Multiwavelength Observations of the Blazar BL Lacertae: A New Fast TeV Gamma-Ray Flare

Combined with measurements made by very-long-baseline interferometry, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL Lacertae observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). with a rise time of ~2.3 hr and a decay time of ~36 min. The peak flux above 200 GeV is (4.2 ± 0.6) × 10^(−6) photon m^(−2) s^(−1) measured with a 4-minute-binned light curve, corresponding to ~180% of the flux that is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in Very Long Baseline Array observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models that invoke relativistic plasma passing stationary shocks

Detection of persistent VHE gamma-ray emission from PKS 1510–089 by the MAGIC telescopes during low states between 2012 and 2017

Context. PKS 1510–089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. To date, very high-energy (VHE, > 100 GeV) emission has been observed from this source either during long high states of optical and GeV activity or during short flares. Aims. We search for low-state VHE gamma-ray emission from PKS 1510–089. We characterize and model the source in a broadband context, which would provide a baseline over which high states and flares could be better understood. Methods. PKS 1510–089 has been monitored by the MAGIC telescopes since 2012. We use daily binned Fermi-LAT flux measurements of PKS 1510–089 to characterize the GeV emission and select the observation periods of MAGIC during low state of activity. For the selected times we compute the average radio, IR, optical, UV, X-ray, and gamma-ray emission to construct a low-state spectral energy distribution of the source. The broadband emission is modeled within an external Compton scenario with a stationary emission region through which plasma and magnetic fields are flowing. We also perform the emission-model-independent calculations of the maximum absorption in the broad line region (BLR) using two different models. Results. The MAGIC telescopes collected 75 hr of data during times when the Fermi-LAT flux measured above 1 GeV was below 3  ×  10^(−8) cm^(−2) s^(−1), which is the threshold adopted for the definition of a low gamma-ray activity state. The data show a strongly significant (9.5σ) VHE gamma-ray emission at the level of (4.27 ± 0.61_(stat))  ×  10^(−12) cm^(−2) s^(−1) above 150 GeV, a factor of 80 lower than the highest flare observed so far from this object. Despite the lower flux, the spectral shape is consistent with earlier detections in the VHE band. The broadband emission is compatible with the external Compton scenario assuming a large emission region located beyond the BLR. For the first time the gamma-ray data allow us to place a limit on the location of the emission region during a low gamma-ray state of a FSRQ. For the used model of the BLR, the 95% confidence level on the location of the emission region allows us to place it at a distance > 74% of the outer radius of the BLR

Constraining the limiting brightness temperature and Doppler factors for the largest sample of radio bright blazars

Relativistic effects dominate the emission of blazar jets complicating our understanding of their intrinsic properties. Although many methods have been proposed to account for them, the variability Doppler factor method has been shown to describe the blazar populations best. We use a Bayesian hierarchical code called {\it Magnetron} to model the light curves of 1029 sources observed by the Owens Valley Radio Observatory's 40-m telescope as a series of flares with an exponential rise and decay, and estimate their variability brightness temperature. Our analysis allows us to place the most stringent constraints on the equipartition brightness temperature i.e., the maximum achieved intrinsic brightness temperature in beamed sources which we found to be $\rm \langle T_{eq}\rangle=2.78\times10^{11}K\pm26\%$. Using our findings we estimated the variability Doppler factor for the largest sample of blazars increasing the number of available estimates in the literature by almost an order of magnitude. Our results clearly show that $\gamma$-ray loud sources have faster and higher amplitude flares than $\gamma$-ray quiet sources. As a consequence they show higher variability brightness temperatures and thus are more relativistically beamed, with all of the above suggesting a strong connection between the radio flaring properties of the jet and $\gamma$-ray emission.Comment: 14 pages, 8 figures, accepted for publication in AP

The broad-band properties of the intermediate synchrotron peaked BL Lac S2 0109+22 from radio to VHE gamma-rays

The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes observed S2 0109+22 in 2015 July during its flaring activity in high-energy gamma-rays observed by Fermi-Large Area Telescope. We analyse the MAGIC data to characterize the very high energy (VHE) gamma-ray emission of S2 0109+22, which belongs to the subclass of intermediate synchrotron peak (ISP) BL Lacertae (BL Lac) objects. We study the multifrequency emission in order to investigate the source classification. Finally, we compare the source long-term behaviour to other VHE gamma-ray emitting (TeV) blazars. We performed a temporal and spectral analysis of the data centred around the MAGIC interval of observation (MJD 57225–57231). Long-term radio and optical data have also been investigated using the discrete correlation function. The redshift of the source is estimated through optical host-galaxy imaging and also using the amount of VHE gamma-ray absorption. The quasi-simultaneous multifrequency spectral energy distribution (SED) is modelled with the conventional one-zone synchrotron self-Compton (SSC) model. MAGIC observations resulted in the detection of the source at a significance level of 5.3σ. The VHE gamma-ray emission of S2 0109+22 is variable on a daily time scale. VHE gamma-ray luminosity of the source is lower than the average of TeV BL Lacs. The optical polarization and long-term optical/radio behaviour of the source are different from the general population of TeV blazars. All these findings agree with the classification of the source as an ISP BL Lac object. We estimate the source redshift as z = 0.36 ± 0.07. The SSC parameters describing the SED are rather typical for blazars

Optical polarisation variability of radio loud narrow line Seyfert 1 galaxies. Search for long rotations of the polarisation plane

