Table Of ContentProceedingsoftheSymposium:“Japan-GermanyWorkshopon AGNandthe X-rayBackground”
heldin Tokyo,Japan,November1-3,1999, eds.T.Takahashiand H.Inoue, ISAS Report,pp.0-0
Warm Absorbers in Active Galactic Nuclei
Stefanie Komossa
Max–Planck–Institut fu¨r extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany
0 email: [email protected]
0
0
2
n Abstract. We first provide a review of the properties of 2251-178(Halpern 1984). With the improvedspectral ca-
a
warm absorbers concentrating on what we have learned pabilities of ROSAT and ASCA, many more warm ab-
J
from ROSAT and ASCA. This includes dusty and dust- sorbers have been found1, and theoretical modeling of
4
free warm absorbers, non-X-ray emission and absorption plasmaunder‘warmabsorber’conditionshasbeenpushed
1
features of warm absorbers, and the possible warm ab- forward2.
1 sorber interpretation of the peculiar 1.1 keV features. Below, we first provide a review on what is known on
v We then discuss facets of warm absorbers by a more warm absorbers after the ‘ROSAT and ASCA epoch’ of
3
6 detailedinvestigationofindividualobjects:Inafirstpart, observations and theoretical modeling (Sect. 2). We then
2 we discuss several candidates for dusty warm absorbers. discussinmoredetailthe possibilitythatseveral(butnot
1 In a second part, we review and extend our earlier study all) warm absorbers contain dust (Sect. 3). In Sect. 4 we
0 ofa possible relationbetweenwarmabsorberandCLRin investigate the possible relation of WA and optical/UV
0 NGC4051, and confirm that both components are of dif- emission line regions with focus on NGC4051. Sect. 5 is
0
ferentorigin(theobservedcoronallinesareunderpredicted concerned with the warm absorber in MR2251-178. In
/
h by the models, the warm absorber is too highly ionized). Sect. 7 we explore whether and under which conditions a
p We then suggest that a potential overprediction of these warm absorber provides an explanation for the dramatic
-
o lines in more lowly ionized absorbers can be avoided if X-ray spectral variability of RXJ0134-4258.
r thesewarmabsorbersaredusty.Inathirdpart,wepresent
t
s first results of an analysis of a deep ROSAT PSPC ob-
a
servation of the quasar MR2251–178, the first one dis-
:
v covered to host a warm absorber. Finally, we summa-
i
X rizeourscrutinyunder whichconditionsawarmabsorber 1 e.g., MCG6-30-15 (Nandra & Pounds 1992, Fabian et al.
r could account for the dramatic spectral variability of the 1994, Otani et al. 1996b, Reynolds et al. 1997, Orr et al.
a Narrow-line Seyfert1 galaxy RXJ0134-4258. 1997, and references therein), NGC3783 (Turner et al. 1993,
George et al. 1995, 1998c), 3C212 (Mathur 1994), 3C351
(Fiore et al. 1993, Mathur et al. 1994, Nicastro et al. 1999b),
NGC3227 (Ptak et al. 1994, Komossa & Fink 1997b, George
1. Introduction et al. 1998b), NGC4051 (e.g., Pounds et al. 1994, Mihara et
al. 1994, McHardy et al. 1995, Guainazzi et al. 1996, Ko-
Warm absorbers (WAs) are an important new diagnostic
mossa & Fink 1997a, Guainazzi et al. 1998, and references
of the physical conditions within the central regions of therein), NGC 4151 (Weaver et al. 1994, Warwick et al. 1995,
active galaxies. They have been observed in 50% of the 1996), IC 4329A (Cappi et al. 1996, Perola et al. 1999), NGC
∼
well-studied Seyfert galaxies and have also been detected 3786(Komossa&Fink1997c),NGC5548(Nandraetal.1993,
in quite a number of Narrow-line Seyfert1 galaxies. So Mathur et al. 1995, 1999, Done et al. 1995), NGC3516 (Kriss
far, they revealedtheir existence mainly in the soft X-ray etal.1996a,Mathuretal.1996),Mrk290(Turneretal.1996),
spectralregionwheretheyshowupbytheircharacteristic Mrk1298(Wangetal.1996,1999,Komossa&Fink1997d,Ko-
metalabsorptionedges.Thestudy ofthe ionizedmaterial mossa & Meerschweinchen 2000), IRAS13349+2438 (Brandt
et al. 1996, 1997, Komossa et al. 1999b, Siebert et al. 1999),
provides a wealth of information about the nature of the
IRAS17020+4544(Leighlyetal.1997,Komossa&Bade1998),
warm absorber itself, its relation to other components of
PG1114+445 (George et al. 1997), 4C +74.26 (Brinkmann
the active nucleus, and the intrinsic AGN X-ray spectral
et al. 1998, Komossa & Fink 1998); see Reynolds (1997) and
shape, and leads to a better understanding of AGN in
Georgeetal.(1998a)forasystematicstudyofWAswithASCA
general. 2 e.g., Halpern 1984, Krolik & Kallman 1984, Netzer 1993,
The presence of an ionized absorber was first discov- 1996, Krolik & Kriss 1995, Murray et al. 1995, Reynolds &
ered based on Einstein observations of the quasar MR Fabian 1995, Komossa & Bade 1998, Nicastro et al. 1999a
2 St. Komossa: Warm Absorbers in AGN
2. Basic properties of warm absorbers: dusty where d is the distance between observer and warm ab-
WAs, dust-free WAs, peculiar 1.1keV features sorber. The mass of the warm absorber is approximately
given by
2.1. Warm absorber models and definitions
The active nucleus is thought to contain an accreting su- Mwa ≃4πr2Nwmpη =0.02r126N22η M⊙ , (4)
permassive black hole (SMBH), surrounded by the pu- with r =1016r cm and N =1022N cm−2.
