TeV Blazars Research Articles

The optical spectropolarimetric behaviour of a selection of high-energy blazars

ABSTRACT At optical/ultraviolet energies, blazars display an underlying thermal (unpolarized) contribution from the accretion disc, torus, and line-emitting regions, diluting the polarized emission from the jet-component. Optical polarimetry can be used to disentangle the thermal and non-thermal components, and place constraints on the particle populations and acceleration mechanisms responsible for the non-thermal emission. We present the results of a linear optical spectropolarimetric observing campaign of 18 blazars (six BL Lac-type objects and 12 flat-spectrum radio quasars) undertaken with the Southern African Large Telescope between 2016 and 2022. This was done to observe these systems during flaring states, as well as long-term monitoring of PKS1510-089, AP Lib, and PKS 1034-293. The observations traced the frequency dependence of the degree and angle of polarization, as well as changes in the spectral line strengths. We investigated possible correlations between the polarization and other observed characteristics for the sources. While an indication of correlation was found between the frequency dependence and the average level of polarization for some sources, a correlation was not found for the population as a whole. These results highlight that continuous observations and in-depth modelling of polarization and its frequency dependence is required to obtain a more holistic view of TeV blazars.

A delayed 400 GeV photon from GRB 221009A and implication on the intergalactic magnetic field

Large High Altitude Air Shower Observatory has detected 0.2 − 13 TeV emission of GRB 221009A within 2000 s since the trigger. Here we report the detection of a 400 GeV photon, without accompanying prominent low-energy emission, by Fermi Large Area Telescope in this direction with a 0.4 days’ delay. Given an intergalactic magnetic field strength of about 4 × 10−17 G, which is comparable to limits from TeV blazars, the delayed 400 GeV photon can be explained as the cascade emission of about 10 TeV gamma rays. We estimate the probabilities of the cascade emission that can result in one detectable photon beyond 100 GeV by Fermi Large Area Telescope within 0.3 − 1 days is about 2% whereas it is about 20.5% within 0.3 − 250 days. Our results show that Synchrotron Self-Compton explanation is less favored with probabilities lower by a factor of about 3 − 30 than the cascade scenario.

TELAMON: Effelsberg monitoring of AGN jets with very-high-energy astroparticle emission

Aims. We introduce the TELAMON program which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, specifically TeV blazars and candidate neutrino-associated AGN. Here, we present and characterize our main sample of TeV-detected blazars. Methods. We analyzed the data sample from the first ∼2.5 yr of observations between August 2020 and February 2023 in the range from 14 GHz to 45 GHz. During this pilot phase, we observed all 59 TeV-detected blazars in the Northern Hemisphere (i.e., Dec > 0°) known at the time of observation. We discuss the basic data reduction and calibration procedures used for all TELAMON data and introduce a sub-band averaging method used to calculate average light curves for the sources in our sample. Results. The TeV-selected sources in our sample exhibit a median flux density of 0.12 Jy at 20 mm, 0.20 Jy at 14 mm, and 0.60 Jy at 7 mm. The spectrum for most of the sources is consistent with a flat radio spectrum and we found a median spectral index (S(ν)∝να) of α = −0.11. Our results on flux density and spectral index are consistent with previous studies of TeV-selected blazars. Compared to the GeV-selected F-GAMMA sample, TELAMON sources are significantly fainter in the radio band. This is consistent with the double-humped spectrum of blazars being shifted towards higher frequencies for TeV-emitters (in particular for high-synchrotron peaked BL Lac type objects), which results in a lower radio flux density. The spectral index distribution of our TeV-selected blazar sample is not significantly different from the GeV-selected F-GAMMA sample. Moreover, we present a strategy to track the light curve evolution of sources in our sample for future variability and correlation analysis.

