TeV Blazars Research Articles

TeV Blazars as the Sources of Ultra High Energy Cosmic Rays

The origin of ultra high energy cosmic rays (UHECRs) is still an open question in astroparticle physics. TeV blazars are a small group of active galactic nuclei (AGNs). They all have been observed in TeV gamma ray band, and show violent variabilities in flux at all wavelengths. So it is believed that they have abilities to produce UHECRs. To judge whether the TeV blazars can be the candidates of the origin of UHECRs, we collect the information of emission region of 38 TeV blazars, and estimate the maximum energy that the charged particle can be accelerated there. The results show that TeV blazars have abilities to accelerate cosmic rays to the energy above 1018 eV, some even higher than 1020 eV, and they may be the sources of UHECRs.

THE INFLUENCE OF PLASMA EFFECTS OF PAIR BEAMS ON THE INTERGALACTIC CASCADE EMISSION OF BLAZARS

The attenuation of TeV γ-rays from distant blazars by the extragalactic background light (EBL) produces relativistic electron-positron pair beams. It has been shown by Broderick et. al. (2012) and Schlickeiser et. al (2012) that a pair beam traversing the intergalactic medium is unstable to linear two-stream instabilities of both electrostatic and electromagnetic nature. While for strong blazars all free pair energy is dissipated in heating the intergalactic medium and a potential electromagnetic cascade via inverse-Compton scattering with the cosmic microwave background is suppressed, we investigate the case of weak blazars where the back reaction of generated electrostatic turbulence leads to a plateauing of the electron energy spectrum. In the ultra-relativistic Thomson limit we analytically calculate the inverse-Compton spectral energy distribution for both an unplateaued and a plateaued beam scenario, showing a peak reduction factor of Rpeak ≈ 0.345. This is consistent with the FERMI non-measurements of a GeV excess in the spectrum of EBL attenuated TeV blazars. Claims on the lower bound of the intergalactic magnetic field strengths, made by several authors neglecting plasma effects, are thus put into question.

Improved characterization of intranight optical variability of prominent AGN classes

The incidence of intra-night optical variability (INOV) is known to to differ significantly among different classes of powerful active galactic nuclei (AGN). A number of statistical methods have been employed in the literature for testing the presence of INOV in the light curves, sometimes leading to discordant results. In this paper we compare the INOV characteristics of six prominent classes of AGN, as evaluated using three commonly used statistical tests, namely the $\chi^2-$test, the modified $C-$test and the $F-$test, which has recently begun to gain popularity. The AGN classes considered are: radio-quiet quasars (RQQs), radio-intermediate quasars (RIQs), lobe-dominated quasars (LDQs), low optical polarization core-dominated quasars (LPCDQs), high optical polarization core-dominated quasars (HPCDQs), and TeV blazars. Our analysis is based on a large body of AGN monitoring data, involving 262 sessions of intra-night monitoring of a total 77 AGN, using 1-2 metre class optical telescopes located in India. In order to compare the usefulness of the statistical tests, we have also subjected them to a `sanity check' by comparing the number of false positives yielded by each test with the corresponding statistical prediction. The present analysis is intended to serve as a benchmark for future INOV studies of AGN of different classes.

Cosmological magnetic field correlators from blazar induced cascade

TeV blazars offer an exciting prospect for discovering cosmological magnetic fields and for probing high-energy processes, including $CP$ violation, in the early Universe. We propose a method for reconstructing both the nonhelical and the helical magnetic field correlators using observations of cascade photons from TeV blazars.

