Anatomy of the AGN in NGC 5548: I. A global model for the broadband spectral energy distribution

19 Pages·2015·0.97 MB·English

Astronomy&Astrophysicsmanuscriptno.NGC5548-PaperI (cid:13)c ESO2015
January7,2015
Anatomy of the AGN in NGC 5548
I. A global model for the broadband spectral energy distribution
M.Mehdipour1,2,J.S.Kaastra1,3,4,G.A.Kriss5,6,M.Cappi7,P.-O.Petrucci8,9,K.C.Steenbrugge10,11,N.Arav12,E.
Behar13,S.Bianchi14,R.Boissay15,G.Branduardi-Raymont2,E.Costantini1,J.Ebrero16,1,L.DiGesu1,F.A.
Harrison17,S.Kaspi13,B.DeMarco18,G.Matt14,S.Paltani15,B.M.Peterson19,20,G.Ponti18,F.PozoNun˜ez21,A.De
Rosa22,F.Ursini8,9,C.P.deVries1,D.J.Walton23,17,andM.Whewell2
5 1 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,theNetherlands
1 e-mail:[email protected]
0 2 MullardSpaceScienceLaboratory,UniversityCollegeLondon,HolmburySt.Mary,Dorking,Surrey,RH56NT,UK
2 3 DepartmentofPhysicsandAstronomy,UniversiteitUtrecht,P.O.Box80000,3508TAUtrecht,theNetherlands
4 LeidenObservatory,LeidenUniversity,POBox9513,2300RALeiden,theNetherlands
n
5 SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore,MD21218,USA
a
6 DepartmentofPhysicsandAstronomy,TheJohnsHopkinsUniversity,Baltimore,MD21218,USA
J
7 INAF-IASFBologna,ViaGobetti101,I-40129Bologna,Italy
6 8 Univ.GrenobleAlpes,IPAG,F-38000Grenoble,France
9 CNRS,IPAG,F-38000Grenoble,France
] 10 InstitutodeAstronom´ıa,UniversidadCato´licadelNorte,AvenidaAngamos0610,Casilla1280,Antofagasta,Chile
E
11 DepartmentofPhysics,UniversityofOxford,KebleRoad,Oxford,OX13RH,UK
H 12 DepartmentofPhysics,VirginiaTech,Blacksburg,VA24061,USA
. 13 DepartmentofPhysics,Technion-IsraelInstituteofTechnology,32000Haifa,Israel
h
14 DipartimentodiMatematicaeFisica,Universita`degliStudiRomaTre,viadellaVascaNavale84,00146Roma,Italy
p
15 DepartmentofAstronomy,UniversityofGeneva,16Ch.d’Ecogia,1290Versoix,Switzerland
-
o 16 EuropeanSpaceAstronomyCentre,P.O.Box78,E-28691VillanuevadelaCan˜ada,Madrid,Spain
r 17 CahillCenterforAstronomyandAstrophysics,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA
t 18 Max-Planck-Institutfu¨rextraterrestrischePhysik,Giessenbachstrasse,D-85748Garching,Germany
s
a 19 DepartmentofAstronomy,TheOhioStateUniversity,140W18thAvenue,Columbus,OH43210,USA
[ 20 CenterforCosmology&AstroParticlePhysics,TheOhioStateUniversity,191WestWoodruffAve.,Columbus,OH43210,USA
21 AstronomischesInstitut,Ruhr-Universita¨tBochum,Universita¨tsstraße150,44801,Bochum,Germany

1 22 INAF/IAPS-ViaFossodelCavaliere100,I-00133Roma,Italy
v 23 JetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDrive,Pasadena,CA91109,USA
8
8 Received20November2014/Accepted18December2014
1
1 ABSTRACT
0
. Anextensivemulti-satellitecampaign on NGC5548 hasrevealed thisarchetypal Seyfert-1galaxytobeinanexceptional stateof
1
persistentheavyabsorption.Ourobservationstakenin2013–2014withXMM-Newton,Swift,NuSTAR,INTEGRAL,Chandra,HST
0
andtwoground-basedobservatorieshavetogetherenabledustoestablishthatthisunexpectedphenomenoniscausedbyanoutflow-
5
ingstreamofweaklyionisedgas(calledtheobscurer),extendingfromthevicinityoftheaccretiondisktothebroad-lineregion.In
1
thisworkwepresent thedetailsofour campaign andthedataobtainedbyalltheobservatories. Wedeterminethespectralenergy
:
v distributionofNGC5548fromnear-infraredtohardX-raysbyestablishingthecontributionofvariousemissionandabsorptionpro-
i cessestakingplacealongourlineofsighttowardsthecentralengine.Wethusuncovertheintrinsicemissionandproduceabroadband
X continuummodelforbothobscured(averagesummer2013data)andunobscured(<2011)epochsofNGC5548.Ourresultssuggest
r thattheintrinsicNIR/optical/UVcontinuumisasingleComptonisedcomponentwithitshigherenergytailcreatingthe‘softX-ray
a excess’. Thiscomponent iscompatible with emissionfrom awarm, optically-thick corona as part of theinner accretion disk. We
theninvestigatetheeffectsofthecontinuumontheionisationbalanceandthermalstabilityofphotoionisedgasforunobscuredand
obscuredepochs.
Keywords.X-rays:galaxies–galaxies:active–galaxies:Seyfert–galaxies:individual:NGC5548–techniques:spectroscopic
1. Introduction the feedback mechanisms between SMBHs and their environ-
ments is important in AGN science, as well as in cosmology.
The supermassive black holes (SMBHs) at the heart of active They can have significant impacts and implications for the
galactic nuclei (AGN) grow through accretion of matter from evolution of SMBHs and star formation in their host galaxies
their host galaxies. This accretion is accompanied by outflows (e.g. Silk & Rees 1998; King 2010), chemical enrichment of
(powerful relativistic jets of plasma and/or winds of ionised their surrounding intergalactic medium (e.g. Oppenheimer &
gas),whichtransportmatterandenergyawayfromthenucleus, Dave´ 2006),andthecoolingflowsatthecoreofgalaxyclusters
thus linking the SMBHs to their host galaxies. Understanding
1
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
(e.g.Ciotti&Ostriker2001). optical/UV reverberation mapping studies of NGC 5548 (e.g.