GeV Variability Properties of TeV Blazars Detected by Fermi-LAT

Variability is a prominent observational feature of blazars. The high-energy radiation mechanism of jets has always been important but is still unclear. In this work, we performed a detailed analysis using Fermi-LAT data across 15 yr and obtained GeV light-curve information for 78 TeV blazars detected by Fermi. We provided annual GeV fluxes and corresponding spectral indices for the 78 TeV blazars and thorough monthly GeV fluxes for a subsample of 41 bright blazars. Our results suggest a strong correlation between the γ-ray photon index and logLγ for the flat spectrum radio quasars (FSRQs) and high-energy peaked BL Lacs. Fourteen sources in our sample show significant GeV outbursts/flares above the relatively stable, low-flux light curve, with six of them showing a clear sharp peak profile in their 5 day binned light curves. We quantified the variability utilizing the fractional variability parameter F var, and found that the flux of the FSRQs showed significantly stronger variability than that of the BL Lacs. The 41 bright blazars in this work are best fit by a log-normal flux distribution. We checked the spectral behavior and found 11 out of the 14 sources show a bluer-when-brighter trend, suggesting this spectral behavior for these TeV blazars at the GeV band arises from the mechanism in which the synchrotron-self Compton process dominates the GeV emission. Our research offers a systematic analysis of the GeV variability properties of TeV blazars and serves as a helpful resource for further associated blazar studies.

Cosmic-void observations reconciled with primordial magnetogenesis

It has been suggested that the weak magnetic field hosted by the intergalactic medium in cosmic voids could be a relic from the early Universe. However, accepted models of turbulent magnetohydrodynamic decay predict that the present-day strength of fields originally generated at the electroweak phase transition (EWPT) without parity violation would be too low to explain the observed scattering of γ-rays from TeV blazars. Here, we propose that the decay is mediated by magnetic reconnection and conserves the mean square fluctuation level of magnetic helicity. We find that the relic fields would be stronger by several orders of magnitude under this theory than was indicated by previous treatments, which restores the consistency of the EWPT-relic hypothesis with the observational constraints. Moreover, efficient EWPT magnetogenesis would produce relics at the strength required to resolve the Hubble tension via magnetic effects at recombination and seed galaxy-cluster fields close to their present-day strength.

Constraining axion-like particles with HAWC observations of TeV blazars

Axion-like particles (ALPs) are a broad class of pseudo-scalar bosons that generically arise from broken symmetries in extensions of the standard model. In many scenarios, ALPs can mix with photons in regions with high magnetic fields. Photons from distant sources can mix with ALPs, which then travel unattenuated through the Universe, before they mix back to photons in the Milky Way galactic magnetic field. Thus, photons can traverse regions where their signals would normally be blocked or attenuated. In this paper, we study TeV γ-ray observations from distant blazars, utilizing the significant γ-ray attenuation expected from such signals to look for excess photon fluxes that may be due to ALP-photon mixing. We find no such excesses among a stacked population of seven blazars andconstrain the ALP-photon coupling constant to fall below ∼4.5×10-11 GeV-1 for ALP masses below 300 neV. These results are competitive with, or better than, leading terrestrial and astrophysical constraints in this mass range.

Investigating the origin of optical flares from the TeV blazar S4 0954+65

Investigating the origin of optical flares from the TeV blazar S4 0954+65

Multiwavelength study of TeV blazar 1ES 1218+304 using gamma-ray, X-ray and optical observations

ABSTRACT We report on a multiwavelength study of the high-synchrotron-peaked BL Lac 1ES 1218+304 using near-simultaneous data obtained during the period from 2018 January 1 to 2021 May 31 (MJD 58119–59365) from various instruments, including Fermi–LAT, Swift–XRT, AstroSat, and optical data from Swift-UVOT and the TUBITAK observatory in Turkey. The source was reported to be flaring in the TeV γ-ray band during 2019, but no significant variation was observed with Fermi–LAT. A sub-hour variability is seen in the SXT light curve, suggesting a compact emission region for the variability. However, hour-scale variability is observed in the γ-ray light curve. A ‘softer-when-brighter’ trend is observed in the γ-ray, and an opposite trend is seen in the X-ray, suggesting that the two emissions are produced through two different processes, as expected from a high-frequency-peaked BL Lac source. We have chosen the two epochs in 2019 January to study and compare their physical parameters. A joint fit of SXT and LAXPC provides a constraint on the synchrotron peak, estimated to be ∼1.6 keV. A clear shift in the synchrotron peak is observed from ∼1 keV to above 10 keV, revealing its extreme nature or behaviour like an extreme blazar-type source. The optical observation provides a colour-index variation as ‘blue-when-brighter’. The broad-band spectral energy distribution is fitted with a single-zone synchrotron-self Compton model, and their parameters are discussed in the context of a TeV blazar and the possible mechanism behind the broad-band emission.

