X-ray emission from GPS and CSS sources

0.23 MB·English

by  Aneta Siemiginowska

Astron.Nachr./AN999,No.88,789–795(2006)/DOIpleasesetDOI!
X-ray Emission from GPS and CSS Sources
AnetaSiemiginowska1,⋆
Harvard-SmithsonianCenterforAstrophysics,60GardenSt.,Cambridge,MA02138,USA
Thedatesofreceiptandacceptanceshouldbeinsertedlater
9
Keywords X-rays:galaxies-active,jets-radio
0
0 Many X-ray observations of GigaHertz Peaked Spectrum and Compact Steep Spectrum sources have been made with
2 Chandra X-ray Observatory and XMM-Newton Observatory over the last few years. The X-ray spectra contribute the
importantinformationtothetotalenergydistributionofthecompact radiosources.Inadditionthespatialresolutionof
n
ChandraallowsforstudiesoftheX-raymorphologyofthesesourcesonarcsecscalesandprovideadirectviewoftheir
a
J environments. This paper gives a review of the current status of the X-ray observations and their contribution to our
understandingofthenatureofthesecompactradiosources.Italsodescribesprimaryphysicalprocessesthatleadtothe
9
observed X-ray emission and summarize X-ray emission properties expected from interactions between an expanding

radiosourceandtheintergalacticenvironment.
]
E
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
H
.
h
p 1 Introduction iesorhalosofsomeactivegalaxies.Therehavebeensome
- failed attempts to detect such emission surrounding GPS
o
A standard picture of a radio source shows a large scale sources in the past searches for a “confinement medium”
r
t emissioninformsoflobesandhotspotsthatareoftencon- of the radio source (Antonelli & Fiore 1997, O’Dea et al.
s
a nectedtotheradiocorebynarrowjets.Suchemissionspans 2000,Siemiginowskaetal. 2003)inthepast.Non-thermal
[ hundreds of kiloparsecs and can be studied in details in emission associated with relativistic particles requires ac-

1 nearby sources, for example in the closest powerful FRII celerationprocessesandisobservedinjets,orshockregions
v radio source Cygnus A (FR II, z = 0.056). However, the resultingfrominteractionsbetweentheradiosourceandthe
7 morphology of the X-ray emission is very different. The ISM. Relativistic particles emit radio synchrotronphotons
2 Chandra X-ray Observatory image of Cygnus A (Wilson, and radioobservationscan giveinformationaboutthe dis-
3
Young & Shopbell2000,Smith et al. 2002)shows a more tribution of particles. X-ray synchrotronemission requires
1
circulardiffuseX-rayemissionsurroundingtheradiosource veryhighenergyelectrons(γ ∼107−108)thathaveshort
.
1 withtwoprominenthotspotsandafilamentarystructure.A (∼ 10 years) synchrotron life times. Therefore any X-ray
0
primarysourceofthisX-rayemissionisahot(afewkeV) synchrotron emission requires an on-going re-acceleration
9
0 thermalclustergas,whiletheradioemissionismainlydue process.
: tonon-thermalparticlesacceleratedinthejet,lobesandhot Studiesofinteractionsbetweentheradiosourceandthe
v
spots. interstellar and intergalacticmediumcan provideinforma-
i
X Thenearby(3.7Mpc)radiosourceCentaurusAismuch tionabouttheenergythatisdepositedintotheIGMbythe
r smallerthanCygnusA.Ithasadoubleloberadiostructure expandingradiosource.Thisisimportanttoourunderstand-
a
only slightly larger than its host galaxy. The source mor- ingofthefeedbackprocess.GPS/CSSsourcesareobserved
phologyisveryrich,showinganabsorbedcore,dustlanes, at the early stage of their growth, where such interactions
jet,counter-jet,radiolobesandshocksinmulti-wavelength arestrong.Inadditiontheoreticalmodelspredictthatarela-
observations.X-rayemittingcomponentscanberesolvedin tivelystrongX-rayemissionshouldbeassociatedwithGPS
X-raysbecauseofthesourceproximity.Theyprovidecrit- andCSSsources.
ical information about the physics and processes that are
happening in this source (e.g. Kraft et al. 2007). GPS ra-
2 X-ray EmissionofRadio Sources: Models
dio sourcesareembeddedwithin the hostgalaxyandtheir
multi-wavelength morphology might be similar to Cen A.