Peterson et al. 2002; Pancoast et al. 2014) have provided one
The ionised outflows are an essential component of the of the most detailed pictures available of the broad-line region
energy balance of AGN and are best studied through high- (BLR) size and structure in this AGN. Prior to our recent
resolution X-ray and UV spectroscopy of their absorption line campaignonNGC5548,theX-raypropertiesandvariabilityof
spectra. The Reflection Grating Spectrometer (RGS) of XMM- NGC5548wereknowntobetypicalofstandardSeyfert-1AGN
Newton and the Low-Energy & High-Energy Transmission sharingcommoncharacteristics(e.g.Bianchiet al. 2009;Ponti
Grating Spectrometers (LETGS and HETGS) of Chandra, etal.2012).
together with the Cosmic Origins Spectrograph (COS) of the
Hubble Space Telescope (HST) have vastly advanced our
In2013–2014wecarriedoutanambitiousmulti-wavelength
knowledgeoftheseoutflowsinrecentyears(seee.g.thereview
campaignonNGC5548,whichwassimilarbutmoreextensive
by Costantini 2010). With current instrumentation, the nearby
than our campaign on Mrk 509. This remarkable campaign
and bright Seyfert type AGN are the best laboratories for
has utilised eight observatories to take simultaneous and
studying these outflows. The majority of the absorbing gas is
frequent observations of the AGN. It incorporates instruments
detectable in the X-ray band. The outflows detected in the soft
onboard five X-ray observatories: XMM-Newton (Jansen et al.
X-rays (referred to as ‘warm absorbers’) are found to have
2001), Swift (Gehrels et al. 2004), NuSTAR (Harrison et al.
column densities of 1020−1024 cm−2 and consist of multiple
2013), INTEGRAL (Winkler et al. 2003), Chandra’s LETGS
phases of photoionised gas with temperatures of 104−106 K,
(Brinkman et al. 2000), as well as the HST COS (Green et al.
travelling at velocities of up to a few 103 km s−1 (see e.g.
2012), and two ground-based optical observatories: the Wise
Blustin et al. 2005). Understanding the origin and launching
Observatory (WO) and the Observatorio Cerro Armazones
mechanismoftheionisedoutflowsinAGN isanactiveareaof
(OCA). These observatories have collected over 2.4 Ms of
research.Ithasbeensuggestedthattheseoutflowsareproduced
X-rayand800ksofoptical/UVobservationtime.Aspreviously
byirradiationofthe dustygastorusstructure,whichsurrounds
reported by Kaastra et al. (2014), NGC 5548 was discovered
theSMBHandaccretiondisk(e.g.Krolik&Kriss2001),orthat
tobeobscuredinX-rayswithmainlynarrowemissionfeatures
they originate as radiatively driven winds from the accretion
imprinted on a heavily absorbed continuum. This obscuration
disk (e.g. Begelman et al. 1983; Proga et al. 2000), or are in
is thought to be caused by a stream of clumpy weakly-ionised
the form of magnetohydrodynamic (MHD) winds from the
gaslocated atdistances ofabout2–7lightdaysfromthe black
accretiondisk(e.g.Ko¨nigl&Kartje1994;Bottorffetal.2000).
holeandpartiallycoveringtheX-raysourceandtheBLR.From
its associatedbroadUV absorptionlinesdetectedin HST COS
Many aspects and physical properties of these outflows
spectra,theobscurerisfoundtobeoutflowingwithvelocitiesof
are still poorly understood. For instance, the location of the
upto5000kms−1.TheintenseSwiftmonitoringonNGC5548
ionisedabsorbersneedstobeestablishedtodistinguishbetween
shows the obscuration has been continuously present for a
the different outflow mechanisms and to determine their mass
few years (at least since Feb 2012). As the ionising UV/X-ray
outflow rates and kinetic luminosities, which are essential
radiationisbeingshieldedbytheobscurer,newweakly-ionised
parameters in assessing their impact on their surroundingsand
featuresofUVandX-rayabsorberoutflowshavebeendetected.
their contribution to AGN feedback (e.g. Hopkins & Elvis
Compared to normal warm absorber outflows commonly seen
2010).ThedistanceofanX-rayorUVabsorbingoutflowtothe
in Seyfert-1s at pc scale distances, the remarkable obscurer in
ionisingsourcecanbedeterminedfromestimatesofthedensity
NGC 5548 is a new breed of weakly-ionised, higher-velocity
of the absorbinggas. The density can be measured fromeither
outflowing gas, which is much closer to the black hole and
density-sensitive UV lines (e.g. Gabel et al. 2005) or from the
extends to the BLR. As reported in Kaastra et al. (2014) the
recombinationtimescale of the ionised absorber(Kaastra et al.
outflowing obscurer is likely to originate from the accretion
2012).IntheX-rayband,thelatterdeliversmorerobustdensity
disk. Based on the high outflow velocity of the obscurer, its
measurements than the use of density-sensitive X-ray lines. In
short-timescaleabsorptionvariability,anditscoveringfractions
therecombinationtimescalemethod,astheintrinsicluminosity
ofthecontinuumandtheBLR,theobscurerisincloseproximity
oftheAGNvariesovertime,theionisationstateoftheabsorber
to the central source, and its geometry extends from near the
changes with a time delay; by measuring this lag, the electron
disktooutsidetheBLR.
density and hence the distance of the absorber to the ionising
source can be obtained. Such a study was first done as part of
our 2009 multi-wavelength campaign on the Seyfert-1/QSO Inthisworkwepresentabroadbandspectralanalysisofthe
Mrk 509 (Kaastra et al. 2012). From the response of three NGC5548data.Thestructureofthepaperisasfollows.Section
absorber components with the highest ionisation in the soft 2 gives an overview of our multi-satellite campaign. In Sect.
X-rays,individualdistancesof5–100pcwerederived,pointing 3 we present lightcurves of NGC 5548 constructed at various
to an originin the narrow-lineregion(NLR) or the AGN torus energiesfromnear-infrared(NIR)tohardX-rays.InSect.4we
region. explain the required steps in determining the spectral energy
distribution(SED)ofNGC5548.InSect.5weexaminethesoft
The archetypal Seyfert-1 galaxy NGC 5548 is one of the X-ray excess in NGC 5548 and present an appropriate model
most widely studied nearby active galaxies. It was one of the for it. In Sec. 6 we describe the modelling of the broadband
12classifiedobjectsintheseminalworkofSeyfert(1943),and continuumin unobscuredandobscuredepochsandpresentour
since the sixties it has been the target of variousAGN studies. results. The thermal stability curves, corresponding to various
In more recent times it was the first object in which narrow ionising SEDs, are presented in Sect. 7. We discuss all our
X-rayabsorptionlinesfromwarmabsorberswerediscoveredby findingsinSect. 8andgiveconcludingremarksinSect.9.The
Kaastra et al. (2000) using a high-resolution Chandra LETGS processing of the data from all the instruments is described in
spectrum. Later on, a detailed study of its warm absorber was AppendixA.