16 w 22
tative molecular torus and two emission line regions, the From direct spectral fits, one first determines U and
broadlineregion(BLR)andthenarrowlineregion(NLR). N .Iftheintrinsiccontinuumturnsouttobevariable,or
w
The physical properties of the two emission line regions if there is evidence that the warmmaterialcontains dust,
arequitewellknown,sinceawealthofinformationcanbe important constraints on the density and distance of the
extracted from emission line intensities, line profiles, line absorberfrom the nucleus canbe obtainedfromreaction-
shifts,andlinevariability(seePeterson1993forareview). timescalearguments,anddustsurvivalarguments,respec-
Concerning the nature and location of the warm ab- tively. In particular, the recombinationtimescale t of a
rec
sorber,severaldifferent models havebeen suggested:(i) a warm absorber is given by
relationoftheWAtotheBLR(ahigh-densitycomponent
1 n 1 T
of the inner BLR, a BLR confining medium, winds from n i ( )X (5)
e ≈ t n A 104
bloated stars, or a matter bounded BLR component; see, rec i+1
e.g., Reynolds & Fabian 1995, Shields et al. 1995, Netzer (Krolik&Kriss1995)wheren/n isthe ionabundance
i i+1
1996), (ii) an accretion disk wind (e.g., K¨onigl & Kartje ratio of the metal ions dominating the cooling of the gas,
1994,Murrayetal.1995),(iii)arelationtothetorus(e.g., n the electron density, and the last term is the corre-
e
Reynolds1997,Komossa&Fink1997b),or(iv)arelation sponding recombination rate coefficient α−1 (Shull &
i+1,i
to the NLR (see, e.g., the two-component WA model of VanSteenberg1982;forcoefficientsA,X seeAldrovandi&
Otani et al. 1996b). The reason for the large variety of Pequignot1973).Availableobservationslocatemostwarm
models discussed is that not all physical properties of the absorbers outside the bulk of the BLR.
warmabsorber(itsdensityn,columndensityN ,covering
w
factor η = ω/4π, distance r from the nucleus, elemental
2.2. X-ray appearance of warm absorbers
abundances Z, its velocity field, and the shape of the il-
luminating continuum) can be directly determined from 2.2.1. Dust-free warm absorbers
X-rayspectralfits,butonlycertaincombinationsofthese
parameters, defined below. Sofar,ionizedabsorbershavebeenmainlyobservedatX-
rayenergies;therefore,mostofourknowledgeonthisma-
The two most important quantities are the column
terialcomesfromtheanalysisofX-rayspectra.Warmab-
density N and ionization parameter U of the warm ab-
w
sorbersshowupinthesoftX-raybandbytheircharacter-
sorber. N is given by
w
istic X-ray absorption edges which are created by K-shell
photoionization of highly ionized metal ions. The warm
N = n dx , (1)
w Z H
wheren isthetotalhydrogendensity,andxthethickness
H
of the absorber.The ionization parameter U is defined as
Q
U = . (2)
4πr2n c
H
Here, Q relates to the spectral energy distribution (SED)
which illuminates the warm absorber, and is the number
rate of photons above the Lyman limit3 (ν = 1015.512
0
Hz), given by
∞
f
Q=4πd2 νdν , (3)
Z hν
ν0 Fig.1. X-ray absorption structure of a typical warm ab-
sorber. The thin solid line corresponds to the intrinsic X-
3 Inrelation towarmabsorbers, someauthorsusean‘X-ray
ray spectrum, the thick solid line shows the spectrum af-
ionization parameter’, counting only X-ray photons, because
terpassageofthe ionizedabsorber.The oxygenOVIIand
theionization state of thewarm material is dominated by the
soft X-ray,not theEUV-partof thespectrum. OVIII edges are marked.
St. Komossa: Warm Absorbers in AGN 3
material is thought to be photoionized by emission from he took as reddening indicator. Except for two outlyers,
the central continuum source in the active nucleus (for a most ofthe objects of his sample showlow reddening and
detailed model invoking shocks (Meier & Demmig 1997) weak OVII absorption,whereas four objects show the op-
in additionto photoionizationsee Contini & Viegas 1999; posite (high reddening, a larger optical depth of OVII)
seealsothediscussioninNicastroetal.1999a).Itsdegree and these latter are suggested to likely host dusty warm
of ionization is higher than that of the bulk of the BLR absorbers. No such trend is found for OVIII.
and the gas temperature is typically of order 105 K. Fig.
1 shows the X-ray spectrum of a typical warm absorber.
2.2.3. Peculiar 1.1 keV features
Themostprominentabsorptionedgesaremarked.Incase
of high coveringfactor of the warmabsorber,X-rayemis- Warm absorbers have also been observed in several
sion and reflection becomes significant and will partially Narrow-line Seyfert1 galaxies (NLSy1s hereafter; e.g.,
fill up the edges (e.g., Netzer 1993 (his Fig. 1-3), 1996). Vaughan et al. 1999, Komossa & Meerschweinchen 2000,
andreferencestherein).Inadditiontothe‘standard’warm
absorbers with strong edges of OVII and OVIII around
2.2.2. Dusty warm absorbers
0.7-0.8 keV, peculiar absorption features have been re-
Warm absorbers have been mostly modeled using the ported around 1 keV (e.g., Brandt et al. 1994, Otani
∼
codesCloudy(Ferland1993,Ferlandetal.1998)andION etal.1996a,Ulrich-Demoulin&Molendi1996,Hayashida
(Netzer 1993, 1996). For several years, the ionized ab- 1997, Leighly et al. 1997, Matt 1998, Ulrich et al. 1999,
sorberswereassumedtobe dust-free;partlybecausethey Vaughan et al. 1999).
