The XMM-Newton HBS28 sample: Studying the obscuration in hard X-ray selected AGNs

22 Pages·2008·0.77 MB·English

A&A416,901–915(2004) Astronomy
DOI:10.1051/0004-6361:20034148 &
(cid:1)c ESO2004 Astrophysics
The XMM-NewtonHBS28 sample: Studying the obscuration
(cid:1),(cid:1)(cid:1)
in hard X-ray selected AGNs
A.Caccianiga1,P.Severgnini1,V.Braito1,2,R.DellaCeca1,T.Maccacaro1,A.Wolter1,X.Barcons3,F.J.Carrera3,
I.Lehmann4,M.J.Page5,R.Saxton6,andN.A.Webb7
1 INAF–OsservatorioAstronomicodiBrera,viaBrera28,20121Milano,Italy
e-mail:[paola;braito;rdc;tommaso;anna]@brera.mi.astro.it
2 DipartimentodiAstronomia,Universita`diPadova,Vicolodell’Osservatorio2,35122Padova,Italy
3 InstitutodeFisicadeCantabria(CSIC-UC),AvenidadelosCastros,39005Santander,Spain
e-mail:[barcons;carreraf]@ifca.unican.es
4 AstrophysikalischesInstitutPotsdam(AIP),AnderSternwarte16,14482Potsdam,Germany
e-mail:[email protected]
5 MullardSpaceScienceLaboratory,UniversityCollegeLondon,HolmburySt.Mary,Dorking,SurreyRH56NT,UK
e-mail:[email protected]
6 X-rayAstronomyGroup,DepartmentofPhysicsandAstronomy,LeicesterUniversity,LeicesterLE17RH,UK
e-mail:[email protected]
7 Centred’E´tudeSpatialedesRayonnements,9avenueduColonelRoche,31028ToulouseCedex04,France
e-mail:[email protected]
Received1August2003/Accepted2December2003
Abstract. This paper presents the analysis of a statistically complete sample of 28 serendipitous X-ray sources selected in
82pointedXMM-Newtonfieldsdowntoacount-rateof0.002countss−1 (4.5–7.5keVenergyband).Thisisthefirstsample
selectedinthisenergyrangetohavecompletespectroscopicidentificationsandredshiftdeterminationsforalltheobjects.Apart
fromoneGalacticsource(aninteractingbinary),alltheobjectsareAGNs.TheiropticalandX-rayproperties(derivedfromthe
spectralanalysisoftheXMM-EPICdata)arecomparedtogether.Thegoodcorrelationbetweentheopticalspectraltypeand
theX-rayabsorptionpropertiessupportstheAGNunifiedmodel.Onlyoneobjectthatdoesnotfittherelationbetweenoptical
andX-rayabsorptionisfound,namelyaSeyfert1.9withnoevidenceofobscurationintheX-rayband(N <1.3×1020cm−2).
H
Intotal,7sourcesoutof27areheavilyobscuredintheX-ray(N >1022cm−2),correspondingtoasurfacedensityof0.7deg−2
H
atthefluxlimitthesample(4−7×10−14 ergs−1cm−2 inthe4.5–7.5 keV energyband). Among theseobscured objects, two
sourcesshowalarge(intrinsic)luminosity(L[2−10keV] > 1044 ergs−1)andarethusclassifiedastype2QSO.Finally,wehave
comparedthefractionofX-rayabsorbedAGNs(26%)withthatpredictedbythecurrentXRBsynthesismodelsatthefluxlimit
ofthesurvey.Wefindthatthemodelssignificantly(∼90%confidencelevel)overpredictthefractionofabsorbedAGNsthus
confirmingalsointhishardenergyband(4.5–7.5keV)similarresultsrecentlyobtainedinthe2–10keVband.
Keywords.galaxies:active–X-rays:galaxies–X-rays:diffusebackground
1. Introduction reproduced by a combination of obscured and unobscured
AGNs(Setti&Woltjer1989;Madauetal.1994;Comastrietal.
Over 80% of the cosmic X-ray background (XRB) has been
1995,2001;Gillietal.2001;Uedaetal.2003).Theobscured
recently resolved in the 2–10 keV energy range by deep
AGNs,inparticular,playakeyroleintheXRBsynthesismod-
Chandra and XMM-Newton observations (Mushotzky et al.
els since they are expected to make up a significant fraction
2000; Hasinger et al. 2001; Brandt et al. 2001; Rosati et al.
ofthepopulationinhardX-rayenergies(seeFabian2003and
2002; Moretti et al. 2003). Its spectrum can be successfully
referencestherein).Onthecontrary,onlyasmallfractionofab-
Sendoffprintrequeststo:A.Caccianiga, sorbedAGNsisfoundinsoftX-raysurveysevenatthefaintest
e-mail:[email protected] fluxes(e.g.∼15%intheROSATUltraDeepSurvey,Lehmann
(cid:1) Based on observations collected at the European Southern etal.2001).
Observatory, La Silla, Chile and on observations obtained with
Many important issues related to this population are still
XMM-Newton,anESAsciencemissionwithinstrumentsandcontri-
butionsdirectlyfundedbyESAMemberStatesandtheUSA(NASA). to be understood, like the number of type 2 QSO, the
(cid:1)(cid:1) Appendix A and B are only available in electronic form at relationship between optical absorption and X–ray obscura-
http://www.edpsciences.org tion (e.g. Maccacaro et al. 1982; Panessa & Bassani 2002;
Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20034148
902 A.Caccianigaetal.:TheXMM-NewtonHBS28sample
Risaliti et al. 1999; Maiolino et al. 2001), the evolutionary discussed.InSect.5theX-rayandopticalpropertiesarecom-
properties of type 2 AGNs (Gilli et al. 2001; Franceschini paredtogetherwhileinSect.6therelativefractionofabsorbed
et al. 2002) and the nature of the “Optically dull X-ray loud versusun-absorbedAGNsiscomparedtothepredictionsbased
galaxies”, firstly discoveredfrom the analysisof Einstein and ontheX-raybackgroundsynthesismodels.Finally,inSect. 7
ROSAT data (e.g. Elvis et al. 1981; Maccacaro et al. 1987; an estimate of the density of type 2 QSO at the flux limit of
Griffithsetal.1995;Tananbaumetal.1997)andmorerecently thesurveyisgivenwhileinSect.8webrieflydiscusstheprob-
foundinlargenumberbyChandraandXMM-Newtonsurveys lemoftheX-rayBrightOpticallyNormalGalaxies.Summary
(Fiore et al. 2000; Barger et al. 2002; Comastri et al. 2002; andconclusionsarepresentedinSect.9.Throughoutthispaper
Severgninietal.2003). H0 =65kms−1 Mpc−1,ΩΛ =0.7andΩM =0.3areassumed.
