X-ray analysis of the proper motion and pulsar wind nebula for PSR J1741-2054

0.35 MB·English

by  Katie Auchettl

DRAFTVERSIONFEBRUARY24,2015
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
X-RAYANALYSISOFTHEPROPERMOTIONANDPULSARWINDNEBULAFORPSRJ1741-2054
KATIEAUCHETTL1,2,PATRICKSLANE1,ROGERW.ROMANI3,BETTINAPOSSELT4,GEORGEG.PAVLOV4,OLEGKARGALTSEV5,C-Y.
NG6,TEATEMIM7,8,MARTIN. C.WEISSKOPF9,ANDREIBYKOV10,DOUGLASA.SWARTZ9
DraftversionFebruary24,2015
ABSTRACT
5 We obtained six observations of PSR J1741-2054 using the Chandra ACIS-S detector totaling ∼300 ks.
1 By registering this new epoch of observationsto an archival observationtaken 3.2 years earlier using X-ray
0 point sources in the field of view, we have measured the pulsar proper motion at µ=109±10masyr- 1 in a
2
direction consistent with the symmetry axis of the observed Hα nebula. We investigated the inferred past

b trajectoryofthepulsarbutfindnocompellingassociationwithOBassociationsinwhichtheprogenitormay
e have originated. We confirm previousmeasurementsof the pulsar spectrum as an absorbed power law with
F photonindexΓ=2.68±0.04,plusablackbodywithanemissionradiusof(4.5+3.2)d km,foraDM-estimated
distance of 0.38d kpc and a temperature of 61.7±3.0 eV. Emission fr- o2.m5 t0h.3e8compact nebula is well
1 0.38
describedbyanabsorbedpowerlawmodelwithaphotonindexofΓ=1.67±0.06,whilethediffuseemission
2
seen as a trail extendingnortheastof the pulsar shows no evidenceof synchrotroncooling. We also applied
] imagedeconvolutiontechniquestosearchforsmall-scalestructuresintheimmediatevicinityofthepulsar,but
E foundnoconclusiveevidenceforsuchstructures.
H
Subjectheadings:pulsars:individual(PSRJ1741-2054)-X-rays:general
.
h
p
- 1. INTRODUCTION dio beam (Camiloetal. 2009). This makes J1741 a transi-
o
r PSRJ1741- 2054(J1741)isoneoftheclosestmiddle-aged tionalobjectbetweenaclassicalradio/γ-rayloudpulsarsuch
st (τc = 390kyr)pulsarsknown. IthasaperiodofP=413ms ainsgVlye,lRaoamndantiheetraald.i(o2-0q1u0ie)tdGeteemcteindgaa-2t0y′p′elopnugl,sanrosn.-rIandteiarteisvte-
a andwasfirstdiscoveredinγ-raysusingtheLargeAreaTele-
[ scope(LAT)ontheFermiGamma-raySpaceTelescopebya Hαbowshocknebulaaroundthepulsar.Modelingofthebow
shocksuggestedthatthepulsaristravelingwithavelocityof
blind search for periodic γ-ray pulsations from Fermi-LAT
2 point sources (Abdo 2009). It was subsequently detected in ∼150kms- 1,whiletheobservationofnegativeradialveloc-
v archivalParkesradiodataandobservedusingtheGreenBank ities from both sides of the nebula imply that the velocityis
5 Telescope (Camiloetal. 2009). The pulsar has a spin down directedoutoftheplaneoftheskyatanangleof15◦±10◦.
2
2 energylossrateofE˙ =9.5×1033ergs- 1whichismoderately Using a short Chandra ACIS-S observation (observation
ID (ObsID): 11251), Romanietal. (2010) detected an X-
3 lowcomparedtothoseofotherγ-raypulsars.Thepulsarhasa
0 verysmalldispersionmeasure(DM)=4.7pccm- 3andamag- lroanygpu(>lsa2r waricnmdinne)bXul-ara(yPWtraNil)awtiathninantghlies Hofα4n5e◦b±ul5a◦anEdasat
1. neticfieldof2.7×1012G.UsingtheNE2001Galacticelectron from North. Romanietal. (2010) also suggested that there
0 density model, the low DM implies a distance of 0.38 kpc are asymmetriesin the small scale structure surroundingthe
5 (Cordes&Lazio 2002). At this distance, its measuredradio pulsar, which they associate with a compact 2.5′′ equatorial
1 fluxat1400MHz(S∼160µJy)makesitoneoftheleastlumi- toroidal structure. Marellietal. (2014) and Karpovaetal.
