Searches for very high energy gamma rays from blazars with CANGAROO-III telescope in 2005-2009

We have searched for very high energy (VHE) gamma rays from four blazars using the CANGAROO-III imaging atmospheric Cherenkov telescope. We report the results of the observations of H 2356-309, PKS 2155-304, PKS 0537-441, and 3C 279, performed from 2005 to 2009, applying a new analysis to suppress the effects of the position dependence of Cherenkov images in the field of view. No significant VHE gamma ray emission was detected from any of the four blazars. The GeV gamma-ray spectra of these objects were obtained by analyzing Fermi/LAT archival data. Wide range (radio to VHE gamma-ray bands) spectral energy distributions (SEDs) including CANGAROO-III upper limits, GeV gamma-ray spectra, and archival data, even though they are non-simultaneous, are discussed using a one-zone synchrotron self-Compton (SSC) model in combination with a external Compton (EC) radiation. The HBLs (H 2356-309 and PKS 2155-304) can be explained by a simple SSC model, and PKS 0537-441 and 3C 279 are well modeled by a combination of SSC and EC model. We find a consistency with the blazar sequence in terms of strength of magnetic field and component size.     

Linkage between accretion disks and blazars

The magnetic field in an accretion disk is estimated assuming that all of the angular momentum within prescribed accretion disk radii is removed by a jet. The magnetic field estimated at the base of the jet is extrapolated to the blazar emission region using a model for a relativistic axisymmetric jet combined with some simplifying assumptions based on the relativistic nature of the flow. The extrapolated magnetic field is compared with estimates based upon the synchrotron and inverse Compton emission from three blazars, MKN 501, MKN 421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-Compton models are inconsistent with the magnetic fields extrapolated in this way. However, in two cases inverse Compton models in which a substantial part of the soft photon field is generated locally agree well, mainly because these models imply magnetic field strengths consistent with an important Poynting Flux component. This comparison is based on estimating the mass accretion rate from the jet energy flux. Further comparisons along these lines will be facilitated by independent estimates of the mass accretion rate in blazars and by more detailed models for jet propagation near the black hole.     

Optical quasi-periodic oscillation and color behavior of blazar PKS 2155–304

PKS 2155-304 is a well studied BL Lac object in the southern sky. The historical optical data during different periods have been collected and compiled. Light curves spanning 35 yr have been constructed. The R-band light curve has been analyzed by means of three methods: the epoch folding method,the Jurkevich method and the discrete correlation function method. It is derived that there is an evident periodic component of 317 d(i.e. 0.87 yr) superposed on a long-term trend with largeamplitude variation in the light curve. The variability of this source is accompanied by a slight color variation,and the brightness and color index are correlated with each other. On a long-term time scale,PKS 2155-304 exhibits a tendency of bluer-whenbrighter,which means the spectrum becomes flatter when the source brightens.     

Acceleration of particles in the vicinity of a massive black hole

Recent results of the gamma-ray Cherenkov astronomy definitely prove the existence of fast variability in the very high energy (V.H.E.) gamma-ray flux of some active galactic nuclei. The BL Lac PKS 2155-304 for instance showed variations down to a few minutes time scale. From standard light travel time argument, these variations put extremely strong constraints on the size of the TeV emitting zone, which has to be of the order of a few Schwarzschild radius, even for high values of the relativistic Doppler factor of the emitting jets. Such discovery is a challenge for particle acceleration scenarios, which have to imagine efficient acceleration processes at work in a very compact zone. Eventually, the immediate vicinity of the central black hole appears as the most conservative choice for the location of the TeV emission region of active galactic nuclei. In this paper, we propose a two-step mechanism for charged particle acceleration in the magnetosphere of a massive black hole surrounded by an accretion disk. Particles first gain energy by a stochastic process during the accretion phase. It is shown that effective proton acceleration up to energies 10¹⁷–10¹⁹ eV is possible in a low-luminosity magnetized accretion disk with 2D turbulent motion. The distribution function of energetic protons over energies is a power law function with typical index ≃−1. Here electrons are not very efficiently accelerated because of their drastic losses by synchrotron radiation. In a second time, part of the fast particles escape from the disk and are then entrained by the magnetic structure above the disk, in the rotating black hole magnetosphere. They thus gain additional energy by direct centrifugal mechanism, up to about 10²⁰ eV for the protons and to 10–100 TeV for the electrons when they cross the light cylinder surface. Such energetic particles can further radiate in the TeV spectral range observed by Cherenkov experiments as HESS, MAGIC and VERITAS. Energetic protons can produce γ-radiation in the energy band 1 GeV–100 TeV and above mainly by nuclei collisions with the disk matter, clouds, or ambient low energy photons. Energetic electrons can also reach the required spectral range by inverse Compton emission. However their acceleration is less efficient due to heavy radiation losses, and only gained by centrifugal process during the second phase of the whole mechanism we describe. Our present analysis would therefore favor hadronic scenarios for TeV emission of active galactic nuclei. It is tempting to relate long term variability over years of TeV active galactic nuclei to the first stochastic acceleration phase, which also provides the needed power law particle distributions, while short term variability over minutes is more likely due to perturbations of the second fast direct acceleration phase.     