were observed in bright, optically unreddened Seyfert1 Mostinterpretationsofthesefeaturefocussedonwarm
galaxies, and there was no necessity at all to suspect this absorbers,albeitinverydifferentways.Thesuggestedex-
highly ionized material to contain dust. planations are: (i) Non-solar metal abundances: Absorp-
The situation changed with the cases of 3C 212 tion around1.1keVcouldbe due to Ne-Kabsorptionbut
(Mathur 1994), IRAS13349+2438 (Brandt et al. 1996), would require a rather peculiar abundance ratio of Ne/O
and NGC3227 (Komossa & Fink 1996, 1997a): 3C 212 forthe Ne absorptionto be muchstrongerthanthe O ab-
is a quasar with a heavily reddened optical spectrum. Its sorption.Inaddition,the strongestNe edgeinmostmod-
X-ray spectrum was successfully fit by several models in- els is located at 1.36 keV at somewhat higher energies
cluding a powerlaw plus excess cold absorption (Elvis et
al. 1994). Since other multi-wavelength observations did
not indicate a coldabsorberof high columndensity along
the line of sight, Mathur (1994) suggested that the dust
that reddens the optical spectrum is mixed with a warm
absorber, instead. She also showed that this material can
consistentlyaccountfortheobservedMgIIUVabsorption
of 3C 212. In the cases of IRAS 13349 and NGC3277 the
largecolumndensityN ofdustygasinferredfromopti-
opt
cal observations is inconsistent with the one (N ) derived
x
fromX-rayspectralfitsinthesensethatN N (here-
x opt
≪
after referred to as ‘N N discrepancy’). The incon-
opt x
−
sistency disappears,if the dust is accompaniedby ionized
gas. Further candidates for dusty warm absorbers which
show the same N N discrepancy emerged quickly:
opt x
−
NGC3786,MCG6-30-15,IRAS17020+4544,andpossibly
4C74.26 (see Tab. 1).
Warm absorber models taking into account the pres- Fig.2. Influence of dust mixed with the warm absorber
ence of dust were first presented by Komossa & Fink on the X-ray absorption structure (logU = 0.5,logNw
−
(1996,1997a)andapplied to the ROSAT X-ray spectrum = 21.6). The straight solid line corresponds to the unab-
of NGC4051 (which turned out to be likely dust-free, see sorbed intrinsic spectrum, the heavy solid line to a dust-
Sect. 4 below), followed by NGC3227, NGC3786 (which freewarmabsorber(WA)withsolarabundances,thedot-
are well fit by dusty warm absorbers; Komossa & Fink ted line to a dust-free WA with the same depleted abun-
1997b,c), and MCG6-30-15 (which likely possess a two- dances as in the dusty model, andthe long-dashedline to
component WA one of which is dusty; Reynolds et al. thechangeinabsorptionstructureafteraddingdusttothe
1997). WA (other model parameters are fixed). Clear changes in
Inanalternativeapproach,Reynolds(1997)correlated the absorption structure are revealed for the models that
the optical depths of OVII and OVIII of a sample of include dust. The abscissa brackets the ROSAT energy
Seyferts with the ‘steepness’ of the UV continuum which range (0.1-2.4 keV); some prominent edges are marked.
4 St. Komossa: Warm Absorbers in AGN
than observed; Komossa & Fink 1998, Ulrich et al. 1999. galaxies (Crenshaw et al. 1999)suggests a common or re-
Alternatively, it could be caused by Fe-L absorption (ap- lated origin of UV and X-ray absorbing material.
plied to PG1404+226this requires an Fe overabundances
of > 25 solar; Ulrich et al. 1999). (ii) A warm absorber
× 2.4. Non X-ray line emission from warm absorbers
in relativistic outflow; the 1 keV feature is then caused
∼
by highly blueshifted OVII or OVIII edges (Leighly et al. Examples of UV-EUV lines that become strong under
1997; see also Brandt et al. 1994 and Otani et al. 1996a). warm absorber conditions were given in, e.g., Krolik &
(iii) Asequence ofresonance absorption lines incombina- Kriss 1995 (their Fig. 4), Shields et al. 1995, Netzer 1996
tion with a steep X-ray spectrum (Nicastro et al. 1999c). (his Fig. 10), and Komossa & Fink 1997d(their Fig.6),
It is important to keep in mind, though, that the ob- 1997e.
served features seem to occur always close to the posi- Observationally, the EUV emission line NeVIIIλ774
tionwherethedecliningsoftexcessemissionintersectsthe was detected in the spectra of high-redshift quasars
powerlawcomponent,andthepossibilityremainsthatthe (Hamann et al. 1995b,c,1998)and was suggested to orig-
absorption feature is only mimicked by a more complex inate from a warm absorber. (X-ray spectra of these ob-
spectralshape.Further,inthe caseofArk564the feature jects are not yet available for a check and more detailed
isbetterdescribedbyanemissionlinethananabsorption modeling).