: nousradiopulsarsknown. Itsγ-raypulsationslagbehindits
v (2014) performed a spectral analysis of the pulsar emission
radio pulsations by δ=0.29P, implying that our line-of-sight
i usingXMM-NewtonandChandraanddeterminedthatatwo-
X tangentiallycutstheγ-raycone,whilenearlymissingthera-
component(blackbodypluspower-law)modelisrequiredto
r obtainsatisfactoryspectralfits.
a 1Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, In this paper, we use ∼300 ks of Chandra data of J1741
Cambridge,MA02138,USA that were obtained as part of the Cycle 14Chandra Vision-
2SchoolofPhysics&Astronomy,MonashUniversity,Melbourne,Vic- ary Project “A Legacy Study of the Relativistic Shocks of
tor3iaD3e8p0a0r,tmAeunsttraolfiaPhysics, Stanford University, Stanford, CA 94305, PWNe”, plusa ∼49ksarchivalobservation,toconstrainthe
USA pulsar motion, and the geometry of the PWN outflow. We
4Department ofAstronomy&Astrophysics, Pennsylvania StateUni- discussourapproachtoimageregistrationandpropermotion
versity,525DaveyLab,UniversityPark,PA,16802,USA measurementin Section 3, followed by a discussion in Sec-
5DepartmentofPhysics,TheGeorgeWashingtonUniversity,72521st
tion4ofourimagedeconvolutioneffortstosearchforsmall-
St,NW,Washington,DC20052
6Department of Physics, The University of Hong Kong, Pokfulam scalestructurearoundthepulsar. InSection5wediscussthe
Road,HongKong,China resultsofourspectralfitsforthepulsaremissionandthatof
7Observational Cosmology Lab, Code 665, NASA Goddard Space theextendedPWN,andcomparethesewithpreviousresults.
FlightCenter,Greenbelt,MD20771,USA InSection6wediscusstheimplicationsofthepropermotion
8CRESST, University of Maryland-College Park, College Park, MD measurements,includingcomparisonswiththeobservedHα
20742,USA
9NASA/Marshall Space Flight Center, ZP12, 320 Sparkman Drive, nebulasurroundingJ1741,anddiscussthelackofevidenceof
Huntsville,AL,35805. forsynchrotroncoolinginthePWNtrail.Ourconclusionsare
10A.F.Ioffe Physical-Technical Institute, St. Petersburg 194021, also summarizedinSection7.
St.PetersburgStatePolitechnicalUniversity,Russia
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Right ascension
Figure1. MergedChandraexposure-corrected0.3-5.0keVACIS-SimageoftheextendedemissionaroundJ1741-2051.Thiswasproducedusingreproject_obs
and flux_obsandincorporatesallavailableChandraobservations. TheimageissmoothedusingaGaussianofwidth3′′andplottedonthelogarithmicscale.
Thereferencesourcesnumbered1through13wereusedforrelativeastrometry,whilethepulsarislabeledasP.Othersourcesseenintheimagearenotsuitable
forastrometrybecause,beingvariable,theywerenotsignificantlydetectedinboththearchivalobservationandoneofthenewepochobservations.Weincluded
acutoutregionaroundthepulsartoshowthepointsource. Themagentalinesshowourderivedpropermotion(solid)anditsuncertainty(dashed),tracedback
fromthepulsarposition.Thebackgroundregionusedforspectralanalysisisshownasthemagentadashedcircle.
2. X-RAYDATAANALYSIS information of ObsID: 11251 (CIAO task: reproject_obs)
and combined all reprojected observations into an exposure
Weobtained282ksofnewChandraACIS-Sexposuretime
correctedimageusingtheCIAOtask flux_obs. Themerged
ofJ1741. Table1liststheparametersforeachofthesixnew
ACIS-S image of the extended emission around the pulsar,
observationsthatweobtained,includingthearchivalobserva-
smoothed with a 3′′ Gaussian", is shown in Figure 1. The
tion.Toreducepileup,theCCDswereoperatedinhalf-frame
pulsarpointsourceliesattheapexofthediffuseX-rayemis-
VFAINT modeso thateventswere readoutevery1.7s. The
sion,whileadiffuse,faintX-raytrailextending∼1.9′isseen
pulsarwasplacedneartheoptimumfocusonthebacksideil-
towardsthenortheastofthepulsar.
