Adv.SpaceRes.Vol. 13,No. 12, pp. (12)135—(12)137,19930273T1177193$6.00+ 0.00

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ROSATOBSERVATIONSOFSINGLEPULSARS

H. Oge1man*~**

* Departmentof Physics,UniversityofWisconsinat Madison,USA** MaxPlanckInstitur für ExtraterrestrischePhysik, Garching,Germany

Abstract

RecentRosatresultson theX-ray emissionof single neutronstarsarereviewedandcomparedto the theexisting information.

Introduction

Accreting (binary) neutron starsare powerful emitters of X-rays andyet the emission is characterizedmostly by theinflow of externalmatterratherthan the internal characteristicsof theneutron star. On theother hand,rotationpowered(single) neutronstarsarenot so luminousin X-raysbut theycontainvaluableinformation on theelectrodynamicsof pulsarinagnetospheresandpossibly the thermalpropoertiesof thesurfaceandthe interiors. In a recentreview abouthalf a year ago, I havesummarizedtheobservationalstatusof theX-ray emission from single pulsars(Ogelman 1993). At that time therewere 10 rotationpoweredpulsarswhichshowedpoint-like X-rayemission;6 ofthesepulsarsshowedpulsedX-rays,indicatingthat theunderlyingprocesseswere taking placeon or nearthepulsar. In the interveningtime RosathasdetectedpulsedX-ray emission from two more pulsars (PSR 1951+32and PSR 1055-52),and has alsodetectedthe gamma-rayemitting pulsar PSR 1706-44. Rather than repeatingthe general argumentspresentedin theabovereview, I haveupdatedthe informationin Table 1 to include thenewresultsandIwill presentthehighlightsof the recentRosatresultsfor theVelapulsar(Ogelman,Finley andZimmermann

1993).

New resultsfor PSR’s 1055-52,1951+32and 1706-44

Preliminaryanalysisof PSR 1055-52(Ôgelmanei at. 1993)showsthat in theHRI exposureit a point-likeobject andthenebularemission abovefew arc secondsresolution of theHRI must be lessthan 10%. ThePSPCexposureshowsclearly that it is pulsedat the radio periodwith 10% of thecountsappearingin abroadpulseprofile. Contraryto theVelapulsarandPSR 0656+14,thepulseappearsstrongerat higherenergies. There is evena phaseshift of about0.4 phasesbetweenthe softerand harderphotons. Theoverall spectrumis still dominatedby soft photons.

PSR 1952+32is apulsarwith a weakmagneticfield (5 x 10” gauss)andlargespindownage (—j 10~yr).It is imbeddedin a supernovaremnantthatappearsto be about io~yr old. The preliminary analysisoftheRosatPSPCexposureshowspulsation of about 15% with asharppeak (Safi-Harbci at. 1993). Thespectrumof thesourceis hardandsynchrotronlike.

PSR 1706-44is a pulsarsimilar to theVela pulsarin age(1.8 x io~yr) andperiod (0.102s). It hasbeenrecently observedas agammaray sourceby EGRET(Kniffen ei at. 1992). Becker ci at. (1992b) havedetectedthis sourcewith theRosat PSPCin a serendipitouspointing. The exposurewas too short todetectany pulsations.If PSR 1706-44is similar to the Vela pulsarin its X-ray emission,thesoft pulsedemissionmay be absorbeddue to the largerinterstellarcolumn density to PSR 1706-44.

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(12)136 H. Ogelman

TABLE 1 X-ray emitting single pulsars (as of Aug 1992)

pulsar ~log E~1~per(s) dist(kpc) logL~2) logL~3)j logL~) log t~5~log B(g

)

0531+21 38.65 0.033 2.0 36.0 36.0 37.4 3.09 12.580540-69 38.17 0.050 55.0 36.4 36.4 37.0 3.22 12.701509-58 37.25 0.150 4.2 34.3 34.3 35.3 3.19 13.190833-45 36.84 0.089 0.5 32.7* 31.7* 334*(6) 4.05 12.531951+32 36.58 0.040 3.0 33.8 33*(9) 34.7 5.02 11.691706~44(8) 36.53 0.102 1.4 32.0* ? ? 4.25 12.490656+14 34.58 0.385 0.4 32.5* 31.7* ? 5.04 12.670630+18~~~34.54 0.237 0.1 30.3* 29.6* ? 5.51 12.21055-52 34.48 0.197 0.9 32.5 31.5*(9) ? 5.73 12.041929+10 33.60 0.227 0.1 28.5 ? ? 6.49 11.710950+08 32.75 0.229 0.1 29.4 ? ? 7.24 11.39

unconfirmeddetections

1813-36 33.14 0.387 3.2 33.0* ? _______ 6.50 11.96

1642-03 33.08 0.388 1.3 31.9 ? ? 6.54 11.931846-06 32.77 1.451 4.4 33.2* ? _______ 5.70 12.920628-28 32.16 1.244 1.3 31.6* ? 7 6.40 12.481648-17 32.12 0.973 1.0 31.2* 7 _______ 6.70 12.241524-39 31.73 2.418 1.0 32.2* ? ? 6.30 12.84

(1) Log of 11111 in erg/s(2) The X-ray luminosity of the point-like object

(3) The pulsed X-ray luminosity of the point-like object(4) The X-ray luminosity of the nebulaplus point-like object

