The blazar GB1428+4217: a warm absorber at z=4.72?

7 Pages·0.37 MB·English

by  A. C. Fabian

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed1February2008 (MNLATEXstylefilev1.4)
The blazar GB1428+4217: a warm absorber at z=4.72?
A.C. Fabian1, A. Celotti2, K. Iwasawa1 and G. Ghisellini3
1Institute of Astronomy, Madingley Road, Cambridge CB3 0HA
2SISSA, via Beirut, 2-4, 34014 Trieste,Italy
3Osservatorio Astronomico di Brera-Merate, viaBianchi 46, 23807 Merate (LC), Italy
1
0
0
2
ABSTRACT

n BeppoSAXobservationsofthe highredshift(z =4.72)blazarGB1428+4217confirm
a the presence of a complex soft X–ray spectrum first seen with the ROSAT PSPC.
J
Flattening below a rest frame energy of 5 keV can be accounted for by absorption

7 from an equivalent column density of (cold) gas with NH ∼ 8×1022 cm−2. Below 2
1 keV a (variable) excess of a factor ∼ 20 above the extrapolated absorbed spectrum

is also detected. These findings are consistent with and extend to higher redshifts
1
the correlation between increasing soft X–ray flattening and increasing z, previously
v
pointed out for large samples of radio–loud quasars. We propose that such features,
8
including X–ray absorption, soft excess emission as well as absorption in the optical
8
2 spectra,canbesatisfactorilyaccountedforbythepresenceofahighly–ionizednuclear
1 absorberwithcolumnNH ∼1023 cm−2,withpropertiespossibly relatedto the condi-
0 tions in the nuclearregionsof the hostgalaxy.High energyX–rayemissionconsistent
1 with the extrapolation of the medium energy spectrum is detected up to ∼ 300 keV
0 (rest frame).
/
h
Key words: galaxies: active - galaxies: individual: GB 1428+4217- X-ray: galaxies.
p
-
o
r
t
s
1 INTRODUCTION to those of lower redshift blazars of similar power and no
a
: strong evidence for different nuclear or jet conditions has
v High redshift Active Galactic Nuclei (AGN) are powerful been found.
Xi tools to study the physical and cosmological evolution of Here we present the results of BeppoSAXobservations
massiveblackholesandtheirrelationship with theirgalaxy of this source. They were in particular focused on: a) de-
r
a hosts. A particularly interesting class is formed by several termining the amount and shape of the flattening and thus
high redshift (z > 4), X-ray bright, radio–loud quasars itsnature,astheMECS allowsagooddeterminationofthe
(Fabianetal.1997,1998;Moran&Helfand1997;Zickgrafet mediumenergyX–rayspectrum;b)thedetectionofthehigh
al.1997;Hook&McMahon1998)whichpresentcharacteris- energyPDScomponent,whichconstrainsthepositionofthe
ticstypicalofblazars.Interestingly,theirX–rayspectrahave γ–ray (inverse Compton) peak, which for GB 1428+4217 is
been recently found to systematically show the presence of predicted – by current models – to be at or below ∼ MeV.
spectralflatteninginthesoftX–rayband.InGB1428+4217 In the next section we present the analysis and results
(Boller et al 2000), RXJ1028.6-0844 (Yuan et al 2000) and oftheBeppoSAXdata,which arethendiscussed in Section
PMN0525-3443 (Fabianetal2000) suchflatteningimplies, 3.
if interpreted as due to intrinsic X-ray absorption by cold
gas,columndensitiesof1.5×1022cm−2,2.1×1023cm−2and
1.8×1023cm−2,respectively.Theseresultsappeartoextend
2 BEPPOSAX DATA
to higher redshift the trend already found in nearer (up to
z=4.2)radio–loudquasarsobservedbyASCAandROSAT GB 1428+4217 was observed by BeppoSAX on 1999, Feb
(Cappietal1997,Fioreetal.1998,Reeves&Turner2000), 4-7(seeTable1).Signalhasbeendetectedinallthreemain
theorigin of which is still unclear. instruments,thetwoimaginginstruments(LECS+MECS)
Inparticularforthemostdistantobject,GB1428+4217 andthePDSone(acollimator),withexposuretimesof27.3
at z = 4.72, previous observations in the X–ray and radio (LECS), 90.3 (MECS) and 46.0 (PDS) ks.
bands have convincingly shown that this AGN is indeed a Afitoverthe(1–10keV)bandwithasimplepower–law
blazar (Fabian et al. 1997, 1998). Its spectral energy dis- statistically agrees with thepreviousASCA results(Fabian
tribution (SED) appears similar, although more extreme, et al. 1997), both in spectral slope and in absolute flux.
