The high-frequency peaked BL Lac object 1ES 0229+200 (z = 0.14) was first detected in very high energy (VHE, E > 100 GeV) $γ$-rays by the H.E.S.S. (High Energy Stereoscopic System) collaboration in 2006. No flux variability was reported in the initial study and its spectral characteristics have been used to derive constraints on the extragalactic background light (EBL) and on the intergalactic magnetic field (IGMF). 1ES 0229+200 has been observed with H.E.S.S. for $\sim$130 hours from 2004 to 2013: the full dataset analysed with a more sensitive method will be presented here. The results indicate that the source is not constant and displays flux variability on yearly and monthly timescales. The existence of flux variability affects the derivation of the constraints on the IGMF. The H.E.S.S. observations cover several simultaneous multi-frequency campaigns and the VHE variations are compared with those reported in different bands.
We present periodicity search analyses on the $γ$-ray lightcurve of the TeV blazar PKS 1510-089 observed by the \textit{Fermi} Large Area Telescope. We report the detection of two transient quasi-periodic oscillations: a 3.6-day QPO during the outburst in 2009 that lasted five cycles (MJD 54906--54923); and a periodicity of 92 days spanning over 650 days from 2018 to 2020 (MJD 58200--58850), which lasted for seven cycles. We employed the Lomb-Scargle periodogram, Weighted Wavelet Z-transform, REDFIT, and the Monte Carlo lightcurve simulation techniques to find any periodicity and the correspon…
BL Lacertae objects (BL Lacs) is the dominant population of TeV emitting blazars. In this work, we investigate whether there is any special observational properties for TeV sources. To do so, we will compare the observational properties of TeV detected BL Lacs (TeV BLs) and non-TeV detected BL Lac objects (non-TeV BLs). From the 3rd $Fermi$/LAT catalog (3FGL), we can get 662 BL Lacs, out of which, 47 are TeV BLs and 615 are non-TeV BLs. Their multi-wavelength flux densities ($F_{\rm R}$, $F_{\rm O}$, $F_{\rm X}$, $F_γ$), photon spectral indexes ($α_{\rm X}^{\rm ph}$, $α_γ^{\rm ph}$), and effec…
We study the requirement on the jet power in the conventional p-$γ$ models (photopion production and Bethe-Heitler pair production) for TeV BL Lac objects. We select a sample of TeV BL Lac objects whose SEDs are difficult to be explained by the one-zone leptonic model. Based on the relation between the p-$γ$ interaction efficiency and the opacity of $γγ$ absorption, we find that the detection of TeV emission poses upper limits on the p-$γ$ interaction efficiencies in these sources and hence minimum jet powers can be derived accordingly. We find that the obtained minimum jet powers exceed the E…
Context. The intrinsic TeV emission of some BL Lacs are characterized by a hard spectrum (the hard-TeV spectrum) after correcting for the extragalactic background light. The hard-TeV spectra pose a challenge to conventional one-zone models, including the leptonic model, the photohadronic model, the proton synchrotron model, etc. Aims. In this work, we study if the one-zone hadronuclear (pp) model can be used to interpret the hard-TeV spectra of BL Lacs without introducing extreme parameters. Methods. We give analytical calculations to study if there is a parameter space and the charge neutrali…
BL Lac objects can be extreme in two ways: with their synchrotron emission, peaking beyond 1 keV in their spectral energy distribution, or with their gamma-ray emission, peaking at multi-TeV energies up to and beyond 10-20 TeV, like 1ES 0229+200. This second type of extreme BL Lacs --which we can name "TeV-peaked BL Lacs" is not well explained by the usual synchrotron self-Compton scenarios for BL Lacs. These sources are also important as probes for the intergalactic diffuse infrared background and cosmic magnetic fields, as well as possible sites of production of ultra-high-energy cosmic rays…
The extragalactic background light (EBL) in the IR to UV bands partly absorbs very high energy (VHE, $E \geq$ 100GeV) $γ-$ray photons travelling over cosmological distances via pair production. In this paper, to get stronger constraints on EBL, we use the deliberate selection of the EBL model and data of five BL Lacs with better statistics and the harder spectra to limit the EBL density and the radiation mechanism of BL Lacs. We constrain the upper limit of the EBL density by fitting the spectral energy distributions (SEDs) of TeV BL Lacs and find that our results are compatible with the publi…