Many of these issues can be studied by using hard X-ray
surveys with complete spectroscopic information, both in the 2. TheXMM-NewtonBrightSerendipitousSource
opticalandintheX-rays,toallowadirectcomparisonbetween sample
the two bands (e.g. Akiyama et al. 2003; Ueda et al. 2003).
The XMM-Newton Bright Serendipitous Source (XMM-
Inparticular,AGNsamplesselectedinthehardestenergyband
Newton BSS) sample is an ongoing project aimed at the se-
currentlyreachablewithimaginginstruments,i.e.the5–10keV
lection of a statistically complete sample of X-ray sources
or5–8keVenergyband,providethebeststartingpointforthis
kindofanalysissincetheyarelessaffectedbyobscuration. serendipitouslydiscoveredin pointedXMM-Newton observa-
tions(seeDellaCecaetal.2002;DellaCeca2002fordetails).
However, the sources found in medium-deep surveys are
TheXMM-NewtonBSSconsistsof2complementarysamples
usually so faintin the opticalthatthe completionof the iden-
basedonthe XMM-NewtonEPIC-MOS2dataandselectedin
tificationprocessishard,ifnotimpossible,toachieve.Forin-
the 0.5–4.5keV and in the 4.5–7.5 keV energyrange respec-
stance, about 25% of the sources discovered in the Chandra
tively.ThereasonsforusingtheMOS2detectorforthedefini-
Deep Fields have optical counterparts fainter than R = 25
tionofthesamplehavebeendescribedindetailsbyDellaCeca
(Giacconietal.2002)makingthedirectredshiftestimatevery
et al. (2002). The XMM-Newton BSS will be released to the
difficultevenwiththelargestopticaltelescopescurrentlyavail-
publiccommunity(DellaCecaetal.inprep.)and,whencom-
able. Furthermore,an accurate X-ray spectral analysisis usu-
bined with medium and deep X-ray surveys, will allow us to
ally not feasible for the faintest sources detected in deep sur-
investigate on a wide range of luminosities and redshifts the
veys.BrightX-raysurveys,forwhichacompletespectroscopic
propertiesof the sources responsible for the XRB at energies
identification in the optical is a feasible task and for which
below10keV.
the X-ray spectra can be easily collected, thus complement
mediumanddeepsurveys.
2.1.Apilotstudy:TheHBS28sample
Withthisgoalinmind,theXMM-NewtonSurveyScience
Centre (SSC)1 is building up a large (∼1000 sources) sam-
Asapilotstudywehaveselectedarepresentativesub-sample
ple of bright (flux limit ∼10−13 ergcm−2s−1) serendipitous
(the HBS28 sample) in the 4.5–7.5keV energyrange.In par-
XMM-Newton sources at high Galactic latitude (|b| > 20◦),
ticularwehaveusedtheXMM-NewtonMOS2fieldsavailable
following well defined criteria so as to allow both a detailed
to the XMM-Newton SSC until December 2002 and selected
study of sourcesof high individualinterest as well as statisti-
accordingto the criteria describedin Della Ceca etal. (2002)
calpopulationstudies(seeDellaCecaetal.2002;DellaCeca
andDellaCeca(2002)i.e.:
2002).
1. |bII|>20◦;
The scope of this paper is to present a first complete 2. Galacticcolumndensity(N )below1021cm−2;
and representative sub-sample of 28 objects, selected in the H
3. exclusion of fields centered on bright and/or extended
4.5−7.5 keV energyrange.Even if small, this sample has the
X-raytargets;
important advantage that a classification based on dedicated
4. exclusionoffieldswhichsufferfromveryhighbackground
optical spectroscopy is available for all the sources. In con-
flares.
trast,recentsamplesselectedinasimilarveryhardenergyband
(∼5−10 keV) using BeppoSAX (Fiore et al. 1999; La Franca Moreover, to define the pilot sample reported here we have
etal.2002)ASCA(Nandraetal.2003),XMM-Newton(Baldi used onlythe fields with α ≤ 5 h or α ≥ 18h. Thislast con-
etal.2002;Fioreetal.2003;Mainierietal.2002)orChandra straint,coupledwiththe|bII|> 20◦criterion,selectsanareaof
data(e.g.Rosatietal.2002)areusuallycharacterizedbyapar- skymostlyintheSouthernhemisphere.
tialidentificationlevel. For the source detection and characterization we have
followed the same pipeline processing used for the First
In Sect. 2 the sample is presented while in Sect. 3 and
XMM-Newton Serendipitous Source Catalogue, which is de-
in Sect. 4 the optical and the X-ray spectra respectively are
scribedindetailsinhttp://xmmssc-www.star.le.ac.uk/.
Thesourcesarethenselectedwiththefollowingconstraints:
1 The XMM-Newton SSC is an international collaboration ap-
1. countrate(4.5−7.5keV)≥0.002countss−1;
pointedbyESAtohelptheSOCindevelopingtheSAS,topipe-line
process all XMM data and to exploit the XMM-Newton serendipi- 2. maximumlikelihood>12 in the 4.5–7.5keV band (corre-
tous detections. See http://xmmssc-www.star.le.ac.uk/ for a spondingtoaprobabilityforarandomPoissonianfluctua-
descriptionoftheSSCactivities. tiontohavecausedtheobservedsourcecountsof6×10−6);
A.Caccianigaetal.:TheXMM-NewtonHBS28sample 903
Table1.TheXMM-NewtonfieldsusedfortheselectionoftheHBS28sample.
