themselves Nandra Seyfert Pounds ected oxygen complex K in ing galaxies as ous X ray repro cessing are is velop studied of by ob jects Mon Accepted ROSAT astro-ph/9411063 16 Nov 1994 W N Max Planck Institut Institute seen edges the ab out sharp er Ginga seen p ower central INTRODUCTION Not a studies fuel hard in uni ed has s were Recent et in soft In whose Brandt half of Di ering over R in NGC law app ear al only in that sp ectral engines Astronomy b een the Astron tail the selected line X ray Ginga of as August of with taxonomy high soft p owers soft mo dest Seyfert PSPC well Seyfert seen emission p er ab ove to amounts of photon features f ur sp ectra X ray data So c X ray as signal to noise have as cent Here keV these by Madingley Extraterrestrische a them A C detail s Received bright through attening of galaxies ROSAT band warm of band prop erties we keV index and of X ray with and ob jects them Mushotzky Warm photo electric analyse observations Seyfert as Compton a Fabian the Key b oth Ginga evidence shows systematically J ies and cently variability Kaz p ortional We ABSTRACT Excess a have absorb ers Kaz Road multiple their variety Seyferts p enetrate with uorescent part in of At ratio steep present has showing absorption individual varies the several its words have revealed July : ve Cambridge b een least data absorption of strong Physik a of X ray galaxies sp ectrum sp ectra re ection for continua X ray Mrk Done Counter keV an large X ray until Four sp ectral absorption which by : into : We spatial found in p er Seyferts the variability K indicating iron e ort galaxies an ionized original D Fe Nandra p er steep er Printed and by nd of the now soft sp ectra cent signature Seyfert CB Kaz obtained with underly revealed is Nandra K galaxies i i and the Pounds the ionized to lead as cent shap es PSPC to vigor found optical temp oral Mrk emission that excess p er b een e g HA have iron intensity well and form line ve p o de re RX sp ectra to of Garching individual as that of November cent of Ark remarkably well narrow line J characteristics Seyfert observations galaxies a they and in warm the than ilar van the Osterbro ck a tively have Boller tentially Gondhalekar ple X ray lation keV in Walter large a sp ectra in of variability May Several C S Seyfert Germany narrow line : the the C the sp ectral also also narrow line narrow line Arakelian has Ark Gronigen are shown number strong and absorb er Einstein variability ux largest b etween Walter X rays et Ark seen individual Seyfert steep s Seyfert has erg a complex similar Reynolds harb our typically Fink of al as photon of We in RE s extremely by analyses Ca a and et the of currently Ark was soft Seyfert the Pogge very IPC RX galaxies sp ectral ROSAT examine al Seyfert galaxies and narrow line Seyfert was Fink or narrow line i i to soft suggest Seyfert further Mrk index galaxies X ray observed MCG Boller Boller emission b oth b e ultra soft steep Mrk a one argue sp ectra of Recent doubling features steep is known s J slop e see ROSAT Osterbro ck and continua mo dels of a Brandt a individual The sp ectral et class keV e g for that Mrk IRAS and p owerful another galaxies Seyfert p ossible the and for galaxies table ROSAT soft in al survey ROSAT > Seyfert and a compactness the often galaxies three time the band W it Seyferts their of p eculiar strong IRAS X ray for Position has ultraviolet as ob jects criteria ROSAT complexity narrow line Lipari of corrob orates of seen Ark connection Mrk other Brinkmann Ark narrow line galaxies all sky narrow a well observations and NGC Puchnarewicz Fabian Fe p eculiarly galaxies keV continua Shuder in outlying individual in i i as Mrk keV Sensitive in s used parameters higher is Seyferts all sky and these emission luminosity survey giving rapid preparation lines classi ed that Ark in NGC Mrk Seyfert sp ectrum for to has the Kaz b etween have and weak Seyferts light galaxies Seyfert energy of survey app ear galax shows X ray exam de ne A it corre NGC steep et have Pro lines rela sim The IZw RX fast one see re for ul al as to of of
2 W N Brandt et al
traviolet continuum b ecause a large part of the narrow line unabsorb ed uxes and Galactic N values The ROSAT ob
H
region is covered by obscuring dust Ark has a narrow servations were p erformed in the standard wobble mo de
line Balmer decrement of van Groningen Walter to avoid accidental shadowing of sources by the coarse wire
Fink grid which forms part of the PSPC entrance window sup
NGC Mrk VV is a morphologically p e p ort structure ROSAT p erforms a slow dithering motion
