Messier 87 (M 87) is one of the nearest radio galaxies with a prominent jet extending from sub-pc to kpc scales. Because of its proximity and the large mass of its central black hole (BH), it is one ...of the best radio sources for the study of jet formation. We study the physical conditions near the jet base at projected separations from the BH of ~7–100 Schwarzschild radii (Rsch). Global mm-VLBI Array (GMVA) observations at 86 GHz (λ = 3.5 mm) provide an angular resolution of ~50 μas, which corresponds to a spatial resolution of only 7 Rsch and reach the small spatial scale. We use five GMVA data sets of M 87 obtained from 2004 to 2015 and present new high angular resolution VLBI maps at 86 GHz. In particular, we focus on the analysis of the brightness temperature, the jet ridge lines, and the ratio of jet to counter-jet. The imaging reveals a parabolically expanding limb-brightened jet which emanates from a resolved VLBI core of ~(8–13) Rsch in size. The observed brightness temperature of the core at any epoch is ~(1–3) × 1010 K, which is below the equipartition brightness temperature and suggests magnetic energy dominance at the jet base. We estimate the diameter of the jet at its base to be ~5 Rsch assuming a self-similar jet structure. This suggests that the sheath of the jet may be anchored in the very inner portion of the accretion disk. The image stacking reveals faint emission at the center of the edge-brightened jet on sub-pc scales. We discuss its physical implication within the context of the spine-sheath structure of the jet.
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.
We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultrahigh angular ...resolution of 50 μas (corresponding to ∼200Rs). We also add complementary multiwavelength data from the Very Long Baseline Array (VLBA; 15 and 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0 and 343.5 GHz). At 86 GHz, we measured a fractional linear polarization of ∼2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (∼0.3−0.7% and <0.1%, respectively). This suggests an increasing linear polarization degree toward shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ∼105–6 rad m2 in the core at ≳43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.
Context.
Controversial studies on the jet collimation profile of BL Lacertae (BL Lac), the eponymous blazar of the BL Lac objects class, complicate the scenario in this already puzzling class of ...objects. Understanding the jet geometry in connection with the jet kinematics and the physical conditions in the surrounding medium is fundamental for better constraining the formation, acceleration, and collimation mechanisms in extragalactic jets.
Aims.
With the aim of investigating the jet geometry in the innermost regions of the BL Lac jet, and resolving the controversy, we explore the radio jet in this source using high-resolution millimeter-wave VLBI data.
Methods.
We collect 86 GHz GMVA and 43 GHz VLBA data to obtain stacked images that we use to infer the jet collimation profile by means of two comparable methods. We analyze the kinematics at 86 GHz, and we discuss it in the context of the jet expansion. Finally, we consider a possible implication of the Bondi sphere in shaping the jet of BL Lac.
Results.
The jet in BL Lac expands with an overall conical geometry. A higher expanding rate region is observed between ∼5 and 10 pc (de-projected) from the black hole. Such a region is associated with the decrease in brightness usually observed in high-frequency VLBI images of BL Lac. The jet retrieves the original jet expansion around 17 pc, where the presence of a recollimation shock is supported by both the jet profile and the 15 GHz kinematics (MOJAVE survey). The change in the jet expansion profile occurring at ∼5 pc could be associated with a change in the external pressure at the location of the Bondi radius (∼3.3 × 10
5
R
S
).
The astrophysical sources of the extraterrestrial, very high-energy neutrinos detected by the IceCube collaboration remain to be identified. Gamma-ray (γ-ray) blazars have been predicted to yield a ...cumulative neutrino signal exceeding the atmospheric background above energies of 100 TeV, assuming that both the neutrinos and the γ-ray photons are produced by accelerated protons in relativistic jets. As the background spectrum falls steeply with increasing energy, the individual events with the clearest signature of being of extraterrestrial origin are those at petaelectronvolt energies. Inside the large positional-uncertainty fields of the first two petaelectronvolt neutrinos detected by IceCube, the integrated emission of the blazar population has a sufficiently high electromagnetic flux to explain the detected IceCube events, but fluences of individual objects are too low to make an unambiguous source association. Here, we report that a major outburst of the blazar PKS B1424-418 occurred in temporal and positional coincidence with a third petaelectronvolt-energy neutrino event (HESE-35) detected by IceCube. On the basis of an analysis of the full sample of γ-ray blazars in the HESE-35 field, we show that the long-term average γ-ray emission of blazars as a class is in agreement with both the measured all-sky flux of petaelectronvolt neutrinos and the spectral slope of the IceCube signal. The outburst of PKS B1424-418 provides an energy output high enough to explain the observed petaelectronvolt event, suggestive of a direct physical association.
