K. E. Gabanyi, T. P. Krichbaum, A. Kraus, A. Witzel, J. A. Zensus
The blazar, S5 2007+77 shows intraday variability in cm wavelengths. Seven epochs of VSOP observations were carried out in 1998 and 1999 at 5 GHz to look for the origin of the variability with the highest achievable angular resolution at cm wavelengths. Here the results of four epochs are analysed, which revealed ~10% variations in polarized flux density.
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Two high-resolution 5 GHz VSOP+VLBA images of the blazar 1633+382 reveal clear morphological change in both the bright inner east-west jet, and in the more diffuse structure further down the jet to the northwest. Here, we discuss the motion of the inner jet as traced by these VSOP images and earlier VLBA observations.
We present a high resolution 1.6 GHz VSOP image of the southern blazar PKS 1921-293. The image shows a typical core-jet morphology, consistent with ground-based VLBI images. However, the addition of data from the space antenna has greatly improved the angular resolution (especially along the north-south direction for this source), and thus allowed us to clearly identify the core. Model fitting reveals an inner jet component ~1.5 mas north of the core. This jet feature may be moving on a common curved path connecting the jet within a few parsecs to the 10-parsec-scale jet. The compact core has …
This contribution presents preliminary results from coordinated polarization sensitive VSOP and VLBA imaging of the blazars 3C~279 and 3C~345 at multiple frequencies.
We present thirty VLBI images of the TeV blazar Markarian 421 (1101+384) at fifteen epochs spanning the time range from 1994 to 1997, and at six different frequencies from 2.3 to 43 GHz. The imaged observations include a high-resolution 5 GHz VLBI Space Observatory Programme (VSOP) observation with the HALCA satellite on 1997 November 14; full-track VLBA observations from 1994 April, 1996 November, and 1997 May at frequencies between 5 and 43 GHz; six epochs of VLBA snapshot observations at frequencies between 2 and 15 GHz from Radio Reference Frame studies; and five geodetic VLBI observations…
We present the first VLBI Space Observatory Programme (VSOP) observations of the gamma-ray blazar 3C 279 at 1.6 and 5 GHz. The combination of the VSOP and VLBA-only images at these two frequencies maps the jet structure on scales from 1 to 100 mas. On small angular scales the structure is dominated by the quasar core and the bright secondary component `C4' located 3 milliarcseconds from the core (at this epoch). On larger angular scales the structure is dominated by a jet extending to the southwest, which at the largest scale seen in these images connects with the smallest scale structure seen…
We present multi-frequency observational results for the Compact Steep-Spectrum (CSS) quasar 3C309.1. The observations were carried out with the VSOP at 1.6 and 4.8GHz and the VLBA at 15.4GHz. The source has a distorted, one-sided radio jet. Relativistics effects and physical properties are discussed. Comparing the predicted and observed X-ray fluxes in the framework of the synchrotron self-Compton model we derive the beaming factor for 3C309.1. The complex structure of the jet may be explained by a relativistic helical flow in a heterogeneous, clumpy ISM.
Given the scientific goals of VSOP-2, including the possibility of observations of the shadows of black holes, we have investigated the fidelity of the recovered images given a typical uv-coverage. We find that we can achieve a dynamic range of better than 1000:1.
