Low-frequency Peaked BL Lac Objects Research Articles

Detection of the blazar S4 0954+65 at very-high-energy with the MAGIC telescopes during an exceptionally high optical state

Aims. The very high energy (VHE ≳100 GeV) γ-ray MAGIC observations of the blazar S4 0954+65, were triggered by an exceptionally high flux state of emission in the optical. This blazar has a disputed redshift of z = 0.368 or z ≥ 0.45 and an uncertain classification among blazar subclasses. The exceptional source state described here makes for an excellent opportunity to understand physical processes in the jet of S4 0954+65 and thus contribute to its classification. Methods. We investigated the multiwavelength (MWL) light curve and spectral energy distribution (SED) of the S4 0954+65 blazar during an enhanced state in February 2015 and have put it in context with possible emission scenarios. We collected photometric data in radio, optical, X-ray, and γ-ray. We studied both the optical polarization and the inner parsec-scale jet behavior with 43 GHz data. Results. Observations with the MAGIC telescopes led to the first detection of S4 0954+65 at VHE. Simultaneous data with Fermi-LAT at high energy γ-ray (HE, 100 MeV < E < 100 GeV) also show a period of increased activity. Imaging at 43 GHz reveals the emergence of a new feature in the radio jet in coincidence with the VHE flare. Simultaneous monitoring of the optical polarization angle reveals a rotation of approximately 100°. Conclusions. The high emission state during the flare allows us to compile the simultaneous broadband SED and to characterize it in the scope of blazar jet emission models. The broadband spectrum can be modeled with an emission mechanism commonly invoked for flat spectrum radio quasars (FSRQs), that is, inverse Compton scattering on an external soft photon fieldfrom the dust torus, also known as external Compton. The light curve and SED phenomenology is consistent with an interpretation of a blob propagating through a helical structured magnetic field and eventually crossing a standing shock in the jet, a scenario typically applied to FSRQs and low-frequency peaked BL Lac objects (LBL).

From radio to TeV: the surprising spectral energy distribution of AP Librae

Following the discovery of high-energy (HE; $E>10\,{\rm MeV}$) and very-high-energy (VHE; $E>100\,{\rm GeV}$) $\gamma$-ray emission from the low-frequency-peaked BL~Lac (LBL) object AP Librae, its electromagnetic spectrum is studied over 60 octaves in energy. Contemporaneous data in radio, optical and UV together with the $\gamma$-ray data are used to construct the most precise spectral energy distribution of this source. The data have been found to be modeled with difficulties with single zone homogeneous leptonic synchrotron self-Compton (SSC) radiative scenarios due to the unprecedented width of the high-energy component when compared to the lower-energy component. The two other LBL objects also detected at VHE appear to have similar modeling difficulties. Nevertheless, VHE $\gamma$ rays produced in the extended jet could account for the VHE flux observed by H.E.S.S.

The challenging SED of AP Librae

AbstractMatching the broad-band emission of active galaxies with the predictions of theoretical models can be used to derive constraints on the properties of the emitting region and to probe the physical processes involved. AP Librae is the third low frequency peaked BL Lac (LBL) detected at very high energy (VHE, E>100GeV) by an Atmospheric Cherenkov Telescope; most VHE BL Lacs (34 out of 39) belong to the high-frequency and intermediate-frequency BL Lac classes (HBL and IBL). LBL objects tend to have a higher luminosity with lower peak frequencies than HBLs or IBLs. The characterization of their time-averaged spectral energy distribution is challenging for emission models such as synchrotron self-Compton (SSC) models.

Optical Spectra Evolution of BL Lac Objects

Many quasi-simultaneous optical observations of nine BL Lac objects are obtained from literature. We study the relationship between the optical spectral index and the luminosity of BL Lac objects, and are tempted to exploit spectral evolution in the optical frequency ranges. Our results show that: (i) The optical spectra index of the low-frequency peaked BL Lac objects (LBLs) is steeper than the high-frequency peaked BL Lac objects (HBLs); (ii) The spectra tend to be softer when the source becomes brighter for LBLs and the intermediate BL Lac objects (IBLs) (i.e., bluer-when-brighter), and the spectra of HBLs does not vary when the brightness of HBLs changes. Possible explanations are briefly discussed for this phenomenon.

TIMING SIGNATURES OF THE INTERNAL-SHOCK MODEL FOR BLAZARS

We investigate the spectral and timing signatures of the internal-shock model for blazars. For this purpose, we develop a semi-analytical model for the time-dependent radiative output from internal shocks arising from colliding relativistic shells in a blazar jet. The emission through synchrotron and synchrotron-self Compton (SSC) radiation as well as Comptonization of an isotropic external radiation field are taken into account. We evaluate the discrete correlation function (DCF) of the model light curves in order to evaluate features of photon-energy dependent time lags and the quality of the correlation, represented by the peak value of the DCF. The almost completely analytic nature of our approach allows us to study in detail the influence of various model parameters on the resulting spectral and timing features. This paper focuses on a range of parameters in which the gamma-ray production is dominated by Comptonization of external radiation, most likely appropriate for gamma-ray bright flat-spectrum radio quasars (FSRQs) or low-frequency peaked BL Lac objects (LBLs). In most cases relevant for FSRQs and LBLs, the variability of the optical emission is highly correlated with the X-ray and high-energy (HE: > 100 MeV) gamma-ray emission. Our baseline model predicts a lead of the optical variability with respect to the higher-energy bands by 1 - 2 hours and of the HE gamma-rays before the X-rays by about 1 hour. We show that variations of certain parameters may lead to changing signs of inter-band time lags, potentially explaining the lack of persistent trends of time lags in most blazars.

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

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

The Accretion Rates and Spectral Energy Distributions of BL Lacertae Objects

We investigate the relationship between accretion rates and the spectral energy distributions (SEDs) of BL Lac objects, using a sample of objects for which published information on the host galaxies, emission-line luminosities, and peak frequencies and luminosities of their SEDs are available. The sample is composed of 43 BL Lac objects which have a relatively continuous distribution of peak frequencies. Under the assumption that the observed emission lines are photoionized by the central accretion disk, we use the line luminosities to estimate the accretion luminosities and hence accretion rates. We find that low frequency-peaked BL Lac objects (LBLs) span a wide range of accretion rates, whereas high frequency-peaked BL Lac objects (HBLs) cover a more restricted range of lower values. There appears to be a continuous distribution of accretion rates between the two subclasses of BL Lac objects. We find that the peak frequency of the SED, $\pknu$, correlates with the accretion rate, approximately with the form $\pknu\propto \Lambda^{-3}$ in HBLs and $\pknu \propto \Lambda^{-0.25}$ in LBLs, where $\Lambda \equiv L_{\rm lines}/c^2$. The peak luminosity of the SED is also correlated with $\Lambda$. These results suggest that the accretion rate influences the shape of the SED in BL Lac objects. They also support models which couple the jet and the accretion disk. We present a physical scenario to account for the empirical trends.