Exploring TeV Candidates of Fermi Blazars through Machine Learning

In this work, we make use of a supervised machine-learning algorithm based on Logistic Regression (LR) to select TeV blazar candidates from the 4FGL-DR2/4LAC-DR2, 3FHL, 3HSP, and 2BIGB catalogs. LR constructs a hyperplane based on a selection of optimal parameters, named features, and hyperparameters whose values control the learning process and determine the values of features that a learning algorithm ends up learning, to discriminate TeV blazars from non-TeV blazars. In addition, it gives the probability (or logistic) that a source may be considered a TeV blazar candidate. Non-TeV blazars with logistics greater than 80% are considered high-confidence TeV candidates. Using this technique, we identify 40 high-confidence TeV candidates from the 4FGL-DR2/4LAC-DR2 blazars and we build the feature hyperplane to distinguish TeV and non-TeV blazars. We also calculate the hyperplanes for the 3FHL, 3HSP, and 2BIGB. Finally, we construct the broadband spectral energy distributions for the 40 candidates, testing for their detectability with various instruments. We find that seven of them are likely to be detected by existing or upcoming IACT observatories, while one could be observed with extensive air shower particle detector arrays.

The “Blazar Sequence” in the TeV Band

The “blazar sequence” has been proposed for more than 20 yr, yet its nature is still unclear. In this work, for the first time we expand this topic to the TeV band by using a sample of 58 TeV blazars, including 48 blazars in the quiescent state and 21 blazars in the flaring state (11 blazars show in both quiescent and flaring state). We investigate the correlation between the TeV luminosity, which has been compensated for attenuation from extragalactic background light, and the synchrotron peak frequency. We note that there is no correlation between TeV luminosity and peak frequency in the quiescent state and a strong anticorrelation in the flaring state for the observed value. However, there is a strong positive correlation in both the quiescent state and the flaring state for the intrinsic value. This indicates that the blazar sequence is shown in the flaring state rather than in the quiescent state for the observed value and the blazar sequence is not present in both states after removing the beaming effect. In addition, to confirm whether the beaming effect results in the blazar sequence, we compare the Fermi γ-ray luminosity between the quiescent state and the flaring state. We find the Fermi γ-ray luminosity in the flaring state is greater than that in the quiescent state, and the Doppler factor in the flaring state is greater. We suggest the blazar sequence in the flaring state may be due to a stronger beaming effect.

Blazar Jets as Possible Sources of Ultra-High Energy Photons: A Short Review

In this paper, I present a qualitative discussion on the prospect of production of ultra-high photons in blazars. The sources are a subclass of active galactic nuclei which host supermassive black holes and fire relativistic jets into the intergalactic medium. The kpc-scale jets are believed to be dominated by Poynting flux and constitute one of the most efficient cosmic particle accelerators, that potentially are capable of accelerating the particles up to EeV energies. Recent IceCube detection of astrophysical neutrino emissions, in coincidence with the enhanced gamma-ray from Tev blazar TXS 0506 + 056, further supports hadronic models of blazar emissions in which particle acceleration processes, such as relativistic shocks, magnetic re-connection, and relativistic turbulence, could energize hadrons, e.g., protons, up to energies equivalent to billions of Lorentz factors. The ensuing photo-pionic processes may then result in gamma-rays accompanied by neutrino flux. Furthermore, the fact that blazars are the dominant source of observed TeV emission encourages search for signatures of acceleration scenarios that would lead to the creation of ultra-high-energy photons.