(cid:13)c 0000RAS
2 A.C. Fabian, et al.
Table 1.Log of the Observations
Object Starttime Endtime
GB1428+4217 99-Feb-0418:04:47 99-Feb-0701:26:40
1ES1426+428 99-Feb-0820:48:57 99-Feb-0921:14:39
No evidence has been found of spectral features associated
withFeemissionaround∼1keV.However,BeppoSAXdata
provide us with interesting information at both the lower
and higher energies.
2.1 Low energy spectrum
The extrapolation of such a power–law below 1 keV (and
down to 0.4 keV) is well above the data, thus indicating
eitherabsorptioninexcessoftheGalacticone(columnden-
sity NH =1.4×1020 cm−2) or an intrinsic flattening of the
spectrum (see Fig. 1). In Table 2 we report the results of
thebestfitswithsuchadditionalcomponents:ifintrinsicto
thesourcethecorrespondingcolumndensityis∼7.8×1022
cm−2.Errors are at the90 percentconfidencelevelfor one
parameter.Thequalityofthedatadoesnotallowtostatisti-
callydistinguishamongthesetwopossibilities.Thespectral
slope and intensity are marginally consistent (at 2σ) with
thoseinferred from theASCAand ROSATdata(Fabian et
al 1997, Boller et al 2000).
However, below 0.4 keV, there appears to be an ex- Figure1.Theratioofthedatatotheabsorbedpower-lawmodel,
cess above the model (see Fig. 1). As this might be due afterremovaloftheintrinsicbutnottheGalacticabsorptioncom-
to residuals in the background subtraction, we performed ponents(toppanel);soft-to-mediumX–rayBeppoSAXspectrum
a more careful analysis and in particular re-extracted the of GB1428+4217 fitted over the 1–10 keV band by a power-law
spectrum byconsidering thelocal background.The low en- withcoldexcess absorption(bottom panel).
ergy excess is still present at the 2σ level and we therefore
considerit,withreasonableprobability,tobearealfeature.
This emission has not been detected in the ROSAT data,
at higher X–ray energies a flatter component dominates in
taken ∼ 2 months before, suggesting that this component
1ES1426+428.
might be variable on intrinsic timescales ∼ 10 days. Note
In the former case, a joint spectral fit to the datasets
that (marginal) indications of variability were also present
forGB1428+4217 and1ES1426+428 withsimplepower-law
duringthe ROSATobservation itself (Boller et al 2000).
models requires GB1428+4217 to vary a factor of ∼ 7 be-
tween thetwoobservations, i.e. overatimescale asshort as
2d,whilethehighredshiftquasardidnotshowsuchstrong
2.2 PDS data
and fast flux variations in the previous observations (note
A strong signal has been also detected in the PDS band. that they correspond to an intrinsic timescale of ∼ 8.4 hr).
However the flux is well above the extrapolation of the In the second case, the hard PDS spectrum of the
LECS+MECS power–law (see Fig. 2). We thus checked for 1ES1426+428 dataset is presumed to intrinsically flatten,
possible contamination in the PDS field of view, and in- similarly to that observed in other BL Lac objects (e.g.
