IC 3639—A New Bona Fide Compton-Thick AGN Unveiled by NuSTAR

23 Pages·2016·1.31 MB·English

Draftversion October 31,2016
PreprinttypesetusingLATEXstyleAASTeX6v.1.0
IC 3639 – A NEW BONA FIDE COMPTON THICK AGN UNVEILED BY NUSTAR
Peter G. Boorman,1 P. Gandhi,1 D. Alexander,2 A. Annuar,2 D. R. Ballantyne,3 F. Bauer,4,5,6 S. E. Boggs,7 W.
N. Brandt,8,9,10 M. Brightman,11 F. E. Christensen,12 W. W. Craig,12,13 D. Farrah,14 C. J. Hailey,15 F. A.
Harrison,13 S. F. Ho¨nig,1 M. Koss,16 S. M. LaMassa,17 A. Masini,18,19 C. Ricci,4 G. Risaliti,20 D. Stern,21 W. W.
Zhang22
6
1 1DepartmentofPhysics&Astronomy,FacultyofPhysicalSciencesandEngineering,UniversityofSouthampton,Southampton, SO171BJ,
0
UK;[email protected]
2
2CentreforExtragalacticAstronomy,DepartmentofPhysics,DurhamUniversity,SouthRoad,Durham,DH13LE,UK
t
c 3CenterforRelativisticAstrophysics,School ofPhysics,GeorgiaInstituteofTechnology, Atlanta,GA30332,USA
O
4Instituto deAstrof´ısicaandCentrodeAstroingenier´ıa,FacultaddeF´ısica,PontificiaUniversidadCat´olicadeChile,Casilla306, Santiago

7 22,Chile
2 5MillenniumInstitute ofAstrophysics(MAS),NuncioMonsen˜orS´oteroSanz100,Providencia,Santiago, Chile
6SpaceScienceInstitute, 4750WalnutStreet, Suite205,Boulder,Colorado80301
]
A 7SpaceSciences Laboratory,UniversityofCalifornia,Berkeley,CA94720,USA
G 8DepartmentofAstronomyandAstrophysics,ThePennsylvaniaStateUniversity,525DaveyLab,UniversityPark,PA16802, USA
. 9InstituteforGravitationandtheCosmos,ThePennsylvaniaStateUniversity,UniversityPark,PA16802, USA
h
p 10DepartmentofPhysics,104DaveyLab,ThePennsylvaniaStateUniversity,UniversityPark,PA16802,USA
- 11CahillCenterforAstronomyandAstrophysics,CaliforniaInstituteofTechnology, Pasadena, CA91125, USA
o
12DTUSpace-National SpaceInstitute, Technical UniversityofDenmark,Elektrovej327,DK-2800Lyngby, Denmark
r
st 13LawrenceLivermoreNationalLaboratory,Livermore,CA94550, USA
a 14DepartmentofPhysics,VirginiaTech,Blacksburg,VA24061, USA
[
15ColumbiaAstrophysicsLaboratory,ColumbiaUniversity,NewYork,NY10027, USA

1 16InstituteforAstronomy,DepartmentofPhysics,ETHZurich,Wolfgang-Pauli-Strasse27,CH-8093Zurich,Switzerland
v
17NASAPostdoctoral ProgramFellow,NASAGoddardSpaceFlightCenter,Code665,Greenbelt, MD20771,USA
7
9 18INAF-OsservatorioAstronomicodiBologna,viaRanzani 1,40127Bologna,Italy
9 19DipartimentodiFisicaeAstronomia(DIFA),Universit`adiBologna,vialeBertiPichat6/2,40127Bologna,Italy
8 20INAFArcetriObservatory,LargoE.Fermi5,50126Firenze,Italy
0
. 21JetPropulsionLaboratory,CaliforniaInstitute ofTechnology, Pasadena, CA91109, USA
0
22X-rayAstrophysicsLaboratory,NASAGoddardSpaceFlightCenter,Greenbelt, MD20771, USA
1
6
1 ABSTRACT
:
v Weanalysehigh-qualityNuSTARobservationsofthelocal(z=0.011)Seyfert2activegalacticnucleus
i (AGN) IC 3639, in conjunction with archival Suzaku and Chandra data. This provides the first
X
broadbandX-rayspectralanalysisofthesource,spanningnearlytwodecadesinenergy(0.5–30keV).
r
a PreviousX-rayobservationsofthesourcebelow10keVindicatedstrongreflection/obscurationonthe
basis of a pronounced iron fluorescence line at 6.4keV. The hard X-ray energy coverage of NuSTAR,
togetherwithself-consistenttoroidalreprocessingmodels,enablesdirectbroadbandconstraintsonthe
obscuringcolumn density ofthe source. We find the sourceto be heavily Compton-thick (CTK)with
an obscuring column in excess of 3.6×1024cm−2, unconstrained at the upper end. We further find
an intrinsic 2–10keV luminosity of log (L [ergs−1]) = 43.4+0.6 to 90% confidence, almost
10 2–10keV −1.1
400 times the observed flux, and consistent with various multi-wavelength diagnostics. Such a high
intrinsic to observed flux ratio in addition to an Fe-Kα fluorescence line equivalent width exceeding
2keV is extreme amongst known bona fide CTK AGN, which we suggest are both due to the high
level of obscuration present around IC 3639. Our study demonstrates that broadband spectroscopic
modelling with NuSTAR enables large corrections for obscuration to be carried out robustly, and
emphasises the need for improved modelling of AGN tori showing intense iron fluorescence.
