Signs of WHIM in the soft X-ray background

0.25 MB·

by  A. M. Soltan

Astronomy & Astrophysics manuscript no. paper˙astroph February 2, 2008
(DOI: will be inserted by hand later)
Signs of WHIM in the soft X-ray background
1 2 2
A. M. So ltan , M. J. Freyberg and G. Hasinger
1Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland
e-mail: [email protected] (AMS)
2 MPI fu¨r extraterrestrische Physik,Giessenbachstraße, 85748 Garching, Germany
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e-mail: [email protected] (MJF)
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e-mail: [email protected] (GH)
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2
Received
n
a
J Abstract. Small angular scale structure of the soft X-ray background correlated with the galaxy distribution is
investigated. An extensive data sample from the ROSAT and XMM-Newton archives are used. Excess emission
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1 below ∼ 1keV extending up to at least ∼ 1.5Mpc around galaxies is detected. The relative amplitude of the
excess emission in the 0.3−0.5keV band amounts to 1.3±0.2% of the total background flux. A steep spec-

1 trum of the emission at higher energies is indicated by a conspicuous decline of the signal above 1keV. The
v XMM-Newton EPIC/MOS data covering widerenergy range than theROSAT PSPC are consistent with ather-
5 mal bremsstrahlung spectrum with kT <∼ 0.5keV. This value is consistent with temperatures of the Warm-Hot
7 Intergalactic Medium derived by several groups from hydrodynamic simulations. Correlation analysis allows for
2 estimateoftheaverageexcessemissionassociated withgalaxiesbutthedataareinsufficienttoconstrainphysical
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parameters of the WHIMand to determine thecontribution of WHIMto thetotal baryonicmass density.
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Key words.X-rays:diffuse background – large-scale structureof Universe
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/
h
p
1. Warm-Hot Intergalactic Medium The density and temperature of the infalling material in-
-
o crease and around mass concentrations arise halos of the
r X-ray telescopes on board the satellites EINSTEIN, hot plasma. The spatial distribution and physical param-
t
s ROSAT andChandra havefirmlyestablishedthe discrete eters of the diffuse gas in the local Universe (z <∼0.4) are
a nature of the extragalactic X-ray background (XRB).
: distinctly different from both the matter in galaxies and
v High angular resolution provided by the imaging optics
plasmainclustersofgalaxies.Thisconstituentofthebary-
i allowed for isolation of various classes of point-like X-ray
X onicmatterisknownasWHIM–Warm-Hot Intergalactic
sources, leaving unresolved only a small fraction of the
r Medium (Dav´e et al. 2000).
a total XRB flux.
The quantitative characteristics of the WHIM as a
While the observational evidences are dominated by
functionofredshiftdependontheratethe gasisaccumu-
the discrete sources, theoretical considerations indicate
lating. The process is determined by the evolution of the
that in the soft X-rays still a non negligible fraction of
gravitationalpotentialandby the non-gravitationalheat-
the XRB should remainin the form of the diffuse compo-
ing, so called feedback (e.g. Zhang & Pen 2002). Details
nent.Apartfromclustersofgalaxieswhichhavebeenrec-
of the feedback, i.e. transfer of energy and matter from
ognized sources of the extended emission since the begin-
galaxies back to the intergalactic medium have been in-
ning of X-ray astronomy, a substantial amount of plasma
vestigated extensively in the literature (e.g. Dav´e et al.
residing in the intergalactic space might be hot enough
2000, Bryan & Voit 2001, Croft et al. 2001). Although
to emit a noticeable amount of the X-ray flux. Following
all these theoretical studies ascertain the WHIM contri-
Cen & Ostriker (1999), several groups performed hydro-
bution to the soft XRB, the quantitative estimates of the
dynamical simulations which have demonstrated that a
WHIM emission are still rather uncertain.
significantfraction of baryonsin the Universe has not yet
accumulated in stars and galaxies. It is estimated that A potential contribution to the soft XRB from the
30 − 40% of the baryonic matter fills the intergalactic diffuse component is strongly constrained by the source
space(e.g.Dav´e et al. 2000).Thisgasslowlyfallstoward counts. It appears that the integrated flux produced by
potentialwellscreatedbythe(non-baryonic)darkmatter. sourcesabovethe Chandra limitamountsto 94.3+−76..07%in
the 0.5−2.0keV band (Moretti et al. 2003). A smooth
Send offprint requests to: A. M. Sol tan extrapolation of counts down to sources fainter by an or-
2 A.M. Sol tan et al.: Signs of WHIM in the soft X-ray background
der of magnitude than the present-daylimit gives 96% of effect wasmeasuredatseparations0◦.3−2◦.1for the Abell
theXRB.Consequently,thediffusefractioncannotexceed DC ≤ 5 clusters and up to 3◦.1 for the Lick counts.
