Peaked BL Lac Objects Research Articles

Search for TeV Gamma-Rays from PKS 2155-304 and PKS 2005-489 during Quiescent States

The high-energy peaked BL Lac objects PKS 2155-304 and PKS 2005-489 have been observed with the CANGAROO-II imaging atmospheric Cherenkov telescope in 2000 and 2001. These sources remained in a quiescent state during our observations. We find no evidence for VHE gamma-ray emission from these sources. We obtained 2σ flux limits of 7.0 x 10 - 1 2 cm - 2 sec- 1 above 570 GeV for PKS 2005-489, and 8.2 x 10- 1 2 cm - 2 sec - 1 (2000) and 9.5 x 10 - 1 2 cm - 2 sec - 1 (2001) above 420 GeV for PKS2155-304, respectively.

The central engines of radio-loud quasars

We have assembled a sample of 37 radio-loud quasars that have been imaged with the Hubble Space Telescopein order to investigate their black hole masses, accretion rates, and the structure of their accretion disks. The black hole masses were estimated from the luminosities of the host galaxies, and the accretion powers were extrapolated from the emission-line luminosities. The majority of the quasars have masses in the range MBH 10 8 10 9 M. Their accretion rates, ˙ M 0:01 1 times the Eddington rate, suggest that most of the objects possess standard optically thick, geometrically thin accretion disks, in some cases perhaps accompanied by an optically thin advection-dominated component. The coexistence of strong radio emission and a standard disk conflicts with recent models for jet formation. We discuss modifications of the standard model that can resolve this discrepancy. We find there is a strong correlation between the accretion rate and the extended radio luminosity. This lends support to the idea that the extended radio emission is somehow linked to the accretion disk. Lastly, we combine the present sample of radio-loud quasars with the sample of BL Lac objects studied by Wang et al. (2002) to reevaluate the unification picture for radio-loud active galactic nuclei. Consistent with current ideas for the unification of radio-loud sources, we find that flat-spectrum radio quasars and FR II radio galaxies indeed seem to belong to the same population, as do BL Lac objects and FR I radio galaxies on the opposite end of the luminosity spectrum. However, some members of the low frequency- peaked BL Lac objects may be more closely associated with FR II rather than FR I radio galaxies. We describe how the various subclasses of radio-loud sources can be viewed as a continuous sequence of varying accretion rate.

BeppoSAXObservations of Synchrotron X‐Ray Emission from Radio Quasars

We present new BeppoSAX Low Energy Concentrator Spectrometer (LECS), Medium Energy Concentrator Spectrometer (MECS), and Phoswich Detector System (PDS) observations of four flat-spectrum radio quasars (FSRQs) having effective spectral indices αro and αox typical of high-energy peaked BL Lac objects. Our sources have X-ray-to-radio flux ratios on average ~70 times larger than "classical" FSRQs and lie at the extreme end of the FSRQ X-ray-to-radio flux ratio distribution. The collected data cover the energy range 0.1-10 keV (observer's frame), reaching ~100 keV for one object. The BeppoSAX band in one of our sources, RGB J1629+4008, is dominated by synchrotron emission peaking at ~2 × 1016 Hz, as also shown by its steep (energy index αX ~ 1.5) spectrum. This makes this object the first known FSRQ whose X-ray emission is not due to inverse Compton radiation. Two other sources display a flat BeppoSAX spectrum (αX ~ 0.7), with weak indications of steepening at low X-ray energies. The combination of BeppoSAX and ROSAT observations, (nonsimultaneous) multifrequency data, and a synchrotron inverse Compton model suggests synchrotron peak frequencies ≈1015 Hz, although a better coverage of their spectral energy distributions is needed to provide firmer values. If confirmed, these values would be typical of "intermediate" BL Lac objects for which the synchrotron and inverse Compton components overlap in the BeppoSAX band. Our sources, although firmly in the radio-loud regime, have powers more typical of high-energy peaked BL Lac objects than of FSRQs, and indeed their radio powers put them near the low-luminosity end of the FSRQ luminosity function. We discuss this in terms of an anticorrelation between synchrotron peak frequency and total power, based on physical arguments, and also as possibly due to a selection effect.