AO 0235+164 is a very compact, flat spectrum radio source identified as a BL Lac object at a redshift of z=0.94. It is one of the most violently variable extragalactic objects at both optical and radio wavelengths. The radio structure of the source revealed by various ground-based VLBI observations is dominated by a nearly unresolved compact component at almost all available frequencies. Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP mission in January-February 1999. The array of the Japanese HALCA satellite and co-observing ground radio telescopes in Australia, …
We report results of a VSOP (VLBI Space Observatory Programme) observation of a high redshift quasar PKS 2215+020 (z=3.572). The ~1 milliarcsecond resolution image of the quasar reveals a prominent `core-jet' structure on linear scales from 5/h to 300/h pc ($H_0=100*h km/(s*Mpc). The brightness temperatures and sizes of bright features identified in the jet are consistent with emission from relativistic shocks dominated by adiabatic energy losses. The jet is powered by the central black hole with estimated mass of ~4*10^9 solar masses. Comparisons with VLA and ROSAT observations indicate a pos…
To locate and image the compact emission regions in quasars, which are closely connected to the phenomenon of IntraDay Variability (IDV), space VLBI observations are of prime importance. Here we report on VSOP observations of two prominent IDV sources, the BL Lac objects S5 0716+714. To monitor their short term variability, these sources were observed with VSOP at 5 GHz in several polarisation sensitive experiments, separated in time by one day to six days, in autumn 2000. Contemporaneous flux density measurements with the Effelsberg 100m radio telescope were used to directly compare the singl…
In 1999, the highly compact and variable BL Lac object AO 0235+164 was identified as the highest brightness temperature active galactic nucleus observed with the VLBI Space Observatory Programme (VSOP), with T_B > 5.8 x 10^{13} K. The sub-milliarcsecond radio structure of this source has been studied with dual-frequency (1.6 and 5 GHz), polarization-sensitive VSOP observations during 2001 and 2002. Here we present the results of this monitoring campaign. At the time of these observations, the source was weakly polarized and characterized by a radio core that is clearly resolved on space-ground…
The physical conditions in the inner parsec of accretion disks believed to orbit the central black holes in active galactic nuclei can be probed by imaging the absorption of background radio emission by ionized gas in the disk. We report high angular resolution observations of the nearby galaxy NGC 4261 which show evidence for free-free absorption by a thin, nearly edge-on disk at several frequencies. The angular width, and probably the depth, of the absorption appears to increase with decreasing frequency, as expected. Because free-free absorption is much larger at lower frequencies, the long…
We studied the radio structure of high-redshift (z>3) quasars with VSOP at 1.6 and 5 GHz. These sources are the most distant objects ever observed with Space VLBI, at rest-frame frequencies up to ~25 GHz. Here we give an account of the observations and briefly highlight the most interesting cases and results. These observations allowed us, among other things, to estimate the mass of the central black holes powering these quasars, to identify large misalignments between the milli-arcsecond (mas) and sub-mas scale radio structures, and to detect apparent superluminal motion at sub-mas scale.
We performed two types of radiation testing on high-speed LSI chips to test their suitability for use in wideband observations by the Japanese next space VLBI mission, VSOP-2. In the total ionization dose experiment we monitored autocorrelation spectra which were taken with irradiated LSI chips and the source current at intervals up to 1,000 hours from the ionization dose, but we could not see any change of these features for the chips irradiated with dose rates expected in the VSOP-2 mission. In the single event effect experiment, we monitored the cross correlation phase and power spectra bet…
A short overview is given of the status of research on young extragalactic radio sources. We concentrate on Very Long Baseline Interferometric (VLBI), and space-VLBI results obtained with the VLBI Space Observatory Programme (VSOP). In 2012, VSOP-2 will be launched, which will allow VLBI observations at an unprecedented angular resolution. One particular question VSOP-2 could answer is whether some of the High Frequency Peakers (HFP) are indeed the youngest objects in the family of GPS and CSS sources. VSOP-2 observations can reveal their angular morphology and determine whether any are Ultra-…
Space-VLBI observations of stellar sources represent a challenge since there are few sources with sufficiently high brightness temperature for detection on space-ground baselines. X-ray binaries (XRB) are among the few types of stellar radio sources that can be detected. Observations of the unusual X-ray and gamma-ray binary system LS I 61 303 obtained with the HALCA satellite and a 20-element ground array are described. The data in this 48-hour experiment represent some of the best quality VLBI observations of LS I 61 303. No fringes were detected on HALCA baselines. 10-minute snapshot images…
To determine the location of the intra-day variable (IDV) emission region within the jet of the BL Lac object S5 0716+714, a multi-epoch VSOP polarization experiment was performed in Autumn 2000. To detect, image, and monitor the short term variability of the source, three space-VLBI experiments were performed with VSOP at 5 GHz, separated in time by six days and by one day. Quasi-contemporaneous flux density measurements with the Effelsberg 100 m radio telescope during the VSOP observations revealed variability of about 5% in total intensity and up to 40% in linear polarization in less than o…