Keywords: galaxies:Seyfert – galaxies:active – galaxies:nuclei – techniques:spectroscopic – X-
2
rays:galaxies– X-rays:individual(IC 3639)
1. INTRODUCTION three main components: a Compton reflection hump,
peaking at ∼30keV; a strong neutral Fe-Kα fluores-
TheoriginofthecosmicX-raybackground(CXB)has
cence line at ∼6.4keV (Matt et al. 2000) (strong gener-
been under study ever since its discovery more than 60
ally refers to an equivalent width EW&1keV); and an
years ago (Giacconi et al. 1962). Spanning from frac-
underlying absorbed power law with an upper cut-off
tions of a keV (soft X-rays) up to several hundreds
of several hundred keV (intrinsic to the AGN, arising
of keV (hard X-rays), the general consensus today is
from the Comptonisation of accretion disc photons in
thatthe majorityofthe CXBarisesfromtheintegrated
the corona). The ability to detect the absorbed power
emission of discrete sources of radiation, with the most
law in the spectrum of a source depends on the level of
prominent contribution arising from active galactic nu-
obscuration - in heavily CTK sources, this component
clei (AGN) (e.g. Mushotzky et al. 2000). The unified
is severely weakened and can be entirely undetectable.
model of AGN (Antonucci 1993; Netzer 2015) predicts
The Compton hump and Fe-Kα line are both reflection
thatthe majordifferencesseenbetweendifferentclasses
features from the putative torus.
of AGN can be attributed to an orientation effect, with
X-ray selection is one of the most effective strategies
theprimaryradiationsourcebeingsurroundedbyanob-
available for detecting CTK sources because hard X-
scuringtorusinclinedrelativetoourline-of-sight(LOS).
ray photons stand a greater chance of escaping the en-
This leads to effectively two types of AGN - those with
shrouding obscuring media due to their increased pen-
a direct view to the nucleus (largely unobscured) and
etrating power. In addition, photons with initial prop-
thosewithanobscuredviewto thenucleusfrombehind
agation directions out of the LOS can also be detected
a putative torus (see Marin 2016 for a recent review on
throughComptonscatteringintoourLOS.Forthisrea-
the orientation of AGN). In addition, obscured AGN
son, the best energy range to observe CTK AGN is
havebeenrequiredtofittheCXB,withSetti & Woltjer
E>10keV. In general, many synthesis models formu-
(1989) requiring a considerable contribution from this
lated to date seem to agree that fitting to the peak
AGNpopulation. Multiple studies haverevealedthis to
flux of the CXB at ∼30keV requires a CTK AGN con-
be the case, although with a dependence on X-ray lu-
tribution in the range 10–25% (Comastri et al. 1995;
minosity (e.g. Lawrence & Elvis 2010). This suggests
Gandhi & Fabian 2003; Gilli et al. 2007; Treister et al.
heavily obscured AGN may be a major contributor to
2009; Draper & Ballantyne 2010; Akylas et al. 2012;
the CXB, and there are many ongoing efforts to study
Ueda et al. 2014). The actual number density of
thispopulation(e.g. Brandt & Alexander2015,andref-
CTK AGN remains unclear, with various recent sam-
erences therein).
ple observations suggesting a fraction exceeding 20%
IntheX-rayband,thetwoimportantinteractionpro-
(Goulding et al. 2011; Lansbury et al. 2015; Ricci et al.
cessesbetweenphotonsandmattersurroundinganAGN
2015; Koss et al. 2016a). Gandhi et al. (2007) and
are photoelectric absorption and Compton scattering.
Treister et al. (2009) discuss degeneracy between the
Photoelectric absorption is dominant at lower energies,
different component parameters (e.g. reflection and ob-
whereas Compton scattering dominates in hard X-rays
scuration) used to fit the CXB. This is why the shape
above ∼10keV up to the Klein-Nishina decline. X-
of the CXB cannot be directly used to determine the
ray photons with energy greater than a few keV are
number of CTK AGN, and further explains the large
visible if the LOS obscuring column density (N ) is
H
.1.5×1024cm−2, and such AGN are named Compton- uncertainty associated with the CTK fraction.
ManyX-raymissionstodatehavebeencapableofde-
thin (CTN) since the matter is optically thin to Comp-
tectingphotonsabove10keV,suchasBeppoSAX,Swift,
ton scattering and a significant fraction of the photons
Suzaku andINTEGRAL.However,duetoissuesinclud-
with E>10keV escape after one or more scatterings.
ing high backgroundlevels, relatively small effective ar-
This leads to only slight depletion of hard X-rays for
eas and low angular resolution, few CTK sources have
CTN sources. Sources with column densities greater
been identified. The Nuclear Spectroscopic Telescope
than this value are classified as Compton-thick (CTK)
Array (NuSTAR)(Harrison et al.2013)is the firstmis-
since even high-energy X-rays can be diminished via
sioninorbitcapableoftrue X-rayimagingintheenergy
Compton scattering, leading to the X-ray spectrum be-
range ∼3–79keV. Since launch, NuSTAR has not only
ingdepressedovertheentireenergyrange. ThehardX-
studied well known CTK AGN in detail (Ar´evalo et al.