In this paper, we compile the very-high-energy and high-energy spectral indices of 43 BL Lac objects from the literature. Based on a simple math model, $ΔΓ_{obs}=α{\rm{z}}+β$, we present evidence for the origin of an observed spectral break that is denoted by the difference between the observed very-high-energy and high-energy spectral indices, $ΔΓ_{obs}$. We find by linear regression analysis that $α\ne 0$ and $β\ne 0$. These results suggest that the extragalactic background light attenuation and the intrinsic curvature dominate on the GeV-TeV $γ$-ray energy spectral break of BL Lac objects. …
Modeling the three-dimensional pair cascade in binaries. Application to LS 5039
LS 5039是一个发射高能和极高能伽马射线的星系双星系统。伽马射线通量调制的轨道周期和TeV的光曲线形状的光子-光子湮灭。所观察到的非常高的能量调制可以用一个简单的轻子模型来重现，但无法解释HESS在超级合点检测到的通量，在那里伽马射线被完全吸收。来自电子-正电子对级联的贡献可能是强的，并且在优合处超过初级通量。所产生的电子对可以通过磁场各向同性化，从而产生三维级联。本文的目的是研究LS 5039中这对级联的γ射线辐射。这个附加的分量可以解释系统中上合点的HESS观测。本文提出了一种计算三维级联辐射的半解析和蒙特卡罗方法，并应用于双星系统。三维级联辐射在TeV光变曲线中的每个轨道相位都有显着的贡献，并且在接近高级结合处占主导地位。伽马射线调制的振幅在轨道倾角约为40度时能正确地再现。初级对应该被注入接近紧凑的对象的位置，否则调制的形状没有解释。此外，X射线中级联的同步加速器发射将环境磁场限制在10 G以下。在LS 5039中，HESS观测到的TeV通量可以用三维对级联辐射来解释，但在低通量下的极高能谱仍难以解释。
Blazars have been pointed out as promising high-energy (HE) neutrinos sources, although the mechanism is still under debate. The blazars with a hard-TeV spectrum, which leptonic models can hardly explain, can be successfully interpreted in the hadronic scenarios. Recently, Aguilar et al. proposed a lepto-hadronic two-zone model to explain the multi-wavelength observations of the six best-known extreme BL Lacs and showed that the hadronic component could mainly interpret very-high-energy (VHE) emission. In this work, we apply this hadronic model to describe the VHE gamma-ray fluxes of 14 extrem…
The spectral energy distribution (SED) of blazars is generally understood through the standard one-zone synchrotron self-Compton (SSC) model, where a strong correlation between X-ray and TeV gamma-ray fluxes is expected. Recently, for Mrk 421 the correlation was confirmed and interpreted within the SSC model arguing that a description of the correlation up to large flux values considerably restricts the value of the magnetic field under the assumption of an electron spectral index independent of the flux state. In this work, we extend the analysis done on Mrk 421 for other two high-energy peak…
The next generation of TeV detectors is expected to have a significantly enhanced performance. It is therefore constructive to search for new TeV candidates for observation. This paper focuses on TeV candidates among the high-synchrotron-peaked BL Lacertae objects (HBLs) reported in the fourth catalog of active galactic nuclei detected by the Fermi's Large Area Telescope, i.e., 4LAC. By cross-matching the Fermi data with radio and optical observations, we collected the multiwavelength features of 180 HBLs with known redshift. The data set contains 39 confirmed TeV sources and 141 objects whose…
We focus our analysis on 55 BL Lac objects with a hard Fermi gamma-ray spectrum, and for which a redshift or a lower limit to it has been determined by a previous study of ours. We extrapolate the spectral fits given by the 4FGL catalogue to the VHE band (>0.1 TeV), which can be explored by imaging atmospheric Cherenkov telescopes. Furthermore, we take into account the absorption due to the extragalactic background light, strongly depending on the redshift. Finally, we compare our results with publicly available sensitivity curves for a selection of imaging atmospheric Cherenkov telescope arra…
We fit the spectral energy distributions (SEDs) of 46 GeV - TeV BL Lac objects in the frame of leptonic one-zone synchrotron self-Compton (SSC) model and investigate the physical properties of these objects. We use the Markov Chain Monte Carlo (MCMC) method to obtain the basic parameters, such as magnetic field (B), the break energy of the relativistic electron distribution ($γ'_{\rm{b}}$) and the electron energy spectral index. Based on the modeling results, we support the following scenarios on GeV-TeV BL Lac objects: (1) Some sources have large Doppler factors, implying other radiation mech…