The broad band spectral index of TeV blazars detected by Fermi LAT

Using γ-ray data detected by Fermi Large Area Telescope (LAT) and multi-wave band data for 35 TeV blazars sample, we have studied the possible correlations between different broad band spectral indices ( $\alpha_{\rm r.ir}$ , $\alpha_{\rm{r.o}}$ , $\alpha_{\rm r.x}$ , $\alpha_{\rm r.\gamma}$ , $\alpha_{\rm{ir.o}}$ , $\alpha_{\rm ir.x}$ , $\alpha_{\rm ir.\gamma}$ , $\alpha_{\rm o.x}$ , $\alpha_{\rm o.\gamma}$ , $\alpha_{\rm r.x}$ , $\alpha_{\rm x.\gamma}$ ) in all states (average/high/low). Our results are as follows: (1) For our TeV blazars sample, the strong positive correlations were found between $\alpha_{\rm r.ir}$ and $\alpha_{\rm{r.o}}$ , between $\alpha_{\rm r.ir}$ and $\alpha_{\rm r.x}$ , between $\alpha_{\rm r.ir}$ and $\alpha_{\rm r.\gamma}$ in all states (average/high/low); (2) For our TeV blazars sample, the strong anti-correlations were found between $\alpha_{\rm r.ir}$ and $\alpha_{\rm x.\gamma}$ , between $\alpha_{\rm{r.o}}$ and $\alpha_{\rm ir.\gamma}$ , between $\alpha_{\rm{r.o}}$ and $\alpha_{\rm o.\gamma}$ , between $\alpha_{\rm{r.o}}$ and $\alpha_{\rm x.\gamma}$ , between $\alpha_{\mathrm{ir.o}}$ and $\alpha_{\rm o.\gamma}$ , between $\alpha_{\rm r.x}$ and $\alpha_{\rm x.\gamma}$ , between $\alpha_{\rm ir.x}$ and $\alpha_{\rm x.\gamma}$ in all states (average/high/low). The results suggest that the synchrotron self-Compton radiation (SSC) is the main mechanism of high energy γ-ray emission and the inverse Compton scattering of circum-nuclear dust is likely to be a important complementary mechanism for TeV blazars. Our results also show that the possible correlations vary from state to state in the same pair of indices, Which suggest that there may exist differences in the emitting process and in the location of the emitting region for different states.

The Stability of BVRI Comparison Stars Near Selected TeV Blazars

ABSTRACTWe have measured Johnson BV and Cousins RI magnitudes for comparison stars near 5 TeV blazars. We compare our values with published values, spanning 25 years in some cases, to identify those stars that are most likely proven stable. To avoid zero-point offsets mimicking long-term variability, we based our analysis on the standard deviation between measurements after a mean offset between data sets was removed. We found most stars to be stable at the 0.04 mag level. We confirm two stars as variable and identify two others as possibly being variable. In each of the five fields there are at least two stars, and typically many more, that show no evidence of variability.

TeV gamma rays from blazars beyondz=1?

At TeV energies, the gamma-ray horizon of the universe is limited to redshifts z<<1, and, therefore, any observation of TeV radiation from a source located beyond z=1 would call for a revision of the standard paradigm. While robust observational evidence for TeV sources at redshifts z>1 is lacking at present, the growing number of TeV blazars with redshifts as large as z~0.5 suggests the possibility that the standard blazar models may have to be reconsidered. We show that TeV gamma rays can be observed even from a source at z>1, if the observed gamma rays are secondary photons produced in interactions of high-energy protons originating from the blazar jet and propagating over cosmological distances almost rectilinearly. This mechanism was initially proposed as a possible explanation for the TeV gamma rays observed from blazars with redshifts z~0.2, for which some other explanations were possible. For TeV gamma-ray radiation detected from a blazar with z>1, this model would provide the only viable interpretation consistent with conventional physics. It would also have far-reaching astronomical and cosmological ramifications. In particular, this interpretation would imply that extragalactic magnetic fields along the line of sight are very weak, in the range 0.01 < fG < 10 fG, assuming random fields with a correlation length of 1 Mpc, and that acceleration of E> 0.1 EeV protons in the jets of active galactic nuclei can be very effective.

