TeV source HESS J1804-216 in X-rays and other wavelengths

0.6 MB·English

by  O. Kargaltsev

SUBMITTEDTOAPJFEBRUARY5,2008
PreprinttypesetusingLATEXstyleemulateapjv.11/26/04
THETEVSOURCEHESSJ1804- 216INX-RAYSANDOTHERWAVELENGTHS
O.KARGALTSEV,G.G.PAVLOV,ANDG.P.GARMIRE
ThePennsylvaniaStateUniversity,525DaveyLab,UniversityPark,PA16802,USA
SubmittedtoApJFebruary5,2008
ABSTRACT
ThefieldoftheextendedTeVsourceHESSJ1804- 216wasserendipitouslyobservedwiththeChandraACIS
detector on 2005 May 4. The data reveal several X-ray sources within the bright part of HESS J1804- 216.
The brightest of these objects, CXOU J180432.4- 214009, which has been also detected with Swift(2005
November3)andSuzaku(2006April6),isconsistentwithbeingapoint-likesource,withthe0.3–7keVflux
7 FX =(1.7±0.2)×10- 13ergss- 1 cm- 2. Itshardandstronglyabsorbedspectrumcanbefittedbytheabsorbed
0 power-lawmodelwith thebest-fitphotonindexΓ≈0.45andhydrogencolumndensitynH≈4×1022 cm- 2,
0 bothwith largeuncertaintiesduetothestrongcorrelationbetweentheseparameters. A searchforpulsations
2 resultedin a106speriodcandidate,whichhoweverhasalowsignificanceof97.9%. We foundnoinfrared-
opticalcounterpartsforthissource. Thesecondbrightestsource,CXOUJ180441.9- 214224,whichhasbeen
n
detected withSuzaku, is either extended or multiple, with the flux F ∼1×10- 13 ergscm- 2 s- 1. We found
a X
J a nearby M dwarf within the X-ray source extension, which could contribute a fraction of the observed X-
rayflux. Theremainingsourcesareveryfaint(F <3×10- 14 ergscm- 2 s- 1), andatleast someofthemare
3 X
likely associated with nearby stars. Although one or both of the two brighter X-ray sources could be faint

1 accretingbinariesorremotepulsarswith pulsarwind nebulae(hencepossible TeV sources), theirrelationto
v HESSJ1804- 216remainselusive. The possibilitythatHESSJ1804- 216ispoweredbytherelativistic wind
9 fromtheyoungpulsarB1800–21,locatedatadistanceof∼10pcfromtheTeVsource,stillremainsamore
6 plausibleoption.
0 Subjectheadings:X-rays: individual(CXOU J180432.4–214009,CXOU J180441.9–214224,HESS J1804–
1 216)—pulsars:individual(PSRB1800–21=J1803–2137)
0
7
0 1. INTRODUCTION nessdistribution;seeAh06fordefinition)isR.A.=18h04m31s,
h/ Recent observations with the High Energy Stereoscopic Decl.=- 21◦42.′0, with a 1.′3 uncertaintyin each of the coor-
p System (HESS) and other modern very high energy (VHE) dinates. ThedistributionoftheTeVbrightnessshowsanex-
- telescopes have revealed a rich population of TeV γ-ray tended source with elongated morphology(see the contours
o in Fig. 17 of Ah06). The size of the source, & 20′×10′,
sources (Aharonian et al. 2005). A significant fraction of
r
substantiallyexceedsthesizeoftheHESSpointspreadfunc-
t thesesourcesareassociatedwithvarioustypesofknownas-
s tion(PSF),≈6′forthisobservation(Ah06).Thelargeextent
a trophysicalphenomena(seeOng2006forareview). Thelist
of the TeV emission rules out its association with an AGN,
: ofGalacticTeVsourceswithfirmassociationsincludeshigh
v whichmeansthatHESSJ1804isaGalacticsource.
massX-raybinaries(HMXBs),supernovaremnants(SNRs),
i Ah06 point out that the TeV emission does not perfectly
X andpulsarwindnebulae(PWNe). ExtragalacticTeVsources
line up with any known sources in the field. Among possi-
aresofarrepresentedonlybyAGNs(mostlyblazars). Many
r
ble counterparts, Ah06 mention the young Vela-like pulsar
a of the newly discovered TeV sources are extended and re-
B1800–21and the SNR G8.7–0.1, both of which have been
solvedin theHESS images. Mostof the identifiedextended
detectedinX-rays(Kargaltsev,Pavlov,&Garmire2006aand
sourcesarePWNeandSNRs,althoughthereisanindication
Finley & Ögelman 1994, respectively). Ah06 also do not
that some HMXBs could also produce extended TeV emis-
dismiss the possibility that HESS J1804 and other unidenti-
sion (e.g., HESS J1632–478; Aharonian et al. 2006a, here-
fiedTeVsourcesbelongtoanewclassofobjectssometimes
after Ah06). Among the known Galactic TeV sources, only
dubbed“darkparticleaccelerators”(Aharonianetal. 2005a)
HMXBsarevariableinTeV,someofthemshowingvariations
becauseoftheapparentlackofcounterpartsoutsidetheTeV
withthebinaryorbitalperiod(e.g.,themicroquasarLS5039,
band.
Aharonianetal.2006b). TheextragalacticAGNsourcesap-
FollowingthediscoveryofHESSJ1804,thefieldwasob-
peartobepoint-likeatTeVenergiesandcanalsobevariable.
A quarter of the ≈ 50 VHE sources known to date1 servedinX-raysbytheSwiftX-rayTelescope(XRT)instru-
ment on 2005 November 3 (Landi et al. 2006) and Suzaku
do not have firm identifications, although possible counter-
X-rayImagingSpectrometers(XIS)on2006April6(Bamba
parts/associations have been suggested for some of them.
etal.2006). Landietal.(2006)detectedthreeX-raysources
HESS J1804–216 (hereafter HESS J1804), the brightest
inthe23.′6×23.′6SwiftXRTdetectorfield-of-view(FOV),at
amongsuchsources,hasbeenrecentlydiscoveredduringthe
distancesof13.′3,7.′4,and2.′0(positionaluncertainty∼5′′–
HESSGalacticplanescanin2004May–October(Aharonian
6′′)fromthe best-fitHESSposition(we willcallthemSw1,
et al. 2005). The “best-fit position” of the source (which is
Sw2,andSw3hereafter). Sw1andSw2hadbeenpreviously
closeto,butmaybedifferentfrom,thepeakintheTeVbright-
detected with ROSAT. Sw1 (= 1RXS J180404.6–215325),
Electronicaddress:[email protected] the brightest of the 3 sources, shows a very soft thermal-
1Seethecatalogsathttp://www.icrr.u-tokyo.ac.jp/m˜orim/TeV-catalog.htm like spectrum (kT ≈ 0.3 keV for an optically thin thermal
andhttp://www.mpi-hd.mpg.de/hfm/HESS/public/HESS_catalog.htm bremsstrahlungmodel),anditispositionallycoincidentwith
2
a bright star outside the extension of the TeV source. The
spectra ofSw2 (= 1WGA J1804.0-2142)and Sw3 couldnot
bemeasuredbecauseofthesmallnumbersofcountsdetected
(22±7and 26±6counts, respectively,in the 11.6ksexpo-
sure). Sw2 could also be associated with a star close to the
boundaryof the XRT error circle, while Sw3, closest to the
centerofHESSJ1804,didnotshowobviouscounterpartsat
otherwavelengths.
