Blazar Sequence Research Articles

VLA Observations of a New Population of Blazars

We present the first deep VLA radio images of flat-spectrum radio quasars (FSRQs) with multiwavelength emission properties similar to those of BL Lacertae (BL Lac) objects with synchrotron X-rays. Our observations of 25 of these sources show that their radio morphologies are similar to those of other radio quasars. However, their range of extended powers is more similar to that of BL Lac objects and extends down to the low values typical of FR I radio galaxies. Five out of our nine lobe-dominated sources have extended radio powers in the range typical of both FR I and FR II radio galaxies, but their extended radio structure is clearly FR II-like. Therefore, we have not yet found a large population of radio quasars hosted by FR I galaxies. Two-thirds of our sources have a core-dominated radio morphology and thus X-rays likely dominated by the jet. We find that their ratios of radio core to total X-ray luminosity are low and in the regime indicative of synchrotron X-rays. This result shows that also blazars with strong emission lines can produce jets of high-energy synchrotron emission, and it undermines at least in part the blazar sequence scenario, which advocates the position that particle Compton cooling by an external radiation field governs the frequency of the synchrotron emission peak.

Spectral energy distributions of a large sample of BL Lacertae objects

We have collected a large amount of multifrequency data for the objects in the Metsahovi Radio Observatory BL Lacertae sample and computed their spectral energy distributions (SED). This is the first time the SEDs of BL Lacs have been studied with a sample of over 300 objects. The synchrotron components of the SEDs were fitted with a parabolic function to determine the synchrotron peak frequency. We checked the dependence between luminosities at several frequency bands and synchrotron peak frequency to test the blazar sequence scenario, which states that the source luminosity depends on the location of the synchrotron peak. We also calculated broad band spectral indices and plotted them against each other and the peak frequency. The range of peak frequencies in our study was considerably extended compared to previous studies. There were 22 objects for which log\nu_{peak}>19. The data shows that at 5 GHz, 37 GHz and 5500 A there is negative correlation between luminosity and nu_{peak}. There is no significant correlation between source luminosity at synchrotron peak and peak frequency. Several low radio luminosity-low energy peaked BL Lacs were found. The negative correlation between broad band spectral indices and nu_{peak} is also significant, although there is substantial scatter. Therefore we find that neither alpha_{rx} nor alpha_{ro} can be used to determine the synchrotron peak of BL Lacs. On the grounds of our results we conclude that the blazar sequence scenario is not valid. In all our results the BL Lac population is continuous with no hint of the bimodality of the first BL Lac samples.

The Infrared‐dominated Jet of 3C 401

We present a Hubble Space Telescope image of the FR II radio galaxy 3C 401, obtained at 1.6 μm with NICMOS, in which we identify the infrared counterpart of the brightest region of the radio jet. The jet has a complex radio structure and brightens where bending occurs, most likely as a result of relativistic beaming. We analyze archival data in the radio, optical, and X-ray bands, and we derive its spectral energy distribution. Unlike all of the previously known optical extragalactic jets, the jet in 3C 401 is not detected in the X-rays, even in a long 48 ks X-ray Chandra exposure, and the infrared emission dominates the overall spectral energy distribution (SED). We propose that the dominant radiation mechanism of this jet is synchrotron. The low X-ray emission is then caused by two different effects: (1) the lack of any strong external photon field and (2) the shape of the electron distribution. This affects the location of the synchrotron peak in the SED, resulting in a sharp cutoff at energies lower than the X-rays. Thus 3C 401 shows a new type of jet, with intermediate spectral properties between those of FR I galaxies, which are dominated by synchrotron emission up to X-ray energies, and FR II galaxies/QSOs, which show strong high-energy emission due to inverse Compton scattering of external photons. This might indicate the presence of a continuous in the properties of large-scale jets, analogous to the blazar sequence already proposed for subparsec-scale jets.

The basic parameters ofγ-ray-loud blazars

We determined the basic parameters, such as the central black hole mass (M), the boosting factor (or Doppler factor) (δ), the propagation angle (Φ) and the distance along the axis to the site of γ-ray production (d) for 23 γ-ray-loud blazars using their available variability timescales. In this method, the absorption effect depends on the γ-ray energy, emission size and property of the accretion disk. Using the intrinsic γ-ray luminosity as a fraction λ of the Eddington luminosity, and the optical depth equal to unity, we can determine the upper limit of the central black hole masses. We found that the black hole masses range between and when and 1.0 are adopted. Since this method is based on gamma-ray emissions and the short time-scale of the sources, it can also be used for central black hole mass determination of high redshift gamma-ray sources. In the case of the upper limit of black hole mass there is no clear difference between BLs and FSRQs, which suggests that the central black hole masses do not play an important role in the evolutionary sequence of blazars.