A 31.3 day Transient Quasiperiodic Oscillation in Gamma-ray Emission from Blazar S5 0716+714

We systematically search for quasiperiodic oscillatory (QPO) signals on the month timescale among the 1525 sources given in the Fermi Large Area Telescope Light Curve Repository. We find a transient QPO of 31.3 ± 1.8 days in the gamma-ray band light curve of the TeV blazar S5 0716+714, which has seven cycles (MJD 55918–56137) for the first time by weighted wavelet Z-transform and Lomb–Scargle periodogram methods. Monte Carlo simulations based on the power spectral density and probability distribution function were used to evaluate the confidence level of the QPO, and the result is ∼4.1σ. Seasonal autoregressive integrated moving average modeling of the light curve revealed it is a significant physical QPO. The physical models to explain the sporadic month-timescale QPOs in the blazar were discussed. Our studies indicate that the helical jet model and blob move helically in a curved jet model to properly explain this kind of transient QPO.

Two-component TeV Emissions for Blazars

For blazars, its radio emissions are composed of two components. In the high energy bands, Tera-electronvolts (TeV; 1012electronvolts) emissions were detected from some blazars. Does the TeV emission also consist of two components? To investigate the TeV emission component, we compiled a sample of 51 TeV blazars with available radio core dominance parameters and applied a two-component (the beamed and the unbeamed) model to study the composition of blazar TeV emission. We obtained the core-dominance parameter at the TeV band through applying a relativistic beaming model, and found that the average TeV core-dominance parameter is for 46 BL Lacs, and that is for 5 FSRQs, which shows an indication that the core dominance parameter in BL Lacs is smaller than that in FSRQs. For the three subclasses of BL Lacs—high synchrotron peaked BL Lacs(HBLs), intermediate synchrotron peaked BL Lacs(IBLs) and low synchrotron peaked(LBLs), is smaller than and . Our results suggest that the blazar TeV emission from the core is thousands times stronger than from the extended region and the TeV emission is more core-dominated for the lower synchrotron peaked sources. In addition, a significant correlation between the TeV core-dominance parameter () and the radio one () , is obtained suggesting that the TeV emission is beamed and the TeV core-dominance parameter is an indicator of beaming effect at the TeV bands, and that the TeV emission has stronger beaming effect than the radio emission.

Long-term Optical Monitoring of the TeV BL Lacertae Object 1ES 2344 + 514

Variability is one of the main observational characteristics of blazars. Studying variability is an efficient method to reveal the nature of active galactic nuclei. In the present work, we report optical R-band photometry observations of a TeV blazar, 1ES 2344 + 514, carried out with a 70 cm telescope in the period of 1998 July–2017 November at Abastumani Observatory, Georgia. Based on the optical R-band observations, the optical variation behaviors on both short timescales and long timescales are investigated. Three methods (Jurkevich, discrete correlation function, and power spectrum analysis) are used to investigate periodicity in the light curve. In addition, combined with multiwavelength data, the jet physical properties are discussed. The following conclusions are drawn: (1) A variability of ΔR = 0.155 mag (15.356 − 15.201 mag) over a timescale of ΔT = 12.99 minutes is detected during our 628 days of monitoring. (2) According to the Kelvin–Helmholtz thermal instability, if the magnetic field intensity (B) for the source is greater than a critical value (B c ), it will reduce the incidence of intraday variations in the light curves. (3) The physical parameters of the dissipation region are obtained by fitting the spectral energy distribution with a one-zone synchrotron self-Compton model for the average and flare states. (4) The three methods show that there are periods of P = 2.72 ± 0.47 yr, P = 1.61 ± 0.18 yr, P = 1.31 ± 0.17 yr, and P = 1.05 ± 0.07 yr. When a binary black hole system is adopted with a period of P = 2.72 ± 0.41 yr, we obtain the orbital parameters for the binary black hole system as follows: M = 8.08 × 109 M ⊙, the sum of the semiaxes is r = 7.18 × 1016 cm, and the lifetime of the binary black hole is τ merge = 6.24 × 102 yr.