ThereareseveraltheoreticalpredictionsoftheX-rayemis-
However, if they are more distant their X-ray components
sionfromevolvingradiosources.Begelman&Cioffi(1989)
maynotberesolved.
following Scheuer (1974), draw a general picture of an
WhattypeofprocessescangenerateanX-rayemission?
evolving radio source where the shocked IGM creates an
Thermalemissionisassociatedwithahotplasmawithtem-
overpressuredcocoonsurroundingadoubleradiostructure
peraturesofafewkeVandisobservedinclustersofgalax-
withlobes,jetsandabowshockboundingthecocoon.The
⋆ Correspondingauthor:e-mail:[email protected] shock heated material emits X-rays. Reynolds, Heinz &
Begelman (2001)presentnumericalsimulationsof a radio
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
790 AnetaSiemiginowska:X-rayEmission
source expansion within a uniform medium with three
phasesoftheevolution:asupersoniccocoon,subsonicside-
waysexpansionwithaweakshockandsupersonicjet,and
afinalsonicboomphase.Eachphaseischaracterizedbya
differentmorphologyof the X-ray emitting gas that is be-
ingheatedbytheexpandingsource.Sutherland&Bicknell
(2007a, 2007b) shows a similar view of the radio source
evolution within a clumpy medium (see also Bicknell &
Sutherland2006).AlsointhissimulationstheX-rayemis- Fig.1 Left: XMM-Newton EPIC X-ray image of
sionistheresultofinteractionsandtheX-rayluminosityde- Mkn668 (z=0.076). A 30′′ radius circle is marked around
pendson the densityandclumpinessofthe medium.They the source. The source is unresolved on this scale. A 30′′
discuss the time dependent (0.1-10 keV) X-ray emissivity scale barequivalentto 44kpcisshownintherightcorner.
with themaximumoftheX-rayluminosityreachedwithin Right: Chandra ACIS-S image of Q1143-245 at z=1.95.
thefirstfew∼104years. 5′′ scalecorrespondingto42.6kpcatthesourceredshiftis
Heinz,Reynolds&Begelman(1998)consideranevolu- markedintheleftcorner.Notethattheradiosourcesizeis
tionofGPSsourceswithintheuniformenvironmentofthe muchsmallerthantheresolutionofthesetwoinstruments.
hostgalaxy.ThesimulatedX-raysurfacebrightnessshows
the shockdiscontinuitymovingoutwardswith time. X-ray
several GPS quasars detected with HEAO-1 and ROSAT
detectionsofsuchdiscontinuityrequirehighspatialresolu-
(see O’Dea 1998 for a summary). During the last decade
tionanddynamicrangeobservationsinnearbysources.The
there have been several efforts to increase the number of
shockofCenAisseenasafactorof∼10jumpat∼7kpc
X-raydetections.Guainazzietal.(2004,2006)andVinket
away from the nucleus and it is ∼ 600 pc wide (Kraft et
al.(2006)presentX-raysamplesofGPSgalaxies,Siemigi-
al. 2007). It has not been detected in any observations of
nowska et al. (2003, 2005, 2008) discuss X-ray emission
GPSsourcessofar,possiblyduetotherequirementofhigh
inGPS/CSS quasars.Worralletal.(2004)andO’Deaetal.
signal-to-noisedataandspatialresolutionthathavenotbeen
(2006)studydetailsoftheX-rayemissioninasingleGPS
obtainedintheexistingobservations.
source.
RecentlyStawarzetal.(2008)presentedaspectralemis-
Chandra and XMM-Newton observational capabilities
sionmodelforGPSsources(seealsoOstoreroetal.inthis
arecomplementary.Chandraexceptionalpointspreadfunc-
Proceedings).Themaincontributiontothespectralenergy
tion(PSF) allowsforstudiesofX-raymorphologywith ∼
distributioncomesfromtheradiolobesandhotspotsofthe
1arcsecresolutionandthehighestdynamicrangeavailable
<1kpcsizeradiosourceembeddedintheradiationfieldof
intheX-raybandtoday(VanSpeybroecketal.1997,Weis-
thehostgalaxy.TheX-rayemissionisduetoInverseComp-
skopf et al. 2000). Because of its low backgroundit gives
ton scattering off the relativistic electrons within the radio
efficientdetectionsofveryfaintsourcesandrelativelygood
source. The UV-diskphotonsand IR-dustphotonsprovide
quality spectra within 0.5-7 keV. The PSF of the XMM-
the external radiation field to the hot spots, lobes and jet.