The subsequent deeper (40 ks) Suzaku XIS observation
revealed two distinct X-ray sources (Suzaku J1804–2142
and Suzaku J1804–2140; Su42 and Su40 hereafter) in the
18′×18′XISFOV.Su40ispositionallycoincidentwithSw3
withinthelarge(∼1′)positionaluncertaintyofSuzakuXIS.
Bamba et al. (2006) found that Su40 is extended (or multi-
ple) while Su42 is unresolvedat theSuzakuresolution(half
power PSF diameter ≈ 2′). Spectral fits with a power-law
(PL) modelshow markedly differentspectral parametersfor
FIG. 1.— Chandra ACIS image (0.5–8 keV; smoothed with a r =
thetwosources. Su42wasfoundtobeunusuallyhard(pho- 6′′ gaussian kernel) of the central part of HESS J1804 with the TeV
tonindexΓ=- 0.3-+00..55,theerrorsareatthe90%confidencefor contours overlayed. The best-fit position of HESS J1804 and its un-
oneinterestingparameter)with amoderate(albeitratherun- certainty are marked by the cross. The arrows show the four bright-
certain) hydrogen column density, nH,22 ≡nH/(1022cm- 2)= (eCstXXO-Uray18s0o4u4rc1e.9s,- 2C1h4122(4CX=OSUu42J1),80C4h332.(4C-X2O14U00J918=04S2w1.35-=21S4u24303)),,Cahn2d
(0n.2+- 20..02=. S11u+4100)sshpoewcetrdumas.oTfhteers(oΓu=rc1es.7h+-a11v..40e),csotmropnagrlaybalebsflourxbeesd, Cimha4ge(C,tXheOpUulJs1a8r0B412830.10-–2211,39an3d2)t,hedeRteOcSteAdTinsotuhreceb1riWghGteAr 1p8ar0t4o.0f- t2h1e4T2e(V=
H,22 - 6 Sw2). (Sw1,thebrightestofthesourcesdetected withSwift,isoutofthe
∼2.5 and 4.3×10- 13 ergs cm- 2 s- 1 in 2–10 keV, for Su42 ACISFOV:itisshownintheROSATimageinFig.8.)
andSu40,respectively. Despiteanappreciableprobabilityof
detector was operated in Full Frame mode which provided
chancecoincidence(obviousfromtheChandraimagesin§2),
time resolution of 3.24 seconds. The data were reduced us-
Bamba et al. (2006) conclude that both Suzakusources are
ingtheChandraInteractiveAnalysisofObservations(CIAO)
physically associated with HESS J1804. They mention that
software(ver.3.2.1;CALDBver.3.0.3).
the harder Su42 could be an HMXB while the softer Su40
couldbeeitheraPWNor,morelikely,itcouldbeassociated 2.1. Chandraimages
withSNRG8.7–0.1.Theauthorsalsopointoutthattheratios
Figure 1 shows the ACIS image of the HESS J1804 field
oftheγ-rayfluxofHESSJ1804totheX-rayfluxesofSu42
with overlaid TeV contours, extracted from Figure 17 of
andSu40aresurprisinglyhighcomparedtothoseseeninTeV
Ah06. The brightest portion of HESS J1804 falls onto the
sourceswithknownassociations,includingSNRsandPWNe.
I3andI2 chips, itsbest-fitpositionis offsetby≈11.′2from
Thus,theSwiftandSuzakudatadonotprovideaconclusive
the aim point. We searched for possible X-ray counter-
resultonthe natureof HESSJ1804,and itsassociationwith
partswithintheHESSJ1804extensionandfoundarelatively
thefoundX-raysourcesremainsunclear.
brightsource,whichwedesignateCXOUJ180432.4- 214009
InthecourseofourobservationofPSRB1800–21andits
(hereafter Ch1), located at R.A. = 18h04m32.s462, decl. =
PWN with the ChandraX-rayObservatory, the most inter-
- 21◦40′09.′′91 (the 1σ centroid uncertainty is 0.′′38 in R.A.
esting part of the HESS J1804 field happened to be within
and0.′′32indecl.;the1σerrorinabsoluteChandraastrometry
the FOV. Detailed results of the PWN/PSR B1800–21study
is≈0.′′4foreachofthecoordinates),wellwithinthebrightest
have been presented by Kargaltsev et al. (2006a). In this
portionof HESS J1804 and just 1.′9 northof the best-fit po-
paper we present the analysis of X-ray sources in the vicin-
sition(Ah06). AlthoughCh1appearstobeslightlyextended
ity of HESS J1804, includingthe two sources detected with
Suzaku2. ThedetailsoftheChandraobservationandthedata intheACISimage,aPSFsimulationshowsthatthisislikely
the result of the off-axis location (off-axis angle θ =10.′3),
analysis, supplementedwith the analysis of optical-infrared-
which is also responsible for the relatively large centroiding
radiodata, arepresentedin §2. We discussthe natureof the
uncertaintyquotedabove. The position of Ch1 is consistent
Chandrasources and the likelihoodof their association with
(withintheuncertainties)withthatofSw3andSu40(see§1).
HESSJ1804in§3,andsummarizeourfindingsin§4.
Therefore, we conclude that Ch1, Sw3, and Su40 represent
2. OBSERVATIONSANDDATAANALYSIS thesamesource,althoughwefoundnoevidenceofthe∼2′–
WeserendipitouslyobservedthefieldofHESSJ1804with 3′extensionreportedbyBambaetal.(2006)forSu40.
the Advanced CCD Imaging Spectrometer (ACIS) on board We barely see some excess counts within the Su42 error
Chandraon2005May4. Thetotalusefulscientificexposure circle in the originalACIS image, scattered overan area ex-
time was30,236s. Theobservationwas carriedoutin Faint ceedingthe PSF evenwith accountfor thelargeoff-axisan-
mode. The aim point was chosen on S3 chip, near the PSR gle, θ ≈14′. However, when we filter out photonswith en-
B1800–21position(seeKargaltsevetal.2006a). Inaddition ergies>8keV(whicheffectivelyreducesthebackgroundby
to S3, the S0, S1, S2, I2, and I3 chips were turned on. The a factor of 2.7) and bin by a factor of 8 (i.e., the new pixel
size is 3.′′9), an extended(or multiple) source becomesvisi-
2 Itshouldbenotedthatafterthis paperhadbeengenerally completed, ble, with a size of ≃1.′5- 2′ (see Fig. 2). The best-fit cen-
Cui&Konopelko (2006)published anApJletter using thesameChandra troid of the source (obtained with the CIAO wavdetect tool)
data.UsingwrongcoordinatesofPSRB1800–21,theycouldnotidentifythe isR.A.=18h04m41.s924,Decl.=- 21◦42′24.′′09;wedesignate
pulsarintheChandraimage,failedtonoticeoneofthetwoSuzakusources, thesourceasCXOUJ180441.9- 214224(hereafterCh2).
and did not provide a thorough analysis of the other Suzaku source. We
correcttheshortcomingsofthatworkinourpaper. In addition to Ch1 and Ch2, we found a dozen
3
FIG. 2.—ChandraACIS-I3 image (in the 0.5–8 keV band; binned by
a factor of 8) of the HESS J1804 central region. The best-fit position of
HESSJ1804fromAh06isshownbythecross. ThepositionoftheM-type
dwarf(see§2.4)isshownbythebox.Twolargercircles(r=1′)arecentered
atthepositionsofSu40andSu42asreportedbyBambaetal.(2006). The
smaller circle (r=44′′)shows theregion used toestimate the count rates
fromCh2whilethesmallellipseshowstheregionusedfortheCh1spectral
extraction. An offset of about 15′′ between the positions of theChandra
sourcesandSuzakusourcesisapparentlyduetoinaccuracyinSuzakuaspect
solution.ThefainterCh3andCh4sources(seetext)arealsomarked.
fainter sources on the I3 and I2 chips, of which
CXOU J180421.5- 214233 and CXOU J180423.1- 213932
(hereafter Ch3 and Ch4, respectively) are the brightest and
theclosesttothebest-fitpositionofHESSJ1809(seeFigs.1
and2). Ch3isconsistentwithbeingpoint-like,whileCh4is
eitherextendedor,morelikely,multiple.
