Context.
IceCube has reported a very-high-energy neutrino (IceCube-170922A) in a region containing the blazar
TXS 0506+056
. Correlated gamma-ray activity has led to the first high-probability ...association of a high-energy neutrino with an extragalactic source. This blazar has been found to be in a radio outburst during the neutrino event.
Aims.
Our goal is to probe the sub-milliarcsecond properties of the radio jet right after the neutrino detection and during the further evolution of the radio outburst.
Methods.
We performed target of opportunity observations at 43 GHz frequency using very long baseline interferometry imaging, corresponding to 7 mm in wavelength, with the Very Long Baseline Array two and eight months after the neutrino event.
Results.
We produced two images of the radio jet of TXS 0506+056 at 43 GHz with angular resolutions of (0.2 × 1.1) mas and (0.2 × 0.5) mas, respectively. The source shows a compact, high brightness temperature core, albeit not approaching the equipartition limit and a bright and originally very collimated inner jet. Beyond approximately 0.5 mas from the millimeter-VLBI core, the jet loses this tight collimation and expands rapidly. During the months after the neutrino event associated with this source, the overall flux density is rising. This flux density increase happens solely within the core. Notably, the core expands in size with apparent superluminal velocity during these six months so that the brightness temperature drops by a factor of three despite the strong flux density increase.
Conclusions.
The radio jet of TXS 0506+056 shows strong signs of deceleration and/or a spine-sheath structure within the inner 1 mas, corresponding to about 70–140 pc in deprojected distance, from the millimeter-VLBI core. This structure is consistent with theoretical models that attribute the neutrino and gamma-ray production in TXS 0506+056 to interactions of electrons and protons in the highly relativistic jet spine with external photons originating from a slower moving jet region. Proton loading due to jet-star interactions in the inner host galaxy is suggested as the possible cause of deceleration.
Aims.
The collimation of relativistic jets in galaxies is a poorly understood process. Detailed radio studies of the jet collimation region have been performed so far in a few individual objects, ...providing important constraints for jet formation models. However, the extent of the collimation zone as well as the nature of the external medium possibly confining the jet are still debated.
Methods.
In this article, we present a multifrequency and multiscale analysis of the radio galaxy NGC 315, including the use of mm-VLBI data up to 86 GHz, aimed at revealing the evolution of the jet collimation profile. We then consider results from the literature to compare the jet expansion profile in a sample of 27 low-redshift sources, mainly comprising radio galaxies and BL Lacs, which were classified based on the accretion properties as low-excitation (LEG) and high-excitation (HEG) galaxies.
Results.
We propose that the jet collimation in NGC 315 is completed on sub-parsec scales. A transition from a parabolic to conical jet shape is detected at
z
t
= 0.58 ± 0.28 parsecs or ∼5 × 10
3
Schwarzschild radii (
R
S
) from the central engine, a distance which is much smaller than the Bondi radius,
r
B
∼ 92 pc, estimated based on X-ray data. The jet in this and in a few other LEG in our sample may be initially confined by a thick disk extending out to ∼10
3
− 10
4
R
S
. A comparison between the mass-scaled jet expansion profiles of all sources indicates that jets in HEG are surrounded by thicker disk-launched sheaths and collimate on larger scales with respect to jets in LEG. These results suggest that disk winds play an important role in the jet collimation mechanism, particularly in high-luminosity sources. The impact of winds on the origin of the FRI and FRII dichotomy in radio galaxies is also discussed.
The IceCube Collaboration has published four years of data and the observed neutrino flux is significantly in excess of the expected atmospheric background. Due to the steeply falling atmospheric ...background spectrum, events at the highest energies are most likely extraterrestrial. In our previous approach we have studied blazars as the possible origin of the High-Energy Starting Events (HESE) neutrino events at PeV energies. In this work we extend our study to include all HESE neutrinos (which does not include IC 170922A) at or above a reconstructed energy of 100 TeV, but below 1 PeV. We study the X-ray and γ-ray data of all (∼200) 3LAC blazars that are positionally consistent with the neutrino events above 100 TeV to determine the maximum neutrino flux from these sources. This larger sample allows us to better constrain the scaling factor between the observed and maximum number of neutrino events. We find that when we consider a realistic neutrino spectrum and other factors, the number of neutrinos is in good agreement with the detected number of IceCube HESE events. We also show that there is no direct correlation between Fermi/LAT γ-ray flux and the IceCube neutrino flux and that the expected number of neutrinos is consistent with the non-detection of individual bright blazars.