ray spectrum of typical CTK AGN is characterised by
2014; Bauer et al. 2015; Marinucci et al. 2016), it has
also helped to identify and confirm numerous CTK
candidates in the local Universe (Gandhi et al. 2014;
[email protected]
3
Balokovi´cet al. 2014; Annuar et al. 2015; Koss et al. spectrum provided by the BeppoSAX satellite, to-
2015, 2016b), as well as carry out variability stud- gether with multi-wavelength diagnostic information
ies focusing on changing-look AGN (Risaliti et al. 2013; (see Risaliti et al. 1999a and references therein for fur-
Walton et al. 2014; Rivers et al. 2015; Marinucci et al. ther details on the modelling used). Additionally,
2016; Ricci et al. 2016; Masini et al. 2016). More- the EW of the Fe-Kα emission-line was reported as
over, deep NuSTAR surveys have resolved a fraction of 3.20+0.98keV. Such high EWs are extreme though not
−1.74
35±5% of the total integrated flux of the CXB in the unheard of, as reported by Levenson et al. (2002). Op-
8–24keV band (Harrison et al. 2015). tical imagesofthe source,aswellassurroundingsource
Detailed modelling of individual highly obscured redshifts,inferIC3639tobepartofatriplemergersys-
sources is the most effective way to understand the tem (e.g. Figure 1a, upper left panel - IC 3639 is ∼1.′5
spectral components contributing to the missing frac- away from its nearest galaxy neighbour to the North-
tion of the peak CXB flux. Here we carry out the East). However, Barnes & Webster (2001) use the HI
first robust broad-band X-ray spectral analysis of the detection in this interacting group to suggest that it is
nearby Seyfert 2 and candidate CTK AGN IC 3639 free of any significant galaxy interaction or merger.
(also called Tololo 1238-364). The source is hosted Miyazawa et al. (2009) analysed the source as part
by a barred spiral galaxy (Hubble classification SBbc1) of a sample of 36 AGN observed by Suzaku, includ-
with redshift z=0.011 and corresponding luminosity ing the higher energy HXD PIN data. The source
distance D=53.6Mpc. This is calculated for a flat cos- was found to have an obscuring column density of
mology with H =67.3 kms−1Mpc−1, Ω =0.685 and 7.47+4.81 ×1023cm−2 withphotonindex1.76+0.52,sug-
0 Λ −3.14 −0.44
Ω =0.315(Planck Collaboration2014). Alluncertain- gesting a CTN nature. This could indicate variability
M
ties are quoted at a 90% confidence level for one inter- betweenthe2007Suzaku observationandthe1999Bep-
esting parameter, unless stated otherwise. This paper poSAX observation reported by Risaliti et al. (1999b).
uses NuSTAR and archival X-ray data from the Suzaku AsoutlinedinSection1,CTKsourcesarenotoriously
andChandra satellites. TheSuzaku satelliteoperatedin hard to detect due to their low count rate in the soft
the energy range ∼0.1–600keV and is thus capable of X-ray band. For this reason, one must use particular
detecting hard X-rays. However,the hard X-ray energy spectralcharacteristics,indicativeofCTKsources. The
rangeofthissatelliteiscoveredbyanon-imagingdetec- first,mostobviousindicationisaprominentFe-Kαfluo-
tor, leading to potential complications for faint sources, rescence line. This can occur if the fluorescing material
as outlined in Section 3.2.2. Chandra has a high en- is exposed to a greater X-ray flux than is directly ob-
ergy limit of ∼8keV, and very high angular resolution served, so that the line appears strong relative to the
with a lower energy limit &0.1keV. Consequently, the continuum emission (Krolik & Kallman 1987).
different capabilities of NuSTAR, Suzaku and Chandra Other CTK diagnostics are provided through multi-
complementeachothersothatamulti-instrumentstudy wavelength analysis. For example, by comparing the
provides a broad-band spectral energy range. MIR and X-ray luminosities of the source, which have
The paper is structured accordingly: Section 2 ex- been shown to correlate for AGN (Elvis et al. 1978;
plainsthetargetselection,withSection3describingthe Krabbe et al. 2001; Horst et al. 2008; Gandhi et al.
detailsbehindeachX-rayobservationofthesourceused 2009; Levenson et al. 2009; Mateos et al. 2015; Stern
as well as the spectral extraction processes. The corre- 2015;Asmus et al.2015). X-rayobscurationisexpected
sponding X-ray spectral fitting and results are outlined to significantly offset CTK sources from this relation.
