We report on the γ -ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 (Mrk 421) during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) γ -ray spectrum above 0.3 GeV can be well described by a power-law function with photon index Γ = 1.78 ± 0.02 and average photon flux F (>0.3 GeV) = (7.23 ± 0.16) × 10 −8 ph cm −2 s −1 . Over this time period, the Fermi-LAT spectrum above 0.3 GeV was evaluated on seven-day-long time intervals, showing significant variations in the photon flux (up to a factor ∼3 from the minimum to the maximum flux) but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in γ -rays measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5 month long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments that provided excellent temporal and energy coverage of the source throughout the entire campaign During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broadband SED was reproduced with a leptonic (one-zone synchrotron self-Compton) and a hadronic model (synchrotron proton blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet powers but very different characteristics for the blazar emission site.

Black holes generate collimated, relativistic jets which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies (active galactic nuclei; AGN). How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGNs is still unknown. Here we show that jets produced by AGNs and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the γ-ray luminosity, with AGNs and GRBs lying at the low and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.Relativistic jets are ubiquitous in the cosmos and have been observed in a diverse range of black hole systems spanning from stellar mass (∼ 10M ) to supermassive scales (∼ 10 5 − 10 10 M ), in particular in the bright flashes of gamma-rays known as GRBs (1, 2), the miniature versions of quasars lurking in our galaxy known as "microquasars" (3) and AGNs (4,5). Despite decades of observations at almost all wavelengths and considerable theoretical efforts, there are still many aspects of black hole jets which remain mysterious: the mechanism(s) responsible for their formation and the nature of their energetics as well as their high-energy radiation (6,7). Jets and outflows from supermassive black holes have important feedback effects on scales ranging from their host galaxies to groups and clusters of galaxies (8). Hence, a better understanding of the physics of jets is required in order to have a more complete picture of the formation and evolution of large-scale structures in the universe and the coevolution of black holes and galaxies (9).One outstanding question is how the jet physics scale with mass from stellar to supermassive black holes. Interestingly, there is evidence that jets behave in similar ways in microquasars and radio-loud AGN (10-12). However, a clear connection between AGN and GRBs has not been established yet, although recent work provides encouraging results (13, 14).As a first step in understanding how the properties of jets vary across the mass scale, we focus on the energetics of jets produced in AGNs and GRBs. Therefore, we searched the literature for published and archival observations that allow us to estimate the jet radiative output and the kinetic power for a sample of black hole systems in which the jet is closely aligned with our line of sight and characterized by a broad range of masses. For this reason, our sample consists of blazars -AGNs with their jets oriented toward Earth (15) -and GRBs, the spectral energy distributions of which are completely dominated by the jet due to beaming effects.We used as a proxy of the jet bolometric luminosity the observed γ-ray luminosity L iso which is isotropically equivalent. In order to estimate the kinetic power P jet , we use extended radio luminosities for the blazars whereas for the GRBs we reli...

We present γ -ray observations with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope of the nearby radio galaxy Centaurus A (Cen A). The previous EGRET detection is confirmed, and the localization is improved using data from the first 10 months of Fermi science operation. In previous work, we presented the detection of the lobes by the LAT; in this work, we concentrate on the γ -ray core of Cen A. Flux levels as seen by the LAT are not significantly different from that found by EGRET, nor is the extremely soft LAT spectrum (Γ = 2.67 ± 0.10 stat ± 0.08 sys where the photon flux is Φ ∝ E −Γ ). The LAT core spectrum, extrapolated to higher energies, is marginally consistent with the non-simultaneous HESS spectrum of the source. The LAT observations are complemented by simultaneous observations from Suzaku, the Swift Burst Alert Telescope and X-ray Telescope, and radio observations with the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry program, along with a variety of non-simultaneous archival data from a variety of instruments and wavelengths to produce a spectral energy distribution (SED). We fit this broadband data set with a single-zone synchrotron/synchrotron self-Compton model, which describes the radio through GeV emission well, but fails to account for the non-simultaneous higher energy TeV emission observed by HESS from 2004 to 2008. The fit requires a low Doppler factor, in contrast to BL Lac objects which generally require larger values to fit their broadband SEDs. This indicates that the γ -ray emission originates from a slower region than that from BL Lac objects, consistent with previous modeling results from Cen A. This slower region could be a slower moving layer around a fast spine, or a slower region farther out from the black hole in a decelerating flow. The fit parameters are also consistent with Cen A being able to accelerate ultra-high energy cosmic-rays, as hinted at by results from the Auger observatory.