obsid position(J2000) t r r obsid position(J2000) t r r
i o i o
(s) ((cid:6)) ((cid:6)) (s) ((cid:6)) ((cid:6))
0125310101 000030.4−250643.4 19163.6 1 13 0067340101 185537.1−463057.6 10653.3 0 11
0101040101 000619.7+201222.8 34046.5 8 13 0081341001 193121.7−723913.3 15288.1 1 13
0127110201 001031.2+105840.7 7558.3 2 13 0081340501 201330.0−414725.7 19196.8 0 13
0111000101 001833.0+162608.0 31363.7 3 13 0111180201 204010.0−005214.7 16495.8 8 13
0001930101 002606.8+104112.5 18033.7 1 13 0111510101 204150.9−322619.8 15046.1 8 13
0050140201 002635.9+170937.8 50383.0 3 13 0130720201 204409.0−104311.2 5312.0 8 13
0065770101 003247.0+393433.3 7360.3 1 13 0111420101 204509.4−312036.6 43323.5 8 13
0125320701 005003.0−520736.7 16525.7 1 13 0112600501 204620.0−024848.4 10550.3 8 13
0110890401 005720.0−222304.1 29743.3 8 13 0083210101 205418.8−155538.2 10435.5 2 13
0112650401 010424.0−062410.6 23697.1 0 13 0112190601 205621.6−043759.4 16639.4 2 13
0103861601 010516.4−582610.4 7127.5 8 13 0081340401 205827.1−423856.9 15002.3 1 13
0101040201 012346.0−584825.8 28995.6 8 13 0041150101 210411.1−112140.0 38736.2 1 13
0109860101 012533.4+014538.2 38535.6 4 13 0038540301 210440.2−122005.6 14698.6 2 13
0112600601 012732.1+191032.3 3994.4 8 13 0088020201 212738.1−444838.5 16094.7 1 13
0084230301 013153.7−133656.4 20447.4 8 13 0103060101 212912.2−153834.7 22042.9 3 13
0032140401 014012.1−674840.3 7611.1 0 13 0109463501 213756.5−434219.8 7577.7 2 13
0093641001 014301.7+133824.4 9329.0 4 13 0061940201 213807.9−423606.6 4868.2 1 13
0101640201 015950.2+002346.8 7543.7 8 13 0008830101 214015.5−233932.1 13986.7 2 13
0084140101 020838.2+352300.2 38003.5 2 13 0103060401 215155.9−302743.5 24072.9 3 13
0112371701 021712.4−043904.2 20002.1 0 11 0124930201 215852.9−301328.8 36246.3 8 13
0112371001 021800.3−045947.8 50802.9 0 11 0130920101 220309.3+185227.9 16491.3 1 13
0112371501 021848.3−043902.9 6684.6 0 11 0012440301 220510.1−015511.2 30937.5 2 13
0112370301 021936.3−045947.7 50386.7 0 11 0106660101 221531.9−174402.6 56911.8 1 13
0098810101 023612.3−521955.6 23451.5 2 13 0009650201 221755.4−082058.0 21027.5 8 13
0075940301 023657.9+243853.7 47428.9 8 13 0049340201 222045.1−244058.1 26842.3 6 13
0067190101 023819.4−521134.1 25628.7 3 10 0018741701 223432.9−374348.1 7080.3 4 13
0111200101 024241.1−000053.7 35486.8 8 13 0111790101 223545.9−260300.4 41304.4 4 13
0111490401 024843.8+310659.2 31030.1 8 13 0103860201 223655.9−221310.1 8489.9 8 13
0056020301 025632.8+000601.7 17251.4 2 13 0006810101 224239.5+294335.6 7009.4 8 13
0041170101 030238.5+000731.9 47278.1 0 13 0109070401 224841.5−510957.9 14722.6 8 13
0042340501 030703.8−284024.1 13285.7 4 13 0112240101 224948.3−642311.2 30608.4 8 13
0122520201 031159.3−765153.0 28296.4 2 13 0081340901 225149.4−175217.0 22398.6 1 13
0110970101 031309.6−550348.4 10240.0 0 13 0112910301 225358.8−173355.8 5301.3 8 13
0105660101 031756.0−441415.1 23151.9 6 13 0112170301 230315.8+085225.9 23349.6 4 13
0108060501 033229.1−274827.1 46835.8 0 13 0109130701 230443.6−084114.5 10615.6 8 13
0099010101 033527.6−254454.5 18777.9 8 13 0033541001 230445.0+031135.6 12461.5 2 13
0055140101 033935.0−352558.1 47637.0 1 11 0123900101 231358.9−424328.3 37401.3 6 13
0111970301 040907.2−711743.9 18741.3 8 13 0109463601 231518.7−591031.7 5403.6 8 13
0112600401 042544.2−571334.4 7627.3 8 13 0112880301 233149.9+195628.5 14318.0 3 13
0103861701 043517.1−780154.3 8010.9 8 13 0093550401 233340.0−151712.2 22258.0 1 13
0112880401 045935.4+014716.0 18915.3 2 13 0100241001 234940.6+362530.6 8739.0 3 13
Column1:observationID;Col.2:skypositionofthecenterofthefield;Col.3:MOS2exposuretime(afterhavingremovedthetimeintervals
characterizedbyahighbackground);Col.4:innerradius(seetextfordetails);Col.5:outerradius(seetextfordetails).
3. exclusionofthesourcestooclosetothegapsbetweenthe limitof0.002countss−1correspondstoanominalfluxlimitof
CCDs(seeDellaCecaetal.,inprep.formoredetails); about 7×10−14 ergs−1cm−2 in the 4.5–7.5 keV energy band
4. exclusionofthetargetoftheobservationandofthesources forapower-lawspectrumwithaphotonindexΓ=2.Wenote,
too close to the edge of the field of view. To this end, we however,thatinsomeobjectthe4.5–7.5keVfluxcomputedon
have defined an inner (r) and an outer (r ) radius which thebasisoftheX-rayspectralanalysis(seeSect.4)arelower
i o
definetheportionofeachfieldusedtoselectthesources. (uptoafactor∼2)thanthisformallimit.Therefore,theactual
fluxlimitofthesampleisbetween4and7×10−14ergs−1cm−2.
In total, the HBS28 sample contains 28 sources selected in ThetotalareacoveredbytheHBS28sampleis9.756deg2.
82XMM-Newton/EPIC-MOS2fields.Thecompletelistofthe The list of 28 sources selected in the sample is presented in
82XMM-NewtonfieldsisreportedinTable1.Thecount-rate Table2.
