You are an expert at summarizing long articles. Proceed to summarize the following text: 
agn classification is still somewhat heuristic , but in the unified model , blazars are believed to be sources viewed close to the axis of a powerful relativistic jet ( * ? ? ? * and references therein ) . they thus have compact flat spectrum radio counterparts , with apparent superluminal motion at vlbi scales . optically , the sources are often variable , exhibit significant polarization and show either broad emission lines ( flat spectrum radio quasar ) or continuum - dominated ( bl lac ) spectra . perhaps the most interesting consequence of the jet / line - of - sight alignment is the observability of a compton scattered component at x - ray to gev or ( for nearby sources ) tev energies . the interaction of the high energy jet particles and radiation with surrounding photon fields allows unique probes of the extragalactic background light ( ebl ) . one of the principal discoveries of the _ egret _ experiment on the _ compton gamma ray observatory _ was the large population of blazars emitting at gev energies . these represented the largest identified population in the third _ egret _ ( 3eg ) catalog ; still many high latitude sources remained unidentified . we have developed a method to extend identifications to fainter radio flux levels @xcite , finding counterparts for 70% of the @xmath3 sources . follow - on observations show that most of these are previously unidentified blazars . upcoming gev @xmath0-ray missions , especially _ glast _ , will have much higher sensitivity and are expected to detect several thousand blazars . this substantially exceeds the total number of blazars cataloged to date @xcite . thus in preparation for this mission , we have selected radio ( & x - ray ) sources with properties like the _ egret _ blazars and are obtaining optical identifications and spectroscopy ( * ? ? ? * srm04 ) .    to prioritize the optical observations , we have also developed a method for estimating the probability that a given position has an excess of @xmath0-ray photons in the _ egret _ survey observations , including the effect of strong variability . the radio blazar candidates selected above show , as a set , a clear excess of @xmath0-ray flux over random sky positions ( srm04 ) ; individually , they are well below the 4@xmath4 criterion for inclusion in the 3eg catalog . for example , radio blazar candidates with @xmath575% probability of _ egret _ @xmath0-ray excess are found at @xmath6 , and thus may represent 10 - 20% of the new sources detectable to _ glast_. these are prime candidates for optical follow - up ; we find that most are indeed flat spectrum radio quasars with redshifts 1 - 2.5 . q0609 + 6930 , however , has a very high redshift of 5.47 . observations described below support a blazar identification for this source ; this would be the highest redshift blazar discovered to date . in selecting @xmath0-ray blazar candidates we start from compact 8.4ghz sources ( from class , meyers et al . 2003 or from our own vla snapshots ) and identify flat / inverted spectrum sources using nvss counterparts . our selection algorithm also applies a weighting toward x - ray sources sources detected in the _ rosat _ all - sky survey ( rass ) ; this is weak as many of the _ egret _ blazars are below the rass survey threshold . this identification scheme differs from other blazar surveys ( eg . dxrbs , * ? ? ? * ) and gives a much higher correlation with the 3eg sources . we follow up with optical spectroscopy obtained with the marcario low resolution spectrograph ( lrs ; * ? ? ? * ) at the hobby - eberly telescope , obtaining source classifications and redshifts . q0906 + 6930 has a strongly inverted spectrum @xmath7 @xmath8 with a class - epoch flux of @xmath9mjy . the radio position is 09:06:30.75 + 69:30:30.8 ( j2000.0 ) . this source was cataloged as compact at the @xmath10 scale , but the short class snapshots provide only limited @xmath11mjy sensitivity to flux on arcsecond scales . this source is just included in our ` sub - threshold ' @xmath0-ray target list , with a likelihood analysis of the @xmath0-ray counts giving a 75% probability for emission in excess of background at this position . we obtained 2@xmath12300s exposure with the het lrs on 1/18/04 , under poor @xmath13 conditions , which nonetheless showed a classic ly@xmath14 forest - absorbed qso spectrum with high @xmath15 redshift . follow - on exposures of 2@xmath12600s on 1/27/04 with a 300l / mm grating , @xmath16 slit and 5150   long - pass filter ( to preclude 2nd order contamination ) and improved @xmath17 seeing produced the spectrum shown in figure 1 . these data were subject to optimal extraction , calibration and telluric correction using standard iraf routines . uncorrectable fringing and imperfect sky subtraction introduce large noise beyond 9500 . the resulting spectrum has a dispersion of 4 / pixel and an effective resolution of 16 , covering @xmath185200 - 10,000 .    