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over the last decade , optical and x - ray observations made with the _ hubble space telescope _ ( e.g. , * ? ? ? * ; * ? ? ? * ) and the _ chandra _ x - ray observatory ( e.g. , * ? ? ? * ) have produced exquisite images of extragalactic kiloparsec - scale jets , completely changing our understanding of their properties . currently , more than 70 ( 30 ) extragalactic jets and hotspots are known in the x - ray ( optical ) ; ] all but the few brightest jets were discovered by _ ( _ hst _ ) .    the origin of the broad - band spectral energy distributions ( seds ) of large - scale quasar jets , constructed using _ hst _ and _ chandra _ data , are the subject of active debate ( for reviews , see * ? ? ? * ; * ? ? ? * ) . in luminous quasars with x - ray jets extending from the quasar nucleus out to hundreds of kiloparsec ( e.g. , * ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) , the x - ray intensity relative to the radio synchrotron flux is generally too high to be explained by a synchrotron - self - compton model unless there is a huge deviation from equipartition @xcite . the radio , optical , and x - ray fluxes of a jet knot generally trace a peaked , inflected broad - band spectrum , which rules out the interpretation of x - rays as due to synchrotron radiation from a single population of electrons . an alternative scenario , inverse - compton ( ic ) scattering of cmb photons by high - energy electrons ( @xmath2 ) in a highly relativistic jet with bulk lorentz factor @xmath3 ( the beamed ic model : * ? ? ? * ; * ? ? ? * ) initially seemed a more natural way to explain the observed x - ray emission , but this process is also not free of problems ( e.g. , * ? ? ? * ) . finally , it is possible that the x - rays arise from synchrotron radiation from extremely energetic protons @xcite . determining which of these emission mechanisms produces the observed x - ray jets in powerful quasars is a strong motivation for more observations of radio - loud quasars , and has resulted in a rapid increase in the number of known x - ray and optical jets . a new window to explore extragalactic large - scale jets has been opened by the _ spitzer space telescope _ , which is capable of detecting jet infrared emission thanks to the excellent sensitivity of the infrared array camera ( irac ; * ? ? ? * ) and multiband imaging photometer ( mips ; * ? ? ? the first example was the detection of infrared synchrotron radiation from jet knots in the quasar pks  0637@xmath0752 with the _ spitzer _ irac at wavelengths of 3.6 and @xmath1 @xcite . in terms of the beamed ic model , the infrared bandpass is particularly interesting since the bulk - comptonization bump produced by cold electrons is expected to appear in the infrared @xcite . the absence of such features in the pks  0637@xmath0752 jet rules out the jet model dynamically dominated by @xmath4 pairs in the guise of the beamed ic model @xcite . the mips observations of the jet in centaurus a @xcite , and most recently , the results from irac and mips imaging photometry of the jet in m  87 @xcite have been reported , which also demonstrate the power of _ spitzer _ to study jet emissions in lower power jets ( see also * ? ? ? * for the m87 jet ) . now the three great observatories collectively offer the possibility to identify the radiation mechanisms operating in powerful quasar jets . in fact , when combined with the data from the _ vla _ , _ hubble _ , and _ @xcite , the _ spitzer _ observation of the bright quasar 3c  273 shed new light on the riddle of x - ray jets @xcite . the _ spitzer _ irac photometry of the jet knots in 3c  273 indicated a two - component spectral energy distribution : a radio - to - infrared synchrotron component and a separate optical - to - x - ray component . the latter also seems likely to be of synchrotron origin , given the similar polarization of optical and radio light . the optical polarization , however , has not yet been measured with high precision , so this conclusion is not yet firm . perhaps such a double synchrotron scenario is applicable to the radiation output from many quasar jets .    in this paper , we present _ spitzer _ irac imaging of the powerful jet in the luminous quasar pks  1136@xmath0135 . together with data from the _ vla _ , _ hst _ , and _ @xcite , our infrared photometry makes the sed of the pks  1136@xmath0135 jet the most detailed and best constrained among _ lobe - dominated _ quasars . the jet in the quasar pks  1136@xmath0135 is reminiscent of the 3c  273 jet @xcite , demonstrating anti - correlation between radio and x - ray brightness , such that the radio intensity increases toward a hotspot while x - ray flux decreases . applying the beamed ic model to the x - ray emission , this has recently been interpreted to imply _ deceleration _ of the jet @xcite . here we analyze the multiwavelength jet emission in the light of the double synchrotron scenario recently outlined for the 3c  273 jet @xcite . the redshift of pks  1136@xmath0135 is @xmath5 , so we adopt a luminosity distance of @xmath6 , for a concordance cosmology with @xmath7 , @xmath8 , and @xmath9 . the angular scale of @xmath10 corresponds to 6.4 kpc . we observed pks  1136@xmath0135 with _ spitzer _ irac @xcite on 2005 june 10 as part of our cycle-1 general observer program ( _ spitzer _ program i d 3586 ) . we used the pair of 3.6 and @xmath1 arrays , observing the same sky simultaneously . the pixel size in both arrays is @xmath11 . the point - spread functions ( psfs ) are @xmath12 and @xmath13 ( fwhm ) for the 3.6 and @xmath1 bands , respectively . the photometry with irac is calibrated to an accuracy of @xmath14 @xcite . we obtained a total of 50 frames per irac band , each with a 30-s frame time . the pipeline process ( version s14.0.0 ) at the _ spitzer _ science center yielded 50 calibrated images ( basic calibrated data ) . these well - dithered frames were combined into a mosaic image with a pixel size of @xmath15 using mopex @xcite , which removes spurious sources such as cosmic rays and moving objects based on inter - frame comparisons . finally , in order to align the irac image with respect to the vla image , we set the center of the quasar in the irac image at the core position in the vla image . in this way irac astrometry is estimated to be accurate to @xmath16 . the infrared fluxes from the quasar core of pks  1136@xmath0135 were measured as 2.2 and 4.0 mjy in the 3.6 and @xmath1 bands , respectively ( table  [ tbl-1 ] ) . these values do not reach the saturation limits of the irac arrays . assuming that the infrared spectrum has a power law with @xmath17 , as consistent with the ratio of the 3.6 and @xmath1 fluxes , the luminosity in the 310 @xmath18 band amounts to @xmath19 . in the model of @xcite , the infrared emission is assumed to arise from the inner jet , while the optical / uv emission originates in the accretion disk . our infrared measurement is consistent with that model . if this is the case , the infrared fluxes determine the apparent luminosity of the synchrotron component . inspection of the @xmath20 irac images clearly reveals the presence of an infrared jet that traces the extended radio jet . the irac @xmath1 image also shows the jet emission , although its quality is worse than the @xmath21 image . the extended psf of the bright quasar core is significant at the location of the jet , amounting to @xmath22 of the jet flux . in order to separate the jet infrared emission from the contaminating psf wings of the core , we subtracted the psf wings by making use of the psf templates in a way similar to @xcite . figure [ fig : image ] shows the irac image of pks  1136@xmath0135 in the @xmath21 band after subtraction of the psf wings of the quasar core . infrared counterparts of the jet knots are clearly visible in the irac image , located @xmath23 from the core ( knot b through hotspot hs ) . a possible counterpart of knot a is marginally found in the irac image , but it may be due to our incomplete psf subtraction , so we regard the knot a flux only as an upper limit . the hotspot at the counter - jet side also suffers from the uncertainties of psf subtraction ; we do not find an infrared counterpart and place an upper limit of @xmath24 at @xmath20 . also , we do not detect extended infrared emissions from the radio lobes .    to construct the broad - band seds of jet knots using multifrequency data , we derived infrared flux densities of some knot features at 3.6 and @xmath1 . given the small separation of adjacent knots , typically @xmath25 , we utilized a psf - fitting method @xcite . specifically , the jet image was fitted with a series of irac psfs , fixing the psf centers at the knot positions in the vla 22 ghz radio image . however , this procedure did not fully determine knot by knot fluxes due mainly to a multiplicity of knots from c to e. we then fixed the flux density of knot e at @xmath26 and @xmath27 . we derived the systematic errors due to this treatment by changing the knot e flux in a range of @xmath28 and @xmath29 . the systematic errors are then added in quadrature to statistical ones . the photometric results determined in this way are listed in table  [ tbl-1 ] , where a combined flux is reported for knots c , d , and e ( referred as knots cde ) . crrrrrrrrr core & 463 & & & @xmath30 & @xmath31 & & @xmath32 & 170 & + a & & @xmath33 & & @xmath34 & @xmath35 & @xmath36 & @xmath37 & @xmath38 & @xmath39 + b & @xmath40 & @xmath41 & @xmath42 & @xmath43 & @xmath44 & @xmath45 & @xmath46 & @xmath47 & @xmath48 + cde & @xmath49 & @xmath50 & @xmath51 & @xmath52 & @xmath53 & @xmath54 & @xmath55 & @xmath56 & @xmath57 + hs & @xmath58 & @xmath59 & @xmath60 & @xmath61 & @xmath62 & @xmath63 & @xmath64 & @xmath65 & +      in figure [ fig : great ] we present a three - color multifrequency image based on _ spitzer _ infrared ( _ red _ ) , _ hst _ optical ( _ green _ ) , and _ chandra _ x - rays ( _ blue _ ) . the vla 8.5 ghz radio contours in a logarithmic scale are overlaid . the infrared photometry at @xmath21 is illustrated as a series of best - fitted psfs of every knot after artificially shrinking the widths of the psfs ( @xmath10 fwhm ) to restore a resolution similar to the x - rays . at the positions of optically bright knots a and b we plot a _ green _ dot of @xmath66 to illustrate clear detections with the _ hst _ stis and acs @xcite . chandra _ x - ray image in 0.38 kev band is smoothed with a gaussian kernel of @xmath67 .    