904 A.Caccianigaetal.:TheXMM-NewtonHBS28sample
Table2.TheHBS28sample.
name ObservationID Gal.NH CR4.5−7.5keV EPICcameras Notes
(×1020) (×10−3)
[cm−2] [s−1]
XBSJ002618.5+105019 0001930101 5.07 2.3 MOS2,MOS1,pn
XBSJ013240.1−133307 0084230301 1.64 3.2 MOS2,MOS1,pn
XBSJ013944.0−674909 0032140401 2.49 2.0 MOS2,MOS1,pn
XBSJ014100.6−675328 0032140401 2.49 70.9 MOS2,MOS1 a
XBSJ015957.5+003309 0101640201 2.59 3.8 MOS2,MOS1,pn
XBSJ021640.7−044404 0112371701 2.42 2.1 MOS2,MOS1,pn
XBSJ021808.3−045845 0112371001∗ 2.52 2.7 MOS2,MOS1,pn
XBSJ021817.4−045113 0112371001∗ 2.52 3.3 MOS2,MOS1,pn
XBSJ021822.2−050615 0112371001∗ 2.52 4.5 MOS2,MOS1,pn
XBSJ023713.5−522734 0098810101 3.28 3.2 MOS2,MOS1,pn
XBSJ030206.8−000121 0041170101 7.16 3.1 MOS2,MOS1,pn
XBSJ030614.1−284019 0042340501 1.36 4.6 MOS2,MOS1 e
XBSJ031015.5−765131 0122520201 8.21 4.4 MOS2,MOS1,pn
XBSJ031146.1−550702 0110970101 2.55 5.9 MOS2,MOS1,pn
XBSJ031859.2−441627 0105660101 2.60 2.2 MOS2,pn c
XBSJ033845.7−352253 0055140101 1.31 2.4 MOS2,MOS1,pn
XBSJ040658.8−712457 0111970301 7.57 3.4 MOS2,MOS1,pn b
XBSJ040758.9−712833 0111970301 7.57 5.0 MOS2,pn c
XBSJ041108.1−711341 0111970301 7.57 2.2 MOS2,MOS1 a,b
XBSJ185613.7−462239 0067340101 5.29 8.2 MOS2,MOS1,pn
XBSJ193248.8−723355 0081341001 5.95 4.5 MOS2,MOS1,pn
XBSJ204043.4−004548 0111180201 6.72 3.2 MOS2,MOS1,pn
XBSJ205635.7−044717 0112190601 4.96 2.1 MOS2,MOS1 e
XBSJ205829.9−423634 0081340401 3.89 3.9 MOS2,MOS1,pn
XBSJ213002.3−153414 0103060101 4.99 2.3 MOS2,MOS1 d
XBSJ213824.0−423019 0061940201 2.68 2.5 MOS2,MOS1 e
XBSJ214041.4−234720 0008830101 3.55 3.3 MOS2,MOS1,pn
XBSJ220601.5−015346 0012440301 6.13 2.3 MOS2,MOS1 d
Column1:sourcename;
Col.2:identificationnumberofthecorrespondingXMMobservation;
Col.3:Galacticcolumndensity;
Col.4:sourcecount-rateinthe4.5–7.5keVenergyband;
Col.5:EPICcamerasusedforthespectralanalysis(seenotes);
Col.6:notes.
a=sourceonagapinthepn.
b=MOS1andMOS2notsummed.
c=sourceonagapinMOS1.
d=sourceoutsidethepnfieldofview.
e=problemswiththepndata.
∗forthespectralanalysiswehavesummedthesedatawiththe0112370101sequence.
3. Opticalidentificationandclassification anothersourceispresentjustoutsidetheX-rayerrorcirclewe
havespectroscopicallyobservedalsothissecondobject.Inall
3.1.Theopticalcounterparts
these cases (4 in total) the object within the error circle has
turned out to be a more reliable counterpart (an AGN) than
Theopticalcounterpartforeachsourceinthesamplehasbeen
theobjectoutsidethecircle(alwaysastar)thusconfirmingthe
searched in the Digitized Sky Survey(DSS) and a magnitude
good accuracy of the X-ray position. All the optical counter-
hasbeencollectedfromtheAPM(AutomaticPlateMeasuring
partsarewithin4(cid:6)(cid:6)fromtheX-rayposition.
machine)catalogue2.Thankstothegoodpositionalaccuracy3
(∼3(cid:6)(cid:6)–4(cid:6)(cid:6))andthebrightnessoftheXMMsources,thereisno For 26 sources out of 28, an APM counterpartwith a red
ambiguity in the detection of the optical counterpart. When magnitude brighter than 20.1 was found. For the remaining
2sources,anopticalcounterpartisvisibleintheacquisitionim-
2 http://www.ast.cam.ac.uk/∼apmcat/ agetakenattheESO3.6mtelescopeduringthespectroscopic
3 see the “EPIC status of calibration and data analysis” report, at run(seebelowforthedetails)andamagnitude(R)iscomputed.
xmm.vilspa.esa.es/ Hence,forthetotalityofthesampleanopticalcounterparthas
A.Caccianigaetal.:TheXMM-NewtonHBS28sample 905
Fig.2. Redshift distribution of the 27 extragalactic sources in the
Fig.1. Red magnitude distribution of the 28 sources in the HBS28
HBS28 sample. The shaded area represents the type 2 objects as
sample. For most of the object (26) the magnitude is the APM
definedinthetext.
red magnitude, while for the 2 faintest sources the R magnitude is
considered.
package. The spectra have been wavelength calibrated using
anhelium-argonreferencespectrum.Arelativefluxcalibration
been found. The magnitude distribution of the 28 sources is
hasbeenobtainedbyobservingthestandardstarsG191-B2B,
presentedinFig.1. BD25+4655(TNG)andLTT7987(ESO).Theweathercondi-
Itisworthnotingthat,giventhesmallX-ray-to-opticalpo- tions(notalwaysphotometric)andtheobservingset-upusually
sitional offset and the brightness of the optical counterparts, do not guarantee a correct absolute flux calibration. For this
theexpectednumberofspuriousopticalidentificationsiscom- reason,thespectrareportedherearegiveninarbitraryunits.
pletelynegligibleintheHBS28sample.Evenconsideringthe
stars,thatrepresentthemostprobablechancecoincidence,the
3.3.Theclassificationcriteria
expectednumberofspuriousmatchesintheHBS28sampleis
belowunity.Indeed,asexplainedbelow,no“normal”starsare
All the 28 sources selected in the HBS28 sample are emis-
foundintheHBS28sample.TheonlyGalacticobjectfoundis
sionlineobjects,includingoneaccretingbinarystar(BLHyi)
an accreting binary star whose X-ray emission properties are
which is the only Galactic source in the sample. We will not
wellknown.
discuss this object any further as the main scientific target of
the HBS28 is the extragalactic population. In Appendix A.1
3.2.Theopticalspectrosocopy webrieflyoutlinethemainpropertiesofthissource.