Wealsoattemptedtosearchforsignaturesofdiffuseemis-
sion(e.g.,anSNR)ontheI3chip. Adirectvisualinspection
oftheACISimagedidnotshowclearsignaturesoflarge-scale
diffuseemission.Weappliedtheexposuremapcorrectionand
smoothedtheimagewithvariousscales,butfailedtofindsta-
tisticallysignificantdeviationsfromauniformbrightnessdis-
tribution.ToestimateanupperlimitontheSNRemission,we
measuredthecountratefromtheentireI3chip(withalliden-
tifiablepointsourcesremoved).Thecountrate,0.266±0.003
counts s- 1 in the 0.5–7 keV band, exceeds the nominal I3
backgroundof0.17countss- 1 (ChandraProposers’Observa-
toryGuide3, v.8,§6.15.2),whichcouldbecausedbyan ele-
vatedparticlebackground,diffuseX-raybackground,orSNR
emission.SinceweseenoclearevidenceofanSNR,wecon-
sider the difference, ≈0.09 counts s- 1, as an upper limit on
FIG.3.—Ch1spectrumfittedwiththePLmodel(top)andthecorrespond-
theSNRcountrateinthe70arcmin2 ofthechiparea,which ingconfidencecontours(68%and90%)obtainedwiththenH heldfixedat
correspondstheaveragesurfacebrightnesslimitof1.3counts thebest-fitvalue(middle)andnH fittedateachgridpoint(bottom). ThePL
ks- 1arcmin- 2. normalization(verticalaxis)isinunitsof10- 5photonscm- 2s- 1keV- 1at1
keV.Thelinesofconstantunabsorbedflux(inunitsof10- 13ergscm- 2s- 1;
2.2. SpectralanalysisoftheChandrasources in0.5–8keVband)areplottedasdashedlines.
We extracted the Ch1 spectrum from the elliptical region
(withtheminorandmajoraxesof4.′′9and10.′′8;seeFig.2), 2keV(thelowestphotonenergyis0.35keV).Theabsorbed
PL model fits the spectrum well, χ2 =0.99 for 7 degreesof
which accounts for the elongated shape of the off-axis PSF ν
freedom,withn ≈3.8,Γ≈0.45,andtheabsorbedandun-
and contains ≈83% of the source counts. The background H,22
absorbedfluxesof (1.7±0.2)and (2.5+0.9) ×10- 13 ergcm- 2
was measured from a larger circular annulus; it contributes - 0.4
about 15% to the total of 127 counts within the source ex- s- 1 in 0.3–7 and 0.3–8 keV, respectively. (Here and below
traction region. We group the spectra into 10 spectral bins the Chandrafluxes, luminosities and PL normalizations are
between 0.3 and 7 keV. The spectrum of the source (shown correctedforvignettingandforthefiniteextractionaperture.)
in Fig. 3) is strongly absorbed, with only five counts below TheuncertaintiesofthefitsarelistedinTable1andillustrated
by confidence contoursin Figures 3 and 4. As one can see,
3Seehttp://asc.harvard.edu/proposer/POG/index.html fixingtheabsorptionatthebest-fitvaluesubstantiallyreduces
4
FIG.5.—Hard(2–7keV;left)andsoft(0.5–2keV;right)bandChandraim-
agesoftheHESSJ1804centralregion. Thebest-fitpositionofHESSJ1804
(Ah06)isshownbythecross. Thecircles(r=1′)arecenteredattheposi-
tionsofthetwoX-raysourcesseenbySuzakuXIS(Bambaetal.2006).The
imagesinthetoppanelsarebinnedbyafactorof8(pixelsize3.′′9)while
FIG. 4.—Confidencecontours(68%and90%)inthenH–Γplaneforthe thesameimagesinthemiddlepanelsarebinnedbyafactorof20(pixelsize
PLfittotheCh1spectrum. Thetoppanelshowscontoursobtainedwiththe 9.′′8).Thebottompanelsshowthesameimagesbinnedbyafactorof20and
PLnormalization heldfixedatthebest-fitvalue(seeTable1)whileinthe smoothedwitha30′′gaussiankernel.
bottompanelthePLnormalizationwasfittedateachgridpointintheΓ–nH
space.
with the Su42 spectrum as reportedby Bamba et al. (2006).
The hard and soft band images are shown in Figure 5. The
theuncertaintiesoftheremainingparameterssinceΓandn
H lowS/N valuesprecludeareliablespectralfitting. Themea-
are strongly correlated. At a fiducial distance of 8 kpc, the
suredcountratescorrespondto the observed0.3–8keV flux
oobfs∼er2ve×d1P0L33fleurgxscso- 1rr.eEspveonndwsitthoathcceouunnatbfosorrtbheedlalrugmeiunnocseitry- oanfd(1t.h0e±u0n.a3b)s×or1b0ed- 13fluexrgsofs-≈1 c1m.5- ×2 i1n0t-h1e3 rer=gs44s-′′1acpmer-t2urien,
tainties, the spectral parameters are in poor agreement with
2–10 keV, using the best-fit spectral parameters reported by
thoseobtainedbyBambaetal.(2006)forSu40,althoughan
Bambaetal.(2006)forSu42.Theestimatedunabsorbedflux
accuratecomparisonisdifficultbecausethoseauthorsdonot
of Ch2 is a factor of ≈ 1.7 smaller than the flux of Su42,
provideconfidencecontours. TheChandraandSuzakuunab- (2.5±0.4)×10- 13 ergs s- 1 cm- 2 reported by Bamba et al.
sorbed fluxes, which are more accurately measured than the
(2006); however, the difference may be due to unaccounted
spectralparameters,areconsistentwithintheiruncertainties:
systematicerrors.