culiar Seyfert One hundred and eight years ago F D diagonal to the detector axes with a p erio d of s and
Leavenworth noted its strong quasi stellar nucleus Stone an amplitude of arcmin
NGC has a giant kp c ring shap ed zone The PSPC detector at the fo cus of the ROSAT X
of strong infrared emission which Ro dr guez Espinosa ray telescop e has a bandpass of keV over a
Stanga suggest arises from hot gas b eing photoionized diameter eld of view Its energy resolution is E E
:
by OB stars formed in an intense episo de of star formation E FWHM where E is the photon energy
Stanga Ro dr guez Espinosa Mannucci have found measured in keV The spatial resolution for the full bandpass
a nuclear double p eaked He i line and also demonstrate that is arcsec FWHM on axis Hasinger et al The
the H and H lines are asymmetric They are able to t PSPC background count rate is very low and is dominated
the line pro les consistently with an optically thick Kep by the cosmic ray induced particle background scattered so
lerian accretion disc mo del but p oint out that this is not lar X rays and the di use X ray background The rejection
the only p ossible interpretation of the lines NGC is e ciency for the particle background is excellent and the
a strong X ray source having a keV luminosity of residual count rate due to cosmic rays is only
erg s in the observation of Krup er Urry count s arcmin keV Snowden et al
Canizares Reduction and analysis of the PSPC image data was
The Seyfert galaxy Kazarian VI I Zw is p erformed with the Starlink ASTERIX X ray data pro cessing
in the pro cess of collision with an ob ject to the north re system
sembling an elliptical galaxy in shap e and colour Kriss
Canizares Hutchings Hickson It lies in an
ANALYSES
irregular blue nebulosity and its nuclear sp ectrum has rel
atively weak O i i i lines and some Fe i i blends Hutchings
Spatial analyses
Hickson The ratio of its p ermitted and forbidden
The keV spatial pro les of all targets are consistent
lines falls away from the nucleus but then rises again in
with those of p oint sources convolved with the ROSAT XRT
outer parts of the nebulosity Kaz has b een observed
and PSPC spatial resp onses Hasinger et al None
in the ROSAT all sky survey Walter Fink and in
of the targets was detected with the ROSAT Wide Field
the Einstein EMSS Kriss Canizares Sto cke et al
Camera All targets lie in the centres of their elds except
but no sp ectral features or variability were observed
for Kaz and RX J Kaz lies arcmin
Markarian MCG is a barred Seyfert
away from the eld centred on Mrk and RX J
and is also an extended kp c three comp onent linear
lies arcmin from the eld centred on Ab ell
radio source Ulvestad Wilson Mazzarella Boro
We have extracted the source counts for all ob jects from
son In addition to its broad p ermitted lines Mrk
circular source cells chosen to b e large enough to ensure that
has strong Fe i i lines as well as the stronger forbidden
all of the source counts are included given the electronic
lines such as O i i O i i i and Ne i i i Oke Lauer
ghost imaging which widens the p oint spread function b e
Mrk was studied by EXOSAT but noise in the EXOSAT
low keV Hasinger et al Background counts
data resulted in a p o or quality sp ectral t Turner Pounds
were subtracted from the source cells using large circular
source free background cells Corrections were included for
RX J NAP lies arcmin on the sky
PSPC dead time vignetting and shadowing by the coarse
away from the z X ray cluster Ab ell It was re
mesh window supp ort We use these data in the temp oral
cently discovered in optical sp ectroscopy by Crawford et al
and sp ectral analyses b elow
and has signi cantly contaminated X ray studies of
Ab ell made using Ginga and HEAO Johnson et al
Temporal analyses
Arnaud et al Fig shows the optical sp ec
trum of RX J which lies at z The Count rates for X ray sources can only b e reliably given as
H line of RX J has easily visible broad and averages over the ROSAT wobble p erio d of s as the
narrow line comp onents and RX J is b est clas path of the source in the detector should rep eat with this p e
si ed as a Seyfert The H broad line FWHM is rio d The wobble path can b e geometrically distorted by dig
km s and the O i i i narrow line FWHM is km s itization uncertainties in the star sensor read out and this
A value of the Hubble constant of H km s can lead to apparent variations in count rate over di erent
Mp c and a cosmological deceleration parameter of q wobble p erio ds However such e ects are generally small
and the accuracy of ux determinations in wobble mo de are have b een assumed throughout
