The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow ...relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.
ABSTRACT
Relativistic amplification boosts the contribution of the jet base to the total emission in blazars, thus making single-dish observations useful and practical to characterize their physical ...state, particularly during episodes of enhanced multiwavelength activity. Following the detection of a new gamma-ray source by Fermi-LAT in 2017 July, we observed S4 0444+63 in order to secure its identification as a gamma-ray blazar. We conducted observations with the Medicina and Noto radio telescopes at 5, 8, and 24 GHz for a total of 12 epochs between 2017 August 1 and 2018 September 22. We carried out the observations with on-the-fly cross-scans and reduced the data with our newly developed Cross-scan Analysis Pipeline, which we present here in detail for the first time. We found the source to be in an elevated state of emission at radio wavelength, compared to historical values, which lasted for several months. The maximum luminosity was reached on 2018 May 16 at 24 GHz, with $L_{24}=(1.7\pm 0.3)\times 10^{27}\ \mathrm{W\, Hz}^{-1}$; the spectral index was found to evolve from slightly rising to slightly steep. Besides the new observations, which have proved to be an effective and efficient tool to secure the identification of the source, additional single dish and very long baseline interferometry data provide further insight on the physics of the source. We estimate a synchrotron peak frequency νpeak = 1012.97 Hz and a Doppler factor in excess of δ ∼ 5.0, with both quantities playing a role in the gamma-ray emission from the source.
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.
We report on high angular resolution polarimetric observations of the nearby radio galaxy M87 using the Very Long Baseline Array at 24 GHz (
λ
= 1.3 cm) and 43 GHz (
λ
= 7 mm) in 2017–2018. New ...images of the linear polarization substructure in the nuclear region are presented, characterized by a two-component pattern of polarized intensity and smooth rotation of the polarization plane around the 43 GHz core. From a comparison with an analogous dataset from 2007, we find that this global polarization pattern remains stable on a time interval of 11 yr, while showing smaller month-scale variability. We discuss the possible Faraday rotation toward the M87 nucleus at centimeter to millimeter wavelengths. These results can be interpreted in a scenario where the observed polarimetric pattern is associated with the magnetic structure in the confining magnetohydrodynamic wind, which also serves as the source of the observed Faraday rotation.
Jetted radio-quiet quasars at z > 5 Sbarrato, T.; Ghisellini, G.; Giovannini, G. ...
Astronomy and astrophysics (Berlin),
11/2021, Letnik:
655
Journal Article
Recenzirano
Odprti dostop
We report on the JVLA observations of three high-redshift active galactic nuclei (AGNs) that have black hole masses estimated to be among the largest known. Two of them, SDSS J0100+2802 and SDSS ...J0306+1853 at redshift 6.326 and 5.363, respectively, are radio-quiet AGNs according to the classic definition, while the third (B2 1023+25 at
z
= 5.284) is a powerful blazar. The JVLA data clearly show a radio structure in the first source and a radio emission with a relatively steep radio spectrum in the second one, indicating the presence of a radio jet and a diffuse component. Therefore, being radio-quiet does not exclude the presence of a powerful relativistic jet, which has important consequences on population studies and on the ratio between jetted and non-jetted AGNs. We can estimate the viewing angle of these jets, and this allows us to find, albeit with some uncertainty, the density of black holes with a mass in excess of 10
10
M
⊙
at high redshifts. We found that their density in jetted AGNs is very large in the redshift bin 5–6 and comparable with the overall AGN population of the same optical luminosity. Jets might thus play a crucial role in the fast formation and evolution of the most massive black holes in the early Universe. They are more common than what is expected from wide radio surveys with milliJansky flux sensitivity. Deeper JVLA or very-long-baseline interferometry observations are key to discovering a possible relativistic jet population hiding in plain sight at very high redshift. The discovery of powerful relativistic jets associated with the most massive black holes in the early Universe revives the question: is the jet instrumental for a rapid growth of the black hole or, instead, is the black hole mass the main driver for the jet formation?
ABSTRACT
According to radiative models, radio galaxies and quasars are predicted to produce gamma rays from the earliest stages of their evolution. Exploring their high-energy emission is crucial for ...providing information on the most energetic processes, the origin and the structure of the newly born radio jets. Taking advantage of more than 11 yr of Fermi-LAT data, we investigate the gamma-ray emission of 162 young radio sources (103 galaxies and 59 quasars), the largest sample of young radio sources used so far for such a gamma-ray study. We separately analyse each source and perform the first stacking analysis of this class of sources to investigate the gamma-ray emission of the undetected sources. We detect significant gamma-ray emission from 11 young radio sources, 4 galaxies, and 7 quasars, including the discovery of significant gamma-ray emission from the compact radio galaxy PKS 1007+142 (z = 0.213). The cumulative signal of below-threshold young radio sources is not significantly detected. However, it is about one order of magnitude lower than those derived from the individual sources, providing stringent upper limits on the gamma-ray emission from young radio galaxies (Fγ < 4.6 × 10−11 ph cm−2 s−1) and quasars (Fγ < 10.1 × 10−11 ph cm−2 s−1), and enabling a comparison with the models proposed. With this analysis of more than a decade of Fermi-LAT observations, we can conclude that while individual young radio sources can be bright gamma-ray emitters, the collective gamma-ray emission of this class of sources is not bright enough to be detected by Fermi-LAT.
We present Director’s Discretionary Time multi-frequency observations obtained with the
Jansky
Very Large Array and the Very Long Baseline Array (VLBA) of the blazar PSO J030947.49+271757.31 ...(hereafter PSO J0309+27) at
z
= 6.10 ± 0.03. The milliarcsecond angular resolution of our VLBA observations at 1.5, 5, and 8.4 GHz unveils a bright one-sided jet extended for ∼500 parsecs in projection. This high-
z
radio-loud active galactic nucleus is resolved into multiple compact sub-components that are embedded in a more diffuse and faint radio emission that enshrouds them in a continuous jet structure. We directly derive limits on some physical parameters from observable quantities such as viewing angle and Lorentz and Doppler factors. If PSO J0309+27 is a genuine blazar, as suggested by its X-ray properties, then we find that its bulk Lorentz factor must be relatively low (lower than 5). This value would be in favour of a scenario currently proposed to reconcile the paucity of high-
z
blazars with current predictions. Nevertheless, we cannot exclude that PSO J0309+27 is seen under a larger viewing angle, which would imply that the X-ray emission must be enhanced, for example, by inverse Compton scattering with the cosmic microwave background. More stringent constraints on the bulk Lorentz factor in PSO J0309+27 and on these factors in the other high-
z
blazars are necessary to test whether their properties are intrinsically different from those of the low-
z
blazar population.
We report the discovery of γ-ray emission from the narrow-line Seyfert 1 (NLSy1) galaxy FBQS J1644+2619 by the Large Area Telescope on board the Fermi satellite. The Third Fermi LAT Source catalogue ...reports an unidentified γ-ray source, detected over the first four years of Fermi operation, 0
$_{.}^{\circ}$
23 from the radio position of the NLSy1. Analysing 76 months of γ-ray data (2008 August 4–2014 December 31) we are able to better constrain the localization of the γ-ray source. The new position of the γ-ray source is 0
$_{.}^{\circ}$
05 from FBQS J1644+2619, suggesting a spatial association with the NLSy1. This is the sixth NLSy1 detected at high significance by Fermi-LAT so far. Notably, a significant increase of activity was observed in γ-rays from FBQS J1644+2619 during 2012 July–October, and an increase of activity in the V band was detected by the Catalina Real-Time Sky Survey in the same period.
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