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June 16, 2010 11:23 WSPC/S0218-2718 142-IJMPD 01714 International JournalofModernPhysicsD Vol.19,No.6(2010)879–885 (cid:1)c WorldScientificPublishingCompany DOI:10.1142/S0218271810017147 MODELING X–RAY EMISSION OF A STRAIGHT y. JET: PKS 0920-397 nl o e s u al on D.A.SCHWARTZ∗,F.MASSARO† andA.SIEMIGINOWSKA‡ s per High Energy Astrophysics Division, mor Smithsonian Astrophysical Observatory, oF c.c2. 60 Garden Street, Cambridge, Massachusetts, 02138, USA dscientifin 10/17/1 ‡asi†efmm∗iadgsaisnsa@orwoh@[email protected] worlY o w.AR D.M.WORRALL§ andM.BIRKINSHAW¶ wR wB Physics Department, Universityof Bristol, m LI TyndallAve. Bristol, BS81TL, UK aded froNICAL ¶Mar§dk..wBoirrkrianlls@[email protected] oH nlC owTE H.MARSHALL(cid:2) andD.EVANS∗∗ 5. DWC) Kavli Institute for Astrophysics, 88U Massachusetts Institute of Technology, 9:879-ER (N NE-80(cid:2),hCerammabnrmid@ges,paMceA.m, 0it2.1ed3u9, USA 0.1NT ∗∗[email protected] 01E 2C D E E.PERLMAN hys. FAR Physics & Space SciencesDepartment, PR J. Mod. EA WA 150 West UFnliovreirdsaiteyIp,nesMrtlimetulabtoneu@orfinfteT.,eedFcuhlonroildoag,y,32901, USA nt. RS IE D J.M.GELBORD N U Physics Department, Durham University, L A South Road, Durham, DH13LE UnitedKingdom V A [email protected] N y b J.E.J.LOVELL School of Mathematics and Physics, Universityof Tasmania, Private Bag 37, Hobart, 7001, Australia [email protected] 879 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3. DATES COVERED 2010 2. REPORT TYPE 00-00-2010 to 00-00-2010 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Modeling X-Ray Emission of a Straight Jet: PKS 0920-397 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION US Naval Observatory, 3450 Massachusetts Ave. REPORT NUMBER NW,,Washington,DC,20392 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES International Journal of Modern Physics D Vol. 19, No. 6 (2010) 879-885 14. ABSTRACT We summarize a study of PKS 0920-397 using our 42 ks Chandra observation in conjunction with our ATCA 20GHz image, and HST/ACS F814W and F475W images. We investigate the hypothesis that the jet X?ray emission is due to inverse-Compton (IC) scattering on the cosmic microwave background (CMB) from the same population of relativistic electrons that give rise to the radio emission. To calculate parameters intrinsic to the source, one must finesse the fact that we do not know the true angle of the jet to our line of sight. Typical assumptions are that the Doppler factor equals the bulk Lorentz factor, or that the Lorentz factor takes some fixed numerical value. While giving useful estimates, neither assumption can be exact in general. We try different constraints to determine the jet quantities. It is plausible that the kinetic flux is constant along the jet prior to a terminal hotspot or lobe, and with minimal bending of the jet. Alternatively because PKS 0920-397 appears straight in projection on the sky, we might assume the jet maintains a constant angle to our line of sight. Either approach gives bulk Lorentz factors of 6 to 8, with kinetic energy flux of order 1046 erg s−1, and with the jet at an angle 2◦to 4◦from our line of sight. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE Same as 8 unclassified unclassified unclassified Report (SAR) Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 June 16, 2010 11:23 WSPC/S0218-2718 142-IJMPD 01714 880 D. A. Schwartz et al. L.GODFREY†† andG.BICKNELL‡‡ Research School of Astronomy & Astrophysics, Mt Stromlo Observatory, Weston P.O. ACT, 2611, Australia ††[email protected] ‡‡geoff@mso.anu.edu.au nly. R.OJHA o e USNaval Observatory, 3450 Massachusetts Ave. NW, s al u Washington D.C. 20392, USA n [email protected] o s er p mor M.HARDCASTLE oF c.c2. Centre for Astrophysics Research, dscientifin 10/17/1 UniversimtHy.ja.othffiaeHrlddecraAtsfoLtlre1d@0shh9ieArretB,s,.CaUco.lKulekge Lane, worlY o w.AR S.JESTER wR wB Max-Planck-Institut fu¨r Astronomie, Ko¨nigstuhl 17, d from CAL LI 691je1s7teHr@eimdeplibae-rhgd,.mGeprgm.daeny adeNI oH nlC S.JORSTAD wE oT Institute for Astrophysical Research, 5. DWC) Boston University,725 Commonwealth Avenue, 88U