carriedoutbySteenbruggeetal.(2005).Furthermore,extensive
2
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
Thespectralanalysisandmodelling,presentedinthiswork, FortheXMM-Newtonobservations,thelowestX-rayfluxes
were done using the SPEX1 package (Kaastra et al. 1996) ver- wereobservedinObs.1:F =1.40×10−12ergcm−2s−1
0.3−2keV
sion 2.05.02.We also made use of tools in NASA’s HEASOFT2 and F = 1.51 × 10−11 ergcm−2 s−1. These fluxes,
2−10keV
v6.14package.Thespectrashowninthisreportarebackground- compared to those from the unobscured 2000 and 2001 ob-
subtracted and are displayed in the observed frame, unless servations, are smaller by factors ranging from 17–27 (for
otherwise stated in the text. We use C-statistics (Cash 1979) 0.3–2 keV) and 2–3 (for 2–10 keV). Later on, in the summer
for spectral fitting (unless otherwise stated) and give errors XMM-Newton campaign, the X-ray fluxes increased from their
at 1σ (68%) confidence level. The redshift of NGC 5548 is minimum in Obs. 1 by a factor of 2.8 (0.3–2 keV) and 2.3
set to 0.017175 (de Vaucouleurs et al. 1991) as given in the (2–10 keV), peaking at XMM-Newton Obs. 4. The X-ray flux
NASA/IPAC Extragalactic Database (NED). The adopted cos- during the summer XMM-Newton campaign was low relative
mologicalparametersforluminositycomputationsin ourmod- to other obscured epochs, as seen from the long-term Swift
ellingareH =70kms−1Mpc−1,Ω =0.70andΩ =0.30. monitoring. The average fluxes of the summer 2013 XMM-
0 Λ m
Newton observations (F = 3.02 × 10−12 ergcm−2 s−1
0.3−2keV
and F = 2.77 × 10−11 ergcm−2 s−1) are smaller by a
2−10keV
2. Multi-wavelengthcampaignonNGC5548
factor of 1.6 (for 0.3–2 keV) and 1.1 (for 2–10 keV) than the
At the core of our campaign in summer 2013 (22 June to 1 average fluxes for the whole obscured epoch observed with
August), there were 12 XMM-Newton observations, of which Swift between February 2012 and July 2014 (Mehdipour et al.
five were taken simultaneously with HST COS, four with inprep).
INTEGRAL and two with NuSTAR observations. Throughout
our campaign and beyond, Swift monitored NGC 5548 on a After the end of the summer XMM-Newton observations
daily basis. There were also optical monitoringswith WO and (Obs.1–12),SwiftkeptonmonitoringNGC5548untilthemid-
OCA. The summer XMM-Newton observations were followed dle of September 2013, when it passed out of Swift’s visibility
by three Chandra LETGS observations taken in the first half window. However,prior to that a sudden increase in the X-ray
of September 2013, one of which was taken simultaneously flux was observedwith Swiftwhich started on 24 August2013
withNuSTAR.TheSeptemberobservationsweretriggeredupon (56528inMJD)andlastedaboutoneweek.Comparedtotheav-
observing a large jump in the X-ray flux from our Swift mon- eragefluxofthesummerXMM-Newtonobservations,thefluxin-
itoring.However,dueto schedulingconstraintsbythe time the creasedbyafactorof3.4(for0.3–2keV)and1.9(for2–10keV).
triggeredChandraobservationsweremade,theweek-longpeak ThisresultedinthetriggeringoftheChandraLETGSobserva-
of high X-ray flux was just missed and the tail end of the flare tions. Later on, the two final XMM-Newton observations(Obs.
was caught. Nonetheless, the X-ray flux was still higher than 13 and 14) were taken in December 2013 and February 2014
during the XMM-Newton observations and improved LETGS (theseareoutsidetherangeofFig.2lightcurves).Comparedto
spectra were obtained. During this autumn period (Sep-Nov the averageXMM-Newton flux during the summer 2013 obser-
2013),NGC5548wasnotvisibletoXMM-Newtonandthusno vations, the Obs. 13 flux was slightly lower by a factor of 1.2
XMM-Newtonobservationscouldbetriggered. (for0.3–2keV)and1.1(for2–10keV),andforObs.14theflux
washigherbyafactorof1.5for0.3–2keVandaboutthesame
A few months later, two more XMM-Newton observations for 2–10 keV. A comprehensive analysis of the XMM-Newton
were taken, one in December 2013 and the other in February lightcurves and their properties is reported in Cappi et al. (in
2014.TheformerobservationwassimultaneouswithHSTCOS prep).
and NuSTAR observations, and the latter close in time to an
INTEGRAL observation. The timeline of all the observations
inourcampaignisdisplayedinFig.1andtheobservationlogs 3.2.NIR/optical/UVlightcurves
areprovidedinTable1.InAppendixA,wedescribetheobser-
vations made by the instruments of each observatory and give In Fig. 3 we show the optical/UV lightcurves from Swift
detailsontheirdatareductionandprocessing. UVOT, XMM-Newton OM, WO and OCA, taken in the I, R,
V, B, U, UVW1, UVM2 and UVW2 filters. Figure 4 shows
the HST COS fluxes variability at six narrow UV energy
3. LightcurvesofNGC5548 bands (which are free of spectral features and represented by
their central wavelengths) between 1160 and 1793 Å for the
Here we present lightcurves of NGC 5548, from NIR to hard
summer 2013 observations. To calculate the flux in broadband
X-rays, obtained from the eight observatories used during our
photometricfilters, onerequiresconvolutionof theresponseof
summer2013campaign.
the instrument and the transmission of the filter as a function
of wavelength, with the shape of the optical/UV spectrum, to
3.1.X-raylightcurves convert from instrumental units to energy flux at an effective
wavelength. For the shape of the optical/UV spectrum, rather
Figure 2 shows the X-ray lightcurves of NGC 5548, from
than using default spectral shapes commonly incorporated in
the Swift, XMM-Newton, NuSTAR, INTEGRAL and Chandra
the data reduction softwares (such as standard stars spectra,
observations, with the average energy flux of each observation
or a power-law), we used the optical/UV spectrum model that
calculated over four X-ray energy bands from soft to hard
we derived for NGC 5548from modelling the OM and UVOT
X-rays. The X-ray fluxes have been calculated by fitting each
Optical and UV grism spectra and the HST COS continuum
datasetovertherequiredenergybandwiththemodeldescribed
data (see Sect. 4 and Fig. 5). This approach allows for a more
lateroninSect.6.
accurate flux calculation in the photometric filters, taking into
account the presence of strong AGN emission lines with a
1 http://www.sron.nl/spex realisticcontinuummodel.Forinstance,forthesixSwiftUVOT
2 http://heasarc.nasa.gov/lheasoft filters, the custom-fit spectral model results in 5 to 15% flux
3
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
Summer/autumn 2013 campaign on NGC 5548
15−Jun−13 1−Jul−13 15−Jul−13 1−Aug−13 15−Aug−13 1−Sep−13 15−Sep−13
OCA
WO
Chandra
INTEGRAL
NuSTAR
HST/COS
XMM−Newton
Swift

20 40 60 80
Days (MJD) − 56455
Winter 2013/2014 campaign on NGC 5548
15−Dec−13 1−Jan−14 15−Jan−14 1−Feb−14
WO
INTEGRAL
NuSTAR
HST/COS
XMM−Newton
Swift

190 200 210 220 230 240
Days (MJD) − 56455
Fig.1. Timeline of our multi-wavelengthcampaign on NGC 5548.The thickness of each rectangularsymbol on the time axis is
indicativeofthelengthofthatobservation.ThedaysinwhichSwift,WOandOCAmadeobservationsareindicatedbycrosses.