G[CLC]e[/CLC]V Emission from T[CLC]e[/CLC]V Blazars and Intergalactic Magnetic Fields

Several high-frequency peaked BL Lac objects such as Mrk 501 are strong TeV emitters. However, a significant fraction of the TeV gamma rays emitted are likely to be absorbed in interactions with the diffuse IR background, yielding electron-positron pairs. Hence, the observed TeV spectrum must be steeper than the intrinsic one. Using the recently derived intrinsic gamma-ray spectrum of Mrk 501 during its 1997 high state, we study the inverse Compton scattering of cosmic microwave photons by the resulting electron-positron pairs, which implies the existence of a hitherto undiscovered GeV emission. The typical duration of the GeV emission is determined by the flaring activity time and the energy-dependent magnetic deflection time. We numerically calculate the scattered photon spectrum for different intergalactic magnetic field (IGMF) strengths and find a spectral turnover and flare duration at GeV energies that are dependent on the field strength. We also estimate the scattered photon flux in the quiescent state of Mrk 501. The GeV flux levels predicted are consistent with existing EGRET upper limits and should be detectable above the synchrotron self-Compton component with the Gamma-Ray Large Area Space Telescope for IGMFs 10-16 G, as expected in voids. Such detections would provide constraints on the strength of weak IGMFs.

X‐Ray Spectral Variability Signatures of Flares in BL Lacertae Objects

We are presenting a detailed parameter study of the time-dependent electron injection and kinematics and the self-consistent radiation transport in jets of intermediate and low-frequency peaked BL Lac objects. Using a time-dependent, combined synchrotron-self-Compton and external-Compton jet model, we study the influence of variations of several essential model parameters, such as the electron injection compactness, the relative contribution of synchrotron to external soft photons to the soft photon compactness, the electron- injection spectral index, and the details of the time profiles of the electron injection episodes giving rise to flaring activity. In the analysis of our results, we focus on the expected X-ray spectral variability signatures in a region of parameter space particularly well suited to reproduce the broadband spectral energy distributions of intermediate and low-frequency peaked BL Lac objects. We demonstrate that SSC- and external-Compton dominated models for the gamma-ray emission from blazars are producing significantly different signatures in the X-ray variability, in particular in the soft X-ray light curves and the spectral hysteresis at soft X-ray energies, which can be used as a powerful diagnostic to unveil the nature of the high-energy emission from BL Lac objects.

BeppoSAX spectral survey of BL Lacs – New spectra and results

We present BeppoSAX LECS, MECS, and PDS spectra of eleven X-ray selected BL Lacertae objects. Combining these sources with the ones presented elsewhere we have a sample of 21 BL Lacs from the Einstein Medium Sensitivity and Einstein Slew Survey. The sample shows strong correlations of several physical parameters with the peak frequency of the synchrotron branch of the spectral energy distribution. In particular the peak frequency is correlated to the X-ray spectral shape: objects with the peak near to the X-ray band show harder and straighter X-ray spectra than those of the low energy peaked sources. This work shows that the recently proposed unification scenario for different types of blazars can hold also within the class of high frequency peaked BL Lac objects.

The spectral energy distribution of AO 0235+164 during two active optical phases

We investigate the spectral variability of the source AO 0235+164 during two active phases. We find that the peak frequencies in the Spectral Energy Distributions (SED) are variable with time within a range of an order of magnitude, which does not cause the source to make a transition from Low-energy peaked BL Lac objects to High-energy peaked BL Lac objects. The correlation of peak frequency and peak flux either for an individual outburst or the combination of several individual outbursts is significant. Our research reveals that in many physical properties AO 0235+164 is much closer to flat-spectrum radio quasars, rather than to High-energy peaked BL Lacs, which in turn implies that Comptonization of external radiation may play quite an important role in γ-ray emission. There are indications that the observed variability may be due to superposition of intrinsic and propagation effects, which is in agreement with the conclusion of Kraus et al. ([CITE]). We also explore the possible origin of the two outbursts and suggest that intrinsic variability, not including the shock effect, probably accounts for the two outbursts.