go o d to within p er cent Brinkmann et al We
have used s binning for all temp oral analysis work In
OBSERVATIONS AND DATA REDUCTION
addition we have conservatively disregarded data from the
rst and last s of each observation interval so as to avoid ROSAT PSPC Tr umper Pfe ermann et al ob
p otential detector voltage and satellite drift problems servations were made of the ve Seyferts on the dates shown
Kaz shows p er cent X ray ux variability in Table Also shown in Table are redshifts exp osure
and its keV light curve corrected for detector dead times raw background subtracted counts absorb ed uxes
ROSAT observations of Ark NGC Kaz Mrk and RX J 3
Figure The optical sp ectrum of the z X ray Seyfert RX J The sp ectrum has not b een de reddened
Table Observational parameters
Seyfert Observation Raw b ckgnd keV keV
Galaxy Exp osure Date Mission subtracted Absorb ed Flux Unabsorb ed Flux Galactic N
H
Name z Time s Phase counts ergs cm s ergs cm s cm
:
Ark Nov AO
:
:
NGC Aug AO
:
:
Kaz Aug AO
:
:
Mrk Sep AO
:
:
RX J Nov AO
:
Fluxes are computed using the preferred mo dels from the text The Galactic column density for Mrk is taken from Elvis Lo ckman
Wilkes The Galactic column densities for all other sources are taken from Stark et al
on time scales shorter than s or whether the observed time vignetting and wire shadowing is shown in Fig
uctuations are the result of the irregular wobble motion The hardness ratio for Kaz do es not change signi cantly
Such analysis will have to await further understanding of when its ux changes Ark has three observation inter
the wobble s e ect on short term source variability vals During the rst the light curve of Ark smo othly
We have extensively searched for instrumental e ects decreases by p er cent over s from count
that might pro duce the variability seen in these ob jects and s to count s By the second observation inter
can exclude them We have examined the light curves of val seconds later Ark registers count
other serendipitous sources in the ROSAT elds of view s During the third observation interval seconds
and they show no signs of correlated variability with the after the second Ark registers count s Ark
ob jects discussed ab ove showed large amounts of variability during the HEAO
all sky survey as well Its keV ux as measured by
HEAO was mC mC and mC on Dec
Sp ectral analyses
June and Dec R Mushotzky pri
Counts from the corrected circular source cells were ex vate communication Due to its short exp osure time Mrk
tracted into channel pulse invariant sp ectra We ig has only two ROSAT observation intervals separated by
nored channels and rebinned the remaining channels s During the rst Mrk gives count
into bins using the ROSAT Standard Analysis Software s and during the second it gives count s
System binning We have veri ed that neglect of the rst At present we cannot state with con dence whether or
and last two p oints of the rebinned sp ectra do es not change not we are seeing real intensity variations of our sources
4 W N Brandt et al
Figure The Kaz keV light curve The data have b een binned into s bins corresp onding to the wobble p erio d and
are corrected for detector dead time vignetting and wire shadowing The count rate of Kaz varies by p er cent over s
see Fig a ts the sp ectrum p o orly yielding the nature of our tting results b elow
Large systematic residuals are clearly visible The derived Systematic errors of p er cent were added in quadra
:
photon index of is signi cantly higher than ture to the data p oint rms errors to account for residual un
:
that seen in higher energy Ginga observations of Seyfert certainties in the sp ectral calibration of the PSPC We have
galaxies Nandra Pounds for example nd the used the January resp onse matrix MPE No for
mean X ray continuum of Seyferts to b e b est mo delled all analysis This matrix corrects for the systematic de cit
by a p ower law with photon index in the range of photons near the PSPC detector s carb on edge that was
A high photon index could b e due to a number of e ects present in earlier matrices cf Turner George Mushotzky
For example sp ectral features such as edges and lines or an The exp ected systematic errors from this matrix are
additional soft continuum comp onent could mimic a steep a few p er cent and are signi cantly smaller than the rele
sp ectrum We have examined these p ossibilities by tting vant residuals in the data discussed b elow We are keenly
mo dels with sp ectral features and additional soft continuum aware however of remaining sp ectral calibration uncertain