Table 1.ObservationlogoftheNGC5548campaign.FortheSwift, OCA andWO monitorings,theObs.numberscorrespondto
daysinwhichobservationsweretaken.ForSwift,onlyrecentobservationstakenin2013–2014arereported,includingallmonitoring
programsduringthisperiod.TheSwiftlengthsinksarethetotallengthoftheobservationsineachyear.ForHSTCOSthespanof
eachobservationinksisgiven.
Starttime(UTC) Length Starttime(UTC) Length
Observatory Obs. ID yyyy-mm-dd hh:mm (ks) Observatory Obs. ID yyyy-mm-dd hh:mm (ks)
XMM-Newton 1 0720110301 2013-06-22 03:53 50.5 HSTCOS 1 lc7001 2013-06-22 13:25 13.0
XMM-Newton 2 0720110401 2013-06-29 23:50 55.5 HSTCOS 2 lc7002 2013-07-12 02:23 14.2
XMM-Newton 3 0720110501 2013-07-07 23:28 50.9 HSTCOS 3 lc7003 2013-07-24 16:43 8.9
XMM-Newton 4 0720110601 2013-07-11 23:11 55.5 HSTCOS 4 lc7004 2013-07-30 15:15 16.0
XMM-Newton 5 0720110701 2013-07-15 22:56 55.5 HSTCOS 5 lc7005 2013-08-01 03:21 12.3
XMM-Newton 6 0720110801 2013-07-19 22:40 56.5 HSTCOS 6 lc7006 2013-12-20 02:49 13.0
XMM-Newton 7 0720110901 2013-07-21 22:32 55.5 NuSTAR 1 60002044002 2013-07-11 09:50 51.6
XMM-Newton 8 0720111001 2013-07-23 22:24 55.5 60002044003 2013-07-12 00:10 52.2
XMM-Newton 9 0720111101 2013-07-25 22:15 55.5 NuSTAR 2 60002044005 2013-07-23 14:25 97.2
XMM-Newton 10 0720111201 2013-07-27 22:06 55.5 NuSTAR 3 60002044006 2013-09-10 21:25 97.5
XMM-Newton 11 0720111301 2013-07-29 21:58 50.4 NuSTAR 4 60002044008 2013-12-20 08:30 98.1
XMM-Newton 12 0720111401 2013-07-31 21:49 55.5 Chandra 1 16369 2013-09-01 00:01 29.7
XMM-Newton 13 0720111501 2013-12-20 14:01 55.3 Chandra 2 16368 2013-09-02 10:33 67.5
XMM-Newton 14 0720111601 2014-02-04 09:33 55.5 Chandra 3 16314 2013-09-10 08:17 122.0
INTEGRAL 1 10700010001 2013-06-29 21:34 100.0 Swift(2013)a 1–160 b 2013-01-04 00:24 326.6
INTEGRAL 2 10700010002 2013-07-11 21:13 102.0 Swift(2014)c 161–291 d 2014-01-02 14:53 182.5
INTEGRAL 3 10700010003 2013-07-15 23:31 106.5 OCAe 1–27 - 2013-05-20 03:17 e
INTEGRAL 4 10700010004 2013-07-23 03:54 128.9 WO(2013)f 1–93 - 2013-06-02 07:22 f
INTEGRAL 5 11200110003 2014-02-09 10:00 38.2 WO(2014)g 94–150 - 2013-12-16 14:38 g
a TheSwiftmonitoringin2013endedon2013-12-3118:19.
b TheSwifttargetIDsofNGC5548in2013:30022,80131,91404,91711,91737,91739,91744,91964.
c TheSwiftmonitoringupto2014-07-0100:00isreportedhere.TheSwiftmonitoringofNGC5548iscurrentlyongoingin2014–2015.
d TheSwifttargetIDsofNGC5548in2014:30022,33204,91964.
e TheOCAmonitoringendedon2013-07-2501:13.ObservationstakenintheB,V,Rfilters,with150sexposureineachfilter.
f TheWOmonitoringendedon2013-09-2404:59.ObservationstakenintheB,V,R,Ifilters,with300sexposureineachfilter.
g TheWOmonitoringendedon2014-04-1408:25.ObservationstakenintheB,V,R,Ifilters,with300sexposureineachfilter.
4
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
7.0
1.2 0.3−2.0 keV 2.0−10 keV
6.0
−2−1m s 01..80 XELERPTITCG−Spn 45..00 XELERPTITCG−Spn
c
g 0.6
er 3.0
−110 0.4 2.0
1
0.2 1.0
0.0 0.0

8.0 10−30 keV 8.0 30−80 keV
−2−1m s 6.0 NuSTAR 6.0 NINuTSETGARRAL
c
g 4.0 4.0
er
−110 2.0 2.0
1
0.0 0.0
0 20 40 60 80 0 20 40 60 80
Days (MJD) − 56450 Days (MJD) − 56450
Fig.2.X-raylightcurvesofNGC5548fromoursummer2013campaign.Thelightcurvesshowtheobservedfluxandaredisplayed
between 7 June and 14 Sep 2013. The XRT data are shown as blue circles, EPIC-pn as magenta stars, LETGS as dark yellow
squares,NuSTARasreddiamondsandINTEGRALdataasgreensquares.Theverticaldashedlinesindicatethe intervalbetween
XMM-NewtonObs.1and12.
−1AÅ 0.8 I WO R WO
−1s OCA
−2m 0.7
c
g
er 0.6
−140
1 0.5

1.6
−1AÅ V UVOT B UVOT
−2−1m s 1.4 OWOMCOA OWOMCOA
g c 1.2
er
−140 1.0
1
4.0
−1AÅ 3.5 U UVW1
−2−1m s 3.0 UOVMOT UOVMOT
c 2.5
g
er 2.0
−140 1.5
1
4.0
−1AÅ 3.5 UVM2 UVW2
−2−1m s 3.0 UOVMOT UOVMOT
c 2.5
g
er 2.0
−140 1.5
1
0 20 40 60 80 0 20 40 60 80
Days (MJD) − 56450 Days (MJD) − 56450
Fig.3.NIR,opticalandUVlightcurvesofNGC5548takenwithvariousfiltersfromoursummer2013campaign.Thelightcurves
show the observed energy flux and are displayed between 7 June and 14 Sep 2013. The UVOT data are shown as blue circles,
OM asmagentastars, WO asredcircles,andOCA dataasgreensquares.The verticaldashedlinesindicatetheintervalbetween
XMM-NewtonObs.1and12.
