Investigating the complex X-ray spectrum of a broad-line 2MASS red quasar: XMM-Newton observation of FTM 0830+3759

0.37 MB·English

by  E. Piconcelli

Investigating the complex X-ray spectrum of a broad-line 2MASS
red quasar:
XMM-Newton observation of FTM 0830+3759
0
1
Enrico Piconcelli1, Cristian Vignali2, Stefano Bianchi3, Fabrizio Nicastro1,4,5, Giovanni
0
2 Miniutti6, and Fabrizio Fiore1

n
a
J

4
1 ABSTRACT

We report results from a 50 ks XMM–Newton observation of the dust-reddened broad-line
]
O quasar FTM 0830+3759 (z = 0.413) selected from the FIRST/2MASS Red Quasar survey. For
C this AGN, a very short 9 ks Chandra exposure had suggested a feature-richX-ray spectrum and
. HST images revealed a very disturbed host galaxy morphology. Contrary to classical, optically-
h
selectedquasars,theX-raypropertiesofred(i.e. withJ−K >1.7andR−K >4.0)broadline
p s s
- quasars are still quite unexplored, although there is a growing consensus that, due to moderate
o obscuration, these objects can offer a unique view of spectral components typically swamped
r
t by the AGN light in normal, blue quasars. The XMM–Newton observation discussed here has
s
definitely confirmed the complexity of the X-ray spectrum revealing the presence of a cold (or
a
[ mildly-ionized) absorber with NH ≈ 1022 cm−2 along the line of sight to the nucleus and a

Compton reflection component accompanied by an intense Fe Kα emission line in this quasar
1
v with a L2−10keV ≈ 5 × 1044 erg s−1. A soft-excess component is also required by the data. The
4 match between the column density derived by our spectral analysis and that expected on the
0 basis of reddening due to the dust suggests the possibility that both absorptions occur in the
4 same medium. FTM 0830+3759 is characterized by an extinction/absorption-corrected X-ray-
2
. to-opticalflux ratio αox = −2.3,that is steeper than expected on the basis of its UV luminosity.
1 Thesefindings indicate thatthe X-raypropertiesofFTM0830+3759differs fromthosetypically
0
observed for optically-selected broad line quasars with comparable hard X-ray luminosity.
0
1 Subject headings: infrared: galaxies – galaxies: active - galaxies: nuclei - X-rays: individual: FTM
v: 0830+3759
i
X 1. Introduction
r 1OsservatorioAstronomicodiRoma(INAF), ViaFras-
a cati33,I-00040MontePorzioCatone,Italy;piconcelli@oa- Selection criteria using near-infrared and mid-
roma.inaf.it. infrared photometry or combination of infrared
2Dipartimento di Astronomia, Universit`a di Bologna, (IR)andmultiwavelengthdatahavedemonstrated
ViaRanzani 1,I-40127Bologna,Italy.
the striking existence of a conspicuous population
3Dipartimento di Fisica, Universit`a degli Studi Roma
of obscured active galactic nuclei (AGNs) that
Tre,viadellaVascaNavale84,I–00146Roma,Italy.
has remained unknown until fairly recently, being
4IESL,FoundationforResearchandTechnology,71110,
Heraklion,Crete,Greece. difficult to detect with traditional selection cri-
5Harvard-SmithsonianCenterforAstrophysics,60Gar- teria in the optical band (e.g., Lacy et al. 2004,
denStreet, MS-04,Cambridge,MA02155, USA. 2007; Leipski et al. 2005; Webster et al. 1995;
6LAEX,CentrodeAstrobiologia(CSIC-INTA)LAEFF, Fiore et al.2008;Donley et al.2008;Lanzuisi et al.
PO Box 78, E-28691 Villanueva de la Can˜ada, Madrid,
2009). On the one hand, dust extinction hampers
Spain.
acompletecensusoftheactiveSMBHsintheUni-
verse in case of optical/UV surveys, on the other
1
hand,the AGNlightis absorbedbythe obscuring cialroleinshapingtheSEDofred2MASSquasars.