comp onents and the results of our tting are shown in Table ties with the PSPC and refer the reader to the discussions
of this issue in app endix A of Brinkmann et al and
Mo dels with simple soft continuum comp onents reduce app endix A and app endix B of Fiore et al Our re
but have other problems For example the p ower law sults b elow agree with the general trend of steep soft Seyfert
and bremsstrahlung mo del yields a negative alb eit with sp ectra and the results for Ark are in line with what has
large error bars note that photon ux is prop ortional to b een seen for other narrow line Seyfert s It is imp ortant
1
E as well as an unusually strong bremsstrahlung com to note that the sp ectra of Ark and other narrow line
p onent A two power law mo del not shown in Table Seyfert galaxies are steep even when compared to those of
yields an unphysical photon index for the secondary p ower other ROSAT observed Seyfert galaxies
law of The values of N derived for b oth the We mo del the the X ray sp ectra of our targets using the
H
bremsstrahlung and blackbo dy soft comp onent mo dels are X ray sp ectral mo dels in the XSPEC sp ectral tting package
to o small to agree with the Galactic column density see Shafer et al
Table For the p ower law and blackbo dy mo del the
:
cm which p er cent con dence N
H
:
Ark
still do es not overlap the Galactic N value This disagree
H
ment is only aggravated by the fact that Ark may have The results of basic sp ectral ts to the Ark sp ectrum are
intrinsic absorption Walter Fink argue for the shown in Table The errors for these and all other ts are
strong absorption of the ultraviolet continuum of Ark quoted for p er cent con dence unless explicitly stated
by dust and gas asso ciated with dust will absorb X rays otherwise taking all free parameters to b e of interest other
Assuming that the X rays and ultraviolet radiation travel than absolute normalization Lampton Margon Bowyer
through the same matter a Galactic dust to gas ratio case Press et al A single absorb ed p ower law mo del
ROSAT observations of Ark NGC Kaz Mrk and RX J 5
Table Basic sp ectral tting to Ark
Mo del N A Other
H
d o f Name cm ph keV cm s Parameters
: : :
Power Law
: : :
: : : :
K Power Law
: : : :
:
Brems k T keV
:
: : : :
Power Law K
: : : :
:
keV Blackbo dy k T
:
: : : :
Power Law
: : : :
:
keV Edge E
Edge
:
: : : :
Power Law K cm s
Line
: : : :
:
keV Gaussian Line E
Line
:
All sp ectral mo dels ab ove are describ ed in Shafer et al and the sp ectral parameters are de ned as follows
N equivalent hydrogen column in atom cm A normalization of p ower law in photon keV cm s photon index
H
of p ower law K dimensionless normalization of bremsstrahlung or blackbo dy k T bremsstrahlung or blackbo dy temp erature in
keV edge absorption depth at threshold E edge threshold energy corrected for redshift K line ux E line
Line Line
Edge
energy corrected for redshift
All error bars are for p er cent con dence assuming that all free parameters are of interest other than the absolute normalization
The Gaussian line is taken to have a xed width of eV
it is worth noting that p ossible absorption by out owing B recombination a density of cm and a temp erature
material which would raise the edge energy predicts an of K the density and temp erature chosen are char
extremely large out ow velocity which would cause kinetic acteristic for the narrow line region and our results do not
out ow p ower to overwhelm radiation p ower unless the out dep end strongly on their values we can estimate E B V
ow were extremely collimated and b e in discord with the Gaskell Ferland Ward et al Osterbro ck
accretion hypothesis The warm absorb er mo del of Yaqoob and then the equivalent N column Bohlin Savage Drake
H
Warwick is rejected at p er cent con dence us Burstein Heiles exp ected from Ark s large
ing the F test Bevington Similarly a warm absorb er narrow line Balmer decrement of see Section We es
mo del constructed using the photoionization co de CLOUDY timate an intrinsic column of N cm consistent
H
Ferland Fabian et al is rejected with p er with the prediction of Walter Fink While our N
H
cent con dence the mo del di ers from that of Yaqoob estimation is admittedly crude and probably breaks down
Warwick in that it includes sp ectral line emission as we argue b elow any intrinsic column adds weight to the
A p ower law and Gaussian line mo del ts the data rea evidence against a simple soft continuum comp onent We
sonably well and is physically acceptable The line energy discuss more complex soft continuum comp onents b elow
:
of keV lies in the centre of the strong O vi i i K The p ower law and edge mo del for the data indicates an
