The the radiation from mildly relativistic outflows

The recent observations in the High Energy (HE; = 100 MeV)?-ray band show that the extragalactic ?-ray sky is dominatedby the emission of Active Galactic Nuclei (AGNs) of differenttypes. Dominant in these are blazars – an extreme class of AGNswhich have jets that are forming a small angle with respect to theline of sight (Urry & Padovani 1995). Blazars are very bright andluminous sources known to emit electromagnetic radiation in almostall frequencies that are currently being observed, rangingfrom radio to Very High Energy (VHE; > 100 GeV) ?-ray bands.Their broadband spectrum is mainly dominated by non-thermalemission produced in a relativistic jet pointing toward the observer.Other important class of ?-ray emitting AGNs observedby Fermi Large Area Telescopes (Fermi LAT) are radio galaxieswith relativistic jets at systematically larger angles (Abdo et al.2010a; Kataoka et al. 2011). Due to larger jet inclination angleas compared with blazars, the jet emission is not significantlyDoppler boosted, making it less prevalent over such componentsas the radiation from mildly relativistic outflows or emissionfrom extended structures. This opened a new window to havean insight into the particle acceleration and emission processesin different components of AGNs.The radio galaxy 3C 120, at a distance of ˜ 144.9 Mpc, is anactive and powerful emitter in all the observed wavebands. Inthe radio band, its characteristics are closer to the Fanaroff-Rileyclass I radio sources (Fanaroff & Riley 1974) with a powerfulone-sided radio jet from sub-pc to 100 kpc scales (Walker et al.1987). The one-sided parsec-scale jet has been studied by longbaseline interferometry and superluminal motion has been observedwith apparent speed up to 4-6 c (Homan et al. 2001;Gómez et al. 1998, 1999). Recently, using X-ray and radio observations,Marscher et al. (2002) found that the dips in the Xrayemission are followed by ejections of bright superluminalknots in the radio jet which clearly establishes an accretiondisk-jetconnection. The kpc-scale jet of 3C 120 has a complexstructure with several knots, k4, k7, s2, s3, and k25 (see Fig.2 of Harris et al. (2004), where on the 1.6 GHz radio contoursthe section of the jet with knot labels is shown), detected in theradio, optical, and X-ray bands (Harris et al. 1999; Harris et al.2004). The knots are labeled by their distance from the corein arcseconds (e.g., k4, k7) and the smooth sections of the jetdetected in the optical band (Hjorth et al. 1995) are labeled ass2 and s3. These knots appeared to have interesting morphologyand spectra, more tricky among which is the X-ray emissionfrom the knot k25: it has a very weak radio flux but it isbright in the X-ray band. It is a real challenge for one-zone synchrotronemission scenario to interpret the emission from k25 incase of which large deviation from the minimal energy conditionis required. It has been suggested that X-rays might be producedthrough the synchrotron radiation of an electron population disArticlenumber, page 1 of 10A&A proofs: manuscript no. 3c120tinct from that responsible for the radio emission (Harris et al.2004). Alternative theories such as proton synchrotron emission(Aharonian 2002) or inverse-Compton scattering of CMB photons(Zhang et al. 2010) have also been proposed. However, it isto date not clear which is the exact mechanism responsible forthe X-ray emission.The core of 3C 120’s jet itself has interesting and peculiar features.It is very bright in the X-ray band with a flux of ˜ 5 ×10-11 erg cm-2s-1at 2-10 keV, variable on time scales from daysto months (Halpern 1985). The ?-rays from 3C 120 had beenalready detected by Fermi LAT during first 15 months’ scan ofthe whole sky (Abdo et al. 2010a) which was then confirmed bythe data accumulated for two years (Kataoka et al. 2011). Alsoa long-term (several months) variability had been found usingthe five-year Fermi LAT data (Sahakyan et al. 2015) with shortperiods (days and hours) of brightening (Tanaka et al. 2015;Janiak et al. 2016). Inverse Compton scattering of synchrotronphotons seems to be the mechanism responsible for the ?-rayemission from 3C 120 (Tanaka et al. 2015; Sahakyan et al. 2015)while the flares and the fast ?-ray variability are explained withinmore complex structured jet scenarios (Janiak et al. 2016).Combining of the data derived at the sub-pc and kilo-parsec regionsof the same jet could greatly help to understand the featuresof powerful extragalactic jets, e.g., shed some light on theevolution and propagation of the jets from the central engine tothe outer regions, where the jet is starting to significantly decelerate.This approach can be fruitfully applied to the sourcesshowing a large-scale jet long enough to be resolved by Chandra.Unfortunately, the best-studied blazars do not tend to havewell-studied large-scale jets, precisely because the blazars aremost closely aligned with the line of sight, reducing the projectedangular dimension of the large-scale jet. Thus, only a fewjets can be studied on both scales. The prominent features of 3C120, e.g., the strong jet well resolved in both small (pc) and large(kpc) scales makes this object an ideal target for investigation ofthe processes occurring in the powerful jet along its propagation.The low statistics in the ?-ray band did not allow to study the fluxchanges on sub-month scales (the light curves contain many upperlimits). The recent update of the Fermi LAT event-level analysisfrom PASS7 to PASS8 has significantly improved the eventreconstruction and classification which increased the sensitivityand improved the angular resolution. Combining this withthe data accumulated for a longer period (8 years) would significantlyimprove the statistics allowing to perform a detailedstudy of the ?-ray flux evolution in time. Also, the analysis ofthe Swift data will allow to explore the emission from the coreregion with the help of contemporaneous Spectral Energy Distributions(SEDs). Moreover, due to several Chandra observationsof the large-scale jet of 3C 120 in 2001-2016, the overall exposureis large enough to perform a spectral analysis of the X-raydata. This motivated us to have a new look on the emission fromthe jet of 3C 120 in small and large scales using the most recentdata available.The paper is stru