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The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4 million (Sagittarius A*(Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below those of active galactic nuclei1. Reflection of X-rays from Sgr A*by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years2. The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A*are consistent with the reflection scenario3–5. If this interpretation is correct, the reflected continuum emission should be polarized6. Here we report observations of polarized X-ray emission in the direction of the molecular clouds in the Galactic Centre using the Imaging X-ray Polarimetry Explorer. We measure a polarization degree of 31% ± 11%, and a polarization angle of −48° ± 11°. The polarization angle is consistent with Sgr A*being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr A*was briefly comparable to that of a Seyfert galaxy.

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation1,2. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4−1.1+0.5Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier1–3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree ΠXof around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.

Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales1–3. This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare4in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet5, plasma instabilities6,7or orbital motion in an accretion disc7,8. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref. 9). BL Lac, the prototype of a subclass of blazars10, is powered by a 1.7 × 108MSun(ref. 11) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref. 12)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities6near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.

Pulsar wind nebulae are formed when outflows of relativistic electrons and positrons hit the surrounding supernova remnant or interstellar medium at a shock front. The Vela pulsar wind nebula is powered by a young pulsar (B0833-45, aged 11,000 years)1and located inside an extended structure called Vela X, which is itself inside the supernova remnant2. Previous X-ray observations revealed two prominent arcs that are bisected by a jet and counter jet3,4. Radio maps have shown high linear polarization of 60% in the outer regions of the nebula5. Here we report an X-ray observation of the inner part of the nebula, where polarization can exceed 60% at the leading edge—approaching the theoretical limit of what can be produced by synchrotron emission. We infer that, in contrast with the case of the supernova remnant, the electrons in the pulsar wind nebula are accelerated with little or no turbulence in a highly uniform magnetic field.

Blazars are active galactic nuclei, which are powerful sources of radiation whose central engine is located in the core of the host galaxy. Blazar emission is dominated by non-thermal radiation from a jet that moves relativistically towards us, and therefore undergoes Doppler beaming. This beaming causes flux enhancement and contraction of the variability timescales, so that most blazars appear as luminous sources characterized by noticeable and fast changes in brightness at all frequencies. The mechanism that produces this unpredictable variability is under debate, but proposed mechanisms include injection, acceleration and cooling of particles, with possible intervention of shock waves or turbulence. Changes in the viewing angle of the observed emitting knots or jet regions have also been suggested as an explanation of flaring events and can also explain specific properties of blazar emission, such as intra-day variability, quasi-periodicity and the delay of radio flux variations relative to optical changes. Such a geometric interpretation, however, is not universally accepted because alternative explanations based on changes in physical conditions—such as the size and speed of the emitting zone, the magnetic field, the number of emitting particles and their energy distribution—can explain snapshots of the spectral behaviour of blazars in many cases. Here we report the results of optical-to-radio-wavelength monitoring of the blazar CTA 102 and show that the observed long-term trends of the flux and spectral variability are best explained by an inhomogeneous, curved jet that undergoes changes in orientation over time. We propose that magnetohydrodynamic instabilities or rotation of the twisted jet cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the extreme optical outburst of 2016–2017 (brightness increase of six magnitudes) occurred when the corresponding emitting region had a small viewing angle. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory.

Context. BL Lacertaeis the prototype of the blazar subclass named after it. Yet, it has occasionally shown a peculiar behaviour that has questioned a simple interpretation of its broad-band emission in terms of synchrotron plus synchrotron self-Compton (SSC) radiation.

Context. Since 2001, the radio quasar 3C 454.3has undergone a period of high optical activity, culminating in the brightest optical state ever observed, during the 2004-2005 outburst. The Whole Earth Blazar Telescope (WEBT) consortium has carried out several multifrequency campaigns to follow the source behaviour.

Aims. The Whole Earth Blazar Telescope (WEBT) consortium has been monitoring the blazar 3C 454.3from the radio to the optical bands since 2004 to study its emission variability properties.

Context. The quasar 3C 279is among the most extreme blazars in terms of luminosity and variability of flux at all wavebands. Its variations in flux and polarization are quite complex and therefore require intensive monitoring observations at multiple wavebands to characterise and interpret the observed changes.

Context.NRAO 150, a compact and bright radio to mm source showing core/jet structure, has been recently identified as a quasar at redshift $z = 1.52$through a near-IR spectral observation.