medium and then re-emitted at near- and mid-IR It is also worth noting that a small (≈10–15%)
wavelengths. percentage of classical, blue quasars is unavoid-
UsingtheTwoMicronAllSkySurvey(2MASS) ably collected, especially at z ∼< 0.4, by the
(e.g.,Skrutskie et al.2006,andreferencestherein), J −Ks > 2 selection criterion (Barkhouse & Hall
Cutri et al. (2002) have recently unveiled a pop- 2001). Also in Kuraszkiewicz et al. (2009a), it
ulation of highly-reddened (J −Ks > 2) quasars is shown that some unreddened (i.e. AV = 0–
at an average redshift of hzi ∼ 0.22 whose num- 1 mag) Type 1 sources with high Eddington ra-
ber density is almost comparable to that of the tios(L/LEdd)andlargeamountsofhotcircumnu-
optically-selected quasars, selected on the ba- cleardustarepickedupwithintheirsampleofred
sis of the UV-blue color excess criterion (e.g., 2MASS quasars. However, since the J −Ks color
Risaliti & Elvis 2004; Hall et al. 2006). Most of selectionisinhomogeneouswithredshift,athigher
these 2MASS sources are previously unidentified redshifts less contamination from blue AGNs will
quasars (Francis et al. 2004; Glikman et al. 2004; occur(asJ−Ks colorresemblesmorearestframe
Leipski et al. 2005; Urrutia et al. 2009). Opti- optical color) and the red J − Ks selection will
cal follow-up observations have revealed that the mostlygatherAGNsthataretrulydust-reddened.
bulk of this extragalactic population is composed More sophisticated selection processes involv-
by broad-line AGNs with a significant number of ing multiwavelength (i.e. radio, optical, infrared)
intermediate type objects (i.e. Type 1.2–1.9). In data have recently allowed to reveal red quasars
particular,ithasbeenfoundthat: (i)theirmedian (i.e. with J − K >1.7 and R − K >4.0)
s s
spectralenergydistribution(SED)isredderinthe up to z ∼> 1, such as in the case of the VLA
optical/UV band than that of the best-studied, FIRST-2MASS(FTMhereafter)surveypublished
classical blue quasars (e.g. Elvis et al. 1994), (ii) by Glikman et al. (2007). They estimate that red
they are primarily reddened by dust (A ∼ 1–5 quasars account for between 25% and 60% of the
V
mag) rather than being intrinsically red sources, total quasar population with K < 14 mag.
s
(iii) these objects usually exhibit a high optical The nature and the evolutionary properties of
polarization level (≈ 3–15%) suggesting that the red quasars are still open issues. It is worth not-
polarizationisduetoscatteringofnuclearlightby ing that the classical, non-red AGN population,
material located close to the active nucleus, but i.e. with a 2MASS color J − K < 2, indeed
s
exterior to the BLR (Kuraszkiewicz et al. 2009a; comprises a mixed bag of objects spanning from
Smith et al.2003). Suchanaspectisveryintrigu- unobscured blue quasars to Seyfert 2-like AGNs
ingsince∼20%of2MASSAGNsexhibitthesame showing only narrow lines in their optical spec-
broademissionlinesbothinthepolarizedfluxand tra and absorbed (even with Compton-thick, i.e.
in the total flux spectrum. This indicates that a NH >1024cm−2,columndensities)atX-rays(e.g.
sizablefractionoftheobservedopticalAGNemis- Watanabe et al. 2004; Alonso-Herreroet al. 1998;
sionmusthavescatteredintoourdirectionand,in Gorjian et al.2004;Zakamska et al.2004). Inpar-
turn, suggests caution in the use of spectral type ticular,whatisthe placeofredbroad-linequasars
(Type 1versusType 2)asindicatoroforientation in the AGN unification scheme and their role in
in AGNs (Schmidt et al. 2007). the contextofAGNandgalaxyco-evolution? Are
The observed properties of red quasars can be they standard systems observed along a special
interpreted in terms of intermediate viewing an- lineofsightforwhichbothnuclearandhostgalaxy
gles where the AGN is viewed through the edge obscuration are important (Kuraszkiewicz et al.