Variability in the southern blazars PKS 1921-293 and PKS 2155-152

We report results of 1435 MHz rapid variability observations of the southern extragalactic sources PKS 1921-293 and PKS 2155-152. Both objects displayed variability in their flux densities on timescales of the order of one and two weeks, respectively. A brief discussion on the origin of this variability is presented, with a comparison of different models that could account for the observations.Fellow of CONICET.     

X-ray observation of AGN's from TENMA

In order to investigate the structure and mechanism of active galactic nuclei through X-ray observation, detailed energy spectra are examined with the gas scintillation proportional counters on board TENMA. For three selected objects, PKS 2155-304, Centaurus A and IC 4329 A, the observational results in the intensity variation, the spectral shapes and the iron line features are presented.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

X-ray and very high energy gamma-ray emission from Mrk 421 during 2005

Results are reported from a study of the variability of the x-ray and very high energy (VHE) γ -ray emission from Mrk 421 during a 10-day period of enchanced source activity at the end of 2005. The TeV data were taken from observations with the Whipple 10-m Cerenkov telescope and the soft x-ray data, from the ASM on board the RXTE orbital observatory. Light curves for this source indicated an obvious variability in the form of flares in both energy ranges. During the flares, the flux of VHE γ -rays exceeded that from the Crab nebula and correlated positively with the observed x-ray emission. The flux of VHE γ - rays varied almost linearly with the soft x-ray flux. The statistical characteristics of the variability were independent of energy and the two emission components were detected quasi-simultaneously. The observed temporal properties of the source can be interpreted in terms of the rapid acceleration of leptons in a jet using a synchrotron self-Compton model. Our studies confirm a flare activity intrinsic to this source on the scale of a day found in earlier studies.     

Is the GeV-TeV emission of PKS 0447-439 from the proton synchrotron radiation?

We study the multi-wavelength emission features of PKS 0447-439 in the frame of the one-zone homogeneous lepto-hadronic model. In this model, we assumed that the steady power-laws with exponential cut-offs distributions of protons and electrons are injected into the source. The non-linear time-dependent kinematic equations, describing the evolution of protons, electrons and photons, are defined; these equations self-consistently involve synchrotron radiation of protons, photon-photon interaction, synchrotron radiation of electron/positron pairs, inverse Compton scattering and synchrotron self-absorption. The model is applied to reproduce the multi-wavelength spectrum of PKS 0447-439. Our results indicate that the spectral energy distribution (SED) of PKS 0447-439 can be reproduced well by the model. In particular, the GeV-TeV emission is produced by the synchrotron radiation of relativistic protons. The physically plausible solutions require the magnetic strength \(10~\text{G}\lesssim B \lesssim 100~\text{G}\). We found that the observed spectrum of PKS 0447-439 can be reproduced well by the model whether \(z = 0.16\) or \(z = 0.2\), and the acceptable upper limit of redshift is \(z=0.343\).     

The X-ray jet and halo of PKS 0521−365

Chandra ACIS observations of PKS 0521−365 find that the X-ray emission of this BL Lac object consists of emission from an unresolved core, a diffuse halo and a 2-arcsec jet feature coincident with the inner radio/optical jet. A comparison with a new ATCA 8.6-GHz map also finds X-ray emission from the bright hotspot south-east of the nucleus. The jet spectrum, from radio to X-ray, is probably synchrotron emission from an electron population with a broken power-law energy distribution, and resembles the spectra seen from the jets of low-power (FR I) radio galaxies. The hotspot X-ray flux is consistent with the expectations of synchrotron self-Compton emission from a plasma close to equipartition, as seen in studies of high-power (FR II) radio galaxies. While the angular structure of the halo is similar to that found by an analysis of the ROSAT High Resolution Imager image, its brightness is seen to be lower with Chandra , and the halo is best interpreted as thermal emission from an atmosphere of similar luminosity to the haloes around FR I radio galaxies. The X-ray properties of PKS 0521−365 are consistent with it being a foreshortened, beamed, radio galaxy.     