BL Lacertaehas been the target of several observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration and is one of the best studied blazars at all accessible wavelengths. A recent analysis of the optical and radio variability indicates that part of the radio variability is correlated with the optical light curve. Here we present an analysis of a huge VLBI data set including 108 images at 15, 22, and 43 GHz obtained between 1995 and 2004. The aim of this study is to identify the different components contributing to the single-dish radio light curves. We obtain separate radio light curves for the VLBI core and jet and show that the radio spectral index of single-dish observations can be used to trace the core variability. Cross-correlation of the radio spectral index with the optical light curve indicates that the optical variations lead the radio by about 100 days at 15 GHz. By fitting the radio time lags vs. frequency, we find that the power law is steeper than expected for a freely expanding conical jet in equipartition with energy density decreasing as the square of the distance down the jet as in the Königl model. The analysis of the historical data back to 1968 reveals that during a time range of 16 years the optical variability was reduced and its correlation with the radio emission was suppressed. There is a section of the compact radio jet where the emission is weak such that flares propagating down the jet are bright first in the core region with a secondary increase in flux about 1.0 mas from the core. This illustrates the importance of direct imaging to the interpretation of multi-wavelength light curves that can be affected by several distinct components at any given time. We discuss how the complex behaviour of the light curves and correlations can be understood within the framework of a precessing helical jet model.

Context.The radio quasar 3C 454.3underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was $R=12.0$, which represents the most luminous quasar state thus far observed ($M_B \sim -31.4$).

We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous and later observations suggests that the overall kinematic and structural properties of the jet are greatly influenced by its activity. Time intervals of enhanced activity, as reported before 1993 and after 2000 by other studies, are followed by highly superluminal motion along a rectilinear jet. In contrast the less active state in which we performed our observations, shows subluminal or slow superluminal jet features propagating through a twisted jet with two sharp bends of about 90° within the innermost three-milliarcsecond jet structure. Proper motion estimates from the data presented here allow us to constrain the jet viewing angle to values ≤$9^\circ$, and the bulk Lorentz factor to be between 2 and 4.

Aims.The aim of this work is to identify and study spectral features in the quasar 3C 273 usually blended by its strong jet emission.

We compare total flux density variations in 27 γ-ray blazars with structural changes in their parsec-scale jets using multi-epoch VLBA observations at 22 and 43 GHz together with data from the Mets$\ddot{\rm a}$hovi quasar monitoring program at 22 and 37 GHz. There is a clear connection between total flux density outbursts and VLBI components emerging into the jet. For essentially every new moving VLBI component, there is a coincident total flux density flare, with evolution similar to that of the component. Furthermore, extrapolated ejection times of the new VLBI components correspond to the beginnings of associated flares. Our results suggest that it is possible to explain all the radio variations as shocks propagating down the jet. A large fraction of the shocks grow and decay within the innermost few tenths of a milliarcsecond and therefore we see them only as “core flares” in the VLBI images. However, with present data we cannot exclude the possibility that the core itself also brightens (and thus contributes to the flare) as a shock passes through it.

We present intensive optical, infrared, and radio monitoring observations of the BL Lac object OJ 287, taken between the years 1993-1998. Two large optical outbursts were detected at the predicted times in November 1994 and December 1995. The detection of these outbursts supports the binary black hole model for OJ 287. Optical and radio polarisation observations show large variability in the degree of polarisation and position angle, very similar to those observed during the 1983/84 outburst in OJ 287. The polarisation position angles show very similar behaviour during these observations, indicating that, at least, the magnetic field orientations in radio and optical bands are related in OJ 287. Optical and infrared light curves show continuous variability in time scales ranging from tens of minutes to years. In the radio bands we have observed some of the lowest ever measured flux levels. During the first optical outburst in November 1994 the observed radio flux was very low, but during the second optical outburst radio bands also showed high flux levels. This is a puzzling observation, which can hopefully be used for discriminating between different outburst models. On top of the large outbursts OJ 287 has displayed flaring activity in time scales from days to weeks and shorter time scale flickering.