Aredshifthasbeensecuredforallthe27extragalacticob-
A search through the literature has produced 5 spectroscopic jectsinthesample.TheredshiftdistributionisshowninFig.2.
identifications (3 QSOs, 1 Narrow Line Radio Galaxy and To classify the emission line objects we have adopted the
1 accretingbinarystar). Three moreidentificationshavebeen criteriapresentedforinstanceinVe´ron-Cetty&Ve´ron(2001)
taken from the AXIS project(Barcons et al. 2002a,b);one of whicharebasedonthelinewidthsandthe[OIII]λ5007Å/Hβ
these 3 objects has been re-observed at the SUBARU tele- flux ratio. In particular, we have divided the extragalactic
scope (Severgnini et al. 2003). The remaining sources have sourcesintotwogroups,i.e.type1andtype2objects:
been observed and spectroscopically identified during dedi-
catedobservingruns.Mostofthesourceswereobservedatthe – Type1objects,including:
ESO3.6minMayandOctober2002usingtheEFOSC2cou- •17sourcesshowingbroad(FWHM > 2000kms−1)
pledwithCCD40andgrismn.13(October)orgrismn.6(May). permitted lines. These are Broad Emission Line AGNs
The slit width rangesfrom1.2(cid:6)(cid:6) up to 1.5(cid:6)(cid:6), dependingon the (BELAGN).ThisclassincludesSeyfert1,BroadEmission
seeing conditions, and was usually oriented according to the Line Radio galaxies(BLRG) and type1QSO. The optical
parallacticangleto avoiddifferentialflux losses, exceptwhen spectraofthe10BELAGNsidentifiedbyourownspectro-
itwasorientedsoastoincludetwoobjects.Fouradditionalob- scopicobservationsareshowninFig.3.
jects were observed at the TNG telescope in November2001 • 2 sources which show permitted lines with
and September 2002, using DOLORES and grism LR-B. For 1000 kms−1 < FWHM < 2000 kms−1 and a
the data reduction we have used the standard IRAF longslit [OIII]λ5007/Hβratiobelow3.Thesesourcesareclassified
906 A.Caccianigaetal.:TheXMM-NewtonHBS28sample
XBSJ002618.5+105019 XBSJ013944.0-674909
1
0.5
0
XBSJ015957.5+003309 XBSJ021640.7-044404
1
0.5
0
XBSJ030614.1-284019 XBSJ185613.7-462239
1
0.5
0
XBSJ205635.7-044717 XBSJ213002.3+153414
1
0.5
0
XBSJ214041.4-234720 XBSJ220601.5-015346
1
0.5
0
4000 5000 6000 7000 8000 4000 5000 6000 7000 8000
Fig.3.Theopticalspectraof10newlydiscoveredBroadEmissionLinesAGNs(FWHM>2000kms−1).
as Narrow Line Seyfert 1 (NLSy1) candidates. The pres- •1object(XBSJ193248.8–723355)whosespectrum
enceofarelativelystrongFeIIλ4570Åhumpfurthersup- presents only narrow lines but the [OIII]λ5007/Hβ ra-
portstheclassificationofthesesourcesasNLSy1.Theop- tio is about 2 thus suggesting a non-AGN origin. Also
tical spectra of the 2 NLSy1 are plotted in Fig. 4. Given the [OII]λ3727 Å/[OIII]λ5007 ratio is quite high (∼3) in
thelow-resolution(∼20Å)andtherelativelylowS/Nratio agreement with what is usually observed in starburst or
ofthesespectra,amoredetailedanalysisofthelineratios HII-region galaxies (e.g. Veilleux & Osterbrock 1987).
(e.g.theFeIIλ4570Å/Hβusedforamorequantitativesep- This source is thus classified as an Emission Line Galaxy
arationbetweenNLSy1andSy1e.g.byVe´ron-Cettyetal. (ELG);itsspectrumisshowninFig.5(bottompanel).As
2001)isnotpossible. describedinthenextsection,theX-raydataclearlyreveal
– Type2objects,including: thepresenceofanobscuredAGNinthisobject;
•5sourceswhichshowonlynarrowlines(FWHM < • 2 sources in which all the observed emission lines
1000 kms−1) and [OIII]λ5007/Hβ > 3. These objects are arenarrow(FWHM < 1000kms−1)exceptforabroadHα
classified as Narrow Emission Line AGNs. This class in- component.Thesesourcesarealsocharacterizedbyahigh
cludesSeyfert2galaxies,NarrowLineRadiogalaxiesand [OIII]λ5007/Hβratio(>3)andareclassifiedasSeyfert1.9
type 2 QSO. The opticalspectrumof the Seyfert2 identi- (Sy1.9).The opticalspectra of the two Sy1.9 are reported
fiedbyusisshowninFig.5(toppanel); in Fig. 6. In one of the sources (XBSJ031146.1–550702)
A.Caccianigaetal.:TheXMM-NewtonHBS28sample 907
z=0.181
z=0.193
z=0.232 z=0.162
Fig.4.Thespectraofthe2NLSy1candidatesdiscoveredinthesam- Fig.6. The spectra of the two Sy1.9 discovered in the sample:
ple:XBSJ023713.5–522734 (top)andXBSJ205829.9–423634 (bot- XBSJ040658.8–712457(top)andXBSJ031146.1–550702(bottom).
tom).Thetwosmallboxesinsidethetwofiguresshowtheexpanded
regionaroundtheFeIIλ4600Åhump.
Inconclusion,thepercentageoftype1 andtype2objects
is70%and30%respectivelyoftheextragalacticsources.The
redshiftdistributionsfortype1andtype2objectstakensepa-
z=0.134 ratelyare onlymarginallydifferent(K-Sprobabilityof6%of
beingdrawnfromthesameparentpopulation).