complex (Vaughan et al. 1999, Turner et al. 1999). A more detailed approach then was to select individ-
ual, well observed AGN and to investigate whether the
physical conditions in one of the observed optical-UV
2.3. Relation to UV absorbers
emission line regions and the X-ray warm absorbers are
The ‘X-rays-only’approachto study warmabsorbers suf- identical, such that the ionized X-ray absorber could be
fersfromthe stilllimitedspectralresolutionofcurrentX- directlyidentifiedwithoneoftheknown(high-ionization)
raydetectorsmakingmeasurementsof,e.g.,red/blueshifts emission line regions. Mathur et al. (1994) presented de-
of the absorption edges difficult. A powerful approach to tailed modeling of the BELR of the quasar 3C351 which
constrain further the properties of warm absorbers is to also shows an X-ray warm absorber, and concluded that
combine the X-ray with UV observations, and to search high-ionization BELR and warm absorber of 3C351 are
forthe signatureofthe warmabsorberssimultaneouslyin not one and the same component. Komossa & Fink (e.g.,
both spectral regions. 1997a,b)studied the casesofNGC4051(summarizedand
The relation between UV and X-ray (cold) absorbers extended in Sect. 4 below) and NGC3227. They first de-
was examined by Ulrich (1988). Mathur and collabora- rivedallwarmabsorberpropertiesthatcanbedetermined
tors(e.g.,Mathur1994,Mathuretal.1994,Mathur etal. fromX-rayspectralfits,thenvariedtheremainingfreepa-
1995) then presented several good cases (3C 212, 3C351, rameters (like metal abundances), predicted for each case
NGC5548) where UV and X-ray warm absorber are very the optical/UV line emission from the X-ray absorbing
likely one and the same component (see also deKool & gas and compared with optical observations. Again, they
Meurs 1994 on PG1416-129). These absorbers are out- concluded that warm absorber and CLR in NGC4051,
flowing with velocities of 1000-3000km/s and are lo- andWAandemission-linecomponentsinNGC3227show
∼
cated outside the BLR. Three further good candidates different physical conditions. Reynolds et al. (1997) fol-
for a common UV–X-ray warm absorber are the high- lowed a similar approach in application to MGC6-30-15
redshift quasar PKS 2351-154 (Schartel et al. 1997), the andfindthat[FeX]and[FeXIV] couldbe explainedbyan
Sy1 galaxyNGC3783(Shields &Hamann 1997),andthe (outer) warm absorber of covering factor 0.08, but that
∼
quasar PG 1114+445 (Mathur et al. 1998). Hamann et this wouldheavilyunderpredictthe observed[FeXI]emis-
al. (1995a) reported the detection of NeVIIIλ774 absorp- sion. Contini & Viegas (1999) presented a detailed multi-
tion in the quasar UM675, and noted that this gas could cloud model in which both, photoionization and shocks
act as X-ray warmabsorber.Telfer et al. (1998) analyzed contribute to the heating and ionization of the clouds,
the very highly ionized BAL system of the quasar SBS and applied their model to NGC4051.They find that the
1542+541 and concluded that it is likely associated with observed X-ray spectrum and the NLR line emission can
an X-ray warm absorber. be successfully reproduced by an ensemble of clouds of
Sometimes, though, the UV absorptionis rather com- different densities. [Predicted emission line intensities for
plex andshows the presence ofdifferent components with coronal lines have not yet been reported.]
differentdegreesofionizationand/orvelocities(e.g.,NGC
3516,Krissetal.1996b,Crenshawetal.1998;NGC5548,
3. Dusty warm absorbers
Mathuretal.1999;NGC7469,Krissetal.1999),andonly
one component(e.g.,NGC5548)ornone (e.g,NGC7469) Below, we describe in more detail the properties of dusty
may be identified with the X-ray warm absorber. warm material, and provide a summary of the sug-
Inanycase,thenearone-to-onematchofthe presence gested candidates for dusty WAs including explicit spec-
ofUV-andX-raywarmabsorbersinasampleofSeyfert1 tral model fits.
St. Komossa: Warm Absorbers in AGN 5
Table 1. Summaryofthe candidatesfordustywarmabsorbers,listedinthechronologicalorderthey weresuggested.
The column “sug.” gives those references that proposed the presence of a dusty warm absorber, whereas the column
“mod.” lists the works that first modelled the X-ray data in terms of a dusty warm absorber. Values of the ionization
parameter U (Eq. 2) givenhere and elsewhere in the text refer to a continuum spectrum with α = 1.4 (between
EUV
−
Lyman-limit and 0.1 keV) and Γ as listed.
x
object type warm absorber fit references comments
Γixntr log U log Nw sug. mod.
3C212 ‘red’ quasar [1] dusty WA not yet fit
df –2.4 –0.5 21.9 [1] best-fit parameters of dust-free WA
IRAS13349+2438 NL quasar –2.9 –0.4 21.21 [2], [3] [10], [11]
df –2.2 0.7 22.7 dust-free WA for comparison
NGC3227 Sy 1.5 –1.9 –0.3 21.8 [4], [5] [4]
NGC3786 Sy 1.8 –1.9 –0.8 21.7 [6] [6]
MCG6-30-15 Sy 1 –2.2 21.7 [7] [7] (>)2-comp. WA
IRAS17020+4544 NLSy1 –2.8 0.7 21.61 [8] [12]
sil –2.4 1.0 21.61 [12] silicate species only
4C+74.26 radio quasar –2.2 –0.1 21.6 [9] [9] excess cold absorption possible
(1) fixedtothevalue Nopt determinedfromoptical reddening.References:[1] Mathur1994, [2,3]Brandt et al. 1996, 1997, [4] Komossa& Fink
1997b(seealsoKomossa&Fink1996),[5]Georgeetal.1998b,[6]Komossa&Fink1997c,[7]Reynoldsetal.1997(thepossibilityofadustyWA
in this galaxy was earlier brieflymentionedin Reynolds1997), [8] Leighly et al. 1997, [9] Komossa & Fink 1998, [10] Komossa & Greiner 1999,
Komossaetal.1999b,[11]Siebertetal.1999,[12]Komossa&Bade1998.
