You are an expert at summarizing long articles. Proceed to summarize the following text: 
blazars are radio - loud active galactic nuclei characterized by polarized , highly luminous , and rapidly variable non - thermal continuum emission ( angel & stockmann 1980 ) from a relativistic jet oriented close to the line of sight ( blandford & rees 1978 ) . as such , blazars provide fortuitous natural laboratories to study the jet processes and ultimately how energy is extracted from the central black hole .    the radio through gamma - ray spectral energy distributions ( seds ) of blazars exhibit two broad humps ( figure 1 ) . the first component peaks at ir / optical in `` red '' blazars and at uv / x - rays in their `` blue '' counterparts , and is most likely due to synchrotron emission from relativistic electrons in the jet ( see ulrich , maraschi , & urry 1997 and references therein ) . the second component extends from x - rays to gamma - rays ( gev and tev energies ) , and its origin is less well understood . a popular scenario is inverse compton ( ic ) scattering of ambient photons , either internal ( synchrotron - self compton , ssc ; tavecchio , maraschi , & ghisellini 1998 ) or external to the jet ( external compton , ec ; see bttcher 1999 and references therein ) . in the following discussion i will assume the synchrotron and ic scenarios , keeping in mind , however , that a possible alternative for the production of gamma - rays is provided by the hadronic models ( proton - induced cascades ; see rachen 1999 and references therein ) . red and blue blazars are just the extrema of a continuous distribution of seds . this is becoming increasingly apparent from recent multicolor surveys ( laurent - muehleisen et al . 1998 ; perlman et al . 1998 ) , which find sources with intermediate spectral shapes , and trends with bolometric luminosity were discovered ( sambruna , maraschi , & urry 1996 ; fossati et al . 1998 ) . in the more luminous red blazars the synchrotron and ic peak frequencies are lower , the compton dominance ( ratio of the synchrotron to ic peak luminosities ) is larger , and the luminosity of the optical emission lines / non - thermal blue bumps is larger than in their blue counterparts ( sambruna 1997 ) . a possible interpretation is that the different types of blazars are due to the different predominant electrons cooling mechanisms ( ghisellini et al . 1998 ) . in a simple homogeneous scenario , the synchrotron peak frequency @xmath0 , where @xmath1 is the electron energy determined by the competition between acceleration and cooling . because of the lower energy densities , in lineless blue blazars the balance between heating and cooling is achieved at larger @xmath1 , contrary to red blazars , where , because of the additional external energy density , the balance is reached at lower @xmath1 . blue blazars are ssc - dominated , while red blazars are ec - dominated . while there are a few caveats to this picture ( urry 1999 ) , the spectral diversity of blazars jets can not be explained by beaming effects _ only _ ( sambruna et al . 1996 ; georganopoulos & marscher 1998 ) , but require instead a change of physical parameters and/or a different jet environment . correlated multiwavelength variability provides a way to test the cooling paradigm since the various synchrotron and ic models make different predictions for the relative flare amplitudes and shape , and the time lags . first , since the same population of electrons is responsible for emitting both spectral components ( in a homogeneous scenario ) , correlated variability of the fluxes at the low- and high - energy peaks with no lags is expected ( ghisellini & maraschi 1996 ) . second , if the flare is caused by a change of the electron density and/or seed photons , for a fixed beaming factor @xmath2 the relative amplitudes of the flares at the synchrotron and ic peaks obey simple and yet precise relationships ( ghisellini & maraschi 1996 ; see however bttcher 1999 ) . third , the rise and decay times of the gamma - ray flux are a sensitive function of the external gas opacity and geometry in the ec models ( bttcher & dermer 1998 ) . fourth , the rise and decay times of the synchrotron flux depend on a few source typical timescales ( chiaberge & ghisellini 1999 ) . fifth , spectral variability accompanying the synchrotron flare ( in x - rays for blue blazars , in optical for red blazars ) is a strong diagnostic of the electron acceleration versus cooling processes ( kirk , riegler , & mastichiadis 1998 ) . when cooling dominates , time lags between the shorter and longer synchrotron wavelengths provide an estimate of the magnetic field @xmath3 ( in gauss ) of the source via @xmath4 ( takahashi et al . 