Chapter 3 39
3.4 Spetra Extration
The spetra extration is performed, using the SAS evselet task. At this stage
the event le has been leaned and ltered in the spae phase of energy, time and
6
The CosmiX-ray Bakground is believed to be mainly produed by an unresolved osmi
inthephotoneventsquality. In termsof the photonenergy, wehaveonlymade use
ofphotons withenergy lyinginthe intervalof0.2keV to7.5keV. Themain reasons
torestrit the photon energies to the former interval, is the pooralibration below
0.2 keV and the presene of instrumentallines above 7.5keV. However, during the
spetraltproedure,theenergyintervalwasonstrainedtotheintervalof0.25keV
to7.5keV.The reason tostart thex-rayspetraltat 0.25keV and not at0.2keV
isrelatedtothe fat,thatthe APECplasmaode hasalak ofemissionlinesbelow
0.25 keV. Therefore, x-ray data below0.25 keV were not inluded during the t.
The event lehas also been ltered with aGTI le, inorder to remove periods
aeted by soft proton ares (SPFs). The photon quality is ensured by using the
keyword (PATTERN==0) in the lter expression. This means, that spetra will
onlybe omposed by single-pixel x-ray photons. If we would allowphotons with a
(PATTERN
>
0), then the spetrum would also be omposed by photons, whihhitthe boundaryof the pixel. When the photon hits a regionnear the boundaries,
partofthe depositedharge islosttoanearby pixelandtwophotons willberead
out, faking a dierent spetrum. Also, in order to avoid problems resulting from
extrating spetra from regions of the detetor aeted by the out-of-time events,
due to bright x-ray soures, the assoiated out-of-time olumns and nearby ones
were removed. By doing so, we ensure that our spetrum is as little as possible
ontaminated by OOTspetra of bright soures.
The analysis of an astrophysial spetrum requires the existene of a proper
instrumentalbakgroundspetrumforsubtrationpurposes. Fortheaseofspetral
analysisof pointsoures itissuient touse somenear regionto thesoure onthe
CCD, to obtain a bakground spetrum. This simple tehnique will subtrat from
the spetrum all spetral features assoiated to the detetor. In priniple, with
this proedure, not only all spetral detetor features are removed, but also the
astrophysialbakground spetrumis removed fromthe soure spetrum.
The tehnique desribed above is not appliable, when analyzing the spetrum
of diuse x-ray soures, whih ll all eld of view of the detetor. And in
partiu-lar, when we are interested in studying the astrophysial bakground itself. The
astrophysialbakground is omposed by several omponents of loal, galati,
in-tergalatiand osmi origin. The loalemission ontribution an be omposed by
somesmallperentageofx-rayemissionduetosolarwindhargeexhange(SWCE).
Thegalationtribution, whihisnot isotropiinnature,isdue tothe Loal
Bub-ble(LB),the hot interstellargas and duetothe hotgalatihalo. Theintergalati
omponentisbelieved tobe due tothe hot baryons, while the osmiomponentis
theresult ofallontributionsonanunresolved populationof AGNs. Therefore, our
spetrum is made by these ontributions, whih must be taken into onsideration
whenmodellingthe spetrum. However, the maindiultydoesnotarrive,in
prin-iple, from these astrophysial omponents themselves, but from the instrumental
bakground, inonjuntion with shortobservation times.
To study these several of x-ray astrophysial bakgrounds, a new method had
tobe developed to deal with the instrumental bakground. The adopted approah
is to onsider, that a representative instrumental bakground spetrum an be
ob-tained froma losed lter EPIC-pn XMM-Newton observation. There is, however,
anumberof requirementsto follow whenusing this approah.
In the rst plae,the same lteringriterionasusedfor the sieneobservation
must be appliedfor the losed observation. Extremely important is the removal of
the exat detetor regions of the losed observations, as the ones orresponding to
the extrationsoures and OOT events.
Seondly,wemustverifythat ourobservationwith thelter wheelinthelosed
positionhadthesameoperatingmodeasthe sieneobservation. Thismeans,that,
if the sieneobservation used, forinstane, the Extended-Full-Frame (eFF) mode,
thenthe properlosedobservationmusthavethesameeFFmode. Theonsequene
ofnotusingalosedobservationwiththesameoperatingmodemeans,thattheOOT
events ontribution will be dierent with a subsequent ontamination of the data,
forinstane.