We have used VSOP space very long baseline interferometry observations to measure the brightness temperature distribution of a well-defined sub-set of the Pearson-Readhead sample of extragalactic radio sources. VLBI which is restricted to Earth-diameter baselines is not generally sensitive to emitting regions with brightness temperatures greater than approximately $10^{12}$ K, coincidentally close to theoretical estimates of brightness temperature limits, $10^{11} - 10^{12}$ K. We find that a significant proportion of our sample have brightness temperatures greater than $10^{12}$ K; many have …
During the 2007/2008 season, VERITAS was used for observations at E>200 GeV of several extragalactic non-blazar objects such as galaxy clusters, starburst and interacting galaxies, dwarf galaxies, and nearby galaxies. In these proceedings, we present preliminary results from our observations of dwarf galaxies and M87. Results from observation of other non-blazar sources are presented in separate papers in the proceedings.
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 2155--304, and PKS 2005--489 are highlighted.
The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four 12m diameter Imaging Atmospheric Cherenkov Technique (IACT) telescopes operated at the base of Mt. Hopkins in southern Arizona. The four-telescope experiment started operation in April, 2007. GeV and TeV gamma-ray observations of blazars can be used to probe the structure and composition of their jets, and to contribute to our understanding of how supermassive black holes accrete matter. In this contribution, we present first VERITAS blazar results obtained with three and four telescopes.
The close relation between ground-based TeV observations and satellite borne $γ$-ray measurements has been important for the understanding of blazars. The observations which involve the TeV component in blazar studies are reviewed.
We selected a sample of a dozen blazars which are the prime candidates for simultaneous multi-wavelength observing campaigns in their outburst phase. We searched for optical outbursts, intra-day variability and short term variability in these blazars. We carried out optical photometric monitoring of nine of these blazars in 13 observing nights during our observing run October 27, 2006 - March 20, 2007 by using the 1.02 meter optical telescope. From our observations, our data favor the hypothesis that three blazars were in the outburst state; one blazar was in the post outburst state; three bla…
The double humped SED (Spectral Energy Distribution) of blazars, and their flaring phenomena can be explained by various leptonic and hadronic models. However, accurate modeling of the high frequency component and clear identification of the correct emission mechanism would require simultaneous measurements in both the MeV-GeV band and the TeV band. Due to the differences in the sensitivity and the field of view of the instruments required to do these measurements, it is essential to identify active states of blazars likely to be detected with TeV instruments. Using a reasonable intergalactic …
We discuss the contribution of the blazar population to the extragalactic background radiation across the electromagnetic (e.m.) spectrum with particular reference to the microwave, hard-X-ray and gamma-ray bands. Our estimates are based on a recently derived blazar radio LogN-LogS that was built by combining several radio and multi-frequency surveys. We show that blazar emission integrated over cosmic time gives rise to a considerable broad-band non-thermal cosmic background that dominates the extragalactic brightness in the high-energy part of the e.m. spectrum. We also estimate the number o…
Since the first TeV blazar Markarian (Mrk) 421 was detected in 1992, the number of established TeV gamma-ray emitting BL Lac objects has grown to 6, with redshifts ranging from 0.031 (Mrk 421) to 0.129 (H 1426+428). The intensive study of these sources has had a major impact on our understanding of the blazar phenomenon. The most notable observational results have been extremely fast large amplitude flux and spectral variability on hour time scales, and a pronounced X-ray - TeV gamma-ray flux correlation. In this paper we discuss recent observational results and report on progress in their the…
Results obtained with BeppoSAX observations of blazars within various collaborative programs are presented. The spectral similarity "paradigm", whereby the spectral energy distributions of blazars follow a sequence, leading to a unified view of the whole population, is briefly illustrated. We concentrate on recent observations of flares and associated spectral variability for three objects at the "blue" end of the spectral sequence, namely PKS 2155-304, Mkn 421 and Mkn 501. The results are discussed in terms of a general analytic synchrotron self-Compton interpretation of the overall spectrum.…
This review paper discusses the past and present observations of gamma-ray blazars with both space and ground-based gamma-ray telescopes (such as EGRET and current TeV telescopes), and with the VLBA. I also discuss the more sensitive gamma-ray telescopes such as AGILE and GLAST that will become available over the next several years, and how observations with these telescopes, together with improved VLBA observations, can increase our understanding of blazar physics.