3.3+1.2×10- 13versus4.3+4.0×10- 13ergss- 1cm- 2inthe2–10
- 0.5 - 1.1 For Ch3 and Ch4, the background-subtractednumbers of
keVband, respectively. TheACISspectrumofCh1 also fits
counts in the 0.5–8 keV band are 19±5 and 44±10, in
anabsorbedblack-body(BB) modelwiththetemperatureof r=7.′′4 and 21′′ apertures, respectively(we chosethe larger
2.3keVandemittingregionradiusof∼30(d/8kpc)m. The
apertureforCh4becauseitlooksextendedormultiple).Their
uncertaintiesoftheBBfitareevenlargerthanthoseofthePL
observed fluxes can be crudely estimated as ∼ 1.2 and ∼
fit. 2.5×10- 14ergscm- 2s- 1,respectively.ThelowS/Ndoesnot
ForCh2,thetotalnumberofbackground-subtractedcounts
allowameaningfulspectralanalysisofthesesources. Figure
within the r = 44′′ aperture centered at the source position
5shows,however,thatbothCh3andCh4arebetterseeninthe
(see Fig. 2) is 73±19 in 0.3–8 keV (the total number of
softband,whichmeansthattheyarelessabsorbed(henceless
counts is 307, of which 234 counts are estimated to come
distant)thanCh1andCh2.Sincetheotheroff-axissourceson
fromthebackground). Restrictingthephotonenergiestothe
I3andI2chipsareevenfainter,theirfluxestimatesarenotre-
hard,2–7keV,bandresultsinasimilarS/N=3.1(55±12net
liable.
sourcecounts), while S/N =2.1 (28±13netsource counts)
in the soft, 0.5–2 keV, band. These numbers indicate a rel-
2.3. TimingofCh1
ativelyhardspectrumofthesource,inqualitativeagreement
5
TABLE1
FITSTOTHESPECTRUMOFCH1
Model nH,22 NaorAreab ΓorkTc χ2/dof LXorLbold
PL 3.8+- 42..25 10.2+- 25.070 0.45+- 21..0455 6.9/6 1.9-+00..37
PL 3.8e 10.2+- 32..67 0.45+- 00..3349 6.9/7 1.9±0.2
BB 2.6+- 11..82 ∼2.9 ∼2.3 6.8/6 ∼7.9
NOTE.— Theuncertaintiesaregivenatthe68%confidencelevelforone
interestingparameter.
aSpectralfluxin10- 6photonscm- 2s- 1keV- 1at1keV
bProjectedareaoftheemittingregionfortheBBmodelin103m2(assuming
an8kpcdistance).
cBBtemperatureinkeV.
dUnabsorbed luminosity in 0.5–8 keV band or bolometric luminosity in
unitsof1033ergs- 1atthedistanceof8kpc.
eThehydrogencolumndensitywasfrozenatthisvalueduringthefit.
FIG. 6.—Top: Lightcurve ofCh1folded withtheperiod of106.12s.
Bottom:UnfoldedlightcurveofCh1.
We searchedforpulsationsofCh1, usingthe arrivaltimes
ofthe127photons(ofwhich≈85%onareexpectedtocome
from the source) recalculated to the solar system barycenter
usingthe CIAO axBarytool. The ACIStime resolutionof
3.24sandthetotaltimespanof30ksallowasearchforpulsa-
tionsina3×10- 5–0.1Hzrange.WecalculatedtheZ2statistic
1
(e.g.,Zavlinetal.2000)at105 equallyspacedfrequenciesν
in the 3×10- 5–0.1Hz range. Thiscorrespondsto oversam-
plingbyafactorofabout33,comparedtotheexpectedwidth
ofT- 1 ≈33µHzoftheZ2(ν)peaks,andguaranteesthatwe
span 1
miss no peaks. The most significant peak, Z2 = 23.70,
1,max
was found at ν = 0.009423Hz±5µHz (P≈ 106.12±0.05
s). The maximum value of Z2 corresponds to the 97.9%
1
(≈2.3σ)significancelevel,forthenumberofindependenttri-
FIG. 7.—Top: 8µmSpitzerIRACimageoftheHESSJ1804fieldwith
alsN =νmaxTspan≈3×103. Thepulseprofilefoldedwiththe TeVcontoursoverlayed. Thebest-fitpositionofHESSJ1804isshownby
abovefrequencyisshowninFigure6(top). Thecorrespond- thecross. ThepositionsofCh1andCh2aremarkedwiththestarandbox
ing observedpulsed fraction is 58%±13%(intrinsic source respectively. ThepositionofPSRB1800–21ismarkedwithadiamond,and
thepossiblenewSNRG8.31–0.09isshownbythearrow. Bottom:Blow-up
pulsedfraction≈67%±15%)Thesignificanceoftheperiod ofthecentralpartoftheimage.Thetwor=10′′circles,thelarger28′′circle,
candidateisratherlow,sotheperiodicityshouldbetestedin andthe14′′×20′′ellipsearecenteredatthepositionsofCh1,Ch3,Ch2,and
alongerobservation. Ch4,respectively.TheMdwarfneartheCh2positionisshownbythebox.
We also producedtheunfoldedlightcurveofCh1 (Fig.8,
bottom) using a 2 ks binning. The light curve indicates that We found no counterparts to Ch1 within 9′′ from its po-
thesourcemayexperiencesomenon-periodicvariabilityona sition in the Two Micron All Sky Survey (2MASS; Cutri et
fewksscale. al. 2003) or Digital Sky Survey (DSS2)4 catalogs up to the
limitingmagnitudesK =15.4,H =16,J=17.5,R=19,and
s
2.4. Optical-IR-radiodata
4seehttp://archive.eso.org/dss/dss
6
yr- 1 (NOMAD1 0682- 0650954;Zachariaset al. 2005), it is
likelyalate-typeMdwarflocatedatd≈10pc.Suchadwarf
could provide an X-ray flux of ∼10- 13–10- 12 ergs cm- 2 s- 1
(see, e.g., Hünsch etal. 1999; Preibisch etal. 2005),similar
to those observed from Ch2/Su42. However, given its large
offsetfromthe brightestpartofCh2(Fig.2),thedwarfcan-
notaccountfortheentireextendedX-rayemission,although
its flare might be responsible for the possible difference be-
tweenthefluxesmeasuredwithSuzakuandChandra(§2.2).
Ch3ispositionallycoincidentwiththeoptical-NIRsource
DENIS J180421.4- 214233,with magnitudes K =11.9, H =
12.3, J = 13.3, I =15.5, R=16.8, V =17.5, and B=19.0,
which is also seen in the 4.5 and 8 µm IRAC images.
WithintheX-rayextentofCh4,therearefiverelativelybright
2MASSandDENISsources(Hmagnitudesrangingfrom10
to 14). Two IR sources within the X-ray extent of Ch4 are
clearlyseen in the IRACimages. One ofthem (northeastof
theX-raycentroidofCh4)ispositionallycoincidentwiththe
DENIS source J180423.6- 213928(J =12.7, H =10.0, and
K =8.8). The other IR source has a NIR counterpart NO-
MAD1 0683–0642056 (V = 17.2, J = 15.4, H = 11.7, and
K=10.0),withthepropermotionof208masyr- 1.