the jet morphology , or brightness pattern along the jet , in the infrared appears to be different from emission at shorter wavelengths . it is interesting to note that optically - bright features ( knots a and b ) are not particularly prominent in the infrared , while an infrared - bright knot d does not emit correspondingly strong optical light . the difference in the brightness pattern between the infrared and optical suggests a dramatic change of the spectral shape in the infrared - to - optical band along the jet . this emphasizes the importance of measuring infrared fluxes , together with the optical fluxes , in quasar jets . putting hotspot hs aside , the jet knots can be divided up into two parts : the _ inner knots _ ( a and b ) and the _ outer knots _ ( c , d , and e ) . the inner knots are bright in both the optical and x - rays , and as such they are  high - energy dominated . " indeed the inner knots are most luminous in the x - rays , while the outer knots seem to have a luminosity peak in the far - infrared . to clarify this , we present the seds of the jet knots in the following section .      ccccc parameter & knot a & knot b & knots cde & hotspot hs + @xmath68 & 0.65 & 0.65 & 0.85 & 1.0 + @xmath69 ( hz ) & @xmath70 & @xmath71 & @xmath72 & @xmath73 + @xmath74 ( @xmath75 ) & @xmath76 & @xmath77 & @xmath78 & @xmath79 + @xmath80 & 0.75 & 0.70 & 0.50 & + @xmath81 ( @xmath75 ) & @xmath82 & @xmath83 & @xmath84 & +    we construct the broad - band seds of the jet knots using the new _ spitzer _ irac data added to the _ vla _ , _ hst _ , and _ chandra _ data already presented in @xcite . once again , the _ spitzer _ irac provides crucial photometric points in characterizing the emissions of quasar jets @xcite . table [ tbl-1 ] summarizes the flux densities obtained with _ spitzer _ , _ hst _ , and _ chandra_. the radio fluxes measured with the vla at frequencies of 4.9 , 8.46 , and 22.5 ghz are taken from @xcite . infrared photometry with the _ spitzer _ irac is described in  [ irac ] . in the optical , we list the flux densities from the _ hst _ acs in two filters @xcite : f475w ( @xmath85 ) and f814w ( @xmath86 ) . we employed a somewhat different method for x - ray photometry compared with @xcite . specifically , we here define two energy bands for photometry : 12 kev and 36 kev . integrating over the entire jet ( from knot a to hotspot hs ) we found 203 and 57 counts in the 12 and 36 bands , respectively ( also , 147 counts in the 0.61 kev band ) . the nominal energy @xmath87 for each band is the mean energy weighted by the effective area : @xmath88 1.56 kev and 4.35 kev . we reanalyzed the _ chandra _ data sets that are fully described in @xcite , and constructed flux images ( in units of @xmath89 ) in the two bands , to which aperture photometry was applied . the object apertures for knots a / b and hotspot are a circle with a radius of @xmath90 centered on each feature , while that for knots cde is a box enclosing knots c , d , and e. to correct for the interstellar absorption of @xmath91 , we multiplied the measured fluxes by a factor of 1.04 for the 12 kev band . the fudge factor slightly depends on the source spectrum such that it ranges from 1.040 to 1.048 for a power law with photon index @xmath92 . the photometry - based spectra are found to be consistent with the usual spectral analysis presented by @xcite . the photometric method is convenient when combining lower frequency data . care should be taken in interpreting the x - ray fluxes . first , the x - ray emission downstream of knot b does not show well discernible knots , and therefore the x - ray flux of knots cde may not be related directly to the lower frequency fluxes . second , in knot a there is an offset of @xmath93 between the soft and hard x - ray peaks @xcite . the x - ray spectrum derived for knot a may be contaminated by some unrelated emission , which makes the spectral shape inconsistent with a power law .    in fig .  [ fig : sed ] , we present the seds from radio to x - rays for knots a , b , cde , and hotspot hs . we note that the infrared fluxes we have measured with the _ spitzer _ irac fill central points in the radio - to - x - ray seds of the jet knots , setting an important constraint on models of the broad - band emission . the _ inner knots _ ( a and b ) radiate most strongly in the x - ray . also , significant ( and probably flatter ) optical emission is observed exclusively from the _ inner knots_. the possible difference in slopes of the optical continuum between the _ inner _ and _ outer knots _ @xcite suggest different origins ; the optical emission in the _ inner knots _ may belong to the same spectral component responsible for the x - ray emission ( as suggested by * ? ? ? * for knot a ) , while in the outer knots the optical emission is related to the radio synchrotron component . these spectral characteristics are analogous to those of the jet in 3c  273 @xcite , for which we identified two spectral components : ( 1 ) the low - energy synchrotron spectrum extending from radio to infrared , and ( 2 ) the high - energy component arising in the optical and smoothly connecting to the x - ray flux . we argued that the second component is likely to be of synchrotron origin as well , thus forming double - synchrotron spectra , because of the similarity of polarization between optical and radio emission ( though the degrees of optical polarization are not established yet ) . given the similarity to 3c  273 , we model the radio - to - x - ray seds phenomenologically by the following function , namely , a double power - law with an exponential cutoff : @xmath94 .