The sedentary survey of extreme high energy peaked BL Lacs

The multi-frequency Sedentary Survey is a deep, statistically complete, radio flux limited sample comprising 150 BL Lacertae objects distinguished by their extremely high X-ray to radio flux ratio ( fx/ fr), ranging from five hundred to over five thousand times that of typical BL Lacs discovered in radio surveys. This large excess of high energy photons compared to radio emission is thought to be due to synchrotron radiation that in these sources reaches the UV or the X-ray band. The name Sedentary Survey originates from the multi-frequency technique used to select the sample that was expected to be so efficient as to allow the conduction of some preliminary statistical studies even without the need to identify the candidates through optical spectroscopy. The details of the selection criteria and the preliminary results have been published in Giommi et al. (1999, MNRAS, 310, 465). In this paper we present the final, 100% identified, catalog together with the optical, X-ray and broad-band Spectral Energy Distributions (SED) constructed combining literature multi-frequency data with non-simultaneous optical observations and BeppoSAX X-ray data, when available. The SEDs confirm that the peak of the synchrotron power in these objects is located at very high energies. BeppoSAX wide band X-ray observations show that, in most cases, the X-ray spectra are convex and well described by a logarithmic parabola model peaking (in a ν f (ν )v s .ν representation) between 0.02 to several keV. Although detailed X-ray spectral data are available for only about one fifth of the sources the observed peaks never reach energies well above 10 keV (as in Mkn 501 during the large X-ray flare of April 1997 and in 1ES 2344+514 in December 1996) implying that hard X-ray synchrotron peak energies are rare and probably associated with strong flaring events. Owing to the high synchrotron energies involved most of the sources in the catalog are likely to be TeV emitters, with the closest and brightest ones probably detectable by the present generation of Cherenkov telescopes. However, only 50% (3 out of 6) of the presently established TeV BL Lacs are actually included in the survey suggesting that the hardest peaks may be associated with secondary synchrotron components that can be detected only above the soft X-ray band. The existence of secondary emission regions is suggested by the strong X-ray spectral curvature that in some objects predicts an optical flux much below the observed emission. The optical spectrum of about one fourth of the sources is totally featureless hampering any redshift or luminosity determination. Because this implies that the non-thermal nuclear emission must be well above that of the host galaxy, these objects are likely to be the most powerful sources in the survey and therefore be examples of the yet unreported high radio luminosity-high energy peaked BL Lacs. The existence of such objects would be at odds with the claimed inverse proportionality between radio power and synchrotron peak energy known as the blazar sequence. At the low-power end of the luminosity dynamical range, where the non-thermal optical continuum falls below the emission from the host galaxy, recognition issues start becoming important since BL Lacs in this luminosity regime can hardly be recognized as such, but rather as radio galaxies or simply as elliptical galaxies. We have found a small sample of bright nearby elliptical galaxies that are candidate low radio power high energy peaked BL Lacs.

The recognition of blazars and the blazar spectral sequence

We analyse a group of radio sources, a subset of the 200-mJy sample, all of which have core‐jet radio structures measured with very long baseline interferometry and have flat spectra stretching from the radio to the millimetre/submillimetre band. Thus the objects have most of the properties expected of blazars. However, they display varied optical properties ranging from ‘Seyfert-like’ objects, through BL Lac objects, to ‘normal’ elliptical galaxies. We investigate the distribution of synchrotron peak frequencies in their spectral energy distributions (SEDs) and find a broad distribution between 10 12 and 10 16 Hz. Our conclusion is that we should consider virtually all objects in the sample as blazars since much of the diversity in their classification based on traditional optical criteria arises from differences in the frequency at which the nonthermal emission begins to decline. Specifically, an object is only classified as BL Lac when its peak frequency falls in the near-infrared/optical range. We determine peak frequencies using the same method for objects from other blazar samples. An important result is that our objects do not follow the blazar spectral sequence proposed by Fossati et al. and Ghisellini et al. in which, on average, peak frequencies increase as the radio luminosity decreases. Most of our low radio luminosity sources have peaks in their SEDs at low frequencies, not at the expected high frequencies. We suggest that at least part of the systematic trend seen by Fossati et al. and Ghisellini et al. results from selection effects. Ke yw ords: galaxies: active ‐ BL Lacertae objects: general.