5
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
4.0 HST COS 4.0 HST COS CCOOSS OObbss.. 11
CCOOSS OObbss.. 22
11116600 AÅAÅ CCOOSS OObbss.. 33
−1AÅ 3.5 11336677 AÅAÅ −1AÅ 3.5 CCCCOOOOSSSS OOOObbbbssss.... 4545
−1s 11448800 AÅAÅ −1s
−2m 3.0 11773355 AÅAÅ −2m 3.0
c 11775500 AÅAÅ c
g g
er 2.5 11779933 AÅAÅ er 2.5
−140 −140
1 1
2.0 2.0
1.5 1.5
10 20 30 40 50 60 1200 1300 1400 1500 1600 1700 1800
Days (MJD) − 56450 Observed Wavelength (ÅA)
Fig.4. HST COS fluxesof NGC 5548at differentUV wavelengthsfromsummer2013observations.Thelightcurves(leftpanel)
and spectra (right panel) show the observed energy flux obtained from narrow energy bands which are free of spectral features.
Thedisplayedrangeofthelightcurvesisbetween16Juneand6August2013,withtheverticaldashedlinesindicatingtheinterval
betweenXMM-NewtonObs.1and12.
improvement in our knowledge of the continuum flux in each and the modelling of the X-ray components (6–9) in Sects.
filter. The calculation of the COS fluxes at the narrow UV 4.6–4.9. We have modelled the grism spectra from OM and
energy bands is described in Appendix A.6. In the lightcurves UVOT together with simultaneous HST COS continuum mea-
of Fig. 3, the UVOT and OM fluxes have been calculated at surements to establish the contributionof each componentand
thefollowingwavelengthsforeachfilter:V (5402Å),B (4329 consequently correct the data taken in the photometric filters.
Å), U (3501 Å), UVW1 (2634 Å), UVM2 (2231 Å), UVW2 For fitting the grism spectra in SPEX we used χ2 minimisa-
(2030 Å). For OCA and WO the fluxes have been given at the tion instead of C-statistics (used for X-ray spectra), as the op-
tical/UV spectra have sufficient counts per bin. In Fig. 5 the
followingwavelengthsforeachfilter:I(8060Å),R(7000Å),V
best-fitmodelto thestackedOM andUVOTgrismspectra and
(5500Å),B(4330Å).
HST COS continua obtained from our simultaneous 2013 ob-
servations are shown. In Fig. 6 we display all the stacked data
As illustrated in the Fig. 3 lightcurves,the NIR/optical/UV
fromoursummer2013campaign.Thefigureshowshowmuch
fluxes were more or less continuously increasing throughout
theaforementionedNIR/optical/UVcorrectionsmodifythedata
our summer XMM-Newton campaign (Obs. 1–12) and beyond,
fromobserved(leftpanel)tocorrected(rightpanel);theemerg-
until the end of our Swift monitoring in middle of September
ing shape of the NIR/optical/UV SED after the corrections is
2013, when the source was not visible to Swift for a couple of
remarkableand exhibits a thermal disk spectrum. In Fig. 6 the
months. In fact the optical/UV fluxes recordedby Swift at mid
displayedX-raydataareonlycorrectedforGalacticinterstellar
SeptemberwerethehighestobservedinalloftheSwiftmonitor-
absorptionandstillincludetheeffectsofheavyabsorptionbythe
ingofNGC5548spanningfrom2005to2014,whichalsocoin-
obscurer and the ‘de-ionised’ warm absorber (i.e. less ionised
cidedwithourtriggeredChandraLETGSobservations.Figure4
warmabsorberduetotheshieldingoftheionisingradiationby
showsthattheCOSUVfluxesincreasedfromtheirminimumat
theobscurer).Similarly,inFig.7thecorrectedoptical/UV(OM)
XMM-NewtonObs.1(COSObs.1)andpeakedatXMM-Newton
and X-ray (EPIC-pn and RGS) data from 2000, 2001 and av-
Obs. 8 (COS Obs. 3), followed by a decrease afterwards. The
eragesummer2013are displayed,demonstratingthatwhilst in
COS flux changefromthe first observationgets largertowards
2013 the optical/UV continuum is higher than during the un-
higher UV energies:factor of 1.8 at 1793 Å to factor of 2.3 at
obscuredepochs,thesoftX-raysare heavilysuppressedbythe
1160Å.
obscurer.
4. Determinationofthecontinuumemission
In order to establish the actual intrinsic continuum in the
4.1.Galacticinterstellarreddening
NIR/optical/UV part of the SED, the following absorption
effects and emission contributions must be taken into account:
(1)Galacticinterstellarreddening;(2)Hostgalaxystellaremis-
sion; (3) Emission lines from the broad-line region (BLR) and Tocorrecttheoptical/UVfluxesforinterstellarreddeninginour
narrow-line region (NLR); (4) Feii blended emission feature; Galaxy,the reddeningcurveof Cardelliet al.(1989)wasused,
(5) Balmer continuum emission. Moreover, in the X-ray band including the update for near-UV given by O’Donnell (1994).
the following must be taken into account: (6) Galactic X-ray TheebvmodelinSPEXappliesthisde-reddeningtothedata.For
absorption; (7) Absorption by the obscurer; (8) Absorption by NGC5548thecolourexcessisE(B−V) = 0.02mag(Schlegel
thewarmabsorber;(9)SoftX-rayemissionlines. etal. 1998).Thescalar specifyingthe ratioof totalto selective
extinctionR ≡ A /E(B−V)wassetto3.1.InFig.5,thered-
V V
In the following subsections, we describe the modelling of dened continuum model is shown in dotted black line and the
the above NIR/optical/UV components (1–5) in Sects. 4.1–4.5 de-reddenedoneindashedblackline.
6
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
4.2.Hostgalaxystellaremission 4.5.Balmercontinuumfeature
To take into accountthe contributionof starlightfrom the host ApartfromFeiithereisanothersignificantemissioncomponent
galaxy of NGC 5548 in our NIR/optical/UV data, we pro- in the ∼2000–4000 Å region of AGN spectra: the Balmer
duced an appropriate model for inclusion in our spectral mod- continuum (see e.g. Grandi 1982). The Balmer continuum for
elling.Bentzetal.(2009,2013)havedeterminedthehostgalaxy an optically-thin emission with constant electron temperature
flux at an optical wavelength for a sample of AGN (includ- is given by FBC =FBEe−h(ν−νBE)/kTe (Grandi 1982), where FBC
ν ν ν
ing NGC 5548) using HST. However, for our observations of is the Balmer continuumflux at frequencyν, FBE is the fluxat
ν
NGC 5548,this host galaxy flux was re-calculatedto take into the Balmer edge ν , h the Planck constant, k the Boltzmann
BE
account the 5 arcsec radius circular aperture used in our pro- constantandT theelectrontemperature.TheBalmeredgeisat
e
cessings. For the HST F550M medium-band V filter, the host theoretical wavelength of 3646 Å. We used this model to take
galaxyfluxina5arcsecradiuscircularaperturewasdetermined into account the contribution of the Balmer continuum in our
tobe6.2×10−15ergcm−2s−1Å−1withanuncertaintyof∼10% opticaldata.ThemodelwasconvolvedwithanintrinsicDoppler
(MistyBentz,privatecommunication).Theeffectivewavelength velocitybroadeningσ andtheresolutionofthegrismsinSPEX.