\ ] ] the first term ( @xmath95 ) of the right - hand - side accounts for the low - energy spectrum and the second term ( @xmath96 ) describes the high - energy part . as there are no firm indication of spectral steepening at x - rays , we set an arbitrary cutoff at @xmath97 hz for the second component . for the inverse - compton model described later (  [ sec : iccmb ] ; see fig . [ fig : iccmb ] ) , however , the high - energy cutoff is located far beyond the x - ray domain , reaching very high - energy gamma - rays . table [ tbl-2 ] lists the spectral parameters that well describe the knot seds shown in fig . [ fig : sed ] . instead of the normalization , @xmath98 , we present the total apparent luminosity ( without taking account of relativistic beaming ) defined as @xmath99 for each component , where the low - energy boundary of the integral is set at @xmath100 hz and @xmath101 hz for the first and second component , respectively . knot b , the most x - ray luminous one , has a luminosity of @xmath102 for the high - energy component , which indeed exceeds the luminosity of the low - energy component . the spectral index of the low - energy component is in the range of @xmath103 , which is determined by the radio spectra , as listed in table [ tbl-2 ] . the cutoff frequency of the low - energy component , as constrained by the irac fluxes , is @xmath104 hz in the cases of knots b , cde , and hotspot hs ; @xmath105 hz for knot a. the second spectral index is found to be @xmath106 . while the low - energy emission from quasar jets is undoubtedly of synchrotron origin , the production mechanism(s ) of strong x - rays remains a matter of debate ( e.g. , * ? ? ? three radiation models have been proposed to explain the high - energy component : inverse - compton scattering by radio - emitting electrons , synchrotron radiation by a second electron population , and synchrotron radiation by energetic protons . in this section we consider each of these in turn . there are reasons to think that jets are relativistic on large scales @xcite and thus beaming may be important . the doppler beaming factor is defined as @xmath107^{-1}$ ] where @xmath108 is the velocity of the jet , @xmath109 is the bulk lorentz factor of the jet , and @xmath110 is the observing angle with respect to the jet direction . a one - sided jet with two - sided hotspots in this source is a sign of substantial relativistic beaming . the doppler factor is , however , not well constrained for this jet . throughout this section , all physical quantities are referred to a jet co - moving frame unless otherwise specified . the exceptions to this are the direction and velocity of the jet itself . it is assumed that a relativistically moving blob ( as noted by knot or hotspot ) has spherical geometry with a radius of @xmath111 kpc ( in the jet frame of reference ) occupied homogeneously with relativistic particles and magnetic fields (  one - zone " model with a filling factor of unity ) .    before we discuss the high - energy emission component of the _ jet knots _ ( like knot b ) , we derive a lower limit on the magnetic field strength in hotspot hs , based on the _ absence _ of the high - energy emission . we do not introduce beaming effects for hotspot hs ( @xmath112 ) . synchrotron photons are compton upscattered by synchrotron - emitting electrons themselves , namely  synchrotron self - compton " ( ssc ) , which is known to be a primary component for the high - energy emission in bright hotspots such as those in cygnus a @xcite . in fact , since the average energy density of synchrotron photons ( see * ? ? ? * ) in hotspot hs , @xmath113 , largely exceeds the cmb energy density of @xmath114 , the ssc flux well exceeds the ic / cmb flux in the hotspot of pks  1136@xmath0135 . we found a lower limit on magnetic field as @xmath115 in order not to violate the x - ray upper limit by the ssc emission . with an equipartition magnetic filed of @xmath116 where the energy density of relativistic electrons is equal to that of magnetic fields , the one - zone ssc flux is an order of magnitude lower than the x - ray upper limit . we first discuss the second , x - ray - dominated component of the sed seen in the jet knots ( knot b , in particular ) in the framework of the beamed inverse - compton model @xcite , which has been advocated in the previous papers for this jet @xcite . in this model , it is assumed that the jet has a highly relativistic bulk velocity , with a lorentz factor @xmath3 out to distances of hundreds of kiloparsecs , so that the cmb field _ seen _ by electrons is enhanced by a factor of @xmath117 in the jet comoving frame . the amplified cmb can be compton up - scattered by high - energy electrons of @xmath118 into the observed x - rays . this model automatically requires that the jet direction is close to the line - of - sight , @xmath119 . ( such a condition , and therefore the beamed ic model itself , would not be favorable for a lobe - dominated