that the through Mon of and W p olar ROSAT to Wills of IRAS to The W N dust have wards cent tinct that ization Internet what et I I I c prototype Institute X ray the al the months the
host astro-ph/9508057 12 Aug 1995 INTRODUCTION Beichmann the optical at a of was have Not mo delled upturn directions is QSO Earth et p olarization direct ma jor Astronomy present p ercentage Frogel a often galaxy narrow al Brandt of selected kap star le ac uk R dusty developed radio quiet To and Astronomy continuum m Astron It in axis but This et seen explain IUE the has and K the is argue inclined et line PSPC al attenuated Group of band hereafter much through which is scattered or a al dust in sp ectra blue probably So c the the region an redshift ux for its p owerful Madingley of infrared bright A C elegant less host rst emission disc Department rises to ab ove p olarization a IRAS density its Lanzetta W bip olar of light attenuated evidence previously galaxy of In that b etween p er AGN towards IRAS infrared observations ab out Barvainis z mo del Fabian infrared Key to count tical infrared scattering dep endent The p ortional edge cent IRAS tion transmission We ABSTRACT E of addition Road is cent from is on W soft geometry B those sp ectrum hence of parallel present lack variability by time Turnshek in and the words at the for Physics rate quasar emission Cambridge found IRAS and X rays V unidenti ed warm the quasars m of p olarized and blue of broad the light other scales Counter of suggests by p olarization is to narrow line signi cant exhibits spatial X ray in and reaches They no inner most a scattered mo del from Beichmann m galaxies the a luminosity ionized escaping contrast high prop erties Printed line and within variability Astronomy These from for factor and see and CB Sandoval U plane parallel suggest regions of sp ectrum K A PSPC band quasar region is p olar Earth either is a temp oral of com W days sect the HA the dis p er the gas to two one of intrinsic this a to in of individual and August soft Seyfert of archetypal of Internet a with University week Pounds warm facts ob ject observations other very in X rays IRAS and the have X ray X rays mo del tion that have ment ROSAT time radio quiet brie y culiar and correlation to It rated Mp c a ab out cold internal have much s is p eculiar We keV prop erties which sp ectral Brinkmann In A and worth wnb ast cam ac uk IRAS suggested see b een broad that of X ray and di cult outlying nd highly p olarized prop erties rep orted value from and this one imp ortant In more Leicester infrared all sky ux shall absorb er their dust erg of was deep er assumed its particular no b etween noting nature a system absorption year pap er absorption the analyses IRAS of cosmological ultraviolet sophisticated s evidence compare without developed sect ob jects by survey The the There in infrared University p ointed and to that the Sieb ert we Walter consequences IRAS has throughout We geometry sp ectral Hubble soft explore of we present large detections there in IRAS lines is acf ast cam ac uk our many a for detect radio quiet together continuum X ray ROSAT the loud very shall evidence observations Despite to deceleration quasar X rays large study Road measured reddening constant slop e is BAL explain with WF Fink and or prop erties a p eculiar quasar also examine variability variability detailed of intrinsic We Leicester of more but with for its and Position test IRAS for evidence slop e see galaxies ROSAT this QSO its use high the of cold our has IRAS of ultraviolet parameter allow the an facets Boroson scattering probably soft internal high H imp ortant the study E may that hereafter scattering plus temp orally not IRAS cold is p olarization LE and ionized absorption in makes large Sensitive B one X ray us all sky observed for wavelength the b een of change RH are absorption has to of V of observed the of electron ROSAT absorp p erform the mirror km oxygen absorp similar the Meyers seen WF an ob ject q survey sepa Pro W were with with and soft soft p er op s p e A to
2 W N Brandt A C Fabian K A Pounds
Temporal analysis OBSERVATIONS AND DATA REDUCTION
Count rates should b e averaged over the s ROSAT wob
ROSAT PSPC Tr umper Pfe ermann et al
ble p erio d when used for source ux determination as vari
observations were made of IRAS starting on
ability on timescales shorter than this can b e in uenced by
Jan RP total raw exp osure of s over
thick and thin wire shadowing The wobble path can b e geo
s in two observation intervals and Dec
metrically distorted by digitization uncertainties in the star
RP total raw exp osure of s in one obser
sensor read out and this can lead to apparent variations in
vation interval Hereafter we shall refer to RP as
count rate over di erent wobble p erio ds However such ef
Pointing or P and RP as Pointing or P
fects are generally small and Brinkmann et al state
IRAS has and raw background sub
that the accuracy of ux determinations in wobble mo de is
tracted counts in P and P resp ectively
go o d to within p er cent We have used s binning
The ROSAT observations were p erformed in the stan
for all temp oral analysis work
dard wobble mo de to avoid accidental shadowing of
We have searched the ROSAT housekeeping data for
sources by the coarse wire grid which forms part of the
instrumental e ects that might pro duce the variability dis
PSPC entrance window supp ort structure ROSAT p er
cussed b elow The asp ect uncertainty ASP ERR is normal
forms a slow dithering motion diagonal to the detector axes
for all observations We have removed data where the mas
with a p erio d of s and an amplitude of arcmin
ter veto rate EE MV We have checked for general
The PSPC detector at the fo cus of the ROSAT X
correlations b etween the count rate of IRAS
ray telescop e has a bandpass of keV over a
and the master veto rate and no correlations are apparent
diameter eld of view Its energy resolution is E E
:
We have done this for b oth P and P jointly as well as for
E FWHM where E is the photon energy
just P and just P to prevent the relatively large source
measured in keV The spatial resolution for the full bandpass
count rate change b etween P and P from confusing our
is arcsec FWHM on axis Hasinger et al The
correlation checking We have examined the count rates of