This article provides an introduction to the use of students' business skills in optimizing teaching opportunities, student learning, and client satisfaction in a primary health care setting at a veterinary teaching hospital. Seven veterinary-student members of the local chapter of the Veterinary Business Management Association (VBMA) evaluated the primary-care service at the University of Georgia (UGA) veterinary teaching hospital and assessed six areas of focus: (1) branding and marketing, (2) client experience, (3) staff and staffing, (4) student experience, (5) time management, and (6) standard operating procedures and protocols. For each area of focus, strengths, weaknesses, opportunities, and threats were identified. Of the six areas, two were identified as areas in need of immediate improvement, the first being the updating of standard operating protocols and the second being time management and the flow of appointments. Recommendations made for these two areas were implemented. Overall, the staff and students provided positive feedback on the recommended changes. Through such a student-centered approach to improving the quality of their education, students are empowered and are held accountable for their learning environment. The fact that the VBMA functions without a parent organization and that the primary-care service at UGA functions primarily as a separate entity from the specialty services at the College of Veterinary Medicine allowed students to have a direct impact on their learning environment. We hope that this model for advancing business education will be studied and promoted to benefit both veterinary education and business practice within academia.

We analyze the behavior of the parsec-scale jet of the quasar 3C 454.3 during pronounced flaring in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors G > 10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the R-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and g-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the millimeter-wave core lies at the end of the jet's acceleration and collimation zone. We infer that the X-ray emission is produced via inverse Compton scattering by relativistic electrons of photons both from within the jet (synchrotron self-Compton) and external to the jet (external Compton, or EC); which one dominates depends on the physical parameters of the jet. A broken power-law model of the g-ray spectrum reflects a steepening of the synchrotron emission spectrum from near-IR to soft UV wavelengths. We propose that the g-ray emission is dominated by the EC mechanism, with the sheath of the jet supplying seed photons for g-ray events that occur near the millimeter-wave core.

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 120 between 2002 and 2007 at X-ray (2-10 keV), optical (R and V bands), and radio (14.5 and 37 GHz) wave bands, as well as imaging with the Very Long Baseline Array (VLBA) at 43 GHz. Over the 5 yr of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. Consistent with this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the radio variations. Furthermore, the total radiative output of a radio flare is related to the equivalent width of the corresponding X-ray dip. This implies that, in this radio galaxy, the radiative state of accretion disk plus corona system, where the X-rays are produced, has a direct effect on the events in the jet, where the radio emission originates. The X-ray power spectral density of 3C 120 shows a break, with steeper slope at shorter timescale and the break timescale is commensurate with the mass of the central black hole (BH) based on observations of Seyfert galaxies and black hole X-ray binaries (BHXRBs). These findings provide support for the paradigm that BHXRBs and both radio-loud and radio-quiet active galactic nuclei are fundamentally similar systems, with characteristic time and size scales linearly proportional to the mass of the central BH. The X-ray and optical variations are strongly correlated in 3C 120, which implies that the optical emission in this object arises from the same general region as the X-rays, i.e., in the accretion disk-corona system. We numerically model multi-wavelength light curves of 3C 120 from such a system with the optical-UV emission produced in the disk and the X-rays generated by scattering of thermal photons by hot electrons in the corona. From the comparison of the temporal properties of the model light curves to that of the observed variability, we constrain the physical size of the corona and the distances of the emitting regions from the central BH. In addition, we discuss physical scenarios for the disk-jet connection that are consistent with our observations.

Context. Since 2001, the radio quasar 3C 454.3 has undergone a period of high optical activity, culminating in the brightest optical state ever observed, during the 2004-2005 outburst. The Whole Earth Blazar Telescope (WEBT) consortium has carried out several multifrequency campaigns to follow the source behaviour.Aims. The GLAST-AGILE Support Program (GASP) was born from the WEBT to provide long-term continuous optical-to-radio monitoring of a sample of ?-loud blazars, during the operation of the AGILE and GLAST (now known as Fermi GST) ?-ray satellites. The main aim is to shed light on the mechanisms producing the high-energy radiation, through correlation analysis with the low-energy emission. Thus, since 2008 the monitoring task on 3C?454.3 passed from the WEBT to the GASP, while both AGILE and Fermi detected strong ?-ray emission from the source.Methods. We present the main results obtained by the GASP at optical, mm, and radio frequencies in the 2008-2009 season, and compare them with the WEBT?results from previous years.Results. An optical outburst was observed to peak in mid July?2008, when Fermi detected the brightest ?-ray levels. A?contemporaneous mm outburst maintained its brightness for a longer time, until the cm emission also reached the maximum levels. The behaviour compared in the three bands suggests that the variable relative brightness of the different-frequency outbursts may be due to the changing orientation of a curved inhomogeneous jet. The optical light curve is very well sampled during the entire season, which is also well covered by the various AGILE and Fermi observing periods. The relevant cross-correlation studies will be very important in constraining high-energy emission models.