luminatedS3chip. Inadditiontooursixobservations,there
isa48.8ksarchivalobservation(ObsID:11251),whichwas
takenon2010May21.Eachnewobservationhasarollangle 3. REGISTRATIONANDTHEPROPERMOTION
similartotherollangleofthe2010epoch(∼90◦),exceptfor Toconstrainthepropermotionofthepulsar,weregistered
ObsID:15544.Thisobservationhasarollangleof∼260◦as eachofthenewChandraimagesto thearchivalimageusing
thenominalrollangleofChandraisrotatedby180◦ during field point sources that were identified using the CIAO tool
thetimeoftheyearthisobservationwascompleted.Thedata wavdetect. Weselectedsourceswithadetectionsignificance
wereanalysedwithCIAO4.6.2afterallobservationswerere- of>3σthatwerefoundontheS3chipinboththe2010obser-
processedusingtheCALDB4.5.9.Noflaringoccurredinany vationandthecorrespondingnewobservation.Thesesources
oftheobservationssothefullexposuretimeswereused. arehighlightedinwhiteandthepulsarislabeledasPinFig-
Using all seven observations we produced a merged, ure1.
exposure-corrected image of J1741 by reprojecting the new Careful consideration and modeling of the point-spread
observationsto a common tangent plane based on the WCS function (PSF) must be undertaken to reduce the effect of
3
0 3
0
9 11251
0.
2
)ce-0.1 -
s
cra( AR 0.903
ni te-0.2 -2
sffO on 03
-0.3 ati 0.9
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cli -
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-0.4 0 0.5 1 1.5 2 2.5 3 D 904
Years 0.
2
-
4
0
0 9
0.
2
-
)c
es-0.1
cra 265.490 265.489 265.489 265.489 265.488 265.488
( C
ED Rightascension
n-0.2
i te
s Figure3. DeconvolvedimageofJ1741-2054fromObsID11251obtained
ffO usingarestoreafter50iterations. ThecyanregioncorrespondstothePSF
-0.3 asymmetryseeninChandradatawhenpushingtosub-ACIS-pixelresolution.
The image has been logarithmically stretched and the colour enhanced to
highlighttheobservedfeatures.
-0.4
0 0.5 1 1.5 2 2.5 3
Years
absorbedMekalmodelinSHERPA.Toimprovetheaccuracy
of the PSF modeling, we increase the normalisation of this
Figure2. OffsetinthepositionofthepulsarinRA(top)andDEC(bottom)
inthenewobservationsfromthepositionofthepulsarinthearchivalobser- spectrumbyafactorof100beforepassingitintoSAOTrace.
vationplottedagainstthetimesincethefirstobservationofPSRJ1741with Amodelofapointsourceatitspositionisobtainedbypass-
Chandra(years).Thedashedlinecorrespondstothelineofbestfitinwhich ing the raytrace from SAOTrace into the program MARX12.
theslopecorrespondstothepropermotionofthepulsar.
EachPSFmodeliscorrectedforthescienceinstrumentmod-
ule(SIM)offsetfromnominallocationandfilteredusingthe
GoodTimeInterval(GTI)datafromtheoriginaleventfile.
Table1 Todeterminethepositionof thesourceswe usethe maxi-
ChandraobservationsofPSRJ1741-2054 mumlikelihood“figureofmerit”(FoM)techniquedeveloped
ObservationID ExposureTime(ks) ObservationDate byVanEttenetal.(2012). Wegeneratea39pixelby39pixel
archival imageofthemodeledPSF, binnedto 1/9ACISpixelresolu-
11251 48.78 2010-05-21 tion.Thisisthencomparedto0.3-5.0keVsourceimagesof
newobservations thesamesizebutbinnedtonativeACISpixelresolution.PSF
14695 57.15 2013-02-06
models and source images are producedfor all observations
14696 54.30 2013-02-19
15542 28.29 2013-04-01 andforeachregistrationsource. ThePSFisshiftedalongthe
15638 29.36 2013-04-02 xandyaxesofthe1/9pixelgridandrebinnedtonativeACIS
15543 57.22 2013-05-15 pixels. We thencomputetheFoMateachoffsetinpixelco-
15544 55.73 2013-07-09
ordinates, giving us a map of the likelihood of the observed
countswithrespecttothexandyposition. Todeterminethe
changes in the PSF shape on the count distribution of our best-fitpositionofthesource,wefitatwodimensionalGaus-
point-sources. To improve the astrometric accuracy and re- sian to the FoM surface, with the minimum of this surface
ducethiseffect,wesimulateaPSFforeachpointsourcepo- providingthebestfitpositionofthesource. Thestandardde-
sition,ineachobservation,anduseittofitforthepositionof viationsalongxandyareestimatedbycalculatingthesquare
thesource. rootoftheeigenvaluesofthecovariancematrix.