v
of thisflux measurementis at an observedwavelengthof5580 Wefittedthismodeltothegrismdataforvariouscombinations
Å. Then, in order to calculate the host galaxy spectrum at the of T and σ values and found the best-fit Balmer continuum
e v
otherwavelengths,weusedatemplatemodelspectrumandnor- componenthasTe ∼ 8000K andσv ∼ 10000kms−1.We note
malisedittoNGC5548hostgalaxyfluxat5580Å.A5arcsec thatthepurposeoffittingtheBalmercontinuuminthisworkis
aperturetakesinonlytheinnermostfewkpcofthehostgalaxy, to obtain its total flux contribution to the optical data in order
and so the galaxy bulge template of Kinney et al. (1996) was to establish the underlying continuum component, so we do
adopted. In Fig. 5, the contribution of the host galaxy stellar notdelveintothe Te and σv parametershere whichcarrylarge
modelisdisplayedasthesolidgreenline. uncertainties. The flux contribution by the Balmer continuum
wasfoundtobeabout5.75×10−12ergcm−2s−1.
4.3.EmissionlinesfromBLRandNLR TheBalmercontinuum,togetherwithFeiiemission,forma
broad and blended feature in AGN spectra dubbed the ‘small-
TotakeintoaccountemissionlinesproducedfromtheBLRand blue-bump’. This is displayed in Fig. 5, in which our best-fit
NLR,wemodelledtheOMOpticalgrismandUVOTUVgrism model to the grism spectra and HST COS continua points are
spectra,coveringarangefrom1895to6850Å.Thesimultane- shown. The small-blue-bump is the feature in magenta colour
ity of the grism spectra with COS is important as it enables between ∼2000–4000 Å; beneath that the Balmer continuum
us to model the grism spectra and COS continuum points to- model(withoutFeiiemission)isdisplayedindarkyellow.
gether. This wider energy band in the optical/UV helps in es-
tablishingthe underlyingoptical/UVcontinuumand modelling
theBLRandNLRlines.Fortheunderlyingoptical/UVcontin- 4.6.GalacticinterstellarX-rayabsorption
uum, the Comptonisationcomponentcomt in SPEX (explained
later in Sects. 5 and 6) was used. The broad and narrow emis- The effects of the Galactic neutral absorption in the interstel-
sionlinesweremodelledusingGaussianlineprofiles.Forbroad lar mediumare includedin ourmodellingby applyingthe hot
lines, multiple Gaussian componentswith different widths, but modelinSPEX.Thismodelcalculatesthetransmissionofgasin
the same wavelength were added until a good fit was obtained collisional ionisation equilibrium.For a given temperature and
foreach line. The best fit obtainedfor the BLR and NLR lines set of abundances, the model calculates the ionisation balance
was saved as a spectral model componentwhich was included and then determines all the ionic column densities by scaling
in all ourbroadbandmodellinglater in Sect. 6, to take into ac- totheprescribedtotalhydrogencolumndensity N .Thetrans-
H
count the contribution of these lines in the photometric filters. missionincludesbothcontinuumandlineopacity.Tomimicthe
ThefluxcontributionofalltheBLR/NLRlinesinthegrismen- transmissionofaneutralplasmaincollisionalionisationequilib-
ergy range shown in Fig. 5 (solid black lines) was found to be rium(suchastheinterstellarmediumofourGalaxy),thetemper-
about7.94×10−12ergcm−2s−1. atureoftheplasmaissetto0.5eV.InourmodellingtheGalactic
HicolumndensityinthelineofsighttoNGC5548wasfixedto
N =1.45×1020cm−2 (Wakkeretal.2011)withLoddersetal.
H
4.4.Feiiblendedemissionfeature (2009)abundances.
The Feii in the BLR produces several thousands of transi-
tions,whichresultinablendedandcomplexspectrumbetween 4.7.Absorptionbytheobscurer
∼2000–4000Å (seee.g.Netzer&Wills1983;Willsetal.1985).
In order to take into account the Feii contribution in our opti- Totakeintoaccountthenewly-discoveredX-rayobscurationin
cal data we took the model calculated by Wills et al. (1985), NGC5548weadoptedthesamemodelfoundbyKaastraetal.
which is convolved to an intrinsic width of 2500 kms−1. This (2014). In this model the obscurer consists of two ionisation
wasthenimportedintoSPEXasaspectralmodelcomponentand phases, each modelled with an xabs component in SPEX. The
convolvedwith the resolutionofthe grisms. The normalisation xabs model calculates the transmission through a slab of pho-
scaling factor of the componentwas left as a free parameterto toionised gas where all ionic column densities are linked in a
allowforfittingthetotalFeiifluxinNGC5548.Thefluxcontri- physically consistent fashion through the CLOUDY photoionisa-
butionbyFeiiinNGC5548wasfoundtobeabout2.79×10−12 tionmodel.Thefittedparametersofanxabsmodelcomponent
ergcm−2 s−1. Thiscomponentis shown in Fig. 5 as partof the aretheionisationparameter(ξ),theequivalenthydrogencolumn
magentacolouredfeature. density (N ), the coveringfractionC of the absorber,its flow
H f
7
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
]
4.0 HST C III g II NGC 5548
COS M (Summer 2013)
UV I]
I
I
−1AÅ 3.0 grism Optical [O
1 α
−s grism H
2
−
m β
c H
g
r 2.0
e
4
1
−
0
1
1.0
0.0
1000 2000 3000 4000 5000 6000 7000
Observed Wavelength (AÅ)
Fig.5.De-reddenedoptical/UVspectrumofNGC5548.ThedisplayeddataarefromsimultaneousUVOTUVgrismspectra(shown
inlightblue)at1896–3414Å,OMOpticalgrismspectra(shownindarkblue)at3410–6836Åandthesix HSTCOScontinuum
points(shownaspurplesquares)between1160–1793Å.ThedisplayeddataaretheaverageofthecontemporaneousOM,UVOT
andHSTCOSobservationstakeninsummer2013andarecorrectedforGalacticreddeningusingthemodeldescribedinSect.4.1.