:
:
Fe xvi i L and Fe xvi i i L line complex and these lines keV In the PSPC band the dom edge energy of
:
would b e mixed together by the limited PSPC energy re inant absorb er is oxygen b ecause of b oth its large abun
sp onse They could b e formed in a photoionized disc see dance Morrison McCammon and its large pho
for example g of Ross Fabian and g a of toionization cross section Daltabuit Cox It has
Zycki et al The value of N is consistent with the K edge energies ranging from keV for O i to
H
Galactic value keV for O vi i i and the edge energy we t is inconsistent
More complex soft continuum mo dels with for example with this energy range While it is p ossible that combined
a two blackbo dy soft excess can also t the sp ectrum well edges from neon so dium and iron the L edge could if
and are entirely plausible Due to the limited number oxygen is fully stripp ed pro duce the feature we detect this
of indep endent PSPC energy bands we are not able to prob e interpretation is implausible for several reasons Both neon
such mo dels in detail and so dium are usually signi cantly less abundant than oxy
gen and also have smaller photoionization cross sections
They would therefore have di culty pro ducing an edge with
NGC
:
In addition there is only a very nar depth
:
NGC is also p o orly tted by a simple p ower law see Fig row range of ionization parameters over which oxygen is
b and we show the results of tting more complex mo dels fully or almost fully stripp ed while neon and so dium can
in Table A p ower law and line mo del do es not describ e still form edges see for example g of Kallman
:
keV these data well as it predicts a strong line at McCray Iron L edges alone cannot pro duce an edge
:
:
While the C vi K line energy is marginally consistent with keV if oxygen is fully stripp ed since feature at
:
this energy it is hard to imagine a plausible scenario where iron will b e in the ionization stages of Fe xvi i i which has
it is formed with such a high intensity without the stronger an L edge energy of keV and ab ove Furthermore
6 W N Brandt et al
Figure Sp ectra for a Ark and b NGC in the keV band Single p ower law ts to the observations and residuals
are also shown Note that neither sp ectrum is well tted by a simple p ower law mo del
:
and are even less consistent with single O K and Fe L lines also b eing formed which would then
int
:
absorb ed p ower law ts to HEAO EXOSAT and Ginga lead to a higher t energy In addition the derived N
H
data see Nandra Pounds g The derived N is low b eing only barely consistent with the Galactic value
H
for Kaz is low and is only marginally consistent with see Table and note that the p er cent con dence N
H
:
the Galactic value note that the p er cent con dence cm
:
:
cm Fits with a xed p ower N Mo dels with soft continuum comp onents do t NGC
H
:
law photon index and extra comp onents do signi cantly im s sp ectrum well and give physically plausible results
prove the s over those for xed photon indices in Table The N for the p ower law and blackbo dy mo del agrees
H
Due to our assumption that a detailed presen very well with the Galactic value and the p ower law and
tation of the sp ectral mo delling of these sources would not bremsstrahlung mo del N is feasible if there is intrinsic ab
H
b e appropriate We do however rep ort the b est secondary sorption The soft comp onents are p owerful dominating the
comp onent ts so as to prob e the sp ectra to the greatest sp ectrum up to ab out keV A p ower law and edge mo del
extent p ossible Kaz is b est tted with a secondary also ts the sp ectrum of NGC well and gives a reason
:
p ower law comp onent It gives N able edge energy A natural mechanism for the formation of
H
:
:
and cm removing the N disagreement such an edge is oxygen absorption in a warm absorb er and
H
:
for degrees of freedom Mrk is b est tted the warm absorb er mo del of Yaqoob Warwick ts
: :
keV secondary bremsstrahlung comp onent with a a the data with an ionization parameter of U
: :
: :
cm and for with N cm a p ower law warm absorb er N of
H H
: :
:
degrees of freedom RX J is b est tted with a photon index of and The photon index
:
blackbo dy secondary comp onent The b est tting parame derived in the warm absorption scenario is again high hint
: :
cm keV N ters are k T ing that if NGC has a warm absorb er then it probably
H
: :
and for degrees of freedom has a continuum soft excess as well
Kaz Mrk and RX J
DISCUSSION AND CONCLUSIONS
The geometry comp osition and ionization state of matter In Table we show the results of single p ower law ts to