Narrow line Seyfert 1 galaxies (NLSy1s) constitute the AGN subclass associated with systematically smaller black hole masses. A few radio loud ones have been detected in MeV -- GeV energy bands by Fermi and evidence for the presence of blazar-like jets has been accumulated. In this study we wish to quantify the temporal behaviour of the optical polarisation, fraction and angle, for a selected sample of radio loud NLSy1s. We also search for rotations of the polarisation plane similar to those commonly observed in blazars. We have conducted R-band optical polarisation monitoring of a sample of 10 RL NLSy1s 5 of which have been previously detected by Fermi. The dataset includes observations with the RoboPol, KANATA, Perkins and Steward polarimeters. In the cases where evidences for long rotations of the polarisation plane are found, we carry out numerical simulations to assess the probability that they are caused by intrinsically evolving EVPAs instead of observational noise. Even our moderately sampled sources show indications of variability, both in polarisation fraction and angle. For the four best sampled objects in our sample we find multiple periods of significant polarisation angle variability. In the two best sampled cases, namely J1505+0326 and J0324+3410, we find indications for three long rotations. We show that although noise can induce the observed behaviour, it is much more likely that the apparent rotation is caused by intrinsic evolution of the EVPA. To our knowledge this is the very first detection of such events in this class of sources. In the case of the largest dataset (J0324+3410) we find that the EVPA concentrates around a direction which is at 49.3\degr to the 15-GHz radio jet implying a projected magnetic field at an angle of 40.7\degr to that axis.Comment: Accepted for publication in section 2. Astrophysical processes of Astronomy and Astrophysic

Optical EVPA rotations in blazars: testing a stochastic variability model with RoboPol data

We identify rotations of the polarization angle in a sample of blazars observed for three seasons with the RoboPol instrument. A simplistic stochastic variability model is tested against this sample of rotation events. The model is capable of producing samples of rotations with parameters similar to the observed ones, but fails to reproduce the polarization fraction at the same time. Even though we can neither accept nor conclusively reject the model, we point out various aspects of the observations that are fully consistent with a random walk process

Investigating the multiwavelength behaviour of the flat spectrum radio quasar CTA 102 during 2013–2017

We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013–2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well as data from the Swift (optical-UV and X-rays) and Fermi (γ-rays) satellites to study flux and spectral variability and the correlation between flux changes at different wavelengths. Unprecedented γ-ray flaring activity was observed during 2016 November–2017 February, with four major outbursts. A peak flux of (2158 ± 63) × 10⁻⁸ ph cm⁻² s⁻¹, corresponding to a luminosity of (2.2 ± 0.1) × 10⁵⁰ erg s⁻¹, was reached on 2016 December 28. These four γ-ray outbursts have corresponding events in the near-infrared, optical, and UV bands, with the peaks observed at the same time. A general agreement between X-ray and γ-ray activity is found. The γ-ray flux variations show a general, strong correlation with the optical ones with no time lag between the two bands and a comparable variability amplitude. This γ-ray/optical relationship is in agreement with the geometrical model that has successfully explained the low-energy flux and spectral behaviour, suggesting that the long-term flux variations are mainly due to changes in the Doppler factor produced by variations of the viewing angle of the emitting regions. The difference in behaviour between radio and higher energy emission would be ascribed to different viewing angles of the jet regions producing their emission

Detection of persistent VHE gamma-ray emission from PKS 1510–089 by the MAGIC telescopes during low states between 2012 and 2017

Context. PKS 1510–089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. To date, very high-energy (VHE, > 100 GeV) emission has been observed from this source either during long high states of optical and GeV activity or during short flares. Aims. We search for low-state VHE gamma-ray emission from PKS 1510–089. We characterize and model the source in a broadband context, which would provide a baseline over which high states and flares could be better understood. Methods. PKS 1510–089 has been monitored by the MAGIC telescopes since 2012. We use daily binned Fermi-LAT flux measurements of PKS 1510–089 to characterize the GeV emission and select the observation periods of MAGIC during low state of activity. For the selected times we compute the average radio, IR, optical, UV, X-ray, and gamma-ray emission to construct a low-state spectral energy distribution of the source. The broadband emission is modeled within an external Compton scenario with a stationary emission region through which plasma and magnetic fields are flowing. We also perform the emission-model-independent calculations of the maximum absorption in the broad line region (BLR) using two different models. Results. The MAGIC telescopes collected 75 hr of data during times when the Fermi-LAT flux measured above 1 GeV was below 3  ×  10^(−8) cm^(−2) s^(−1), which is the threshold adopted for the definition of a low gamma-ray activity state. The data show a strongly significant (9.5σ) VHE gamma-ray emission at the level of (4.27 ± 0.61_(stat))  ×  10^(−12) cm^(−2) s^(−1) above 150 GeV, a factor of 80 lower than the highest flare observed so far from this object. Despite the lower flux, the spectral shape is consistent with earlier detections in the VHE band. The broadband emission is compatible with the external Compton scenario assuming a large emission region located beyond the BLR. For the first time the gamma-ray data allow us to place a limit on the location of the emission region during a low gamma-ray state of a FSRQ. For the used model of the BLR, the 95% confidence level on the location of the emission region allows us to place it at a distance > 74% of the outer radius of the BLR

Multiwavelength Observations of the Blazar BL Lacertae: A New Fast TeV Gamma-Ray Flare

Combined with measurements made by very-long-baseline interferometry, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL Lacertae observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). with a rise time of ~2.3 hr and a decay time of ~36 min. The peak flux above 200 GeV is (4.2 ± 0.6) × 10^(−6) photon m^(−2) s^(−1) measured with a 4-minute-binned light curve, corresponding to ~180% of the flux that is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in Very Long Baseline Array observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models that invoke relativistic plasma passing stationary shocks