Particle-acceleration time-scales in TeV blazar flares

Observations of minute-scale flares in TeV Blazars place constraints on particle-acceleration mechanisms in those objects. The implications for a variety of radiation mechanisms have been addressed in the literature; in this paper, we compare four different acceleration mechanisms: diffusive shock acceleration, second-order Fermi, shear acceleration and the converter mechanism. When the acceleration time-scales and radiative losses are taken into account, we can exclude shear acceleration and the neutron-based converted mechanism as possible acceleration processes in these systems. The first-order Fermi process and the converter mechanism working via synchrotron self-Compton (SSC) photons are still practically instantaneous, however, provided sufficient turbulence is generated on the time-scale of seconds. We propose stochastic acceleration as a promising candidate for the energy-dependent time delays in recent gamma-ray flares of Markarian 501.     

Multifrequency emission analysis of TeV blazars H 2356-309 and 1ES 1218+304

The multiband nonthermal emissions in radio,X-ray,and very high-energy (VHE)γ-ray bands from two distant blazars,H 2356-309 and 1ES 1218+304,have been detected,and,especially from recent observations with the Suzaku,MAGIC and VERITAS telescopes,clearly reveal nonthermal power-law spectra. We study the broadband nonthermal spectra of the two sources by using a combination of a onezone homogeneous synchrotron self-Compton (SSC) model and an inhomogeneous conical jet model,where the new external background light (EBL) model is taken into account. The results show that (1) the nonthermal emissions of the two blazars,ranging from X-rays to VHE γ-rays,are from the homogeneous zone whereas the emissions in the radio bands can be explained as the radiation from the inhomogeneous conical jet; (2) a strict lower-limit EBL model can be used to explain their observed spectra well.     

Early results from the Chandra X-ray observatory

We present some early results on AGN from the Chandra X-ray Observatory, highlighting high resolution spectroscopy using the High Energy Transmission Grating Spectrometer (HETGS). The quasar PKS 0637–752 was found to have a very bright X-ray jet whose shape is remarkably similar to that of the radio jet on a size scale of 100 kpc, but the X-ray emission is still inexplicably bright. Two BL Lac objects, PKS 2155–304 and Mrk 421, observed with the spectrometer were found to have no strong absorption or emission features. Other radio loud AGN observed with the HETGS show simple power law spectra without obvious features.     

Optical variability of gamma-ray loud blazars

Blazars display flux variability on diverse timescales ranging from minutes to months. In our blazar monitoring project carried out on the 1.2meter telescope at Gurushikhar, Mount Abu, India, we selected someγ-ray loud blazars to study the variability over both the short and the long time scales. In this paper we have reported results based on photometric monitoring on 8 nights during the observing run in the first half of the year 2000 for 5 blazars: S5 0716+714, OJ 287, S4 0954+658, Mrk421 and PKS 2155–304 in B and R passbands. Microvariability and rapid variability has been observed in these sources suggesting that variability is a common property in these objects. This revised version was published online in July 2006 with corrections to the Cover Date.     

The optical spectral slope variability of 17 blazars

Many quasi-simultaneous optical observations of 17 blazars are obtained from previous papers published over the last 19 years in order to investigate the spectral slope variability and understand the radiation mechanism of blazars. The long-period dereddened optical spectral slopes are calculated. We analyse the average spectral slope distribution, which suggests that the spectra of flat spectrum radio quasars (FSRQs) and high energy peaked BL Lac objects (HBLs) are probably deformed by other emission components. The average spectral slopes of low energy peaked BL Lac objects(LBLs), which scatter around 1.5, show a good accordance with the synchrotron self-Compton (SSC) loss-dominated model. We present and discuss the variability between the spectral slope and optical luminosity. The spectra of all HBLs and LBLs get flatter when they turn brighter, while for FSRQs this trend does not exist or may even be reversed. This phenomenon may imply that there is a thermal contribution to the optical spectrum for FSRQs. For the FSRQ 1156+295, there is a hint that the slope gets flatter at both the brightest and faintest states. Our result shows that three subclasses locate in different regions in the pattern of slope variability indicator versus average spectral slope. The relativistic jet mechanism is supported by the significant correlation between the optical Doppler factor and the average spectral slope.     