Boston, MA 02215-1401, USA 9:879-ER (N [email protected] 0.1NT L.STAWARZ 01E D 2E C Kavli Institute for Particle Astrophysics and Cosmology, hys. FAR Stanford Usntiavwerasrizt@y,slSatca.nstfaonrdfo,rCd.Aed9u4305, USA PR od. WA J. MEA We summarize a study of PKS 0920-397 using our 42 ks Chandra observation in con- nt. RS junctionwithourATCA20GHzimage,andHST/ACSF814WandF475Wimages.We IDE investigate the hypothesis that the jet X–ray emission is due to inverse-Compton (IC) N scatteringonthecosmicmicrowavebackground(CMB)fromthesamepopulationofrel- U L ativisticelectronsthatgiverisetotheradioemission.Tocalculateparametersintrinsic A V tothesource,onemustfinessethefactthatwedonotknowthetrueangleofthejetto A ourlineofsight.TypicalassumptionsarethattheDopplerfactorequalsthebulkLorentz N y factor,orthattheLorentzfactortakessomefixednumericalvalue.Whilegivinguseful b estimates, neither assumption can be exact in general. We try different constraints to determinethejetquantities.Itisplausiblethatthekineticfluxisconstantalongthejet, priortoaterminalhotspotorlobe,andwithminimalbendingofthejet.Alternatively, because PKS 0920-397 appears straight inprojection on the sky, wemight assumethe jet maintains a constant angle to our lineof sight. Either approach gives bulk Lorentz factors of6to 8,withkinetic energyflux oforder 1046 ergs−1,and withthe jetatan angle2◦to4◦fromourlineofsight. Keywords:Quasarjets;X–rayjets;jetemissionmechanisms. June 16, 2010 11:23 WSPC/S0218-2718 142-IJMPD 01714 Modeling X–Ray Emission of the PKS0920-397 Jet 881 1. Introduction Chandra has enabled the study of X–ray jets,1,2 starting with the serendipi- tous detection of the jet in the quasar PKS 0637–7513 during the very first pointedobservation.WhileX–rayjetemissionfromFRIradiogalaxiesisnormally explainedviathesynchrotronmechanism,thequasarjetsinitially presentedapuz- zle.Observations,orupper limits, to opticalemissionfromquasarjets usually pro- y. nl hibitanextensionoftheradiosynchrotronspectrumtotheX–rayregime.However, o e estimates of the magnetic fields via minimum energy assumptions result in such s u al large magnetic field strengths that inverse-Compton (IC) emission would not be n so expected to be significant.This dilemma wasresolvedby realizing4,5 that if the jet er mor p was relativistically beamed with bulk Lorentz factor Γ, then the Γ2 increase of the c.co2. F cosmic microwave background (CMB) energy density in the jet rest frame6 would ntifi17/1 allow the CMB to dominate the magnetic energy density and thus for IC/CMB to dscien 10/ dominate the relativistic electron energy loss. worlY o w.AR 2. Images of PKS 0920-397 wR wB m LI PKS0920-397isaquasaratredshift0.591.WeoriginallyobserveditwithChandra d froCAL for 5ks7,8 as part of a survey7 of flat spectrum quasars with good radio imag- adeNI ing.9,10 Because it was clearly detected along the 10(cid:1)(cid:1) length of the radio emission, oH nlC and because of its straight appearance, we proposed an additional 40ks Chandra wE oT 85. DWC) ostbasnetrvgaetoiomne.tWrice faascstuomrsedaltohnagttthhee sjettr,aiegvhetnmtohropuhgohlotghyewaonuglldeatlolotwheuslitnoeuosfescigohnt- 8U 9:879-ER (N was unknown. Figure 1 shows images of the quasar PKS 0920-397 (position noted 0.1NT 01E D 2E C PKS 0920-397 ys. AR 5" 5" hF PR od. WA J. MEA nt. RS IE D N U L A V A N y F814W 20 GHz b 0.5 to 7 keV Fig. 1. Images of the PKS 0920-397 quasar and jet at 20GHz (right panel) inthe 0.5 to 7keV X–rayband(middlepanel)andwiththeHST/WFCfilterF814W(leftpanel).TheX–rayimage shows counts in a 0.2(cid:4)(cid:4) square bin, with the faintest color being one count. The rectangle shows theskyregionusedtoprojecttheradioandX–raydatainFig. 