Thebest-fitmodel(describedinSect.4)isshowninredandvariouscomponentscontributingtothemodelarealsodisplayed.The
underlyingcontinuummodel(comt)is the dashedblack curve.The dotted blackcurveis the reddenedversionof the continuum
toillustratethecorrectionforreddening.Thebroadandnarrowemissionlinecomponents(Sect.4.3)areshownatthebottomand
someoftheprominentemissionlinesarelabelled.Thebroadfeatureinmagentaisthe‘small-blue-bump’:blendedFeiiemission
(Sect. 4.4) with Balmer continuum (Sect. 4.5). The model for the Balmer continuum alone is shown in dark yellow below the
small-blue-bump.ThecontributionfromthehostgalaxyofNGC5548(Sect.4.2)isshowningreen.
1013 NGC 5548 1013 NGC 5548
(Summer 2013) (Summer 2013)
Uncorrected Corrected
Hz) 1012 OM Optical grism 1012
y UVOT UV grism
J
νF (ν IU RV WV 1B U I R V B U
UVM2 XMM RGS UVW1 XMM RGS
UVW2 XMM EPIC−pn UVM2 XMM EPIC−pn
HST COS NuSTAR UVW2 NuSTAR
1011 1793 AÅ 1750 AÅ INTEGRAL 1011 1793 AÅ 1750 AÅ INTEGRAL
1735 AÅ 1480 AÅ 1735 AÅ 1480 AÅ
1367 AÅ 1160 AÅ 1367 AÅ 1160 AÅ
0.001 0.01 0.1 1 10 100 0.001 0.01 0.1 1 10 100
Observed Energy (keV) Observed Energy (keV)
Fig.6.Averagemulti-wavelengthdataofNGC5548fromoursummer2013campaign.Thedataaredisplayedbefore(leftpanel)
andafter(rightpanel)correctionsforGalacticreddening(Sect4.1),hostgalaxystellaremission(Sect4.2),emissionlinesfromthe
BLRandNLR(Sect4.3),blendedFeiiemission(Sect.4.4),Balmercontinuum(Sect.4.5)andGalacticinterstellarX-rayabsorption
(Sect4.6).Thespectrahavebeenbinnedforclarityofpresentation.ThedisplayedX-raydataincludetheeffectsofheavyabsorption
bytheobscurerandthewarmabsorber.TheshapeoftheNIR/optical/UVdataafterthecorrections(rightpanel)isremarkableand
exhibitsathermaldiskspectrum.
8
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
0.33 to 2.67, σ from 20 to 210 kms−1 and outflow velocities
v
1013 rangingfromabout250to1220kms−1.Forthe2000and2001
NGC 5548
XMM-Newton data, when the warm absorber would have been
exposed to normal ionising flux, parameters of the warm ab-
sorber were taken from a re-analysis of their RGS data, using
the same methodas in Kaastra et al. (2014).Thisworkwill be
1012 reportedinaforthcomingpaperonourcampaign(Ebreroetal.
Hz) 2000 (24 Dec) inprep),wherethelong-termvariabilityofthewarmabsorberis
Jy 2001 (09 Jul) goingtobepresented.
F (ν 2001 (12 Jul)
ν
Summer 2013
4.9.SoftX-rayemissionlines
1011 The2013RGSdataclearlyshowthepresenceofseveralnarrow
emission lines and radiative recombinationcontinuafrom pho-
toionisedgas(Kaastraetal.2014).Theseemissionfeaturesare
0.01 0.1 1 10 moreapparentin2013asthesoftX-raycontinuumissuppressed
Observed Energy (keV) duetoobscuration,buttheyarealsopresentatearlierepochs.In
orderto take into accounttheir flux contributionin our contin-
Fig.7.Optical/UV(OM)andX-ray(RGSandEPIC-pn)dataof uummodellingweusedthesamemodelandparametersreported
NGC5548fromunobscured(2000,2001)andobscured(2013) inKaastraetal.(2014).Thecontributionoftheselinestotheto-
epochs.Theoptical/UVdatahavebeencorrectedfortheeffects tal0.3–2.0keVfluxis1.4%(in2000),0.8–1.3%(in2001)and
described in Sects. 4.1–4.5. The X-ray spectra have been cor- 7.7%(in2013).Sowhilstintheunobscuredepochstheircontri-
rectedforGalacticabsorption(Sect.4.6),butnotforabsorption butionisrathersmall,in obscuredepochstheycontributemore
bytheobscurerandthewarmabsorber.ThedisplayedX-raydata tothesoftX-rayflux.Inanotherforthcomingpaperonourcam-
between0.3–1.4are RGS andathigherenergiesEPIC-pn.The paign by Whewell et al. (in prep), a detailed study of the soft
X-ray spectra have been binned for clarity of presentation. In X-ray emission features as detected by RGS in NGC 5548 is
the2013data,thesoftX-rayspectrumisthelowestoneandthe reported.
optical/UVdatathehighestone.
5. ThesoftX-rayexcessinNGC5548
vandturbulentσ velocities.Theionisationparameterξ(Tarter
v The AGN ‘softX-rayexcess’ is an excesscontinuumemission
etal.1969)isdefinedas
above the intrinsic X-ray power-law at the soft X-ray energies
L (below ∼ 2 keV). Since its discoveryby Singh et al. (1985)in
ξ ≡ (1)
n r2 HEAO-1 observations of Mrk 509 and also by Arnaud et al.
H
(1985) in EXOSAT observations of Mrk 841, it has been ob-
where L is the luminosity of the ionising source over the 1– served in manySeyfert-1 AGN. For example,in the Catalogue
1000 Ryd (13.6 eV to 13.6 keV) band in erg s−1, nH the hy- of AGN In the XMM-NewtonArchive(CAIXA, Bianchiet al.
drogen density in cm−3 and r the distance between the ionised 2009), this component was commonly found in about 80% of
gas and the ionising source in cm. The first component of the the AGN. In NGC 5548 the soft excess has been previously
obscurercoversabout86%ofthecentralX-rayemittingregion, detected(e.g.Kaastra&Barr1989;Magdziarzetal.1998)and
withlogξ = −1.2and NH =1.2×1022cm−2.Thesecondcom- itspresenceintheunobscuredstateisapparent.Figure8shows
ponent of the obscurer covers 30% of the X-ray source and is the EPIC-pnspectrum obtainedin 2000.The continuumabove
almost neutral (logξ = −4.0) with NH =9.6×1022cm−2. The 3.0 keV has been fitted with a Galactic-absorbed power-law
parametersoftheobscurerwerefixedtothoseofKaastra etal. component (Γ∼1.8), including a reflection component for
(2014)inourbroadbandspectralmodellingofthestackedsum- modelling the Fe Kα line. The best-fit is then extrapolated
mer2013data.Ofcoursefortheunobscuredepochs(2000and to lower energies, displaying the presence of the soft excess
2001 XMM-Newton data), the obscurer components were ex- belowabout2keV.However,thesoftexcessinNGC5548only
cluded from our modelby setting Cf of both obscurer compo- stands out in data before the X-ray obscuration began (i.e. <
nentstozero. 2011).Becauseofthenewlydiscoveredobscurer(Kaastraetal.