Internal shocks and the blazar sequence

We consider the internal shock model as the dissipation mechanism responsible for the emission in blazars. It has been shown that this model is successful in reproducing the observed spectral energy distribution and the variability properties of a powerful blazar like 3C 279. However, the blazar family covers a wide range of spectral characteristics which appear to be correlated and the whole class can be seen as a sequence: the frequency and the intensity of the low energy versus high energy peak intensity increase with decreasing luminosity. We show that the internal shock model can satisfactorily account also for the properties of the low power blazars like BL Lac and Mkn 421 and it is successful in reproducing the blazar sequence.

The high energy view of blazars

Abstract Beppo SAX contributed substantially to our understanding of the physics of blazars. This has been made possible mainly by its wide energy range and especially by its high energy detector. Together with the information coming from still higher energies we know at last the entire spectral energy distribution (SED) of blazars. Different subclasses of blazars have different SEDs, which seem to form a sequence, whose main parameter is the bolometric luminosity. Physically, the blazar sequence can be the result of different cooling rates by those electrons emitting most of the radiation we see. Blazars are among the most active sources we know of, and the coordinated variability at high energies gives very important clues about the physics of their jets: initially, they must transport energy in a dissipationless way. They radiate most of the entire bolometric luminosity we see at a preferred distance of a few hundreds of Schwarzschild radii. For powerful blazars, the emitted radiation must be a small fraction of the power transported by the jet, since the bulk of it is required to energize the radio lobes. These are the jets which likely have bulk relativistic speeds up to hundreds of kiloparsecs, as suggested by the bright X–ray knots detected by Chandra in many extended jets.

The CLASS blazar survey: testing the blazar sequence

We discuss the properties of the sources in the CLASS blazar survey, which aims at the selection of low radio power (P 5 GHz < 10 25 W Hz −1 ) blazars. We use Very Large Array (VLA) data from available catalogues and from our own observations to constrain the radio core-dominance of the sample which, together with the flat radio spectral index, is a signature of the blazar activity. X-ray data from the ROSAT All Sky Survey were also collected in order to constrain the radio-to-X-ray luminosity ratio (α RX ) of the sources. The data analysis shows that more than 30 per cent of sources at low radio power (P 5 GHz < 10 25 W Hz −1 ) have an α RX steeper than that expected in the framework of the ‘blazar sequence’ recently put forward to unify the high- and low-power blazars. The possibility that this result is influenced by contaminating sources in the current sample is discussed. The conclusion is that, even if a number of non-blazars (typically CSO/GPS sources) are expected in the survey, it is unlikely that this constitutes the sole reason for the observed deviation. In particular, we show two examples for which the blazar nature is confirmed from VLBI data and for which the steep α RX (suggesting a synchrotron peak frequency below 10 15.5 Hz) and the low radio power (0.6–2 × 10 24 W Hz −1 ) put these sources outside the ‘blazar sequence’. The results presented here show the importance of a correct and unbiased sampling of the low-power regime of the blazar population.

Extreme blazars

TeV emission can be a common characteristic of low power blazars. This is in line with the sequence of blazars relating the observed bolometric luminosity with their overall spectral energy distribution. Detecting new TeV blazars, possibly at different redshifts, is important for studying the particle acceleration process operating in the jet, and it is even more important for studying the far infrared-optical background. Recent studies of low power BL Lacs suggest that the particle acceleration mechanism may be a two-step process, analogous to what is invoked to explain the spectra of γ-ray bursts. I will briefly discuss these findings, together with a simple scheme for selecting good TeV candidates and I will briefly comment on the very fast TeV and X-ray variability observed in BL Lacs.

The Mass-Luminosity Relation, Accretion Rate-Luminosity Relation, and Evolutionary Sequence of Blazars

A sample of 39 blazars each with a well-established rapid variability timescale and bolometric luminosity has been compiled from the literature. Based on the assumption that central supermassive black holes (SMBHs) are Kerr black holes, the upper limits of the SMBHs were estimated. The masses ranged from 10(7.2) to 10(9.4) M-circle dot, showing a distribution of three subclasses: massive flat-spectrum radio quasars (FSRQs) and smaller mass BL Lac objects occupying separate regions, while medium-mass FSRQs and BL Lac objects bridge the gap. We found that the mass-intrinsic luminosity relations for FSRQs and BL Lac objects are the same. Analysis reveals a relationship of log (L-in/L-circle dot)=1.225 log (M/M-circle dot) + 1.500, which mimics a relationship found in the main sequence. We found that the intrinsic accretion rates are quite different between FSRQs and BL Lac objects. The diagram of the intrinsic accretion rate-luminosity relation shows that FSRQs occur in the earlier, high-luminosity, violent phase of the galactic evolution sequence, while BL Lac objects occur in the low-luminosity, late phase of the galactic evolution sequence. Of note is that the results of the mass-luminosity relation of blazars are consistent with the accretion rate-luminosity relation, and together they prove that active galactic nuclei evolve from FSRQs to BL Lac objects. The evolution diagram of blazars derived in this paper seems to be a Hertzsprung-Russell diagram of star evolution.