in Sections 4 and 5, respectively. Finally, Section6 out- Indeed, IC 3639 shows an observed weak X-ray (2–
lines broadband spectral components determined from 10keV)luminositycomparedtothepredicted valuefrom
the fits, the intrinsic luminosity of the source and a this correlation. Another multi-wavelength technique
multi-wavelength comparison with other CTK sources. compares emission lines originating in the narrow line
We conclude with a summary of our findings in Section region (NLR) on larger scales than the X-ray emis-
7. sion, which arises close to the core of the AGN. Of
the multitude of emission lines available for such anal-
2. THE TARGET
ysis, one well studied correlation uses the optical [OIII]
The first published X-ray data of IC 3639 were re- emission-line flux at λ=5007˚A. Panessa et al. (2006)
portedbyRisaliti et al.(1999b),wheretheysuggestthe and Berney et al. (2015), among others, study a cor-
sourcetobeCTKwithcolumndensityN >1025cm−2. relation between the observed [OIII] emission-line lu-
H
This lower limit was determined from a soft X-ray minosity and X-ray (2–10keV) luminosity for a group
of Seyfert galaxies, after correcting for obscuration. IC
3639 again shows a weak X-ray flux compared to the
1 http://leda.univ-lyon1.fr observed [OIII] luminosity. This indicates heavy obscu-
4
rationdepletingtheX-rayluminosity. Foracomparison from the corresponding event files. Background counts
betweentheratiosofMIRand[OIII]emissionfluxtoX- were extracted from annular regions of outer radius 2.′5
rayflux with the averagevalue for (largely unobscured) and inner radius 0.′75, centred on the source regions to
Seyfert 1s, see LaMassa et al. (2010, Figure 2). avoidanycross-contaminationbetweensourceandback-
Dadina (2007) reports that the BeppoSAX obser- groundcounts. Thebackgroundregionwaschosentobe
vation of IC 3639 in both the 20–100keV and 20– as large as possible in the same module as the source.
50keV bands had negligible detection significance, and The extracted spectra for FPMA and FPMB were
place an upper bound on the 20–100keV flux of then analysed using the xspec version 12.9.0 software
F 69.12×10−12ergs−1cm−2. Unfortunately, package3. The energy range was constrained to the op-
20–100keV
IC 3639 lies below the Swift/BAT all-sky survey limit timum energy range of NuSTAR and grouped so that
of ∼1.3×10−11ergs−1cm−2 in the 14–195keV band each bin contained a signal-to-noise ratio (SNR) of at
(Baumgartner et al. 2013). least 4. The resulting spectra are shown in count-rate
units in Figure 2.
3. OBSERVATIONS & DATA REDUCTION Figure 1a shows the comparison of the NuSTAR
Archival observations for IC 3639 used in this paper FPMA image with an optical Digitised Sky Survey
were all extracted from the HEASARC archive2. To- (DSS) image. The blue regions highlight the counter-
partsofthemergingtriple,clearlyvisibleintheoptical.
gether, we use Suzaku (XIS & HXD), Chandra and re-
cent NuSTAR data in this study. Table 1 shows the However, there is no detection of the separate galaxies
details of each of these observations. in the NuSTAR image, with the primary emission orig-
inating from IC 3639.
Satellite Obs. ID Date /y-m-d Exp./ks PI 3.2. Suzaku
NuSTAR 60001164002 2015-01-09 58.7 P. Gandhi When fully operational, Suzaku had four CCD X-ray
Suzaku 702011010 2007-07-12 53.4 H.Awaki imaging spectrometers(XIS) anda hardX-raydetector
Chandra 4844 2004-03-07 9.6 R.Pogge (HXD).TheXIScoveredanenergyrangeof0.4–10keV
with typical resolution 120eV.4 During the lifetime of
Col. 1: Satellite name; Col. 2: Corresponding observation ID;
Col. 3: Date of observation; Col. 4: Unfiltered exposure time Suzaku, one of the four XIS detectors became non-
for the observation; Col. 5: Principal Investigator (PI) of the operational,leavingtwofrontilluminated(FI)detectors
observation.
(XIS0 and XIS3), and one back illuminated (BI) detec-
tor (XIS1). HXD was a non-imaging instrument de-
Table 1. Details of observations used to analyse IC 3639.
signedforobservationsintheenergyrange10–700keV.
3.2.1. XIS
First, the ximage software package5 was used to cre-
3.1. NuSTAR
ate an image by summing over the three XIS cleaned
Data from both focal plane modules (FPMA &
event files. Next source counts were extracted from a
FPMB) onboard the NuSTAR satellite were pro- circularregionofradius2.′6,withbackgroundcountsex-
cessed using the NuSTAR Data Analysis Software tracted from an annular region of inner radius 2.′6 and
(NuSTARDAS) within the heasoft package. The outer radius 5.′0. The background annular region was
corresponding caldb files were used with the NuS-
again centred around the source region to avoid source
TARDAS task nupipeline to produce calibrated and
and background count contamination. xselect was
cleaned event files. The spectra and response files were
then used to extract a spectrum for each XIS detector
produced using the nuproducts task, after standard
cleaned event file using the source and background re-
datascreeningprocedures. Thenetcountrateinthe3–
gions defined above. Lastly, we used the addascaspec
79keVbandforFPMA&FPMBwere(9.413±0.549)×
commandto combine the twoFI XISspectra. The final
10−3countss−1 and (8.018 ± 0.544)× 10−3countss−1,
result was two spectra: one for the FI cameras (XIS0
fornetexposuresof58.7ksand58.6ks,respectively(this
+ XIS3, referred to as XIS03 herein) and one for the
corresponds to total count rates of (1.686 ± 0.054)×
single BI camera (XIS1). The net exposure times for
10−2countss−1 and (1.648 ± 0.053)× 10−2countss−1
XIS03 and XIS1 were 107.8ks and 53.4ks, respectively.
for FPMA & FPMB, respectively). Circular source re-
gions of radius 0.′75 were used to extract source counts
3https://heasarc.gsfc.nasa.gov/xanadu/xspec/XspecManual.pdf
4 isas.jaxa.jp/e/enterp/missions/suzaku/index.shtml
2 https://heasarc.gsfc.nasa.gov/cgi-
bin/W3Browse/w3browse.pl 5 https://heasarc.gsfc.nasa.gov/xanadu/ximage/ximage.html
5
Figure 1a. (Left)DigitisedSkySurvey (DSS)opticalimageoftheinteractingtriplegalaxygroupsystem,withIC3639shownin
thecenter. TheNuSTARextractionregionsforsourceandbackgroundareshowninredwithsolidanddashedlinesrespectively.
