A Stellar Wind Bubble Coincident with the Anomalous X-ray Pulsar 1E 1048.1-5937: Are Magnetars Formed From Massive Progenitors?

0.12 MB·English

by  B. M. Gaensler

TOAPPEARINTheAstrophysicalJournal(Letters)
PreprinttypesetusingLATEXstyleemulateapjv.6/22/04
ASTELLARWINDBUBBLECOINCIDENTWITHTHEANOMALOUSX-RAYPULSAR1E1048.1–5937:ARE
MAGNETARSFORMEDFROMMASSIVEPROGENITORS?
B.M.GAENSLER,1N.M.MCCLURE-GRIFFITHS,2M.S.OEY,3M.HAVERKORN,1J.M.DICKEY4ANDA.J.GREEN5
ToappearinTheAstrophysicalJournal(Letters)
ABSTRACT
Wepresent21-cmHIobservationsfromtheSouthernGalacticPlaneSurveyofthefieldaroundtheanoma-
lous X-ray pulsar 1E 1048.1–5937, a source whose X-ray properties imply that it is a highly magnetized
neutron star (a “magnetar”). These data reveal an expanding hydrogen shell, GSH 288.3–0.5–28, centered
on 1E 1048.1–5937,with a diameter of 35×23 pc (for a distance of 2.7 kpc) and an expansion velocity of
5
0 ≈7.5kms- 1. WeinterpretGSH288.3–0.5–28asawindbubbleblownbya30–40M⊙star,butnosuchcentral
0 starcanbereadilyidentified.WesuggestthatGSH288.3–0.5–28isthewindbubbleblownbythemassivepro-
2 genitorof1E1048.1–5937,andconsequentlyproposethatmagnetarsoriginatefrommoremassiveprogenitors
thandoradiopulsars. Thismaybeevidencethattheinitialspinperiodofaneutronstariscorrelatedwiththe
n
massofitsprogenitor,andimpliesthatthemagnetarbirthrateisonlyasmallfractionofthatforradiopulsars.
a
J Subjectheadings: ISM:bubbles—ISM:individual(GSH288.3–0.5–28)—pulsars: individual(1E1048.1–
5937)—radiolines:ISM—stars: winds,outflows—stars: neutron
6
2

1. INTRODUCTION Theenvironmentsofmagnetarsprovideuniqueconstraints
1
v The last decade has revealed remarkable diversity in the on their origin and evolution. Here we present HI obser-
vations of the interstellar medium (ISM) surrounding the
3 young neutron star population: radio pulsars, soft γ-ray re-
6 peaters (SGRS), anomalous X-ray pulsars (AXPs) and cen- AXP 1E 1048.1–5937 (Gavriil&Kaspi 2004, & references
5 tral compact objects (CCOs) are all now known to be po- therein), which show that this source is possibly associated
1 tential compact remnants of core-collapse supernovae (e.g., withalargeexpandingHIshell. Thisassociationstrengthens
0 thepossibilitythatmagnetarsareformedfrommoremassive
Kaspi&Helfand2002). While the CCOs remainenigmatic,
5 progenitorstarsthanareradiopulsars.
the AXPs and SGRs are now both believed to be popula-
0
tions of “magnetars”, neutron stars whose persistent X-ray
/ 2. OBSERVATIONS
h emissionandoccasionalX-andγ-rayburstsarepoweredby
p the energy associated with extreme surface magnetic fields, The HI data shown here were taken from the Southern
- &1015G(seeWoods&Thompson2005,forareview). GalacticPlaneSurvey(SGPS;McClure-Griffithsetal.2005),
o The low Galactic latitudes and the associations of some which has mapped HI and 1.4-GHz continuum emission in
tr of these sources with supernova remnants (SNRs) make it the range 253◦ ≤ l ≤ 358◦, |b| ≤ 1.◦5, at a resolution of
as clear that magnetarsare youngneutron stars (Gaensleretal. ≈2′.ThesurveycombinesinterferometricdatafromtheAus-
traliaTelescopeCompactArraywithsingle-dishdatafromthe
: 2001). However,whatisnotcleariswhysomeneutronstars
v Parkes64-mradiotelescope,givingsensitivitytoawiderange
are “normal” radio pulsars, while others are X- and γ-ray
i ofspatialscales. Thefinalspectrallinedata-sethasavelocity
X emitting magnetars. The physical distinction cannot simply
be the strength of the dipole magnetic field, since there is resolutionof∼0.8kms- 1andasensitivityof∼2K.