4%, though this estimate is subject to large uncertain- However,the signal amplitude was weak and we were not
ties. Deep counts in the Lockman Hole by Worsley et al. able to put strong constraints on the temperature of the
(2004) using the XMM EPIC cameras show that sources emission.Ataconfidencelevelof90%thetemperaturekT
above the detection threshold of few ×10−16ergcm−2s−1 was below 1.0keV (assuming the thermalbremsstrahlung
generate in the 0.5 − 2.0keV band ∼ 90% of the inte- mechanism).
gral XRB. The resolved fraction below 0.5keV is consis- Accordingtosimulations,thesurfacebrightnessofthe
tent with 100%, although, with large uncertainty due to WHIM emission increases sharply as the distance to the
lack of precise measurement of the total flux. Moreover, gravitational center gets smaller. In the present investi-
So ltan (2003)estimatedthatroughly1%ofthesoftXRB gation we analyze the distribution of the XRB intensity
constitutes purely diffuse component (unrelated to the aroundgalaxiesatfew arcminscales,where the expected
WHIMemission)duetoThomsonscatteringofX-raypho- signal should by stronger. In Sect. 2 the observational
tons.Thus,theupperlimitsestablishedfortheWHIMare material is described. Calculations and main results are
rather restictiveand impose important constraintsonthe presented in Sect. 3 while in Sect. 4 we shortly discuss
feedback models (Bryan & Voit 2001). limitations of the present work.
Despite the poor understanding of processes involved
andlimitedprecisionofcalculations,hydrodynamicalsim-
2. Observational data
ulations provide information on physical parameters and
spatial structure of the WHIM. The advanced models are Bothsimulations(Croft et al. 2001)andextrapolationof
flexibleenoughtoconformtotheobservationallimits.For our results based on the RASS (So ltan et al. 2002) show
instance, the simulations by Croft et al. (2001) predict that the strength of the WHIM emission is low in com-
that in the 0.5−2.0keV energy band ∼ 6% of the XRB parison with the total soft XRB even within 10 arc min
is generatedby the intergalacticgaswithtemperaturesin from a typical galaxy in the sample. It appears that the
the range of 105−107K. amplitude of the the cross-correlation function (CCF) of
The low emission strength makes the direct de- thesoftXRBandgalaxydistributiongeneratedbyWHIM
tection of the WHIM signal extremely difficult. To at separations above 2′−4′ would not exceed a couple of
search for traces of the extragalactic thermal radiation, percent. To detect fluctuations of the XRB at such low
Kuntz et al. (2001) analyzed the surface brightness dis- level, scrupulous analysis of an extensive data sample is
tribution of the ROSAT All-Sky Survey (RASS) at high required.
galactic latitudes. They found that after subtraction of
galactic and extragalactic sources and the local extended
2.1. PSPC data
emission, the data are consistent with the smooth ther-
mal component with kT = 0.23keV and amplitude of ThecentralareaoftheROSATPSPCfieldofview(unob-
1.20×10−8erg/(scm2srkeV) in the 3keV band. In the structedbythewindowsupportingstructure)isacircleof
4
discussion the authors stated that the observed signal is ∼27′diameter.Similarsize,albeitofmorecomplexshape,
probably a mixture of the WHIM and the Galactic halo have the field of view of the XMM-Newton EPIC/MOS
emission. cameras. The excellent characteristics of the ROSAT X-
All the simulations show that the WHIM emission is ray telescope – PSPC combination make this instrument
stronglycorrelatedwiththegalaxydistribution.Thus,the well suited for searches of the low amplitude fluctuations
straightforward way to look for the WHIM presence is of the XRB. The PSPC has strikingly smooth sensitivity
to measure the XRB flux around galaxies and clusters. over the entire field of view. The X-ray telescope intro-
Recently we have investigated the cross-correlations be- duces practically negligible vignetting within the central
tween the soft X-ray maps of RASS and the galaxy dis- areaandcontaminationoftheacceptedcountsbythepar-
tributionusingtheLickcounts(Shane & Wirtanen 1967) ticle background is also quite low.