3.1. Modifications of the X-ray absorption structure al. 1993) is only of the order of a few eV (Greaves et
al. 1984).]
Several changes occur in the presence of dust mixed with
– There is a stronger temperature gradient across the
thewarmabsorber:(i)Gas-dustinteractionsinfluencethe
absorber, with more gas in a colder state. This and
thermal conditions in the gas and change its ionization
the previous effects lead to an effective flattening of
state(e.g.,Draine&Salpeter1979).Inparticular,forhigh
the X-ray spectrum (Fig. 2)
ionization parameters dust effectively competes with the
– The increasedsensitivity of dust to radiationpressure
gas in absorbing photons (Laor & Draine 1993). (ii) Dust
(e.g., Chang et al. 1987, Binette et al. 1997) likely
scatters and absorbsthe incident radiationand X-rayab-
causes the gas to outflow (if not otherwise confined)
sorption edges are created by inner-shell photoionization
leading to temporal changes of the absorber proper-
of metals bound in the dust (see Figs 4,5 of Martin &
ties. [Indeed, strong variability in X-ray count rate of
Rouleau 1991).
factors 10-15has been detected from the dusty-warm-
Underthefollowingassumptionswe(Komossa&Fink
absorber candidates NGC 3227 and NGC 3786 (Ko-
1997b,Komossa&Bade1998)havecalculatedasequence
mossa & Fink 1997b,c.)]
of dusty warm absorber models using Ferland’s (1993)
– The warm absorber emission in certain optical/UV
code Cloudy: (i) The dust composition (graphite and as-
lines, in particular the iron coronal lines, is much re-
tronomicalsilicate)andgrainsizedistributionwerechosen
ducedbecauseofthedepletionofironintodustgrains.
like in the Galactic diffuse interstellar medium (Mathis
[This likely also alleviates the problem of the recently
et al. 1977) if not mentioned otherwise, as incorporated
reported potential over-prediction of these lines from
in Cloudy, (ii) the metal abundances were depleted corre-
some warm absorbers; see Sect. 4 below].
spondingly(ameanofCowie&Songaila1986;seeFerland
1993, Baldwin et al. 1991 for details), (iii) the warm ma-
terial was assumed to be of constant density and ionized 3.2. Results from X-ray spectral fits: List of
by a ‘mean Seyfert’ IR to gamma-ray continuum (with suggested/modeled candidates
αuv−x=–1.4 in the EUV).
Themainconsequencesofthepresenceofdustinternal We have investigated most suggested dusty-WA candi-
to the warm absorber can be summarized as follows: dates with our models. We get successful X-ray spectral
fits.Insomecasesthebest-fitparameterscandeviatesub-
– Absorption edges in the X-ray spectrum from neutral stantially fromthose obtained by fitting a dust-free warm
metals bound in the dust, like a carbonCI edge (from absorber, though. Table 1 gives a summary of the sug-
the graphite species) and an oxygen OI edge (from gested dusty warm absorber candidates and the model
silicate) are predicted. [The shift in edge energy due results. The ionization parameters are found to be some-
to solid state effects (e.g., Martin 1970, Adamietz et what lower than those of dust-free warm absorbers, with
6 St. Komossa: Warm Absorbers in AGN
the exceptionofIRAS17020.Itis interestingto note that 3.4. Model uncertainties
the dusty warm absorbers show a rather narrow range in
The line of arguments in favor of dusty warm absorbers
columndensitiesN whichdifferbylessthanafactor 2.
w
∼ (reviewed in Sect. 2.2.2; see Brandt et al. 1996 and Ko-
Thisislikelypartlycausedbyselectioneffects:Firstly,X-
mossa & Fink 1997b for many further details) relies on
ray warm absorbers of lower column density are difficult
severalassumption.Firstly,opticalandX-rayobservations
to detect presently, and small deviations of the Balmer
were not performed simultaneously. Variability in the op-
decrement from the recombination value are less straight
tical reddening on the timescale of years is possible in
forwardly interpreted in terms of reddening. Secondly, in
Seyferts (e.g., Goodrich 1989,1995),but we note that for
case of higher column density the dusty gas becomes op-
both, IRAS17020 and IRAS13349 optical spectra taken
tically thick, and the concept of the standard extinction
at many different epochs are available and they do not
curve can no longer be applied.
show evidence for changes in reddening (e.g., Siebert et
al. 1999). Secondly, optical-UV and X-ray photons may
not travel along the same l.o.s, and they may therefore
3.3. Origin of the dusty material
‘see’differentamountsofdustandgas.This wouldbe the
case if the X-rays were seen mainly in scattered light, or
In case dust is mixed with the warm absorber, impor-
if there was a strong starburst contribution. Since most
tant constraints on its density and locations can be ob-
of the Seyferts with dusty warm absorber candidates are
tainedfromtherequirementofdustsurvival:Firstly,dust
rapidlyvariableinX-rays(e.g.,afactorof 2within800s
isheatedanddestroyedbytheradiationfieldofthecentral ∼
and a factor 10 within a few years in case of NGC3227;
source if at distances less than rev √L46 (e.g., Netzer ∼
≈ Komossa& Fink 1997b)these twopossibilities canbe ex-
1990;see Laor & Draine 1993for a dependence ofthis re-
cluded.