deedfoundthepresenceoftheBLLacobject1ES1426+428 PKS2155–304, Chiappetti et al 1999), where the (flat)
which,fortuitously,hasbeenobservedbySAXfourdaysaf- inverse Compton component starts dominating over the
ter GB 1428+4217 (see Table 1). In collaboration with the (steep)synchrotronone.Wethenestimatedthecontribution
1ES1426+428 proposing team, we thustried to disentangle from 1ES1426+428 to the PDS data of the GB1428+4217
thecontributions of the two sources in thePDS. observationiteratively,byassumingthatthefluxesfromthe
Also the spectrum of 1ES1426+428 is well described twosourcesareconstantbetweentheobservationsandthat
by a power-law below 10 keV (Costamante et al 2000), the spectrum of GB1428+4217 is a single power-law across
with a photon index steeper than that of GB1428+4217, the BeppoSAXenergy range (from 0.5 to 100 keV). Uncer-
Γ ∼ 1.93. The two observed PDS spectra are however very tainties in the relative normalization factors between PDS
similar and significantly flatter than 1.9 (thedata from the andMECS areaproblem:inordertoreducethenumberof
1ES1426+428 dataset are even slightly harder). Therefore freeparametersand,consistentwiththeassumptionofcon-
this implies that either the dominant contribution in the stant fluxes, we fixedthese factors to 0.8 for both datasets.
PDScomesfromGB1428+4217 (assumingitsspectrumcan Fig. 3 shows the 1ES1426+428 data with the best-
beextrapolatedfromtheLECS+MECSpower–law)orthat fit power-law model for the LECS+MECS extrapolated
(cid:13)c 0000RAS,MNRAS000,000–000
BeppoSAX observations of GB1428+4217 3
Table 2.Results of the spectral fits (0.4 –10 keV)
Model NH Γ1 Ebreak,obs Γ2 χ2/d.o.f.
(1022 cm−2) (keV)
Power-law+freeintrinsicNH (0.4-10keV) 7.8+−86..70 1.45±0.10 – – 37.4/47
Brokenpower–law(0.4-10keV) NH,gal −1.5±2.5 0.86+−10..6189 1.41±0.09 36.7/46
Figure2.TheBeppoSAXspectrumofGB1428+4217.Theplot Figure3.Thespectrumof1ES1426+428withthebest-fitpower-
shows the best-fit power-law model to the LECS+MECS data. lawmodeltotheLECS+MECSdata.Thecontributionexpected
The surplus emission in the PDS band is likely due to contami- from GB 1428+4217 in the PDS band has been taken into ac-
nationfrom1ES1426+428. count (dotted line). There is still surplus emission in the PDS
band which is probably due to the BL Lac object itself, unless
GB 1428+4217 brightened by a factor of ∼ 7 since it was ob-
to the PDS energy range and with a contribution from servedwithBeppoSAXtwodaysbefore.
GB1428+4217 derived from its spectral fit (as in Fig. 2)
⋆
. Excess flux is still present in thePDS band.The surplus
emissionisattributedtoaflatspectralcomponentemerging
above10keVfrom1ES1426+428, i.e.theentirebroadband
spectrum of the BL Lac is represented by a broken power-
law. Once the estimated contribution from GB1428+4217
(Γ=1.45±0.10) isaddedthismodelgivesagoodfittothe
data (χ2 = 242.2 for 263 d.o.f., see Fig. 4). Photon indices
belowandabovethebreakenergyof9.26keVareΓ1 ≃1.93
and Γ2 ≃ 1.34, respectively. The corresponding spectrum
for GB1428+4217 is well fitted with the single power-law
(χ2 = 40.8 for 54 d.o.f., Fig. 5). Although the contribution
from 1ES1426+428 may be slightly overestimated (see the
residuals in the PDS band in Fig. 5), the PDS data in the
GB1428+4217 observation appear to be dominated by the
contamination from 1ES1426+428.
Theobserverframe2–10keVfluxesforthetwosources
are 2.76×10−12 ergcm−2 s−1 for GB1428+4217 and 2.0×
Figure 4. The spectrum of 1ES1426+428 fitted with a broken
10−11 ergcm−2 s−1 for 1ES1426+428 (compatiblewith the power-lawmodel.Theplotshowsthebest-fitmodelincludingthe
resultsfromtheASCAobservationof1994Feb6byKuboet contributionfromGB1428+4217(seethecaptionoftheprevious
al. 1998). The 20–100 keV fluxes are 1.1×10−11 erg cm−2 figure).
s−1 and 6.3×10−11 erg cm−2 s−1 for GB1428+4217 and
1ES1426+428, respectively.