4. X-raydata
On the basis of the MOS2 data, used for the sample defini-
tion,wehavefirstlystudiedthehardnessratiosofthesourcesin
thesample.Thisisusefulasreferenceforother(deeper)sam-
plesforwhichtheavailablenumberofcountspersourceisnot
enough to perform a detailed spectral analysis. The hardness
z=0.287
ratios have been computed using the data in the 0.5–2.0 keV,
2.0–4.5keVand4.5–7.5keVenergyintervalsadoptingthefol-
lowingstandarddefinitions:
C(2.0−4.5)−C(0.5−2.0)
HR2=
C(2.0−4.5)+C(0.5−2.0)
C(4.5−7.5)−C(2.0−4.5)
HR3=
C(4.5−7.5)+C(2.0−4.5)
Fig.5. Top panel: the spectrum of the NEL AGN (a Seyfert 2)
where C represents the MOS2 corrected count-rates (see
XBSJ040758.9–712833. Bottom panel: The spectrum of the ob-
http://xmmssc-www.star.le.ac.uk/ for details) in the
ject XBSJ193248.8–723355 optically classified as a Emission Line
Galaxy (ELG, i.e. a starburst or a LINER, see text for details). As energyintervalsindicatedinparenthesis(inkeV).Thehardness
explainedinthetext,theX-rayanalysisindicatesthepresenceofan ratiosusedareweaklydependentontheGalacticNH (whichis
AGNinthisobject. below 1021 cm−2 in the fields used for the sample selection).
In Fig. 7 the HR3 is plotted against HR2. The figure clearly
the broad Hα component is clearly evident while in shows that the optical classification and the hardness-ratio
(HR2inparticular)arecoupled:nearlyall(7/8)thetype2ob-
XBSJ040658.8–712457itspresenceislessobviousandit
jects show HR2 > −0.3 while all but one type 1 sources
wouldrequireaproperde-blendingtobeclearlyquantified.
have HR2 values below this value. The two exceptions are:
InTable3theopticalpropertiesofthe28sourcesintheHBS28 XBSJ031146.1−550702, a Sy1.9 with HR2 = −0.58 and
samplearepresentedwhilethebreakdownoftheopticalclas- XBSJ031859.2−441627,atype1AGNwithHR2=−0.21.As
sificationissummarizedinTable4. discussedbelow,theX-rayanalysishasconfirmedtheabsence
908 A.Caccianigaetal.:TheXMM-NewtonHBS28sample
Table3.TheopticalpropertiesofHBS28.
name X-rayposition opticalposition id type z mag idref
(J2000) (J2000)
XBSJ002618.5+105019 002618.50+105019.3 002618.70+105019.5 BELAGN 1 0.473 17.5 TNG11/01
XBSJ013240.1–133307 013240.11–133307.8 013240.27–133307.2 NELAGN 2 0.562 20.0 ESO10/02
XBSJ013944.0–674909 013944.02–674909.4 013943.77–674908.4 BELAGN 1 0.104 16.7 ESO10/02
XBSJ014100.6–675328 014100.66–675328.9 014100.23–675327.6 E.L.star – – 16.4 (a)
XBSJ015957.5+003309 015957.52+003309.7 015957.60+003310.5 BELAGN 1 0.310 18.3 TNG11/01
XBSJ021640.7–044404 021640.74–044404.9 021640.66–044404.7 BELAGN 1 0.873 16.5 TNG11/01
XBSJ021808.3–045845 021808.34–045845.7 021808.20–045845.7 BELAGN 1 0.712 17.7 AXIS
XBSJ021817.4–045113 021817.40–045113.3 021817.43–045113.0 BELAGN 1 1.070 19.5 AXIS
XBSJ021822.2–050615 021822.28–050615.7 021822.15–050614.5 NELAGN 2 0.044 14.8 (b)
XBSJ023713.5–522734 023713.54–522734.4 023713.66–522734.7 NLSy1 1 0.193 17.1 ESO10/02
XBSJ030206.8–000121 030206.86–000121.2 030206.76–000121.7 BELAGN 1 0.641 18.8 (c)
XBSJ030614.1–284019 030614.19–284019.9 030614.21–284019.3 BELAGN 1 0.278 18.5 ESO10/02
XBSJ031015.5–765131 031015.56–765131.5 031015.88–765133.8 BELAGN 1 1.187 17.6 (d)
XBSJ031146.1–550702 031146.12–550702.5 031146.18–550659.9 Sy1.9 2 0.162 17.3 ESO10/02
XBSJ031859.2–441627 031859.29–441627.6 031859.41–441626.7 BELAGN 1 0.140 15.9 (b),ESO10/02
XBSJ033845.7–352253 033845.77–352253.4 033846.02–352252.7 NELAGN 2 0.113 17.0 (e)
XBSJ040658.8–712457 040658.87–712457.7 040658.88–712459.8 Sy1.9 2 0.181 18.7 ESO10/02
XBSJ040758.9–712833 040758.97–712833.5 040758.58–712832.2 NELAGN 2 0.134 17.0 ESO10/02
XBSJ041108.1–711341 041108.10–711341.1 041108.78–711342.8 BELAGN 1 0.923? 20.3∗ ESO10/02
XBSJ185613.7–462239 185613.75–462239.2 185613.78–462237.1 BELAGN 1 0.768 19.0 ESO05/02
XBSJ193248.8–723355 193248.80–723355.2 193248.77–723353.3 ELG 2 0.287 18.8 ESO05/02
XBSJ204043.4–004548 204043.46–004548.2 204043.46–004551.9 NELAGN 2 0.615 21.2∗ ESO10/02
XBSJ205635.7–044717 205635.75–044717.9 205635.67–044717.2 BELAGN 1 0.217 17.3 TNG09/02
XBSJ205829.9–423634 205829.97–423635.0 205829.89–423634.2 NLSy1 1 0.232 18.3 ESO10/02
XBSJ213002.3–153414 213002.32–153414.1 213002.30–153413.2 BELAGN 1 0.562 17.3 ESO10/02
XBSJ213824.0–423019 213824.03–423019.2 213824.03–423017.5 BELAGN 1 0.257 16.6 (f)
XBSJ214041.4–234720 214041.45–234720.1 214041.53–234719.3 BELAGN 1 0.490 18.4 ESO10/02
XBSJ220601.5–015346 220601.50–015346.9 220601.45–015346.1 BELAGN 1 0.211 20.1 ESO10/02
Column1:name;
Col.2:X-rayposition;
Col.3:opticalposition;
Col.4:spectroscopicclassification(BELAGN=BroadEmissionLineAGN;NELAGN=NarrowEmissionLineAGN;NLSy1=Narrow
LineSeyfert1;Sy1.9=Seyfert1.9;ELG=EmissionLineGalaxy);
Col.5:opticalspectraltype;
Col.6:redshift;
Col.7:APMredmagnitude;theasteriskindicatesthatthemagnitude(R)hasbeenderivedfromourownobservations;
Col. 8: reference for the identification i.e. the observing run, if the source has been identified by us, or a reference, if the source has been
identifiedfromtheliterature.Thenotesare:
(a)V*BLHyi.SeeAppendixA.1;
(b)identificationfromSevergninietal.(2003);
(c)identificationfromLaFrancaetal.(1992);
(d)identificationfromFioreetal.(2000);
(e)identificationfromCarter&Malin(1983);
(f)identificationfromHewettetal.(1995);
AXIS:identificationfromtheAXISproject(Barconsetal.2002a,b).