We revisit the concept of a blazar sequence that relates the synchrotron peak frequency (ν peak ) in blazars with synchrotron peak luminosity (L peak , in νL ν ) using a large sample of radio-loud AGN. We present observational evidence that the blazar sequence is formed from two populations in the synchrotron ν peak − L peak plane, each forming an upper edge to an envelope of progressively misaligned blazars, and connecting to an adjacent group of radio galaxies having jets viewed at much larger angles to the line of sight. When binned by jet kinetic power (L kin ; as measured through a scaling relationship with extended radio power), we find that radio core dominance decreases with decreasing synchrotron L peak , revealing that sources in the envelope are generally more misaligned. We find population-based evidence of velocity gradients in jets at low kinetic powers (∼ 10 42 − 10 44.5 erg s −1 ), corresponding to FR I radio galaxies and most BL Lacs. These low jet power 'weak jet' sources, thought to exhibit radiatively inefficient accretion, are distinguished from the population of nondecelerating, low synchrotron-peaking (LSP) blazars and FR II radio galaxies ('strong' jets) which are thought to exhibit radiatively efficient accretion. The two-population interpretation explains the apparent contradiction of the existence of highly core-dominated, low-power blazars at both low and high synchrotron peak frequencies, and further implies that most intermediate synchrotron peak (ISP) sources are not intermediate in intrinsic jet power between LSP and high synchrotron-peaking (HSP) sources, but are more misaligned versions of HSP sources with similar jet powers.

TeV emission from BL Lacertae (BL) objects is commonly modeled as Synchrotron-Self Compton (SSC) radiation from relativistically moving homogeneous plasma blobs. In the context of these models, the blob Lorentz factors needed to reproduce the corrected for absorption by the diffuse IR background (DIRB) TeV emission are large (δ 50). The main reason for this is that stronger beaming eases the problem of the lack of ∼ IR-UV synchrotron seed photons needed to produce the de-absorbed ∼ few TeV peak of the spectral energy distribution (SED). However, such high Doppler factors are in strong disagreement with the unified scheme, according to which BLs are FR I radio galaxies with their jets closely aligned to the line of sight. Here, motivated by the detection of sub-luminal velocities in the sub-pc scale jets of the best studied TeV blazars, MKN 421 and MKN 501, we examine the possibility that the relativistic flows in the TeV BLs decelerate. In this case, the problem of the missing seed photons is solved because of Upstream Compton (UC) scattering, a process in which the upstream energetic electrons from the fast base of the flow 'see' the synchrotron seed photons produced in the slow part of the flow relativistically beamed. Modest Lorentz factors (Γ ∼ 15), decelerating down to values compatible with the recent radio interferometric observations, reproduce the ∼ few TeV peak energy of these sources. Furthermore, such decelerating flows are shown to be in agreement with the BL -FR I unification, naturally reproducing the observed BL/FR I broad band luminosity ratios.

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We report the Fermi-LAT discovery of high-energy (MeV/GeV) γ-ray emission positionally consistent with the center of the radio galaxy M87, at a source significance of over 10σ in ten-months of all-sky survey data. Following the detections of Cen A and Per A, this makes M87 the third radio galaxy seen with the LAT. The faint point-like γ-ray source has a >100 MeV flux of 2.45 (±0.63) × 10 −8 ph cm −2 s −1 (photon index = 2.26 ± 0.13) with no significant variability detected within the LAT observation. This flux is comparable with the previous EGRET upper limit (< 2.18 × 10 −8 ph cm −2 s −1 , 2σ), thus there is no evidence for a significant MeV/GeV flare on decade timescales. Contemporaneous Chandra and VLBA data indicate low activity in the unresolved X-ray and radio core relative to previous observations, suggesting M87 is in a quiescent overall level over the first year of Fermi-LAT observations. The LAT γ-ray spectrum is modeled as synchrotron self-Compton (SSC) emission from the electron population producing the radio-to-X-ray emission in the core. The resultant SSC spectrum extrapolates smoothly from the LAT band to the historical-minimum TeV emission. Alternative models for the core and possible contributions from the kiloparsec-scale jet in M87 are considered, and can not be excluded.

Abstract. Galactic binary systems that contain a black hole candidate show evidence of radio jets in their hard X-ray states. Unavoidably, photons from the companion star and/or the accretion disk are Compton-scattered by relativistic electrons in the jet, producing beamed X-rays and possibly gamma-rays. The importance of this process depends on the jet power and the Doppler boosting factor. For plausible values of these parameters, we show that the jet emission can contribute significantly to the hard state X-ray luminosity. Two sources -XTE J1118+480 and Cygnus X-1 -are modelled as representatives of black holes with low and high luminosity companion stars respectively. In XTE J1118+480, weak reflection features indicate that the jet emission is comparable to coronal emission. In Cygnus X-1, strong reflection features indicate coronal emission in the X-ray band, but the jet emission may emerge in the gamma-ray band. The absence of reflection features in the spectra of the ultraluminous compact X-ray sources in nearby galaxies suggests that they are dominated by jet emission. We show that a viable model for these sources is a stellar mass black hole with a high luminosity companion and a favourably oriented jet.