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cygnus a is the nearest example ( @xmath2=0.056 ) of a powerful fanaroff - riley type - ii radio galaxy . as such , it contains some of the brightest radio hotspots to study these classical ` working surfaces ' of relativistic jets ( hargrave & ryle 1974 ; blandford & rees 1974 ) . although the hotspots are well - studied at radio and x - ray wavelengths (   2 ) , cygnus a is viewed through the galactic plane , making them difficult to study at optical wavelengths ( 1.26 mag of extinction at v - band ; meisenheimer et al . additionally , it is well known that bright foreground stars lie in the direction of cygnus a , and this has also hindered optical studies of the hotspots . to circumvent these obstacles , we obtained new _ spitzer space telescope _ images ( figure  1 ) resulting in successful mid - infrared detections of the hotspots . these new data , together with a wealth of published high - quality measurements at other frequencies , give us some of the most complete hotspot seds for a powerful radio galaxy . this contribution is a summary of our recently published study ( stawarz et al . we obtained new @xmath3/irac @xcite observations simultaneously at 4.5 and 8.0 @xmath4 m for 3.6 hrs . galactic extinction amounts to only 0.020.03 mag at these bands . a foreground star is known to lie within an arcsecond of the position of hotspot a  extrapolating its spectrum into the irac bands , its contamination to the hotspot ir fluxes is expected to be negligible ( see fig . 2 g in meisenheimer et al . radio data were obtained from published vla and bima measurements and maps @xcite . we focus our discussion on the two brightest hotspots ( a and d ) , whose spectral energy distributions ( seds ) are shown in figure  [ fig-2 ] . we modeled the synchrotron ( radio - to - infrared ) portion of the hotspot seds with a double power - law that decays exponentially at the higher frequencies :    @xmath5    the fitted parameters ( @xmath6 , @xmath7 , @xmath8 , @xmath9 ) are ( 0.28 , 1.2 , 2.6@xmath1010@xmath11 hz , @xmath123.3@xmath1010@xmath13 hz ) for hotspot a , and ( 0.21 , 1.1 , 3.1@xmath1010@xmath11 hz , 0.9@xmath1010@xmath13 hz ) for hotspot d. the minimum frequency , @xmath14 , can be extrapolated down to 74 mhz with the vla detections of the hotspots by lazio et al . ( 2006 ) at this frequency . the hotspots emit strong x - ray emission by the synchrotron self - compton ( ssc ) process as discovered with @xmath15 ( harris et al . 1994 ) . assuming this for the @xmath16 detection of the hotspots ( wilson et al . 2000 ; wright & birkinshaw 2004 ) , we infer magnetic field strengths of @xmath170.20.3 mg . the emitting plasma is close to equipartition with @xmath18 ( figure  3 ) , consistent with the previous fits of the @xmath16 data . an @xmath19 optical measurement of hotspot d ( nilsson et al . 1997 ) lies near the low energy extrapolation of the ssc spectrum ; this makes it one of the few hotspots with detected optical ssc emission ( brunetti 2004 ) . increasing the low energy spectral index ( @xmath6 ) of hotspot d from its observed value to the ` canonical ' value of 0.5 would overproduce the observed optical and ir emissions , so the observed flat spectra are likely intrinsic .    with the magnetic field and observed spectral fits , we can reconstruct the underlying electron energy spectra , @xmath20 ( @xmath21 ; figure  3 ) . they have the following features : _ ( i ) the critical break energy corresponds almost exactly to the ratio of the proton to electron masses , @xmath22 . ( ii ) the low - energy segment of the electron distributions continues down to at least @xmath170.1 @xmath23 , with flat spectra ; the power - law slopes are @xmath241.41.6 . ( iii ) the new ir data constrain the high - energy electron spectra , with indices @xmath25@xmath123 and maximum energies @xmath2650 @xmath23 . _ the `` standard '' electron spectrum expected from the diffusive ( 1st order fermi ) shock acceleration in the non - relativistic test - particle limit , @xmath27 , is not observed . the overall curved spectra are likely to be intrinsic rather than due to any absorption / cooling effects . terminal shocks of powerful jets , like those in cygnus a , are probably ( mildly ) relativistic , with an oblique magnetic field configuration . as such , it should not be surprising that the diffusive acceleration process known from the non - relativistic test - particle models are not directly applicable , i.e. , the lack of a canonical shock - type spectrum @xmath28  see begelman & kirk ( 1990 ) and niemiec et al . ( 2006 ) for relevant discussions . the break frequency corresponding to the @xmath23 mass ratio preceded by a flat - spectrum power - law is also an interesting finding , which confirms some previous evidences for such in other powerful hotspots ( see leahy et al . 1989 ) .    our results , although inconsistent with the simplest modeling of terminal hotspots in radio galaxies as sites of the efficient 1st order fermi acceleration , are instead in good agreement with our current understanding of relativistic shock waves . in particular , a flat power - law electron spectrum up to @xmath23 energies is consistent with the simple ( 1d ) resonant acceleration model discussed by amato & arons ( 2006 ) for relativistic shocks . their simulations show that as a result of resonant absorption of cyclotron emission produced by cold protons reflected from the shock front , a flat power - law electron energy distribution emerges between energies @xmath29 and @xmath30 , the latter possibly reduced by a factor of a few due to thermal dispersion in the upstream proton momenta ( @xmath31 is the bulk lorentz factor of the upstream medium ) . the slope of this distribution is not well constrained by the simulations , but can be relatively flat and appears to depend on the plasma content ( i.e. , on the ratio of proton to electron number density ) . although these particular simulations involve a large @xmath31 plasma , analogous processes are expected to take place also at mildly relativistic shocks considered in hotspots like cygnus a ( @xmath32 few ) , so the model results can at least be applied qualitatively . if such an association is correct , and if the results of amato & arons ( 2006 ) are applicable , then it would automatically imply that ( _ i _ ) the jets in powerful radio sources like cygnus  a are made of an electron - proton rather than electron - positron plasma , ( _ ii _ ) a significant fraction of the jet kinetic power is carried by cold protons , and ( _ iii _ ) the number density of protons within the jets is most likely lower than that of the electron - positron pairs ( see the discussion in stawarz et al . we note that these conclusions regarding the jet content would then be consistent with those presented for quasar jets ( e.g. , sikora & madejski 2000 ) , which are widely regarded as beamed counterparts of fr  ii radio galaxies like cygnus a.    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 a contract with nasa . the nrao is operated by associated universities , inc . under a cooperative agreement with the national science foundation . s. and m.  o. were supported by mein through the research project 1-p03d-003 - 29 in years 2005 - 2008 . the work at sao was partially supported by _ spitzer _ grant 1279229 .
we report new detections of the hotspots in cygnus a at 4.5 and 8.0 microns with the _ spitzer space telescope_. together with detailed published radio observations and synchrotron self - compton modeling of previous x - ray detections , we reconstruct the underlying electron energy spectra of the two brightest hotspots ( a and d ) . 
 the low - energy portion of the electron distributions have flat power - law slopes ( @xmath0 ) up to the break energy which corresponds almost exactly to the mass ratio between protons and electrons ; we argue that these features are most likely intrinsic rather than due to absorption effects . beyond the break , 
 the electron spectra continue to higher energies with very steep slopes @xmath13 . 
 thus , there is no evidence for the ` canonical ' s=2 slope expected in 1st order fermi - type shocks within the whole observable electron energy range . 
 we discuss the significance of these observations and the insight offered into high - energy particle acceleration processes in mildly relativistic shocks .