Newton is too large for studies of the X-ray morphology
Thisfielddominatesinsmallersources,whilethesynchro-
ofGPSsources(Kirschetal.2004),buta veryhigheffec-
ton photon field intrinsic to the lobes dominates in larger
tive area of the telescope giveshigh quality spectra within
sources,andGPS/CSSsourcesexceeding1kpcinsizewill
0.5-10keVenergyrange.
be dominated by the synchrotron self-Compton emission
ThespatialresolutionofmodernX-raytelescopesisstill
processintheX-rayband.ThepredictedX-rayluminosity
much worse than the resolution obtained by radio obser-
depends on a few model parameters such as a jet power,
vations. Figure 1 shows the X-ray image of a nearby GPS
photon fields and the density of the ISM. Note that this
sourceMkn668atz=0.076(1arcsec=1.46kpc).Adouble
modelrelates the X-ray emission to the radiosource com-
radio structure (< 10 pc) is fully enclosed within the X-
ponentswhilethesimulationsdescribedaboveconsiderthe
ray point source observed with XMM-Newton EPIC cam-
emissionfromthehotgasheatedbytheradiosource.
era. The highest redshfit GPS source in the X-ray sample
Both type of the X-ray emission, thermal and non-
isplottedinFigure2.Q1143-245(z=1.95)isanunresolved
thermal, will be present in GPS sources. Hot thermal gas
X-raypointsourceintheChandraACIS-Sobservationand
willproduceanX-rayspectrumwithemissionlines,while 1.5′′correspondsto∼12kpcatthequasardistance.
thenon-thermalcontinuumdoesnothaveanyspectralfea-
Table1liststhephysicalsizeinkpcforasourcelocated
tures.HighresolutionX-rayspectraareneededinorderto
atdifferentredshifts.1arcseccorrespondstoaphysicalsize
disentanglethesetwoemissioncomponents.
of179parsecinCenA,whileitcorrespondsto8kpcfora
sourceatredshift1.Theredshiftdistributionforasampleof
3 X-ray Observations ofGPS/CSSSources sourcesobservedsofarinX-raysisplottedinFigure2.The
peakdistributionoftheGPSgalaxiesisatz ∼0.3−0.4and
BeforeChandraandXMM-NewtontherewasonlyoneGPS foraradiosourcesmallerthan<10kpccannotberesolved
galaxydetectedinX-raysbyASCA(O’Deaetal.2000)and withthecurrentX-raytelescopes.FortheGPSquasarsthe
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2006) 791
Table1 PhysicalSizevs.Redshfit
Redshift 1′′ 5′′ Example
kpc kpc GPSSource
6
0.00087 0.0179 0.0895 CenA
Quasars
0.01 0.202 1
s
e
0.076 1.46 7.3 Mkn668 c
0.1 1.8 9 Sour 4 Galaxies
0.668 7 35 0108+338 er of
b
1 8 407 m
u
N
2
upper limits
0
1 2 3 4
log N [1020 cm−2]
H
Fig.3 Distribution of the intrinsic absorption column
in GPS galaxies. The histogram includes both Chandra
and XMM-Newton observations from Guainazzi et al.
(2004,2006),Vinketal.(2006),Siemiginowskaetal.2008,
Tengstrand et al. (2008). Detections are plotted with the
solidredcurveandupperlimitswiththedashedbluecurve.
TheobservedcolumnsinGPSquasarsreportedbySiemigi-
nowskaetal.(2008)aremarkedbyabigarrowandallare
belowN <1021atomscm−2.
H
Fig.2 RedshiftdistributionofGPSandCSSsourcesob-
servedinX-rays.ThehistogramincludesbothChandraand 3.1 CompactSource:X-raySpectra
XMM-Newtonobservationsandconsistsof19galaxiesand
GPS sources are faint in X-rays and have varying quality
13quasars.Thegalaxiesaremarkedbyasolidredlineand
quasarsbyadashedbluelineandashadedregion.5′′scale of the X-ray spectrum, from a detection of a source with
corresponding to size < 10 kpc and > 40 kpc is marked justafewcountsinashort∼ 5ksecChandraexposureto
a relatively good spectra with > 1000 counts in a longer
byarrows.ThedatapointsarefromGuainazzietal.(2004,
20-30 ksec XMM-Newton observations. Most spectra do
2006),Vinketal.(2006)andSiemiginowskaetal.(2008).