Nonthermal Radiation of the Extreme TeV Blazar 1ES 0229+200 from Electromagnetic Cascades on Infrared Photon Field

Extreme TeV blazars (ETBs) are active galactic nuclei with jets presumably pointing towards the observer having their intrinsic (compensated for the effect of γ-ray absorption on extragalactic background light photons) spectral energy distributions (SEDs) peaked at an energy in excess of 1 TeV. These sources typically reveal relatively weak and slow variability as well as higher frequency of the low-energy SED peak compared to other classes of blazars. It proved to be exceedingly hard to incorporate all these peculiar properties of ETBs into the framework of conventional γ-ray emission models. ETB physics have recently attracted great attention in the astrophysical community, underlying the importance of the development of self-consistent ETB emission model(s). We propose a new scenario for the formation of X-ray and γ-ray spectra of ETBs assuming that electromagnetic cascades develop in the infrared photon field surrounding the central blazar engine. This scenario does not invoke compact fast-moving sources of radiation (so-called “blobs”), in agreement with the apparent absence of fast and strong variability of ETBs. For the case of the extreme TeV blazar 1ES 0229+200 we propose a specific emission model in the framework of the considered scenario. We demonstrate that this model allows to obtain a good fit to the measured SED of 1ES 0229+200.

Exploring the voids: Luminosity functions and magnetic field

We present semi-analytical models for magnetisation of the void inter-galactic medium (IGM) by outflows from void galaxies. The number density of dark matter haloes in an under-dense region (i.e., void) is obtained using the excursion set method extended for such low density environment. The star formation in such haloes has been estimated, taking account of the negative feedback by supernovae. The galaxy formation/evolution model is tuned to provide the r-band luminosity function, the stellar mass function and also the color of void galaxies as obtained from recent observations. This star formation model is used to study possible outflows from void galaxies driven by the hot thermal gas and cosmic ray pressures. These outflows drag the magnetic fields present in those galaxies to the void IGM. We show that such a model can magnetise ∼30% of the void IGM with the magnetic field strength of 10−12−10−10 G while considering only magnetic flux freezing condition. Along with this, the megapersec size of individual outflows can explain the non-detection of GeV photons in TeV blazars that put a lower limit of 10−16 G for void IGM magnetic field with a Mpc coherent length scale.

Broad-band study of BL Lac during flare of 2020: spectral evolution and emergence of HBL component

ABSTRACT BL Lacertae (BL Lac) is categorized as TeV blazar and considered as a possible source of astrophysical neutrinos. In 2020, the brightest X-ray flare ever detected from it. A detailed study can answer many puzzling questions related to multiband emissions and fast-flux variability often seen in this kind of source. We have performed the temporal and spectral analysis of the brightest flare. The variability is characterized by the fractional variability amplitude and the variability time. We found that the source has crossed all its previous limits of flux and reached to a maximum ever seen from it in optical and X-rays. It is highly variable in X-rays with fractional variability above 100 per cent (1.8397 ± 0.0181) and the fastest variability time of 11.28 h within a day. The broad-band light-curves correlation with X-ray suggest a time lag of one day. A broad-band SED modelling is pursued to understand the possible physical mechanisms responsible for broad-band emission. Modelling requires two emission regions located at two different sites to explain the low and high flux states. A significant spectral change is observed in the optical-UV and X-ray spectrum during the high state, which eventually leads to shifts in the location of the synchrotron peak towards higher energy, suggesting an emergence of a new HBL component.

Modeling Gamma-Ray SEDs and Angular Extensions of Extreme TeV Blazars from Intergalactic Proton-Initiated Cascades in Contemporary Astrophysical EGMF Models

