Aims. Compact symmetric objects (CSOs) are thought to represent the first step in the evolutionary path of radio galaxies. In the present study, we investigated the X-ray emission of two CSOs ...confirmed to emit at GeV energies: PKS 1718–649 and TXS 1146+596. Unveiling the origin of their observed high-energy emission is crucial to establishing the physical parameters of the radio source and understanding how CSOs interact with the surrounding medium. Methods. We combined archival and new NuSTAR observations of PKS 1718–649 and TXS 1146+596 to have broadband X-ray coverage. For both sources, we modeled the broadband spectral energy distribution (SED) from the radio band up to γ -rays in order to derive their physical parameters. We also discuss the role of the ambient medium in confining the source expansion, which we investigate using X-ray obscuration. Results. For the first time, we report X-ray detections of PKS 1718–649 and 1146+596 with NuSTAR at energies higher than 10 keV. Combining Chandra and NuSTAR observations of TXS 1146+596, we reveal the presence of a multitemperature thermal component dominating the soft X-ray spectrum, and we interpret this finding as indicative of an AGN feedback process in action in this source. In addition, we show that two emitting electron populations are necessary to reproduce the observed broadband SED of TXS 1146+596: in our models, the X-ray emission could either be produced by synchrotron radiation or by a weak X-ray corona, or could be an ADAF-type emission. Interestingly, an additional X-ray component, namely a weak corona, is also required for PKS 1718–649. Moreover, we argue that heavily obscured and possibly frustrated sources tend to show different radio sizes with respect to those that are unobscured and free to expand.
ABSTRACT
Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in ...2019 December–2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales $\lesssim 0.2$ d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10−3 pc. In contrast, flux changes on time-scales $\gtrsim 0.5$ d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin.
ABSTRACT
We report on results of a multiband monitoring campaign from radio to γ-rays of the high-redshift flat spectrum radio quasar S5 0836 + 710 during a high-activity period detected by the Large ...Area Telescope on board the Fermi Gamma-ray Space Telescope. Two major flares were detected, in 2015 August and November. In both episodes, the apparent isotropic γ-ray luminosity exceeds 1050 erg s−1, with a doubling time-scale of about 3 h. The high γ-ray activity may be related to a superluminal knot that emerged from the core in 2015 April at the peak of the radio activity and is moving downstream along the jet. The low variability observed in X-rays may indicate that X-ray emission is produced by the low-energy tail of the same electron population that produces the γ-ray emission. The analysis of full-polarization pc-scale radio observations suggests the presence of a limb-brightened polarization structure at about 1 mas from the core in which a rotation measure gradient with a sign change is observed transverse to the jet direction. These characteristics are consistent with a scenario in which Faraday rotation is produced by a sheath of thermal electrons with a toroidal magnetic field surrounding the emitting jet.
Context.
Blazars are the most numerous class of high-energy (HE;
E
∼ 50 MeV−100 GeV) and very high-energy (VHE;
E
∼ 100 GeV−10 TeV) gamma-ray emitters. Currently, a measured spectroscopic redshift is ...available for only about 50% of gamma-ray BL Lacertae objects (BL Lacs), mainly due to the difficulty in measuring reliable redshifts from their nearly featureless continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars.
Aims.
This paper is the first in a series of papers that aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground-based gamma-ray observatory.
Methods.
Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the
Fermi
-LAT telescope still lacking redshift measurements, but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium- to high-resolution spectroscopy of 19 blazar optical counterparts with the Keck II, SALT, and ESO NTT telescopes. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux.
Results.
We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482, one tentative redshift, three redshift lower limits including one at
z
≥ 0.449 and another at
z
≥ 0.868. Four BL Lacs show featureless spectra.
ABSTRACT On 2015 June 16, Fermi-LAT observed a giant outburst from the flat spectrum radio quasar 3C 279 with a peak >100 MeV flux of ∼3.6 × 10−5 photons cm−2 s−1, averaged over orbital period ...intervals. It is historically the highest γ-ray flux observed from the source, including past EGRET observations, with the γ-ray isotropic luminosity reaching ∼1049 erg s−1. During the outburst, the Fermi spacecraft, which has an orbital period of 95.4 minutes, was operated in a special pointing mode to optimize the exposure for 3C 279. For the first time, significant flux variability at sub-orbital timescales was found in blazar observations by Fermi-LAT. The source flux variability was resolved down to 2-minute binned timescales, with flux doubling times of less than 5 minutes. The observed minute-scale variability suggests a very compact emission region at hundreds of Schwarzschild radii from the central engine in conical jet models. A minimum bulk jet Lorentz factor (Γ) of 35 is necessary to avoid both internal γ-ray absorption and super-Eddington jet power. In the standard external radiation Comptonization scenario, Γ should be at least 50 to avoid overproducing the synchrotron self-Compton component. However, this predicts extremely low magnetization (∼5 × 10−4). Equipartition requires Γ as high as 120, unless the emitting region is a small fraction of the dissipation region. Alternatively, we consider γ rays originating as synchrotron radiation of γe ∼ 1.6 × 106 electrons, in a magnetic field B ∼ 1.3 kG, accelerated by strong electric fields E ∼ B in the process of magnetoluminescence. At such short distance scales, one cannot immediately exclude the production of γ-rays in hadronic processes.