quasar like pks  1136@xmath0135 . later in this section , we shall briefly discuss this issue . ) it is interesting to test the beamed ic / cmb hypothesis , particularly for the pks  1136@xmath0135 jet . the jet exhibits the evolution of multiwavelength emission along the jet , namely the increase of radio brightness accompanied by the decrease of x - ray brightness towards downstream . as detailed in @xcite , such a behavior can be interpreted , within the framework of the beamed ic radiation , as being due to deceleration of the jet @xcite . specifically , @xcite concluded that the bulk lorentz factor of the jet decreases from @xmath120 ( knot b ) to @xmath121 ( knot e ) . if the beamed ic scenario is confirmed , we gain a new probe of the flow structure of the large - scale jets . let us apply a synchrotron plus beamed ic / cmb model of @xcite to the broadband sed of knot b. we also include ssc calculation for completeness . the both models adopt  one - zone " emission , assuming an emitting blob with homogeneously filled with relativistic particles and magnetic fields . it is interesting to note that in the case of no - beaming ( @xmath112 ) , again , the average energy density of synchrotron photons in the knot , @xmath122 , exceeds the cmb energy density . the energy distribution of radiating electrons is assumed to be of the form @xmath123 , having a low - energy cutoff at @xmath124 and a high - energy exponential cutoff at @xmath125 , where @xmath126 denotes the electron s lorentz factor . the number index of electrons is set to be @xmath127 based on a typical radio index in this jet ( @xmath128 ) , and the high - energy cutoff is determined to be @xmath129 to produce the infrared flux . the magnetic field strength is adopted as @xmath130 , where @xmath131 g ; this relation ensures equipartition between relativistic electrons and magnetic fields , and keeps the peak frequency of the synchrotron radiation , @xmath132 . the bulk lorentz factor itself does not explicitly affect the discussion made here and it is adopted to be @xmath133 . figure  [ fig : icssc ] shows the ic / cmb and ssc x - ray fluxes as a function of @xmath134 , given the radio flux observed for knot b. since the normalization of the electron distribution scales as @xmath135 and the beaming pattern of the ic / cmb emission follows @xmath136 @xcite , the ic / cmb flux scales as @xmath137 . the ssc flux scales as @xmath138 , where @xmath139 . as such , if the jet is heavily beamed , @xmath140 , the ic / cmb flux is dominant . it reaches the observed x - ray flux at @xmath141 ; the corresponding sed is shown in the top panel of fig . [ fig : iccmb ] . the ssc becomes a main component for @xmath142 . even if we invoke a particle - dominated case of @xmath143 , the ssc model requires a significant de - beaming of @xmath144 to explain the observed x - rays , which is quite unlikely as the on - axis version of such jet emission becomes unacceptably luminous . the radio - emitting electrons emit gev @xmath126-rays through the ic / cmb process ( fig . [ fig : iccmb ] ) . the total radiative output is dominated by the multi - gev @xmath126-rays and their predicted flux just reaches the sensitivity offered by the upcoming glast satellite ( see * ? ? ? * for the case of 3c 273 ) . the quasar core is expected to show a similar level of @xmath126-ray emission @xcite ; steadiness and hard spectrum are signatures of the large - scale emission . the observation of pks  1136@xmath0135 ( or quasar jets in general ) with glast may be able to test the ic / cmb hypothesis . note that the sub - tev domains are subject to intergalactic absorption , which makes it difficult to investigate the jet emission with future ground - based cherenkov telescopes . the doppler factor of @xmath145 required by the ic / cmb model implies an uncomfortably small angle between the jet and the line - of - sight , @xmath146 . @xcite also derived the maximum angle permitted as @xmath147 for this jet . the small angle would make the jet quite long , @xmath148 , and the total source extent would be as large as the largest quasar . the core - to - lobe flux ratio at 5 ghz , @xmath149 , is an indicator of jet orientation . in the case of 1136@xmath0135 , the core - to - lobe ratio of @xmath150 is obtained @xcite , thus designated as a _ lobe - dominated _ quasar . according to @xcite , a sample of radio - loud quasars with a median value of @xmath151 have an average angle to the line - of - sight of @xmath152 with simple beaming models . also , the mean angle for steep - spectrum radio quasars is estimated as @xmath152 @xcite . in this respect , a very small viewing angle @xmath146 for a lobe - dominated quasar like pks  1136@xmath0135 is not favored in the light of simple unification schemes of radio - loud quasars and radio galaxies @xcite ; a similar statement was made by @xcite . however , it should be kept in mind that _ chandra_-detected jets can be biased toward smaller jet angles , which alleviates a problem of the jet angle . we need a more direct means to distinguish currently proposed models ( see  [ sec : discri ] ) . the large - scale jet in pks  1136@xmath0135 is in many respects similar to the well - known jet of quasar 3c  273 . as @xcite have pointed out , in both jets , the