To simulate a PSF of a point source, we use the software Prior to registration we checked each source for any opti-
suiteSAOTrace11whichisdesignedtosimulatethepropaga- cal counterparts using VizieR13. Sources 6, 8, 9, 10 and 13
tionofphotonsfromastronomicalobjectsthroughtheoptics allhaveopticalcounterpartswithin2′′ofthePSF-fitposition.
oftheChandraX-raysatellite. Weusetheaspectsolutionfile Source8and13haveestimatedpropermotionsof(µRA,µDEC)
ofeachobservationandprovidedspectralinformationforthe = (22,- 62) masyr- 1 and (20,-22) masyr- 1, respectively (er-
raytracebyextractingthespectrumofeachstellarsourceus- rorsδµ ∼7masyr- 1,δµ ∼15masyr- 1). Wecorrected
RA DEC
ingtheCIAOtaskspecextract.Wemodelthespectrausingan
12http://space.mit.edu/CXC/MARX/
11http://cxcoptics.cfa.harvard.edu/SAOTrace/Index.html 13http://vizier.u-strasbg.fr/viz-bin/VizieR
4
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-20.90 0.880
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Declinatio -20.90304 Declinatio -20.89
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05 - 3
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265.510 265.500 265.490
265.492 265.491 265.490 265.489 265.488 265.487
Rightascension
Right ascension
Figure4. Logarithmicallyscaled,mergedChandraexposure-corrected0.3-5.0keVACIS-Simageofthepulsaranditsextendedemission. Left: Thegreen
circular regiondefines the0.70′′ regionusedtoextract thespectrum ofthepulsar fromallavailable observations. Thecyanrectangular regiondefines the
regionusedtoextractthespectrumofthepulsar’scompactnebulafromallobservations. Right:Thegreenregionsdefinetheaperturesusedtolookforspectral
variabilityintheextendedemissionofthepulsar.Thepulsarwasexcludedfromthespectralextractionofregion1.
for each frame was then registered by applying the best-fit
Table2
transformations(Table2).
Frameshiftsandtheiruncertaintiesusedforregistration.
To quantify the proper motion of the pulsar, we plot in
ObsID RA DEC
Figure 2 the offset in the position of the pulsar between the
(arcsec) (arcsec)
archival observation and the new observations against the
14695 - 0.02±0.02 0.48±0.02
14696 - 0.01±0.02 0.25±0.02 number of years since the archival observation. The uncer-
15542 0.02±0.04 0.43±0.04 taintiesintheoffsetarecalculatedbyaddinginquadraturethe
15638 - 0.06±0.03 0.36±0.03 uncertaintiesinthefitpositionsofthepulsar,theuncertainty
15543 0.23±0.02 0.29±0.03
15544 - 0.11±0.03 - 0.29±0.03 in the frame shifts and the systematic uncertaintyassociated
withchoosingaparticulartangentplanewhencreatinganim-
ageinskycoordinates.Wefittheoffsetusingalinearfunction
forthesenominalpropermotions,butalsoconfirmedthatour thatcorrespondstothepositionalshiftofthepulsarbetween
final astrometric solution was insignificantly changed if we thearchivalobservationandthenewobservationsandthisis
excludedthesetwostarsfromtheanalysis. seenasthedashedlineinFigure2.
Using the FoM positions of our registration sources and We obtain a proper motion of µ cos(δ) = - 63±12
RA
their uncertaintiesas a referencegrid to performthe relative masyr- 1 and µ =89±9 masyr- 1. This corresponds to
DEC
astrometry,wedeterminedthebesttranslationtransformation a totalpropermotion of 109±10masyr- 1. Assuming a dis-
neededto registerthe imagesfromthetwo epochsusing the
tanceof0.38kpctothepulsar,thistranslatestoa transverse
CIAOcommandwcs_match. Foreach observationwe adopt velocityof(196±18)d kms- 1. Thepositionangleofthe
the Chandra-determined roll angle. In Table 2 we list the propermotionis215◦±0.368◦ eastofnorth. Thepropermotion
best-fitframeshiftsandtheiruncertainties. Theuncertainties
axispointsintheoppositedirectionoftheextendedX-raytrail
intheseshiftswerecalculatedbyaddinginquadraturetheer-
asexpected(seeFigure1).