(or atmosphere) of the torus or a clumpy accre- 2009a),i.e. alternativetothatoftwoclassicalpop-
tion disk wind. Accordingly, the observed broad- ulations of unobscured broad-lined quasars and
band emission is the likely combination of direct, that of heavily obscured, narrow-lined quasars?
reprocessed and scattered emission components Alternatively, can they (especially if they are at
(e.g., Smith et al. 2003; Wilkes et al. 2008). In z ∼> 0.4–0.5) be interpreted as a peculiar dust-
addition, as pointed out by Kuraszkiewicz et al. cocoonedstageinthelifecycleofquasars,possibly
(2009a), both obscuration and emission from the associated with the assembly of the host galaxy?
circumnucleargasand the hostgalaxyplay a cru- Intriguingly, HST images available for a sam-
2
ple of dust-reddened FTM quasars with M ≤ ter their discovery, a handful of programs aimed
B
−23 and 0.4∼< z ∼< 1 show disturbed opti- at studying the X-ray properties of these AGNs
cal morphologies in the vast majority (85%) of have been undertaken. Chandra (Wilkes et al.
them with evidence of ongoing merging, inter- 2002; Hall et al. 2006; Urrutia et al. 2005, here-
actions and multiple nuclei (Urrutia et al. 2008; after U05) and XMM–Newton shallow obser-
Hutchings et al. 2006), unlike blue quasars being vations (Wilkes et al. 2005; Pounds et al. 2005)
mostly hosted in undisturbed elliptical or bulge- of ∼20-30 2MASS quasars indicate the quasi-
dominatedgalaxiesasfoundinHSTimagingstud- ubiquitous presence of cold absorption with
ies of optically-selected luminous quasars at z 1021 ∼< NH ∼< 1023 cm−2 regardlessofopticaltype.
< 1 (e.g., Dunlop et al. 2003; Floyd et al. 2004; This matches with the finding that only ≈10% of
Sanchez et al. 2004, see also Bennert et al. 2008). the red 2MASSAGNs aredetected inthe ROSAT
Evidence is mounting that distant red quasars Faint Source Survey (Cutri et al. 2002).
might indeed represent a dust-enshrouded phase Bearing in mind the small sample size to
in quasar evolution linked with the host galaxy date, broad-line red quasars seem to show a wide
assemblyviarepeatedmergers(Georgakakis et al. range of spectral types in the X-ray band unlike
2009; Urrutia et al. 2009). This phase should be their blue counterparts. In particular, two re-
characterized by the presence of massive nuclear sults from X-ray spectroscopy of red quasars can
windsexpelling/heatingmostofthecoldgasreser- be considered of special interest: (i) a number
voir and merger debris in the host galaxy, and of these sources have an unusually flat hard X-
so rendering progressively visible the quasar as ray continuum with Γ2−10<1.5 (e.g. Vignali et al.
an optically-bright source (the so-called feedback 2000; Wilkes et al. 2005; Pounds & Wilkes 2007;
processes, e.g. Silk & Rees (1998); Hopkins et al. Wilkes et al. 2008), probably due to the presence
(2006)). of an intense Compton reflection component, and
A deep exploration of the X-ray spectral prop- (ii) some broad-line red AGNs exhibit a ”soft-
erties of red quasars is therefore important to im- excess” component below ∼1-2 keV that can be
proveourunderstandingofthestructureofAGNs interpreted as the result of absorption and re-
and their cosmological evolution. The combina- emission from the same weakly ionized (outflow-
tion of a favorable line of sight inclination and ing) gas (Pounds et al. 2005; Pounds & Wilkes
obscuration can indeed provide a unique view 2007).
of spectral components usually overwhelmed by In this paper, we present the first high-quality
the AGN light in blue quasars (Schmidt et al. X–ray spectrum, obtained with XMM–Newton,
2007; Pounds et al. 2005). The X-ray spec- of the luminous red quasar FTM 0830+3759 at
tral properties of unobscured, optically-selected, z=0.413. A very short 9 ks Chandra observation
radio-quiet quasars have been largely investi- revealed a very steep continuum and the possible
gated in the past (Laor et al. 1997; George et al. presence of puzzling emission/absorption line-like
2000; Piconcelli et al. 2005), especially in case features at energies which do not correspond to
of medium-luminosity objects showing an X-ray any obvious rest-frame atomic transition (U05).
luminosity of LX < 5 × 1044 erg s−1. They WeassumethroughoutthispaperH0 =70kms−1
appear to be quite homogeneous and without Mpc−1, ΩΛ = 0.73 and ΩM= 0.27 (Spergel et al.
any evidence for significant evolution with z 2007).
(Just et al.2007;Page et al.2005;Piconcelli et al.