We report first observational results of multifrequency campaigns on the prominent Virgo blazars 3C 273 and 3C 279 which were carried out in January and February 1999. Both blazars are detected from radio to gamma-ray energies. We present the measured X- to gamma-ray spectra of both sources, and for 3C 279 we compare the 1999 broad-band (radio to gamma-ray) spectrum to measured previous ones.
We will present a summary of the observations of blazars by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO). EGRET has detected high energy gamma-ray emission at energies greater than 100 MeV from more that 50 blazars. These sources show inferred isotropic luminosities as large as $3\times 10^{49}$ ergs s$^{-1}$. One of the most remarkable characteristics of the EGRET observations is that the gamma-ray luminosity often dominates the bolometric power of the blazar. A few of the blazars are seen to exhibit variability on very short time-scales of …
The EGRET instrument onboard of CGRO detected a number of gamma-ray flares from blazars, and it is expected that GLAST, to be launched in 2006, should detect many more such flares. These flares should be preceded by X-ray precursors, and the MAXI All-Sky Monitor, to be deployed around the time of launch of GLAST, is likely to detect such precursors. This paper highlights the interesting new insights into the structure of jets in blazars that may result from simultaneous X-ray and gamma-ray observations. In particular, it addresses the expected temporal relationship between the precursors and f…
We review 6 years of INTEGRAL observations of blazars, from Target-Of-Opportunity (TOO) to normal observations to coordinated campaigns, from the new and unexpected discoveries to the improvements in this research field. We also shortly review what is expected from INTEGRAL for the forthcoming years.
The gamma-ray emission of the blazar Markarian 421 above 250 GeV has been observed by the CAT Cherenkov imaging telescope since December, 1996. We report here results on the source variability up to April, 1998, with emphasis on the 1998 campaign. For the flaring periods of this year, the energy spectrum was derived from 330 GeV up to 5.2 TeV: it is very well represented by a simple power law, with a differential spectral index of 2.96 +/- 0.13.
We report on observations of Markarian 421 made with the Whipple 10m Cerenkov telescope and the RXTE satellite during a multi-wavelength campaign in December 2002 and January 2003, initiated by a Whipple target of opportunity. The observations revealed several flares with flux levels between 1 and 2 times the flux of the Crab Nebula. We will discuss the temporal properties, including evidence of X-ray/TeV gamma-ray flux correlation.
The CELESTE atmospheric Cherenkov detector ran until June 2004. It has observed the blazars Mrk 421, 1ES 1426+428 and Mrk 501. We significantly improved our understanding of the atmosphere using a LIDAR, and of the optical throughput of the detector using stellar photometry. The new data analysis provides better background rejection. We present our light curve for Mrk 421 for the 2002-2004 season and a comparison with X-ray data and the 2004 observation of 1ES 1426+428. The new analysis will allow a more sensitive search for a signal from Mrk 501.