of a significantabsorptionin the firstsourceandthe presence 4.1.X-rayspectralanalysis
ofamildabsorptioninthesecondone.
Qualitatively, the correlation between the hardness-ratio Thanks to the relatively bright flux limit which characterizes
and the optical spectral type is expected in the contextof the thesample,mostofthesourceshaveenoughcountstoperform
unifiedmodels.Amorequantitativeassessmentofthiscorrela- areliableX-rayspectralanalysis.Exceptfor2sources,thetotal
tionisdoneinthefollowingSectiononthebasisofadetailed countsin the MOS2rangefrom100to 3900.The availability
spectralanalysis. oftheMOS1and,inmostcases,ofthepnincreasesthecounts
A.Caccianigaetal.:TheXMM-NewtonHBS28sample 909
Table4.Spectroscopicclassification.
class Spectraltype Number %extragalactic
Type1
BroadEmissionLineAGN 17 63%
NLSy1 2 7%
19 70%
Type2
NarrowEmissionLineAGN 5 19%
ELG 1 4%
Seyfert1.9 2 7%
8 30%
Accretingbinary 1
Total 28
simultaneouslytheMOSandpndata,leavingtherelativenor-
malization free to vary. During the fitting procedure only the
data in the 0.3−10 keV and 0.4–10 keV energy ranges are
considered for the MOS and the pn respectively. Errors are
given at the 90% confidence level for one interesting param-
eter(∆χ2 =2.71).
Initially, we attemptedto fit a simple absorbedpower-law
modeltoallthe27spectratakingintoaccountboththeGalactic
hydrogencolumndensityalongthelineofsight(fromDickey
& Lockman 1990) and a possible intrinsic absorption at the
sourceredshift.
For20sourcestheresultingfitisgoodenough(i.e.nosys-
tematic trends are observed in the residuals) and kept as the
best-fit modelwhile for the remaining7 sources a simple ab-
sorbedpower-lawmodeldoesnotgiveanacceptablefit.
In most (13) of the 20 objects that are well fitted by a
simple absorbed power-law model, no intrinsic absorption is
foundwhilefor7sourcesthebest-fitintrinsicN islargerthan
H
thelocalGalacticcolumndensity.Amongtheselattersources,
4 present an hydrogen column density larger than 1022 cm−2
andtherelatedbestfitphotonindexisflatter(Γ<1.5)thanthe
Fig.7. The hardness-ratio HR3 versus HR2. Filled circles represent
flattestΓfoundfortheunabsorbedAGNs(seebelow).Thisis
thetype2objectswhiletheopencirclesarethetype1objects.
probablyduetothepoorstatisticsavailableforspectralanaly-
sis which donotallow usto adequatelyconstrainat the same
by a factor ∼2 (if only the MOS1 is available) or ∼4 (if both timeboththespectralindexandtheabsorption.Inordertobet-
MOS1andpnareavailable)theactualnumberofcountsavail- ter constrain the correct value of N we have fixed for these
H
able for the spectralanalysis. For 7 sourcesthe pn was either 4sourcestheΓto1.9(whichcorrespondstotheaveragevalue
notavailableortheobjectwasoutsidethefieldofvieworun- foundforthetype1AGNs, seebelow)andderivedthecorre-
derabadcolumn.Inanothercasethesourcefallsunderagap spondingcolumndensityN .Thisprocedureisjustifiedinthe
H
in the MOS1 camera. In Table 2 the EPIC detectors used for contextoftheunifiedschemeaccordingtowhichtheabsorbed
the X-rayanalysisofeachsourcearesummarized.Beforethe AGNs should have the same intrinsic power-law and a larger
extraction of the spectra, we have removed the time intervals absorptionwith respect to the type 1 AGNs. Since the values
characterizedbyahighbackground. of Γ found for the type 1 AGNs have an intrinsic dispersion,
Allthespectrahavebeenextractedusingacircularregion wehavecheckedthevariationofN whenextremevaluesofΓ
H
witharadiusof20(cid:6)(cid:6)–30(cid:6)(cid:6),dependingonthesourceoffaxisdis- (withintheobserveddistribution)areassumed.Wehavefound
tance.Thebackgroundisextractedinanearbysource-freecir- thatthevariationofN isusuallywithinthereportederrors.
H
cularregionofaradiusafactor∼2largerthantheoneusedto Finally, for the 7 sources for which the simple absorbed
extract the source counts. When the same blocking filters are power-law model does not offer a good representation of the
usedinthetwoMOScamerasthedataarecombinedafterthe data,animprovedfitisobtainedwithamorecomplexmodel,
extraction. like a “leaky absorbed power-law” model4 or by adding a
The MOS and pn data have been re-binned in order to
have at least 15–25 counts per channel depending on the 4 This model consists of two power-laws, one absorbed and one
brightness of the source. XSPEC 11.2 was then used to fit unabsorbed,havingthesamephotonindex.