radio emission brightens monotonically towards the terminal hotspot , while the upstream knots are brighter in x - rays . the two - component seds are also similar ( cf , * ? ? ? . in the case of 3c  273 , since both the radio and optical emission are linearly polarized to a similar degree and in the same direction , it seems reasonable that the high - energy component , which contributes more than half the optical flux , should also arise from the synchrotron process @xcite . therefore we discuss the optical - to - x - ray component of 1136@xmath0135 in terms of synchrotron radiation produced by a second population of high - energy electrons . the suggestion of the presence of a second synchrotron component has been made in previous studies of other large - scale jets ( e.g. , * ? ? ? ? * ; * ? ? ? let the energy distribution of _ accelerated _ electrons be @xmath153 for an energy interval of interest . the power - law slope of electrons is derived from the radiation slope as @xmath154 [ see eq . ( [ eq : fnu ] ) ] in a synchrotron - cooling regime as appropriate for optical and x - ray emission ; the _ cooled _ electrons responsible for the observed radiation have a steeper number index of @xmath155 ( see e.g. , * ? ? ? then the spectral index of the high - energy component , @xmath106 corresponds to @xmath156 . however , within the framework of diffusive shock acceleration , the hardest possible electron distribution of acceleration would be @xmath157 , in other words , @xmath158 ( see * ? ? ? the inferred index @xmath156 would violate such a theoretical limit . this brings into question the idea that the second electron distribution is formed through the diffusive shock acceleration or that there is a second electron distribution at all . note that the jet in 3c  273 has spectral index of @xmath159 corresponding to @xmath160 , which was considered to be compatible with the shock acceleration theory @xcite . the second synchrotron component in the pks  1136@xmath0135 jet may instead be due to turbulent acceleration operating in the shear layers @xcite . the coexistence of two distinct types of acceleration , shock and turbulent acceleration , in the knots would naturally give rise to a double - synchrotron spectrum . more importantly , unlike the shock acceleration , a hard spectrum , @xmath161 , can be expected to form in the case of turbulent acceleration ( i.e. , second - order fermi acceleration ) . in fully turbulent shear layers , the time scale of turbulent acceleration can be estimated as @xmath162 , where @xmath163 is an electron gyroradius and @xmath164 denotes the alfvn velocity . the turbulent acceleration has to compete against synchrotron losses , which is presumed to dominate over ic losses , with a timescale of @xmath165 where @xmath166 is the thomson cross section , and @xmath167 is the energy density of the magnetic field . by equating @xmath168namely , balancing the acceleration and synchrotron loss rates one obtains the maximum attainable energy limited by synchrotron losses : @xmath169 where @xmath170 . to account for the observed x - rays with synchrotron radiation , one needs @xmath171 ( see * ? ? ? * ) , thus requiring @xmath172 . this condition is reasonable with typical jet parameters . finally , we argue the possibility that the optical - to - x - ray emission may be due to synchrotron radiation by very high energy _ protons _ @xcite . this model requires that protons in the jet are somehow accelerated to very high energies , @xmath173 or more . also , the magnetic field strength must be of the order @xmath174 , so that energy equipartition between the relativistic _ protons _ and magnetic fields can be ( roughly ) realized . the knots and hotspots in the relativistic jets of powerful quasars and radio galaxies are indeed one of a few potential sites of cosmic - ray acceleration up to @xmath175 @xcite . for example , the turbulent acceleration in the shear layer may be able to accelerate such ultra high energy protons @xcite . the characteristic frequency and cooling time of proton synchrotron radiation can be written as @xmath176 \left ( \frac{b}{\rm mg}\right )   \left ( \frac{e_{p}}{10^{18}\ \rm ev } \right)^2\   \rm hz,\ ] ] and @xmath177 respectively @xcite . ( note that the synchrotron cooling time is much shorter than the photo - meson cooling time at energies relevant here . ) a total energy content of the magnetic field @xmath178 integrated over a spherical knot with a radius of @xmath179 amounts to @xmath180 . if we assume equipartition between radiating protons and magnetic fields , @xmath181 , the x - ray luminosity of synchrotron radiation by ultra - high - energy protons of @xmath173 can be estimated roughly as @xmath182 where @xmath183 denotes the fraction of kinetic energy of protons responsible for the x - ray emission . relativistic beaming is not taken into account in this estimate . roughly speaking , the luminosity scales with @xmath184 and @xmath134 such that @xmath185 . the expected luminosity agrees with typical ( apparent ) luminosity of the jet knots of quasars , @xmath186 , if @xmath187 . thus , very high energy protons rather than a second population of electrons , may explain the second synchrotron component extending from the optical to x - rays provided that both ultra - high - energy protons of @xmath173 and a magnetic field of @xmath174 are present in the jet knots . under the proton - synchrotron hypothesis , the low - energy synchrotron emission may be accounted for by accompanying electrons .    