lationongrainsize).Secondly,dustisheatedbycollisions
As far as the modeling of dusty warm absorbers is
with gas particles (e.g., Baldwin et al. 1991)to a temper-
concerned, one uncertainty is the applicability of the
ature thatexceeds the evaporationtemperature ifthe gas
codeCloudy. Moreprecisely,radiationpressuremaydrive
density is high enough. Both approaches locate the dusty
strong outflows of the dusty gas. In case of large velocity
warm absorbers outside the BLR.
gradients,thetreatmentofoverlappingresonancelinesbe-
One component that suggests itself to be identified comes inaccurate (Ferland 1993),and the heating-cooling
with the dusty warm absorber is an atmosphere of the balance might be disturbed by expansional cooling of the
molecular torus which plays an important role in unifi- gas.
cation schemes. The torus has the advantage that it is Another uncertainty are the dust properties: Is the
‘known’ to be present (no need to invent a new compo- dust in Seyfert galaxies sufficiently similar to Galactic
nent),andwouldprovideanaturalreservoirfordustclose ISM-like dust (and is the latter really ‘MRN’-like in na-
to the central power source (but far enough away to en- ture4) ? Firstly, galactic ISM-like dust was also assumed
suredustsurvival).Oneisthenleadtoaviewinggeometry to estimate the amount of dust from the optical spec-
such that our line of sight grazes the torus atmosphere in tra. Therefore, making the same assumption in modeling
those Seyferts that show dusty warm absorbers (see also the X-ray data is a good starting point. Secondly, obser-
the discussion in Brandt et al. 1996, Reynolds 1997, and vations by ISO suggest that the dust/gas ratio in other
Komossa & Fink 1997b,c). galaxies does not deviate much from the Galactic value
(e.g., Bianchi et al. 1999). Thirdly, even for other dust
LookingatTab.1,onequestionarisesimmediately:the
compositionsorstructures,theX-rayabsorptionstructure
dustywarmabsorbersseemtocomeinall typesofSeyfert
wouldnotchangestrongly,becausethe X-rayspectrumis
galaxies, which is not what would be expected in a sim-
notdominatedbycomplexmoleculartransitionsandsolid
ple viewing-geometry-scenario. The sample is still small,
state andchemicaleffects (these mayhavesome influence
though, and it might be interesting to re-investigate this
on the heating-cooling balance, though), but by K-shell
possibility once the dusty warm absorbers are confirmed,
photoionization of metals bound in the dust.
and further ones are known.
The other already earlier suggested dusty component
3.5. Future perspectives
of the active nucleus, and thus potential dusty warm ab-
sorbercounterpart,isthe dusty transitionregionbetween Given the presence of X-ray absorption edges of neutral
BLR and NLR ‘invented’ and studied by Netzer & Laor dust-bound metals like the carbon edge, dusty warm ab-
(1993). The ionization parameter of this region should sorbers,if confirmed5 with XMM, AXAF and ASTRO-E,
show a smooth transition to that of the BLR and NLR,
though; whereas the ionization parameters of warm ab- 4 see, e.g., Laor & Draine 1993, and the recent review by
sorbers are typically much higher. Witt 1999
5 EvenifthedustyX-raywarmabsorbersarenotconfirmed,
thislikelytellsusalotaboutthedustinthesegalaxies, which
St. Komossa: Warm Absorbers in AGN 7
will play an important role not only in probing compo- ability arguments we derive a limit on the density of the
nents of the active nucleus, like the dusty torus, but also warm absorber, n < 3 107cm−3, which translates into
H
are they a very useful diagnostic of the (otherwise diffi- a distance from the∼nucl×eus of r >3 1016 cm.
cult to determine) dust properties and dust composition ∼ ×
in other galaxies.
4.3. Prediction of coronal lines
We then checked, for this best-fit warm absorber model,
4. The warm absorber in NGC4051, and the
the strengths ofthe optical/UVlines originatingfromthe
(missing) connection between WA and CLR
ionized material; in particular, the Fe lines [FeVII]λ6087,
4.1. Introduction [FeX]λ6374,[FeXI]λ7892,and [FeXIV]λ5303 (see Tab. 2).
We find that all of them are much weaker than observed!
Given the several good candidates for dusty warm ab-
We then variedthose parametersthat notstronglyin-
sorbers the question emerges whether all warm absorbers
fluence the X-rayabsorptionstructure,but couldhavean
contain dust. We think this is unlikely because (i) warm
effectonthepredictedFelinestrengths;namely:themetal
absorbers have also been observed in several Seyfert1
abundances, the gas density, the EUV continuum shape,
galaxies that do not show evidence for optical reddening,
the IR continuum strength. We find that in all cases, the
and (ii) in some cases the modifications in the X-ray ab-
lines [FeVII]–[FeXI] remain much weaker than observed
sorptionspectrumduetothepresenceofdustaresostrong
byseveralordersofmagnitude.Thereasonforthisisthat
that no successful X-ray spectral fit is possible.
the warmabsorberis alwaystoo highly ionized, with a to-
Dust-free warm absorbers might contribute signifi-
tally negligible amountofFe9+ andFe10+ ions inthe gas.
cantly to the emission in optical/UV iron coronal lines.
Therefore,changesincollisionalstrengthsforthe relevant
In the following we shall examine in detail the warm ab-
Fe transitions, which are still poorly known, are not ex-
sorberintheNLSy1galaxyNGC4051withrespecttothe
pected to alter this result. We conclude that, for the case
questionwhetherthisionizedmaterialcanaccountforthe
of NGC4051, warm absorber and coronal line region are
opticalcoronallinesobservedinthisgalaxy.Thisisanup-
not one and the same component, but of different origin.
date of our earlier work on this subject (see Komossa &
This is consistent with the recent findings of Nagao et al.