We explore the variability and cross-frequency correlation of the flux density and polarization of the blazar OJ287, using imaging at 43 GHz with the Very Long Baseline Array, as well as optical and near-infrared (near-IR) polarimetry. The polarization and flux density in both the optical waveband and the 43 GHz compact core increased by a small amount in late 2005, and increased significantly along with the near-IR polarization and flux density over the course of 10 days in early 2006. Furthermore, the values of the electric vector position angle (EVPA) at the three wavebands are similar. At 43 GHz, the EVPA of the blazar core is perpendicular to the flow of the jet, while the EVPAs of emerging superluminal knots are aligned parallel to the jet axis. The core polarization is that expected if shear aligns the magnetic field at the boundary between flows of disparate velocities within the jet. Using variations in flux density, percentage polarization, and EVPA, we model the inner jet as a spine-sheath system. The model jet contains a turbulent spine of half-width 1.deg2 and maximum Lorentz factor of 16.5, a turbulent sheath with Lorentz factor of 5, and a boundary region of sheared field between the spine and sheath. Transverse shocks propagating along the fast, turbulent spine can explain the superluminal knots. The observed flux density and polarization variations are then compatible with changes in the direction of the inner jet caused by a temporary change in the position of the core if the spine contains wiggles owing to an instability. In addition, we can explain a stable offset of optical and near-IR percentage polarization by a steepening of spectral index with frequency, as supported by the data.

We present results from a multiyear monitoring campaign of the broad-line radio galaxy 3C 120, using the Rossi X-ray Timing Explorer for nearly five years of observations. Additionally, we present coincident optical monitoring using data from several ground-based observatories. Both the X-ray and optical emission are highly variable and appear to be strongly correlated, with the X-ray emission leading the optical by 28 days. The X-ray power density spectrum is best fit by a broken power law, with a low-frequency slope of -1.2, breaking to a high-frequency slope of -2.1, and a break frequency of log nb = -5.75 Hz, or 6.5 days. This value agrees well with the value expected based on 3C 120's mass and accretion rate. We find no evidence for a second break in the power spectrum. Combined with a moderately soft X-ray spectrum (G = 1.8) and a moderately high accretion rate ($\dot{m}/\dot{m}_{\rm Edd} \sim 0.3$), this indicates that 3C 120 fits in well with the high/soft variability state found in most other active galactic nuclei. Previous studies have shown that the spectrum has a strong Fe Ka line, which may be relativistically broadened. The presence of this line, combined with a power spectrum similar to that seen in Seyfert galaxies, suggests that the majority of the X-ray emission in this object arises in or near the disk, and not in the jet.

Context. Since 1997, BL Lacertae has undergone a phase of high optical activity, with the occurrence of several prominent outbursts. Starting from 1999, the Whole Earth Blazar Telescope (WEBT) consortium has organized various multifrequency campaigns on this blazar, collecting tens of thousands of data points. One of the main issues in the study of this huge dataset has been the search for correlations between the optical and radio flux variations, and for possible periodicities in the light curves. The analysis of the data assembled during the first four campaigns (comprising also archival data to cover the period 1968?2003) revealed a fair optical-radio correlation in 1994?2003, with a delay of the hard radio events of ~100?days. Moreover, various statistical methods suggested the existence of a radio periodicity of ~8?years.Aims. In 2004 the WEBT started a new campaign to extend the dataset to the most recent observing seasons, in order to possibly confirm and better understand the previous results.Methods. In this campaign we have collected and assembled about 11?000 new optical observations from twenty telescopes, plus near-IR and radio data at various frequencies. Here, we perform a correlation analysis on the long-term R-band and radio light curves.Results. In general, we confirm the ~100-day delay of the hard radio events with respect to the optical ones, even if longer (~200?300?days) time lags are also found in particular periods. The radio quasi-periodicity is confirmed too, but the ?period? seems to progressively lengthen from?7.4 to 9.3?years in the last three cycles. The optical and radio behaviour in the last forty years suggests a scenario where geometric effects play a major role. In particular, the alternation of enhanced and suppressed optical activity (accompanied by hard and soft radio events, respectively) can be explained in terms of an emitting plasma flowing along a rotating helical path in a curved jet.