DETERMINATION OF THE POINT-SPREAD FUNCTION FOR THEFERMILARGE AREA TELESCOPE FROM ON-ORBIT DATA AND LIMITS ON PAIR HALOS OF ACTIVE GALACTIC NUCLEI

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from 20 MeV to > 300 GeV. The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests. The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of \gamma rays from pulsars and active galactic nuclei (AGN). Above 3 GeV, the PSF is found to be broader than the pre-launch PSF. We checked for dependence of the PSF on the class of \gamma-ray source and observation epoch and found none. We also investigated several possible spatial models for pair-halo emission around BL Lac AGN. We found no evidence for a component with spatial extension larger than the PSF and set upper limits on the amplitude of halo emission in stacked images of low and high redshift BL Lac AGN and the TeV blazars 1ES0229+200 and 1ES0347-121.

BLAZAR ANTI-SEQUENCE OF SPECTRAL VARIABILITY FOR INDIVIDUAL TeV BLAZARS

We compile from literature the broadband SEDs of twelve TeV blazars observed simultaneously or quasi-simultaneously with Fermi/LAT and other instruments. Two SEDs are available for each of the objects and the state is identified as a low or high state according to its flux density at GeV/TeV band. The observed SEDs of BL Lac objects (BL Lacs) are fitted well with the synchrotron + synchrotron-self-Compton (syn+SSC) model, whereas the SEDs of the two flat spectrum radio quasars (FSRQs) need to include the contributions of external Compton scattering. In this scenario, it is found that the Doppler factor δ of FSRQs is smaller than that of BL Lacs, but the magnetic field strength B of FSRQs is larger than that of BL Lacs. The increase of the peak frequency of the SEDs is accompanied with the increase of the flux for the individual sources, which seems opposite to the observational phenomena of the blazar sequence. We refer this phenomenonto blazar anti-sequence of spectral variability for individual TeV blazars. However, both the blazar sequence from FSRQs to BL Lacs and blazar anti-sequence of the spectral variability from low state to high state are accompanied by an increase of the break Lorentz factor of the electron's spectrum γ b and a decrease of B. We propose a model in which the mass accretion rate Ṁ is the driving force behind both the blazar sequence for ensembles of blazars and the blazar anti-sequence for individual blazars. Specifically we suggest that the differences in 〈Ṁ〉 of different blazars produce the observed blazar sequence, but ΔṀ in each blazar results in the observed blazar anti-sequence.

The Parsec-Scale Structure of the Newer TeV Blazars

We expand our previous studies of the parsec-scale structure of TeV blazars by presenting first-epoch images from VLBA monitoring of ten newer TeV HBLs. All ten sources were successfully detected and imaged, and all showed a one-sided core-jet structure. Many display a morphology common to TeV HBLs: a short, collimated jet followed by a transition to low surface brightness extended emission with a much broader opening angle. The newly detected TeV HBLs tend to be fainter in the radio; the median core flux density was 22 mJy, and the median brightness temperature was 8x10^9 K. The brightness temperatures are well below the equipartition limit, and thus the VLBI cores do not require strong beaming, consistent with the modest values of Doppler and Lorentz factors found in the VLBI jets of TeV HBLs by other studies, and contrasting with the strong beaming generally required by the TeV emission. We study the full sample of TeV HBLs that have been observed with VLBI, and find a correlation between TeV flux and VLBI core brightness temperature, suggesting different but correlated beaming factors for the TeV and radio emission. We present a discussion of these observations in the context of velocity structures in the jets of the TeV HBLs.