2014), the soft X-ray flux is heavily absorbed in 2013. This
demonstratesthatthesoftexcessemissionisproducedinasmall
4.8.Absorptionbythewarmabsorberoutflows
regioninteriortothe obscurer(e.g.theBLR) andisnotfroma
Sincetheobscurerislocatedbetweenthecentralionisingsource large-scalescatterer. Yet, the softX-raybandshowssignificant
and the warm absorber, it prevents some of the ionising radia- variability which can be attributed to both the obscurer and/or
tionfromreachingthewarmabsorber.Thusthedifferentphases soft excess component. Therefore, for any variability study of
ofthewarmabsorberbecomelessionised,resultinginmoreX- the obscurer, the variability of the soft excess component also
ray absorption than when NGC 5548 was unobscured. In our needs to be established. This means an appropriate model for
modellingoftheobscureddataweusedthede-ionisedwarmab- thesoftexcessisrequiredwhichwedescribebelow.
sorber modelobtainedby Kaastra et al. (2014),which consists
of six different phases of photoionisation.Each phase is repre- There are different interpretationsand models in the litera-
sented by an xabs componentin SPEX with N , ξ, flow v and tureforthenatureoftheenigmaticsoftexcessemissioninAGN
H
turbulentσ velocitieskeptfrozenthroughoutourmodellingto (seeSect.8.1).ForthecaseofNGC5548,evidence(described
v
the values given in Kaastra et al. (2014). The warm absorber below) suggests that the soft X-ray excess is primarily part
phases have N ranging from 2 to 57 ×1020 cm−2, logξ from of an optical/UV/soft X-ray continuum component, produced
H
9
M.Mehdipouretal.:AnatomyoftheAGNinNGC5548.I.
NGC 5548 0.8 NGC 5548
EPIC-pn XXMMMM ((22000000))
XXMMMM ((22000011))
o
ati 0.6 SSwwiifftt ((22000077))
ux r XXMMMM ((22001133))
y fl
a
soft excess X−r
d 0.4
ar
H
oft /
S
0.2
0.5 1.0 1.5 2.0
UVW2 flux (10−14 erg cm−2 s−1 AÅ−1)
Fig.8.EPIC-pnspectrumofNGC5548from2000,fittedabove −1s) NGC 5548
3.0 keV with a Galactic-absorbedpower-law(Γ∼1.8), includ- erg 8.0 XXMMMM ((22000000))
ingareflectioncomponentfortheFeKα.Thefitisextrapolated 420 XXMMMM ((22000011))
1
to lower-energies displaying the presence of a soft excess be- y ( SSwwiifftt ((22000077))
lowabout2keV.Thefitresiduals,(observed−model)/model,are nosit 6.0 XXMMMM ((22001133))
mi
displayedinthebottompanel. u
V) l
ke 4.0
2
−
by ‘warm Comptonisation’: up-scattering of the seed disk 0.3
photons in a Comptonising corona, which is distinct from the s (
es 2.0
one responsible for the hard X-ray power-law (i.e. the hot xc
e
corona),andischaracterisedbyalowertemperatureandhigher oft
S
optical depth (e.g. Magdziarz et al. 1998). Firstly, without any
modelling, the unobscured NGC 5548 data (Fig. 9, top panel) 0.5 1.0 1.5 2.0
UVW2 flux (10−14 erg cm−2 s−1 AÅ−1)
show that as the UV flux increases, the ratio of soft to hard
X-ray flux (‘softness’ ratio) also increases. This indicates the Fig.9. Top panel:Ratio of observedsoft(0.3–2.0keV) to hard
UV and soft excess are related to each other. In Fig. 9 (top (2.0–10 keV) X-ray flux plotted versus the observed UVW2
panel)theX-raysoftnessratioisplottedversustheUVfluxfor (2030 Å) flux. Bottom panel: Intrinsic luminosity of the soft
the unobscured data (XMM-Newton 2000 and 2001 and Swift excess componentbetween 0.3–2.0 keV plotted versus the ob-
2007), and also for the obscured XMM-Newton data from the
servedUVW2(2030Å)flux,andfittedwiththepower-lawfunc-
average of the summer 2013 observations. The softness ratio
tiongiveninEq.2.
corresponds to the observed flux, without any modelling and
correctionsforabsorption.Remarkably,fortheobscuredepoch
in2013,thesoftnessratioisatitslowestleveleventhoughthe InFig.9(bottompanel)weshowthesoftexcessluminosity
UV flux is at its highest. The reason for this sharp drop in the inthe0.3–2.0keVbandversustheUVflux,showingtherelation
softnessratioisthepresenceoftheheavysoftX-rayabsorption between the soft excess and UV. The soft excess luminosity is
bytheobscurerin2013(seeFig.7). calculated using the model described in Sect. 6 (i.e. corrected
forobscurer,warmabsorberandGalacticabsorption).Thedata
The link between the UV and soft excess flux was also inFig.9(bottompanel)havebeenfitted withafunctionwhich
seen in Mrk 509 (Mehdipour et al. 2011). From broadband isfoundtobeapower-law,givenby
modelling of its continuum, Mehdipour et al. (2011) and
Petrucci et al. (2013) argued warm Comptonisation to be the Lsoftexcess =1.261(FUVW2)2.869 (2)
most reasonable explanation for the soft excess. Furthermore,
where L is the intrinsic luminosity of the soft excess
warm Comptonisation has already been suggested for the soft softexcess
componentinthe0.3–2.0keVbandinunitsof1042 ergs−1 and
excessinNGC5548,inoneofthefirstpapersinvestigatingthis
F istheUVW2filterfluxinunitsofergcm−2s−1Å−1.This
interpretation, by Magdziarz et al. (1998). From broad-band UVW2
fluxintheUVW2filterisatthewavelengthof2030Åforboth
spectral analysis of NGC 5548, using data from IUE, Ginga,
UVOTandOM andisthe observedflux withoutanymodifica-
ROSAT, CGRO and OSSE, Magdziarz et al. (1998)found that
tionssuchasde-reddening,buttakesintoaccounttheoptical/UV
thesoftexcessrequiresaseparatecontinuumcomponentwhich
spectrumshapeofNGC5548(showninFig.5)intheUVW2fil-
canbefittedbyComptonisationofthermalphotonsfromacold
disk (T ∼ 3.2 eV), in a warm (∼ 0.3 keV), optically thick terbandpass.WefurtherdiscussthesoftexcessinSect.8.1.
seed
(τ∼30)corona.Theauthorsalsofoundtheoptical/UVandsoft
X-ray fluxes in NGC 5548 to be closely correlated. Therefore
6. BroadbandcontinuummodellingofNGC5548
consideringtheaboveevidenceweadoptwarmComptonisation
astheappropriatemodelforthesoftexcess. Here we describe our continuum modelling of the NGC 5548
data. The archival XMM-Newton data from 2000 and 2001
10