Table Of ContentDetection ofthermalX-ray emissioninthe haloofthe plerionic
5 supernovaremnant G21.5-0.9
0
0
F.Bocchinoa
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aINAF- OsservatorioAstronomico di Palermo, Piazza delParlamento 1, 90134 Palermo, Italy
n
a
J
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2 Abstract
1 ThedetectionofasoftthermalX-raycomponentinthespectrumofabrightknotinthehalooftheplerionG21.5-
v 0.9 is reported. Using a collisional ionization equilibrium model for an hot optically thin plasma, a temperature
7 kT =0.12−0.24kev,amassof0.3–1.0M⊙ andadensityof1.6-6cm−3 isderived.Thespectralanalysissuggests
1 apossibleoverabundanceofSiliconwithrespecttothesolarvalueintheknot;ifthiswillbeconfirmedthisobject
5 may beaclump ofshocked ejecta.
1
0
5 Keywords: PWN,ejecta, G21.5-0.9, plerion, X-rays
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p
1. Introductionand data analysis namelythe NorthSpur (Warwicketal.2001).
-
o For XMM-Newton, the data consist of the
tr The plerionG21.5-0.9has beenthe subject observation of G21.5-0.9 with the source posi-
s ofvariousstudiesinradio(Bocketal.2001andref- tioned on the axis and off the axis of the EPIC
a
: erencestherein),infrared(GallantandTuffs1998) cameras(observationID0122700101,0122700201,
v
andX-ray(Safi-Harbetal.2001;LaPalombaraandMer0e1g2h2e7tt0i0301,0122700401,0122700501),whichhave
i
X 2002).However,itsweakX-rayhalowasdiscovered atotalsummedlivetimeof142ksforPNand146
r only recently (Slane etal. 2000; Warwicketal. ks for the MOS cameras. The observations were
a 2001) thanks to the larger sensitivity of Chan- takenonApril7–17,2000.Theoriginaleventfiles
dra (Weisskopfetal. 2000) and XMM-Newton werescreenedtoeliminatethecontributionofsoft
(Jansenetal. 2001) with respect to previous X- proton flares. After the screening, the summed
rayastronomysatellites.Theweaknessofthehalo exposure time were 79 ks for PN and 116 ks for
has prevented a detailed study of its features up MOS. All the analysis of XMM data was per-
to now. Luckily, G21.5-0.9 is a calibration source formed with the software SAS v6.0. The XMM-
for both Chandra and XMM-Newton and a large Newton spectra of the North Spur, collected in
number of calibration datasets have been accu- the single observations using a circular region of
mulated and is still being accumulated to this 20arcsecradius centeredatlocation18h33m32.7s
purpose. In this paper, this large collection of RA and -10d32m49s DEC (J2000), were summed
publicdataisusedtoinvestigatethenatureofthe and their response averaged. The spectra of the
brightest spot in the X-ray halo of this plerion, two MOS cameras were also summed and their
Preprintsubmitted toElsevier Science 2February2008
effective areas and response matrices were aver- sources.Thebackgroundwasselectedinthesame
aged.ThefitwasperformedseparatelyforthePN wayastheXMM-Newtonbackground.Allthefits
andMOS,andthen, after checkingthatthey give wereperformedusingXSPECv11.3.
consistentresults,theywerefittedsimultaneously.
The spectra have been rebinned to a minimum of
20counts per bin andthe 0.5–9keV energyrange
2. Resultsofspectralfitting
was selected for the spectral fitting. The back-
groundusedforthespectralanalysiswascollected
in an annulus centered on 18h33m33.8s RA and The spectra were first fitted with a power
−10d34m07s DEC J2000 between 85′′ and 120′′ law model modified by the interstellar absorption
fromthecenter(thisregionisinsidetheX-rayhalo ofMorrisonandMcCammon(1983)andareason-
at the same average off-axis angle as the North ablefitwasfound.Allthe best-fitparametersand
Spur). This method subtracts the contribution of their uncertainties are reported in Table 1. How-
the underlying continuum due to diffuse emission ever, given the non uniform distribution of resid-
of the X-ray halo from the source spectrum, and uals in the soft band and the possibility that the
concentrate the study on the intrinsic spectral North Spur may show signs of thermal emission
propertiesofthe NorthSpur. duetohotplasma,aspartofashellorejectaasso-
For Chandra, 21 observations from the ciatedtoG21.5-0.9,the spectrawerefittedwitha
Chandra archivefor which the sourcewas located combinationofapowerlawmodelandthemekal
on the ACIS S3 chip were selected, in order to optically-thinthermal plasma model (Mewe etal.