PSPC background count rate is very low and is dominated
a serendipitous source near the centre of the ROSAT eld
by the cosmic ray induced particle background scattered so
of view and it shows no signs of correlated variability with
lar X rays and the di use X ray background The rejection
IRAS b etween P and P this source was the
e ciency for the particle background is excellent and the
brightest one in the eld after IRAS The
residual count rate due to cosmic rays is only
source is to o faint to allow us to sensitively lo ok for cor
count s arcmin keV Snowden et al
related variability within just p ointing P It is lo cated at
Reduction and analysis of the PSPC image data was
h m s d m s It is not listed
p erformed with the Starlink ASTERIX X ray data pro cessing
in NED and has a count rate of ab out counts s We
system
have rep eated some of the temp oral analysis of the data us
ing the IDL ROSAT analysis software system and obtain
ANALYSES
the same general results
Fig shows the IRAS full ROSAT band
Spatial analysis
light curves during the P and P ROSAT p ointings The
The keV spatial pro le of IRAS in b oth
weighted mean count rate during P is a factor of times
observations is consistent with that of a p oint source con
that that during P and during P IRAS has
volved with the ROSAT XRT and PSPC spatial resp onses
b ecome one of the brighter AGN in the ROSAT sky The
Hasinger et al IRAS lies in the cen
end of the P p ointing and the start of the P p ointing are
tres of the ROSAT elds of view for b oth observations
separated by ab out days
The X ray centroid of the source we identify with IRAS
A comparison of the data from the rst and second ob
is h m s d m s
servation intervals of P suggests further variability Exactly
during P and its centroid is h m s
the same source and background regions were used for b oth
d m s during P These p ositions are consistent
observation intervals in making this comparison and b oth
with the optical p osition of h m s
observation intervals were pro cessed together A constant
d m s and the p ositional error of the ROSAT PSPC
mo del is a p o or description of the P data giving
of arcsec
for seven degrees of freedom we have included p er cent
We have extracted source counts for IRAS
ux determination errors as p er Brinkmann et al in
from circular source cells chosen to b e large enough to en
this analysis This can b e rejected at greater than
sure that all of the source counts are included given the
p er cent con dence
electronic ghost imaging which widens the p oint spread
We have examined ratios of keV count rate to
function b elow keV Hasinger et al The radii
keV count rate and there is no strong evidence for
of the circular source cells were not a function of channel
hardness ratio variability either b etween P and P or b e
energy Background counts were subtracted from the source
tween the two observation intervals of P
cells using large nearby circular source free background cells
Corrections were included for PSPC dead time vignetting
Sp ectral analysis
and shadowing by the coarse mesh window supp ort We
Counts from the corrected circular source cells were ex have conservatively disregarded data from the rst s and
tracted into channel pulse invariant sp ectra We ig last s of each observation interval so as to avoid p otential
nored channels and rebinned the extracted sp ectra so detector voltage and satellite drift problems We use these
that at least source photons were present in each bin data in the temp oral and sp ectral analyses b elow
ROSAT observations of the infrared loud quasar IRAS 3
Figure The ROSAT full band light curves of IRAS for the rst observation interval of P left the second observation
interval of P middle and the P observation right The P and P observations have di erent time axis zero p oints and times are
measured in seconds from the starts of the P and P observations The last data p oint in the left panel and the rst data p oint in the
middle panel are separated by ab out days and the last data p oint in the middle panel and the rst data p oint in the right panel are
separated by ab out days 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 plotted error bars include the p er cent ux determination error mentioned in
the text IRAS brightened b etween P and P b ecoming one of the brighter AGN in the ROSAT sky
aware however of remaining sp ectral calibration uncertain The widths of the bins that resulted were smaller than the
ties with the PSPC and refer the reader to the discussions PSPC s sp ectral resolution and this oversampling obviates
of this issue in app endix A of Brinkmann et al and p otential biasing problems that could arise as a result of
app endix A and app endix B of Fiore et al sharp sp ectral changes Rebinning the extracted sp ectra
We mo del the X ray sp ectra of IRAS us with the requirement that at least source photons were
ing the X ray sp ectral mo dels in the XSPEC sp ectral tting present in each bin did not materially change our results
package Shafer et al The errors for all ts shall b e b elow
quoted for p er cent con dence unless explicitly stated Systematic errors of p er cent were added in quadra
otherwise conservatively taking all free parameters to b e of ture to the data p oint rms errors to account for residual
interest other than absolute normalization Lampton Mar uncertainties in the sp ectral calibration of the PSPC We
gon Bowyer Press et al According to Stark have used the March resp onse matrix MPE No
et al the Galactic neutral hydrogen column den for the P sp ectral tting and the January resp onse
sity towards IRAS is N cm matrix MPE No for the P sp ectral tting these are
H
Sect of Laor et al indicates that an appropriate the recommended matrices according to the PSPC team
Galactic N uncertainty is N cm A recent These matrices correct for the systematic de cit of photons
H H
precise radio measurement of the Galactic column towards near the carb on edge of the PSPC detector that was present
IRAS gives N cm in earlier matrices see Turner George Mushotzky
H
A Laor and J Lo ckman p ers comm We shall rst The exp ected systematic errors from these matrices are a
t the data from the second ROSAT p ointing P since it few p er cent and are signi cantly smaller than the rele