Cosmic opacity for very high-energy gamma rays ($E>10$ TeV) due to the interaction with the extragalactic background light can be strongly reduced because of possible Lorentz-violating terms in the particle dispersion relations expected, e.g., in several versions of quantum gravity theories. We discuss the possibility to use very high energy observations of blazars to detect anomalies of the cosmic opacity induced by LIV, considering in particular the possibility to use -- besides the bright and close-by BL Lac Mkn 501 -- extreme BL Lac objects. We derive the modified expression for the optica…
Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. In blazars, the spectral energy distribution (SED) is dominated by the non-thermal emission of the relativistic jet, and consists of two main broad humps. For the EHBLs, these two components peak in the X-ray and GeV-TeV bands, respectively. Although the number of TeV detected extreme blazars is very limited, recent observations by Imaging Atmospheric Cherenkov Telescopes (IACTs) have revealed that in some of them the energy of the second peak exceeds several TeV (e.g. 1ES 0…
Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic TeV spectrum, locating the peak of their gamma-ray emission in the spectral energy distribution (SED) above 2-10 TeV. Such high energies are problematic for the Compton emission, using a standard one-zone leptonic model. We study six examples of this new type of BL Lacs in the hard X-ray band with the NuSTAR satellite. Together with simultaneous observations with the SWIFT satellite, we fully constrain the peak of the synchrotron emission in their SED, and test the leptonic synchrotron self-Compton (SSC) model. We con…
Observations of fast TeV $γ$-ray flares from blazars reveal the extreme compactness of emitting regions in blazar jets. Combined with very-long-baseline radio interferometry measurements, they probe the structure and emission mechanism of the jet. We report on a fast TeV $γ$-ray flare from BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and a decay time of about 36 minutes. The peak flux at $>$200 GeV measured with the 4-minute binned light curve is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photons} \;\text{m}^{-2}\, \text{s}^{-1}$, or $\sim$180% the Crab Nebula flux. Variabil…
Very high energy (VHE; $E \geq$ 100GeV) $γ-$ray from cosmological distances are attenuated by the extragalactic background light (EBL) in the IR to UV band. By contrasting measured versus intrinsic emission, the EBL photon density can be derived. However, neither the intrinsic spectra nor the EBL is separately known~-~except their combined effect. Here we first present a flexible model-dependent optical depth method to study EBL by fitting the emission spectra of TeV BL Lacs objects via a one-zone leptonic synchrotron self-Compton model (SSC). We have little information about electron energy d…
Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. The non-thermal emission of the relativistic jet peaks in the spectral energy distribution (SED) plot with the synchrotron emission in X-rays and with the gamma-ray emission in the TeV range or above. These high photon energies may represent a challenge for the standard modeling of these sources. They are important for the implications on the indirect measurements of the extragalactic background light, the intergalactic magnetic field estimate, and the possible origin of ext…
We present a model-dependent method to estimate the redshifts of three TeV BL Lac objects (BL Lacs) through fltting their (quasi-) simultaneous multi-waveband spectral energy distributions (SEDs) by one-zone leptonic synchrotron self-Compton (SSC) model. Considering the impact of electron energy distributions (EEDs) on the results, we use three types of EEDs, such as the power-law EED with exponential cut-ofi (PLC), the log-parabola (PLLP) and the broken power-law (BPL) EEDs, to flt the SEDs. We also use a parameter fi to describe the uncertainties of the extragalactic background light (EBL) m…