The other two galaxies present in the triple system are highlighted with blue circles, each of radius 0.′76. (Right) NuSTAR
FPMA event file with source and background regions again shown in red solid and dashed lines respectively. The locations of
theothertwogalaxiesintheinteractingtriplesystem(visibleinopticalimages)areshownbythebluecircles. Clearly,NuSTAR
does not significantly detect these other galaxies in theX-rayenergy range.
Figure 1b. (Left) Hubble Space Telescope (HST) WFPC2 image of IC 3639, with superimposed regions defined from the
Chandra image. Theimageconfirmsthebarredspiralclassification describedinSection1,orientatedalmostcompletelyface-on
to our LOS. (Right) Full band Chandra image with 0.′5 radius extraction regions for background and annular count extraction
region for off-nuclear emission superimposed. The annular extraction region has an inner radius of 0.′05. Scale was determined
assuming a distance tothe source of 53.6Mpc.
6
1
0
0.
1
−
V
ke 0−3
1 1
−
s
s
nt
u
o
c
4
−
10 NuSTAR FPMA
NuSTAR FPMB
Suzaku XIS1
−5 Suzaku XIS03
0
1
1 2 5 10 20
Energy (keV)
Figure 2. The NuSTAR FPMA and FPMB spectra plotted together with the Suzaku XIS spectra in count rate units. All are
binned with SNR of 4. The data has been normalised by the area scaling factor present in each response file - this was unity
for each observation. XIS1 refers to the single BI detector on Suzaku used to collect the spectral counts, whereas XIS03 refers
to thecombined spectra from the two FI detectors, XIS0and XIS1- see Section 3.2.1 for furtherdetails.
7
ThedatawereagaingroupedwithaminimumSNRof4.
Additionally the XIS spectral data in the energy range
1.7–1.9keV and 2.1–2.3keV were ignored due to in-
strumentalcalibrationuncertaintiesassociatedwiththe 0.01
silicon and gold edges in the spectra6.
V−1
The correspondings3p.e2c.2tr.umHXfDor the HXD instrument counts s ke−1 10−3
was generated with the ftools command hxdpinxbpi.
The data were then binned to allow a minimum of
500 counts per bin. The energy range 10–700keV 10−4
of HXD is achieved with Gadolinium Silicate (GSO)
counters for >50keV and PIN diodes for the range
15 20 25 3"0 35 40
15–50keV. The GSO instrument is significantly less Energy (keV)
sensitive than NuSTAR and thus not used here. For Figure 3. The Suzaku HXD spectra plotted in count
rate units for gross counts (source+background), 5% gross
the PIN instrument, a model has been designed to
counts(tocomparetheapproximatethresholdweusetoclas-
simulate the non-X-ray background (NXB). In the
sify a weak detection - see text for details) and net source
15–40keV range, current systematic uncertainties in countsshowninblack,greenandred,respectively. TheHXD
the modelled NXB are estimated to be ∼3.2%. A 15–40keVspectral countsare shown not tobesignificantly
detected bySuzaku, as discussed in Section 3.2.2.
tuned NXB file for the particle background is provided
by the Suzaku team, whereas the CXB is evaluated
separatelyandaddedtothetunedbackgroundresulting trum is less than ∼5% of the tuned background, the
in a final total background. The modelled CXB is detection is weak, and a source spectrum flux lower
∼5% of the total background for PIN. The ftool than ∼3% of the background would require careful as-
command hxdpinxbpi then uses the total background sessment. The IC 3639 source counts are found to be
to produce a dead-time corrected PIN source and 0.8−+00..98% of the tuned background counts in the 15–
background (NXB+CXB) spectrum. The net source 40keV range. For this reason, we do not use the HXD
counts for IC 3639 are shown in red in Figure 3. The datainourspectralanalysisforIC3639. Thisvaluecon-
gross counts (source+background) are considerably tradictsMiyazawa et al.(2009)whoreporta15–50keV
higher than the net source counts and are shown in fluxofF15−50keV = 1.0×10−11ergs−1cm−2 –approx-
black on the same figure for comparison. The source imatelytwoordersofmagnitudehigherthanwefind, as
flux was calculated in the energy range 10–40keV for well as greater than the upper limit attained from the
a simple power-law model. The corresponding fluxes BeppoSAX satellite mentioned in Section 2.
for source+background (F ) and source alone We further find this result to be inconsistent
B,15-40keV
(F ) are: with NuSTAR. Using a simple powerlaw+gaussian
S,15-40keV
model fitted to the NuSTAR data, we obtain
FB,15-40keV =1.41 ± 0.01 × 10−10 ergs−1cm−2, and F F15P−M5A0keV = 3.0−+00..59 × 10−12ergs−1cm−2 and
FS,15-40keV =8.20−+80..2407 × 10−13 ergs−1cm−2. F F15P−M5B0keV = 3.1−+01..40 × 10−12ergs−1cm−2. Extrap-
olating these fluxes to the 20–100keV band gives
The CXB is known to vary between different instru- F 20−100keV ∼ 1.8 × 10−12ergs−1cm−2 for both Fo-
ments on the order of ∼10% in the energy range con- cal Plane Modules, which is fully consistent with
sidered here. For this reason, as a consistency check, the upper limit found with the BeppoSAX satellite
we compared the background uncertainty from Suzaku (F2B0e–p1p0o0SkAeXV 69.12×10−12ergs−1cm−2).