has more counts and shall then t the data from the rst vant residuals in the data discussed b elow We are keenly
4 W N Brandt A C Fabian K A Pounds
Table Basic sp ectral tting to IRAS during Pointing P RP
Mo del N A Other
H
Name cm ph keV cm s Parameters d o f
: : :
Power Law
: : :
: : : :
K Power Law
: : : :
:
keV Blackbo dy k T
:
: : : :
Power Law
: : : :
:
keV Edge E
Edge
:
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 at keV
H
photon index of p ower law K dimensionless normalization of blackbo dy we have used the bb o dy XSPEC mo del
see Shafer et al for a de nition of the normalization s meaning k T blackbo dy temp erature in keV edge
absorption depth at threshold E edge threshold energy corrected for redshift
Edge
In the last column d o f stands for degrees of freedom
All error bars are for p er cent con dence assuming that all free parameters are of interest other than the absolute
normalization
p er cent con dence but p er cent con dence For the p ointing P in light of what we have learned
sake of completeness we have also considered mo dels with Sp ectral ts to the keV sp ectrum of IRAS
a p ower law and bremsstrahlung soft excess as well as a during P are shown in Table and Fig
p ower law and narrow eV Gaussian emission line A A simple p ower law t is p o or and systematic residuals are
p ower law and bremsstrahlung soft excess mo del is unstable apparent ab ove keV It can b e ruled out with greater
and is statistically a p o or t A p ower law and Gaussian than p er cent con dence A p ower law and blackbo dy
line is also statistically a p o or t with Further t is statistically acceptable but reduces the t N value to
H
more we have xed the cold column at cm see where it is inconsistent with the Galactic column we shall
Sect for our motivation for choosing this value as well discuss this further b elow Any cold absorption intrinsic
as cm half of cm and searched to IRAS would only exacerbate this discrep
without success for statistically acceptable mo dels using for ancy In addition weak systematic residuals remain visible
example double blackbo dy and blackbo dy bremsstrahlung ab ove keV A p ower law and edge mo del is statisti
soft excess mo dels For example a p ower law plus double cally acceptable removes all systematic residuals and yields
:
blackbo dy mo del that also allows for the presence of an ion keV an edge energy corrected for redshift of
:
ized oxygen edge with N xed at cm yields In the PSPC band the dominant ionized absorb er is oxy
H
for degrees of freedom compare this value gen b oth b ecause of its large abundance Morrison Mc
with the values in Table This value can b e ruled out Cammon and its large photoionization cross section
with greater than p er cent con dence and systematic Daltabuit Cox It has K edge energies ranging
residuals are clearly visible at low energies Deleting sets from keV for O i to keV for O vi i i and
of data p oints at low energies shows that the small intrin the edge energy we t is consistent with absorption by a
sic cold column we measure is insensitive to individual data combination of O vi i which has an edge energy of
p oints but rather arises from the robust overall shap e of the keV and O vi i i see equation of Mewe Schrijver
low energy X ray sp ectrum for oxygen edge energies Absorption by warm ionized
Sp ectral tting of a p ower law with cold absorption to oxygen has b een seen in several other AGN and we will
:
N IRAS during P gives consider more sophisticated ionized gas absorption mo dels
H
:
:
cm and with degrees of b elow Fitting a p ower law with b oth a blackbo dy soft ex
:
freedom Note that these values are consistent with those cess and an edge is an overparametrization of the ROSAT
obtained from tting a p ower law with cold absorption to the data although it is probably physically realistic but when
P data This value of can b e rejected at the p er cent it is done the presence of the edge is robust in the sense
con dence level and edge like residuals are still marginally that the edge is strong while the blackbo dy normalization is
:
visible If an edge is included we obtain N greatly reduced The parameters of the t are p o orly con
H
:
: : :
E cm strained but for the record their b est t values are
Edge
: : :
corrected for redshift and with degrees of N cm K k T keV
H
freedom here we have quoted the edge parameters assuming and E keV with for de
Edge
two parameters to b e of interest Note again that the b est grees of freedom We have checked whether the addition
t edge parameters agree well with absorption by ionized of an edge to the p ower law and blackbo dy mo del of Table
oxygen is a statistically signi cant improvement using the F test
For the reasons stated ab ove we prefer the edge mo del for the two additional parameters see sect of Bev
to the blackbo dy soft excess mo del However it must b e ington We calculate and F which
noted that while the mo del with a soft excess yields a t N indicates that the addition of an edge is signi cant at H
ROSAT observations of the infrared loud quasar IRAS 5
Figure The ROSAT sp ectrum of IRAS during the P p ointing The sp ectrum has not b een corrected for redshift A
single p ower law mo del t and the data to mo del ratio are also shown Note the systematic residuals ab ove keV and the lack of a
cold absorption drop o at low energies
indep endent of the details of the sp ectral mo delling b elow the Galactic column PSPC calibration uncertainties
<
Using the p ower law mo del describ ed ab ove the mean might account for this small cm discrep
absorb ed keV X ray ux for IRAS dur ancy it is crucial to note here that PSPC calibration uncer
ing P is erg cm s This corresp onds to a tainties cannot account for the immensely larger observed
keV unabsorb ed ux of erg cm s and versus exp ected N discrepancy we discuss in Sect
H
an isotropic luminosity of erg s H km Furthermore X ray absorption by cold gas and dust can
s Mp c and q b e extremely complex e g Czerny et al and hence as p er Sect Using the p ower law
limitations in the cold absorption mo del we t must b e con and edge mo del describ ed ab ove the mean absorb ed
sidered Therefore we shall further examine b oth mo dels in keV X ray ux for IRAS is erg
our discussion and we note that our main results b elow are cm s This corresp onds to a keV unabsorb ed
6 W N Brandt A C Fabian K A Pounds