r
a now a small population of radio pulsars whose dipole field TheSGPSrevealsastrikingcavityinHI, almostcentered
strengths(asinferredfromtheirspindown)areatmagnetar- on the position of 1E 1048.1–5937 (namely l = 288.◦3,b =
likelevels,butwhichhavecompletelydifferentX-raycharac- - 0.◦5),asshowninFigure1. Theshellhasacentralvelocity
relativetotheLocalStandardofRest(LSR)V =- 28kms- 1;
teristics (Pivovaroffetal. 2000; Kaspi&McLaughlin 2005).
we thus designate this object GSH 288.3–0.5–28. At this
Amongstthepossibilitiessuggestedtoexplainthedifferences
central velocity, the major and minor axes of GSH 288.3–
betweenthesetwopopulationsarethatthemagneticfieldsof
0.5–28 are 45′ and 29′, respectively. The images of l ver-
radio pulsars and magnetars differ in their geometry or ori-
susV and b versusV, also shown in Figure 1, demonstrate
entation(e.g., Zhang&Harding2000; Kulkarnietal. 2003),
thatmagnetarsoriginatefrommoremassiveprogenitorsthan that GSH 288.3–0.5–28 is expanding, at a velocity Vexp ≈
doradiopulsars(Eikenberryetal.2004),thathigh-fieldradio 7.5 km s- 1. GSH 288.3–0.5–28is seen in multiple velocity
pulsarsarequiescentmagnetars(Kaspi&McLaughlin2005), planes, shows a high contrastbetween its walls and interior,
orthatradiopulsarsandmagnetarsforman evolutionaryse- andchangesinangularextentasafunctionofvelocity.Itthus
quence(Lyne2004;Heyl&Hernquist2005). meets all the standard criteria for HI shells in the ISM (see
McClure-Griffithsetal.2002).
1Harvard-SmithsonianCenterforAstrophysics,60GardenStreetMS-6, Standardrotationcurves(e.g.,Brand&Blitz 1993) forbid
Cambridge,MA02138;[email protected] systemic motions having the central velocity observed for
2AustraliaTelescopeNationalFacility,CSIRO,POBox76,Epping,NSW GSH288.3–0.5–28,indicatingsignificantdeviationsfromcir-
1710,Australia
3DepartmentofAstronomy,UniversityofMichigan,830DennisonBuild- cularrotationinthisregion. However,theshell’scentralve-
ing,AnnArbor,MI48103 locity matches the observed terminal velocity in this direc-
4PhysicsDepartment,UniversityofTasmania,GPOBox252-21,Hobart, tion, which allows us to estimate a distance of 2.7 kpc from
Tasmania7001,Australia geometry alone. We assume that random cloud motions on
5SchoolofPhysics,UniversityofSydney,NSW2006,Australia theorderof6kms- 1 contributetoanerrorinthedistancees-
2 GAENSLERETAL
timateontheorderof40%,or1kpc. Incorporatingboththis 100–200pcacross,hasbeenidentifiedinionized,neutraland
distance uncertainty and the non-circular shape of the shell, moleculargas(Cowieetal.1981;Rizzo&Arnal1998). Any
weadoptaradiusR=14±7pc. Byintegratingalonganum- individualstarinthisvicinitywillnotsweepupaneutralshell
beroflinesofsightthroughtheshellwallsandthensubtract- ofitsownbutwillcontributetotheoverallionizedsupershell.
ingameanbackground,wefindanaveragehydrogencolumn SincethepropertiesofGSH288.3–0.5–28areconsistentwith
density through the shell walls of (2.5±0.6)×1020 cm- 2. a single star sweeping up neutral gas, it seems unlikely that
If this material uniformly filled the region into which the thisshellisinthevicinityofCarOB1.