2.Fivedistinctopticalknots are detectedalongtheradiojet(arrowsinleftandrightpanels).The“plus”and“asterisk”intheleft paneldenotetheregions2.1(cid:4)(cid:4)and5.9(cid:4)(cid:4)fromthequasarforwhichthespectralenergydistributions areconstructedinFig.3. June 16, 2010 11:23 WSPC/S0218-2718 142-IJMPD 01714 882 D. A. Schwartz et al. y. nl o e s u al n o ers Fig. 2. Projection of the X–ray and radio data. Points plot the number of X–ray counts per omFor p 0w.i2t(cid:4)h(cid:4) bthine.XT-hreaysoplridofillien.eTphloeytsatrheew2i0thGinHzadfaactato,rarobfit2raorvielyr maudlitsitpalniecdebfryom3502(cid:4)(cid:4)totobe5t.t5e(cid:4)(cid:4)rfcroommptahree ntific.c17/12. quasar.Representative statisticalerrorsshownarethesquarerootofnumberofX–raycounts. dscien 10/ by the cross) and its jet extending to the south. Both the ATCA 20.16GHz image worlY o (right panel) and the 40ks Chandra 0.5 to 7 keV image (center panel) show a gap w.AR wR past the end of the jet, followed by two hotspots in the southern lobe. wB m LI The radio and X–ray profiles appear very similar. Figure 2 compares those aded froNICAL pquroafisalers.,Tphroejbecrtigehdt±q1u(cid:1)a(cid:1)spaerrcpoernedcicounltaarmtionathteesjteht,eaXs–arafyunimctaiogneoofutditsotaanbcoeuftro2m(cid:1)(cid:1). tWhee oH nlC arbitrarily renormalize the 20.16GHz data to show that it tracks the X–ray flux wE 5. DoWC) T dweitthecinteadfjaectt.oroftwoouttoatleast5.5(cid:1)(cid:1),whichappearstobetheendoftheX–ray 8 8U 9-N 9:87ER ( 0.1NT 3. X–ray Emission Mechanism D 201E CE Figure3presentsthe spectralenergydistributionofthe regions2.1(cid:1)(cid:1) and5.9(cid:1)(cid:1) from ys. AR the quasar (indicated by the “+” and “*” signs in the left panel of Fig. 1). We hF PR know that the radio emission is synchrotronbecause we measure polarization.It is od. WA evident that the 20 GHz flux density cannot be extrapolated to the X–ray region, J. MEA since it would greatly exceed the HST measurements of optical flux density. Int. ERS GiventhattheX–raysarenotacontinuationoftheradiosynchrotronemission, D N thesimplesthypothesisistoaddonesingleparameter,namelythebulkLorentzfac- U L torΓ,whichthenenablesthesameelectronpopulationtoemitX–raysviaIC/CMB. A V Sinceweknowthejetsareinrelativisticbulkmotionfromthefactthattheyareone- A N y sided, it would take some fine tuning to keep Γ small enough to prevent IC/CMB. b Whileasynchrotronorigincannotberejectedabsolutely,wealsohaveadditional observationalargumentsfavoringIC/CMB,asfollows.IftheX–raysdidresultfrom synchrotron radiation, they would be emitted from an independent population of relativistic electrons which did not extend to low enough energies to radiate at GHz frequencies. In that case, the correlation of the radio and X–rays would be difficult to understand. The minimum energy estimates of magnetic field strength are of order 100µG, so that synchrotron X–rays would be produced by electrons June 16, 2010 11:23 WSPC/S0218-2718 142-IJMPD 01714 Modeling X–Ray Emission of the PKS0920-397 Jet 883 y. nl o e s u al n o s er p omFor Fig. 3. Spectral energy distributions of the two regions 2.1(cid:4)(cid:4) and 5.9(cid:4)(cid:4) from the quasar. The c.c2. solid line is an eyeball-fit of a power-law plus exponential cutoff, drawn to demonstrate that a ntifi17/1 synchrotron spectrum cannot be extended from the radio region to explain the X–ray emission. dscien 10/ Tinhdeex“.bowties” through the two X–ray points indicate the allowed range of the X–ray spectral worlY o ww.RAR with Lorentz factors γ ≈2×107, and have lifetimes of order 100 years. Thus they wB m LI would need to be accelerated continuously in space at the right intensity to match d froCAL the independent population of radio emitting electrons. adeNI oH nlC wE 4. Calculation of the IC/CMB Emission oT 5. DWC) Figure 4 showshow we divide the jet into four spatially distinct cylindricalregions 8 8U 9-N for analysis. For the present work, we make the same assumptions of Ref. 8, with 9:87ER ( the following modifications. We will calculate the minimum energy magnetic field 0.1NT by assuming intrinsic limits to the relativistic electron spectrum, γmin = 30 to 01E 2C D E ys. AR hF PR od. WA J. MEA nt. RS IE D N U L A V A N y b Fig. 4. Rectangular boxes define the distinct cylindrical regions for which the jet structure is calculated.IdenticalspatialregionsareusedfortheX–rayandradiodata.Thelargecircleindicates the point response of the quasar, while the smaller circle is coincident with the southern radio lobe. June 16, 2010 11:23 WSPC/S0218-2718 142-IJMPD 01714 884 D. A. Schwartz et al. γmax =106, instead of limits to the radio spectrum received at earth.11 The