to be specific , we modeled the high - energy spectral component of knot b with proton synchrotron radiation . we adopt mild beaming of @xmath188 ( @xmath189 ) . the low - energy spectral component was simultaneously modeled by electron synchrotron radiation with taking account of the effects of significant synchrotron cooling . it is assumed that during @xmath190 years , protons and electrons are injected continuously into the emission volume ( @xmath191 ) with the distributions characterized by number index @xmath192 and @xmath193 and by the maximum energy @xmath194 and @xmath195 : @xmath196 . in the bottom panel of fig .  [ fig : iccmb ] , we present the broadband sed reproduced by the proton synchrotron model with the equipartition magnetic field of @xmath197 , the index of the power - law distribution of _ accelerated _ particles @xmath198 , and the maximum energy of @xmath199 and @xmath200 . the injection rate of electrons , in terms of energy content @xmath201 , is about 5% of that of protons . the injected electrons suffer from severe synchrotron cooling . even radio - emitting electrons have a cooling time of only @xmath202 years , which requires _ in - situ _ acceleration of electrons . the infrared - emitting electrons have a lifetime of @xmath203 years , implying that each knot is a currently ( within @xmath204 years ) active site of particle acceleration . the energy density of protons @xmath205 , which is equal to @xmath206 , can be calculated by @xmath207 with @xmath208 , since synchrotron cooling of protons is not effective . the kinetic power of the jet , estimated as @xmath209 , amounts to @xmath210 . this estimate is not sensitive to the choice of @xmath211 as long as the equipartition condition is fulfilled . a large power has to be carried by the protons and magnetic fields in the jet to produce the x - ray emission via a proton - synchrotron process . the black hole mass that powers the jet is estimated to be @xmath212 @xcite using the empirical relation to the width of the h@xmath213 line and the optical continuum luminosity . the black hole mass corresponds to the eddington luminosity of @xmath214 . therefore , the proton synchrotron model requires the jet to be _ super - eddington _ , @xmath215 . this issue would pose a problem for the proton synchrotron model , though it may be alleviated by assuming that the knots present the locations of _ power peaks _ due to modulated activity of the central engine . we emphasize here that optical polarimetry can be an effective way of discriminating the radiation models responsible for the optical - to - x - ray emission of the jet in pks  1136@xmath0135 , and of quasar jets in general . in the beamed ic interpretation , if the optical fluxes belong to the ic component , the optical and x - ray emission are due to compton up - scattering off the amplified cmb by high - energy electrons of @xmath216 ( optical ) and @xmath217 ( x - ray ) . unlike in the case of synchrotron models , the x - rays are expected to be _ unpolarized _ and the optical light is nearly unpolarized at most a few percent of polarization ( uchiyama & coppi , in preparation ) ( in the jet frame ) exists , namely  bulk comptonization " , can yield in principle a high degree of polarization for a jet angle @xmath218 @xcite . for @xmath219 of a few as relevant here , however , the polarization is largely suppressed . ] . precise polarization measurements in the optical can in principle verify ( or discard ) the beamed ic model . unfortunately , there have been no useful polarization observations of quasar jets with _ hst _ so far . only for 3c 273 , early _ hst _ polarimetry of the jet was done but with the pre - costar foc and low significances @xcite . as was recently performed for nearby radio galaxies by @xcite , new , deep polarimetry of quasar jets on large - scales is highly encouraged , although the optical emission from the jet knots is generally faint , say @xmath220 @xcite . we have detected the infrared emission from the kiloparsec - scale jet in the quasar pks  1136@xmath0135 with the _ spitzer space telescope_. using the new _ spitzer _ data together with deep _ chandra _ , _ hubble _ , and multi - frequency vla observations from @xcite , we construct the broadband spectral energy distributions along the large - scale jet . the seds of the jet knots are comprised of two components : radio - to - infrared synchrotron emission and a separate high - energy component responsible for x - rays , which may extend down to the optical . in total , each component has a similar apparent luminosity of the order of @xmath221 . the origin of the high - energy component has significant implications to the properties of particle acceleration in a relativistic jet as well as jet dynamics . we consider three radiation models that have been proposed to explain the high - energy component : inverse - compton scattering in a highly relativistic jet , synchrotron radiation by a second electron population , and synchrotron radiation by highly energetic protons .    