shell has expanded, the implied ambient number density is Second, the visual extinctiontoward the stars in Car OB1
n ≈17±9cm- 3. islow,A .3(Humphreys1978;Hoekzemaetal.1992). At
0 V
thisextinctionanddistance, a massivestar shouldbeclearly
3. INTERPRETATION detected at the center of GSH 288.3–0.5–28. For exam-
3.1. TheNatureofGSH288.3–0.5–28 ple, the O6Vstar consideredabovewouldhavea magnitude
V . 9. We have performed an exhaustive search for such
Shells like GSH 288.3–0.5–28 are common, and result
sources (e.g., Humphreys 1978; Forte&Orsatti 1981; Reed
from the effect of massive stars on the ISM. Specifically,
1998;Kaltcheva2003),fromwhichtheonlycataloguedmas-
GSH288.3–0.5–28couldbeanHIIregion,aSNR,orastellar
sive star projected near the center of GSH 288.3–0.5–28 is
windbubble.Belowwebrieflyconsiderthesepossibilities
IX Car (HD 94096; see Fig. 1), an M2Iab star with a mass
The photoionizing flux needed to maintain an HII region
is N∗ =4πR3n20α/3, where α≈3×10- 13 cm3 s- 1 is the re- oanfd∼it2s0LMSR⊙.raTdhiaeldviesltoacnictyeoesft–im17atkedmtso- 1IX(HCuamrpohfr∼ey1s.169k7p8c)
combinationcoefficientforhydrogen(Osterbrock1989). For
suggestthatit is a foregroundobject. Other massive stars in
t1h0e49obs-s1e.rveTdhivsaliusecsoonfsiRsteanntdwni0t,hwtheereioqnuiizreinNg∗fl∼ux(e3s±of2)th×e the vicinity,such as HD 93843,HD 94230,HD 305599and
LS 1976, all lie on the very edge or outside of GSH 288.3–
earliest-typestars, butsuchasourcewouldalsohavearadio
0.5–28inprojection.
flux of >100 Jy at decimeter wavelengths. A radio contin-
Finally, the photoionization from stars in Car OB1, most
uumimageofthisregionshowsnosuchemissionassociated
notablytheO5IIIstarHD93843,shouldfullyionizetheshell.
withGSH288.3–0.5–28(Greenetal.1999).
If we assume that the shell thickness is 20% of its radius,
For the observed values of n , R and V , a SNR in the
0 exp the edges of the shell should show Hα emission at a sur-
Sedovorradiativephasesrequiresaninitialexplosionenergy
face brightness of thousands of rayleighs. We have exam-
of ∼5×1049 or ∼1050 ergs, respectively. These estimates
inedsensitiveimagesofthisregion(e.g.,Buxtonetal.1998;
are both well below the typical supernova explosion energy
Gaustadetal. 2001); these show no Hα emission associated
of1051ergs,makingthisinterpretationunlikely.Thereisalso
withGSH288.3–0.5–28downtomuchlowersurfacebright-
noevidenceindeeparchivalobservationsforanyradioorX-
ness limits. We therefore believe that GSH 288.3–0.5–28
raySNRassociatedwith1E1048.1–5937(seeGaensleretal.
is not exposed to the high ionizing fluxes associated with
2001).
Car OB1, but likely has a higher extinction than and is be-
The remaining possibility is that GSH 288.3–0.5–28 is
hindthesecoincidentstars.
a wind-driven bubble. Using the wind bubble solution of
Animportantpointisthatthehighextinctionthatwehave
Weaveretal. (1977), during the energy-conservingphase of
invokedonlyneedimplyaslightlylargerdistancefortheHI
expansion we expect that R = 0.76(L /ρ )1/5t3/5, where t
w 0 shellthanforCar OB1, since extinctioninthisdirectionhas
is the age of the bubble, L is the wind luminosity of the
w little correspondence with distance. This is clear from con-
central star, and ρ0 is the density of the ambient medium. sideration of the ∼30 Wolf-Rayet (WR) stars within 3◦ of
SinceV =dR/dt,wefindL ≈(4±2)×1035ergss- 1 and
exp w GSH288.3–0.5–28(vanderHucht2001). Forstarsfromthis
t = 1.1±0.5 Myr. The luminosity of the wind is a strong
samplewithdistancesintherange1.7–3.7kpcasappropriate
functionofmass,allowingareasonabletypingoftheassoci-
forGSH288.3–0.5–28,A canbeashighas6–7.
ated centralobject(assumingonly one star powersthis bub- V
ble).Inthiscase,anappropriatecentralsourceisanO6Vstar, 3.3. TheCentralSourceAssociatedwithGSH288.3–0.5–28
with a mass-loss rate M˙ ≈2×10- 7 M⊙ yr- 1 (deJageretal. If GSH 288.3–0.5–28 is not associated with any of the
1988),awindvelocity≈2500kms- 1(Prinjaetal.1990),and
knownbrightstarsinthisdirection,whatispoweringit?Ifthe
azero-agemainsequence(ZAMS)massof30–40M (e.g.,
⊙ extinctiontoGSH288.3–0.5–28issignificantlyhigherthanto
Masseyetal.2002;Ostrov&Lapasset2003).
thestarsinCarOB1,itsassociatedstarmightnotbeobvious.