Abstract
The discovery of γ-ray emission from radio-loud narrow-line Seyfert 1 (NLSy1) galaxies has questioned the need for large black hole masses ( ≳ 108 M⊙) to launch relativistic jets. We present ...near-infrared data of the γ-ray-emitting NLSy1 FBQS J1644+2619 that were collected using the camera CIRCE (Canarias InfraRed Camera Experiment) at the 10.4-m Gran Telescopio Canarias to investigate the structural properties of its host galaxy and to infer the black hole mass. The 2D surface brightness profile is modelled by the combination of a nuclear and a bulge component with a Sérsic profile with index n = 3.7, indicative of an elliptical galaxy. The structural parameters of the host are consistent with the correlations of effective radius and surface brightness against absolute magnitude measured for elliptical galaxies. From the bulge luminosity, we estimated a black hole mass of (2.1 ± 0.2) × 108 M⊙, consistent with the values characterizing radio-loud active galactic nuclei.
Abstract The detection of γ-ray emission from narrow-line Seyfert 1 galaxies (NLSy1) has challenged the idea that large black hole (BH) masses (≥108 M⊙) are needed to launch relativistic jets. We ...present near-infrared imaging data of the γ-ray-emitting NLSy1 PKS 1502+036 obtained with the Very Large Telescope. Its surface brightness profile, extending to ∼20 kpc, is well described by the combination of a nuclear component and a bulge with a Sérsic index n = 3.5, which is indicative of an elliptical galaxy. A circumnuclear structure observed near PKS 1502+036 may be the result of galaxy interactions. A BH mass of ∼7 × 108 M⊙ has been estimated by the bulge luminosity. The presence of an additional faint disc component cannot be ruled out with the present data, but this would reduce the BH mass estimate by only ∼30 per cent. These results, together with analogous findings obtained for FBQS J1644+2619, indicate that the relativistic jets in γ-ray-emitting NLSy1 are likely produced by massive black holes at the centre of elliptical galaxies.
Context. Blazars, which include BL Lacs and flat-spectrum radio quasars, represent the brightest persistent extragalactic sources in the high-energy (HE; 10 MeV–100 GeV) and very-high-energy (VHE; E ...> 100 GeV) γ -ray sky. Due to their almost featureless optical/UV spectra, it is challenging to measure the redshifts of BL Lacs. As a result, about 50% of γ -ray BL Lacs lack a firm measurement of this property, which is fundamental for population studies, indirect estimates of the extragalactic background light, and fundamental physics probes (e.g., searches for Lorentz-invariance violation or axion-like particles). Aims. This paper is the third in a series of papers aimed at determining the redshift of a sample of blazars selected as prime targets for future observations with the next generation, ground-based VHE γ -ray astronomy observatory, Cherenkov Telescope Array Observatory (CTAO). The accurate determination of the redshift of these objects is an important aid in source selection and planning of future CTAO observations. Methods. Promising targets were selected following a sample selection obtained with Monte Carlo simulations of CTAO observations. The selected targets were expected to be detectable with CTAO in observations of 30 h or less. We performed deep spectroscopic observations of 41 of these blazars using the Keck II, Lick, SALT, GTC, and ESO/VLT telescopes. We carefully searched for spectral lines in the spectra and whenever features of the host galaxy were detected, we attempted to model the properties of the host galaxy. The magnitudes of the targets at the time of the observations were also compared to their long-term light curves. Results. Spectra from 24 objects display spectral features or a high signal-to-noise ratio (S/N). From these, 12 spectroscopic redshifts were determined, ranging from 0.2223 to 0.7018. Furthermore, 1 tentative redshift (0.6622) and 2 redshift lower limits at z > 0.6185 and z > 0.6347 were obtained. The other 9 BL Lacs showed featureless spectra, despite the high S/N (≥100) observations. Our comparisons with long-term optical light curves tentatively suggest that redshift measurements are more straightforward during an optical low state of the active galactic nucleus. Overall, we have determined 37 redshifts and 6 spectroscopic lower limits as part of our programme thus far.
ABSTRACT
Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very high-energy (VHE) gamma rays to date with the imaging atmospheric Cherenkov telescopes. ...Apart from four nearby radio galaxies and two AGNs of unknown type, all known VHE AGNs are blazars. Knowledge of the cosmological redshift of gamma-ray blazars is key to enabling the study of their intrinsic emission properties, as the interaction between gamma rays and the extragalactic background light (EBL) results in a spectral softening. Therefore, the redshift determination exercise is crucial to indirectly placing tight constraints on the EBL density, and to studying blazar population evolution across cosmic time. Due to the powerful relativistic jets in blazars, most of their host galaxies’ spectral features are outshined, and dedicated high signal-to-noise (S/N) spectroscopic observations are required. Deep medium- to high-resolution spectroscopy of 33 gamma-ray blazar optical counterparts was performed with the European Southern Observatory, New Technology Telescope, Keck II telescope, Shane 3-metre telescope, and the Southern African Large Telescope. From the sample, spectra from 25 objects display spectral features or are featureless and have high S/N. The other eight objects have low-quality featureless spectra. We systematically searched for absorption and emission features and estimated, when possible, the fractional host galaxy flux in the measured total flux. Our measurements yielded 14 firm spectroscopic redshifts, ranging from 0.0838 to 0.8125, one tentative redshift, and two lower limits: one at $z > 0.382$ and the other at z > 0.629.
The Large Area Telescope on board the Fermi satellite observed a gamma-ray flare in the Crab Nebula lasting for approximately nine days in April of 2011. The source, which at optical wavelengths has ...a size of approx =11 lt-yr across, doubled its gamma-ray flux within eight hours. The peak photon flux was (186 + or - 6) x 10 super(-7) cm super(-2) s super(-1) above 100 MeV, which corresponds to a 30-fold increase compared to the average value. During the flare, a new component emerged in the spectral energy distribution, which peaked at an energy of (375 + or - 26) MeV at flare maximum. The observations imply that the emission region was likely relativistically beamed toward us and that variations in its motion are responsible for the observed spectral variability.
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