andAbellclustercatalog(Abell 1958,Abell et al. 1989). InasingleROSATobservationwiththeexposuretime
Coarse pixel binning of the RASS (12′ × 12′) and Lick of10000s,typically400PSPCcounts arerecordedin the
counts (10′×10′) allowedonly for the analysisof the cor- central area in the band R5 and R6, and ∼ 300 counts
relation signal at relatively large angular separations, ef- in R7 (if there are no bright sources in the field). To de-
fectively above 0◦.3 (So ltan et al. 2002; references to ear- tect relative fluctuations with the amplitude of ∼0.01 at
lier work on this subject therein). Our investigation was high significance level, one needs a large number of ob-
also impeded by the narrow useful energy range and the servations. In order to maximize the signal-to-noiseratio,
low energy resolution of the RASS maps. The data we all the ROSAT PSPC pointings at high galactic latitude,
used were binned in three overlapping energy bands: R5, devoid of bright or known extended sources, available in
R6 and R7 centered at 0.8, 1.1, and 1.5keV, respectively the archive have been used.
(Snowden et al. 1994). Since we concentrate on the diffuse extragalactic
Our calculations revealed a clear excess of the soft signal, only pointings satisfying stringent criteria have
emission associated with the galaxies and clusters. The been included in the analysis. In particular, pointings at
A.M. Sol tan et al.: Signs of WHIM in the soft X-ray background 3
known extended sources (SN remnants, clusters of galax- have been used, for band X2 – pointings with N <
H
ies, nearby galaxies) have been excluded. In selecting 1.5×1021cm−2,andforbandsX3andX4–pointingswith
“good”observationswegenerallyfollowedcriteriausedby N < 2×1021cm−2. All the data in the public archive
H
So ltan et al. (2001)tobuildasampleofobservationssuit- obtained with the MOS1 and MOS2 detectors have been
ablefortheinvestigationoftheautocorrelationfunctionof used. After excluding pointings with bright sources and
the soft XRB. In the present analysis we used also obser- knownextendedsources,thetotalnumberofusablepoint-
vations at high galactic latitudes from the south galactic ings reached 150. All point-like enhancements present in
hemisphere. the data have been removed in a similar way as in the
From nearly 300 PSPC observations we have selected PSPC observations.
217 for further processing. The sample contained point- Becauseofthevignettingthecountdistributionwithin
ings with the Galactic hydrogen column density below the field of view is highly nonuniform. Exposure maps
7×1020cm−2.Next,allthesourcesdetectedineachenergy created for each energy band include vignetting effects.
bandhavebeenremoved.We haveapplied the Poissonian To compensate vignetting, one should correct the image
statistics to search for point-like enhancements exceeding focused by the X-ray telescope using the exposure map.
the3σ threshold.Suchprocedureeffectivelyeliminatesall However, amount of contamination by the non-vignetted
high amplitude non-homogeneities of the count distribu- counts varies in time and for each pointing is different
tion and substantially reduces noise of the CCF. Also, (Read & Ponman 2003). In effect, the vignetting correc-
circular areas around all clusters and groups of galaxies tions are not adequately described by the exposure map
foundinthe NASA/IPAC ExtragalacticDatabase(NED) andhavetobedeterminedforeachobservationseparately.
have not been used in the CCF calculations. After the Also, the source extraction introduces some additional
removal of all the sources and clusters, total numbers of biasforthevignettingamplitude.Amountoftheremoved
counts in the data amount to 119000 in the band R5, photons depends on the exposure time (for longer expo-
102000 in R6 and 69000 in R7. suresalargerfractionoftheXRBisdecomposedintorec-
ognized sources). Thus, the deep exposures are relatively
more contaminated by the non-vignetted counts.