Hard X-ray properties of blazars

We have considered all blazars observed in the X{ray band and for which the slope of the X{ray spectrum is available. We have collected 421 spectra of 268 blazars, including 12 archival unpublished ASCA spectra of 7 blazars whose analysis is presented here. The X{ray spectra of blazars show trends as a function of their power, conrming that the blazar overall energy distribution can be parameterized on the basis of one parameter only, i.e. the bolometric luminosity. This is conrmed by the relatively new hard (2{10 keV) X{ray data. Our results conrm the idea that in low power objects the X{ray emission mechanism is the synchrotron process, dominating both the soft and the hard X{ray emissions. Low energy peaked BL Lac objects are intermediate, often showing harder spectra in the hard X{ray band, suggesting that the synchrotron process dominates in the soft band, with the inverse Compton process dominating at high energies. The most powerful objects have X{ray spectra that are flat both in the soft and in the hard band, consistent with a dominating inverse Compton component.

The Properties of the X-Ray-selected EMSS Sample of BL Lacertae Objects

We present updated and complete radio, optical, and X-ray data for BL Lac objects in the Einstein Medium Sensitivity Survey (EMSS). The complete sample first presented in Morris et al. is updated to include 26 BL Lac objects in total, and we define a new, virtually complete sample consisting of 41 EMSS BL Lac objects (the sample). New high signal-to-noise ratio, arcsecond-resolution VLA observations are also presented for 11 EMSS BL Lac objects, completing VLA observations of the M91 sample. The addition of four new objects, as well as updated X-ray flux and redshift information, has increased the Ve/Va value for the M91 sample to 0.399 ± 0.057 and Ve/Va = 0.427 ± 0.045 for the newly defined D40 sample. In conjunction with other studies of X-ray–selected BL Lac (XBL) samples, these results solidify negative evolution for XBLs, especially for more extreme high-energy–peaked BL Lac objects, for which we find Ve/Va = 0.271 ± 0.077. The observed Ve/Va, spectral, and radio properties of XBLs are completely consistent with being the beamed population of low-luminosity, FR-1 radio galaxies. However, our VLA observations do confirm that XBLs are too core dominated to be consistent with a beamed population of FR-1's seen at intermediate angles, as suggested by the unified model, if XBLs have moderate outflow velocities (γ ~ 5).

ROSAT/ASCAObservations of a Serendipitous BL Lacertae Object PKS 2316−423: The Variable High‐Energy Tail of Synchrotron Radiation

We present the analysis of archival data from ROSAT and ASCA of a serendipitous BL Lac object PKS 2316-423. Because of its featureless nonthermal radio/optical continuum, PKS 2316-423 has been called a BL Lac candidate in the literature. PKS 2316-423 was evidently variable over the multiple X-ray observations; in particular, a variable high-energy tail of the synchrotron radiation is revealed. The X-ray spectral analysis provides further evidence of the synchrotron nature of its broadband spectrum: a steep and downward-curving spectrum between 0.1 and 10 keV, typical of high-energy peaked BL Lac objects (HBLs). The spectral energy distribution (SED) through radio to X-ray yields the synchrotron radiation peak at frequency νp = 7.3 × 1015 Hz, with integrated luminosity of Lsyn = 2.1 × 1044 ergs s-1. The averaged SED properties of PKS 2316-423 are very similar to those intermediate BL Lac objects (IBLs) found recently in several deep surveys, such as Deep X-ray Radio Blazar, Radio-Emitting X-ray, and ROSAT-Green Bank surveys. We suggest that PKS 2316-423 is an IBL though it also shows some general features of an HBL. Actually, this double attribute of PKS 2316-423 provides a good test of the prediction that an IBL object can show either synchrotron or inverse Compton characteristics in different variability states.

A Rapid X-ray Flare from Markarian 501.

We present X-ray observations of the BL Lacertae object Mrk 501 that were taken with the Rossi X-Ray Timing Explorer in 1998 May as part of a multiwavelength campaign. The X-ray light curve shows a very rapid flare in which the 2-10 keV flux increased by approximately 60% in less than 200 s. This rapid rise is followed by a drop-off in the 2-10 keV flux of approximately 40% in less than 600 s. The 10-15 keV variation in this flare is roughly a factor of 2 on similar timescales. During the rise of the flare, the 3-15 keV spectral index hardened from 2.02+/-0.03 to 1.87+/-0.04, where it remained during the decay of the flare. This is the fastest variation ever seen in X-rays from Mrk 501 and among the fastest seen at any wavelength for this object. The shift in the energy at which the spectral power peaks (from less, similar3 keV before the flare to greater, similar30 keV during the flare) is also among the most rapid shifts seen from this object. This flare occurs during an emission state (2-10 keV flux approximately 1.2x10-10 ergs cm-2 s-1) that is approximately 25% of the peak flux observed in 1997 April from this object but that is still high compared to its historical average X-ray flux. The variations in the hardness ratio are consistent with the low-energy variations leading those at high energies during the development and decay of the flare. This pattern is rare among high-frequency peaked BL Lac objects like Mrk 501, but has been seen recently in two other TeV-emitting BL Lac objects, Mrk 421 and PKS 2155-304. The hard lag is consistent with a flare dominated by the acceleration timescale for a simple relativistic shock model of flaring.