Aims. We report the detection by the AGILE satellite of a rapid ?-ray flare from the source 1AGL J1511-0908, associated with the powerful ?-ray quasar PKS 1510-089, during a pointing centered on the Galactic center region from 1 March to 30 March 2008. This source has been continuosly monitored in the radio-to-optical bands by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). Moreover, the ?-ray flaring episode triggered three ToO observations by the Swift satellite in three consecutive days, starting from 20 March 2008. The quasi-simultaneous radio-to-optical, UV, X-ray and ?-ray coverage allows us to make a detailed study of the multifrequency time evolution, the spectral energy distribution of this source, and its theoretical interpretation based on the synchrotron and inverse Compton?(IC) emission mechanisms.Methods. During the radio-to-optical monitoring provided by the GASP-WEBT, AGILE observed the source with its two co-aligned imagers, the gamma-ray imaging detector (GRID) and the hard X-ray imager (SuperAGILE), which are sensitive in the 30?MeV?30?GeV and 18?60?keV energy bands, respectively.Results. In the period 1?16?March 2008, AGILE detected ?-ray emission from PKS 1510-089 at a significance level of 6.2-? with an average flux over the entire period of (84 ?17) ?10-8?photons?cm-2s-1for photon energies above 100?MeV. After a predefined satellite re-pointing, between 17 and 21 March 2008, AGILE detected the source at a significance level of 7.3-?, with an average flux (E > 100?MeV) of (134 ?29) ?10-8?photons?cm-2?s-1and a peak level of (281 ?68) ?10-8?photons?cm-2?s-1with daily integration. During the observing period January-April 2008, the source also showed an intense and variable optical activity, with several flaring episodes and a significant increase in the flux was observed at millimetric frequencies. Moreover, in the X-ray band, the Swift/XRT observations seem to show a harder-when-brighter behavior of the source spectrum.Conclusions. The flat spectrum radio quasar PKS?1510-089 showed strong activity between January and April 2008, with episodes of rapid variability from radio to ?-ray energy bands, in particular with a rapid ?-ray flaring episode. The spectral energy distribution of mid-March 2008 is modeled with a homogeneous one-zone synchrotron self Compton (SSC) emission plus contributions from inverse Compton scattering of external photons from both the accretion disk and the broad line region. Indeed, some features in the optical-UV spectrum seem to indicate Seyfert-like components such as the little and the big blue bumps.

We present the results of extensive multi-wave band monitoring of the blazar 3C 279 between 1996 and 2007 at X-ray energies (2-10 keV), optical R band, and 14.5 GHz, as well as imaging with the Very Long Baseline Array (VLBA) at 43 GHz. In all bands the power spectral density corresponds to "red noise" that can be fit by a single power law over the sampled timescales. Variations in flux at all three wave bands are significantly correlated. The time delay between high- and low-frequency bands changes substantially on timescales of years. A major multifrequency flare in 2001 coincided with a swing of the jet toward a more southerly direction, and in general the X-ray flux is modulated by changes in the position angle of the jet near the core. The flux density in the core at 43 GHz--increases in which indicate the appearance of new superluminal knots--are significantly correlated with the X-ray flux. We decompose the X-ray and optical light curves into individual flares, finding that X-ray leads optical variations (XO) in six flares, the reverse (OX) occurs in three flares, and there is essentially zero lag in four flares. Upon comparing theoretical expectations with the data, we conclude that (1) XO flares can be explained by gradual acceleration of radiating electrons to the highest energies, (2) OX flares can result from either light-travel delays of the seed photons (synchrotron self-Compton scattering) or gradients in maximum electron energy behind shock fronts, and (3) events with similar X-ray and optical radiative energy output originate well upstream of the 43 GHz core, while those in which the optical radiative output dominates occur at or downstream of the core.