2.2. EPIC/MOS data To minimize gradients of the count distribution gen-
erated by the telescope the following procedure has been
AlargecollectiveareaoftheX-raytelescopesoftheXMM- applied. We assumed that some, a priori unknown, frac-
Newton Observatory gives higher count rates as com- tion of counts, f, is subject to the vignetting while the
pared to the ROSAT PSPC. Also a wide energy range remaining 1−f counts are free from the vignetting and
andgoodenergyresolutioncontributetotheuniquequal- are distributed randomly in the field of view. Thus, the
ity of the data. However, investigation of the extended large scale variationsobservedin the integralcounts have
diffuse emission is hampered by strong vignetting of the smaller relative amplitude than implicated by the expo-
telescope, highly variable non–X-ray background and im- sure map, EM. The effective exposure correction, EC,
perfections of the CCD detectors. All these effects could applicable to the actual observationwas assumed to have
be accounted for with relative ease in case of the point the form:
source analysis, but pose a serious problem for the inves-
EC =(1−f)·t +f ·EM, (1)
tigation of the smoothly varying signal. Elaborated pro- av
cedures to deal successfully with the instrumental effects where t is the average exposure time. The observed
av
in the context of the background measurements are pre- counts were divided by the EC array and the resulting
sented by Lumb et al. (2002), Read & Ponman (2003) “corrected”countswerefittedtotheflatdistribution.The
and De Luca & Molendi (2004). These investigations al- value of f was adjusted using the maximum-likelihood
lowtomeasuretheabsolutelevelofthecosmicsignalwith method.Thecalculationshavebeenperformedseparately
the accuracy of several percent. In the present investiga- for eachobservation,energyband (X1,..., X4)anddetec-
tion we do not attempt to determine the net XRB flux, tor (MOS1 and MOS2)
but we concentrate on the XRB fluctuations correlated
with the galaxy distribution. Although the expected am-
2.3. Galaxy data
plitude of this signal is smaller than uncertainties of the
total XRB measurements, the correlation analysis can be For all the X-ray observations (PSPC and EPIC/MOS)
used to isolate effectively the WHIM emission. the NED has been searched for galaxies brighter than 20
To select observations suitable for the present inves- magorwithintheredshiftrangeof0.02−0.20.Nearly3400
tigation, criteria analogous to the PSPC pointings have objects have been extracted from the database. Clearly,
been applied. In preparationof the data we generally fol- the selected galaxies do not constitute a complete or ho-
lowed prescriptions by Snowden et al. (2002). The data mogeneous sample. Pointings are covered by a variety of
have been divided into four energy bands: 0.3−0.5keV galaxysurveys.Ineffect,thenumberofgalaxieswithinthe
(band X1), 0.5 − 1.0keV (X2), 1.0 − 2.0keV (X3) and fieldofviewoftheindividualpointingdependsonthespe-
2.0−4,5keV (X4). For the band X1 only pointings with cific survey characteristics and to less extent reflects sta-
theGalactichydrogencolumndensityN <5×1020cm−2 tisticalpropertiesofthe galaxypopulationattheselected
H
4 A.M. Sol tan et al.: Signs of WHIM in the soft X-ray background
Fig.1.ThecountratedistributioninthreeROSATPSPC Fig.2. The X-ray “colour-colour”diagram for the PSPC
energy bands vs. angular distance to the galaxy averaged observations. The excess emission surrounding galaxies is
over the galaxy sample. Power law fits are shown with shown with the full dot; solid ellipses indicate 68%, 90%
dashed lines. and 99% confidence limits. The cross above the center of
the plot indicates the colours of the average total XRB.
The thermal bremsstrahlung emission (upper curve) and
magnitude limits. Nevertheless,galaxies found within the
theplasmaemissionwithmetalabundancesat20%ofthe
field of view of each pointing separately represent an un-
cosmicabundances(lowercurve)areshown;labelsdenote
biased set of objects suitable for calculations of the CCF
temperatureinkeV.Coloursoftheenhancementsfoundin
with the X-ray photon distribution.
theRASSmapsareshownwiththeopensymbols.Square
andcirclesindicateemissionofhalosaroundAbellclusters
3. Diffuse emission in the PSPC and EPIC/MOS
(with the 90% confidence region defined by the dotted
observations ellipse) and galaxies in the Lick counts, respectively (see
thetextfordetails).Starsattheupperleftshow25bright
To estimate the average intensity of the XRB at distance
ROSAT clusters of galaxies.