(3− 5%)7 to the error in the total background found Figure 2 shows the Suzaku XIS spectra over-plotted
when the CXB flux component carried a 10% uncer- with the NuSTAR data. The spectral data for Suzaku
tainty. This altered the tuned background error to XIS and NuSTAR are consistent with each other in
2.9%–4.8%. Thus, within acceptable precision, the to- shape within the common energy range 3–10keV. The
tal background appears to be unaltered by potential composite spectrum formed spans approximately two
CXB cross-instrument fluctuations. If the source spec- dex in energy,from 0.7–34keV, as a result of the mini-
mum SNR grouping procedures for each data set.
3.3. Chandra
6http://heasarc.gsfc.nasa.gov/docs/suzaku/analysis/abc/node8.html
7http://heasarc.gsfc.nasa.gov/docs/suzaku/analysis/abc/node10.html, The Chandra level 2 event file was obtained from the
7.5.1 heasarc database. A fraction of the total collecting
8
area of the detector was used in the timed exposure the flux of the CPL in the Chandra energy band (0.5–
mode setting, where the CCD collects data for a set 8keV) was F CPL = 7.17 × 10−14ergs−1cm−2.
0.5−8.0keV
frame time. Selecting a frame time less than the de-
faultvalue(3.2sforChandra) reducestheprobabilityof 4. X-RAY SPECTRAL FITTING
pile-up. Thisiswheremorethanonephotoninapartic- The resulting spectral data sets for Suzaku XIS and
ular bin from the same event is detected. An exposure NuSTAR were used in the energy ranges 0.7–9.0keV
timeof0.4sperframewasusedintheobservationofIC and3.0–34.0keV,respectively. Data abovethis thresh-
3639,whichgivesareducedpredictedpile-upfractionof old were excluded due to low SNR. Initially, the Suzaku
∼0.3%. ThissettingwaschosenintheoriginalChandra XIS and NuSTAR data were fitted independently of
observationproposaldue to the previously unknown X- eachotherwithasimplepowerlaw+gaussianmodel
ray flux of the source, to minimise the risk associated togivethefollowingfluxesinthe2–10keVenergyband:
with pile-up.
Spectral extraction from the Chandra data was car- NuSTAR:
ried out with the ciao 4.7 software package8. We pri- F FPMA =1.81+0.16 × 10−13ergs−1cm−2,
2−10keV −0.41
marily investigated the Chandra image of IC 3639 for F FPMB =1.86+0.17 × 10−13ergs−1cm−2.
2−10keV −0.40
potential contaminants located within the Suzaku and
NuSTAR extraction regions. However, no particularly Suzaku XIS:
prominentcontaminatingsourceswerevisibleintheim- F XIS03 =1.84 ± 0.24 × 10−13ergs−1cm−2,
2−10keV
mediatevicinityoftheAGN.Acomparativelylargecir- F XIS1 =2.12+0.34 × 10−13ergs−1cm−2.
2−10keV −0.32
cular source region of 2.′6 was used with the XIS image
due to its larger point spread function (PSF) relative Theoverallmatchbetweenthesefluxesimplieswecan
to Chandra. As a result, the XIS spectra will contain analysethedatasetstogether. AChandra spectrumwas
some flux from non-AGN related activity, unresolvable extracted from a 3′.′9 circular extraction region. How-
by that instrument. To account for this, the Chandra ever,there wereonly 35countspresentinthe 2–10keV
imagewasusedtomodelasmuchoftheunresolvednon- band. Byusingthesamepowerlaw+gaussianmodel
AGN activity from the Suzaku XIS image as possible. todeterminethefluxaswiththeotherdatasets,weget:
An annular Chandra source region with outer radius as F Chandra =9.51+7.83 × 10−14ergs−1cm−2.
2−10keV −9.51
closetothatofthecircularXISsourceregionaspossible This fluxisonlymildly inconsistentwiththe otherdata
was created with inner region of radius 0.′05, excluding sets,butalsoconsistentwithzero,duetothelowSNRof
the central AGN. A simple power law was fitted to this the data. Giventhis low SNR, we do not use the Chan-
spectrum, and was added to the model used with the dra AGNdataforfurtherspectralanalysis,butthecon-
Suzaku XIS data. This power law is referred to as the sistency found between this and Suzaku/NuSTAR data
contamination power law (CPL) hereafter. sets suggests that we are classifying IC 3639 as a bona
Ideally, the outer radius of the annular extraction ra- fide CTK AGN robustly.