around Seyfert engines are of central imp ortance when Kaz Mrk and RX J For the xed pho
trying to understand Seyfert sp ectra We have analysed ton index ts we use the Nandra Pounds mean
p ointed ROSAT observations of ve Seyfert galaxies and intrinsic photon index of While the three ts with
nd evidence in all ve for complex soft emission This variable sp ectral indices yield formally acceptable values of
emission may b e mo delled in our various Seyfert s as O their steep sp ectra b elie this simple interpretation All
vi i i K Fe xvi i L and Fe xvi i i L line emission p owerful three are inconsistent with the Nandra Pounds
ROSAT observations of Ark NGC Kaz Mrk and RX J 7
Table Basic sp ectral tting to NGC
Mo del N A Other
H
d o f Name cm ph keV cm s Parameters
: : :
Power Law
: : :
: : : :
K Power Law
: : : :
:
Brems k T keV
:
: : : :
Power Law K
: : : :
:
keV Blackbo dy k T
:
: : : :
Power Law
: : : :
:
keV Edge E
Edge
:
: : : :
Power Law K cm s
Line
: : : :
:
keV Gaussian Line E
Line
:
All sp ectral mo dels ab ove are describ ed in Shafer et al and the sp ectral parameters are de ned as follows
N equivalent hydrogen column in atom cm A normalization of p ower law in photon keV cm s photon index
H
of p ower law K dimensionless normalization of bremsstrahlung or blackbo dy k T bremsstrahlung or blackbo dy temp erature
in keV edge absorption depth at threshold E edge threshold energy corrected for redshift K line ux E
Line Line
Edge
line energy corrected for redshift
All error bars are for p er cent con dence assuming that all free parameters are of interest other than the absolute normalization
The Gaussian line is taken to have a xed width of eV
amount of dust will b e reduced by thermal sputtering pho soft continuum emission and intrinsic warm absorption
to destruction at cloud surfaces and sho cks from high sp eed The combination of unusual observational parameters
cloud collisions Krolik Begelman section of Laor for Ark makes it a challenge to mo del We will rst
Draine This mo del do es not make any attempt to summarize Ark s features and will then examine the
connect the generally steep soft X ray sp ectra b eing seen in explanatory extent and feasibility of various mo dels Ba
narrow line Seyfert galaxies with the fact that they have sic sp ectral tting of Ark consistently yields very steep
slimmer optical emission lines than normal Seyfert s sp ectra for this p owerful its keV luminos
The assumption that the X rays and ultraviolet emis ity is erg s narrow line Seyfert This is in
sion travel through the same matter might also break down line with what has b een seen in other narrow line Seyfert
This certainly seems likely at least to some degree in the galaxies see Section In the absence of attening at en
inhomogeneous neighborho o d thought to exist around a ergies b elow keV extrap olated soft X ray sp ectra over
Seyfert nucleus Gas and dust near the central engine predict the dereddened Savage Mathis A and
will b e made inhomogeneous as they are photo destroyed A uxes Walter Fink by factors and
photoionized clump ed and blown around by anisotropic resp ectively A naive estimation see Section
radiation pressure and sup ersonic winds Wolfe X ab ove of Ark s intrinsic column suggests a p ossible N
H
rays emitted near the central engine might for example of up to cm from gas asso ciated with the dust
escap e relatively unabsorb ed down an ionization cone that creating its narrow line Balmer decrement see Section
has its ap ex at the nucleus and extends b eyond the classical yet we see negligible X ray absorption over the Galactic col
narrow line region e g Unger et al Wilson Ward umn Ark has strong optical Fe i i lines p erhaps not
Hani Acosta Pulido et al The more generally surprising in light of its steep X ray sp ectrum and g of
extended ultraviolet emission will su er more more absorp Wilkes Elvis McHardy The line emission from its
tion and will b e weakened relative to the X rays However broad line region is relatively narrow for a Seyfert galaxy
this mo del cannot easily explain the lack of very broad lines mirabile dictu its classi cation as a narrow line Seyfert
without invoking a somewhat ad ho c arrangement of dust galaxy indicating that the line emitting clouds have lower
and gas around the broad line region Potential radiation velocities along the line of sight than do typical Seyfert
scattering mo dels are limited by the fact that Ark shows galaxies Ark s keV ux varies by p er cent
no evidence for p olarization Go o drich This lack of in s