High-frequency-peaked BL Lacs (HBLs) dominate the extragalactic TeV sky, with more than 50 objects detected by the current generation of TeV observatories. Still, the properties of TeV-emitting HBLs as a population are poorly understood due to biases introduced by the observing strategies of Cherenkov Telescopes, limiting our ability to estimate the potential contribution of TeV blazars to the diffuse neutrino, gamma-ray, and cosmic-ray background as well as their role in the late-stage evolution of active galactic nuclei. The VERITAS telescope array has designed a program to quantify and mini…
The emission of the so-called extreme BL Lacs poses challenges to the particle acceleration models. The hardness of their spectrum, $\lesssim 2$, in the high-energy band demands unusual parameters using the standard one-zone synchrotron self-Compton (SSC) model with a deficient magnetized plasma. Some authors use either two-zone or hadronic/lepto-hadronic models to relax these atypical values. In this work, we present a lepto-hadronic two-zone model to explain the multi-wavelength observations of the six best-known \textit{extreme} BL Lacs. The very-high-energy gamma-ray observations are descr…
We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. We wish to understand if there are differences in the BL Lac objects that are detected with the current-generation TeV instruments compared to those that have not yet been detected. The mean polarization fraction of the TeV-detected BL Lacs is 5% while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host gal…
We investigate the physical properties of \textit{Fermi} TeV BL Lac objects jets by modeling the quasi-simultaneous spectral energy distribution of 29 \textit{Fermi} TeV BL Lacs in the frame of a one-zone leptonic synchrotron self-Compton model. Our main results are the following: (i) There is a negative correlation between $B$ and $δ$ in our sample, which suggests that $B$ and $δ$ are dependent on each other mainly in Thomson regime. (ii) There are negative correlations between $ν_{\text{sy}}$ and $r$, the $ν_{\text{IC}}$ and $r$, which is a signature of the energy-dependence statistical acce…
The production site of gamma rays in blazars is closely related to their interaction with the photon fields surrounding the active galactic nucleus. In this paper, we discuss an indirect method that may help to unveil the presence of ambient structures in BL Lac objects through the analysis of their gamma-ray spectrum. Gamma rays, passing through structures at different distances from the black hole, interact via $γγ$ pair production with the corresponding photon fields and produce absorption features in their spectral energy distribution. An interaction with a putative broad-line region may r…
Aim: We have collected the most complete multi-wavelength ($6.0 - 6.0 \times 10^{-18} cm$) dataset of very high energy (VHE) $γ$-ray emitting (TeV) BL Lacs, which are the most numerous extragalactic VHE sources. Using significant correlations between different bands, we aim to identify the best TeV BL Lac candidates that can be discovered by the current and next generation of imaging air Cherenkov telescopes. Methods: We formed five datasets from lower energy data, i.e. radio, mid-infrared, optical, X-rays, and GeV $γ$-ray, and five VHE $γ$-ray datasets to perform a correlation study between d…
The very high energy (VHE) gamma ray spectral index of high energy peaked blazars correlates strongly with its corresponding redshift whereas no such correlation is observed in the X-ray or the GeV bands. We attribute this correlation to a result of photon-photon absorption of TeV photons with the extragalactic background light (EBL) and utilizing this, we compute the allowed flux range for the EBL, which is independent of previous estimates. The observed VHE spectrum of the sources in our sample can be well approximated by a power-law, and if the de-absorbed spectrum is also assumed to be a p…
The wealth of recent data from imaging air Cherenkov telescopes (IACTs), ultra-high energy cosmic-ray experiments and neutrino telescopes have fuelled a renewed interest in hadronic emission models for gamma-loud blazars. We explore physically plausible solutions for a lepto-hadronic interpretation of the stationary emission from high-frequency peaked BL Lac objects (HBLs). The modelled spectral energy distributions are then searched for specific signatures at very high energies that could help to distinguish the hadronic origin of the emission from a standard leptonic scenario. By introducing…