not show any strong emission lines, except for Mkn 668
(Guainazzietal.2004),andcan be characterizedbyan ab-
sorbedpowerlawmodel.Suchamodelprovidesanestimate
distribution spans evenly the redshift between 0.4−2. At ofanX-rayfluxandprobestheX-rayabsorptionproperties.
redshiftz > 1 the 5 arcsec size of a pointsourcecontains Figure3showsadistributionoftheintrinsicabsorption
a source exceeding > 40 kpc in size. GPS radio structure columninGPSgalaxiesobservedsofarinX-rays.Thereis
is smallerthan< 1 kpc,CSS < 10kpc,so anyassociated a peakat∼ 1022 atomscm−2 in thedistributionandmost
X-rayemission(asinStawarzetal.2008model)wouldnot of the galaxies are absorbed in X-rays. On the other hand
beresolved. thequasarsshownointrinsicabsorptionwithdetectionlim-
itsbelow<1021atomscm−2(Siemiginowskaetal.2008).
AnunresolvedX-raypointsourcewillcontainasource The GPS galaxies show an anticorrelationbetween the X-
smallerthan<3′′ inChandraand<15′′ inXMM-Newton rayabsorptioncolumnandtheradiosourcesize(Guainazzi
observations. There, in most GPS/CSS we can only study etal.2006,Vinketal.2006)similartotheonenoticedinthe
X-ray spectral properties and may be able to disentangle observations of HI 21 cm absorption by Pihlstro¨em, Con-
different emission components, evaluate absorption prop- way &Vermeulen(2003).Thismayindicatean evolution-
erties and intrinsic source luminosity. However, an X-ray ary phase in radio sourcegrowthin which the initially en-
emission on scales largerthan the PSF size can be studied shroudednucleusisbeing“clearout”byanexpandingradio
with Chandrain nearbysources(see Fig.2),e.g.a diffuse source.
emissionassociatedwithcocoon,NarrowEmissionLinere- GiventheX-rayspectraandtheobservedabsorbingcol-
gions,superwinds,X-rayclusterorrelicemission.Inaddi- umnonecandeterminethesourceintrinsicX-rayluminos-
tionlargerscalejetsassociatedwiththeradiosourcecould ity. The new observations indicate that the GPS galaxies
bedetectediftheirlengthexceeds>3′′. are not X-ray quiet. Their X-ray luminosity corrected for
www.an-journal.org (cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
792 AnetaSiemiginowska:X-rayEmission
Wecansummarizethemainobservationalresultsbased
ontheX-rayspectra:
• Absorption:(1) high (> 1022 cm−2) column densities
are commonin galaxies;(2) there is a tentativedepen-
dence of the column density on the distance between
the hot spots/lobes X-ray; (3) X-ray column density is
alwayslargerthantheHIcolumnobservedinradio.
• IntrinsicX-rayLuminosity:(1)GPSgalaxiesarenotX-
rayquiet;(2)GPS/CSSquasarsshowanindicationthat
theyareX-rayweak.
• X-rayloudness:(1)X-raytoradioluminosityratiosare
similar to the ratios observed in FRI galaxies;
Fig.4 Radio loudness of the GPS quasars from
(2)GPS/CSSquasarsarethemostradioloudobjects.
Siemiginowska et al.(2008) compared to the other radio-
loudquasarsfrom Elvis et al.(1994).GPS/CSS quasars are
3.2 X-rayMorphology(onkpcscales)
markedbyshadedhistogramandtheyaremoreradioloud.
GPS radio structure is much smaller than the spatial reso-
lution capabilities of the current X-ray instruments, so the
GPSsourceremainsunresolved(eveninthelowestredshift
sources) in all X-ray observationsto date. X-ray morphol-
ogy on scales exceedingthe GPS source size can be stud-
iedwithChandra.SeveraltypesofX-raymorphologyhave
beendetectedintheGPSandCSSsources:
• X-rayjetsweredetectedonlargescalesuptohundreds
kpcawayfromtheGPSsource:PKS1127-145(Siemigi-
nowska et al. 2002, 2007), B2 0738+393 (Fig.6,
Siemiginowskaetal.2003).