1996 ; urry et al . 1997 ) . * the role of rxte . * with its wide energy band coverage ( 2250 kev ) , rxte plays a crucial role in monitoring campaigns of blazars , since it probes the region where the synchrotron and compton component overlap in the seds ( figure 1 ) , allowing us to quantify their relative importance in the different sources . its high time resolution and good sensitivity are ideal to detect the smallest x - ray variability timescales , study the lags between the harder and softer x - rays , and to follow the particle spectral evolution down to timescales of a few hours or less , pinning down the microphysics of blazars jets . one of the best monitored red blazars is 3c279 . from the simultaneous or contemporaneous seds in figure 1a , it is apparent that the largest variations are observed above the synchrotron peak in ir / optical ( not well defined ) and the compton peak at gev energies , supporting the synchrotron and ic models . the gev amplitude is roughly the square of the optical flux during the earlier campaigns , supporting an ssc interpretation ( maraschi et al . 1994 ) or a change of @xmath2 in the ec models , while in 1996 large variations were recorded at gamma - rays but not at lower energies ( wehrle et al . 1998 ) . during the latter campaign , the rapid decay of the gev flare ( figure 2a ) favors an ec model ( bttcher & dermer 1998 ; wehrle et al . note in figure 2a the good correlation , within one day , of the egret and rxte flares , which provides the first evidence that the gamma - rays and x - rays are cospatial ( wehrle et al . 1998 ) . = 7.5 cm = 7.5 cm    another candidate for future gamma - ray monitorings is bl lac itself . in 1997 july it underwent a strong flare at gev and optical energies ( bloom et al . the gamma - ray light curve shows a strong flare possibly anticipating the optical by up to 0.5 days ; however , the poor sampling does not allow firmer conclusions . contemporaneous rxte observations showed a harder x - ray continuum ( madejski et al . 1999 ) than in previous asca measurements . the sed during the outburst is best modeled by the ssc model from radio to x - rays , while an ec contribution is required above a few mev ( sambruna et al . 1999a ) . a similar mix of ssc and ec is also required to fit the seds of pks 0528 + 134 ( sambruna et al . 1997 ; mukherjee et al . 1998 ; ghisellini et al . 1999 ) . mrk 501 , one of the two brightest tev blazars , attracted much attention in 1997 april when it underwent a spectacular flare at tev energies ( catanese et al . 1997 ; aharonian et al . 1999 ; djannati - atai et al . this was correlated to a similarly - structured x - ray flare observed with rxte ( figure 2b ) , with no delay larger than one day ( krawczynski et al . these results are consistent with an ssc scenario where the most energetic electrons are responsible for both the hard x - rays ( via synchrotron ) and the tev ( via ic ) . figure 1b shows the seds of mrk 501 during the 1997 april tev activity , compared to the `` quiescent '' sed from the literature . an unusually flat ( photon index , @xmath5 ) x - ray continuum was measured by sax and rxte during the tev flare ( pian et al . 1998 ; krawczynski et al . 1999 ) , implying a shift of the synchrotron peak by more than two orders of magnitude . this almost certainly reflects a large increase of the electron energy ( pian et al . 1998 ) , or the injection of a new electron population on top a quiescent one ( kataoka et al . later rxte observations in 1997 july found the source still in a high and hard x - ray state ( lamer & wagner 1998 ) , indicating a persistent energizing mechanism .    an interesting new behavior was observed during our latest 2-week rxte - hegra monitoring of mrk 501 in 1998 june ( sambruna et al . 