p olarization also hints that Ark has only a weakened in From the large N disparity we conclude that one of the
H
ner broad line region if any where broad line clouds would assumptions used in the conversion from Balmer decrement
move most rapidly as opp osed to a hidden broad line re to N must break down see Section One p ossibil
H
gion This mo del also do es not establish a connection b e ity is that the assumption that a Galactic dust to gas ratio
tween the steep X ray slop e and relatively slim optical emis is appropriate for Ark is invalid This p ossibility has
sion lines b een explored by Walter Fink If true it indi
The assumption of case B recombination might fail for cates that Ark has a dust to gas ratio of at least
Ark due to its p owerful soft X ray ux A strong soft X times the Galactic value This high value while not imp os
ray comp onent will increase the amount of X ray heating of sible is somewhat surprising in light of the fact that the
8 W N Brandt et al
Table Power law ts to Kaz Mrk and RX J
Seyfert N A
H
d o f Name cm ph keV cm s
: : :
Kaz
: : :
: :
Fixed
: :
: : :
Mrk
: : :
: :
Fixed
: :
: : :
RX J
: : :
: :
Fixed
: :
All sp ectral mo dels ab ove are describ ed in Shafer et al and the sp ectral parameters are
de ned as follows
N equivalent hydrogen column in atom cm A normalization of p ower law in photon
H
keV cm s photon index of p ower law
All error bars are for p er cent con dence assuming that all free parameters are of interest
other than the absolute normalization The word xed by a parameter means that it was held
xed during the t
or mechanically heated outer accretion disc Collin Sou rin neutral gas and move the Balmer decrement away from the
Dumont Joly or inner torus O vi i i K Fe standard case B value toward a value more appropriate for
xvi i L and Fe xvi i i L lines could b e formed in the inner pure collisional excitation Gaskell Ferland This
radiation dominated part of the same disc To make the solution unlike the mo dels mentioned ab ove can explain
ROSAT and ultraviolet observations compatible the sp ec the N discrepancy without requiring a p eculiar dust to
H
trum must break b elow keV We have found that sim gas ratio or sp ecial arrangement of matter The steep soft
ple mo dels with for example a two blackbo dy soft excess X ray sp ectrum might also b e directly connected with the
can pro duce the required ultraviolet X ray sp ectrum Such lack of broad optical lines in narrow line Seyfert galaxies
sp ectra are of course p o orly constrained by our data but a large number of soft X ray photons might interfere with
are plausible A break at less than keV do es not rein broad line cloud con nement and formation near the central
state the thermal instability mentioned ab ove as we have source where cloud velocities would b e largest In a sim
veri ed with CLOUDY Ferland computations ple pressure con ned broad line cloud mo del for example a
Another p ossibility based on orientation e ects is that steep soft X ray sp ectrum removes the double valued b ehav
Ark and other similar narrow line Seyfert galaxies ior of the curve of gas temp erature as a function of ionization
might b e those Seyfert s where we view the disc in a rela parameter see g of Guilb ert Fabian McCray
tively face on manner If the general ow of matter in the and the asso ciated discussion and thereby eliminates the
vicinity of the disc were largely p erp endicular to the line of thermal instability asso ciated with multiphase cloud con
sight this would explain narrower optical lines Ark s nement Clouds may also have di culty forming via small
steep X ray slop e might also b e understo o d via orientation p erturbations in an environment without the thermal insta
due to the angularly dep endent soft X ray ux and re ec bility This would hinder broad line formation in mo dels
tivity of an accretion disc see g of Madau g of where many clouds are b orn and die over the time required
Sun Malkan g of Zycki et al and the as for an emission line cloud to cross the broad line region
so ciated discussions Statistical analysis of a large number While a simple pressure con nement mo del is almost cer
of Seyfert s might b e used to prob e this p ossibility tainly inappropriate generally similar b ehaviour might b e
Ark s r ole as a bright X ray Seyfert with narrow exp ected in mo dels with more complex cloud con nement
optical emission lines Osterbro ck Shuder can b e e g magnetically aided pressure con nement evaporation
compared with the recent work of Boyle et al which aided pressure con nement Kallman Mushotzky
suggests that narrow line X ray galaxies NLXGs may b e and cloud formation e g sho ck induced cloud formation