latter limits are not observable in a narrow jet by conceivable techniques, whereas γmin in principle could be detected by a low energy X–ray cutoff.12 We use the super- snapshotspecialrelativitytransformations13 whichconsiderthe relativetime delay along the jet. Finally, we relax the assumption that the apparent Doppler factor, δ =1/(Γ(1−βcosθ)) equals the bulk Lorentz factor Γ. y. With those refinements we have two possible ways to calculate the magnetic nl e o field in the jet rest frame. The minimum energy formalism gives s personal u Hmin ∝(cid:1)fνδ−2−αsinθθ(lγθm1r2−ax2α−γm1−in2α)(cid:2)1/(3+α) (1) c.com2. For while the original IC/CMB calculation of HFM14 is modified to ntifi17/1 HCMB ∝ΓHFM (2) w.worldscieARY on 10/ bweheirnetrθinlsaicnadllθyrcayrleintdhreicaaln,gzuliasrtlhenegrtehdsahnidft,rafdνiuasnodfftxheajreettehleemreandtio, aasnsudmXe–dratoy wR flux densities, and α is the radio and X–ray spectral index. Equating (1) and (2) wB m LI andwiththe definition ofδ givestwoequationsandthree unknowns.This is where d froCAL observers have usually assumed a fixed value for Γ or that δ =Γ. adeNI Instead we continue the calculation using θ as an unknown free parameter. oH nlC Figure 5 shows the result of the calculation of the kinetic energy flux applied to wE oT 85. DWC) tishecofnosutranretgaiolonnsgotfhFeigle.n4g.thAopflatuhseibjelet.aTsshuims ipstiboanseids tohnatthteheexktirneemtiec ienneeffirgcyienflcuyx, 8U 9-N 10−4–10−5, of the radiative energy losses.8 A counter-argument is the known fact 9:87ER ( thatquasarsarevariableonscalesofyearsormonthssotheirjetoutputneednotbe 010.1ENT constant. Still, considering that the present X–ray angular resolution is averaging 2C D E overelementsthatcontainatleasttenthousandyearsoutput,itmaywellturnout ys. AR that the quasar accretionis a stationary process over that time-scale. From Fig. 5, hF PR od. WA J. MEA nt. RS IE D N U L A V A N y b Fig.5. KineticenergyfluxthroughthefourregionsdefinedinFig.4,asafunctionoftheunknown angletoourlineofsight.Forangleslessthan4.5◦ thefourregionsarestatisticallyconsistent(at 90%confidencelevelindicatedbytheverticalline)withakineticenergyfluxof3×1046 ergs−1 orless. June 16, 2010 11:23 WSPC/S0218-2718 142-IJMPD 01714 Modeling X–Ray Emission of the PKS0920-397 Jet 885 the kinetic flux could be constant if the jet angle is less than 4.5◦ from our line of sight, while even 10 times variation would still require less than a 6.5◦ angle. 5. Summary The X–ray emission from the jet in PKS 0920-397 most likely arises from inverse- y. Comptonscattering onthe cosmic microwavebackground.This is largelybasedon onl the tight correlation of the X–ray and radio profiles along the continuous portion e us of the jet, and the fact that the spectral energy distribution from radio through al n optical to X–ray rules out that the X–rays are synchrotron emission from a single o s er power-lawpopulationofelectronsthatalsoproducestheradio.WithintheIC/CMB p mor hypothesis, determination of Γ, δ and θ are crucial to determine the structure and oF c.c2. dynamics of the jet. We know that the hypothesis Γ = δ cannot be exact. In this ntifi17/1 paper we showed that the assumption of constant kinetic flux along the jet gives dscien 10/ a plausible fit to the observations. It results in a somewhat smaller value of the worlY o kinetic energy flux and requires that the jet be quite close to the line of sight. It w.AR therefore remains to be seen whether this approach gives consistent results when wR wB applied to a larger, well defined population of X–ray jets. m LI d froCAL adeNI Acknowledgments oH nlC wE This work was supported by NASA contract NAS8-03060 and SAO grants GO9- oT 5. DWC) 0121B and GO7-8107X. F. Massaro acknowledges a grant from the Foundation 8 8U BLANCEFLOR Boncompagni-Ludovisi,n’ee Bildt. 9-N 9:87ER ( 0.1NT References 01E 2C D E 1. D.A.Schwartz,ChandracreatesthestudyofX-rayjets,inProceedingsoftheNational ys. AR Academy of Sciences, eds. N. A.Bahcall and H.Tananbaum (2010) P. xxx. PhRF 2. D. M. Worrall, Astron. Astrophys. Rev. 17 (2009) 1. od. WA 3. D. A.Schwartz et al.,Astrophys. 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