The third requirement to follow is, to verify that both, siene and losed
ob-servations, havethe same detetor operating settings.
The fourth and until now undeteted problem, to deal with, is the radiation
bakgroundlevelduringeahofthe observations-sieneand losed. Asmentioned
inthebeginningofthishapter,theenvironmentinwhihthespaeraftisembedded
is a harsh one. This means, that the XMM-Newton satellite is subjeted to a
multitude of types of radiation: osmi rays, soft protons, ionizing partiles and
others.
During the studyof thex-raysky bakground spetraobtained bythe EPIC-pn
amera,itwasrealized,thatthe parametersderived forthe power-law, representing
theCXB, haddierentvaluesfromtheones obtainedonotheranalyses tothe CXB
by Gendreau etal. [161℄ and Lumbet al.[158℄.
The dierene between these derived values was found to be the result of the
subtrationofthe skyspetrumfromanimproper detetor bakground spetrum.
Theonitbetweenthese dierentanalyseswasresolved afterrealizing,that,most
likely, not all MIP events had been properly deteted/removed on-board of the
XMM-Newtonspae observatory.
Sine the launhof the XMM-Newton satellite,the environmentradiationlevel
hasbeenmonitorizedusingtheolleteddatafromthenumberofmaximumionizing
partiles (MIPs) [159, 156℄. If the event amplitude, whih is read out for a pixel,
exeedsagiven threshold (3000adu
∼
15 keV),then not onlythe olumn,inwhihthis pixel is loated, but also the left and right olumns are rejeted by the
on-board software for the Full-Frame (FF), for the Extended-Full-Frame (eFF) and
for Large-Window (LW) modes. The number of rejeted olumns is written as
Disard Line Counter (NDISCLIN) to the PNAUX2 extension into the auxiliary
le(...AUX.FIT). The observed number of rejeted olumns, whih basially gives
the number of olumns in eah quadrant of the EPIC-pn amera, where a MIP is
deteted, were used toderive a numerial value, that ould beused asa orretion
fatorfor the exess ofdeposited hargeonthe EPIC-pn amera. This issue willbe
developedfurtherin
§
3.6whendisussingthemethod,usedtoalulatetheaveragenumberof rejeted olumns,and whereit isexplained how toorret it.
The next step ofthe algorithmisthe extrationof spetrafor sienti analysis
EPIC-pneld ofviewindetetor oordinates (DETX,DETY). The exatregions in
detetor oordinates of the losed observation are used to extrat a proper
instru-mental bakground spetrum. By doing so, we avoid the introdutionof problems
that would appear, when using distint detetor regions for the instrumental
bak-ground spetra relative to the sienti ones. The objetive of this proedure is
to redue possible eets arising from the detetor spatial behaviour, see Freyberg
etal.[156℄.
The three nal steps of the algorithm onsist on the reation of a
redistribu-tion matrix le(RMF) with the SAS rmfgen taskand the reationof ananillary
response le (ARF), with the SAS arfgen task for eah individual spetrum. The
SASrmfgen taskgenerates the appropriateresponses of the instrument asa
fun-tionof energy and PI hannelfor the extrated spetra, whilethe SASarfgentask
is used to alulate the proper eetive area, lter transmission and quantum
ef-ieny for eah spetrum. These spetra are then binned and orreted with the
suitabledetetor bakground spetra with the response of the instrument and with
the anillaryresponse using the HEASARC FTOOLS grppha 7
task.
We ould then proeed for spetral analysis with the X-Ray Spetral Fitting
Pakage 8
(XSPEC). However, beforethat,aninvestigationof the spetralsignature
of softproton ares wasonduted.
In the EPIC-mos ameras there is a substantial ative CCD area outside the
nominal eld of view, and dened by the optial bloking lter, that is normally
used to obtain the instrumental bakground spetrum. In general, it is assumed
thatthis zone isrepresentative of thetrue EPIC-mos amerasinternal bakground,
beauseitis not expetedthat the soft protonsand sky photons are foused by the
x-ray mirrorsto this region of the detetors.