θ from a randomly chosen galaxy we use the formula:
Pn
cnt
ρ(θ)= , (2)
Pn ·t
pxl exp
hypothesis that ρ(θ) = const. over the separation range
where the sums extendoverallpointings andallgalaxies, of 2′ − 15′ is rejected at 5.4σ in the band R5, 4.5σ in
ncnt denotes total number of counts recorded in npxl pix- R6, and 2.9σ in R7. The S/N ratio is assessed under the
els separated by angle θ from the galaxy and texp is the assumptionthat uncertaintiesof allpoints (for eachband
exposuretimeoftheobservation.AllthePSPCdatawere separately)areequal.Totesttheefficiencyofouranalysis,
binned into 8′′×8′′ pixels and EPIC/MOS into 4′′×4′′. analogous calculations have been performed in which we
In the absence of any signal correlated with galaxies, one searched for the possible enhancements of the soft X-ray
should expect ρ(θ)=ρ, the average XRB flux. emissionaroundquasarsandknownweak X-rayssources.
Sincethegreatmajorityofobjectsinthosetwocategories
are much more distant than objects in the galaxy sam-
3.1. PSPC
ple,wehavenotexpectedtodetectanycorrelationsignal.
The distributions of ρ(θ) in the three energy bands, R5, Unfortunately, the number of objects in this “test sam-
R6, and R7, are shown in Fig. 1. Scatter of points for ple” was much smaller than number of galaxies. Because
each energy band is slightly larger than the statistical ofthatuncertaintiesoftheCCFestimateshavebeensub-
noise implied by the number of “galaxy – X-ray count” stantiallylarger.Nevertheless,thedifferencesbetweenthe
pairs.Additionalfluctuationsoftheρ(θ)aregeneratedby galaxy and the test sample were significant.No X-ray en-
weak sources not removed from the data. A quantitative hancements around the test objects have been detected
analysis of plots in Fig. 1 reveals weak but statistically and in the soft band R5 the test data were incompatible
significant systematic trends in all three bands. The null with the galaxy signal at the level of 3.9σ.
A.M. Sol tan et al.: Signs of WHIM in the soft X-ray background 5
To measure the excess emission correlated with the
galaxy distribution linear fits to the plots in Fig. 1 have
been calculated. The slopes of those fits for three energy
bands determine the enhancements potentially generated
bytheWHIM.Thesedatawerethenusedtoconstructthe
X-ray“colour-colour”diagramshowninFig.2inthesame
way as the diagrams for the RASS maps in So ltan et al.
(2002). Alargesoliddotindicatesa positionofthe excess
emission. Solid ellipses define 68%, 90% and 99% con-
fidence limits based on the simulations. The large cross
situated above the center of the plot gives the average
colours of the integral XRB.
CurvesinFig.2showthelociofthe thermalemission:
the upper curve indicates colours of the bremsstrahlung,
whilethe lowercurve–plasmaemissionwithmetalabun-
dances reduced to 20% of cosmic abundances. Labels
give temperatures in keV. The X-ray colours of the en-
hancements found by So ltan et al. (2002) in the RASS
maps are shown with open symbols. The open square de-
notes the colours around the Abell clusters (the dotted
ellipse represents the 90% confidence limits), and open
circles – around galaxies. The enhancements are inte-
Fig.3. The cross-correlation function of the ROSAT
gratedbetween0◦.3and2◦.1forclusters,whilefortheLick
PSPC bands R5 + R6 and the galaxy sample: the best
counts the results are shown for 4 separationbins: <0◦.3,
fit - thick line, selection of power law fits allowedby 68%
0◦.3−0◦.7,0◦.7−1◦.5,and1◦.5−3◦.1,wherethesmallersymbol
confidence limits - thin lines; plots for separationsgreater
in Fig. 2 refers to the larger separation.For both samples ′
than15 areextrapolated.Pointswiththeerrorbarsshow
◦
1 corresponds to several Mpc and it is likely that some
the CCF of the RASS maps (R5 + R6) with the Lick
contribution to the X-ray halos detected in the RASS is
counts of galaxies (see the text for details).
generatednotonlybythediffuseWHIMemission,butalso
by the hot gas in poor groups of galaxies which surround
the rich Abell clusters and have typical temperatures of
the order of 107K.