The broad band X-ray spectra of intermediate BL Lac objects

We present recent X-ray observations of intermediate BL Lac objects (IBLs), i.e. BL Lacs which are located between high-energy and low-energy peaked BL Lac objects with respect to α rx. We briefly discuss the statistical properties of IBLs from the RGB sample and then focus on a detailed broad band spectral analysis of two objects, namely 1424+2401 and 1055+5644, which were observed with ASCA and SAX, respectively. In both cases the spectra are steep and we find significant curvature in 1424+2401 in the sense that the spectrum gets flatter at both low (< 1 keV) and high energies (> 5 keV). Our results are in line with the hypothesis of a continuous distribution of synchrotron peak frequencies among BL Lac objects.

X-ray variability in BL Lac objects

X-ray variability in high energy peaked BL Lac objects (HBLs) is characterized by a correlation between flux and index and by soft lag. It is shown that these properties can quantitatively be expressed by Fermi acceleration and synchrotron cooling model and physical parameters in accelerating and decelerating regions are derived by fitting the observed light curves with the model.

X-ray observations of TeV blazars and multi-frequency analysis

The nonthermal spectra of blazars, observed from radio to GeV/TeV γ-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 γ-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 to the jet (SSC process). In the quasar-hosted blazars (QHBs), on the other hand, the X-ray band emission is still dominated by the SSC process, while the MeV to GeV range is produced by Comptonization of external photons such as the emission line light. In the context of this three-component model, we derive the magnetic field of 0.1–1 Gauss for all classes of blazars. Lorentz factors γ peak of electrons radiating at each peak of the vF( v) spectra are estimated to be ∼ 10 5 for HBLs; this is much higher than ∼ 10 3 for QHBs. This difference is consistent with the fact that the four sources that are known to emit TeV γ-rays (TeV blazars) are all classified as HBLs. Among the TeV blazars, Mkn 421 is one of the brightest and most variable emitters from ultraviolet (eV) to hard γ-ray (TeV) energies, and its correlated inter-band variability suggests that both keV and TeV spectral regimes are produced by the same, most energetic end of the electron population radiating via the synchrotron process in the keV, and the SSC process in TeV band. The multi-frequency observations including TeV energy band provide the best opportunity to understand high-energy emission from blazar jets. In this paper, we discuss results of multi-frequency analysis and review the results of intensive campaigns for Mkn 421 from 1994 to 1998.

ASCAObservations of Blazars and Multiband Analysis

We present data for 18 blazars observed with ASCA, half of which were also observed contemporaneously with EGRET as parts of multi-wavelength campaigns. The observations show a clear difference in the spectra between three subclasses of blazars, namely the High-energy peaked BL Lac objects (HBLs), Low-energy peaked BL Lac objects (LBLs), and quasar-hosted blazars (QHBs). We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Synchrotron-Self-Compton (SSC) emission, with the Doppler factor consistent with the VLBI and/or gamma-ray variability data. For many QHBs, on the other hand, the gamma-rays cannot be solely due to the SSC mechanism unless the Doppler factor is significantly in excess of that inferred from VLBI data. We consider an alternative scenario consistent with the measured values of the Doppler factor, where the SSC component is still present in QHBs and it dominates in the X-ray band, but it is below the observed gamma-ray spectrum. With an assumption that the peak of the SSC emission is on the extrapolation of the X-ray spectrum, and adopting the Doppler factor of 10, we infer the magnetic field to be 0.1 - 1 Gauss, and Lorentz factors of electrons radiating at the peak of the nu F(nu) spectrum of 10^3 for QHBs; this is much lower than 10^5 for HBLs. This difference is most likely due to the large photon density expected in QHBs.