Context. The quasar-type blazar 3C 454.3 underwent a phase of high activity in summer and autumn?2007 that was intensively monitored in the radio-to-optical bands by the Whole Earth Blazar Telescope (WEBT). The ?-ray satellite Astro-rivelatore Gamma a Immagini LEggero (AGILE) detected this source first in late July, and then in November?December 2007.Aims. We present the multifrequency data collected by the WEBT and collaborators during the second AGILE observing period, complemented by contemporaneous data from the UltraViolet and Optical Telescope (UVOT) onboard the Swift satellite. The aim is to trace in detail the behaviour of the synchrotron emission from the blazar jet, and to investigate the contribution from the thermal emission component.Methods. Optical data from about twenty telescopes have been homogeneously calibrated and carefully assembled to construct an R-band light curve containing about 1340?data points over 42?days. This extremely well-sampled optical light curve allows us to follow the dramatic flux variability of the source in detail. In addition, we show radio-to-UV spectral energy distributions (SEDs) at different epochs, which represent different brightness levels.Results. In the considered period, the source varied by 2.6?mag over two weeks in the R?band. Many episodes of fast (i.e.?intranight) variability were observed, most notably on December?12, when a flux increase of about 1.1?mag in 1.5?h was detected, followed by a steep decrease of about 1.2?mag in 1?h. The contribution by the thermal component is difficult to assess, due to the uncertainties in the Galactic, and possibly also intrinsic, extinction in the UV?band. However, polynomial fitting of radio-to-UV?SEDs reveals an increasing spectral bending towards fainter states, suggesting a UV?excess likely due to the thermal emission from the accretion disc.Conclusions. Once the AGILE data are completely analysed, the low-frequency observations presented in this letter will offer a powerful tool to investigate optical-? flux correlations, i.e.?the relationship between the synchrotron and inverse-Compton emission components.

We report an attempt to detect the electron-positron annihilation line from the radio galaxy 3C 120, in which the jet interacts strongly with interstellar clouds. Such interactions should cause most of the jet plasma to mix with the gas in the clouds. This will thermalize the majority of any positrons in the jet, leading to continuous annihilation with ambient electrons. We derive the number density of the combined electron-positron population in the core and compact knots in the jet of 3C 120 using ultrahigh-resolution observations with the Global millimeter-VLBI Array at 86 GHz and the Very Long Baseline Array at 43 GHz, along with the millimeter-wave continuum spectrum and a computational code that maps the synchrotron intensity of a model jet. If the jet contains a pure pair plasma, the production rate of positrons required to produce this density plus the efficiency of eventual annihilation predict the emission of a narrow spectral line at a rest energy of 511 keV, or 495 keV in the observer's frame. Our spectral observations with the SPI instrument on INTEGRAL failed to detect the line. The upper limit, which is 30% lower than our rather uncertain prediction, does not significantly constrain the positron-to-proton ratio in the jet of 3C 120. However, our procedure provides a robust method for determining the flux of electrons and positrons in a jet that will be useful when more sensitive soft g-ray spectrometers and millimeter-wave VLBI arrays become available.