What Types of Jets Does Nature Make? A New Population of Radio Quasars

We use statistical results from a large sample of about 500 blazars, based on two surveys, the Deep X-Ray Radio Blazar Survey (DXRBS), nearly complete, and the ROSAT All-Sky Survey-Green Bank Survey (RGB), to provide new constraints on the spectral energy distribution of blazars, particularly flat-spectrum radio quasars (FSRQs). This reassessment is prompted by the discovery of a population of FSRQs with spectral energy distribution similar to that of high-energy-peaked BL Lac objects. The fraction of these sources is sample dependent, being ~10% in DXRBS and ~30% in RGB (and reaching ~80% for the Einstein Medium Sensitivity Survey). We show that these "X-ray-strong" radio quasars, which had gone undetected or unnoticed in previous surveys, indeed are the strong-lined counterparts of high-energy-peaked BL Lac objects and have synchrotron peak frequencies, νpeak, much higher than "classical" FSRQs, typically in the UV band for DXRBS. Some of these objects may be 100 GeV to TeV emitters, as are several known BL Lac objects with similar broadband spectra. Our large, deep, and homogeneous DXRBS sample does not show anticorrelations between νpeak and radio, broad-line region, or jet power, as expected in the so-called blazar sequence scenario. However, the fact that FSRQs do not reach X-ray-to-radio flux ratios and νpeak values as extreme as BL Lac objects and the elusiveness of high-νpeak high-power blazars suggest that there might be an intrinsic, physical limit to the synchrotron peak frequency that can be reached by strong-lined, powerful blazars. Our findings have important implications for the study of jet formation and physics and its relationship to other properties of active galactic nuclei.

Low power BL Lacertae objects and the blazar sequence

The spectral properties of blazars seem to follow a phenomenological sequence according to the source luminosity. By inferring the source physical parameters through (necessarily) modeling of the blazar spectra, we have previously proposed that the sequence arises because the particles responsible for most of the emission suffer increasing radiative losses as the luminosity increases. Here we extend those results by considering the widest possible range of blazar spectral properties. We find a new important ingredient for shaping the spectra of the lowest power objects, namely the role of a finite timescale for the injection of relativistic particles. Only high energy particles radiatively cool over such a timescale leading to a break in the particle distribution: particles with this break energy are those emitting most of the power, and this gives rise to a link between blazar spectra and total energy density inside the source, which controls the cooling timescale. The emerging picture requires two phases for the particle acceleration: a first pre–heating phase in which particles reach a characteristic energy as the result of balancing heating and radiative cooling, and a more rapid acceleration phase which further accelerate these particles to form a power law distribution. While in agreement with standard shock theory, this scenario also agrees with the idea that the luminosity of blazars is produced through internal shocks, which naturally lead to shocks lasting for a finite time.

Evidence for the Evolutionary Sequence of Blazars: Different Types of Accretion Flows in BL Lacertae Objects

The limits on the mass of the black hole in 23 BL Lac objects are obtained from their luminosities of the broad emission line Hbeta on the assumption that broad emission lines are emitted from clouds ionized by the radiation of the accretion disk surrounding a black hole. The distribution of line luminosity L-Hbeta of all these BL Lac objects suggests a bimodal nature, although this cannot be statistically proven on the basis of the present, rather small sample. We found that standard thin disks are probably in the sources with L-Hbeta > 10(41) ergs s(-1). The central black holes in these sources have masses of 10(8)-10(10) M., if the matter is accreting at the rate of 0.025(M) over dot (Edd.). For the sources with L-Hbeta < 10(41) ergs s(-1), the accretion flows have transited from standard thin-disk type to the advection- dominated accretion flow type. The lower limits on the mass of the black hole in these sources are in the range of (1.66-24.5) x 10(8) M.. The results support the evolutionary sequence of blazars from flat-spectrum radio quasar to low-energy-peaked BL Lac object to high-energy-peaked BL Lac object.