InFig.3weplottedalsotheCCFoftheRASSwiththe
Stars in the upper left corner show the colours of 25
galaxy distribution of the Lick counts (see So ltan et al.
“normal” clusters of galaxies selected from the ROSAT
(2002)forthedetailsofcalculations).Errorbarsrepresent
archives. Clear separation of clusters and the extended
1σ uncertainties based on the simulations. Near perfect
emission in Fig. 2 demonstrates that at least substantial
agreementbetweentheROSATpointingsandtheAll-Sky
fraction of the diffuse emission correlated with galaxies
Surveyistosomeextentaccidental,sincethegalaxysam-
does not originate in the “unknown” clusters but repre-
plesinbothinvestigationshavedifferentstatisticalproper-
sents a distinct component of the soft XRB.
ties;inparticulartheyhavedifferentmagnitudelimitsand
Next,wehavetriedtofittheobservedvariationsofthe
undoubtedly different redshift distributions. It is evident,
the X-ray surface brightness enhancements around galax-
however, that estimates of the CCF at small separations
ies by a power law:
areconsistentwith the extrapolationbasedon the RASS.
ρ(θ)=aθγ +ρ , (3)
o
with three a priori unknown parameters a, γ and ρo, 3.2. EPIC/MOS
where ρ represents the XRB component uncorrelated
o
with galaxies. In this case, however, the signal-to-noise SearchfortheWHIMemissionintheEPIC/MOSobserva-
ratio was too low to obtain restrictive limits for the in- tionswasrealizedinasimilarwayaswiththePSPCdata.
teresting parameters. The best fit values for the slope γ First, we have determined the ρ(θ) in four energy bands.
were equal to −0.7 for bands R5 and R6 and −1.3 for Te eliminate residual effects introduced by vignetting, we
R7, but the range of uncertainties was wide at any rea- have compared the real data with data from simulations.
sonable significance level. To improve statistics, we have In a single simulation run the distribution of galaxies in
repeated all the calculations using merged bands R5 and eachpointingwasrandomizedandthecalculationsofρ(θ)
R6. The allowed range of fits still covers a large area in wereperformedinthesamewayasfortherealdata.Then,
parameterspace(Fig. 3). This is mostly because the data the CCF of the X-ray and galaxy distributions was ob-
cover a narrow range of separations. Additionally, we do tainedbysubtractionoftheaveragedsimulatedcountrates
not have a priori sufficiently accurate information on ρ ρ(θ)fromtheobserveddata.Forfourenergybands30full
o
andallthree parametershaveto be fitted simultaneously. simulationrunshavebeenperformed.Thermsscatterbe-
6 A.M. Sol tan et al.: Signs of WHIM in the soft X-ray background
Fig.4. The cross-correlation function of the galaxy sam- Fig.5.Fullcircleswith1σerrorbars–averageexcesssur-
ple and the EPIC/MOS data in the energy band 0.3− facebrightnessmeasured2′ fromthegalaxyinthesample
0.5keVforseparations1′−15′.Thedashedlineshowsthe in 4 energy bands based on EPIC/MOS data; remaining
′ ′
linear fit to the data points between 2 and 15, dashed symbols–expectedcountratesofthermalbremsstrahlung
curve – the power law fit with γ =−1. for 0.2, 0.5, 1.0 1nd 2.0keV with normalization fitted to
the observational points.
Table 1. Excess countrates
Band Energy (keV) Countrates [cnt/pxl/s] Thespectrumoftheobservedenhancementscorrelated
X1 0.3 – 0.5 (1.80±0.23)×10−8 withthegalaxysampleisroughlyconsistentwiththether-
X2 0.5 – 1.0 (1.50±0.30)×10−8 mal emission with kT ∼ 0.2keV and becomes incompat-
X3 1.0 – 2.0 (0.37±0.42)×10−8 ible with kT >∼ 0.5keV. The amplitude of the signal is,
X4 2.0 – 4.5 (0.62±0.38)×10−8 however, very weak. In the band X1 (0.3 − 0.5keV) it
amounts roughly just to 1.3% of the total countrate.