2003; Vignali et al. 2003a), although the strength 2. Optical and Radio Properties of FTM
of spectral features due to the reprocessing of 0830+3759
the primary continuum (such as Fe Kα emis-
FTM0830+3759(J−K=2.28,e.g.Glikman et al.
sion line and Compton hump) is still largely
2007) is the X-ray brightest object included in
unconstrained for high luminosity quasars with
LX ∼> 1045 ergs−1 (Mineo et al.2000;Page et al. the U05 sample of 12 broad-line, dust-reddened
quasars selected from the FTM survey. The ob-
2005; Jimenez-Bailon et al. 2005).
jects in this sample need to satisfy three main
Onthe contrary,the X-raypropertiesofredlu-
criteria: being a FIRST radio source; belonging
minous quasars still remain poorly explored. Af-
3
Table 1
Multiwavelength properties of FTM 0830+3759
R.A. Dec. za Flux1.4GHza J−Kb E(B−V)c Mc
(hms) (◦′′′) (mJy) (mag) (mag) (maBg)
083011.12 +375951.8 0.413 6.4 2.28 1.29±0.12 −23.07
Note.—Allmagnitudes areintheVegasystem.
aDatafromUrrutiaetal.(2005).
bDatafromGlikmanetal.(2007).
cNuclearvaluesfromUrrutiaetal.(2008)basedonHST/ACSWideFieldCameraobser-
vations.
to the 2MASS point-source catalog and having Finally, there is no detailed information on the
a R − K ∼> 4.4 (we refer to U05 for further optical classification of FTM 0830+3759 in the
details on the optical/near-IR selection process). literature besides it is a broad-line AGN. How-
All these luminous red quasars lie in the redshift ever, the optical spectrum reported in Fig. 1 of
range 0.4∼< z ∼< 2.7, exhibit broad lines in their Urrutia et al. (2008) suggests that an intermedi-
optical spectra and they are radio-quiet or radio- ate (i.e. Type 1.5) classification for this source
intermediate, i.e. with a ratio of radio to optical may be considered plausible, although a proper
emission RL=F5GHz/FB <100. The value of RL investigation on this issue is required.
derivedforFTM0830+3759is≈30,andthesteep
radiospectralindex(α =−1.06)de- 3. XMM-Newton Observation and Data
1.4GHz/8.3GHz
rived by Glikman et al. (2007) also supports the Reduction
moderate radio-loud nature of this quasar.
We observed the quasar FTM 0830+3759with
FTM 0830+3759 is also included in the HST
XMM–Newton (Jansen et al. 2001) on November
study of a sample of 13 dust-reddened FTM
8, 2008 for about 52 ks (Obs. ID.: 0554540201).
quasars performed by Urrutia et al. (2008). HST
The observation was performed with the EPIC
ACS Wide Field Camera images reveal that the
PN and MOS cameras operating in Full-Window
host galaxy of FTM 0830+3759 shows a lot of ir-
mode and with the THIN filter applied. Data
regularitiesnearthe nucleusalongthe majoraxis.
werereducedwithSASv8.0.0usingstandardpro-
Aprominentstructurewithaionizationconemor-
cedures and the most updated calibration files
phologyispresent: itcanbe interpretedasdue to
available at the date of the analysis (April 2009)
merger-induced star formation regions or to the
were used. X-ray events corresponding to pat-
presenceofoutflowinggas. AfteranaccuratePSF
terns 0–4(0–12) for the PN(MOS) cameras were
fitting and host galaxy subtraction, Urrutia et al.
selected. The event lists were filtered to ignore
(2008) measured a M = −23.07 mag and a red-
B periods of high background flaring according to
deningofE(B−V)=1.29forthequasarinFTM
the method presented in Piconcelli et al. (2004)
0830+3759. The inferred value of the nucleus
based on the cumulative distribution function of
to host ratio for the I magnitude (F814W filter)
background lightcurve count-rates. After screen-
was 0.55. Interestingly, Urrutia et al. (2008) also
ing the finalnetexposuretimes were41and48ks
found a possible correlation between the magni-
for PN and MOS, respectively.
tude of galaxy interactions and the level of ob-
The source photons were extracted for the
scuration affecting the quasar. This led them to
PN(MOS) camera from a circular region with a
provide a possible explanation of the red nature
radius of 29(30) arcsec centered at the peak of
of FTM quasars in terms of reddening occurring
the emission, while the background counts were
in the host galaxy.