The intrinsic spectra of some distant blazars known as “extreme TeV blazars” have shown a hint at an anomalous hardening in the TeV energy region. Several extragalactic propagation models have been proposed to explain this possible excess transparency of the Universe to gamma-rays starting from a model which assumes the existence of so-called axion-like particles (ALPs) and the new process of gamma-ALP oscillations. Alternative models suppose that some of the observable gamma-rays are produced in the intergalactic cascades. This work focuses on investigating the spectral and angular features of one of the cascade models, the Intergalactic Hadronic Cascade Model (IHCM) in the contemporary astrophysical models of Extragalactic Magnetic Field (EGMF). For IHCM, EGMF largely determines the deflection of primary cosmic rays and electrons of intergalactic cascades and, thus, is of vital importance. Contemporary Hackstein models are considered in this paper and compared to the model of Dolag. The models assumed are based on simulations of the local part of large-scale structure of the Universe and differ in the assumptions for the seed field. This work provides spectral energy distributions (SEDs) and angular extensions of two extreme TeV blazars, 1ES 0229+200 and 1ES 0414+009. It is demonstrated that observable SEDs inside a typical point spread function of imaging atmospheric Cherenkov telescopes (IACTs) for IHCM would exhibit a characteristic high-energy attenuation compared to the ones obtained in hadronic models that do not consider EGMF, which makes it possible to distinguish among these models. At the same time, the spectra for IHCM models would have longer high energy tails than some available spectra for the ALP models and the universal spectra for the Electromagnetic Cascade Model (ECM). The analysis of the IHCM observable angular extensions shows that the sources would likely be identified by most IACTs not as point sources but rather as extended ones. These spectra could later be compared with future observation data of such instruments as Cherenkov Telescope Array (CTA) and LHAASO.

Observable spectral and angular distributions of γ-rays from extragalactic ultrahigh energy cosmic ray accelerators: the case of extreme TeV blazars

ABSTRACT Ultrahigh energy protons and nuclei from extragalactic cosmic ray sources initiate intergalactic electromagnetic cascades, resulting in observable fluxes of γ-rays in the GeV–TeV energy domain. The total spectrum of such cascade γ-rays of hadronic nature is significantly harder than the one usually expected from blazars. The spectra of some sources known as ‘extreme TeV blazars’ could be well-described by this ‘intergalactic hadronic cascade model’ (IHCM). We calculate the shape of the observable point-like spectrum, as well as the observable angular distibution of γ-rays, for the first time taking into account the effect of primary proton deflection in filaments and galaxy clusters of the extragalactic magnetic field assuming the model of Dolag et al. (2005). We present estimates of the width of the observable γ-ray angular distribution derived from simple geometrical considerations. We also employ a hybrid code to compute the observable spectral and angular distributions of γ-rays. The observable point-like spectrum at multi-TeV energies is much softer than the one averaged over all values of the observable angle. The presence of a high-energy cutoff in the observable spectra of extreme TeV blazars in the framework of the IHCM could significantly facilitate future searches of new physics processes that enhance the apparent γ-ray transparency of the Universe (for instance, γ → ALP oscillations). The width of the observable angular distribution is greater than or comparable to the extent of the point spread function of next-generation γ-ray telescopes.

X-Ray Intraday Variability of the TeV Blazar PKS 2155–304 with Suzaku during 2005–2014

Abstract We have examined 13 pointed observations of the TeV-emitting high synchrotron peak blazar PKS 2155−304, taken by the Suzaku satellite throughout its operational period. We found that the blazar showed large-amplitude intraday variabilities in the soft (0.8–1.5 keV) and the hard (1.5–8.0 keV) bands in the light curves. Spectral variability on intraday timescales is estimated using the hardness ratio. The blazar usually becomes harder when brighter and vice versa, following the typical behavior of high synchrotron peak blazars. The power spectral density analyses of 11 out of 13 light curves in the total energy (0.8–8.0 keV) are found to be dominated by red noise, with power-law spectral indices that span a large range, from −2.81 to −0.88. Discrete correlation function analyses of all the 13 light curves between the soft and the hard bands show that they are well correlated and peak at, or very close to, zero lag. This indicates that the emissions in soft and hard bands are probably cospatial and emitted from the same population of leptons. Considering fluxes versus variability timescales, we found no correlation on intraday timescales, implying that X-ray emission from PKS 2155−304 is not dominated by simple changes in the Doppler factor. We briefly discuss the most likely emission mechanisms responsible for the observed flux and spectral variabilities and place constraints on magnetic field strength and Lorentz factors of the electrons emitting the X-rays in the most likely scenario.