A large rotation of the polarization angle of the TeV Blazar W Comae

Optical R-band polarimetric variability observations of the TeV Blazar W Comae are presented. Data obtained from 2008 February to 2013 May (∼5.2 years) are analyzed. The source presented a maximum flux variability of∼3 mJy. The minimum variability time scale displayed by the source during the monitored period ist∼ 3.3 d. A maximum linear polarisation degree value of P= (33.8± 1.6)% was observed in 2013 May 12. A rotation of the position angle from 78 ◦ (2008 March 10) to 229 ◦ (2008 July 11) was observed. This rotation corresponds to a large change of �� ∼ 150 ◦ in a period of 123 d or to a rotation of∼1.2 ◦ per day. After the high activity state observed in 2008, the position angle shows a preferential value of ∼ 56 ◦ , with variations of ∼60-120 ◦ . From the Stokes parameters we infer the existence of two optically-thin synchrotron components that contribute to the optical polarized flux. One of them is stable, with P∼11%.

First Results on Long-Term Multiwavelength Variability Analysis in Mrk 421

Due to the broad-band nature of the emission from blazars, strong efforts are being placed in recent years on simultaneous multiwavelength campaigns as a way to understand the mechanisms responsible for the acceleration of ultra-relativistic particles in these objects. The present work aims at providing a systematic study of the broadband long-term variability properties of the TeV blazar, Mrk 421, by gathering archival multiwavelength data spanning nearly two decades in time over several energy bands, from radio frequencies through x-rays to high and very high γ -ray energies. The lightcurves at these energies have been characterized over long periods (≈months) using several statistical methods, in order to study variability over long-term timescales and the dependence of this variability with flux and energy, aiming ultimately at shedding some light onto the physical mechanisms that drive blazar emission. We present in this work the preliminary results of the variability analysis for Mrk 421, and it will be extended in the future to see if the properties found are particular of this source or if they can be extended to the blazar class.

RAPID TeV GAMMA-RAY FLARING OF BL LACERTAE

We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was observed during a 34.6-minute exposure, when the integral flux above 200 GeV reached $(3.4\pm0.6) \times 10^{-6} \;\text{photons}\;\text{m}^{-2}\text{s}^{-1}$, roughly 125% of the Crab Nebula flux measured by VERITAS. The light curve indicates that the observations missed the rising phase of the flare but covered a significant portion of the decaying phase. The exponential decay time was determined to be $13\pm4$ minutes, making it one of the most rapid gamma-ray flares seen from a TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft, with a photon index of $3.6\pm 0.4$, which is in agreement with the measurement made previously by MAGIC in a lower flaring state. Contemporaneous radio observations of the source with the Very Long Baseline Array (VLBA) revealed the emergence of a new, superluminal component from the core around the time of the TeV gamma-ray flare, accompanied by changes in the optical polarization angle. Changes in flux also appear to have occurred at optical, UV, and GeV gamma-ray wavelengths at the time of the flare, although they are difficult to quantify precisely due to sparse coverage. A strong flare was seen at radio wavelengths roughly four months later, which might be related to the gamma-ray flaring activities. We discuss the implications of these multiwavelength results.

Intranight optical variability of -ray-loud narrow-line Seyfert 1 galaxies

The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope has detected γ-ray emission in about half a dozen narrow-line Seyfert 1 (NLSy1) galaxies. This indicates the presence of relativistic jets in these sources similar to blazars and radio galaxies. In an attempt to have an idea of the intranight optical variability (INOV) characteristics of these γ-ray-loud NLSy1 galaxies, we have carried out optical flux monitoring observations of three NLSy1 galaxies detected by Fermi/LAT: 1H 0323+342, PMN J0948+0022 and PKS 1502+036. These optical monitoring observations in RC band carried out during 2012 January–May showed the presence of rapid optical flux variations in these sources. The intranight differential light curves of these sources have revealed flux variations on time-scales of hours with amplitudes of variability >3 per cent for most of the time. However, for one source, PMN J0948+0022, we observed amplitude of variability as large as 52 per cent. On using the F-statistics to classify the variability nature of these sources, we obtained a duty cycle (DC) of INOV of ∼85 per cent. Alternatively, the more commonly used C-statistics gave a DC of INOV of ∼57 per cent. Such high DC of INOV is characteristics of the BL Lac class of active galactic nucleus. The results of our monitoring observations thus indicate that there is similarity in the INOV nature of γ-ray-loud NLSy1 galaxies and BL Lac objects, arguing strongly for the presence of relativistic jets aligned closely to the observers line of sight in γ-ray-loud NLSy1s. Moreover, our dense monitoring observations on some of the nights have led to the clear detection of some miniflares superimposed on the flux variations during the night over time-scales as short as 12 min. The detection of short time-scale flux variability in the sources studied here is clearly due to stronger time compression leading to the jets in these sources having large Doppler factors, similar to that of the inner jets of TeV blazars.