estimated from a 52(49) arcsec radius source-
4
free region on the same chip and close to FTM Fig. 1 (left panel) shows the result of this simple
0830+3759 without being contaminated by the fit extrapolated to 0.3 keV. The broad and deep
target itself. deficit in the soft X-ray band clearly suggests the
Theredistributionmatrixfilesandancillaryre- presence of strong absorption. Fitting the data
sponse files were created using the SAS task RM- overthe0.3-9keVbandwithanunrealisticpower-
FGEN and ARFGEN, respectively. As the dif- law model yielded a Γ∼0.72.
ference between the MOS1 and the MOS2 re- To reproduce the shape of the soft X-ray spec-
sponsematricesisafewpercent,wecreatedacom- trum we initially applied a model consisting of a
bined MOS spectrum and response matrix. The power law modified by intrinsic, rest-frame cold
background-subtractedspectraforthePNandthe matterabsorption. This fitwasstatisticallyunac-
combinedMOScameraswerethensimultaneously ceptable with χ2/dof = 1.95/324, revealing addi-
ν
fitted. Spectra were rebinned so that each energy tionalspectralcomplexity. Inparticular,anexcess
bin contains at least 20 counts to allow us to use in the data/model residuals emerges at energies
the χ2 minimization technique in spectral fitting. below 0.6 keV (e.g. Fig. 1, right panel).
During the XMM–Newton observation the flux To further explore the nature of the obscur-
ofFTM0830+3759remainedsteady,withnovari- ing material along our line of sight we modeled
ation exceeding 2σ from the average count-rate the low-energy drop by replacing the neutral ab-
level in both soft- and hard-X-ray band. Since sorber with a photoionized absorber component.
nosignificantspectralchangesoccurred,the spec- The latter was modeled in XSPEC adopting the
tral analysiswas performed on the spectrum inte- publicly available output table “grid 18”1 of the
grated over the full exposure time. XSTAR code (Kallman & Bautista 2001). In this
model the state of the warm absorber is a func-
4. Spectral Analysis tion of the ionization parameter ξ defined as ξ =
L/nr2, where L is the isotropic luminosity of the
In this Section we present the spectral analysis
power-law ionizing source in the interval 13.6 eV
ofthe EPICobservationofFTM0830+3759 that
to 13.6 keV, n is the density of the plasma and
was carried out using the XSPEC v11.3 software
r represents the radial distance from the central
package(Arnaud1996). TheGalacticcolumnden-
source. Inthefitsolarelementalabundanceswere
sity of NGal = 3.61 × 1020 cm−2 derived from
H assumedand ξ was left as a free parameter. How-
Kalberla et al. (2005) was adopted in all the fits. ever, this fit, with χ2/dof=1.05/323, was not en-
ν
In the following, errors correspond to the 90% tirely effective in reproducing the EPIC data as
confidence level for one interesting parameter, i.e.
broadpositiveresidualswerestillpresentbetween
∆χ2= 2.71 (Avni 1976).
0.5 and 0.7 keV.
4.1. Preliminary Fits
4.2. Complex models
As a first step we fitted the hard portion of In order to account for the soft-excess compo-
the EPIC spectra at E>2.5 keV (corresponding
nent, we then included an additional unabsorbed
to 3.5 keV in the source frame) with a power law
power-law fixing its photon index to that of the
to achieve a preliminary description of the X-ray
absorbed power law but with a different normal-
primary continuum emission in an energy range
ization. Such a parametrization typically pro-
expected to be much less affected by absorption
vides an excellent description of the X-ray spec-
than soft X-rays. The emission line, observed at
trum of many well-studied Compton-thin AGNs,
∼ 4.5 keV (6.4 keV in the quasar frame), was inwhichthe soft-excessisduetoacombinationof
modelled with a narrow Gaussian line with en-
emission from scattered continuum photons and
ergy and normalization free to vary (see Sect. 4.3
distant photoionized gas (e.g. Turner et al. 1997;
for more details). This model is a good fit to the
Winter et al. 2009; De Rosa et al. 2008). This fit
EPIC data, i.e. χ2ν/dof= 1.02/174. We measured resulted in a χ2/dof = 0.99/323 yielding a dra-
ν
a Γ = 1.37±0.08 that is significantly flatter than
the averagehardX-rayslope hΓi ≈1.85found for
1Seehttp://heasarc.gsfc.nasa.gov/docs/software/xstar/xstar.html
optically-selected quasars (Piconcelli et al. 2005). forfurtherdetails
5
V 1
e 0.
k
c/
e
s
s/
nt 1
u 0
co 0.
d
e
z
ali
norm 10−3
2
5
ratio 11.