maximize the instrument response in the soft X- 1985,Liedahletal.1995).Theresults,alsoshowed
rayband,andatoff-axisangle<5′.Thesequence in Table 1, indicate that the two components fit
numberoftheselectedobservationare0159,1433, is better then the power law only fit. The decre-
1554, 1717, 1769, 1771, 1839, 2873, 3693, 4353, ment of χ2 is significant according to the F-test
5166,1230, 1553, 1716, 1718, 1770, 1838, 1840, (probability∼10−4),andthedistributionofresid-
3474,3700,4354.The datasetsinclude the dataset uals is more homogeneous. The discrepancy be-
used by Safi-Harbetal. (2001), the latest report tween XMM-Newton and Chandra derived fluxes
on X-ray data of the North Spur. However, they areduetodifferentextractionregionareaandsug-
usedonly6observationforatotalof65ks,whereas gests that the source is extended (especially its
we also include all the more recent observation non-thermalemission).
that have been done, for a total of 196.5 ks. This The XMM-Newton and Chandra spectra
is the reasonwhy they did not detect the thermal are shown in Fig. 1 and 2, along with the best-fit
emission reported in this work. The observations two component model. Emission lines of Mg XI
were reprocessed using CIAO 3.1 and CALDB at 1.34 keV and Si XIII at 1.86 keV may be iden-
2.28 in order to apply the most recent calibra- tified in FIg. 2. Other expected emission lines at
tions to all the data, including time dependent different energies are not evident due to limited
gaincorrection.Afterwards,thedatawerefiltered statistics of these highly absorbed spectra and
to retain grade 0,2,3,4,6 and the status column moderateCCDspectralresolution.
equal zero. All the filtered datasets were merged Fig. 3 shows the χ2 contour levels in the
together using the CIAO merge all task. The temperature-emission measure plane correspond-
spectrum of the North Spur was extracted using ingtoconfidencelevelsof68%,90%and99%onpa-
a circular region centered at the same location as rameteruncertainties. Fig. 4 shows the same con-
before, but with a smaller radius (13 arcsec) to tourslevelsofFig.3,but forthe Mg andSiabun-
exploit the high resolution of the Chandra mir- danceswhentheseparametersareleftfreetovary.
rors. The spectrum was rebinned like the EPIC Theχ2obtainedinthiscaseisbetterthanthecase
spectrum and the band 0.5–8 keV was used. The of abundances fixed to solar values (F-test proba-
responsematrixandeffectiveareaweregenerated bility of 2×10−3), but the solar values for both
using the CIAOtasksfor the analysisofextended elementalabundancesarestillwithinthe99%con-
2
Table1
Summaryof spectral fitting results to the X/rayspectrum of the brightknot “North Spur.
Model NH γ Norm kT fluxa χ2/dof
cm−2 photon cm−2 s−1 keV−1 keV ergcm−2 s−1
×1022 index ×10−4 ×10−15
XMM-NewtonPN and MOS
Power-law 1.45±0.131.91±0.12 13.9±2.5 - - 442/407
P.L.+MEKAL 2.63±0.302.30±0.15 29.3±6.5 0.15±0.03 12.0±4.5 423/405
P.L.+VMEKALb2.63±0.392.26±0.13 28.5±7.6 0.17±0.03 11.4±5.6 410/403
Chandra ACISS3
Power-law 1.15±0.091.72±0.10 9.3±1.2 - - 237/187
P.L.+MEKAL 1.78±0.322.04±0.16 15.4±3.7 0.17±0.04 5.3±1.3 217/185
P.L.+VMEKALc 1.81±0.242.01±0.14 14.8±3.0 0.15±0.03 7.9±2.3 206/183
a Absorbed fluxdue to the thermal component only, inthe 0.5–2.0 keV.
b OnlyMgand Si abundances werefitted (best-fit values and 90% uncertainties are 1.0(0.1–
2.3) and 4(1.3–8), respectively). Flux of Mg XI line at 1.34 keV is 3.4+−21..08×10−6 ph cm−2
s−1, whereas flux of SiXIII lineat 1.86keV is 8.7+−65..23×10−6 phcm−2 s−1.
c OnlyMgandSi abundances werefitted (90% confidence rangeis>0.6forMgand>5for
Si). Flux of Mg XI line at 1.34 keV is 2.8+−12..55×10−6 ph cm−2 s−1, whereas flux of Si XIII
lineat1.86 keV is9.5+−34..72×10−6 phcm−2 s−1.
fidence level.