3 DISCUSSION
We have presented the results of the BeppoSAX observa-
tionsofthemostdistantradio–loudX–rayluminousquasar
GB1428+4217. This hasallowed ustostudyandrevealin-
teresting features at the extremes of the BeppoSAX X–ray
⋆ Weadoptedatransmissionefficiencyof46percentinthePDS energy window.
fieldofviewduringthe1ES1426+428 observation. At the higher energy end of the BeppoSAX broad
(cid:13)c 0000RAS,MNRAS000,000–000
4 A.C. Fabian, et al.
and therefore in the following we also discuss the possible
role of absorption of intergalactic origin.
Weconsideritunlikelythatlarge(galactic) scalegasis
responsiblefortheX–rayabsorption,giventhelargemasses
of gas implied bysuch hypothesisas well as the lack of any
clear connection with the radio–loud phenomenon. There-
fore in the following we concentrate on the nuclear and/or
cosmological properties which could account for such fea-
ture.
A further key piece of information is the presence of a
systematic trend of the flattening in the spectra of radio–
loud quasars to increase with redshift (Cappi et al 1997,
Fiore et al. 1998, Reeves & Turner 2000). The inclusion of
the recent results on RXJ1028.6-0844 (Yuan et al 2000),
PMN0525-3343 (Fabian et al 2000) and GB1428+4217 it-
self strengthens and extends this behavior up to z ≥ 4.7,
Figure 5.Thespectrum of GB 1428+4217 fitted withapower-
as shown in Fig. 6. Although no correlation between the
lawmodelincludingthecontributionfrom1ES1426+428(dotted
flattening and other spectral property has been previously
line)whichisbasedonthebrokenpower-lawfit(seepreviousfig-
ure).ThecontributionfromtheBLLacobjectappearstodomi- found, we stress the possible presence of a significant trend
natethedetected PDScounts above20keV. ofincreasingNH with increasinghardX–ray(intrinsic2–10
keVband)luminosity,as can beargued from Fig. 7.Unfor-
tunately small statistics do not allow us to disentangle the
band spectrum, the contamination of the PDS data from
redshift and luminosity dependences.
1E1426+428 unfortunately does not allow us to determine
InFig.6wealsoshowtheline-of-sightvalueofNH due
the spectrum of GB 1428+4217 with sufficient accuracy to
to the intergalactic medium (IGM) assuming solar abun-
constrainanycurvaturewhichmightindicatethepositionof
dances. If the IGM was enriched by redshifts of about 4
the high energy γ–ray peak. However, from the decontami-
to the same metallicity as clusters of galaxies then the cor-
nation we performed, the spectrum appears consistently to
relation with redshift could be explained. As discussed in
extendtothehigherdetectedintrinsicenergies,i.e.280keV
the ROSAT PSPC work on GB 1428+4217 (Boller et al
in theblazar rest frame.
2000) this conclusion does not agree with observations of
the metallicity of the Lyman α forest. Only if there was a
strong correlation between enrichment and temperature of
3.1 Low energy spectrum
theIGMphasemightsomeagreement occur,buteventhen
At the lowest energies, < 1 keV, the BeppoSAX observa- theenrichment requirementswould be huge.
tions robustly confirmed the presence of a flattening in the A more plausible interpretation of the apparent corre-
spectrum found in the ROSAT PSPC spectrum (Boller et lation shown in Fig.6would thenbethatit arises from the
al.2000).Secondly,below0.4keVemission afactor ∼20in detectionlimitforabsorptioninpresentX-raydetectors.For
excessoftheflattenedspectrumhasbeenfound(seeFig.1). example, a limit of a few times 1020 cm−2 at z = 0 would
The origin oftheflatteninghasbeendiscussed in some correspondstoafew times(1+z)31020 cm−2 atredshift z,
detailbyseveralauthors(Cappietal1997,Fioreetal1998, which roughly scales with thepoints in Fig. 6.