5
0.
0.5 1 2 5
channel energy (keV)
8
6
o
ati
r
4
2
0.5 1 2 5
channel energy (keV)
Fig. 1.— Left: Continuum power-law fit (+ a Gaussian emission line at ∼ 6.4 keV rest-frame) to the 2.5-9
keV band of the PN (top) and MOS (bottom) spectra of FTM 0830+3759 extrapolated over the 0.3-9 keV
band. Thelowerpanelshowsthedata/modelratioresiduals. Right: Thedata/modelratioresidualsresulting
from fitting an absorbed power law to the 0.3-9 keV EPIC data (circles and empty squares are the PN and
MOS residuals, respectively). The shape of the residuals strongly suggests the presence of a soft-excess
emission component below ≈0.7 keV.
6
matic improvement in fit quality over the simple 2002).
absorbed power-law model at the >99.99% confi- Unfortunately, the combination of EPIC reso-
denceinanF−test. Wemeasuredaphotonindex lution, intermediate column density of the X-ray
of Γ = 1.51±0.06 and a column density of NH = absorber and moderate soft X-ray flux of FTM
(1.86±0.16)× 1022 cm−2 of the neutral absorber, 0830+3759heavilyhampersthedetectionofthese
while the ratio between the normalization of the emissionlinesinthe XMM–Newton spectrumpre-
unabsorbed and absorbed PL is fs = 0.13±0.02. sentedhere. Thisimpliesthatwewerenotableto
Table2liststhebest-fitvaluesoftherelevantspec- discriminate between the two proposed scenarios
tral parameters obtained by this fit, hereafter re- for the soft-excess emission and, in turn, provide
ferred as NP model. unambiguous constraints on the covering fraction
Awarmabsorbermodel(WP)wasthentested, of the absorber in FTM 0830+3759. This aspect
using again the “grid 18” XSTAR table. This fit should be borne in mind while interpreting all
yielded χ2/dof = 0.97/322 with an improvement the results from the spectral analysis presented
ν
at>95.6%confidencelevelaccordingtoanF−test hereafter. Furthermore, we cannot use the high
once compared with the NP model. We derived resolution RGS data to shed light on this issue
the following best-fit values for the physical pa- since FTM 0830+3759 is too faint at soft X-rays
rameters of the warm gas: a column density of to be detected by the RGS with sufficient signal
NH ∼ 2.4 × 1022 cm−2 and an ionization param- to allow a useful spectral analysis.
eter of ξ ∼ 15 erg cm s−1.
We note that leaving the photon index of the Giventheflatnessofcontinuumslopemeasured
unabsorbed power law free to vary in both NP with the NP and WP models, i.e. Γ ∼1.5, we
and WP models gave values that are consistent attempted to explain it as the result of a com-
with the photon index of the continuum and did bination of an underlying steep continuum modi-
not produce any appreciable improvement in the fied by a cold/warmabsorber plus a Compton re-
quality of the fits. flection component from neutral matter (pexrav
modelinXSPEC,Magdziarz & Zdziarski(1995)),
It is worth noting that a fitting model with an with the metal abundances of the reflector fixed
absorber completely covering the nucleus plus an to the solar value and its inclination angle fixed
extra-continuum X-ray emission component (like to 65 deg. The addition of this spectral com-
bothNPandWPmodel)isnumericallyequivalent ponent produced a steeping of the photon index
to a model with a partially-covering absorber. In to Γ ≈1.85-1.9. However the measured value of
the partial covering scenario the soft X-ray emis- the reflection fraction R ∼ 3 (defined as Ω/2π,
sion is alternatively interpreted as a portion of where Ω is the solid angle subtended by the re-
primary radiation leaking through the absorber flector for isotropic incident emission) is largely
with a covering fraction equal to C = (1 − f ); unconstrained and implies an unphysical cover-
f s
see Table 2. Instead, in case of fully-covering ab- ing factor of neutral reflecting material of Cf>
sorber, the soft excess is explained in terms of 1 and/or anisotropy in the irradiation. Such a
emission due to reprocessing of the nuclear con- value is well outside the range typically observed
tinuumbysurroundingmaterial. Thisreprocessed for radio-quiet broad-line AGNs, i.e. 0.6∼< R
component may be electron-scatteredemissionby ∼< 1.2 (Perola et al. 2002; Deluit & Courvoisier
highly ionized matter extending above the “hole” 2003; Nandra et al. 2007; Panessa et al. 2008). A
of the torus or Compton reflected by cold mate- fitwithRfixedatunityresultedinaχ2(ν)similar
rial, and/or it may arise from large-scale (∼0.1-1 to that obtained with R left free to vary, then we
kpc;seeBianchi et al.(2006,2007a))photoionized decided to use R≡1 in the following fits. The ad-
gas as pointed out in most of high-resolution soft dition of a reflection component in the NP(WP)
X-ray spectra of heavily-obscured Type 2 AGNs model caused χ2 to decrease by 9(6), see model
which are dominated by a wealth of strong emis- refl-NP(refl-WP) in Table 2. The photon index
sion lines from hydrogen- and helium-like ions of steepens to Γ ≈1.65 in both absorption scenar-
the most abundant metals, from carbon to sulfur ios (e.g. Fig. 2 (left panel) for the iso-χ2 con-
(Guainazzi & Bianchi 2007; Kinkhabwala et al. tour plot of NH versus Γ for the refl-NP model).