(5) Log of spin-downageP/2P in years(6) the 2’ compactnebula(7) Geminga;Halpern andHolt 1992; Bertschet al. 1992(8) Becker et al. 1992; IAU cir #5554(9) New ROSATresults(Ogelmanet al. to be published)All X-ray luminositiesarelog of erg/sin the EINSTEINbandpassof .1-4keV unlessmarkedwith (*) which indicatesthe ROSATobservationsin thebandpassof 0.1-2.4keV. Other confirmed pulsarX-ray datacomesfrom theEINSTEIN resultsassummarizedby SewardandWang(1988). TheX-raydataon unconfirmedpulsarsis from Becker et al. (1992a)

ROSATObservationsofSinglePulsars (12)137

The Vela Pulsar

TheVelapulsaris anearby,young,activeneutronstarthatdisplayspulsedemissionin theradio,opticalandgamma-raybands(seeOgelmanandZimmermann1989for areview). Measurementswith imagingdetectorsin the soft X-ray baudby the Einstein(Harndenci at. 1985)andExosat(OgelmanandZiiumerniann1989)observatorieshaverevealedapoint-like sourceat the position of thepulsarimbeddedin a compactnebulaof about~ diameter.Thedetectionof pulsationshas,however,remainedelusive. Rosathasaccomplishedthefirst definitive detectionof pulsedX-raysfrom the Velapulsarandtherebyput to restthis long standingenigma(Ogehuan,Finley andZimmermanu1993). The pulsedsignal is soft and appearspredominantlyat energiesbelow 1 keV and it constitutesabout 10% of thepoint-sourceemission. The X-ray pulse iscomplex and does not match the shapesof either the gamma-ray,optical or the radio pulse. The 0.5’spatial resolution of the RosatPSPChasalso allowed the determinationof thespectralfeaturesof thepoint-like sourceandthe 2’ diametercompactnebulaseparately:thepoint-like emissioncenteredon thepulsaris also soft while thecompactnebulaaroundthepulsaris harder. Despite thefact that it is treatedasan archetypalyoungpulsarin asupernovaremnant,in X-rays, the Vela pulsarappearsmorelike the iO~yr old pulsars.Among the 8 knownpulsarswhich emit pulsedX-rays (seeTable 1), theVeIn pulsarhasanintermediateagethat is in betweentheyoung, io~yr agesof the Crab pulsar,PSR 0540-69,PSR 1509-58

andtheold, 10’ yr spin-downagesof PSR 0656+14,Gensinga(HalpernandHolt 1992), PSR 1055-52(Ogelmanci al. 1993)andPSR 1951+32(Safi-Harbet at. 1993). The young pulsarsall havepower-lawtype spectrainterpretedas themagnetosphericemissionof the relativistic electrons andpositrons. Theolder pulsarshaveasoft, black body like spectrumwith sonicresidualexcessat higherenergies. In X-raystheVelapulsar, is verysimilar to the olderpulsarsalthougha factor10 younger.TheX-ray pulsedfractionof Vela is also comparableto PSR 0656+14 (Finley, OgelmanandKizilo~lu1992)andGensingawith 11%,14% and 15% respectively. The combination of parametersthat determinethe transition of theX-rayspectrumfrom thehard niagnetosphericemission to thesoft blackbody like emission must havealreadyexceededtheboundaryfor the Vela pulsar. Black body fits to thespectrumof the Vela point sourcegive

surfacetemperaturesin the 1.5 — 1.6 x 10’K rangewhich is compatiblewith somecooling scenariosandreheatingprocesses,howevertheobservedluminosities of 4 — 5 x 1032 erg s1 imply that the radiusofthe Vela pulsar is only 3-4 km in comparisonto the expected10-18 km (at co). This discrepancymaybe resolvedwith thespectralmodificationof theneutronstar atmosphereincluding the effect of strong

magneticfields.

This detectionof theVela pulsartogetherwith therecentRosatdiscoveriesof Geminga,PSR0656+14, PSR1055-52andPSR 1951+32as pulsedsoft X-ray sourcesimplies that Rosatcan studytheX-ray spectrumandpulse profiles of io~to 10’ yr old neutronstarswithin few kiloparsecsfrom us. We anticipateothersuch discoveriestogetherwith observationsat other wavelengthswill allow us to constructa consistentpicture of magnetosphericemissionandcooling propertiesof neutronstars.

Becker,W. ci at. , 1993a,to appearin Physicsof IsolatedPulsars,Proceedingsof Workshop,Taos, N.M.Becker,W. ci at. , 1992b,I.A.U. C’ir. # 5554Bertsch,D.L.et at. , 1992, Nature,357, 306Finley, J.P.,Ogelinan, H. andKizilo~lu,U., 1992, Asirophys.J. (Letters)394, L21Halpern, J.P.andHolt, S.S.,1992, Nature,357, 222Harnden,F.R.Jr.,Grant,P.D., andSeward,F.D., 1985, Astraphys.J. 299, 828Kniffen, D.A. ci al. , 1992, I.A.U. Cir. # 5485Ogelman,H. andZimmerinann,H.-U., 1989, Astron. Astrophys.214, 179Ogelman,H., 1993, to appearin Physicsof IsolatedPulsars, Proceedingsof Workshop, Taos,N.M.Ogelman,H., Finley, J .P. andZimnier,nann,H., 1993, Nature, , in pressOgelman,H., Finley, J.P., 1993, in preparationSafi-Harb,S., Finley, J.P. andOgelman,H., 1993, in preparationSeward,F.D. andWang, Z.R., 1988, Astrophys.J. 332, 199