rors in the differencesbetween each source before and after
shift. Addingarotationtothetransformationdidnotproduce
4. X-RAYIMAGING
astatistically significantimprovementonthebest-fittransla-
tion. Usingthe archivalChandraobservationofJ1741(ObsID:
Tocalculatethepositionofthepulsarafterregistration,we 11251), Romanietal. (2010) performed a PSF subtraction
first calculate the position of the pulsar in the unregistered of the pulsar point source to look for any small-scale struc-
frames. We simulate a PSF at the position of the pulsar in ture surroundingthe pulsar. Theydiscoveredthat the region
around the pulsar appears to be slightly extended and they
eachobservationusingthemethoddescribedfortheregistra-
tionsources.TodefinetheenergydependenceofthePSF,we associate this feature with the equatorial torus of the PWN.
Using the same data as analyzed here, Karpovaetal. (2014)
extract a spectrum of the pulsar and fit it with an absorbed
searchedforevidenceofsuchstructurebyperformingfitstoa
power law plus blackbody model (see Section 5). For each
observation, we generate a 6 pixel by 6 pixel image of the two-dimensionalGaussian convolvedwith PSF modelsgen-
erated for each observation. They find no evidence for any
PSFmodelofthepulsarthatisbinnedto1/4ACISpixelres-
small-scaleextendedfeaturesotherthanforasmallemission
olution. Usinganimageofthe0.3-5.0keVpulsareventsof
thesamesizeandbinning,wefitforthepositionofthepulsar feature associated with a known mirror artifact14. We have
carriedoutasimilarinvestigationusingimagedeconvolution
inSHERPA. Inthisfit, weusedadeltafunction(thepulsar)
plus a two dimensional Gaussian (the circumpulsar PWN),
bothconvolvedwiththePSF.Theresultingpulsarfitposition 14http://cxc.harvard.edu/ciao/caveats/psf_artifact.html
5
Table3
SpectralfitvaluesoftheextendedemissionofthepulsardefinedbythecyancolouredregioninFigure4aandthegreenregionsdefinedinFigure4b.All
uncertaintiesareoneσ.
Region NH(×1021)cm- 2∗ Γ AbsorbedFlux UnabsorbedFlux χ2/dof
10- 14ergcm- 2s- 1 10- 14ergcm- 2s- 1
compac1234tPWN 1.20----+- 00..0087 1.11116....095567073±+-+-+-+00000000.......1111211.287650920 21104.....5583864156+-+-+-++- 000000000.........111101212325081870 11134...861..8117015±±±++- 000000......000311497361 00000.....9998890940
5 - 1.70+- 00..1101 5.40+- 00..2195 6.03+- 00..0164 1.16
∗Fixedatvaluefromjointcompactnebula. - 0.11 - 0.36 - 0.16
techniques. We simulate a PSF of the pulsar in each obser- withtheabovestudies,andadoptthisvalueinallourmodels
vation using MARX following re-reduction of ourChandra of the pulsar spectrum. We use the Wilms et al. abundance
observationswithCALDB4.4.7tomatchthecalibrationdata tablethroughoutouranalysis(Wilmsetal. 2000). Modeling
usedforMARX.WedefinetheenergydependenceofthePSF thespectrumwithapower-lawplusblackbodymodel,weob-
as described in Section 3 and use the dither pattern of the tained a photon index of Γ = 2.68±0.04 and a blackbody
observation with an aspect blur of 0.07, which corresponds spectrum with a temperature of kT =61.7±3.0 eV, in ex-
eff
to the uncertainty in the telescope pointing15. We also cor- cellent agreement with the results reported by Marellietal.
rect for SIM offset. Using this PSF we deconvolve a 0.3 - (2014) and Karpovaetal. (2014). Omitting the pile-up cor-
5.0keVpulsarimageusingtheLucy-Richardsondeconvolu- rectionyieldssimilarvalues. Weusethebest-fitvaluesabove
tionalgorithm(Lucy1974)implementedusingtheCIAOtask forPSFmodelingofthepulsar.