We have observed five quasars (0827+243, 0923+392, 1222+216, 1317+520, and 2209+080) with Chandra at 0.2–8 keV and with the VLA at 5 and 15 GHz. The X-ray emission is detected from all the radio jets except for 0923+392, whose double radio jet only extends out to the edge of the X-ray PSF. We model the spectral energy distribution of the jet features assuming that the X-ray emission originates via inverse Compton scattering off the cosmic microwave background by relativistic electrons. For two sources (0827+243 and 1222+216) proper motion on parsec scales is consistent with the Doppler factors derived from the model, which implies that no deceleration occurs between parsec and kiloparsec jets. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The radio galaxy 3C 120 appears as one of the best sources to study the emission and propagation of relativistic jets in active galactic nuclei. Muti-wavelength multi-epoch observations suggest that this Seyfert 1 galaxy is probably the result of a merger and presents a jet in precession that originates from the accretion of material onto a super massive central black hole. Thanks to its proximity and high activity it is possible to monitor the rapid changing rich inner jet structure with unprecedented spatial and temporal resolutions through very long baseline interferometry observations. The jet is pointing close to the line of sight, and is most probably interacting very dynamically with a cloud of properties intermediate of those of the broad and narrow emission-line regions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It appears that relativistic jets are produced by accreting black holes. We might therefore expect that events observed in jets, such as the appearance of bright radio knots moving at apparent superluminal speeds, originate as disturbances in the central engine. This is observed in a number of compact X-ray binary systems in our Galaxy, manifested as changes in the X-ray emission state followed by superluminal ejections. The author and collaborators have detected somewhat similar events in the radio galaxies 3C 120 (type Fanaroff-Riley I) and 3C 111 (FR II). The data both confirm the disk-jet connection in accreting supermassive black holes and provide constraints on the structure of the jet between the central engine and the millimeter-wave core. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we extend our approach to modeling multifrequency emission variability from blazars to include external radiation Compton (ERC) emission and electron energy losses from inverse Compton scattering of seed photons originating outside the jet. We consider seed photon emission from a dusty molecular torus and the broad-line region (BLR) surrounding the central engine. We establish constraints on the properties of the molecular torus and BLR under which synchrotron self-Compton (SSC) emission dominates such that the results obtained in our previous paper are applicable. The focus of this study is on relative time delays between the light curves observed at different frequencies. For definiteness, we consider emission resulting from a collision between relativistic shocks, but the results apply more generally to conditions involving acceleration of relativistic electrons and/or magnetic field amplification at any type of front. Unlike SSC emission, ERC flares involving a constant field of seed photons are not delayed by light-travel time of the seed photons. The main cause of delays is from radiative energy losses, which result in frequency stratification behind the front and quench the flare first at the highest frequencies, then progressing to lower frequencies as time advances. However, if the spectrum of electrons injected at the shock front is characterized by a relatively high value of the minimum energy (a Lorentz factor gmin ~ 100 is sufficient), the ERC flare in the X-ray band can be delayed and may even peak after the injection has ceased. This effect is strongly frequency dependent, with a longer lag at lower frequencies and an X-ray spectral index that changes rapidly from positive (inverted spectrum) to steep values.

We present Chandra X-ray (0.2-8 keV) and Very Large Array radio (15 and 5 GHz) images of the g-ray-bright, superluminal quasar 0827+243. The X-ray jet bends sharply--by ~90deg, presumably amplified by projection effects--5'' from the core. Only extremely weak radio emission is detected between the nuclear region and the bend. The X-ray continuum spectrum of the combined emission of the knots is rather flat, with a slope of -0.4 +- 0.2, while the 5-15 GHz spectra are steeper for knots detected in the radio. These characteristics, as well as nondetection of the jet in the optical band by the Hubble Space Telescope, pose challenges to models for the spectral energy distributions (SEDs) of the jet features. The SEDs could arise from pure synchrotron emission from either a single or dual population of relativistic electrons only if the minimum electron energy per unit mass gmin [?] 1000. In the case of a single population, the radiative energy losses of the X-ray-emitting electrons must be suppressed owing to inverse Compton scattering in the Klein-Nishina regime, as proposed by Dermer & Atoyan. Alternatively, the X-ray emission could result from inverse Compton scattering of the cosmic microwave background photons by electrons with Lorentz factors as low as g ~ 15. In all models, the bulk Lorentz factor of the jet flow G [?] 20 found on parsec scales must continue without substantial deceleration out to 800 kpc (deprojected) from the nucleus, and the magnetic field is very low, [?]2 mG, until the bend. Deceleration does appear to occur at and beyond the sharp bend, such that the flow could be only mildly relativistic at the end of the jet. Significant intensification of the magnetic field occurs downstream of the bend, where there is an offset between the projected positions of the X-ray and radio features.

We model rapid variability of multifrequency emission from blazars occurring across the electromagnetic spectrum (from radio to gamma rays). Lower energy emission is produced by the synchrotron mechanism, whereas higher energy emission is due to inverse Compton scattering of the synchrotron emission. We take into account energy stratification established by particle acceleration at shock fronts and energy losses due to synchrotron emission. We also consider the effect of light-travel delays for the synchrotron emission that supplies the seed photons for inverse Compton scattering. The production of a flare is caused by the collision between a relativistic shock wave and a stationary feature in the jet (e.g., a Mach disk). The collision leads to the formation of forward and reverse shocks, which confine two contiguous emission regions resulting in complex profiles of simulated flares. Simulations of multifrequency flares indicate that relative delays between the inverse Compton flares and their synchrotron counterparts are dominated by energy stratification and geometry of the emitting regions, resulting in both negative and positive time delays depending on the frequency of observation. Light-travel effects of the seed photons may lead to a noticeable delay of the inverse Compton emission with respect to synchrotron variability if the line of sight is almost perfectly aligned with the jet. We apply the model to a flare in 3C 273 and derive the properties of shocked plasma responsible for the flare. We show that the pronounced negative time delay between the X-ray and IR light curves (X-rays peak after the maximum in the synchrotron emission) can be accounted for if both forward and reverse shocks are considered.