dius used in the Chandra image would equal the ra- The power-law slope of the composite spectrum
diusofthecircularsourceregionusedinthe XISimage. (Suzaku+NuSTAR)ishard,withphotonindexΓ∼1.8
However,asnotedabove,dataweretakenwithacustom and the EW of the Fe-Kα line is very large (EW∼
1/8 sub-array on ACIS-S3. This meant that Chandra 2.4keV). These are consistent characteristics of a heav-
only observed part of the sky covered by the other in- ily obscured AGN. Due to the high EWs found for the
struments. As such,the image producedcouldnothave Fe-Kα line witha power-law+Gaussianmodel, we pro-
a source regionwider than ∼0.′5, as opposed to the XIS ceeded to fit more physically motivated models as fol-
source region of radius 2.′6. Accordingly, we used an lows.
annular region of outer radius 0.′5 for the Chandra im- A general model structure was used with each spec-
age. The annular counts extraction region and circular trum, included in Equation 1. However, not all models
background region of equal radius used with the Chan- used in this study required all the components listed in
dra image are shown in the right panel of Figure 1b. the template.
Thespecextractcommandwasusedtocreatethespec-
Template = const × phabs[gal] × [apec + cpl + spl +
tral and response files for use in xspec. The Chandra
spectrum was grouped with greater than or equal to 20 (refl + obsc × ipl + f lines)] (1)
countsperbinpriortothecplmodelling. Furthermore,
Below, we give explicit details for each component in
Equation 1:
8 http://cxc.harvard.edu/ciao/ • const: multiplying constant used to determine
the cross-calibration between different instru-
9
ments. The NuSTAR FPMA constant was frozen absorptionthroughtheobscurerviathemul-
to unity and the other three constants left free tiplying obsc term.
(Madsen et al. 2015 report cross-normalisation
– f lines: component describing fluorescence
constants within 10% of FPMA).
lines believed to arise from photon interac-
• phabs[gal]: component used to account for tions with the circumnuclear obscurer.
photoelectric absorption through the Milky Way,
based on H measurements along the LOS 4.1. Slab models: pexrav and pexmon
I
(Dickey & Lockman 1990). This is represented Slab models describe X-ray reflection off an infinitely
as an obscuring column density in units of cm−2, thick and long flat slab, from a central illuminating
and assumed constant between instruments (and source. pexrav (Magdziarz & Zdziarski 1995) com-
so was frozen for each data set). The determined prises an exponentially cut-off power law illuminating
value was 5.86×1020cm−2. spectrum reflected from neutral material. To acquire
• apec (Smith et al. 2001): model component used the reflection component alone, with no direct trans-
mitted component, the reflection scaling factor param-
as a simple parameterisation of the softer energy
eter is set to a value R<0. Other parameters of inter-
X-rayemissionassociatedwithathermallyexcited
est include the power law photon index; cutoff energy;
diffusegassurroundingtheAGN.Detailedstudies
abundance of elements heavier than helium; iron abun-
of brighter local AGN indicate that photoionisa-
dance (relative to the previous abundance) and inclina-
tionmayprovideabetterdescriptionofthesoftX-
tion angle of the slab (90◦ describes an edge-on config-
rayemissioninAGNspectra(e.g. Guainazzi et al.
uration; 0◦ describes face-on). The model gave far bet-
2009; Bianchi et al. 2010), but such modelling
terreducedchi-squaredvaluesforareflection-dominated
would require a higher SNR and better spectral
configuration, and as such the reflection scaling factor
resolution than currently available for IC 3639.
was frozen to −1.0, corresponding to a 50% covering
The low-energy spectral shape for IC 3639 found
factor. pexrav does not self-consistently include fluo-
with Suzaku XIS is far softer than for the higher
rescent line emission. As such, a basic Gaussian com-
energy portion of the spectrum.
ponent was initially added to account for the strong
• cpl: component referring to the contamination Fe-Kα line, resulting in an EW∼ 1.4−3.0keV. Alter-
power law, used to account for the unresolved natively, the pexmon model (Nandra et al. 2007) com-
non-AGNemissioncontaminating theSuzaku XIS bines pexrav with approximatedfluorescencelines and
spectralcounts. SeeSection3.3forfurtherdetails. anFe-KαComptonshoulder(Yaqoob & Murphy2011).
The fluorescencelinesinclude Fe-Kα,Fe-Kβ andnickel-
• spl: component referring to the scattered power
Kα. All analysis with slab models refer to the pex-
law. This accounts for intrinsic AGN emission
mon model hereafter,andaredenoted asmodel P. The
that has been scattered into our LOS from re-
high EWs found for the Fe-Kα line with the reflection-
gions closer to the AGN, such as the NLR. The
dominatedpexrav+Gaussianmodelinadditiontothe
power law photon index and normalisation were
power-law+Gaussian model, we next considered more
tied to the intrinsic AGN emission as a simplifi-
physicallymotivatedself-consistentobscuredAGNmod-
cation. However, a constant multiplying the spl
els.
componentwasleftfreetoallowavariablefraction
of observed flux arising from scattered emission.