WefirstconsiderKorMsupergiants,whichshouldbepromi-
3.2. GSH288.3–0.5–28andCarinaOB1
nentinthenear-infrared—weestimatea1.3-µmmagnitude
WehaveshownthatGSH288.3–0.5–28ismostlikelypow- J .6.5 for a red supergiant at the distance to GSH 288.3–
eredbythewindofamassivestaratadistanceof2.7±1kpc. 0.5–28. However, using the 2MASS point source catalog
Butcan we identifythe centralstar responsibleforthis bub- (Cutrietal. 2003), we find no candidates for such a source
ble? Therearemanymassivestarsinthisdirectionandatthis insideGSH 288.3–0.5–28,otherthanIXCar (see §3.2). We
distance,mostofwhicharepartoftheassociationCarinaOB1 searched for faint OB stars by considering the Tycho-2 cat-
atadistanceof∼2.5kpc(e.g.,Humphreys1978). However, alog (Høgetal. 2000), which is 90% complete down to aV
we now argue that GSH 288.3–0.5–28is unassociated with, magnitude of 11.5. We assume that any massive star with
andprobablymoredistantthan,thestarsinCarOB1. A .4inthiswell-studiedregionwouldhavebeenclassified
V
First, thecollectivewindsofthemassivestarsinCarOB1 inearlierefforts,andsoonlyconsiderstarswithA &4. For
V
correspondtoatotalluminosityof∼8×1037ergss- 1(Abbott standard reddening, this implies a color excess EB- V & 1.3
1982). Combinedwith the powerfulwindsfrom stars in the andhencefor OB stars B- V ≥1. Just nine such starsfrom
nearby clusters Tr 14 and Tr 16, this should power a much the Tycho-2catalog lie within the perimeter of GSH 288.3–
larger expanding superbubble. Indeed such a larger shell, 0.5–28,ofwhichthreeareKorMstarswithlowextinction,
AREMAGNETARSFORMEDFROMMASSIVEPROGENITORS? 3
four have high proper motions which indicate that they are son, this should also be the age of the surrounding bubble.
foregroundobjects, one is a foregroundB1V star (LS1956) However, in §3.1, we esimated an age for the shell of 1.1±
which is heavily reddened, and one has near infrared mag- 0.5 Myr. There is clearly a large discrepancybetween these
nitudes from 2MASS which are inconsistent with it being a twoestimates. However,thissameageproblemhasbeenob-
massivestaratthedistancetoGSH288.3–0.5–28. served for many other HI shells around individual massive
It is certainly possible that stars too faint to be in the stars (e.g., Gervais&St-Louis 1999; Cappa&Herbstmeier
Tycho-2 catalog could also be candidates. While we thus 2000). There is no simple resolutionto this discrepancy,al-
accept that we cannot make a definitive identification using though in some cases it may be explained by a “blow out”
available data, there is one remaining source, namely the intoalowerdensityenvironment(Oey&Smedley1998). We
AXP 1E 1048.1–5937, which we argue below represents a concludethatanydiscrepancybetweentheagesestimatedfor
viable association with GSH 288.3–0.5–28. If 1E 1048.1– GSH288.3–0.5–28andfortheprogenitorof1E1048.1–5937
5937generatesawindpoweredbyitsspin-down,asdomany isnotastrongargumentagainsttheirassociation.
radio pulsars, then its spin parameters imply only L ∼4× Finally, we consider direct physical evidence that
w
1033ergss- 1(Gavriil&Kaspi2004). Thisisfartoolowtobe 1E 1048.1–5937 and GSH 288.3–0.5–28 are associated.
responsibleforthesurroundingshell. However,anintriguing YoungneutronstarsshouldbeembeddedinyoungSNRs,but
alternative is that GSH 288.3–0.5–28was blown by the star for1E1048.1–5937nosuchSNRisobserved(Gaensleretal.