in the spectrum ly@xmath14 is strong , but absorbed ; we estimate a pre - absorption rest equivalent width of 50 - 60 . the redshift , z=5.47@xmath19 , is best constrained by the nv and oiv lines ; the oiv rest ew is 22 and the kinematic width is @xmath20km / s . the uv continuum luminosity at 1350 is @xmath21erg / s ( we use @xmath22km / s / mpc , @xmath23 , @xmath24 ) . civ is not clearly detected , so if we use the oiv kinematic width in the @xcite uv m - l relationship we obtain @xmath25 \approx 10.2 $ ] . this is near the upper end of masses inferred for high z sources and formation of such a high @xmath26 black hole after @xmath11gyr is difficult to understand . there are several candidate civ absorption line systems redward of ly@xmath14 . two candidate systems with apparent damped ly lines are marked . the presence of intervening absorption raises the possibility that q0906 + 6930 is lensed , which could inflate our estimate of the bh mass . however , class finds no radio lens , there is no extended emission visible on the poss and the source appears point - like in a short pre - spectrum acquisition image . improved optical / near ir spectroscopy is needed to tighten up the mass estimate , and to improve the identification of the absorption systems as strong civ doublets .      to search for evidence of compact jet structure that would support the blazar designation of q0906 + 6930 , we obtained snapshots with the vlba at 2 cm and 7 mm wavelengths ( figure 2 ) . with 10 stations recording two 8-mhz bandpasses at 15.36ghz , each in two polarizations , we obtained an average of 60 minutes of on - source per baseline under nominal observing conditions on 2004 february 27 ( mjd 53062 ) . we used 0716 + 714 to calibrate phases and delays prior to self - calibration of the blazar , using aips . we constructed deconvolved images with natural weighting of the data ( half - power beam width 1.55@xmath120.47mas at -2@xmath27 ) . the rms background noise in the image was @xmath28 above the thermal noise limit of 0.18 mjy . the 7 mm observations recorded @xmath29  minutes on - source per baseline on 2004 march 3 ( mjd 53067 ) , tuned to 43.21 ghz . about 30% of the data could not be self - calibrated because of rapid ( @xmath30minute ) tropospheric fluctuations . in addition , we obtained no usable data from the los alamos antenna , due to weather , and from one polarization - baseband combination recorded at the saint croix antenna . the half - power beamwidth was @xmath31  mas at @xmath32 for natural weighting . the compact core was unresolved at both wavelengths . in the 2 cm image a clear jet component is seen at pa 225@xmath33 . for a two component gaussian model we fit a core flux of 115@xmath34mjy and a jet flux of 6.3@xmath35mjy . the jet is marginally detected in our 7 mm image with a maximum @xmath36 beams from the position of the 2 cm peak , although it is only comparable to the largest peaks in the image background . the two component fit gives a core flux 42@xmath37mjy and jet flux @xmath38mjy , for a 3.7@xmath4 detection . our combined _ egret _ likelihood analysis finds a nominal @xmath39 excess of @xmath0-ray photons at the radio position . in the viewing period with the most significant detection , vp0220 , the source produced 1.12@xmath40(e@xmath1100mev ) . however , if other sources beyond the standard 4@xmath4 3eg sources are present in this region the fit flux is even lower . at this point it is best to employ the mission - averaged upper limit @xmath41(e@xmath1100mev ) . we can collect the existing data into a crude spectral energy distribution ( sed , figure 3 ) . comparing to the well known blazar 3c279 placed at z=5.47 , q0906 + 6930 has a brighter uv flux , fainter cm wavelength emission and ( potentially ) a gev peak power nearly 10@xmath12 greater . the source has by far the highest radio loudness @xmath42 of any @xmath43 qso . we also guide the eye with a simple synchrotron - compton spectrum @xcite for a broken power - law approximating a cooled electron spectrum . the synchrotron component is broadly similar to that of 3c279 . the compact core