arestore. Allimageswerebinnedtoquarter-ACISpixel. We We alsoranfitsusingmagnetisedneutronstaratmosphere
ran arestore multiple times using a numberof differentiter- models (nsa and nsmax in SHERPA) for the thermal com-
ationsbetween10and200todetermineconvergenceofnew ponent. These gave somewhat different temperatures and
features.Nonewstructuresappearedafter50iterations. emitting areas, but did not significantly improve the qual-
The deconvolvedimage fromone observationis shownin ity of the fit. For example a magnetic carbon atmosphere
Figure 3. The emission is well described by a point source, model(nsmaxmodel12006,Mori&Ho(2007))gaveatem-
withtheexceptionoftheartifactfeature(identifiedincyan). perature of kT = 86.0±9.0 eV and emission radius of
eff
Wethusseenoconclusiveevidenceofothersmall-scalestruc- R = (4.90+3.0)d km. The power law component was
emis - 2.3 0.38
ture in the immediate region surroundingthe pulsar, consis- onlyslightlyaffectedwithΓ=2.63±0.03.
tentwiththeresultsreportedbyKarpovaetal.(2014).
5.2. PWNanditsextendedX-raytrail
5. SPECTRALANALYSISOFTHEX-RAYEMISSIONOFTHE
To analyse the spectrum of the compact nebula described
PULSARANDITSTRAIL
above,we extractedspectra fromeachobservationusingthe
5.1. Pulsar
cyanrectangularregioninFigure4aandcombinedtheseus-
As noted above, modeling of theChandra PSF for use in ingspecextract. We used the same backgroundspectrumas
determining an accurate pulsar position requires knowledge forthepulsarspectrum. Thecompactnebulacontainsatotal
ofthesourcespectrum. XMM-Newtonobservationsestablish of ∼900 countsand we binned the combinedspectrum with
a two-componentspectrum for the pulsar, with a blackbody a minimum of 20 counts per spectral bin. The PWN spec-
accompanied by a power law (Marellietal. 2014). Similar trumisconsistentwithanabsorbedpowerlawwithanindex
resultswerederivedbyKarpovaetal.(2014)usingthesame ofΓ=1.60±0.20.
Chandradatareportedhere. We havere-analyzedthesedata Todeterminewhetherthereisanyspectralvariationinthe
by extracting events from a 0.70′′ radius region centered on PWN and its extendedtrail, we extractspectra fromthe five
thepulsar,shownasthegreencircleinFigure4a. Thesizeof regions defined in Figure 4b. These regions correspond to
this region was chosen to minimise the contamination from roughly the same spectral regions reported by Marellietal.
the PWN, and subsequent modeling results have been cor- (2014),exceptthatweinvestigatesmallerregionsinthecom-
rected for the finite encircled energy fraction. All spectra pactnebulanearthepulsar.Intheouterportionsofthenebula,
were grouped with a minimum of 20 counts per bin, and a thecountrateistoolowtoobtaingoodspectrainsmallerre-
backgroundspectrumwasobtainedusingasource-freecircu- gions. We modeleachregionindividuallyusinganabsorbed
larregionwitharadiusof30′′,asshownbythemagentacircle powerlaw,wherewefixcolumndensitytothevaluederived
inFigure1.UsingtheCIAOpileup_map,wedeterminedthat earlierfrom fits to the innernebularegionbutlet photonin-
approximately5%ofthepulsareventssufferfrompileup.We dexandthenormalisationvary. WehavelistedinTable3the
thus included a pileup model in our spectral fits, where the absorbed flux, unabsorbed flux and the reduced χ2 for each
frametimeis1.7sandthePSFfractionisallowedtovary.All region,aswellasthebest-fitparametersfromfittingthetrail
other parameters in the pileup model were frozen at default spectra. There is no evidence of systematic variation in the
values. photonindexofthecompactnebulaandtrail(regions1-5).
To constrain the column density, we fit spectra from the The photon index of region 1 is slightly higher than that of
compactnebulatothenortheastofthepulsar(seeSection5.2) the other four regions, but is consistent within uncertainties
using an absorbed power law (more details in section 5.2). withallregionsexceptforregion2. Thisslightvariationbe-
WeobtainedNH =(1.20+- 00..0087)×1021cm- 2,ingoodagreement tween region 1 and 2 could suggest that region1 is affected
byleakageofthesofteremissionfromthepointsource.Mod-
15http://space.mit.edu/CXC/marx/news.html elingthephotonindexasafunctionofdistancefromthepul-
6
Figure5. Thepasttrajectory(red)ofPSRJ1741- 2051inGalacticcoordinates. Positionsaremarkedevery105y,withuncertaintyellipsesatτc=3.91×105y
andat106y. ForcomparisonweshowthelocationsandsizesofOBassociations fromtheMel’Nik&Efremov(1995)catalog(bluedottedellipses)andthe
HipparcossampleofOBstars(greencircles). AsubstantialconcentrationofOBstarsat∼0.3- 0.6kpcliesnearthepulsartrackat∼τc andamoredistant
(d=1.45kpc)OBassociationoverlapswiththetrackuncertaintyatthisage.