in terms of the double synchrotron scenario , a flat spectrum of the second synchrotron component , @xmath222 , would turn down diffusive shock acceleration as the mechanism of electron acceleration to very high energies , @xmath223 tev . instead turbulent acceleration in the shear layers may explain the second population of electrons giving rise to the flat spectrum . in the beamed ic interpretation , a jet orientation with respect to the observer of @xmath146 is required , which is not quite favorable given the small core - to - lobe ratio of this object . future optical polarimetry or @xmath126-ray observations with glast may be able to test the beamed ic model . the proton synchrotron model requires a relatively large magnetic field of @xmath224 mg , and consequently large kinetic power of @xmath225 , with efficient acceleration of protons to @xmath226 . if this is the case , one can probe particle acceleration at very high energies , higher than any other known objects in the universe . we thank the referee , eric perlman , for his careful reading of the manuscript and valuable suggestions . this work is based on observations made with the spitzer space telescope , which is operated by the jet propulsion laboratory , california institute of technology under nasa contract 1407 . support for this work was provided by nasa through contract number rsa 1265389 issued by jpl / caltech . the national radio astronomy observatory is operated by associated universities , inc .  under a cooperative agreement with the national science foundation . aharonian , f. a. 2002 , , 332 , 215 atoyan , a. m. , & dermer , c. d. 2004 , , 613 , 151 bahcall , j. n. , kirhakos , s. , schneider , d. p. , davis , r. j. , muxlow , t. w. b. , garrington , s. t. , conway , r. g. , & unwin , s. c. 1995 , , 452 , l91 band , d. l. , & grindlay , j. e. 1985 , , 298 , 128 begelman , m. c. , & sikora , m. 1987 , , 322 , 650 brookes , m.  h. , lawrence , c.  r. , keene , j. , stern , d. , gorijan , v. , werner , m. , & charmandaris , v.  2006 , , 646 , l41 celotti , a. , ghisellini , g. , & chiaberge , m. 2001 , , 321 , l1 chartas , g. , et al . 2000 , , 542 , 655 cheung , c. c. , stawarz ,  . , & siemiginowska , a.  2006 , , 650 , 679 crane , p. , et al . 1993 , , 402 , l37 dermer , c. d. 1995 , , 446 , l63 dermer , c. d. , & atoyan , a. m. 2002 , , 568 , l81 fazio , g. , et al.2004 , , 154 , 10 gambill , j.  k. , sambruna , r.  m. , chartas , g. , cheung , c.  c. , maraschi , l. , tavecchio , f. , urry , c.  m. , & pesce , j.  e.  2003 , , 401 , 505 garrington , s.  t. , leahy , j.  p. , conway , r.  g. , & laing , r.  a.  1988 , , 331 , 147 georganopoulos , m. , & kazanas , d. 2004 , , 604 , l81 georganopoulos , m. , kazanas , d. , perlman , e. , & stecker , f. w. 2005 , , 625 , 656 georganopoulos , m. , kirk , j. g. , & mastichiadis , a. 2001 , , 561 , 111 georganopoulos , m. , perlman , e. , kazanas , d. , & mcenery j.2006 , , 653 , 5 hardcastle , m.  j. , birkinshaw , m. , & worrall , d.  m.  2001 , , 326 , 1499 hardcastle , m.  j. , kraft , r.  p. , & worrall , d.  m.  2006 , , 368 , l15 harris , d. e. , carilli , c. l. , perley , r. a. 1994 , , 367 , 713 harris , d. e. , & krawczynski , h. 2006 , , 44 , 463 harris , d. e. , mossman , a. e. , & walker , r. c. 2004 , , 615 , 161 hillas , a. m.1984 , , 22 , 425 jester , s. , harris , d.  e. , marshall , h.  l. , & meisenheimer , k.  2006 , , 648 , 900 jester , s. , rser , h .- j . , meisenheimer , k. , & perley , r. a. 2002 , , 385 , l27 jester , s. , rser , h .- j . , meisenheimer , k. , & perley , r. a. 2005 , , 431 , 477 jorstad , s. g. , & marscher , a. p. 2004 , , 614 , 615 kataoka , j. , leahy , j.  p. , edwards , p.  g. , kino , m. , takahara , f. , serino , y. , kawai , n. , & martel , a.  r.  2003 , , 410 , 833 kataoka , j. , & stawarz ,  . 2005 , , 622 , 797 laurent - 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we present _ spitzer _ irac imaging of the large - scale jet in the quasar pks  1136@xmath0135 at wavelengths of 3.6 and @xmath1 , combined with previous _ vla _ , _ hst _ , and _ 
 chandra _ observations . 
 we clearly detect infrared emission from the jet , resulting in the most detailed multifrequency data among the jets in lobe - dominated quasars . 
 the spectral energy distributions of the jet knots have significant variations along the jet , like the archetypal jet in 3c  273 . 
 the infrared measurements with irac are consistent with the previous idea that the jet has two spectral components , namely ( 1 ) the low - energy synchrotron spectrum extending from radio to infrared , and ( 2 ) the high - energy component responsible for the x - ray flux . 
 the optical fluxes may be a mixture of the two components . 
 we consider three radiation models for the high - energy component : inverse compton scattering of cosmic microwave background ( cmb ) photons by radio - emitting electrons in a highly relativistic jet , synchrotron radiation by a second distinct electron population , and synchrotron radiation by ultra high energy protons . 
 each hypothesis leads to important insights into and constraints on particle acceleration in the jet , as well as the basic physical properties of the jet such as bulk velocity , transporting power , and particle contents .