1999b ) , when 100% overlap between the x - rays and tev light curves was achieved ( figure 3a ) . a strong short - lived ( @xmath6 two days ) tev flare was detected , correlated to a flare in the very hard ( 2050 kev ) x - rays , with the softer x - rays being delayed by up to one day . as in 1997 , large x - ray spectral variations are observed , with the x - ray continuum flattening to @xmath7 at the peak of the tev flare , implying a similar shift to @xmath8 50 kev of the synchrotron peak ( figure 3b ) . however , while in 1997 the tev spectrum hardened during the flare ( djannati - atai et al . 1997 ) , as it did in the x - rays , we did not observe significant variability in the tev hardness ratios during the flare ( figure 3a , panel ( e ) ) ; instead the spectrum softens 1 - 2 days later . the correspondence between the x - ray and tev spectra is no longer present during the 1998 june flare . x - ray monitorings of blue blazars are a powerful diagnostic of physical processes occurring in these sources . this is because in these objects the x - rays are the high - energy tail of the synchrotron component where rapid and complex flux and spectral variability is expected depending on the balance between escape , acceleration , and cooling of the emitting particles ( kirk et al . 1998 ) . an ideal target for x - ray monitorings is pks 2155304 , one of the brightest x - ray blazars ( treves et al . 1989 ; sembay et al . 1993 ; pesce et al . interest in this source was recently revived due to a tev detection ( chadwick et al . 1999 ) during a high x - ray state ( chiappetti et al . asca and sax observations detected strong x - ray variability , with the softer energies lagging the harder energies by one hour or less ( chiappetti et al . 1999 ; kataoka et al . 1999b ; zhang et al . 1999 ) , and are consistent with a model where the electron cooling dominates the flares . this implies magnetic fields of @xmath9 gauss ( for @xmath10 ) , similar to mrk 421 ( takahashi et al . 1996 ) . a new mode of variability was discovered during our rxte monitoring of pks 2155304 in 1996 may , as part of a larger multifrequency campaign ( sambruna et al . the sampling in the x - rays was excellent ( figure 4a ) , and complex flux variations were observed , with short , symmetric flares superposed to a longer baseline trend . inspection of the hardness ratios ( the ratio of the counts in 620 kev over the counts in 26 kev ) versus flux shows that different flares ( separated by vertical dashed lines in figure 4a ) exhibit hysteresis loops of opposite signs , both in a `` clockwise '' and `` anti - clockwise '' sense ( labeled as c and a in figure 4a , respectively ) . applying a correlation analysis to each flare separately , we find that the c loops corresponds to a soft lag ( softer energies lagging ) and the a loop corresponds to a hard lag ( harder energies lagging ) , of the order of a few hours in both cases . two examples are shown in figure 5b for the may 18.520.2 and 24.226.9 flares , respectively . we interpreted the data using the acceleration model of kirk et al . ( 1998 ) . here loops / lags of both signs are expected depending on how fast the electrons are accelerated compared to their cooling time , @xmath11 . if the acceleration is instantaneous ( i.e. , @xmath12 ) , cooling dominates variability and , because of its energy dependence , the harder energies are emitted first , with c loops and soft lags . if instead the acceleration is slower ( @xmath13 ) , the electrons need to work their way up in energy and the softer energies are emitted first , with a loops and hard lags predicted . a close agreement between the rxte light curve and the model is found by increasing @xmath14 by a factor 100 going from a c to an a loop , when @xmath14 becomes similar to the duration of the flare , and by steepening the electron energy distribution ( see sambruna et al . 1999d for details ) . thus we reach the important conclusion that we are indeed observing electron acceleration , and together with cooling this is responsible for the observed x - ray variability properties of pks 2155304 . the complex spectral behavior of pks 2155304 in 1996 may is in contrast to the remarkable simplicity observed in earlier x - ray observations of this source ( e.g. , kataoka et al . 1999b ) and of other objects . figure 5 summarizes two epochs of rxte monitoring of another bright blue blazar , pks 2005489 , which has an sed very similar to pks 2155304 and is a tev candidate ( sambruna et al . 