Constraining Intergalactic Magnetic Field with Fermi-LAT Observation of Cascade Radiation for TeV Blazars

The problem concerning the origin of the Intergalactic Magnetic Field (IGMF) is one of the long-standing problems of astrophysics and cosmology, and direct measurements are difficult. TeV photons emitted by TeV blazars produce electron-positron pairs because of interactions with the extragalactic background light (EBL). These pairs emit secondary cascade gamma-rays via Inverse Compton scattering of the cosmic microwave background (CMB) photons. In this process, the trajectories of the pairs are deviated by the IGMF, and the cascade gamma-ray emission appears as extended emission around TeV source. We used the EBL, synchrotron self-Compton (SSC) model and the observed limits on Lorentz factor of electron-positron pairs to calculate the cascade-radiation spectrum, and then to fit the observed GeV to TeV and multi-waveband spectra of TeV blazars to constrain the IGMF. We obtained the GeV energy spectra of three TeV blazars by analyzing the Fermi-LAT data of the past $ \sim$ 3 yr. The flux upper limits of Fermi-LAT in the 90% significant level of 1ES 0229$ +$ 200 suggests that the IGMF is stronger than 2 $ \times$ 10$ ^{-18}$ G for an engine time of TeV activity with three years. The relationships between the deduced lower limits of IGMF and various engine times for 1ES 0229$ +$ 200 and 1ES 0347$-$ 121 are presented by us.

THE PAIR BEAM PRODUCTION SPECTRUM FROM PHOTON-PHOTON ANNIHILATION IN COSMIC VOIDS

Highly beamed relativistic e ±-pair energy distributions result in double photon collisions of the beamed gamma rays from TeV blazars at cosmological distances with the isotropically distributed extragalactic background light (EBL) in the intergalactic medium. The typical energies k 0 10–7 in units of mec 2 of the EBL are more than 10 orders of magnitude smaller than the observed gamma-ray energies k 1 ≥ 107. Using the limit k 0 k 1, we demonstrate that the angular distribution of the generated pairs in the lab frame is highly beamed in the direction of the initial gamma-ray photons. For the astrophysically important case of power-law distributions of the emitted gamma-ray beam up to the maximum energy M interacting with Wien-type N(k 0)∝kq 0exp (– k 0/Θ) soft photon distributions with total number density N 0, we calculate analytical approximations for the electron production spectrum. For distant objects with luminosity distances dL r 0 = (σ T N 0)–1 = 0.49N –1 0 Mpc (with Thomson cross section σ T ), the implied large values of the optical depth τ0 = dL /r 0 indicate that the electron production spectra differ at energies inside and outside the interval [(Θln τ0)–1, τ0/Θ], given the maximum gamma-ray energy M Θ–1. In the case M Θ–1, the production spectrum is strongly peaked near E Θ–1, being exponentially reduced at small energies and decreasing with the steep power law ∝E –1 – p up to the maximum energy E = M – (1/2).

Seminal magnetic fields from inflato-electromagnetic inflation

We extend some previous attempts to explain the origin and evolution of primordial magnetic fields during inflation induced from a 5D vacuum. We show that the usual quantum fluctuations of a generalized 5D electromagnetic field cannot provide us with the desired magnetic seeds. We show that special fields without propagation on the extra non-compact dimension are needed to arrive to appreciable magnetic strengths. We also identify a new magnetic tensor field $B_{ij}$ in this kind of extra dimensional theories. Our results are in very good agreement with observational requirements, in particular from TeV Blazars and CMB radiation limits we obtain that primordial cosmological magnetic fields should be close scale invariance.