sar using a linear regression fit with a constant function in can be identified. However, there are certainly many plau-
SHERPA,weobtainΓ=1.67±0.06forthetrail. Theglobal sibly associated massive stars consistent with our preferred
PWN indexderivedfromthe SHERPAfit andthe valueswe d∼0.4kpc,especiallyconsideringthatsomepulsarprogeni-
obtainedinTable3areconsistentwithKarpovaetal.(2014) torsmaybeOBrunawayswithsignificantoffsetduringtheir
and Marellietal. (2014), who derived Γ = 1.74±0.07 and pre-explosionlifetime.
Γ=1.78±0.15respectivelyforthePWN. NeitherourdeconvolvednorourPSF-subtractedimagesin-
dicateconclusiveevidenceofsmall-scalestructuresurround-
6. DISCUSSION ing the pulsar that might be associated with a torus or jet-
UsingChandraobservationsofPSRJ1741-2054thatspan likefeature. TheequatorialtorusstructurethatRomanietal.
∼ 3.2 year period, relative astrometry measurements have (2010) associate with a diagonal excess seen ∼0.75′′ from
identifiedapropermotionofµ=109±10masyr- 1.Thiscorre- the core of the pulsar image seems to have arisen from the
spondstoamodestvelocityof(196±18)d kms- 1,which mirror asymmetry. Karpovaetal. (2014) perform a similar
0.38 analysisandcometothesameconclusion. However,itisin-
agreeswellwiththevelocityderivedbyRomanietal.(2010)
terestingtocomparethenebulaheadandpropermotionwith
usingHαspectroscopy. ThelargerdistanceofKarpovaetal.
(2014)givesatransversevelocityof∼400km/s,inconsistent the Hα structure described in Romani et al (2010). In Fig-
ure6we see thatthe pulsarlies verycloseto thebowshock
with that obtained from optical spectroscopy. The direction
of the proper motion is 205◦±6◦ east of north, opposite the limb(accuracylimitedbyourrelativeX-ray/opticalastrome-
try). Interestingly,ourmeasuredpropermotionis consistent
elongated X-ray trail. Ng&Romani (2004), Johnstonetal.
with, although nominally slightly south of, the Hα nebula’s
(2005) and Ng&Romani (2007) found that the direction of
symmetry axis. However the X-ray PWN trail fills only the
propermotionofa pulsaris, generally,approximatelyparal-
southernhalfoftheapparentHαcavity,punchingoutthrough
leltoitsrotationaxis.
a gap at the back end of the Hα emission and continuingto
In Figure 5 we plot the pulsar track (red) in Galactic co-
thearcmin-scaletrailbeyond.Theoriginofthisasymmetryis
ordinates (note the expanded b scale). This has the pulsar
unclear,butacluemaybeseenintheX-raycontours,whose
skimmingabovethe plane. Itdoesnotintersectthe planeit-
seevlefru,nthleestsraocnkeestxatrrtaspwoliathteisnatnheun∼re5a0sopncabOlBe∼st2ar×sc1a0l6ey;hheiogwh-t rtiiodngeblyin≈el3ie5s◦a.tTPAhi≈ss7u0g◦g,ei.set.s,ma siseacloignndesdywmimthetthreyparxoipseirnmthoe-
fort ∼τ and distances d <1kpc. For comparison we plot PWN,possiblyduetoapulsarjetorotheroutflowconcentra-
c tion. ThisdirectstheshockedPWNplasmatothesoutheast,
thepositionsoftheHipparcoscatalogOBstars(green),with
preferentiallyfillingthishalfoftheHαcavity. Amorecom-
circlesizeproportionaltotheparallax. Thissetisquitecom-
plete,withusefulparallaxes,to∼500pc,andincreasinglyin- pletediscussionofthePWNgeometry,includingthe3DHα
kinematics,isinpreparation.