Supermassive black holes (SMBH) are essential for the production of jets in radio-loud active galactic nuclei (AGN). Theoretical models based on (Blandford & Znajek 1977, MNRAS, 179, 433) extract the ...rotational energy from a Kerr black hole, which could be the case for NGC 1052, to launch these jets. This requires magnetic fields on the order of 103G to 104G. We imaged the vicinity of the SMBH of the AGN NGC 1052 with the Global Millimetre VLBI Array and found a bright and compact central feature that is smaller than 1.9 light days (100 Schwarzschild radii) in radius. Interpreting this as a blend of the unresolved jet bases, we derive the magnetic field at 1 Schwarzschild radius to lie between 200 G and ~ 8.3 × 104 G consistent with Blandford & Znajek models.
Context.
With the increase in the sensitivity and resolution of radio interferometry within recent years, the study of the collimation and acceleration region of extragalactic jets in active galactic ...nuclei (AGN) has come into focus. Whereas a large fraction of AGN jets show a change from parabolic to conical collimation profile around the Bondi radius, there is a small number of sources that display a deviation from this standard picture, including the radio galaxy NGC 1052.
Aims.
We study the jet width profile, which provides valuable information about the interplay between the central engine and accretion disk system and the formation, acceleration, and collimation of the jets.
Methods.
We observed the double-sided, low-radio-power active galaxy NGC 1052 at six frequencies with the VLBA in 2017 and at 22 GHz with RadioAstron in 2016. These data are combined with archival 15, 22, and 43 GHz multi-epoch VLBA observations. From ridge-line fitting we obtained width measurements along the jet and counter-jet which were fitted with single and broken power laws.
Results.
We find a clear break point in the jet collimation profile at ∼10
4
R
S
(Schwarzschild radii). Downstream of the break, the collimation is conical with a power-law index of 1.0 − 1.2 (cylindrical 0; parabolic 0.5; conical 1) for both jets. On the other hand, the upstream power-law index of 0.36 for the approaching jet is neither cylindrical nor parabolic, and the value of 0.16 for the receding jet suggests this latter is close-to cylindrical. For both jets we find a large opening angle of ∼30° at a distance of ∼10
3
R
S
and well-collimated structures with an opening angle of < 10° downstream of the break.
Conclusions.
There are significant differences between the upstream collimation profiles of the approaching (eastern) and receding (western) jets. Absorption or scattering in the surrounding torus as well as an accretion wind may mimic a cylindrical profile. We need to increase the observing frequencies, which do not suffer from absorption, in order to find the true jet collimation profile upstream of 10
4
R
S
.
Aims. We investigate the dependence of γ-ray brightness of blazars on intrinsic properties of their parsec-scale radio jets and the implication for relativistic beaming. Methods. By combining ...apparent jet speeds derived from high-resolution VLBA images from the MOJAVE program with millimetre-wavelength flux density monitoring data from Metsähovi Radio Observatory, we estimate the jet Doppler factors, Lorentz factors, and viewing angles for a sample of 62 blazars. We study the trends in these quantities between the sources which were detected in γ-rays by the Fermi Large Area Telescope (LAT) during its first three months of science operations and those which were not detected. Results. The LAT-detected blazars have on average higher Doppler factors than non-LAT-detected blazars, as has been implied indirectly in several earlier studies. We find statistically significant differences in the viewing angle distributions between γ-ray bright and weak sources. Most interestingly, γ-ray bright blazars have a distribution of comoving frame viewing angles that is significantly narrower than that of γ-ray weak blazars and centred roughly perpendicular to the jet axis. The lack of γ-ray bright blazars at large comoving frame viewing angles can be explained by relativistic beaming of γ-rays, while the apparent lack of γ-ray bright blazars at small comoving frame viewing angles, if confirmed with larger samples, may suggest an intrinsic anisotropy or Lorentz factor dependence of the γ-ray emission.
Blazar jets are extreme environments, in which relativistic proton interactions with an ultraviolet photon field could give rise to photopion production. High-confidence associations of individual ...high-energy neutrinos with blazar flares could be achieved via spatially and temporally coincident detections. In 2017, the track-like, extremely high-energy neutrino event IC 170922A was found to coincide with increased γ-ray emission from the blazar TXS 0506+056, leading to the identification of the most promising neutrino point-source candidate so far. We calculate the expected number of neutrino events that can be detected with IceCube, based on a broadband parameterization of bright short-term blazar flares that were observed in the first 6.5 yr of Fermi/Large Area Telescope observations. We find that the integrated keV-to-GeV fluence of most individual blazar flares is far too small to yield a substantial Poisson probability for the detection of one or more neutrinos with IceCube. We show that the sample of potentially detectable high-energy neutrinos from individual blazar flares is rather small. We further show that the blazars 3C 279 and PKS 1510−089 dominate the all-sky neutrino prediction from bright and short-term blazar flares. In the end, we discuss strategies to search for more significant associations in future data unblindings of IceCube and KM3NeT.