a signi cant contributor to the soft X ray background Ark Perry Dyson mechanisms For example a steep
s Balmer decrement is consistent with the large Balmer X ray sp ectrum increases the di erence of b etween hot
decrements seen in NLXGs and the NLXGs of Boyle et al and co ol states see g of Guilb ert et al thereby
also show little evidence for N values in excess of the increasing the sho ck velocity required to form co ol clouds
H
Galactic value despite their large Balmer decrements B J from sho cked hot gas A steep sp ectrum would not a ect
Boyle private communication Ark s place as a p oten purely magnetic con nement
tial low redshift counterpart to these ob jects is questionable In many broad line region mo dels optical Fe i i emis
however due to the fact that its steep soft X ray sp ectrum sion is created when X rays p enetrate deeply into broad line
is not generally observed and the fact that its O i i i to H clouds and create a warm partially ionized zone at high op
ratio of Stirp e is much larger than is generally tical depths Kwan Krolik The strong Fe i i lines
seen in these ob jects of Ark can still b e plausibly formed even in the absence
We do not detect repro cessed X ray line emission in of broad line clouds near the central engine by an irradiated
ROSAT observations of Ark NGC Kaz Mrk and RX J 9
Fiore F Elvis M McDowell J C Siemiginowska A Wilkes NGC from the p ostulated accretion disc of Stanga et al
B J ApJ submitted
but do nd evidence for a p owerful soft X ray ex
Gaskell C M Ferland G J PASP
cess which may arise via disc repro cessing and re ection
Go o drich R W ApJ
and or a warm absorb er oxygen edge Kaz Mrk and
Gondhalekar P M Kellett B J Pounds K A Matthews L
RX J all have steep keV sp ectra that
Quenby J J MNRAS
are b est mo delled with continuum soft excess comp onents
Guilb ert P W Fabian A C McCray R ApJ
The variety of soft comp onent shap es seen in these ob jects
Hasinger G Turner T J George I M Bo ese G Legacy
presages the rich physics that will b e further accessible to
The Journal of the High Energy Astrophysics Science
soft X ray detectors of higher energy resolution
Archive Research Center NASA GSFC
Helou G Madore B F Schmitz M Bicay M D Wu X
Bennett J in Egret D Albrecht M eds Databases
and On Line Data in Astronomy Kluwer Dordrecht p
ACKNOWLEDGMENTS
Hutchings J B Hickson P AJ
We thank Steve Allen Thomas Boller Brian Boyle Carolin
Johnson M W Cruddace R G Ulmer M P Kowalski M P
Crawford Chris Done Alastair Edge Liz Puchnarewicz and
Woo d K S ApJ
Joly M A A
Martin Ward for useful discussions and Tahir Yaqoob for
Kallman T McCray R ApJS
providing the warm absorb er mo del We thank Thomas
Kallman T Mushotzky R ApJ
Boller for providing us with the X ray data for Kaz and
Kriss G A Canizares C R ApJ
Carolin Crawford for providing us with the optical sp ec
Krolik J H Begelman M C ApJ
trum of RX J We thank R Mushotzky for his
Krup er J S Urry C M Canizares C R ApJS
kind help with the HEAO data base We gratefully ac
Kwan J Krolik J H ApJ
knowledge help from members of the Institute of Astronomy
Lampton M Margon B Bowyer S ApJ
X ray group and nancial supp ort from the United States
Laor A Draine B T ApJ
National Science Foundation and the British Overseas Re
Madau P ApJ
search Studentship Programme WNB the PPARC KN
Mazzarella J M Boroson T A ApJS
Morrison R McCammon D ApJ CSR and the Royal So ciety ACF The ROSAT pro ject
Mushotzky R F Done C Pounds K A ARA A
is supp orted by the Bundesministerium f ur Forschung und
Nandra K Pounds K A MNRAS
Technologie BMFT Much of our analysis has relied on
Nandra K et al MNRAS
the Starlink ASTERIX X ray data pro cessing system and the
Oke J B Lauer T R ApJ
XSPEC X ray sp ectral tting software and we thank the
Osterbro ck D E Astrophysics of Gaseous Nebulae and
p eople who have created and maintain this software This
Active Galactic Nuclei Univ Science Bo oks Mill Valley
research has made use of data obtained from the UK ROSAT
Osterbro ck D E Pogge R W ApJ
Data Archive Centre at the Department of Physics and As
Osterbro ck D E Shuder J M ApJS
tronomy University of Leicester and has also made use of
Perry J J Dyson J E MNRAS
the NASA IPAC extragalactic data base Helou et al
Pfe ermann E et al Pro c SPIE
which is op erated by the Jet Propulsion Lab oratory Cal
Press W H Flannery B P Teukolsky S A Vetterling W T
Numerical Recip es in Pascal Cambridge Univ Press
tech
Cambridge
Puchnarewicz E M Mason K O C ordova F A Kartje J
Branduardi Raymont G Mittaz J P D Murdin P G
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