reddening of E B V When they interpret this in ux of erg cm s and an isotropic luminosity
terms of their scattered sp ectrum plus transmitted sp ectrum of erg s
mo del they derive an E B V for the transmitted sp ectrum Using the p ower law and edge mo del describ ed ab ove
in the range to If we assume a Galactic dust to the mean absorb ed keV X ray ux for IRAS
cold gas ratio the cold hydrogen column that corresp onds to during P is erg cm s This
E B V is cm and the cold hydrogen col corresp onds to a keV unabsorb ed ux of
umn that corresp onds to E B V is cm erg cm s and an isotropic luminosity of erg
see sect VI of Burstein Heiles The dust to cold s Using the p ower law and blackbo dy mo del describ ed
gas ratio appropriate for the centre of IRAS is ab ove the mean absorb ed keV X ray ux for IRAS
not known and we shall examine this matter in more detail is erg cm s This corresp onds
later For comparison Allen Fabian recently found to a keV unabsorb ed ux of erg cm
evidence for a roughly Galactic dust to cold gas ratio in the s and an isotropic luminosity of erg s
obscured quasar C A similar situation exists for the The ROSAT all sky survey isotropic luminosity was
low redshift radio galaxy Cygnus A which has an intrinsic X erg s Brinkmann Sieb ert The sur
:
ray column of cm Ueno et al and vey observation was taken over Dec and a count
:
A Ward et al Some of the metals asso ciated rate has not b een published for the survey observation Note
V
with the hydrogen in the absorption column may b e lo cated that the all sky survey observation was made b efore either
in dust grains see sect of Martin Sect b of of our p ointed observations and that IRAS was
Spitzer suggests that dust grains contain ab out one brighter during this observation than during either of our
third of the C N and O atoms in addition to most of the p ointed observations hence showing further X ray variabil
heavier element atoms in the Galactic interstellar medium ity
see also So a Cardelli Savage However as long In Fig we show the observed sp ectral energy distribu
as these grains are not so large as to b e individually opti tion of IRAS from radio to X ray energies The
cally thick to X rays the amount of absorption by the grains m optical infrared isotropic luminosity of IRAS
is not much di erent than that by an equivalent mass of is erg s Beichman et al
material in gaseous form e g sect of Martin converted to H km s Mp c
Thus at rst lo ok the intrinsic column suggested by the
E B V data and the intrinsic column we measure ap
The Wide Field Camera WFC data
p ear to b e highly inconsistent Four p ossibilities that might
The ROSAT WFC all sky survey data show a signal of
resolve this large discrepancy suggest themselves
sigma in the WFC S A eV band at
The X ray and IR optical radiation do not travel
h m s d m s The corre
through the same matter
sp onding p er cent WFC error circle has a radius of arc
The gas and dust we are lo oking through in IRAS
sec This source just failed to meet the criteria for inclusion
is very di erent from the typical cold gas
in WFC RE Source Catalogue Pye et al where the
and dust in the Galactic interstellar medium For exam
intention was to have very few false sources It is however
ple the absorbing gas we are lo oking through could b e
included in Mason et al where further WFC survey
highly ionized and or the dust grains in the absorbing
information on IRAS may b e found During the
matter could b e individually optically thick to X rays
p ointed observations the WFC was less sensitive and IRAS
and thus have large self blanketing factors see sect III
was not detected
of Fireman The dust to cold gas ratio in the cen
We note that the detection by the WFC indep endently
tre of IRAS could b e very di erent from the
argues for a small cold X ray column
Galactic one
The obscuration is a strong function of time
The metho ds used to derive E B V are unre
DISCUSSION
liable
X ray absorption by cold gas
We shall examine the rst two p ossibilities b elow but ad
dress the other two here As stated in Sect the Galactic neutral hydrogen col
While we cannot strictly rule out changes in the obscu umn density towards IRAS is N
H
ration b etween the observations of W and our observa cm Stellar reddening studies see sect of
tions none of our sp ectral ts show evidence for changes in W indicate a Galactic E B V towards IRAS
the column density The WF column density is consistent Our acceptable ts to the P ROSAT data all
with our values as well However we cannot rule out time yield total N values of less than cm at p er
H
dep endent partial covering by very optically thick material cent con dence These ts are sensitive to neutral gas b oth
W used the apparent reddening of the broad H in atomic and molecular form Thus we nd an intrinsic cold
H and Pa lines to derive E B V given that column of less than cm in IRAS red
the intrinsic line ratios for IRAS are typical of shift e ects on the cold absorption do not change this result
those for luminous QSOs This reddening correction when as we have veri ed using the zwabs redshifted absorption
applied to the continuum made the corrected IR optical mo del in XSPEC The most probable intrinsic column sug
sp ectrum match those of other QSOs While broad line gested by our favoured p ower law and edge mo del for the
decrements must b e treated with caution due to line trans P observation is less than cm
fer e ects collisional ionization and other uncertainties see W have studied the IR optical reddening intrinsic to
Gaskell Ferland sect of Netzer the fact IRAS see their sect and nd an apparent
ROSAT observations of the infrared loud quasar IRAS 7
Figure Sp ectral energy distributions of IRAS solid dots and arrows and for comparison the radio quiet quasar PG
op en squares Sp ectral data for IRAS are taken from Beichman et al WF and our ROSAT p ointings
We plot our ROSAT data from b oth the P and P p ointings The P data have b een unfolded through the PSPC resp onse using the
single p ower law mo del t and the P data have b een unfolded using the p ower law and edge t from Table Sp ectral data for PG
are taken from Nandra et al and have b een divided by a factor of for clarity of presentation Observations are not
simultaneous The strong ux decrease b etween Hz and Hz is due to photo electric absorption by cold gas and reddening