Although, both the PSPC and EPIC/MOSdata show
tween simulated functions ρ(θ) was used as estimator of
traces ofthe WHIM emission,the amplitude of the signal
uncertainties in the real data.
obtainedfromthePSPCisroughlybyafactorof2greater
Despite the large number of pointings used in the
thanfromthe EPIC/MOS.We havecomparedthe ampli-
present calculations, the signal-to-noise ratio of the cor-
tudes of the correlated emission in the band R5 and X2,
relation measurement is still low. In Fig. 4 the CCF of
as they cover similar energy ranges. Assuming a temper-
the energyband X1 (0.3−0.5keV) is shownwith the two
′ ature of the bremsstrahlung emission kT = 0.5keV, and
bestfits tothe datapoints forseparationsbetween2 and
15′:linear(dashedline)andpowerlawwithγ =−1(solid correcting for effects of the absorption by cold gas in the
Galaxy1 the excess surface brightness in the energy band
curve).Clearly,the S/Nratioistoolowtodistinguishbe-
0.5 − 1.0keV measured by ROSAT is equal to (1.41 ±
tween those two solutions. Nevertheless, the CCF signal
0.26)× 10−13erg/(cm2·s·deg2), while the XMM-Newton
is detected at a decent significance level. The slope of the
data provide (0.71±0.14)×10−13erg/(cm2·s·deg2). Two
linear fit differs from zero by more than 7σ.
factorscouldcontributetothisdifference.First,theobser-
Using the slope determination we estimated the aver-
′ vational material is heterogeneous. It spans a wide range
age excess countrate at separation of 2 above the level
′ of exposure times. In particular, the XMM-Newton sam-
measured at 15. The results for four energy bands are
plecoversmorethan17deg2,buthalfofthecountscomes
listed in the Table 1 and shown in Fig. 5. Then, we have
fromjust above3.6deg2.The distribution ofexposures in
convolved the thermal bremsstrahlung spectra with the
theROSATsampleisalsoverynonuniform.Consequently,
effective area of the X-ray telescope/EPIC MOS detec-
the effective area of the survey is much smaller than the
tors system and fitted the resulting counts in four energy
bandstotheobservedsignal.Thepredictedcountsofther- 1 TheaveragehydrogencolumndensityintheROSATsam-
mal emission for kT = 0.2, 0.5, 1.0 and 2.0keV are also ple amountsto 2.25×1020cm−2, while in theXMM sample –
plotted in Fig. 5. 2.77×1020cm−2.
A.M. Sol tan et al.: Signs of WHIM in the soft X-ray background 7
total area and it is possible that the effect of cosmic vari- ACKNOWLEDGMENTS. The ROSAT project
ance is partially responsible for different results. has been supported by the Bundesministerium fu¨r
Second,thegalaxiesselectedfromNEDalsoconstitute Bildung, Wissenschaft, Forschung und Technologie
highlyheterogeneoussample.Althoughtheselectioncrite- (BMBF/DARA) and by the Max-Planck-Gesellschaft
riaforthePSPCandEPIC/MOSobservationswereiden- (MPG). We thank all people involved in the XMM-
tical, the average apparent magnitudes and surface den- Newton project for making the XMM Science Archive
sityofgalaxiesaresubstantiallydifferentinbothsamples. and Standard Analysis System friendly environments.
TheaveragegalaxymagnitudeselectedintheEPIC/MOS AMS thanks the Max-Planck-Gesellschaft for sup-
dataisfainterby0.4magthaninthePSPCdata.Itisdif- port. This research has made use of the NASA/IPAC
ficult to assess impact of those systematic effects, but it Extragalactic Database (NED) which is operated by
is likelythatgalaxiesselectedinthe EPIC/MOSobserva- the Jet Propulsion Laboratory, California Institute of
tions areonthe averagemore distantthanin theROSAT Technology,undercontractwiththeNationalAeronautics
observationsand the measuredCCF signal at fixed angu- and Space Administration. This work has been partially
lar separation is weaker. supported by the Polish KBN grant 1 P03D 003 27.
The amplitudes measured by both instruments differ
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