Fink 1997a;KF97a hereafter).
(2000) that the [FeXI] emission of NGC4051 is not con-
fined to the nucleus, but widely extended (out to at least
4.2. Observed properties of NGC4051 150 pc).
∼
We first give a brief summary of previous observational
properties of NGC4051 relevant for the present study 4.4. Comparison with subsequent studies
(takenfromKF97):(i)Tomodelthewarmabsorberandto
Recently Porquet et al. (1999; P99 hereafter) presented
predictitslineemission,wecompiledtheobservedSEDof
a parameter space study of the strengths of Fe coronal
thegalaxyfromtheliterature(seeFig.1ofKF97a);based
lines that originate from warm absorbers. They conclude
on photon counting arguments, we found evidence for an
that Fe lines in low-density absorbers (they studied the
EUV bump in the spectrum of NGC4051. (ii) Intensity
density range logn = 8–12) are over-predicted for part
ratiosofcoronallinesweretakenfromthe literature(e.g., H
of the parameter space. Comparing our earlier results on
de Robertis & Osterbrock 1984, Penston et al. 1984, Pe-
NGC4051 (Komossa & Fink 1997a) with their recent re-
terson et al. 1985,Wilson & Nath 1990,Osterbrocket al.
sults, we find both to be consistent: For warm absorbers
1990,Erkens et al. 1997,Nagao et al. 2000).(iii) A warm
dominated by OVIII absorption and high ionization pa-
absorber fit to our ROSAT PSPC spectrum of NGC4051
rameters, no overprediction in line emission occurs (their
yieldslogU =0.4andlogN =22.7.6 Further,fromvari-
w Tab. 1).
thenwouldhaveratherdifferentpropertiesfromGalacticISM- The questionremains whetherthe ‘OVII absorbers’of
likedust,namelyadifferentsizedistributionwithadominance P99 do indeed overpredict the Fe lines and thus are in
of small grains (which more effectively scatter, thus more ef- conflict with observations. We suggest that most of the
fectively redden theoptical/UV spectrum),or averydifferent strong OVII absorberslikely contain dust (which was not
gas/dust ratio !
included in the models of P99), as proposed by Reynolds
6 We have explored the possibility that the warm absorber
(1997). The strong depletion of Fe into dust grains then
inNGC4051containsdust.TheopticalspectrumofNGC4051
results in weaker gas phase emission in the Fe coronal
is not heavily reddened, but the Balmer decrement of broad
lines.
Hα/Hβ deviates from the recombination value. If we try to
modeltheX-rayspectrumofNGC4051withaone-component
dusty warm absorber we do not get a successful spectral fit.
The possibility remains, though, that there is a second, dusty free warm absorber. Since these could also be of instrumental
warm absorber of low column density present. In fact, there origin, higher spectral resolution data are required to search
aresomeresidualsaroundtheCarbonedgeafterfittingadust- for a second, dusty,warm absorber in NGC4051.
8 St. Komossa: Warm Absorbers in AGN
Table 2. Predicted iron lines (Komossa & Fink 1997a) compared to the observed ones of NGC4051 assuming a
covering factor of the warm absorber of 50%; all lines are given relative to the observed Hβ luminosity of NGC4051.
Somenotes:(i)ThestrongestpredictedamongtheFelinesinmostmodelsisFe19λ1118;(ii)modelresultsarerobust
againstthesmalluncertaintiesinthereddeningcorrection(e.g.,weassumedA =0.7mag,Nagaoetal.(2000)usedA
v v
=1mag);(iii)allnon-‘standard’modelsequenceswerefirstre-fittothedata,toderivethenewbest-fitWAparameters
(which sometimes slightly differed from the oldones) before the lines were predicted [if this is notdone, the predicted
Fe lines change more strongly!], (iv) recent NLR(and torus-)related multi-component photoionization models of Sy1s
can successfully account for [FeVII] (e.g., Komossa & Schulz 1997, Murayama & Taniguchi 1998a,b).
model line predictions comments
[FeVII],[FeX],Fe[XI]/Hβobs [Fe XIV]/Hβobs
‘standard’ <510−5, ≪obs. 0.004 observed:Fe14/Hβ <0.06 (Nagao et al. 2000)
var.abundances ′′ change≤ fact.2-4 0.3≤Z ≤1
HeIIλ4686 becomes strongest opt. line
var.density ′′ ′′ 4 ≤lognH ≤9.5
′′ ′′
var.IR conti. up to theobserved IR data points (from host
galaxy) → increased f-f heating
′′ ′′
var.EUV conti. additional soft excess to account for LHβ
logTbb=5,Qbb=Qpl
′′
‘91 McHardy data 0.02 logU =0.2, logNw =22.47 (our re-fit of thedata
originally presented by McHardy et al. 1995)
4.5. A warm absorber relation to any optical/UV sample study of ASCA observationsof AGN reported the
emission line component ? detection of OVII and OVIII absorption edges in the X-
ray spectrum of MR2251.
Finally,we note thatourmore generalsearchfora warm-
Here, we present first results from a detailed analysis
absorber origin of of one of the emission-line regions ob-
of a deep archival ROSAT PSPC (Tru¨mper 1983, Briel
served in the UV/optical spectra of Seyfert galaxies did
et al. 1994) observation of this source performed in Nov.
notgiveasingleexamplewithaone-to-onematch(seeKo-
1993 with a duration of 18 ksec.
mossa & Fink 1997e for a brief overview); emission lines
from warm absorbers are always weaker than observed
lines. However,the possibility of a (weak) warm-absorber
5.2. X-ray analysis: results
contribution to individual emission lines (e.g., CIV in the
case of NGC3227; Komossa & Fink 1997b) remains. X-raydatareductionwascarriedoutinastandardmanner
(see Komossa 2000, in prep.). In total, 36 X-ray sources
were detected with a likelihood 10 within the field of
5. The warm absorber of MR2251–178 ≥
view. The positions of those in the vicinity of MR2251
5.1. Introduction areshowninFig.5,overlaidonanopticalimagefromthe
digitized Palomar sky survey.