We observed the TeV blazar 1ES 1218+304 with the X-ray astronomy satellite Suzaku in May 2006. At the beginning of the two-day continuous observation, we detected a large flare in which the 5-10 keV flux changed by a factor of ~2 on a timescale of 5x10^4 s. During the flare, the increase in the hard X-ray flux clearly lagged behind that observed in the soft X-rays, with the maximum lag of 2.3x10^4 s observed between the 0.3-1 keV and 5-10 keV bands. Furthermore we discovered that the temporal profile of the flare clearly changes with energy, being more symmetric at higher energies. From the sp…
We present new Chandra and HST data for four gamma-ray blazars selected on the basis of radio morphology with the aim of revealing X-ray and optical emission from their jets at large scales. All the sources have been detected. Spectral Energy Distributions of the large scale jets are obtained as well as new X-ray spectra for the blazar cores. Modeling for each object the core (sub-pc scale) and large-scale (>100 kpc) jet SEDs, we derive the properties of the same jet at the two scales. The comparison of speeds and powers at different scales supports a simple scenario for the dynamics and propa…
In Spring 2005, the blazar 3C454.3 underwent a dramatic outburst at all wavelengths from mm to X-rays. This prompted INTEGRAL observations, accomplished in 15-18 May 2005. The source was detected by the INTEGRAL instruments from 3 to 200 keV in a bright state (~5 x 10E-10 cgs), at least a factor of 2-3 higher than previously observed. This is one of the brightest blazar detections achieved by INTEGRAL. During the 2.5 days of INTEGRAL monitoring, we detected a ~20% decrease in the hard X-rays (20-40 keV), indicating that we have sampled the decaying part of the flare. The decrease is less appar…
Jets, paving their way outward through the inner regions of active nuclei, Compton-interact with the UV radiation from an accretion disc and broad emission line region. We calculate the predicted properties of the resulting spectral signatures of this bulk-Compton process, noting that they are independent on the fractional proton content or kinetic power of the jet, and use the presence or absence of such signatures to put constraints on the structure of jets near their bases.
In eight years, during the blazar monitoring program performed with the Perugia automatic telescope, we were able to collect about 20000 $BVR_cI_c$ photometric points, contributing to get knowledge on the optical flux history of many sources brighter than magnitude $V=17.0$. This intensive and constant optical monitoring is essential to perform studies and multiwavelength campains about this class of AGN. We illustrate our program and some examples of data and preliminary results.
We report first results of a multifrequency campaign from radio to hard X-ray energies of the prominent gamma-ray blazar 3C 279 during the first year of the INTEGRAL mission. The variable blazar was found at a low activity level, but was detected by all participating instruments. Subsequently a multifrequency spectrum could be compiled. The individual measurements as well as the compiled multifrequency spectrum are presented. In addition, this 2003 broadband spectrum is compared to one measured in 1999 during a high activity period of 3C 279.
The Very High Energy (VHE) gamma-ray emission of the BL Lacertae objects Markarian 501 and Markarian 421 has been observed by the CAT Imaging Atmospheric Cherenkov Telescope in 1997 and 1998. The spectrum extraction method is presented, and the spectral properties of both sources are compared in different activity states. Theoretical implications for jet astrophysics are discussed.