AllthestarswefoundwithintheX-rayextentsofCh3and
Ch4 exhibit extremely red colors. Explaining such colors
solelybyextinctionwouldrequireaverylargeabsorbingcol-
umn that would absorb any soft X-rays(.2 keV) from this
direction, in contradiction with the fact that we do see such
FIG. 8.—Top: 10ksROSATPSPCimageoftheHESSJ1804field. The X-raysfromCh3 andCh4. The extremelyredcolorscanbe
whitecircle(r=12′)iscenteredatthebest-fitpositionofHESSJ1804.The
naturallyexplainediftheNIR/IRobjectsareyoungpre-main-
diameterofthecircleroughlycorrespondstotheextentoftheγ-rayemis-
sion(seeAh06)Thepositionsoftheothersourcesdiscussedinthetextare sequence(TTauri)starssurroundedbydustydisksorinfalling
alsomarked.ThebrightestsourceSw1(=1RXSJ180404.6–215325)ismost envelopes(e.g.,Hartmanetal. 2005). Indeed,theIRACim-
likely a nearby star DENISJ180403.2–215336 with magnitudes R=12.0, agesshowthattheCh3andCh4regionsareimmersedinthe
J=10.1, and Ks =9.1. Sw2 (=1WGA 1804.0- 2142) corresponds to the extendeddiffuseIRemission(seeFig.7)thatmaybeassoci-
brightsourceattheverybottomoftheACIS-S3chipinFig.1;itpositionally
coincides with the other bright star, DENISJ180400.6–214252 (R=13.9, atedwithanearbystarformingregion. Thelargepropermo-
J=11.3, K=10.15). Bottom: NVSSλ=20cmimageofthesamesize tionofNOMAD10683–0642056suggestsasmalldistanceto
(0b.0e9amSNFRWcHaMnd=id4a5t′e′.).TThheemounclyhbfariingthetrsNouVrSceSwJ1it8h0in43t4h–e2c1ir4c0l2e5is(stheee§G28..43)1i–s thisstar,d≈100(v⊥/100kms- 1)pc. Sincethecolorsofthis
notdiscernibleinthisimage. star are similar to those of the otherstars around, itis likely
that most of them belong to the same group, which is, per-
B=21. However,sincetheinterstellarextinctiontowardsthe haps, oneof the nearestregionsofstar formation. Although
innerGalacticbuldgeisverylarge(A ≃18inthedirectionof the nearby T Tauri stars can easily accountfor the observed
V
Ch1[l=8.◦429,b=- 0.◦018];Schultheisetal.1999),thelim- X-rayfluxesfromCh3 andCh4 (e.g., Preibischetal. 2005),
its are not very restrictive. We also examinedthe publically suchstars cannotproduceTeV emission and, therefore,Ch3
availabledatafromtheSpitzerGLIMPSE-IIsurvey5covering andCh4arenotassociatedwithHESSJ1804.
thevicinityofHESSJ1804(seethe8µmIRACimageinFig. The IRAC images of the field (e.g., Fig. 7, top) reveal a
7,top)butfoundnoIRsourceswithin10′′fromtheCh1posi- large-scalediffuseemissionwithcomplexmorphology.How-
tion,downtolimitingfluxesof5and6µJyat4.5and8 µm, ever,theIRbrightnessdistributiondoesnotcorrelatewiththe
respectively. TeV brightness (shown by the contours in the same figure),
The closest match to Ch1 in radio catalogs was found in norwiththelarge-scaleX-raybrightnessdistributionseenin
the NRAO VLA Sky Survey(NVSS) catalog (Condonet al. thearchivalROSATPSPC image(Fig. 8, top). Therecently
1998). Thecatalogpositionoftherelativelyfaint(27.6±3.8 discoveredradiosourceG8.31–0.09(seetheNVSSimagein
mJy)radio source, NVSSJ180434–214025,is offsetby 32′′ Fig. 8, bottom), classified as a possible SNR (Brogan et al.
from the Ch1 position, less than the NVSS beam size (45′′ 2006),coincideswellwiththeshell-likestructureseeninthe
FWHM). However, the apparently extended NVSS source IRACimages(Fig.7,top),thusconfirmingthatthesourceis
(linear size ∼1.′5) looks like a part of a larger (∼4′ in di- indeedanewSNRwithaninterestingIRmorphology.
ameter)diffusestructure,barelydiscernibleintheNVSSim-
age. SincetheimageofNVSSJ180434- 214025showssome 3. DISCUSSION.
artificialstructures,wecannotconsideritasatrueradiocoun- We see fromtheChandraACISimage(Fig.1)thattheX-
terpartofCh1untilitisconfirmedbydeeperobservations. rayskyintheregionofHESSJ1804isrichwithpointsources
The optical/NIR source nearest to Ch2 is located ∼ 28′′ withfluxesof∼10- 14- 10- 13ergcm- 2s- 1,mostofwhichare
away from the best-fit X-ray centroid (see Fig. 7). Having possibly stars. Therefore, it is not surprising to find a few
the magnitudes B = 14.54, V = 13.30, R= 12.19, J = 8.64, sources located relatively close to each other in this region
H=8.05,andK=7.67,andthepropermotionof≈10.6mas of the sky. However, Ch1 doesnothave a knownIR/optical
counterpart while Ch2 appears to be extended, and both of
5http://www.astro.wisc.edu/sirtf/glimpsedata.html themarelocatedwithinthebrightestpartofHESSJ1804(1.′9
7
and 2.′5 from the best-fit TeV position). This raises a possi- ofsecondtothousandsofseconds,theHMXBinterpretation
bilitythatatleastonethemisassociatedwiththeTeVsource. providesaplausibleexplanationfortheputative106speriod-
BelowwediscusswhetherCh1orCh2couldbeX-raycoun- icityin Ch1, whichwouldbedifficultto interpretotherwise.
terpartsofHESS1804,forseveralpossibleinterpretationsof Ontheotherhand,thelackofanIR/NIRcounterpartissome-
the TeV source. Since the large extent of the TeV emission whatsurprising,althoughtheupperlimitsontheunabsorbed
rulesoutassociationwithextragalacticsources,welimitour IR/NIRfluxes(see§2.4andFig.9)stillcannotruleoutaB-
considerationtotheGalacticsourcesonly. giantatadistanceof&8kpc.
WefoundnoCGROEGRETcounterpartsforCh1andother
3.1. AHighMassX-rayBinary?
sourcesintheHESSJ1804field. ThenearestEGRETsource
As there are several HMXBs among the identified TeV (Hartman et al. 1999) is located ≃2.◦2 from the Ch1 posi-
sources(seeexamplesinTable2),wecanconsiderthepossi- tion, too far to be associated with HESSJ1804or theChan-
bilitythatHESSJ1804isanHMXBandthereforemayhave drasources. However, only three HMXBs (PSR B1259–63,
a compact X-ray counterpart, such as Ch1 or Ch2. It is be- LS5039andLSI+61◦303)havebeenidentifiedwithEGRET
lievedthatinHMXBsparticlescanbeacceleratedupto∼10 sources so far. The upper limit on GeV flux, obtained from
TeVorevenhigherenergieseitherinjetsproducedasthere- the EGRET upper limits map (Fig. 3 from Hartman et al.
sult of accretion onto a compact object (e.g., Bosch-Ramon 1999),isnotdeepenoughtotesttheconnectionbetweenthe
2006andreferencestherein)orinthepulsarwind,ifthecom- X-rayspectrumofCh1andtheTeVspectrumofHESSJ1804
pactobjectisanactivepulsar(e.g.,Dubus2006). Examples (seeFig.9). TheIntergalISGRIupperlimit(A.Bykov2006,
ofsuch systemsarethe famousHMXBwith the youngPSR priv. comm.), shown in the same figure, appears to be even
B1259- 63and the microquasarsLS 5039 and LSI+61◦303, lessrestrictive. FromFigure9wecanonlyconcludethatthe
forwhichthenatureofthecentralengine(NSorBH)isstill TeVspectrumofHESSJ1804breakssomewherebetweenthe
under debate. So far these are the only HMXB firmly de- EGRETandHESSenergyranges,asobservedformanyTeV
tectedinboththeTeV andGeVbands. Theultra-relativistic sourcesofdifferentkinds.