Elvisetal1998,Bolleretal2000,Yuanetal2000,Fabianet Let us now examine the alternative hypothesis of an
al 2000, Reeves & Turner 2000), but no definite conclusion intrinsic origin of the flattening. As discussed by Fabian et
could be drawn. Certainly, the detection of such feature by al (2000), on nuclear scales this might arise because of: a)
different instruments (ROSAT, ASCA, BeppoSAX) argues the characteristic shape of the energy distribution of parti-
against any systematic mis-calibration effect. cles and/or photons which are believed to be responsible –
Thefirstinterestingpointwhichhasbeenmadeisthat throughinverseComptonscatteringonsoftexternalphoton
the flattening seems to be associated only with radio–loud field–forthehighenergyspectralcomponentinblazars;b)
objects, therefore suggesting its origin to be intrinsic. The absorption by nuclear, highly ionized, gas.
numberofgoodqualityX-rayspectraofradio–quietquasars Let us therefore consider in turn how these two possi-
at z >1.5 is unfortunately limited. However, the recent re- bilitiescanaccountforthespectralcharacteristicsdiscussed
sults by Vignali et al (2000), combined with the findings sofar.These,includingtheresultsofthepresentanalysison
by Reeves & Turner (2000), imply that in only 2 out of 15 GB1428+4217, canbesummarizedas:aspectralflattening
radio–quiet quasars (in the range z = 1.8−2.5) a positive below∼5keV,softexcessemissionbelow∼2.3keV,apos-
detection of flattening corresponding to NH ∼ 1022 cm−2 sibleconnection with radio–loudness/jets and with thered-
has been established, despite the biased selection of X–ray shiftand/orX–rayluminosity,anopticalspectrumshowing
loud sources. For comparison, 5 out of 6 radio–loud objects evidenceforabsorptionfeatures(Lyα-NVandCIV)(Hook
inthesameredshiftrangehaveequivalentNH ≥1022 cm−2 &McMahon1998;seeFabianetal2000forthesimilarcase
(seeFig.6).Wethereforefavourthepossibilitythattheflat- of PMN 0525-3433). We recall that the latter (optical) fea-
teningisduetoanintrinsicpropertyofthesource,possibly ture is indeed typically seen in outflows (Brandt, Laor &
associated with the radio–loudness phenomenon. Neverthe- Wills 2000).
less we note that radio-loud objects tend to be more X–ray Within scenario a), which implicitly links theobserved
luminous and so yield the best spectra at a given redshift propertieswith blazar–typesources,theflatteningcouldbe
(cid:13)c 0000RAS,MNRAS000,000–000
BeppoSAX observations of GB1428+4217 5
Figure6.Absorptioncolumndensity-asmeasureofthesoftX–
rayflattening -vsredshiftofradio–loudquasars.Theycomprise
the ASCA results reported by Reeves & Turner (2000) (filled
squares) and the three sources above z > 4 (GB 1428+4217,
PMN0525-3343, RXJ1028.6-0844, filled triangles). The empty
symbolsindicatetheGalacticcolumnforeachobject.Thedashed
line corresponds to the IGM column density, for Ω0 = 0.3,
Ωb=0.06andsolarmetallicity).
Figure 8. a) Fit with awarm absorber of the BeppoSAXdata,
foracolumndensityNH=1023.5cm−2andionizationparameter
ξ = 650; b) the corresponding model. Edges at ∼ 1.5 keV (Fe),
∼0.25(Mg)and0.3keV(Si)areclearlyvisible.Solarmetallicity
hasbeenassumed.
Figure 7. Absorption column density - as measure of the soft
X–rayflattening-vshardX–rayluminosity(2–10keV).Symbols distribution below its peak, which translates into a flatten-
andobjects asinFig.6.
ingin thescattered inverseCompton component at soft X–
ray energies. Also the presence of a soft X–ray excess, as
seen in GB 1428+4217 and RXJ1028.6-0844 could be con-
caused by several effects, such as: an intrinsic break in the
sistentwiththisscenarioasduetoeitherComptonizationof
low energy part of the non–thermal electron distribution
a cold electron component in the jet (bulk Compton emis-
(e.g. caused by incomplete cooling) which would require a
breakenergy∼fewmec2;apaucityinthe(relative)amount sion, Sikora et al. 1994) or the high energy tail of the soft
radiation field.
of synchrotron photons which are scattered at higher ener-
gies(synchrotronself–Comptonemission)andthustypically Neverthelessaseriousproblemariseswiththisscenario,
if the soft X–ray spectrum is produced by low energy elec-
contributetothesoft-to-mediumX–raybandflux,fillingthe
‘depression’ between the synchrotron and inverse Compton trons. In fact, while the uncertainties in the determination
componentsin blazars(e.g. Sikora,Begelman & Rees1994) of the spectral shape/absorption in the high redshift ob-
† jects does not allow the presence of any spectral variation
;theintrinsicflatteningoftheexternalsoft photonenergy
tobefound,softX–rayfluxvariabilityhasbeenestablished
for GB1428+4217 on timescales ∼ 1 d (Fabian et al. 1999).