by dust Note the giant infrared bump of IRAS
changes are seen without strong sp ectral variability causes that the H H and Pa H ratios for IRAS
trouble for simple analogy schemes b etween Galactic black are among the largest observed for QSOs as well as its con
hole candidates and AGN where the soft ux would b e ex tinuum shap e strongly suggest that IRAS do es
p ected to vary less e g sect of Boller Brandt show a large amount of reddening Additionally its infrared
Fink hereafter BBF sect of Fiore Elvis Fe i i Ca i i and Na i emission suggest large amounts of cold
The variability and soft sp ectrum we see echoes the relation gas and dust are present in the nucleus see sect of
found in sect of Green McHardy Lehto W although these last indicators do not necessarily show
X ray scattering around the obscuring matter seen by that the cold gas and dust lie along the line of sight
W might allow X rays to come to us without the signature
of cold absorption If the X rays are scattered this makes
The soft X ray variability and X ray
electrons the most probable scatterers see sect of
scattering
W However the soft X ray variability we see presents
a challenge to mo dels where most of the X rays are scat The ux changes we see in IRAS are somewhat
tered to us The factor of variability in ab out days surprising for such a p owerful ob ject Studies of quasar soft
and causality arguments assuming no b eaming note IRAS X ray variability e g Zamorani et al sect of
is radio quiet allow us to place an upp er limit Laor et al generally nd that the soft X ray ux do es
on the size of the region from which the X rays come of not vary by more than a factor of two from the mean over
p c This upp er limit could b e increased somewhat if a timescales shorter than a few years The X ray variabil
small part of the region varied by a larger amount than the ity p ower sp ectrum of IRAS may atten more
mean variation but the large variability amplitudes implied gradually than for typical quasars The fact that large ux
8 W N Brandt A C Fabian K A Pounds
However as we show in the next paragraph explaining our so on b ecome extreme for an ob ject of this luminosity Ad
X ray observations in the context of the W mo del by in ditionally the p er cent variability suggests an X ray
voking di erent X ray and IR optical light paths proves dif source region with an extent of order p c or less These
cult dimensions are to b e compared with the size of the scatter
X rays and p erhaps UV ux from the compact central ing mirror in the much less luminous Seyfert galaxy NGC
source might escap e through a b orehole of low extinction in where photoionization mo delling Miller Go o drich
a patchy torus see sect of W while the more gener Matthews and Hubble Space Telescope multiaperture
ally extended IR optical emission do es not If the X ray UV sp ectrop olarimetry Antonucci Hurt Miller indi
optical and IR emissions originate from successively greater cate a mirror size of p c or more
radii of an accretion disc a p eculiar relatively p erp endicu Another argument against scattering of most of the soft
lar orientation of the disc and torus axes would seem to b e X rays is the very large intrinsic soft X ray luminosity or
required in this mo del esp ecially if the disc is geometrically extreme mirror prop erties that would b e required for IRAS
thick Such p erp endicular orientations have not b een iden if its X rays were predominantly scattered We
ti ed in other ob jects Comparisons of ionization cones and shall consider the P observation when the ROSAT isotropic
radio structures suggest that discs and tori may b e highly luminosity was erg s see Sect The
coplanar see sect of Wilson but see Mundell et al observed L L was we note L is dominated by
X
Bol Bol
for a tentative counterexample The warm absorbing the infrared luminosity which generally lacks signi cant time
gas discussed ab ove might ll the b orehole and the large variability Cutri et al Neugebauer et al The
ux variability see Sect could arise via time dep endent re ected fraction of incident ux by an electron scattering
partial covering by a very thick torus cloud If our line of mirror f is approximately where is the
T
re
sight grazes the edge of the torus as is suggested in W the solid angle subtended by the mirror ab out the nucleus and
b orehole might not b e a hole but rather a serration in its is its Thomson depth corrections for the fact that Thom
T
edge although again the geometry is di cult to envisage son scattering is dip olar and geometrical corrections such as
If one chooses to abandon the geometry of W then those from sect of Miller Go o drich Matthews
di erent X ray and IR optical light paths can b e accommo do not change the essential results Thus unless f
re
dated more easily However the price of this abandonment the L L ratio b ecomes extremely large for a radio quiet
X
Bol
is a heavy one since the W mo del explains many of the AGN see table of Padovani Rafanelli A value of
prop erties of IRAS with grace f would b e times higher than that observed in
re
One p ossible alternative geometry p ostulates that there NGC Miller Go o drich Matthews For f
re
is reddening intrinsic to the source of optical photons A to b e this requires to b e or more if the op ening
T
thick dusty accretion disc might self redden the photons it half angle of the torus is degrees as p er sect of
emits at large radii and wavelength dep endent p olarization W then This would b e inconsistent with
could arise by transmission through dust although W the optically thin mirror assumption of sect of W
have shown that this dust would have to have a grain size which was predicated on the observed high degree of p olar
distribution di erent from that in the interstellar medium of ization A Thomson depth of within p c gives a
our Galaxy However such a mo del has serious di culty fairly high minimum electron density of cm for
explaining the similar p olarization p ercentage of the H line the scattering region This density may b e compared with
since this presumably comes from BLR clouds and not the the cm upp er limit on a typical Seyfert Broad
disc itself Furthermore in such a mo del the fact that the Line Region BLR intercloud medium imp osed by the fact