particles can produce TeV emission via the inverse Comp- Thespectralparametersof theCh2 sourceareveryuncer-
tonscattering(ICS)oftheoptical-UVphotonsemittedbythe tain. Althoughthe fluxmeasuredwithChandraissomewhat
non-degenerate companion or through the synchrotron self- lowerthanthatmeasuredwithSuzakuayearlater(see§2.2),
Compton(SSC)process. thedifferenceisonlymarginalbecauseofthelargeuncertain-
HMXBsproduceX-rayseitherinthecourseofaccretionof tiesofthemeasurements.However,ifconfirmed,thevariabil-
the matter from the secondary companion onto the compact itywouldbeanargumentsupportinganHMXBinterpretation
objector via the synchrotronradiationin theshockedpulsar ofCh2. Onthecontrary,theratherlargeX-rayextentofCh2
wind. WeseefromTable2thattheTeV-to-X-ray(1–10TeV [∼1′=2(d/7kpc)pc]arguesagainsttheX-raybinaryinter-
to1–10keV)fluxratio, fγ/fX,is.1forallthefourHMXBs pretation6. AlthoughpossibleX-rayemission froma nearby
withmoreorlesssecureTeVassociations,muchsmallerthan M-dwarf(§2.4)maycontributetoCh2,itcannotaccountfor
fγ/fX ∼ 30 and 50 for Ch1 and Ch2, respectively. How- theentireemissionfromthisextendedormultiplesource.
ever, given the small size of the HMXB sample in Table 2 Even if either of the X-ray sources is an HMXB, a ma-
andthefactthat fγ/fX variesbyatleastafactorof10within jor problem with its association with HESSJ1804 is the ex-
the sample, it is possible that some HMXBs have a higher tended morphology of the latter. Although, there is no an
fγ/fX. Indeed, most of accreting binaries are strongly vari- a priori reason to believe that HMXBs cannot produce ex-
ableX-raysources,someofthembeingX-raytransients. For tended TeV emission, the observational evidence for that is
instance, IGR J16358–4726,which is likely associated with currently rather weak. So far, among the TeV sources pos-
HESS1634–472(Ah06),isastronglyvariableX-raysource, sibly associated with HMXBs, only two, HESS J1632–478
withthe 2- 10keVfluxvaryingbya factorof&4000(Patel and HESS J1634–472, might show extended TeV emission
et al. 2004; Mereghetti et al. 2006). This example demon- (Ah06),andtheevidencefortheextensionismarginalinboth
stratesthatthe fγ/fXratioinHMXBsmayvarydramatically, cases.
especiallyinthecaseswhentheTeVandX-rayfluxesarenot Thus, althoughan HMXBata distanceof∼8–15kpc re-
measuredsimultaneously. Thus, the rathermodestX-raylu- mainsa plausible interpretationfor Ch17 and somewhatless
minositiesofCh1andCh2couldbeexplainedassumingthat plausibleforCh2,theassociationbetweenthemandtheTeV
eitherofthemisanHMXBinthelow/hardstate. source is very questionable. An HMXB origin of Ch1 or
Thehard(Γ∼0.5)X-rayspectrumofCh1is stronglyab- Ch2wouldbefirmlyestablishedifthe periodic(and/ornon-
sorbed; the hydrogen column density, nH,22 ≃4, is a factor periodic)variabilityisconfirmedforCh1(orfoundforCh2)
of2–3largerthanthetotalGalacticHIcolumn(≃1.5×1022 inadeeperX-rayobservation,oracompanionstarisdetected
cm- 2; Dickey & Lockman 1990) and a factor of 2–4 larger in the IR-optical. At the same time, a deeperon-axisobser-
thanthen ∼1.4inferredfromtheX-rayspectrumofPSR vation withChandracan measure the true extentand spatial
H,22
B1800–21(and its PWN) located at the distance of ≈4 kpc structureofCh2.
(Kargaltsev et al. 2006a). Taking into account that the n
H
value deduced from an X-ray spectrum under the assump- 6Toourknowledge,extendedX-rayemissionhasbeenreportedonlyfrom
tion of standard element abundances generally exceeds the threeHMXBs:SS433(Migliari,Fender,&Méndez2002),CygX-3(Heindl
etal.2003),andXTEJ1550–564(Corbeletal.2002).Thisemissionisoften
n measured from 21 cm observations by a factor of 1.5–3
HI attributedtojets. InthesesystemstheangularextentoftheresolvedX-ray
(e.g., Baumgartner & Mushotzky 2005), the large nH (con- emissionrangesfrom3′′to30′′correspondingtophysicallengthsof0.1to
sistentwithA ∼20;e.g.,Predehl&Schmitt1995)suggests 0.8pcatthenominaldistancestothesesystems.NoTeVemissionhasbeen
V
thatCh1iseitherlocatedwithin(orevenbeyond)theGalac- reportedfromtheseHMXBsyet.
7 We should mention that, based on the strongly absorbed, hard X-ray
ticBuldgeoritshowsintrinsicabsorption,oftenseeninX-ray
spectrum,thissourcecanalsobeabackgroundAGN.
spectraofHMXBs(e.g.,Walteretal.2006).
AsthespinperiodsofNSsinHMXBsrangefromafraction
8
Lorentz factor of the electron that upscatters the CMB pho-
tontotheenergyE isγ≈108(E /9TeV)1/2. Electronswith
γ γ
suchLorentzfactorsemitsynchrotronphotonswithenergies
E ∼0.5γ2(B/10µG)keV∼0.5(E /9TeV)(B/10µG)keV.
syn 8 γ
Therefore,theobservedTeVspectrumofHESSJ1804,span-
ning from 0.2 to 10 TeV (Ah06), would correspond to the
≈0.01–0.6keVrangeofthesynchrotronphotonenergiesin
B=10 µG. These EUV and soft X-ray synchrotronphotons
are heavily absorbed at n &1022 cm- 2 and hence are dif-
H
ficult to detect. Thus, if the swept-up wind is cold enough
[e.g.,γ.108(B/10µG)- 1/2], the sackmaybebrightinTeV
but faint in the Chandraband. Furthermore, the magnetic
field insidethe sack is lowerthanthatin the compactPWN,
leadingtoalowersynchrotronbrightnesssincethelatterde-
pends on the magnetic field strengths as B(p+1)/2 for the PL
distribution of electrons, dn =Kγ- pdγ. This could explain
e
why the TeV emitting region is dimmer in X-rays than the
compact PWN populated with more energetic electrons, but
FIG. 9.—UnabsorbedspectraofCh1andHESSJ1804(Ah06),together it doesnotexplainwhythe compactPWN shows lowersur-
withtheCGROEGRETandINTEGRALIBIS/ISGRIupperlimits.Theopen face brightnessin TeV than the extendedasymmetric PWN.
trianglesshowtheupperlimitsonthedereddenedNIRfluxesintheKs,H, ThebrightnessoftheTeVemissionproducedviatheICSon
andJbands(see§2.4).