1995 ) . during 1998 september , our rxte monitoring detected a general trend of flux increase of 30% amplitude in 3.5 days ( figure 5a ) . despite gaps in the sampling , it is clear that the variability at harder energies is faster than at softer energies , consistent with cooling dominating the flares . this is confirmed by the analysis of the hardness ratios versus flux , where only clockwise loops are observed . a consistent behavior was also observed one month later ( figure 5b ) during a much larger , longer - lasting flare , when spectral variations occurred on timescales of a few hours ( perlman et al . recent multiwavelength campaigns of blazars expanded the current available database , from which we are learning important new lessons . detailed modeling of the seds of bright gamma - ray blazars of the red and blue types tend to support the current cooling paradigm , where the different blazars flavors are related to the predominant cooling mechanisms of the electrons at the higher energies ( ec in more luminous sources , ssc in lower - luminosity ones ) . however , several observational biases could be present which can be addressed by future larger statistical samples , especially in gamma - rays . in particular , it will be important to expand the sample of tev blazars , which currently includes only a handful ( 5 ) of sources , with only two bright enough to allow detailed spectral and timing analysis . a promising diagnostic for the origin of the seed photons for the ic process is the shape of the gamma - ray flare . this awaits well - sampled gamma - ray light curves , which will be afforded by the next higher - sensitivities missions ( glast , agile in gev and hess , veritas , magic , cangaroo ii in tev ) . broader - band , higher quality gamma - ray spectra will also be available , allowing a better location of the ic peak , a more precise measure of the spectral shape at gamma - rays , and its variability . more correlated x - ray / tev monitorings are necessary , in which rxte and sax have crucial roles , to add to the current knowledge of the variability modes .    finally , the current data show that acceleration and cooling are the dominant physical mechanisms responsible for the observed variability properties from blazars jets . we are starting to study these processes well in the x - rays for blue blazars , where rxte has the potential to determine the shortest x - ray variability timescales and lags , probing into even more detail the jets microphysics . it would also be interesting to perform similar studies in the optical for red blazars , to compare the nature of the acceleration and cooling in the two subclasses . this work was supported by nasa contract nas38252 and nasa grant nag57276 . i thank laura maraschi for a critical reading of the manuscript , felix aharonian and the hegra team for allowing me to report the 1998 tev data of mrk 501 , and lester chou for help with the rxte data reduction .
results from recent multiwavelength observations of blazars are reviewed , with particular emphasis on those involving the _ rossi x - ray timing explorer _ 
 ( rxte ) . 
 i discuss blazars spectral energy distributions , their correlated variability at various energies , and the insights they offer on the physical processes in the jet . 
 new results on mrk 501 , pks 2155304 , and pks 2005489 are highlighted . 
 # 1 # 2 _ mem . 
 soc . 
 astron . 
 it . _ * # 1 * , # 2 # 1 # 2 _ the messenger _ * # 1 * , # 2 # 1 # 2 _ astron . 
 nach . _ * # 1 * , # 2 # 1 # 2 _ astron . 
 astrophys . 
 _ * # 1 * , # 2 # 1 # 2 _ astron . 
 astrophys . 
 lett . _ * # 1 * , l#2 # 1 # 2 _ astron 
 . astrophys . 
 rev . _ * # 1 * , # 2 # 1 # 2 _ astron . 
 astrophys . 
 suppl . 
 ser . _ * # 1 * , # 2 # 1 # 2 _ astron . 
 j. _ * # 1 * , # 2 # 1 # 2 _ ann . 
 rev . 
 astron . 
 astrophys . _ * # 1 * , # 2 # 1 # 2 _ astrophys . 
 j. _ * # 1 * , # 2 # 1 # 2 _ astrophys .. j. lett . _ * # 1 * , l#2 # 1 # 2 _ astrophys . 
 j. suppl . 
 _ * # 1 * , # 2 # 1 # 2 _ astrophys . 
 space sci . 
 _ * # 1 * , # 2 # 1 # 2 _ adv . 
 space res . 
 _ * # 1 * , # 2 # 1 # 2 _ bull . 
 astron . 
 inst . 
 czechosl . _ * # 1 * , # 2 # 1 # 2 _ j. quant . 
 spectrosc . 
 . transfer _ * # 1 * , # 2 # 1 # 2 _ mon . not . 
 r. astr . 
 soc . 
 _ * # 1 * , # 2 # 1 # 2 _ mem . 
 r. astr . 
 soc . _ * # 1 * , # 2 # 1 # 2 _ phys . 
 lett . 
 rev . _ * # 1 * , # 2 # 1 # 2 _ publ . astron 
 . soc 
 . japan _ * # 1 * , # 2 # 1 # 2 _ publ . 
 astr . 
 soc . 
 pacific _ * # 1 * , # 2 # 1 # 2 _ nature _ * # 1 * , # 2 epsf.sty