Context. Few active galactic nuclei (AGN) reveal double-sided jet systems. However, these systems are crucial to understand basic physical properties of extragalactic jets. Aims. We address the ...questions whether jets in AGN are symmetric in nature, how well they are collimated on small scales, and how they evolve with time. Methods. We monitored the sub-parsec scale morphology of NGC 1052 with the Very Long Baseline Array at 43 GHz from 2005 to 2009. Results. A detailed study of 29 epochs show a remarkable asymmetry between both jets. A kinematic analysis of the outflows reveals higher apparent velocities for the eastern (approaching) jet as compared to the western (receding) jet, i.e., βej = 0.529 ± 0.038 and βwj = 0.343 ± 0.037, respectively. Contradictory to previous studies, we find higher flux densities for the western jet as compared to the eastern. The distribution of brightness temperature and jet width features well-collimated jets up to 1 mas distance to the dynamic center and a nearly conical outflow further outward. By combining flux density ratios and velocities of the jet flows, we were unable to find a combination of intrinsic velocities and inclination angles of the jets that is consistent for all four years of observation; this contradicts findings for symmetrically evolving jets. Spectral index maps between quasi-simultaneous 22 GHz and 43 GHz observations support the existence of an optically thick absorber covering the innermost ≃1.6 mas around the 43 GHz central feature and an optically thin jet emission with a spectral index of ≤−1. Conclusions. Our results fit into a picture in which we expect larger internal energy and/or magnetic flux in the western jet and higher kinetic energy in the eastern jet. Previous observations at lower frequencies have found slower velocities of the moving jet features as compared to this work. Considering the different velocities in different areas, we suggest a spine-sheath structure with a faster inner layer and slower outer layer.
We discuss the jet kinematics of a complete flux-density-limited sample of 135 radio-loud active galactic nuclei (AGNs) resulting from a 13 year program to investigate the structure and evolution of ...parsec-scale jet phenomena. Our analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained between 2002 and 2007, but includes our previously published observations made at the same wavelength, and is supplemented by VLBA archive data. In all, we have used 2424 images spanning the years 1994-2007 to study and determine the motions of 526 separate jet features in 127 jets. The data quality and temporal coverage (a median of 15 epochs per source) of this complete AGN jet sample represent a significant advance over previous kinematics surveys. In all but five AGNs, the jets appear one-sided, most likely the result of differential Doppler boosting. In general, the observed motions are directed along the jet ridge line, outward from the optically thick core feature. We directly observe changes in speed and/or direction in one third of the well-sampled jet components in our survey. While there is some spread in the apparent speeds of separate features within an individual jet, the dispersion is about three times smaller than the overall dispersion of speeds among all jets. This supports the idea that there is a characteristic flow that describes each jet, which we have characterized by the fastest observed component speed. The observed maximum speed distribution is peaked at ~10c, with a tail that extends out to ~50c. This requires a distribution of intrinsic Lorentz factors in the parent population that range up to ~50. We also note the presence of some rare low-pattern speeds or even stationary features in otherwise rapidly flowing jets that may be the result of standing re-collimation shocks, and/or a complex geometry and highly favorable Doppler factor.
Catching the radio flare in CTA 102 Fromm, C M; Ros, E; Perucho, M ...
Astronomy and astrophysics (Berlin),
09/2013, Letnik:
557
Journal Article
Recenzirano
Odprti dostop
The temporal and spatial spectral evolution of the jets of active galactic nuclei (AGN) can be studied with multi-frequency, multi-epoch very-long-baselineinterferometry (VLBI) observations. The ...combination of both morphological (kinematical) and spectral parameters can be used to derive source-intrinsic physical properties, such as the magnetic field and the nonthermal particle density. Furthermore, we can trace the temporal variations in the source-intrinsic parameters during the flare, which may reflect the interaction between the underlying plasma and a traveling shock wave. In the first two papers of this series (Papers I and II), we analyzed the singledish light curves and the VLBI kinematics of the blazar CTA 102 and suggested a shock-shock interaction between a traveling and a standing shock wave as a possible scenario to explain the observed evolution of the component associated to the 2006 flare. In this paper we investigate the core shift and spectral evolution to test our hypothesis of a shock-shock interaction.
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