7
Table 2
Best Fit Continuum Parameters
Model ModelName ModelParameters χ2/d.o.f.
APL(neutral absorber)+ NP NH=(1.86±0.16)×1022cm−2;fs =0.13±0.02 321/323
PL Γ=1.51±0.06
APL(warmabsorber)+ WP NH =(2.40+−00..1158)×1022cm−2;fs =0.10±0.04 313/322
PL log(ξ)=1.20±0.11ergcms−1;Γ=1.48±0.05
NP+Compton refl-NP NH =(1.94±0.15)×1022cm−2;fs =0.11±0.01 312/323
Reflection(neutral) Γ=1.67±0.06
(Magdziarz&Zdziarski1995) R=Ω/2π ≡1
WP+Compton refl-WP NH =(2.45±0.15)×1022 cm−2;fs =0.09±0.03 307/322
Reflection(neutral) log(ξ)=1.16+−00..1116 ergcms−1;Γ=1.64+−00..0068
(Magdziarz&Zdziarski1995) R=Ω/2π ≡1
Ionizedabsorber/emitter +PL+ refl-wabs NH =(1.26±0.11)×1022 cm−2;fs =0.07±0.02 309/321
ComptonReflection(neutral) log(ξ)=0.79+−00..1181 ergcms−1;Γ=1.52±0.03
(Magdziarz&Zdziarski1995) AFe ≡3;R=Ω/2π ≡1
refl-NP+thermal th-NP NH =(8.50±0.17)×1022 cm−2;fs =0.11±0.01 303/321
emission(Kaastraetal.1996) Γ=1.64±0.06;R=Ω/2π ≡1
kTa <0.13keV
Note.—APL:absorbedpowerlaw. PL:unabsorbedpowerlaw.
aThethermalemissionmekalcomponenthasa0.5–2keVunabsorbedluminosityof(2.1)×1042ergs−1andcontributes
<1%ofthefluxinthe0.5–2keVband.
8
GiventhelargeluminosityofFTM0830+3759we fore included a mekal component (Kaastra et al.
also tested the possible presence of a reflection 1996) with solar abundance in the refl-NP model.
componentfromionizedmaterialby replacingthe The resulting χ2/dof was excellent with 303/321,
pexrav component in the refl-NP and refl-WP significant at 99.1% confidence in an F−test (e.g.
model with the reflion model of Ross & Fabian model th-NP in Table 2). However, the best-fit
(2005). This model is self-consistent as it also in- temperature of the emitting plasma was pegged
corporates fluorescent emission lines from ionized at the minimum value allowed of kT ≈ 0.08 keV
species. Accordingly we removed from the fitting with an upper limit of 0.13 keV. It is also impor-
model the Gaussian line used in all the previous tanttonotethatthederived0.5-2keVluminosity
fitstodescribetheline-likefeaturearound6.4keV of 2.1 × 1042 erg s−1 for this thermal component
(quasar-frame). The best-fit values of the ioniza- evenexceedsthehighestvaluesmeasuredforstar-
tion parameter of the reflector were found to be burst regions in ultra-luminous infrared galaxies
consistent with the minimum permitted value of (e.g., Franceschini et al. 2003).