CMB photonsdoes notdependon the magnetic field; there-
fore,thesimplestexplanationcouldbethatthesackcontains
3.2. APulsarWindNebula?
alargernumber(andperhapsahighercolumndensity)ofthe
Among other types of Galactic X-ray sources, only SNR
swept-upTeV-emittingelectronscomparedtothosewithinthe
shocks and PWNe are believed to be able to produce ex-
compactPWN.
tended TeV emission. In fact, the second highest (persis-
One could try to apply the above interpretation to
tent)TeV-to-X-rayfluxratio, f /f =3.4,inTable2belongs
γ X HESS J1804, assuming that Ch1 (or Ch2) is a pulsar with
to the PWN G18.0–0.7 around the Vela-like pulsar B1823–
aPWN.Theoff-axispositionmaynotallowonetoresolvea
13 (E˙ ≈3×1036 erg s- 1; d ≈4 kpc), likely associated with compactPWN.Furthermore,Bambaetal.(2006)reportSu40
HESSJ1825–137(Ah06). AlthoughnoSNRhasbeenasso- (=Ch1) as an extended source, which could mean that the
ciatedwith thispulsar, itpowersa luminousextendedX-ray moresensitive(onlargeangularscales)SuzakuXISobserva-
PWN (LX ∼3×1033 erg s- 1, angularsize &5′; Gaensler et tionhasdetectedafainterextendedPWNcomponent(similar
al.2003). Inadditiontotheextendedlow-surface-brightness to theXMM-Newtonobservation of B1823–13; Gaensler et
component, G18.0–0.7 has a much more compact (5′′–10′′) al.2003). ThefaintnessofapossibleextendedPWNcompo-
brightercore,resolvedbyChandra(Teteretal.2004;Kargalt- nentcouldbeatleastpartlyattributedtothestrongX-rayab-
sevetal.2006b).TheTeVemissiondetectedwithHESScov- sorptioninthisdirection. Ontheotherhand,Ch2isresolved
ersa muchlargerareathantheX-rayemissionfromG18.0– byChandraintoanextendedX-raysource,whichmightbea
0.7,extendingupto1◦southwardfromthepulsar(Aharonian PWN. However, the low S/N and the off-axis location ham-
et al. 2006c). However, both the TeV and the low-surface- pertheassessmentofthespatialstructureandthespectrumof
brightnessX-rayemissionhavesimilarlyasymmetricshapes, Ch2.
and they are offset in the same direction with respect to the The 3.24 s time resolution of the ACIS observation also
pulsar position. A similar picture is observed around the precludes a search for sub-second pulsations expected from
Vela pulsar (E˙ ≈7×1036 erg s- 1; d ≈300 pc). An X-ray a young pulsar (the putative 106 s period of Ch1 is cer-
bright, compact (∼ 40′′ in diameter) PWN centered on the tainlytoo longfor a youngisolated pulsar andhenceshould
pulsar is accompaniedby a muchlarger(∼50′) but dimmer beattributedtoastatisticalfluctuationinthisinterpretation).
asymmetricdiffuseX-raycomponent(sometimesreferredto Keeping in mind the above examplesof Vela X and G18.0–
as“VelaX”),whichalsohasaTeVcounterpart(Aharonianet 0.7,the largeextentofHESSJ1804shouldnotbe alarming.
al.2006d). Alackofstrongasymmetrywithrespecttothepulsar,which
The asymmetry in the extended PWN componentscan be isthedistinctivefeatureofalltheotherextendedTeVPWNe
causedbythereverseSNRshockthathadpropagatedthrough (Table2; de Jager 2006), couldbe attributed to the low sen-
theinhomogeneousSNRinteriortowardstheSNRcenterand sitivity of the Chandraobservation to extended emission of
reachedonesideofthePWNsoonerthantheotherside(e.g., low surface brightness or to the projection effect (i.e., the
Blondin, Chevalier, & Frierson 2001). The wind, produced TeV PWN couldbe displacedfromthe pulsar alongthe line
by the pulsar over a substantial period of time (up to a few of sight). The large f /f values cast additional doubts on
γ X
kyrs) and therefore occupying a substantial volume, could the PWN interpretation; however,evena luminousextended
be swept up by the reverse shock wave into a smaller vol- X-raycomponentoflowsurfacebrightnesscouldremainun-
ume on one side of the PWN. The swept-up wind confined detectedintherelativelyshallowoff-axisACISexposure. A
within the formed “sack” emits synchrotron radiation in X-
rays. At the same time, the wind can produce TeV radia- π0 beingproducedwhentherelativistic protonsofthepulsarwindinteract
tionviatheICSofthecosmicmicrowavebackground(CMB) withtheambientmatter(Horns2006).Althoughthepresenceofthehadronic
and synchrotron photons off the relativistic electrons8. The componentinthepulsarwindhasnotyetbeenestablishedobservationally,it
isexpectedtobepresentaccordingtosomepulsarwindaccelerationmodels
(e.g.,Arons2005).
8 Analternative TeVproduction mechanism isπ0 →γ+γ decay, with
9
deeper on-axis observation with Chandrawould test the na- ment against such an interpretation9. Nevertheless, a possi-
ture of Ch1 and Ch2 and the PWN interpretation. Overall, bility that an SNR (so far undetected in X-rays) could pro-
although not excluded, the possibility that Ch1 or Ch2 are ducetheobservedTeVemissioninHESSJ1804(seeFatuzzo,
thepulsarspoweringtheTeVPWNdoesnotlookverycom- Melia,&Crocker2006)cannotberuledoutiftheTeVsource
pellingatthispoint. isnotassociatedwithCh1,Ch2,orPSRB1800–21.
Ontheotherhand,theassociationofHESSJ1804withthe Indeed,contrarytotheconclusionbyBambaetal.(2006)10,
Vela-like pulsar B1800–21 remains a plausible option. To the close match in the sky positions of Ch1 (or Ch2) and
date, young Vela-like pulsars have been found in the vicin- HESS J1804 can merely be a chance coincidence, and
ity of ∼10 extendedTeV sources(e.g., de Jager 2006; Gal- HESSJ1804mayhavenopoint-likeX-raycounterpartsdown
lant 2006). Since both pulsarsand TeV sourcesare concen- to the 3σ limiting flux of .1×10- 14 ergs s- 1 cm- 2 within
trated in the Galactic plane, and the extended TeV sources theTeVbrightregion.However,onecannotexcludethepres-
have typical sizes of ∼ 5′- 15′, one could attempt to ex- enceoffaintdiffuseX-rayemission,e.g.fromanSNRwhose
plain this by a chance coincidence. However, the probabil- image size exceeds the chip size. Since it is difficult to es-
ity of chance coincidence is low. For instance, within the timate whichfractionofthe observeddiffusecountrate (1.3
≃ 300 square degrees area of the Galactic plane, surveyed counts ks- 1 arcmin- 2 in the I3 chip; see §2.1) comes from
by HESS (Ah06) the surface density of young (≤100 kyrs) the backgroundand what is the natureof the remainingflux
pulsars is ≈ 0.13 deg- 2 (based on the ATNF Pulsar Cata- (e.g., thermal emission from an SNR or nonthermal emis-
log data; Manchester et al. 2005). On the other hand, the sionfromanextendedPWN),wecanonlyputanupperlimit
sameareaincludesfourextendedTeVsources(HESSJ1825– of 2.5×10- 12 ergs s- 1 cm- 2 on the 2–10 keV flux in the I3
137,HESSJ1809–193,HESSJ1804–216,andHESSJ1616– chiparea,correspondingto f /f &4(thisestimateassumes
γ X
508)locatedwithin15′fromoneoftheyoungpulsars. Since n =1andaPLmodelwithΓ=1.5). However,wedonot
H,22
the probabilityof finding a youngpulsar within an arbitrary see any significant large-scale (in comparison with the off-
placedR=15′circleisonly2.6%,theprobabilityofacciden- axisPSFsize)X-raybrightnessvariationsintheACISimage.