The S-star cluster in the Galactic center allows us to study the physics close to a supermassive black hole, including distinctive dynamical tests of general relativity. Our best estimates for the ...mass of and the distance to Sgr A* using the three stars with the shortest period (S2, S38, and S55/S0-102) and Newtonian models are MBH = (4.15 0.13 0.57) × 106 M and R0 = 8.19 0.11 0.34 kpc. Additionally, we aim at a new and practical method to investigate the relativistic orbits of stars in the gravitational field near Sgr A*. We use a first-order post-Newtonian approximation to calculate the stellar orbits with a broad range of periapse distance rp. We present a method that employs the changes in orbital elements derived from elliptical fits to different sections of the orbit. These changes are correlated with the relativistic parameter defined as rs/rp (with rs being the Schwarzschild radius) and can be used to derive from observational data. For S2 we find a value of = 0.00088 0.00080, which is consistent, within the uncertainty, with the expected value of = 0.00065 derived from MBH and the orbit of S2. We argue that the derived quantity is unlikely to be dominated by perturbing influences such as noise on the derived stellar positions, field rotation, and drifts in black hole mass.
Abstract
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We have analyzed a comprehensive ...submillimeter (including new observations simultaneous with NIR monitoring), NIR, and 2–8 keV data set. Submillimeter variations tend to lag those in the NIR by ∼30 minutes. An approximate Bayesian computation fit to the X-ray first-order structure function shows significantly less power at short timescales in the X-rays than in the NIR. Less X-ray variability at short timescales, combined with the observed NIR–X-ray correlations, means the variability can be described as the result of two strictly correlated stochastic processes, the X-ray process being the low-pass-filtered version of the NIR process. The NIR–X-ray linkage suggests a simple radiative model: a compact, self-absorbed synchrotron sphere with high-frequency cutoff close to NIR frequencies plus a synchrotron self-Compton scattering component at higher frequencies. This model, with parameters fit to the submillimeter, NIR, and X-ray structure functions, reproduces the observed flux densities at all wavelengths, the statistical properties of all light curves, and the time lags between bands. The fit also gives reasonable values for physical parameters such as magnetic flux density
B
≈ 13 G, source size
L
≈ 2.2
R
S
, and high-energy electron density
n
e
≈ 4 × 10
7
cm
−3
. An animation illustrates typical light curves, and we make public the parameter chain of our Bayesian analysis, the model implementation, and the visualization code.
We present results from the high-precision timing analysis of the pulsar-white dwarf (WD) binary PSR J1012+5307 using 15 years of multitelescope data. Observations were performed regularly by the ...European Pulsar Timing Array (EPTA) network, consisting of Effelsberg, Jodrell Bank, Westerbork and Nançay. All the timing parameters have been improved from the previously published values, most by an order of magnitude. In addition, a parallax measurement of π= 1.2(3) mas is obtained for the first time for PSR J1012+5307, being consistent with the optical estimation from the WD companion. Combining improved 3D velocity information and models for the Galactic potential, the complete evolutionary Galactic path of the system is obtained. A new intrinsic eccentricity upper limit of e < 8.4 × 10−7 is acquired, one of the smallest calculated for a binary system and a measurement of the variation of the projected semimajor axis also constrains the system's orbital orientation for the first time. It is shown that PSR J1012+5307 is an ideal laboratory for testing alternative theories of gravity. The measurement of the change of the orbital period of the system of is used to set an upper limit on the dipole gravitational wave emission that is valid for a wide class of alternative theories of gravity. Moreover, it is shown that in combination with other binary pulsars PSR J1012+5307 is an ideal system to provide self-consistent, generic limits, based only on millisecond pulsar data, for the dipole radiation and the variation of the gravitational constant .
Context. The γ-ray BL Lac object OJ 287 is known to exhibit inner-parsec “jet-wobbling”, high degrees of variability at all wavelengths and quasi-stationary features, including an apparent (≈100°) ...position-angle change in projection on the sky plane. Aims. Sub-50 micro-arcsecond resolution 86 GHz observations with the global mm-VLBI array (GMVA) supplement ongoing multi-frequency VLBI blazar monitoring at lower frequencies. Using these maps, together with cm/mm total intensity and γ-ray observations from Fermi-LAT from 2008−2014, we aim to determine the location of γ-ray emission and to explain the inner-mas structural changes. Methods. Observations with the GMVA offer approximately double the angular resolution compared with 43 GHz VLBA observations and enable us to observe above the synchrotron self-absorption peak frequency. Fermi-LAT γ-ray data were reduced and analysed. The jet was spectrally decomposed at multiple locations along the jet. From this, we could derive estimates of the magnetic field using equipartition and synchrotron self-absorption arguments. How the field decreases down the jet provided an estimate of the distance to the jet apex and an estimate of the magnetic field strength at the jet apex and in the broad line region. Combined with accurate kinematics, we attempt to locate the site of γ-ray activity, radio flares, and spectral changes. Results. Strong γ-ray flares appeared to originate from either the so-called core region, a downstream stationary feature, or both, with γ-ray activity significantly correlated with radio flaring in the downstream quasi-stationary feature. Magnetic field estimates were determined at multiple locations along the jet, with the magnetic field found to be ≥1.6 G in the core and ≤0.4 G in the downstream quasi-stationary feature. We therefore found upper limits on the location of the VLBI core as ≲6.0 pc from the jet apex and determined an upper limit on the magnetic field near the jet base of the order of thousands of Gauss.
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.
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