We present Very Long Baseline Array images of 42 g-ray bright blazars, including 36 with polarization vectors, obtained during the course of a multiepoch monitoring program. Each object was observed at either 43 or 22 GHz, with some objects observed at both frequencies and/or at 15 or 8.4 GHz. The morphologies are varied, with some of the blazars displaying long, thin jets, others short, broad jets, and still others containing cores with only very weak features that are probably knots in faint jets. The polarization of the cores ranges from less than 1% to 8.6%, with electric vector position angles (EVPAs) that are split between nearly parallel and nearly transverse to the jet axis. The polarization of knots in the jets covers a much broader range, from less than 2% to tens of percent. The EVPA of the brightest compact feature in each jet ranges from 0deg to 80deg from the jet position angle, with roughly half measuring less than 20deg. The distribution is consistent with intrinsically oblique magnetic fields whose observed directions are altered by relativistic aberration.

We examine the coincidence of times of high g-ray flux and ejections of superluminal components from the core in EGRET blazars based on a Very Long Baseline Array (VLBA) monitoring program at 22 and 43 GHz from 1993 November to 1997 July. In 23 cases of g-ray flares for which sufficient VLBA data exist, 10 of the flares (in eight objects) fall within 1 s uncertainties of the extrapolated epoch of zero separation from the core of a superluminal radio component. In each of two sources (0528+134 and 1730-130), two successive g-ray flares were followed by the appearance of new superluminal components. We carried out statistical simulations that show that if the number of coincidences is [?]10, the radio and g-ray events are associated with each other at greater than 99.999% confidence. Our analysis of the observed behavior, including variability of the polarized radio flux, of the sources before, during, and after the g-ray flares suggests that the g-ray events occur in the superluminal radio knots. This implies that the g-ray flares are caused by inverse Compton scattering by relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine.

During the first 3 years of the Compton Gamma Ray Observatorymission, the blazar CTA 26 was observed 10 times by EGRET and not significantly detected. We report an observation in 1995 when CTA 26 flared to a peak ?-ray flux of (4.9 ± 1.5) × 10-6cm-2s-1(E> 100 MeV), the third brightest of all EGRET blazars. Following the ?-ray flare, extensive VLBA and single-dish radio observations were obtained. We find two components of a milliarcsecond jet moving with apparent transverse velocities of 12 ± 1 h-1c, and 5 ± 2 h-1c(H0= 100 hkm s-1Mpc-1, q0= 0.1). The position angle of VLBI components appears to change with time. The slowest VLBI component's motion is consistent with ejection at the time of the 1995 ?-ray flare. A weak radio flare is also seen in Metsähovi millimeter radio monitoring data, peaking within weeks of the ?-ray flare.

We present the first space VLBI observations of 3C 371, carried out at a frequency of 4.8 GHz. The combination of the high resolution provided by the orbiting antenna Highly Advanced Laboratory for Communications and Astronomy (HALCA) and the high sensitivity of the VLBA allows imaging of the jet of 3C 371 with an angular resolution of approximately 0.26 mas, which for this relatively nearby source corresponds to ~0.4 h-1 pc. Comparison between two epochs separated by 66 days reveals no apparent motions in the inner 7 mas jet structure above an upper limit of ~1.4 h-1 c. This value, the absence of detectable counterjet emission from the presumably symmetric jet, plus the presence of an extended double-lobe structure are consistent with the knots in the jet being stationary features such as standing shocks. The jet intensity declines with the angular distance from the core as ph-1.68. This is more gradual than that derived for 3C 120, ph-1.86, for which there is evidence for strong interactions between the jet and ambient medium. This suggests that in 3C 371 there is a greater level of in situ acceleration of electrons and amplification of magnetic field. We interpret sharp bends in the jet at sites of off-center knots as further evidence for the interaction between the jet and external medium, which may also be responsible for the generation of standing recollimation shocks. These recollimation shocks may be responsible for the presumably stationary components. The radio properties of 3C 371 are intermediate between those of other radio galaxies with bright cores and those of BL Lacertae objects.

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