spectrum ( inset ) appears to turn over at cm wavelengths and does not connect to the falling optical spectrum . this is not however unexpected , as most blazars show extra radio components at cm wavelengths . if the 7 mm jet detection is trusted , the @xmath44 of the jet component is somewhat flatter than that of the core @xmath45 . this is unusual , although not unique for pc - scale jets ( a. marscher , private comm . ) ; additional vlbi observations will be needed to see if this is temporary due to jet component brightening at 7 mm . with only a marginal gev detection and an x - ray upper limit , the amplitude of the compton component is poorly constrained . however , it is interesting to note that the broad line region flux is much larger than that of 3c279 with with a @xmath46 brighter optical continuum and rest equivalent line widths @xmath47 larger . since comptonized blr flux is generally believed to dominate the gev emission , a bright @xmath0-ray peak may be motivated , as shown by the dotted curves . we expect the compton scattered emission of blazar jets to be highly beamed . thus , few blazar x- and @xmath0-ray sources are visible , but these may appear quite bright . the glast mission will have a large duty cycle and a survey sensitivity @xmath48(e@xmath1100mev ) , so even if the source is a flaring 3c279 analog , the prospects for detection are good . if the gev spectrum of this source can be measured , then glast can observe the effect of pair attenuation of the @xmath1gev flux by the optical - ir ebl @xcite . at its high redshift @xmath4910gev observations of q0906 + 6930 will be probing @xmath50 photons from high @xmath51 star formation . this ebl is presently poorly understood , but an approximate attenuation curve based on the lower @xmath51 ebl is applied for the combined spectrum ( full line ) . the cut - off is well within the glast range . it may be measured if , for example , a stable absorption cutoff is seen on a gev spectrum whose slope and amplitude varies with time .    in the x - ray regime , core emission probes the low energy compton component , with the best connection to the radio . however , most interesting is the possibility of detecting a resolved @xmath17 ( @xmath52kpc ) scale x - ray jet produced by compton up - scatter of cmb photons . such extended ( @xmath13 ) jet structure has been seen in _ cxo _ observations of the qso gb 1508 + 5714 @xcite at z=4.3 . this feature is also seen as a faint radio jet @xcite and the flux ratio @xmath53 supports the cmb up - scatter hypothesis . extended emission x - ray observed @xmath5420@xmath55 from the @xmath56 qso j1306 + 0356 @xcite may represent a second high - z jet , although radio emission is not seen . with a @xmath57 increase in the energy density of the cmb seed photons , such jets may be detectable to very high redshifts@xcite . q0906 + 6930 at z=5.47 experiences @xmath58 the local cmb energy density . as we already have evidence for an energetic jet , prospects for detection of a compton up - scatter component are good and we are pursuing the required x - ray and radio observations to test this hypothesis . if seen , the link to the external seed field gives x - ray / radio measurements unique power to probe the particle and field populations in this high - z jet . the hobby - eberly telescope ( het ) is a joint project of the university of texas at austin , the pennsylvania state university , stanford university , ludwig - maximilians - universitt mnchen , and georg - august - universitt gttingen . the het is named in honor of its principal benefactors , william p. hobby and robert e. eberly . the marcario low resolution spectrograph is named for mike marcario of high lonesome optics who fabricated several optics for the instrument but died before its completion . the lrs is a joint project of the hobby - eberly telescope partnership and the instituto de astronomia de la universidad nacional autonoma de mexico . the national radio astronomy observatory is a facility of the national science foundation operated under cooperative agreement by associated universities , inc .
we report the discovery of a radio - loud flat - spectrum qso at z=5.47 with properties similar to those of the _ egret _ @xmath0-ray blazars . 
 this source is the brightest radio qso at z@xmath15 , with a pc - scale radio jet and a black hole mass estimate @xmath2 . 
 it appears to be the most distant blazar discovered to date . 
 high energy observations of this source can provide powerful probes of the background radiation in the early universe .