† Notethatanincreaseinthedominanceoftheexternal Comp- Although strong and fast variability is of course expected
ton component would be also naturally associated with an in- in blazars thankstotheenhancementcaused byrelativistic
creaseinthesourceX–rayluminosity. beaming,thecoolingtimescalesofelectronsemittingatsoft
(cid:13)c 0000RAS,MNRAS000,000–000
6 A.C. Fabian, et al.
ionization parameter is defined as ξ ≡ L/(nR2); L is the
ionizingluminosityfrom1to1000Ryd).InFig.8wereport
the results of the spectral fitting to the BeppoSAX data
with such a model, as calculated using the photoionization
codeCLOUDYC90.04 (Ferland et al. 1998). The predicted
model spectrum (see Fig. 8) clearly shows the presence of
edges at ∼ 1.5 keV (FeXX-FeXXIV) and ∼ 0.25−0.3 keV
(Mg). Most absorption is due to oxygen. Solar metallicity
has been assumed in thesenuclear regions. χ2 =29.2/40.
We further explored the viability of such scenario by
considering previous observations of GB 1428+4217. In
Fig.9thecontourplotsforthetwocriticalparametersofthe
model–thegascolumndensityandtheionizationstate–are
shown for the BeppoSAX, ASCA and ROSAT PSPC data
(Fabian et al. 1997, 1998). A common range of (marginally
consistent) parameters, around ξ ∼ 600−700 and column
densities ∼ 3×1023, can be found for the three datasets.
Some of the variation may be due to spectral variability
among theobservations.
Note that if this picture is correct the lack of signifi-
cant extinction in the optical–UV bands (similar to what
seen in PMN 0525-3433) also requires that the absorbing
gas is highly ionized and/or rich in metals and lacking of
dust. (The observed intense and fast variability is inconsis-
tent with being caused by the postulated warm absorber
itself and therefore has to be ascribed to the primary emis-
sion from relativistic plasma.)
Theionizationparameterξ,columndensityNH andlu-
minosityLalongourlineofsight toGB1428+4217 require
the warm absorber to be located at an estimated distance
from the nucleusof less than about 200 pc. The increase of
the amount of absorption with redshift but also with lumi-
nosity, argues for an even larger increase in the amount of
gas,plausiblyassociatedwithalarge(super–Eddington)ac-
cretionrateofinfallinggasintheearlystagesofnuclearac-
tivity,and/oroutflowspossiblyconfiningpowerfulradiojets.
Thewarm absorbermayalso betheinnerpartofthedense
interstellar medium of the young host galaxy (see Fabian
1999 for a more general discussion).
Ifthisgascompletelyenvelopesthesourcethenitsmass
is∼109r2200 M⊙,wheretheradiusis200r200 pc.Itispossi-
ble that in directions well away from the axis of the blazar
jetthesurroundinggasisoflowionizationandsoopaqueto
allbuthardX-raysandthatonlyinthejetdirectiontheX-
rayintensityispowerfulenoughtoionizethegas.Therefore
in more extreme/high z sources, such gas might completely
hide the presence of activity in the soft-to-medium X–ray
Figure 9. Confidence contours for the two parameters NH and band.Undersuchconditionsofhighopticaldepthsalso the
ξ describing the warm absorber properties, fitting BeppoSAX, nucleardiskradiation mightbepartlyreflectedbackbythe
ASCAandROSATdataofGB1428+4217(seereferencesinthe gas, thus enhancing the nuclear radiation energy density.
text). A (marginally)consistent solutiondescribingall the three Thehighamount ofscattering gas shouldalso allow signifi-
datasets can be found around NH = 1023.3−23.5 cm−2 and ξ ∼ cantdetectionofnuclearemissioninthemostpowerfulradio
600−700.
galaxies.