4.2. bntorus
• The final term in Equation 1 collectively consists Two tabular models are provided by
of three parts: Brightman & Nandra (2011) to describe the ob-
scurer self-consistently including the intrinsic emission
– refl: reflected component, arising from the
and reflected line components. The spherical version
primary nuclear obscurerand has been mod-
describes a covering fraction of one in a geometry
elled in varying ways. In this work, we use
completely enclosing the source. The presence and
slab models (pexrav and pexmon) as well
morphology of NLRs in a multitude of sources favours
astoroidalgeometrymodels(torusandmy-
a coveringfactor <1, implying an anisotropicgeometry
torus),describedinSections4.1,4.2and4.3
for the shape of the obscurer in most Seyfert galaxies.
respectively.
For this reason, preliminary results were developed
– obsc × ipl: Most models include the direct with this model, before analysis was carried out with
transmitted component (intrinsic power law toroidal models. For further discussion of the NLR of
or ipl), after accounting for depletion due to IC 3639, see Section 6.1.
10
The second bntorus model (model T hereafter) was toroidal geometries with high SNR data, refer to the
used extensively in this study. This models a toroidal publicly available mytorus examples,9 or see Yaqoob
obscurer surrounding the source, with varying opening (2012).
and inclination angles. Here, the opening angle de-
scribes the conical segment extending from both poles
model M = const × phabs × (apec + spl + cpl +
of the source (i.e. the half-opening angle). Because the
pow * etable {trans. absorption} +
obscurerisasphericalsectioninthismodel,thecolumn
density along different inclination angles does not vary. atable{scattered} +
The rangeof opening angles studied is restrictedby the atable{fluor lines}) (3)
inclinationanglesinceforinclinationangleslessthanthe
5. RESULTS FROM SPECTRAL FITTING
openingangle,the sourcebecomesunobscured. Thusto
allowexplorationofthe full rangeofopening angles,we In this section, we present the results of our X-ray
fixed the inclination angle to the upper limit allowed spectral fitting of IC 3639 together with model-specific
by the model: 87◦ (Brightman et al. 2015). The tables parameters shown in Table 2. Figures 4a and 4b show
providedforbntorusarevalidintheenergyrange0.1– the spectra and best-fit models attained for models T
320keV,uptoobscuringcolumndensitiesof1026cm−2. and M, respectively. First we consider the EW of the
Equation2describestheformofmodelTusedinxspec Fe-Kα line. As previously mentioned, an EW of the
- allproperties associatedwith the absorberare present order of 1keV can be indicative of strong reflection.
in the torus term, and collectively used in the mod- Risaliti et al. (1999b) found the EW for IC 3639 to be
elling process. 3.20+0.98keV. In order to determine an EW for the Fe-
−1.74
Kα line here, we modelled a restricted energy range
of ∼3–9keV with a simple (powerlaw+gaussian)
model T = const × phabs × (apec + spl +
model. Here the power law was used to represent the
cpl + torus) (2)
underlying continuum, and the Gaussian was used as
Liu & Li (2015) report model T to over-predict the a simple approximation to the Fe-Kα fluorescence line.
reflection component for edge-on geometries, resulting Additionally, all four data sets were pre-multiplied by
in uncertainties. However,varying the inclination angle cross-calibrationconstantsinthesamewayasdescribed
did not drastically alter our fits and consistent results in the template model.
wereacquiredbetweenbothmodels TandM,described Due to low signal-to-noise, the continuum normali-
next. sation had a large uncertainty. As such, a robust er-
ror could not be directly determined on the EW us-
4.3. mytorus
ing xspec. Alternatively, we carried out a four dimen-
The mytorus model, developed by sional grid to step over all parameters of the model in
Murphy & Yaqoob (2009), describes a toroidal-shaped xspec, excluding line energy, which was well defined
obscurer with a fixed half-opening angle of 60◦, and and frozen at 6.36keV in the observed frame. The EW
free inclination angle. However, because the geometry was calculated for each grid value, and the correspond-
here is a doughnut as opposed to a sphere, the LOS ing confidence plot is shown in Figure 5a. The hori-
column density will always be less than or equal to the zontal black line represents the 90% confidence region
equatorial column density (with equality representing for the chi-squared difference from the best-fit value,
an edge-on orientation with respect to the observer). ∆χ2. Here, the 90% confidence level refers to the chi-
The full computational form of this model is shown squareddistribution for four free parameterswith value
in Equation 3, and encompasses the energy range 0.5– ∆χ2=7.779. Figure 5b shows the model used, fitted to
500keV, for column densities up to 1025cm−2. Three the four data sets. This gave an EW of 2.94+−21..7390keV,
separate tables are used to describe this model: the consistentwithRisaliti et al.(1999b). Thisiswellabove
transmitted absorption or zeroth order continuum, al- the approximate threshold of 1keV typically associ-
teredbyphotoelectricabsorptionandComptonscatter- ated with the presence of CTK obscuration. However,
ing; the scattered component, describing Compton scat- Gohil & Ballantyne (2015) find the presence of dust in
tering off the torus; and the fluorescence emission for the obscuring medium can enhance the Fe-Kα line de-
neutralFe-KαandFe-Kβ togetherwiththeirassociated tection even for CTN gas. This is a further reason for
Compton shoulders. This study uses the coupled mode theimportanceofconsistentmodellingtodeterminethe
forthismodel,wheremodelparametersaretiedbetween column density more robustly. Additionally, the er-
differenttablecomponents(referredtoasmodelMhere-
after). Forfurtherdetailsonthedecoupledmode,which
is often used for sources showing variability or non- 9 http://mytorus.com/mytorus-examples.html