On proton synchrotron blazar models: the case of quasar 3C 279

In the present work we propose an innovative estimation method for the minimum Doppler factor and energy content of the gamma-ray emitting region of quasar 3C 279, using a standard proton synchrotron blazar model and the principles of automatic photon quenching. The latter becomes relevant for high enough magnetic fields and results in spontaneous annihilation of gamma-rays. The absorbed energy is then redistributed into electron-positron pairs and soft radiation. We show that as quenching sets an upper value for the source rest-frame gamma-ray luminosity, one has, by neccessity, to resort to Doppler factors that lie above a certain value in order to explain the TeV observations. The existence of this lower limit for the Doppler factor has also implications on the energetics of the emitting region. In this aspect, the proposed method can be regarded as an extension of the widely used one for estimating the equipartition magnetic field using radio observations. In our case, the leptonic synchrotron component is replaced by the proton synchrotron emission and the radio by the VHE gamma-ray observations. We show specifically that one can model the TeV observations by using parameter values that minimize both the energy density and the jet power at the cost of high-values of the Doppler factor. On the other hand, the modelling can also be done by using the minimum possible Doppler factor; this, however, leads to a particle dominated region and high jet power for a wide range of magnetic field values. Despite the fact that we have focused on the case of 3C 279, our analysis can be of relevance to all TeV blazars favoring hadronic modelling that have, moreover, simultaneous X-ray observations.

Gamma-ray and neutrino backgrounds as probes of the high-energy universe: hints of cascades, general constraints, and implications for TeV searches

Recent observations of isotropic diffuse backgrounds by Fermi and IceCube allow us to get more insight into distant very-high-energy (VHE) and ultra-high-energy (UHE) gamma-ray/neutrino emitters, including cosmic-ray accelerators/sources. First, we investigate the contribution of intergalactic cascades induced by gamma-rays and/or cosmic rays (CRs) to the diffuse gamma-ray background (DGB) in view of the latest Fermi data. We identify a possible ``VHE Excess" from the fact that the Fermi data are well above expectations for an attenuated power law, and show that cascades induced by VHE gamma rays (above ∼ 10 TeV) and/or VHECRs (below ∼ 1019 eV) may significantly contribute to the DGB above ∼ 100 GeV. The relevance of the cascades is also motivated by the intergalactic cascade interpretations of extreme TeV blazars such as 1ES 0229+200, which suggest very hard intrinsic spectra.This strengthens the importance of future detailed VHE DGB measurements. Then, more conservatively, we derive general constraints on the cosmic energy budget of high-energy gamma rays and neutrinos based on recent Fermi and IceCube observations of extragalactic background radiation. We demonstrate that these multi-messenger constraints are useful and the neutrino limit is very powerful for VHE/UHE hadronic sources. Furthermore, we show the importance of constraints from individual source surveys by future imaging atmospheric Cherenkov telescopes such as Cherenkov Telescope Array, and demonstrate that the cascade hypothesis for the VHE DGB can be tested by searching for distant emitters of cascaded gamma rays.

Constraints on the intergalactic magnetic field from gamma-ray observations of TeV blazars

We discuss different approaches to infer the properties of the intergalactic magnetic field (IGMF) from gamma-ray observations of blazars. In particular, we investigate constraints on the IGMF strength and spacial distribution, resulting from studies of TeV blazars by imaging atmospheric Cherenkov telescopes and the Fermi-LAT instrument. We demonstrate that the non-observation of GeV gamma-rays from powerful TeV blazars indicates that more than 60% of space is filled by magnetic fields with stength ≳ 10−15 G, favoring the primordial IGMF origin.