whosecollapseformed1E1048.1–5937. Asforassociations 2001). A simple explanation is that the associated SNR is
betweenpulsarsandSNRs, thelikelihoodofthisassociation expanding into a low density bubble, so that it does not yet
shouldbejudgedbygeometriccorrespondence,agreementin produceobservableemission (e.g., Ciotti&D’Ercole 1989).
distanceandage,andevidenceforphysicalinteraction. Thus from the absence of a SNR, it is reasonable to expect
First considering geometry, it is clear from Figure 1 that that1E1048.1–5937shouldshowevidenceforasurrounding
1E 1048.1–5937 is close to the center of GSH 288.3–0.5– cavity:GSH288.3–0.5–28clearlyfulfillsthisprediction.For
28 in projection. Isolated neutron stars generally have high aSNRshockvelocityof5000kms- 1, theblastwaveshould
space velocities, & 300 km s- 1(Arzoumanianetal. 2002),6 impactthe shell walls ∼3 kyr after core collapse. The lack
so that any neutron star older than ∼ 100 kyr will have of any radio or X-ray emission from this event requires the
moved far from its progenitor’s wind bubble. Thus the as- neutronstartobeyoungerthanthis,consistentwiththesmall
sociation is only viable if we can argue independently that agesexpectedforAXPs.
1E1048.1–5937isextremelyyoung.Indeedthereisgoodev- To summarize, a very young neutron star with a massive
idencethatthisisthecase, sinceassociationswithSNRsfor progenitor should be centrally located in an expanding HI
other AXPs argue that these sources all have ages .10 kyr shell, with no evidence for a surrounding SNR. This is ex-
(Gaensleretal. 1999, 2001). In this case the AXP should actlywhatweobserveforGSH288.3–0.5–28and1E1048.1–
be centrally located in any progenitor wind bubble, as ob- 5937. Bothsourcesareconsistentwithbeingatadistanceof
served. We note that the “characteristic age” of 1E 1048.1– ∼3kpc,behind∼6magnitudesofopticalextinction.
˙
5937,P/2P≈4kyr(Gavriil&Kaspi2004), isindeedsmall
(wherePisthestar’sspinperiod).7 However,wecautionthat 4. IMPLICATIONFORMAGNETARS
thisandothermagnetarsdonotspindownlikenormalradio As explained in §1, a fundamentally unresolved issue in
pulsars(Thompsonetal.2000;Gavriil&Kaspi2004),sothis the study of compact objects is why some neutron stars are
ageestimatedoesnotprovideindependentevidenceforyouth. ordinary radio pulsars, while others are magnetars. Since
Second,we considerdistanceestimates. The systemicve- theinitialmassfunction(IMF)sharplydeclineswithincreas-
locity of GSH 288.3–0.5–28puts it at 2.7±1 kpc. No dis- ing ZAMS mass, most neutron star progenitors will have
tanceestimatesexistfor1E1048.1–5937,exceptforthefact masses near the minimum mass for core collapse, i.e., 8–
thattheabsorbingcolumnofhydrogeninferredfromitsX-ray 9 M⊙. We have presented evidence here that the progeni-
spectrum,N ≈1.1×1022cm- 2 (e.g.,Tiengoetal.2002),is torofAXP1E1048.1–5937wasconsiderablymoremassive
H
muchhigherthanthatfortheCarinacomplex(whichincludes thanthis. ForsomeSGRs,possibleassociationswithmassive
CarOB1)atadistanceof≈2.5kpc. Ithasthusbeenargued star clusters similarly argue for high-mass progenitors (e.g.,
that2.5kpcisafirmlowerlimitonthedistancetothissource Kloseetal.2004;Eikenberryetal.2004).
(Özeletal.2001). However,the highcolumnto1E1048.1– We thus propose that the difference between normal pul-
5937isstillcompatiblewiththedistancetoGSH288.3–0.5– sarsandmagnetarsistheprogenitormass. Wenotethatmas-
28. For standard gas to dust ratios, NH ≈1.1×1022 cm- 2 sive (& 25 M⊙) stars do not always form black holes: for
implies a visual extinction A ≈ 5.8 (Wang&Chakrabarty solar metallicity, mass loss causes single stars heavier than
V
2002). The WR catalogue of vanderHucht (2001) demon- ∼60 M⊙ to form neutron stars (Hegeretal. 2003); a pro-
stratesthatthislevelofextinctionisconsistentwithdistances genitoroflowermass(25- 40M⊙)whosebinarycompanion
intherange∼2- 12kpcinthisregion,soiseasilyreconciled stripsitsouterenvelopebeforecorecollapsecanalsoforma
withthedistancetoGSH288.3–0.5–28. neutronstar(Fryer&Kalogera2001).