We present a targeted search for blazar flux-correlated high-energy ($\varepsilon_ν> 1$ TeV) neutrinos from six bright northern blazars, using the public database of northern-hemisphere neutrinos detected during "IC40" 40-string operations of the IceCube neutrino observatory (April 2008 to May 2009). Our six targeted blazars are subjects of long-term monitoring campaigns by the VERITAS TeV gamma-ray observatory. We use the publicly-available VERITAS lightcurves to identify periods of excess and flaring emission. These predefined intervals serve as our "active temporal windows" in a search for …
Extreme high synchrotron peaked blazars (EHBLs) are amongst the most powerful accelerators found in nature. Usually the synchrotron peak frequency of an EHBL is above $10^{17}\,$Hz, i.e., lies in the range of medium to hard X-rays making them ideal sources to study particle acceleration and radiative processes. EHBL objects are commonly observed at energies beyond several TeV, making them also powerful probes of gamma-ray absorption in the intergalactic medium. During the last decade, several attempts have been made to increase the number of EHBL detected at TeV energies and probe their spectr…
A new look at the extragalactic Very High Energy sky: searching for TeV-emitting candidates among the X-ray bright, non-Fermi detected blazar population
TeV emission from blazars can be used to probe the intergalactic magnetic fields and measure their intensity, coherence length, and helicity. Intergalactic magnetic fields deflect the electron-positron pairs produced by very-high-energy gamma-rays from blazars, resulting in broadened beams of cascade gamma-rays developing along the projected direction of the blazar jet. We present an analysis that uses for the first time the jet orientation of 12 high-synchrotron-peaked (HSP) BL~Lac-type blazars from VLBA radio observations to search for signatures of pair haloes in the Fermi-LAT data. Our sea…
Over seventy blazars identified as sources of TeV gamma ray emission have been detected, including approximately sixty BL Lac objects and seven flat spectrum radio quasars. The distribution in space of these objects can be described by a luminosity function, which gives their comoving space density as a function of their luminosity. We investigate a source selection method to be used for determining the luminosity function of TeV gamma-ray blazars using observations from VERITAS, a ground-based gamma-ray observatory consisting of an array of four atmospheric Cherenkov telescopes located in sou…
We report on the 1.6 GHz (18 cm) VLBI observations of the unresolved, steady TeV source HESS J1943+213 located in the Galactic plane, performed with the European VLBI Network (EVN) in 2014. Our new observations with a nearly full EVN array provide the deepest image of HESS J1943+213 at the highest resolution ever achieved, enabling us to resolve the long-standing issues of the source identification. The milliarcsecond-scale structure of HESS J1943+213 has a clear asymmetric morphology, consisting of a compact core and a diffuse jet-like tail. This is broadly consistent with the previous e-EVN …
We present the results of photometric observations of three TeV blazars, 3C 66A, S5 0954+658 and BL Lacertae, during the period 2013--2017. Our extensive observations were performed in a total of 360 nights which produced $\sim$6820 image frames in BVRI bands. We study flux and spectral variability of these blazars on these lengthy timescales. We also examine the optical Spectral Energy Distributions of these blazars, which are crucial in understanding the emission mechanism of long-term variability in blazars. All three TeV blazars exhibited strong flux variability during our observations. Th…
We investigate the spectroscopic optical properties of gamma-ray sources detected with high significance above 50 GeV in the Third Catalog of Hard Fermi-LAT Sources (3FHL) and that are good candidates as TeV emitters. We focus on the 91 sources that are labelled by the Fermi team as BL Lac objects (BLL) or Blazar candidates of uncertain type (BCU) , that are in the Northern hemisphere, and are with unknown or uncertain redshift. We report here on GTC spectra (in the spectral range 4100 - 7750 AA) of 13 BCU and 42 BLL. We are able to classify the observed targets as BLL and each source is brief…