p olarization is parallel to the ma jor axis of the host Galaxy that it must b e Thomson thin see sect of Perry Dyson
must b e taken to b e a coincidence we conservatively consider a BLR intercloud medium
While we cannot strictly rule out even more complicated with and a radius of cm
T
alternative geometries e g a shell like torus around the We therefore note that while some of the soft X rays
BLR with a tiny X ray b orehole that happ ens to lie along our from IRAS might b e scattered by a mirror such
line of sight such mo dels su er from dynamical di culties as that in NGC a very atypical mirror which is b oth
e g it is unclear how a shell like torus might b e created very compact and Thomson thick is required if most of its
and maintained are generally anti Copernican e g they X rays are scattered to us The IR optical ux may still b e
require tiny X ray b oreholes to b e p ointed right at Earth scattered by a mirror as p er W see b elow
and have di culty naturally explaining why the p olarization
is parallel to the ma jor axis of the host Galaxy
The X ray data and IRAS
mo dels
Possibility The gas and dust we are looking We now discuss the two p ossibilities of Sect that we did
through in IRAS is very di erent from the not elab orate on there
typical cold gas and dust in the Galactic interstel lar
medium
Possibility The IR optical photons su er
extinction by dust but the X rays escape through a
Having found the other three p ossibilities of Sect incom
borehole
mo dious we consider the p ossibility that the gas and dust
we are lo oking through in IRAS is very di er The assumption that the X rays and IR optical emission
ent from that in the cold interstellar medium of our Galaxy travel through the same matter may b e incorrect This cer
We shall rst consider absorption by cold matter with an tainly seems likely to at least some degree in the inhomo
increased amount of dust p er unit gas We shall then con geneous environment thought to exist around AGN nuclei
ROSAT observations of the infrared loud quasar IRAS 9
together with an N cm are p ossible even sider the e ects of gas ionization and shall nally consider
H
when the dust is lo cated in cold gas However it must b e the e ects of changes in the dust grain size distribution
remembered that H and He remain predominantly gaseous Cold matter with a large dust to gas ratio could have a
and these elements play a ma jor role in determining the smaller N E B V ratio than that of the lo cal interstellar
H
ROSAT band t N We have again used XSPEC to simu medium of our Galaxy In this paragraph we shall consider
H
late sp ectra as in the previous paragraph but now take the the p ossibility that cold matter cloaking the nucleus of IRAS
column of N cm to contain only H and He has more dust p er unit of gas Even in the most
H
Neglecting the other elements is equivalent to taking them conservative case the dust content in IRAS
>
all to b e in dust grains with very e ective self blanketing must b e enhanced by a factor of relative to that of
factors Fitting these sp ectra with a p ower law plus stan our lo cal interstellar medium Given the same fraction of
dard cold column mo del we obtain values of N metal depletion into dust see Sect this would require
H
cm much larger than observed Hence it seems unlikely the metal abundance to also b e enhanced by a factor of
>
that by itself a p eculiar dust grain size distribution can ex Our X ray sp ectral data cannot rule out such an en
plain our data Ionization of at least the asso ciated H and hancement H and He dominate the t column see the next
He app ears to b e required paragraph but such an extreme metal abundance enhance
ment seems contrived Even allowing for a total depletion
We have seen that the absorption in IRAS
b elow keV may b e explained by dusty gas in which at of metals into dust the required metal abundance enhance
>
least H and He are ionized In addition at X ray energies Total metal depletion ment is an extreme factor of
into dust would make the ionized oxygen edge we see hard
ab ove keV we have indep endent evidence for a highly
to explain
ionized oxygen edge While asso ciation of these two gases
as one and the same would b e the simplest assumption and Dust grains lo cated in ionized rather than cold gas could
explain the lack of large intrinsic cold X ray absorption at
seems physically p ossible see b elow it must b e noted that
energies b elow keV without an extreme enhancement of
we cannot prove this assumption to b e true There could
for example b e dust free gas very close to the central en metals If the gaseous H He C N and O or just some of
gine that pro duces the tentative oxygen edge as well as less these elements were highly ionized this would reduce the
cold absorption opacity signi cantly esp ecially at the im
highly ionized but still ionized enough so that H and He are
stripp ed dusty warm gas further out Below we shall state p ortant low energy end of the ROSAT band see Krolik
clearly whether we are talking ab out the tentative oxygen Kallman We have examined the e ects of ioniza
tion of the gaseous H and He by using the XSPEC fakeit
edge gas the dusty warm gas or b oth as one and the same
command to simulate ROSAT keV observations of
sources with p ower law sp ectra of various photon indices
X ray absorption by warm gas
absorb ed by columns of cm see Sect to
understand why we choose this value However in these
Modelling of the oxygen edge gas
sp ectra we take all of the H and He to b e fully stripp ed we
In this section we shall t more sophisticated mo dels to the use the p ower law mo del with prop erly redshift corrected
structure in our data ab ove keV see Fig
variable abundance absorption zvarabs and set the H and
A one zone warm gas absorption mo del constructed He normalizations equal to zero to do this We also in
using the photoionization co de CLOUDY Ferland clude the cold Galactic column We consider observations
Reynolds et al is a go o d t to the P data with
of the same length and count rate as the P observation
for degrees of freedom The derived t param When we t the resulting simulated sp ectra with a p ower
: :
eters are N cm law plus standard cold column mo del we obtain values of
H;cold
: :
:
log N and Note that the
N cm and generally acceptable values of
H;warm
H
:
lower limit on N is larger than the cm A mo del where we take a redshifted intrinsic column of
H;warm
of Sect which may suggest a dust to gas ratio that is cm to have H and He fully stripp ed ts our
smaller than the Galactic one However limitations of cur
P data well provided that we also allow for the presence
rent warm absorbing gas mo dels must b e kept in mind when of an ionized oxygen edge We include the Galactic column
the results from them are interpreted For example they of N cm in our tting and obtain
H
assume strict photoionization equilibrium thermal balance
Fit parameters for the p ower law and edge are p o orly
and a simple gas density pro le Krolik Kriss ar constrained but are not physically implausible
gue theoretically that all of these assumptions may well b e In Sect we considered absorption by silicate and
invalid and the results of Reynolds et al hint that
graphite dust with a comp osition and grain size distribution
some are invalid as well
similar to that in the Galactic di use interstellar medium
The fact that the ROSAT isotropic luminosity of IRAS e g Mathis Rumpl Nordsieck Deviations from
varies by a large amount b etween P and P a Mathis Rumpl Nordsieck dust grain size distri
suggests that we may b e able to learn the mo de by which the bution in the vicinity of an AGN have b een considered in
gas that imprints the oxygen edge is ionized In particular sect of Laor Draine and they nd that a higher
if this gas were primarily photoionized and in photoioniza relative abundance of large grains is p ossible Larger grains
tion equilibrium we might exp ect signi cantly deep er edges will reduce the extinction in the IR and optical see g
in the P observation since the ionization parameter of the of Laor Draine and will also lead to self blanketing
oxygen edge gas would b e times less this would b e which reduces the amount of X ray photo electric absorption
true only if the intrinsic isotropic luminosity of the source by a given mass of dust Fireman One might sp ec
varied changes only in the fraction of sky covered by very ulate that dust grains that can give an E B V
10 W N Brandt A C Fabian K A Pounds
by sputtering the gas is probably photoionized rather than thick material would invalidate the argumentation of this
collisionally ionized paragraph On the other hand if collisional ionization were
dominant or the warm absorbing gas were far from photoion
ization equilibrium then we would not necessarily exp ect to
The warm dusty gas as a torus out ow or a torus
see a change in edge depth If we take the b est tting warm
atmosphere
gas absorption mo del for P and allow only its normaliza
If our line of sight to the central source in IRAS tion to change we can t the P data with for
grazed the edge of a torus as suggested by W then we degrees of freedom a go o d t This is suggestive of a
could b e lo oking through dusty gas that is in the pro cess warm absorb er that is not in simple photoionization equi
of b eing torn from the torus and turned into an ionized librium To test this p ossibility further we have t the P
out ow see sect I I I a of Krolik Begelman A geo data using the P mo del allowing all parameters except
metrically thin torus or accretion disc with an out ow that and the p ower law normalization to vary within their p er
blo cks photons at high latitudes might simulate a geomet cent con dence regions from the P t and the p ower
rically thick torus e g K onigl Kartje It could law normalization are allowed to vary b etween their P t
thereby match ionization cone and other observations and lower b ounds divided by and their P t upp er b ounds
also avoid the dynamical di culties asso ciated with geomet divided by Unfortunately we are not able to rule out
rical thickness Thomson thick chunks of matter p erhaps this t with statistical signi cance and therefore we cannot
clouds from the torus in this out ow could lead to the X formally prove that the oxygen edge gas is not in simple
ray variability we see as they pass through the line of sight photoionization equilibrium Thus our sp ectral tting con
see Wachter Strauss Filipp enko The ionized gas straints allow the oxygen edge gas to b e either photoionized
we see would then p erhaps b e the thermal wind describ ed or collisionally ionized but see the next section for further
by Weymann Turnshek Christiansen to accelerate p ossible constraints
BAL clouds Evidence for a warm absorbing out ows have We note that the ionized X ray absorption we see is in
b een seen in other high luminosity ob jects such as C line with the free free radio absorption describ ed by Beich
Mathur et al Alternatively we could b e lo oking man et al as a way of explaining the sharply p eaked
through less rapidly moving gas in a torus atmosphere see radio sp ectrum of IRAS Searches for sharply
g of Tsvetanov Kriss Ford ASCA observations p eaked radio sp ectra in other sources with prominent X ray
may b e able to distinguish b etween these two p ossibilities by absorption edges e g MCG NGC NGC
checking for shifts of the oxygen edge energies In addition by ionized gas would b e worthwhile
ASCA will b e able to lo ok for temp oral variability of oxy
gen edges and will b e able to prob e for edges from elements
Constraints on the dusty warm gas
b esides oxygen
The fact that we app ear to see dust asso ciated with at least
some of the warm absorbing gas in IRAS al
lows us to make inferences ab out the otherwise p o orly con
IRAS and narrow line Seyfert s
strained distance from the central engine and temp erature
IRAS app ears to mix prop erties typically asso
of this gas
ciated with Seyfert s such as fairly rapid soft X ray vari
Silicate dust grains sublime at K and graphite
ability and little cold X ray absorption with prop erties typ
grains sublime at K Laor Draine state that
ically asso ciated with Seyfert s such as high optical ob
the radius at which even large graphite dust grains exp osed
scuration and wavelength dep endent p olarization It has O 1
2
to an AGN sp ectrum sublime is R L p c where
sublime
i i i A H and Shuder Osterbro ck have
L L erg s Using this equation we derive
Bol
shown that Seyferts with O i i i A H are usu
R p c for IRAS Netzer Laor
sublime
ally Seyfert s It is worth seeking out other similar ob jects
state that a go o d guess for the average BLR radius