The properties of jet in luminous blazars under the equipartition condition

In this work, we study the physical properties of the high-energy (HE) emission region by modeling the quasi-simultaneous multi-wavelength(MWL) spectral energy distributions (SEDs) of 27 Fermi-LAT detected low-synchrotron-peaked (LSP) blazars. We model the jets MWL SEDs in framework of a well accepted single-zone leptonic model including synchrotron self-Compton and external Compton (EC) processes for the jets in a state of equipartition between particle and magnetic field energy densities. In the model the GeV γ-ray spectrum is modeled by a combination of two different external Compton-scattered components: (i) EC scattering of photons coming from disk and broad line region (BLR), and (ii) EC scattering of photons originating from the dust tours (DT) and BLR. We find that the SEDs can be well reproduced by the equipartition model for the most majority of the sources, and the results are in agreement with many recent studies. Our results suggest that the SEDs modelling alone may not provide a significant constraint on the location of the HE emission region if we do not know enough about the physical properties of the external environment.     

Quasi-Periodic Oscillations in the X-ray Light Curves of Blazars

Any quasi-periodic variations discovered in blazar light curves would contain important information on the location and nature of the processes within the emission regions. In non-blazar active galactic nuclei, particularly Seyfert galaxies, any such fluctuations are very likely to be associated with the accretion disks, but in blazars they would almost certainly have to emanate from jets. This brief review summarizes recent claims for the presence of quasi-periodic variability in the X-ray emission of several AGN, focusing on blazars. Although no individual claim of the presence of a QPO in AGN X-ray light curves is absolutely convincing, there are some good cases for the presence of QPOs, including the Narrow Line Seyfert 1 galaxy, RE J1034+396, the quasar, 3C 273 and the BL Lac, PKS 2155?304.     

Multi-component study of extended sources with a likelihood method

The spectral and morphological analysis for gamma-ray sources with multiple emission components remains a major challenge for Cherenkov telescopes due to background emission from diffuse gamma rays. Current methods of background suppression, usually based on the bin-by-bin subtraction of OFF-source data do not allow an analysis of the various background components. As an alternative, we present an approach based on an event-by-event likelihood fit of ON-source data, using a combined spectral model for the source emission as well as for the gamma-like background obtained from fits of the OFF-source data. Multiple emission components are separated by successive fits in different energy regimes and spectral variations inside the extended source is derived. The performance of this approach is evaluated with toy Monte-Carlo studies. For the application to real data, two well-studied H.E.S.S. sources are re-examined: the extragalactic point-source PKS 2155-304 and the extended pulsar wind nebula HESS J1825-137. For the latter, radial variation of the emission spectral index was evaluated, confirming earlier findings by the H.E.S.S. collaboration.     

The brightness temperature problem in extreme intra-day variable quasars: a model for PKS 0405-385

I re-examine the brightness temperature problem in PKS 0405-385, which is an extreme intra-day variable radio quasar with an inferred brightness temperature of  ∼5 × 10¹⁴ K  at 5 GHz, well above the Compton catastrophe limit of  ∼10¹¹ K  that is reached when the synchrotron photon energy density exceeds the energy density of the magnetic field. If one takes into account the uncertainty in the distance to the ionized clouds responsible for interstellar scintillation causing rapid intra-day variability in PKS 0405-385, it is possible that the brightness temperature could be as low as  ∼10¹³ K  at 5 GHz, or even lower. The radio spectrum can be fitted by optically thin emission from mono-energetic electrons, or an electron spectrum with a low-energy cut-off such that the critical frequency of the lowest energy electrons is above the radio frequencies of interest. If one observes optically thin emission along a long narrow emission region, the average energy density in the emission region can be many orders of magnitude lower than calculated from the observed intensity if one assumed a spherical emission region. I discuss the physical conditions in the emission region and find that the Compton catastrophe can then be avoided using a reasonable Doppler factor. I also show that MeV to 100-GeV gamma-ray emission at observable flux levels should be expected from extreme intra-day variable sources such as PKS 0405-385.     

Stochastic Particle Acceleration in Blazar Jets

1 INTRoDUCTIONB1azars are rwho-loud AGNs characterized by emissions of strong and raPidiy wriablenOllthermal radiation over the elltire electromagntic spectrum. Syndritron ehasha followedby inverse ComPton scattering in a re1aivistic jet and beamd inio one directiOn is generallythought to be the IneCha8m powering these Objects (Kollgaard 1994; Urry & Paded 1995).All blazars have a sPectral energy distribution (SED) with tWO peak8 in a uFv rePesentation(von Montigny et al. 1995; S…