• an irregularly shaped diffuse X-ray emission:
PKSB1345+125(Siemiginowskaetal.2008,Guainazzi
Fig.5 Radio loudness in comparison to the B-band lu- etal.inpreparation)
minosity for a different classes of sources in Sikora et al. • a secondary X-ray emission associated with a radio
(2007).ThelocationoftheGPSquasars(circle)andgalax- sourcelocated20′′tothewestofadoublenucleusGPS
ies(triangle)ismarkedontheoriginaldiagramofSikoraet galaxyPKS0941-080(Siemiginowskaetal.2008)
al.MostGPSsourcesaremoreradioloud,opticallybrighter • anX-rayemissionassociatedwithaclusterofgalaxies
and located outside a range(markedby a dashedline) ob- attheGPSsourceredshift:CSSquasar3C186isinthe
served in the other sources. Note B-luminosity for GPS centerofanX-raycluster(Siemiginowskaetal.2005)
galaxiesistypicallyundetermined,becauseoftheintrinsic
A large scale X-ray emission associated with the GPS
absorptionofthenucleus.
source may also indicate intermittency of a central AGN.
Baumetal.(1990)postulatedthatanextendedradioemis-
siondetected∼ 20′′ awayfromtheGPSgalaxy0108+388
absorption exceeds 1042 ergs s−1 and matches the X-ray
isarelicofanearlieractivephaseofthesource.Reynolds
luminosities of GPS quasars and FRII type radio galaxies
&Begelman(1997)arguedthatintermittentactivitycanex-
(Guainazzietal.2006,Tengstrandetal.2008).
plain the observedstatistics of GPS sources. Intermittency
Withthenewobservationswecanalsostudythespectral or“modulatedjetactivity”hasbeenconsideredasamecha-
energydistributionoftheGPSsourcesfromradiotoX-rays. nismshapingthemorphologyoflargescalejets(fordetails
Such analysisindicate thatthe GPS/CSS quasarsare more see Stawarz et al. 2004 and examples: 3C273 by Stawarz
radio loud than the other radio loud quasars (see Fig. 4), 2004,andJesteretal.2006;PKS1127-145Siemiginowska
where the radio loudnessis defined as a ratio between the et al. 2007). The newborn portion of the jet resembles the
radio and optical (B band) luminosity. A comparison with GPSsource,butatlatertimeitwillbeobservedasaknotin
a larger sample of variety of radio loud objects in Sikora, theextendedjet.
Stawarz & Lasota (2007) shows that the GPS quasars and A large scale emission allows also for studies of inter-
galaxiesare most radio luminousfor a givenoptical lumi- actions between the radio source and the environment. A
nosity(seeFig.5).Thismaysuggestahigherradiativeeffi- detection of an X-ray cluster in the Chandra observation
ciencyin ayounger(smaller)GPSsourceaspostulatedby of3C186(Fig.7)providedawaytostudythesurroundings
thetheory(seeBegelman,1997). oftheradiosource.Themeasuredtemperatureanddensity
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2006) 793
Fig.8 Smoothed Chandra ACIS-S images of
Fig.6 A smoothed Chandra ACIS-S (0.3-7 keV) image
PKS 1345+125.Left: The soft (0.5-2 keV) band showing
of B2 0738+393overlayedwith the radio contours(VLA-
theextendedemissionsurroundingthecore.Them10′′scale
1.4 GHz). The X-ray jet is narrow, curvesand follows the
ismarkedattheleftcorneranditisthesameinbothimages.
radiostructuretothesouth.Thejetendswitha hotspotat
thesouthernmostpartoftheradiolobe.Aknotat∼13arc- Right: The hard(2-7keV) band image. It is dominatedby
theunresolvedcore.
sec away from the core andits emission is consistentwith
theX-raysbeingcreatedbytheinverseComptonscattering
of the cosmic microwave background(CMB) photonsand
requiresjetbulk Lorentzfactorsofa few (Γ ∼ 5−7) Holt,Tadhunter&Morganti(2003).TheX-rayemissionis
bulk
also stretched towards the South-West similarly to the op-
(seeSiemiginowskaetal.(2003)fordetails).
ticalemissionandisalignedalsoagreeswiththeVLBIjet
axis(Stanghellinietal.2001)suggestingthatitmaybere-
lated to the expanding GPS source. More detailed discus-
sionispresentedinGuainazzietal.(2008inpreparation).