910 A.Caccianigaetal.:TheXMM-NewtonHBS28sample
Table5.Best-fitparametersfortheX-rayspectralanalysis(extragalacticobjects).
name id Model Γ NH kT redχ2/d.o.f. f2−10keV LogL2−10keV
(×1022) (×10−13)
XBSJ002618.5+105019 BELAGN PL 2.02+0.07 <0.011 ... 1.15/97 2.8 44.44
−0.06
XBSJ013240.1−133307 NELAGN PL 1.9(frozen) 2.54+0.71 ... 1.11/19 1.7 44.41
−0.56
XBSJ013944.0−674909 BELAGN PL 1.94+0.14 <0.018 ... 0.91/34 1.2 42.56
−0.12
XBSJ015957.5+003309 BELAGN PL 2.14+0.09 <0.010 ... 1.19/99 2.9 44.03
−0.09
XBSJ021640.7−044404 BELAGN PL 2.24+0.08 <0.020 ... 1.17/67 1.1 44.74
−0.09
XBSJ021808.3−045845 BELAGN PL+BB 2.01+0.04 <0.007 0.16+0.01 1.07/527 2.3 44.79
−0.04 −0.01
XBSJ021817.4−045113 BELAGN PL 1.84+0.04 <0.04 ... 1.07/346 2.7 45.22
−0.03
XBSJ021822.2−050615a NELAGN Leaky 1.66+0.34 20.54+0.36 ... 1.41/63 3.0 42.76
−0.36 −0.44
XBSJ023713.5−522734 NLSy1 PL+BB 1.90+0.18 <0.18 0.11+0.02 0.86/80 2.7 43.52
−0.13 −0.02
XBSJ030206.8−000121 BELAGN PL 1.90+0.06 <0.015 ... 1.14/125 2.2 44.63
−0.06
XBSJ030614.1−284019b BELAGN PL 1.56+0.26 <0.048 ... 1.15/16 2.9 43.87
−0.13
XBSJ031015.5−765131 BELAGN PL 1.92+0.04 <0.022 ... 0.90/189 3.4 45.47
−0.04
XBSJ031146.1−550702 Sy1.9 PL 2.08+0.13 <0.013 ... 0.93/44 2.8 43.37
−0.12
XBSJ031859.2−441627 BELAGN PL 1.58+0.29 0.33+0.43 ... 0.73/18 1.7 42.99
−0.29 −0.19
XBSJ033845.7−352253 NELAGN Leaky+RS 1.9(frozen) 28.5+6.7 0.16+0.04 0.83/29 1.7 43.21
−10.0 −0.08
XBSJ040658.8−712457 Sy1.9 PL 1.9(frozen) 21.97+17.67 ... 1.25/4 1.6 43.56
−12.78
XBSJ040758.9−712833 NELAGN Leaky 1.9(frozen) 28.23+21.96 ... 1.13/5 2.5 43.44
−14.64
XBSJ041108.1−711341 BELAGN PL 1.95+0.48 <0.61 ... 0.99/6 0.8 44.59
−0.36
XBSJ185613.7−462239 BELAGN PL 2.18+0.27 <0.058 ... 1.15/26 1.4 44.69
−0.24
XBSJ193248.8−723355 ELG PL 1.9(frozen) 1.14+0.25 ... 1.06/35 1.9 43.76
−0.21
XBSJ204043.4−004548 NELAGN PL 1.9(frozen) 3.00+1.15 ... 1.35/16 1.7 44.50
−0.90
XBSJ205635.7−044717 BELAGN PL+BB 1.91+0.51 <0.56 0.15+0.03 1.18/21 1.7 43.42
−0.35 −0.03
XBSJ205829.9−423634 NLSy1 PL 1.91+0.09 0.09+0.04 ... 0.76/120 3.2 43.76
−0.09 −0.03
XBSJ213002.3−153414 BELAGN PL 2.10+0.23 0.08+0.12 ... 0.82/26 1.8 44.45
−0.21 −0.08
XBSJ213824.0−423019c BELAGN PL 2.14+0.25 <0.057 ... 0.86/26 1.6 43.60
−0.17
XBSJ214041.4−234720 BELAGN PL 2.19+0.10 <0.012 ... 1.06/93 1.7 44.28
−0.10
XBSJ220601.5−015346 BELAGN PL+BB 1.56+0.16 <0.670 0.06+0.01 0.72/15 1.7 43.36
−0.09 −0.04
Column1:sourcename;
Col.2:opticalidentification;
Col. 3: best-fit model (PL = absorbed power-law; Leaky = leaky absorbed power-law; PL + BB = absorbed power-law plus Black-Body
spectrum;Leaky+RS=leakyabsorbedpower-lawplusaRaymond-Smithspectrum)
Col.4:best-fitphotonindexofthepower-lawcomponent;
Col.5:best-fitintrinsichydrogencolumndensity[cm−2];
Col.6:temperatureofthethermalcomponent(ifpresent)[keV];
Col.7:reducedChi-squaredanddegreesoffreedomofthebest-fit;
Col.8:observed2–10keVflux(correctedfortheGalacticabsorption)[ergs−1cm−1];
Col.9:logarithmoftheintrinsicluminosityinthe2–10keVenergyrange[ergs−1];
aresultsfromtheX-rayanalysistakenfromSevergninietal.(2003);
binthisobjectthepresenceofanemissionlineat1.9keV(sourcerestframe,consistentwiththeSiXIIIat1.86keV)isrequired;
cinthisobjectthepresenceofanemissionlineat6.4keV(sourcerestframe,consistentwiththeFeIKα)isrequired.
thermalcomponent.Inonecase(XBSJ040758.9–712833)the since,formostofthesesources,thevalueofΓhasbeenfixed
value of Γ in the “leaky absorbed power-law” has been fixed tothevalue1.9tobetterconstraintheabsorption.
to1.9tobetterconstraintheabsorption. Wediscussbelowingreaterdetailtheresultsforthesingle
classesofextragalacticobjects.
TheresultsoftheX-rayspectralanalysisfortheextragalac-
ticsourcesaresummarizedinTable5.TheX-rayspectraofthe
extragalacticsourcesarereportedinAppendixB.1. 4.2.Type1AGN
Thedistributionofthephotonindicesofthetype1objects Broad Emission Line AGNs. For 12 out of 17 Broad
isshowninFig.8.Themeanvalueis<Γ>=1.97withastan- Line AGNs the simple absorbed power-law model gives
darddeviationof0.16consistentwiththemeanvaluefoundin an acceptable fit (reduced χ2 below 1.2) and a column
othersimilarsamples(e.g.Mainierietal.2002).Thedistribu- density consistent with the Galactic value. In one object
tionofthephotonindicesofthetype2objectsisnotpresented (XBSJ031859.2–441627) an intrinsic column density (see