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the measurement of electromagnetic observables in relativistic nuclear collisions has constituted a vigorous experimental effort at different facilities around the world for several decades , and it is fair to write that this program has lived up to its promises .
indeed , photons ( real and virtual ) carry information complementary to that in hadronic observables , as they suffer essentially no final state interaction .
one may show @xcite that the photon emission rate in a finite - temperature medium is directly related to the in - medium retarded self - energy of the photon : a valuable quantity . however , the measured signal is comprised of several contributions which have to be under simultaneous theoretical control before significant claims can be substantiated .
as in nucleon - nucleon interactions , nucleus - nucleus collisions will produce prompt photons during the early collisions of the nuclear constituents .
the prompt ( direct + fragmentation ) photon spectrum can be calculated in @xmath2 collisions at next - to - leading order @xcite .
the direct contribution comprises both compton and annihilation components : @xmath3 the nlo effects are contained in the k - factors ; their list of arguments has been omitted for brevity . in principle , the prompt photon spectrum depends on the choice of renormalization , factorization ( @xmath4 ) , and fragmentation ( @xmath5 ) scales ; in the above the first has been set equal to the second . choosing them to be @xmath6 , where @xmath7 is the photon transverse momentum , yields a photon spectrum in proton - proton collisions in good agreement with observation @xcite .
this can be then be used as a baseline for calculations of photon production in nucleus - nucleus collisions . in this case , isospin effects are included in the parton distribution function , by using the parton species for the proton or for the neutron , as appropriate .
shadowing corrections are also taken into account .      in a thermal environment , whether composed of partons or of hadrons in the confined phase , the photons produced are calculated simply by performing an integral over the space - time history of the strongly interacting system : @xmath8 the differential photon production rate , @xmath9 , is evaluated in the local thermal rest frame , and at a given time will consist of partonic and/or hadronic contributions .
another process which depends on the temperature is that of jet - thermal photon production .
this happens when a jet crossing the hot medium undergoes a photon - producing interaction with a thermal parton .
this can further be divided in two classes , those which will benefit from collinear enhancements , and those which will not @xcite .
all of the rates introduced so far need to be integrated using with a realistic time - evolution model of the relativistic nuclear collision .
we will use a two - dimensional relativistic hydrodynamical approach - azhydro - which provides a good global description of hadronic observables @xcite and , importantly , is not tuned to specifically reproduce electromagnetic measurements .
the initial jet energy profiles calculated from qcd are evolved in time according to amy @xcite .
the evolution of the parton energy profile , @xmath10 , is determined by solving a set of coupled fokker - planck rate equations which have the following generic form : @xmath11\end{aligned}\ ] ] the transition rate for processes where partons of energy @xmath12 loose energy @xmath13 is @xmath14 . the energy gain channels are included through the possibility for @xmath13 to be negative .
also , amy smoothly handles the crossover between the bethe - heitler and landau - pomeranchuk - migdal regimes @xcite .
a free parameter of the model is @xmath15 , the strong coupling , which is adjusted to reproduce @xmath16 .
the calculated photon spectra , together with their different components , are shown in figure [ pho_spec ] .
the reader may judge of the good agreement between theory and data .
no parameters have been tuned to generate these figures .
two other useful representations of the experimental data are displayed in figure [ r_aav2 ] .
the first ( left panel ) is the ubiquitous @xmath0 variable , while the second is a plot of the photon azimuthal asymmetry , @xmath1 .    in both panels ,
the data does not yet permit an accurate determination of model parameters and ingredients .
however , the fact that @xmath17 1 at high values of @xmath18 is a consequence of the fragmentation photons being affected by the energy loss of the fragmenting jets ( as well as by an isospin effect @xcite ) : should this trend be confirmed experimentally , a link would exist between the nuclear modification factor of photons and that of strongly interacting particles also borne out of fragmentation .    turning to @xmath1
, one observes that the azimuthal anisotropy associated with the jet - plasma photons is indeed negative , as originally suggested @xcite .
however , the net value of @xmath1 appears to be essentially zero for @xmath19 4 gev .
the reason for this is two - fold .
first , the geometric anisotropy of the medium in the realistic 2d+1 model is rather small initially , and then proceeds to shrink as time evolves .
second , the geometry of the initial jet profile is evaluated here with a realistic woods - saxon distribution , rather than using a hard sphere profile .
however , these results do show that the photon @xmath20 is rather sensitive to early time dynamics in relativistic nuclear collisions : precise measurements thus have the potential to stringently constrain the time - evolution scenarios .
a global picture is emerging where the physics of qcd jets and the generation of electromagnetic radiation in nuclear collisions need to be treated simultaneously and consistently .
the results displayed here are being generalized to include energy loss by elastic collision processes @xcite , and the effects of a 3d+1 hydrodynamical evolution .
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we model the hot and dense strongly interacting mater produced in high energy heavy ion collisions using relativistic hydrodynamics . 
 several different sources of real photons produced during these collisions are considered and their relative importance is assessed . 
 we include contributions from qcd jets , which are allowed to loose and gain energy as they proceed through the hot matter . 
 this is treated within the amy formalism . 
 we obtain photon spectra , @xmath0 , and @xmath1 in agreement with measurements performed by the phenix collaboration .