MR2251-178 at redshift z=0.068 was the first quasar to
bediscoveredinX-rays,inthecourseoftheArielVall-sky The meansourcecountratewas3.1cts/s,afactor of3
survey (Cooke et al. 1978, Ricker et al. 1978). It turned stronger than during the ROSAT all-sky survey observa-
out to be an outstanding object in many respects. It has tion performedin 1990.The X-ray lightcurve is displayed
a high ratio of L /L , is surrounded by a giant HII en- in Fig. 6.
x opt
velope observed in [OIII] emission, and was the first ob- First,we fit asingle powerlawto the totalX-rayspec-
ject in which a warm absorber was discovered (Halpern trum. Cold absorptionwas fixed to the Galactic value to-
1984).BasedonEXOSATandGinga observationsPanet wards MR2251, N = 2.77 1020 cm−2 (Lockman &
Gal
×
al.(1990)andMineo&Stewart(1993),again,interpreted Savage 1995), since it is underpredicted otherwise. This
and modeled the data in terms of the presence of a warm gives Γ = 2.3 and clearly is a very poor description of
x
−
absorber. In contrast, Walter & Courvoisier (1992), in an the data (χ2 =5.4;Fig. 7). A Raymond-Smith emission
red
independent analysis explained the observations in terms model does not fit the spectrum at all, even if temper-
of a variable powerlaw spectral shape, and concluded the ature, normalization, metal abundances and the amount
presence of a warm absorber was unnecessary to explain of cold absorption are all treated as free parameters. We
the X-ray data. Reynolds (1997), in the course of a large then tried two-componentspectral fits involving a power-
St. Komossa: Warm Absorbers in AGN 9
lawplussoftexcessparametrizedbydifferentmodels.The the RASS observation,and has (nearly) disappeareddur-
quality of the fit remains unacceptable, though. ing the later PSPC observation (see KM2000 for details).
The presence of a warm absorber markedly improves In summary, we find that a warm absorber fits well
the fit. Performing a two-edges fit with edge energies the ROSAT survey observation,and residuals around the
fixed at the theoretical values of OVII and OVIII we expectedlocationofabsorptionedgesarestillpresentdur-
obtain τ = 0.26 0.12, τ = 0.20 0.12, and ing our later pointed PSPC observation.7 In addition,
OVII OVIII
± ±
Γ = 2.20 0.02. The not yet totally satisfactory qual- the presence of high-ionization UV absorption lines in
x
− ±
ityofthefit,χ2 =1.6,canbetracedbacktoadeviation this object was recently reported by Goodrich (talk at
red
of the low energy part of the spectrum (between 0.1–0.18 the Narrow-line Seyfert1 workshop in Bad Honnef, Dec.
keV) from the model prediction which already caused an 1999),andtherecentstudybyCrenshawetal.(1999)sug-
underprediction of the galactic N in the pure powerlaw gests a near one-to-one match of the presence of UV and
H
fit. If this partofthe spectrum is excludedfromthe spec- X-ray warm absorbers.
tral fitting, we obtain χ2red = 0.9, τOVII = 0.22± 0.11, We conclude, that the presence of a time-variable
τOVIII =0.24 0.12, andΓx = 2.21 0.02.We conclude warm absorber is a viable explanation for all available X-
± − ±
that a warm absorber is clearly present in this source. ray observations of RXJ0134-4258. High-spectral resolu-
tion data taken during episodes of repeated spectral vari-
6. The X-ray transient AGN RXJ0134-4258 ability of this source are required to distinguish this from
other possible scenarios like real changes in the steepness
6.1. Summary of the observations
ofthepowerlaworapowerlawplussoftexcessdescription
The Narrow-line Seyfert1 galaxy RXJ0134-4258is one of (see Komossa & Meerschweinchen 2000).
the rare sources which showeddramatic spectral variabil-
ity. Its spectrum changed from ultrasoft (Γ = 4.3) in Acknowledgements. ItisapleasuretothankGaryFerland for
x
the ROSAT all-sky survey (RASS) to flat (Γ =− 2.2) providing Cloudy, Smita Mathur for very useful and pleas-
x
− ant discussions, and the organizers for carrying out this ex-
in our pointed PSPC observation taken two years later
citing workshop. This workshop was funded by the Inter-
while its countrate remained nearly constant (Greiner
Research Centers Cooperative Program of the JSPS and the
1996, Komossa & Fink 1997d, Komossa & Greiner 1999,
Deutsche Forschungsgemeinschaft DFG. The ROSAT project
Komossaetal.1999b,Grupeetal.2000,Komossa&Meer-
hasbeensupportedbytheGermanBundesministeriumfu¨rBil-
schweinchen 2000).
dung,ForschungundTechnologie(BMBF/DLR)andtheMax-
Onepossibleexplanationforthiskindofspectralvari- Planck-Society.
ability is the presence of a warm absorber. In particu- Preprintsof this andrelated papers can beretrieved from our
lar, we have studied the following two scenarios (details webpage at http://www.xray.mpe.mpg.de/∼skomossa/
are given in Komossa & Meerschweinchen 2000; KM2000
hereafter):
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