Fig. 1. XMM-Newton PN and MOS spectra of the bright
knot named North Spur in the X-ray halo of the plerion
G21.5-0.9. Solid line corresponds to the best-fit obtained
with a thermal+non thermal model. We also show the
contribution of thermal and non-thermal components to
the total best-fit spectra (forMOSonly). Fig. 2. Sameas Fig. 1but forChandraACIS S3.
3
otherhand,Mgabundanceisfoundtobecompat-
0.25
ible withthe solarvalue.
The best-fit Si abundance is not compati-
blewith the abundancesfoundinshocksofyoung
eV) 0.20 SNR interacting with the circumstellar material
k
e ( (see, e.g., Parketal. 2004). Instead, it may indi-
ur
at cate that the North Spur is an ejecta knot. The
er
mp 0.15 fact that the Mg abundance ranges between 0.3
Te and 2 with a best-fit of ∼ 1 may imply that we
are seeing at a Si-rich knot like the one observed
in Cas A (Laming andHwang 2003) and in other
0.10
remnant. However, given the lack of other emis-
12 13
10 10
Normalization (cm-5) sionlineinthespectrumandtheoverallweakness
ofthe thermalcomponent,a full comparisonwith
theabundancepatternsobservedinejectaofother
Fig.3.68%,90%and99%confidencelevelcontoursforthe
plasma temperature and emission measure of the thermal young SNRs is not possible, so the interpretation
componentusedinthefitoftheNorthSpurXMM-Newton of North Spur as a Si-rich ejecta knot needs more
EPIC(thinlines)andChandraACIS-S(thicklines)spec- datato be confirmed.
tra.
Assuming a reasonable line of sight of the
emitting plasma in the North Spur (about 1 pc),
adensityofthe knotof3.8(1.6−6)×d−1/2 cm−3
5
10.0 were derived (where d5 is the distance in units of
5 kpc, BeckerandSzymkowiak 1981), and a total
ce massofthethermalplasmaof0.7(0.3−1)×d5−5/2
n
da M⊙.Bothdensityandmassareconsistentwithan
n
u ejectaofaTypeIISN.However,themeasuredtem-
b 1.0
a
Si perature (between 0.12 and 0.2 keV, considering
both the EPIC and ACIS measurements) is lower
then typical ejecta knot temperature measured in
youngSNRs(e.g.LamingandHwang2003forCas
0.1 A).
1
Mg abundance This discrepancy can be eliminated if we
assume that the ejecta we are observing have not
Fig. 4. Same as Fig. 3 but for Mg and Si abundances been heated by the passage of the reverse shock,
relativeto solar. butbythe supersonicshockofthe expandingpul-
sar wind nebula. The expansion of a PWN in the
3. Discussion
freelyexpandingejectaofitsSNRhasbeenstudied
by several authors (Reynolds andChevalier 1984,
The best-fit Si abundances obtained from
vander Swaluwetal. 2001, Chevalier 2004). The
the fit is around 4 and its 90% confidence level
X-rayemissionassociatedtothe PWNsupersonic
range is between about 2 and 10 for the XMM-
shockintotheejectahavebeenobserveduptonow
Newton EPIC and above 5 for Chandra ACIS-S.
only in 3C58 (Bocchinoetal. 2001), while for the
Even though the solar value cannot be excluded
Craband0540-69hasbeenobservedintheoptical
at a 99% confidence level for XMM-Newton, the
(SankritandHester 1997, Kirshneretal. 1989).
fact that both Chandra and XMM-Newton inde-
Therearenootherestablishedobservationalsigna-
pendentlygiveabest-fitSiabundanceabovesolar
tureofPWNexpandinginto ejecta (see Chevalier
canbeconsideredahintthatthiselementissome-
2004forareview).In3C58,Bocchinoetal.(2001)
what overabundant with respect to solar. On the
4
measuredanX-raytemperatureof0.2keV,which MillimeterWavelengths,ApJ,561,L203–L206,
is remarkably similar to the one found here in 2001.
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sume a similar evolutionary stage for G21.5-0.9, pernovaremnantG21.5-0.9,PASJ,40,347–356,
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