Multi-wavelength observations with TeV photons are an essential diagnostic tool to study the physics of TeV sources. The complex variability of blazars, however (timescales from years down to minutes, with different patterns and SED behaviours), requires a great effort on simultaneous campaigns, which should be performed possibly over several days. Spectral information is essential, and now with the new TeV and X-ray telescopes it can be obtained on timescales less than one hour. The insights from such observations can be tremendous, since recent results have shown that the X-ray and TeV emiss…
In the present paper, we have reported the result of simultaneous multi-wavelength observations of the TeV blazar Mrk 421 during February $-$ March 2003. In this period, we have observed Mrk 421 using Pachmarhi Array of Čerenkov Telescopes (PACT) of Tata Institute of Fundamental Research at Pachmarhi, India. Other simultaneous data were taken from the published literature and public data archives. We have analyzed the high quality X-ray (2-20 keV) observations from the NASA Rossi X-Ray Timing Explorer (RXTE). We have seen a possible correlated variability between X-ray and J band (1.25 $μ$) ne…
We summarize the results of BeppoSAX ToO observation of blazars that were known to be in a high state from observations carried out in the optical or X-ray or TeV bands.In some of the observed sources, two spectral components were detected, which are interpreted as synchrotron and inverse Compton emission, respectively. Fast variability was detected in three sources (ON 231, BL Lac and S5 0716+714), but always only for the synchrotron component. Most of the triggers are from optical observations, consequently most of the observed sources are LBL or intermediate objects. They were in a high sta…
A ToO observation of the TeV-emitting blazar Mrk 421 with INTEGRAL was triggered in June 2006 by an increase of the RXTE count rate to more than 30 mCrab. The source was then observed with all INTEGRAL instruments with the exception of the spectrometer SPI for a total exposure of 829 ks. During this time several outbursts were observed by IBIS and JEM-X. Multiwavelength observations were immediately triggered and the source was observed at radio, optical and X-ray wavelengths up to TeV energies. The data obtained during these observations are analysed with respect to spectral evolution and cor…
We have measured peak apparent speeds of 25.6 +/- 7.0c, 25.6 +/- 4.4c, and 28.2 +/- 6.6c in the jets of 0235+164, 0827+243, and 1406-076, respectively, based on six epochs of high-sensitivity VLBA observations at 22 and 43 GHz during 2002 and 2003 (H_{0}=71 km/(s Mpc), Omega_{m}=0.27, and Omega_{Lambda}=0.73). These blazars had been identified as potentially having apparent speeds exceeding 40c in an earlier VLBA survey of EGRET blazars by Jorstad et al. We therefore confirm (with high confidence in 0827+243, and lower confidence in 0235+164 and 1406-076) the presence of highly relativistic pa…
The blazar Mkn 421 has been observed, as part of the AO1 Core Program, five times from 2 to 7 May 1997. In the LECS+MECS energy band the spectrum shows convex curvature, well represented by a broken power--law. Flux variability (more than a factor 2) has been detected over the entire 0.1--10 keV range, accompanying which the spectrum steepens with the decrease in intensity. Mkn 421 has also been detected with the PDS instrument. Our preliminary analysis indicates that the PDS spectrum lies significantly above the extrapolation from the MECS, suggesting a contribution from a flatter high energy…
Our recent progress on time-dependent modeling of the multiwavelength spectra and variability of blazars with leptonic and hadronic jet models is reviewed. Special emphasis is placed on X-ray spectral variability of low-frequency peaked (LBLs) and intermediate BL Lac objects (IBLs). As an example, recent observational and modeling results of an extensive multiwavelength campaign of BL Lacertae in 2000 are presented. It is demonstrated how combined spectral and variability modeling of LBLs and IBLs can significantly constrain emission models and potential variability scenarios. In the case of B…
We have investigated the MeV behaviour of the \gray blazars 3C~454.3 and CTA 102 by analyzing all COMPTEL observations of this sky region during the complete CGRO mission. Both sources are detected by COMPTEL at the upper COMPTEL energies; although their flux estimates may be uncertain by possible minor contribution of nearby unidentified EGRET \gray sources. While CTA 102 was only detected at energies above 10 MeV during the early mission, 3C~454.3 is most significantly detected in the COMPTEL 3-10 MeV band in the sum of all data. Time-resolved analyses indicate a weak (near COMPTEL threshold…