Third, we need to determine if the age of GSH 288.3– Why should massive stars form magnetars?
0.5–28is consistent with that expectedfor the progenitorof Duncan&Thompson (1992) and Thompson&Duncan
1E1048.1–5937.AstarwithaZAMSmassof∼30- 40M⊙ (1993) argue that magnetars result from rapidly rotating
andsolarmetallicitylivesfor∼5- 6Myr(e.g.,Meynetetal. (P∼1 ms) proto-neutron stars, in which an efficient large-
1994). Since the age of the AXP is negligible in compari- scale dynamo operates in the first few seconds after birth,
generating a super-strong magnetic field with significant
6Wenotethatnovelocitymeasurementsyetexistspecificallyformagne- multipolar components. Hegeretal. (2005) have recently
tars(Gaensleretal.2001). carried out a series of calculations of differentially rotating
7 1E 1048.1–5937 experiences significant torque variations; we have
˙ magnetized supernova progenitors. They find that magnetic
adoptedthelong-termaverageforP.
torques are especially effective at transferring angular mo-
4 GAENSLERETAL
mentumawayfromthestellarcoreduringtheredsupergiant we infer that the magnetarbirth rate is ∼10%of that of ra-
and helium burning phases of evolution. For progenitors diopulsars. Thebirthrateestimatedfromobservationsofthe
with masses ∼ 10- 15 M⊙, this results in neutron stars existingpopulationsofAXPsandSGRsisalreadycompara-
with initial periods ∼ 10- 15 ms, too slow to generate ble to this(Kouveliotouetal. 1994;Gaensleretal. 1999). If
magnetar-like fields. However for more massive stars, the magnetarsindeedformfrommassive progenitors,we antici-
reduced time interval between hydrogen depletion and the patethatthereisnosubstantialpopulationofsuchsourcesyet
supernova results in limited braking of the core, producing tobeidentified(cf.,Ibrahimetal.2004).
much more rapidly spinning neutron stars. For example, a
35 M progenitor results in a neutron star of initial period
⊙
3ms,intherangeneededtogivebirthtoamagnetar.
Within this scenario, we can immediately make a predic- WethankCristinaCappa,ChrisFryer,AndrewMelatosand
tionastotherelativebirthratesofradiopulsarsversusmag- Chris Thompson for useful discussions, and Paul Price for
netars. For argument’s sake, we suppose that the mass cut supplyingHαdata. TheAustraliaTelescopeisfundedbythe
betweennormalpulsarsandmagnetarsisataZAMSmassof Commonwealth of Australia for operation as a National Fa-
25M⊙. ThenforanIMFofslopeα=2.35formassivestars cilitymanagedbyCSIRO.ThispaperhasusedtheCDSSIM-
(dN/dM∝M- α; Massey 2003), only∼20% of progenitors BAD andVizieRCatalogueServices, andNASA’sADSand
canpotentiallyformmagnetars. Since∼50%ofprogenitors SkyView facilities. B.M.G. is supported by NASA through
heavierthan25M willformblackholes(Hegeretal.2003), LTSAgrantNAG5-13032.
⊙
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AREMAGNETARSFORMEDFROMMASSIVEPROGENITORS? 5
FIG.1.—HIinthefieldoftheAXP1E1048.1–5937,fromSGPSdata.ThethreepanelsplotthesurfacebrightnessofHIasafunctionofGalacticlongitude,
GalacticlatitudeandLSRvelocity. Eachgreyscalerangesbetween–10and+78Jybeam- 1,asshownbythescalebarontheupperright. Thespatialresolution
ofthedataare142′′×125′′,whilethevelocityresolutionis0.82kms- 1. TheupperrightpanelshowstheimageplaneatanLSRvelocityof–28kms- 1;the
positionsof1E1048.1–5937andofthestarIXCararemarkedbyacrossandastar,respectively. Theleftpanelshowsab- vdiagramatl=288.◦25;thelower
panelshowsal- vdiagramatb=- 0.◦52.