ξ =30ergcms−1 (withanupperlimitofξ
refl refl
∼< 40 erg cm s−1), which yielded χ2ν/dof values 4.3. The Iron Kα emission line
slightly worse than those with the pexrav com-
Thepresenceofanexcessaround6.4keV(rest-
ponent. This clearly indicates that the reflection
frame) in the XMM–Newton spectrum of FTM
medium is almost neutral. Using the formula (1)
0830+3759is clearly evident. Fig. 2 (right panel)
reportedinBrenneman et al.(2009),wewereable
shows the data/model ratio in the observer Fe K
toprovideanestimateofthereflectionfractionR.
emissionlineenergyrangewhenfittingtherefl-NP
Regardless the physical state of the absorber, we
modelover0.3-9keVandignoringthe3.8-5.2keV
measuredanRvalueof≈0.6,thatsupportsasce-
band.
nario in which the hard X-ray spectrum of FTM
We have modeled this spectral feature with a
0830+3759is not largely reflection-dominated.
narrow Gaussian line. The inclusion of this com-
Furthermore, we examined the intriguing pos-
ponentinthefitresultedinverysignificantstatis-
sibility that the soft-excess could instead be the tical improvement (∆χ2 = 13 for two additional
result of a combination of scattered continuum
free parameters), i.e. at 99.99% confidence level
and multiple emission lines arising from the same
accordingto an F–testonce comparedto a model
mildly ionized gas responsible of the absorption
without the line. We concentrated only on PN
(e.g., Pounds et al. 2005; McKernan et al. 2007;
data for a more accurate determination of the Fe
Longinotti et al. 2008). To model this soft X-ray
Kα line parameters since the PN CCD has a bet-
emitter we again used the XSTAR output table
ter sensitivity and energy resolution over the 4-6
grid 18, with the ionization parameter ξ and the
keV energy range than the MOS CCD. The rest-
column density of the emitter tied to the values frame energy centroid of the line is E = 6.42+0.04
of the absorber. We also added to this model a −0.06
keV. This energy indicates a low ionization state,
neutral reflection component with R fixed to 1.
Fe I–XVIII, of the emitting material. Allowing
This model(e.g. refl-wabsin Table 2)gavea very
the width of the line in FTM 0830+3759 to vary
goodfit to the EPIC data with χ2/dof= 309/321
did not improve the fit over a narrow-line model
when the ironabundance was fixed to AFe≡3 (i.e. (σ ∼< 0.18keV).Wemeasuredaredshift-corrected
itsbest-fitvalue)forthe warmgas. We deduceda
equivalentwidthEW=168±60eVassumingthat
lowionizationparameterofξ≈6ergcms−1anda
the line and continuum are both absorbed, while
columndensityofNH ≈1022cm−2 fortheabsorb- the EW calculated with respect to the reflected
ing/emitting gas. The photon index of the power
continuum is EW = 800±280eV.
law resulted Γ = 1.52±0.03, i.e. flatter than that
measured with the refl-WP model (Γ≈1.65).
4.4. X-ray Fluxes and Luminosities of
Finally,weconsideredamodelwhereathermal FTM 0830+3759
plasmacomponent(mekalinXSPEC)isresponsi-
Oncemodelrefl-NPwasassumed,wemeasured
ble for a fraction of the soft X-ray positive excess
an observed(intrinsic) 2-10 keV flux of 8.9(9.4) ×
shownin Fig. 1 (left panel), under the hypothesis
10−13ergcm−2s−1anda0.5-2keVfluxof1.3(4.1)
of an origin from starburst activity. We there-
9
2
2.
m)−2 2
c
022
1
(H
N
8
1.
6
1.
1.6 1.7 1.8
Photon Index
2
o
ati 5
r 1.
1
4 4.5 5
channel energy (keV)
Fig. 2.— Left: Confidence contour plot showing the column density (in units of 1022 cm−2) of the cold
absorber against the photon index of the power-law continuum obtained by applying model refl-NP (e.g.
Table2,Sect. 4.2). Thecontoursareat68%,90%,and99%confidencelevelsfortwointerestingparameters,
respectively. Right: Close-up of the EPIC PN data/model ratio residuals in the Fe Kα emission line energy
range (observer-frame) when fitting the refl-NP model over 0.3-9 keV and ignoring the 3.8-5.2 keV band.
The vertical dashed line represents the energy of 6.4 keV in the quasar rest-frame.
10