for comparison The class of ob jects called narrow line
as suggested by reverberation mapping studies is R
BLR
Seyfert s e g Osterbro ck Pogge Go o drich
1
2
Puchnarewicz et al BBF and references therein L p c Using this equation we derive R p c
BLR
hereafter NLS has at least some members that are similar for IRAS Thus it app ears that the dusty warm
Mrk for example has b oth rapid soft X ray variability gas we are lo oking through is probably lo cated outside the
Molendi Maccacaro Schaeidt as well as a high op BLR p erhaps near its outer edge e g Netzer Laor
tical p olarization fraction p er cent Go o drich Its sect of Go o drich This is in agreement with the
optical p olarization p osition angle is approximately p erp en deductions of Barvainis and W
dicular to its radio axis Ulvestad Antonucci Go o drich Thermal sputtering destroys internal dust once gas elec
and this is the typical orientation seen for Seyfert s tron temp eratures reach K Draine Salp eter
It deserves mention that Mrk like IRAS sect of Laor Draine so the dusty warm gas must
is one of the p eculiar outlying ob jects in g of WF It have a gas electron temp erature lower than this if the dust
shares the E B V versus X ray column discrepancy we is going to exist for an extended p erio d of time The gas
see in IRAS as do es Ark another NLS temp erature in warm absorbing gas where oxygen is highly
and WF outlying ob ject Brandt et al ionized is thought to b e K if it is primarily photoion
A comparison of the prop erties of IRAS ized and K or more if collisional ionization plays a
with those tabulated for NLS in the recent systematic study signi cant role e g Krolik Kallman Thus if the
of NLS by BBF reveals a strikingly large number of dusty warm gas and the oxygen edge gas are one and the
similarities same and the dust is not in the pro cess of b eing destroyed
ROSAT observations of the infrared loud quasar IRAS 11
We do not detect large amounts of X ray absorption by A simple p ower law t to the ROSAT sp ectrum of
neutral matter This is in contrast to what might b e IRAS yields a steep photon index of
:
exp ected due to this ob ject s large E B V We do
:
nd evidence for absorption by ionized oxygen
IRAS has rather rapid soft X ray variabil
The X ray variability and large X ray luminosity argue ity when compared to other ob jects of its luminosity
against having most of the X rays b e electron scattered
IRAS has lines from its high density region
towards Earth
that are relatively narrow km s W
A warm absorb er that has internal dust can explain
IRAS has strong optical Fe i i emission
the optical and X ray absorption prop erties of this
W
source We may b e lo oking through an ionized out
IRAS shows a de cit of ultraviolet emission
ow p erhaps a BAL QSO wind IRAS has
relative to its soft X ray sp ectrum WF
many prop erties that are similar to those of BAL QSOs
IRAS has weak narrow line emission see
or a torus atmosphere into the nucleus of this source
g of W
IRAS mixes prop erties typically asso ci
IRAS satis es the Go o drich criteria for
ated with Seyfert s with prop erties typically asso ci
classi cation as a NLS and when plotted in g of BBF
ated with Seyfert s It has a strikingly large num
it lies among the NLS The strong Fe i i emission and weak
b er of similarities to narrow line Seyfert s We ex
O i i i emission agree with the relationship embo died in the
amine these similarities and discuss their implications
fundamental eigenvector of Boroson Green
The absorption prop erties of the well studied ob ject
In light of the similarities of IRAS and
MCG may also b e explained with a dusty
NLS it is worth investigating whether IRAS
warm absorb er
might b e pro tably thought of as a NLS and whether
the absorption physics we have used to explain IRAS
might apply to all NLS Absorption by warm
gas can lead to steep NLS ROSAT band sp ectra e g
ACKNOWLEDGMENTS
BBF Czerny et al Dusty warm gas could lead to
We thank S Allen M Begelman Th Boller A Laor
optical BLR lines with relatively small FWHM by extinc
K Nandra C Otani L Puchnarewicz C Reynolds M
tion without large amounts of cold X ray absorption see
Ward and B Wills for useful discussions H Eb eling for
sect and g of BBF It could also e ectively ab
help with IDL and C Reynolds for the use of his warm
sorb ultraviolet ux This p ossibility may b e further tested
absorb er mo dels We thank an anonymous referee for an
with infrared observations of NLS BLR lines to see if they
illuminating and voluminous rep ort We gratefully acknowl
are systematically broader than their optical BLR lines due
edge help from members of the Institute of Astronomy X
to the smaller infrared extinction Dusty warm absorption
ray group and nancial supp ort from the United States
do es not seem to provide a natural explanation for the rapid
National Science Foundation and the British Overseas Re
ux and sp ectral variability of many NLS BBF Otani
search Studentship Programme WNB and the Royal So ci
but see Sect for interesting sp eculation and
ety ACF The ROSAT pro ject is supp orted by the Bun
warm absorption edges are seen in some e g Ark
desministerium f ur Forschung und Technologie BMFT and
Brandt et al NGC Mihara et al but
the Max Planck so ciety Much of our analysis has relied
not all NLS BBF
on the Starlink ASTERIX X ray data pro cessing system and
The well studied ob ject MCG su ers from
the XSPEC X ray sp ectral tting software and we thank
a similar E B V versus N discrepancy shows an X ray
H
the p eople who have created and maintain this software
absorption edge largely due to ionized oxygen e g Nan
This research has made use of data obtained from the UK
dra Pounds has a weak ultraviolet ux given the
ROSAT Data Archive Centre at the Department of Physics
rest of its sp ectral energy distribution g of WF
and Astronomy University of Leicester and has also made
Reynolds Fabian and has a high wavelength de
use of the NASA IPAC extragalactic data base Helou et al
p endent p olarization with a p osition angle parallel to the
which is op erated by the Jet Propulsion Lab oratory
ma jor axis of the host galaxy Thompson Martin
Caltech
It has E B V table of Busko Steiner
and an X ray measured intrinsic N cm These
H
similarities suggest that much of our dusty warm absorption
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