Interestinglythereissofarnodetectionofahot(kT∼ a
fewkeV)cocoonthatispredictedbytheoreticalmodelsof
the expandingradio source.Isthis becausewe did notob-
servetherighttargetsorwearelimitedbythespatialreso-
lutionofthetelescopes.O’Deaetal.(2006)reportedather-
mal componentin an XMM-Newtonspectrumof 3C303.1
(z=0.267). They suggest that this thermal emission could
beduetoplasmaheatedbytheshockoftheexpandingra-
diosource.ThesourceisunresolvedinthisX-rayobserva-
tionandonlythespectrumcouldbeusedtodisentanglethe
Fig.7 SmoothedChandraACIS-S(0.3-7keV)imageof emissioncomponents.FutureChandraobservationmayal-
3C 186 The diffuse cluster emission extends to ∼120 kpc lowformoredetailedinvestigationsoftheX-rayproperties
from the central quasar. The 10 arcsec (82 kpc) size is onafewarcsecscales.
marked. The direction of an unresolved 2 arcsec radio jet
isalsomarked.
4 Conclusions andFuture Perspectives
A numberof pointed X-ray observationsof GPS and CSS
of the X-rayemitting hotcluster gas allow for an estimate sources has increased significantly over the last decade.
of the gas pressure. This in turn could be comparedto the However,acompletesampleisstillnotavailableandthere
pressuremeasuredintheradiolobes.In3C186thepressure isthereforenostatisticallysolidandsystematicstudyofthe
in the radio lobes exceeded the thermal gas pressure by 2 X-ray properties of such a sample. The current observed
orders of magnitude (Siemiginowska et al.2005). This ob- trends need to be confirmed in the future. We are contin-
servation therefore allowed to conclude that the jet is not uingoureffortstoobtainX-rayobservationsofalltheGPS
frustratedandtheradiosourceexpandswithnodisruptions. sources in Stanghellini et al. (1998) and making a slow
Fig.8 shows smoothed Chandra ACIS-S images of progress.
PKSB1345+125inthesoft(0.5-2keV)andhard(2-10keV) Therearemanyremainingquestionsaswedonotknow
bands.ThediffuseX-rayemissionseeninthesoftbandmay answers to the observed correlations, the processes domi-
originate as a thermalemission associated with the galaxy nating X-ray emission, radio source environment.The ba-
halo.Itssizeagreeswiththesizeoftheextendedopticalline sicquestionsaboutthenatureoftheactivityisstillnotan-
regionstudiedkinematicallyintheopticalanddescribedby swered:Istheradiosourceintermittent?Isthesourceshort
www.an-journal.org (cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
794 AnetaSiemiginowska:X-rayEmission
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tionsthathavebeenaskedatthisworkshopwererelatedto concelli,E.,AzubikeUgwoke,C.:2006,A&A,446,87
the source nature and evolution. They have to be investi- Guainazzi, M., Siemiginowska, A., Rodriguez-Pascual, P.,
gated using multi-wavelengthdata. Samples that cover ra- Stanghellini,C.:2004,A&A,421,461
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Acknowledgements. SOC and LOC members are acknowledged
Smith,D.A.,Wilson,A.S.,Arnaud,K.A.,Terashima,Y., Young,
fororganizationofagreatmeeting.Thisresearchisfundedinpart
A.J.:2002,ApJ,565,195
byNASAcontractNAS8-39073andgrantNNX07AQ55G.Partial Stanghellini,C.,O’Dea,C.P.,Dallacasa,D.,Baum,S.A.,Fanti,
support for this work was provided by the National Aeronautics R., Fanti,C.:1998,A&AS,131,303
andSpaceAdministrationthroughChandraAwardNumberGO5- Stanghellini,C.,Dallacasa,D.,O’Dea,C.P.,Baum,S.A.,Fanti,
6113X,GO7-8103X-RissuedbytheChandraX-RayObservatory R., Fanti,C.:2001,A&A,377,377
Center, whichisoperated bytheSmithsonianAstrophysical Ob- Stawarz, Ł., Ostorero, L., Begelman, M. C., Moderski, R.,
servatoryforandonbehalfofNASAundercontractNAS8-39073. Kataoka,J., Wagner,S.:2008,ApJ,680,911
Stawarz,Ł.:2004,ApJ,613,119
Stawarz,Ł.,Sikora,M.,Ostrowski,M., Begelman,M.C.:2004,
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