intrinsic2-10keVluminosityis∼8.4×1044 ergs−1.Thesource
features that we see in the optical spectrum could be associated isdetectedinalltheWISEbands;therearenoradiodetectionsat
with any relevant emission lines from an AGN in the case of thesourceposition.WeclassifythissourceasaType2QSO.
z=1.87.
Attheproposedlowerredshift(z=0.763)theX-rayspectrumis
describedbyanabsorbedpower-lawmodelhavingaveryflatpho-
tonindex,Γ = 1.31±0.23,andanintrinsicN = 3.6+1.03×1022
H −0.92
cm−2 .Theintrinsic2-10keVluminosityis∼ 1.9×1045 ergs−1
. Since the best-fitting photon index is rather flat we test the
stability of the measured N and luminosity assuming a typi-
H 5 DISCUSSION
cal AGN photon index (Γ = 1.9); we derive an intrinsic N =
H
5.9+0.73 × 1022 cm−2 and an intrinsic 2-10 keV luminosity of HavingdefinedasmallbutrepresentativesampleofbrightEXO50
−0.65
∼ 2.3×1045 ergs−1 ingoodagreementwiththepreviousvalues. objects, itisnow instructivetocompare theirbroad band proper-
Wenotethat2XMMJ123204.9+215254isnotonlytheobjectwith tieswiththoseofothersamplesofAGN,bothabsorbedandunab-
thehighest f /f inthissample, but (toour knowledge) itisthe sorbed,fromtheliterature.
x o
(cid:13)c 0000RAS,MNRAS000,1–16