completeat largerdistances. Atlargedistancethe cataloged
The X-ray spectrum of the pulsar requires a combination
OB associations (Mel’Nik&Efremov 1995) provide plausi-
of non-thermal and thermal model components. The emis-
ble pulsar birthsites, and their cataloged extent is plotted by
sionisdominatedbythenon-thermalcomponent(∼75%of
the blue dotted ellipses. Intriguingly, one association over-
unabsorbed flux), indicating that the majority of the X-ray
lapsthe pulsar track, butthis is ata likely unreasonablecat-
emission is magnetospheric in nature. The emitting radius
alogdistanceof1.45kpc. We concludethat, withthepulsar
implied by the blackbody model for J1741 corresponds to
motionpassingalong the Galactic plane, there will be many
(4.5+3.2)d km. This is substantially smaller than any vi-
superposedmassivestarlocations,andnodefinitivebirthsite - 2.5 0.38
7
observedphotonenergy of 1 keV. This is comparableto the
45.0 (length/propermotion)=108′′/109∼103yrrequiredforthe
53:50.0 ptiuolns.aTrthoutsraitviesrsneotthseurbprrigishinttgrathilawttihtheroeuirsonbosedrrvaemdaptircosppeercmtroa-l
steeping along the trail. If the PWN electrons flow at even
55.0 fasterspeedswithinthetrail,thisconclusionisevenstronger.
-20:54:00.005.0 Using ∼300 ks ofC7h.aCnOdNrCaLAUCSIIOSN-S observationsof PSR
J1741-2054,wewereabletodeterminethepropermotionof
10.0 the pulsar with a detection significance>3σ. The direction
ofthepropermotionisalignedwiththeextendedPWNemis-
15.0 sion,andcorrespondswellwithasymmetryaxisoftheasso-
ciated Hα nebula. The diffuse X-ray emission immediately
20.0 behindthe pulsar is concentratedin the southeasternportion
oftheHαnebula,possiblysuggestinganotherflowaxisfrom
25.0 ajetortorusin thepulsarsystem. Thetrajectoryofthepul-
sar,extrapolatedoverthecharacteristicage,doesnotprovide
59.5 59.0 58.5 17:41:58.0 57.5 57.0 56.5
a compellingcorrelation with known OB associations at the
Figure6. TheheadofthePSRJ1741- 2051nebula.Theredchannelshows distanceofthepulsar,althoughtherearemanymassivestars
Hαemissionat6562- 6564Åwhilethegreenchannelshowsaverageofthe
consistentwiththisdistancethatcouldpotentiallyhavehada
continuum6551- 6561,6565- 6575Å withdataabstractedfromanAATSPI-
RALIFUobservation. Thebluechannelshowthe0.3- 7keVX-rayphotons commonorigin.
fromourcombinednew observation. Greencontours helponetoreadthe The pulsar spectrum is well described by an absorbed
X-raystructure,whiletheyellowarrowsshowourmeasuredpropermotion, powerlawaccompaniedbyablackbodywithanemissionra-
withthearrowheadsmarkingthepositionin100yr. diusof(4.5+3.2)d kmanda temperatureofkT =61.7±
- 2.5 0.38 eff
3.0 eV, as found in earlier works. The thermal component,
ableneutronstarradius(Lattimer&Prakash2007),suggest- a hot region on the neutron star surface, is augmented by a
ingthatthisthermalemissionarisesfromhotspotsonthesur- magnetospheric or unresolved PWN power law component.
face,plausiblynearthemagneticpoles(Ho&Heinke2009). The PWN plus its extended trail can be well described us-
Infact,Marellietal.(2014)dodetectapulsedthermalcom- ing an absorbedpower law and there is no evidenceof vari-
ponentforPSRJ1741,alsosupportingsuchasurfacetemper- ationin thephotonindexwithdistancefromthepulsar. The
atureinhomogeneity. integrated luminosity of the PWN over the 0.5 - 10 keV is
TheX-rayemission fromthecompactnebulaandthetrail 2.36×1030ergs- 1. Thisrepresents0.02%ofthepulsarspin
is consistent with an absorbed power law. There is no dis- downpower,whichisnotatypical.Wefindnoconclusiveev-
cernibleevidenceofspectralvariationwithdistancefromthe idenceofsmall-scalestructuresurroundingthepulsarthatwe
pulsar and the spectrum of the entire tail can be described canassociatewithatorusorjet-likestructure.
by Γ = 1.67±0.06. We compute the minimum (equiparti-
tion) energy by approximating the X-ray emission from the
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