Multicolor (UBVRIJHK) observations of the blazar AO 0235+16 are analyzed. The light curves were compiled at the Turin Observatory from literature data and the results of observations obtained in the framework of the WEBT program (http://www.to.astro/blazars/webt/). The color variability of the blazar was studied in eight time intervals with a sufficient number of multicolor optical observations; JHK data are available for only one of these. The spectral energy distribution (SED) of the variable component remained constant within each interval, but varied strongly from one interval to another. …
We present the first observations at medium-hard X-rays with ASCA in 1998 August--November of four Flat Spectrum Radio Quasars (FSRQs), characterized by unusually steep soft X-ray spectra (photon index, Γ_{0.2-2.4 keV} \sim 2-2.5), as previously measured with ROSAT. Such steep X-ray slopes are similar to those observed in synchrotron-dominated BL Lacs and are unexpected in the context of the recent blazar paradigm, where sources with strong emission lines (such as FSRQs) are dominated in soft X-rays by a flat inverse Compton tail. We find that the ASCA spectra of the four FSRQs are consistent …
We present new, simultaneous, multifrequency observations of 45 out of the 55 candidate High Frequency Peakers (HFP) selected by Dallacasa et al. (2000), carried out 3 to 4 years after a first set of observations. Our sub-sample consists of 10 galaxies, 28 stellar objects (``quasars'') and 7 unidentified sources. Both sets of observations are sensitive enough to allow the detection of variability at the 10% level or lower. While galaxies do not show significant variability, most quasars do. Seven of them no longer show the convex spectrum which is the defining property of Gigahertz Peaked Spec…
The MAGIC telescope has performed long term monitoring observations of the bright TeV Blazars Mrk421, Mrk501 and 1ES1959+650. Up to 40 observations, 30 to 60 minutes each have been performed for each source evenly distributed over the observable period of the year. The sensitivity of MAGIC is sufficient to establish a flux level of 25% of the Crab flux for each measurement. These observations are well suited to trigger multiwavelength ToO observations and the overall collected data allow an unbiased study of the flaring statistics of the observed AGNs.
Recent observations in the TeV band challenge the simplest models developed to describe the overall emission of blazars and radiogalaxies. In particular, the observation of variable TeV emission from M87 and the fast variability shown by PKS 2155-304 challenge the standard framework. We discuss how the existence of a radial structure in the sub-pc scale jet, with faster a component ("spine" or "needles") embedded in a slower layer can explain the basic phenomenology of these sources.
We have analyzed the two blazars of 3C 454.3 and CTA 102 using all available COMPTEL data from 1991 to 1999. In the 10-30 MeV band, emission from the general direction of the sources is found at the 4$σ$-level, being consistent with contributions from both sources. Below 10 MeV only 3C 454.3 is significantly detected, with the strongest evidence (5.6 $σ$) in the 3-10 MeV band. Significant flux variability is not observed for both sources, while a low emission is seen most of the years in the 3-10 MeV light curve for 3C 454.3. Its time-averaged MeV spectrum suggests a power maximum between 3 to…
We present the discovery of very high energy (VHE) gamma-ray emission from the high-frequency-peaked BL Lac object 1ES 0806+524 (z=0.138) and the intermediate-frequency-peaked BL Lac object W Comae (z=0.102) with VERITAS. VHE emission was discovered from these objects during the 2007/2008 observing campaign, with a strong outburst from W Comae detected in mid-March, lasting a few days. Quasi-simultaneous spectral energy distributions are presented, incorporating optical (AAVSO), and X-ray (Swift/RXTE) observations. We also present the energy spectrum of the distant BL Lac (z=0.182) 1ES 1218+30…
The non-thermal spectra of blazars, observed from radio to GeV/TeV gamma-rays, reveal two pronounced components, both produced by radiation by energetic particles. One peaks in the IR - to soft X-ray band, radiating via the synchrotron process; the other, peaking in the high-energy gamma-rays, is produced by the Compton process. These spectra -- and, in particular, the \asca data -- suggest that the origin of the seed photons for Comptonization is diverse. In the High-energy peaked BL Lac objects (HBLs), the dominant seed photons for Comptonization appear to be the synchrotron photons internal…
We address several criticisms by Amelino-Camelia of our recent analyses of two observational constraints on Lorentz violation at order E/M_{Planck}. In particular, we emphasize the role of effective field theory in our analysis of synchrotron radiation, and we strengthen the justification for the constraint coming from photon annihilation.
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