tallyhavingallthefourTeVsourceswithinthe15′ distances (see Figs. 1 and 2). Although such uniformity is somewhat
fromtheyoungpulsarsisnegligible,0.0264≈5×10- 7. This unusualforanSNR,wenotethattheinterioroftheshell-type
stronglysuggestsaphysicalconnectionbetweenthetwophe- SNR RX J1713.7–3946 (resolved into a ≈1◦ shell in TeV;
nomena(e.g.,deJager2006). Furthermore,thereareseveral Aharonianetal.2004)isrelativelyfaintandhomogeneousin
pairs,suchasPSRB0833–45/HESSJ0835–455,PSRB1509– X-rays(Hiragaetal.2005). Furthermore,followingKargalt-
58/HESSJ1514–591, and PSRB1823–13/HESSJ1825–137, sevetal.(2006a),weconcludethatiftheX-rayspectrumand
for which the connectionis supportedby the correlationbe- luminosityoftheundetectedSNRaresimilarto thoseofthe
tweentheTeVandX-raybrightnessdistributions. Note,that Vela SNR, the expected off-axisACIS-I3 surface brightness
inthesepairsthepulsarsareoffsetby10′–15′fromthepeaks is<0.3countsks- 1 arcmin- 2 inthe0.5–7keVband(forthe
oftheTeVbrightness. Raymond-SmiththermalplasmaemissionmodelswithT <3
From the theoretical perspective, the “crushed PWN” hy- MKandn =1),i.e.atleastafactorof4belowtheobserved
H,22
pothesis (Blondin et al. 2001), briefly discussed above, pro- upperlimit(see§2.1.1).
vides a possible explanation for the observed offsets. From On the other hand, the TeV brightness distribution in
theobservationalpointofview,theassociationsaresupported HESSJ1804poorlycorrelateswiththeradiobrightnessdistri-
by the detection of large, asymmetric X-ray structures cor- bution. AlthoughlocatedwithintheboundariesofG8.7- 0.1,
related with the TeV brightness distributions and apparently the region around HESS J1804 in the radio image is much
connectedtothepulsars.However,inseveralpossibleassoci- dimmer than the northeast part of G8.7- 0.1 that also emits
ationstheexistingX-rayimagesarenotdeepenoughtoreveal X-raysobservedwithROSAT(seeFig.8). This,inourview,
an extended PWN component. In particular, the X-ray im- arguesagainsttheHESSJ1804andG8.7- 0.1association(see,
agesofthePWNaroundB1800–21(Kargaltsevetal.2006a) however, Fatuzzo et al. 2006, who argue that the TeV emis-
showahintofadim,asymmetricPWNcomponentextended sion can be producesby a shockin the G8.7–0.1interacting
towardHESSJ1804,butthesensitivityoftheChandraobser- withamolecularcloud).
vation was possibly insufficient to detect the PWN beyond A possibility that HESS J1804 is associated with the re-
15′′–20′′ from the pulsar. This is similar to PSR B1823– cently discovered faint radio (and IR) source G8.31–0.09,
13, where the arcminute-scale PWN was well seen only in likely an SNR (Brogan et al. 2006), is not attractive either.
a long XMM-Newtonobservation, and only a posteriori a G8.31–0.09 is outside the ACIS FOV, and it is not seen in
hint of it was found in the Chandradata (Kargaltsev et al. the archival ROSATPSPC image (Fig. 8; top). However,
2006b). Hence, there is a good chance that PSR B1800–21 G8.31–0.09 is far from the peak of the TeV brightness dis-
also has a dim, asymmetric PWN. It could be detected in a tribution(see theSpitzerimage in Fig. 7). Furthermore,the
deepXMM-Newtonexposure,therebyestablishingtheasso- sizeoftheshell-likeG8.31–0.09intheSpitzerimageismuch
ciationbetweenHESSJ1804andPSRB1800–21. smallerthantheTeVextentofHESSJ1804andhence,even
ifG8.31–0.09isindeedanSNR,itisunlikelytoberelatedto
9Forinstance,theRXJ1713.7–3946andG266.6-1.2SNRshavecompa-
3.3. AnSNRshock? rablesizesinX-raysandTeV.
10Theseauthorsstatethattheexpectednumberofsourceswithinthearea
WhilediscussingtheSu40(=Ch1)andSu42(=Ch2)asso-
definedbytheerrorbarsoftheHESSJ1804best-fitpositionshouldbevery
ciation with HESS J1804, Bamba et al. (2006) suggest that small,(4- 9)×10- 3. First,oneshouldnotusetheuncertaintyofthebest-fit
theX-rayandTeVemissioncouldcomefromanSNRshock TeVpositionforsuchanestimatewhentheTeVsourceisclearlyextended
(possibly in G8.7- 0.1). In our opinion, the fact that the an- andasymmetric. Second, asweseefromFig.1,theprobability offinding
gular extent of the two X-ray sources is much smaller than anX-raysourcewithafluxof10- 14–10- 13ergss- 1cm- 2withinanarbitrary
placedr=1′circleisquitehigh.
the extentof the TeV emission (see Fig. 1) is a strong argu-
10
HESSJ1804. remote SNR. In the former case, there remains a possibility
ofassociationofCh2withHESSJ1804. Furtheron-axisob-
4. SUMMARY servationswithChandraACISareneededtofirmlyestablish
We serendipitouslydetected several X-ray sources, whose thenatureofthetwosources.
positions are close to the maximum of the TeV bright- Itis possiblethatneitherCh1norCh2 areassociatedwith
ness distribution of the extended VHE source HESSJ1804. HESS J1804. In this case the most plausible interpretation
Amongthesesources,onlyCh1andCh2mightberelatedto of HESS J1804 is that the TeV emission comes from an X-
HESSJ1804.ThefactthatHESSJ1804isanextendedsource raydimpartoftheasymmetricPWNcreatedbyPSRB1800–
rulesoutanextragalactic(i.e.AGN)origin,anditalsoargues 21. A longer observation with XMM-Newtonor Chandra,
againstanHMXBinterpretation. combinedwithdeephigh-resolutionimagingintheradioand
On the other hand, the marginal detection of 106 s pulsa- IR, will finally differentiate between these possibilities and
tionsinCh1suggeststhatCh1mightbeanHMXBunrelated establish the nature the two Chandrasources as well as the
to HESSJ1804. Therealso remainsa possibility thatCh1 is originoftheTeVemission.
anewobscuredpulsar/PWNcouple,possiblyassociatedwith
G8.7- 0.1.Inthiscasenovariabilityisexpectedontimescales
& 1 s, but one could expect to see an X-ray PWN, which OurthanksareduetoAndreyBykovforprovidingtheInte-
hasnotbeendetectedintheChandraobservationpossiblybe- gralIBIS/ISGRI upper limit for the HESS J1804 flux. We
causeoftheoff-axisplacementontheACISdetector. are also grateful to Konstantin Getman for the useful dis-
ApossiblevariabilityofCh2onayeartimescalemightalso cussions about multiwavelength emission from young stars.
suggestthatCh2isanaccretingbinary,whichmakestheas- ThisworkwaspartiallysupportedbybyNASAgrantsNAG5-
sociationwith HESSJ1804unlikely. On the otherhand, the 10865andNAS8-01128andChandraawardsAR5-606Xand
extended appearance of Ch2 argues in favor of a PWN or a SV4-74018.
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