Thedecreaseinabsorptionseeninobjectsbelowared-
shift ∼1 (seeFig. 6) may beassociated with thisgas being
X–ray energies - as discussed in scenario a) - would greatly consumed by star formation, or having been blown out of
exceed the observed variability timescale. thehostbyaquasarwind.Thepredominanceofabsorption
In scenario b), i.e. absorption by small scale gas, such inradio–loud objects could berelated tothemhavingmore
as nuclearabsorbing winds/outflows, theflatteningand the massive host galaxies (and thus it being more difficult to
soft X–ray excess can be successfully accounted for by the blow the gas away) and/or to the radio phase being associ-
presence of a warm/highly ionized absorber with a column ated with the quasar youth.
density of the order of 1023 cm−2 and ξ ∼ 102 (where the Observations of GB 1428+4217 and similar high red-
(cid:13)c 0000RAS,MNRAS000,000–000
BeppoSAX observations of GB1428+4217 7
shift blazars with high sensitivity and energy resolution in-
struments,suchasthoseprovidedbyXMM,willsoon allow
us to determine whether these conditions are ubiquitous in
highzsources.Theyshouldrobustlytesttheproposedwarm
absorberscenariobydetectingtheedgespredictedinFig.8
andsoenableustoinferthephysicalconditionsintheactive
nuclear regions of primeval galaxies.
4 ACKNOWLEDGMENTS
We thank Gary Ferland for the use of his code CLOUDY.
TheRoyalSociety (ACF)and theItalian MURSTandASI
grant ARS-99-74(AC) are thanked for financial support.
REFERENCES
BollerTh.,FabianA.C.,BrandtW.N.,FreybergM.J.,2000,MN-
RAS,315,L23
BrandtW.N.,LaorA.,WillsB.,2000, ApJ,528,637
CappiM.,MatsuokaM.,ComastriA.,BrinkmannW.,ElvisM.,
PalumboG.G.C.,VignaliC.,1997, ApJ,478,492
Chiappetti L.,etal,1999,ApJ,521,552
CostamanteL.,etal.,2000,A&A,submitted
ElvisM.,FioreF.,GiommiP.,Padovani P.,1998,ApJ,492,91
FabianA.C.,1999, MNRAS,308,L39
FabianA.C.,BrandtW.N.,McMahonR.G.,HookI.,1997,MN-
RAS,291,L5
Fabian A.C., Iwasawa K., McMahon R.G., Celotti A., Brandt
W.N.,HookI.M.,1998, MNRAS,295,L25
FabianA.C.,CelottiA.,IwasawaK.,CarilliC.L.,McMahonR.G.,
Brandt N.W., Ghisellini G., Hook I.M., 2000, MNRAS, in
press
Fabian A.C., Celotti A., Pooley G., Iwasawa K., Brandt N.W.,
McMahonR.G.,HoenigM.D.,2000,MNRAS,308,L6
FerlandG.J,KoristaK.T.,VernerD.A.,fergusonJ.W.,Kingdon
J.B.,VernerE.M.,1998,PASP,110,761
FioreF.,ElvisM.,GiommiP.,Padovani P.,1998,ApJ,492,79
HookI.,McMahonR.G.,1998,MNRAS,294,L7
Kubo H., Takahashi T., Madejski G., Tashiro M., Makino F.,
InoueS.,TakaharaF.,1998,ApJ,504,693
MoranE.C.,HelfandD.J.,1997,ApJ,484,L95
Reeves J.N., Turner M.J.L., 2000, MNRAS, in press (astro-
ph/0003080)
SikoraM.,BegelmanM.,ReesM.J.,1994,ApJ,421,153
Vignali C., Comastri A.,Cappi M., Palumbo G.G.C., Matsuoka
M.,2000(astro-ph/0007163)
YuanW.,MatsuokaM.,WangT.,UenoS.,KuboH.,MiharaT.,
2000,ApJ,submitted(astro-ph/0006148)
ZickgrafF.-J.,VogesW.,KrautterJ.,ThieringI.,AppenzellerI.,
MujicaR.,SerranoA.,1997, A&A,323,L21
(cid:13)c 0000RAS,MNRAS000,000–000