Using the tilted-pump-pulse-front scheme, we generate single-cycle terahertz ͑THz͒ pulses by optical rectification of femtosecond laser pulses in LiNbO 3 . In our THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pump-to-THz conversion efficiency. By using an uncooled microbolometer-array THz camera, it is found that the THz beam leaving the output face of the LN crystal can be regarded as a collimated rather than point source. The designed focusing geometry enables tight focus of the collimated THz beam with a spot size close to the diffraction limit, and the maximum THz electric field of 1.2 MV/cm is obtained. © 2011 American Institute of Physics. ͓doi:10.1063/1.3560062͔Recent successful developments in efficient high-power terahertz ͑THz͒ pulse generation has created many promising applications such as in large-scale object imaging, medical diagnosis and treatment, and remote sensing techniques for security issues.1,2 In addition, intense THz pulses allow for study of unexplored nonlinear phenomena such as coherent THz manipulation of quantum states, 3,4 high-order harmonic generation, 5 nonlinear optical processes, and nonlinear transport phenomena in solids. [6][7][8][9][10][11][12] Since Hebling et al.13 ͑2002͒ proposed a tilted-pumppulse-front scheme for efficient phase-matched THz pulse generation using LiNbO 3 crystals, the technique has been rapidly developing. This technique has demonstrated the possibility of THz pulse generation with energies on the scale of 10 J by using an amplified Ti:sapphire laser with low repetition frequencies.14,15 To make the technique versatile for applications and useful for study of unexplored nonlinear phenomena, a generation setup to obtain optimal THz beam characteristics and a maximized THz peak field is required. A recent detailed analysis of the scheme predicted that the imaging errors in the setup consisting of a grating and lenses can lead to distortion in THz intensity profile after the LiNbO 3 output surface. 16 This distortion could create strong and ambiguous divergence in the THz beam, causing inaccuracy in optimal optis design for THz measurement, thereby limiting its applications.In this paper, we report the generation of single-cycle THz pulses using the tilted-pump-pulse-front scheme with a 1.3 mol % MgO-doped stoichiometric LiNbO 3 ͑LN͒ crystal. In the THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pumpto-THz conversion efficiency. The propagation characteristics of the THz beam leaving the output face of the LN crystal were measured by an uncooled microbolometer-array THz camera. The results show that the THz beam had divergence of 52Ϯ 5 mrad in the horizontal direction for 1 THz.The designed focusing geometry for the collimated THz beam enables tight focus onto the electro-optic ͑EO͒ crystal with a spot size ...

Powerful radio jets from active galactic nuclei are thought to be powered by the accretion of material onto the supermassive black hole (the 'central engine'). M87 is one of the closest examples of this phenomenon, and the structure of its jet has been probed on a scale of about 100 Schwarzschild radii (R(s), the radius of the event horizon). However, the location of the central black hole relative to the jet base (a bright compact radio 'core') remains elusive. Observations of other jets indicate that the central engines are located about 10(4)-10(6)R(s) upstream from the radio core. Here we report radio observations of M87 at six frequencies that allow us to achieve a positional accuracy of about 20 microarcseconds. As the jet base becomes more transparent at higher frequencies, the multifrequency position measurements of the radio core enable us to determine the upstream end of the jet. The data reveal that the central engine of M87 is located within 14-23R(s) of the radio core at 43 GHz. This implies that the site of material infall onto the black hole and the eventual origin of the jet reside in the bright compact region seen on the image at 43 GHz.

We report on results from new high-sensitivity, high-resolution 86 GHz (3.5 millimeter) observations of the jet base in the nearby radio galaxy M87, obtained by the Very Long Baseline Array in conjunction with the Green Bank Telescope. The resulting image has a dynamic range exceeding 1500 to 1, the highest ever achieved for this jet at this frequency, resolving and imaging a detailed jet formation/collimation structure down to ∼10 Schwarzschild radii (R s ). The obtained 86 GHz image clearly confirms some important jet features known at lower frequencies, i.e., a wide-opening angle jet base, a limb-brightened intensity profile, a parabola-shape collimation profile and a counter jet. The limb-brightened structure is already well developed at < 0.2 mas (< 28 R s , projected) from the core, where the corresponding apparent opening angle becomes as wide as ∼100 • . The subsequent jet collimation near the black hole evolves in a complicated manner; there is a "constricted" structure at tens R s from the core, where the jet cross section is locally shrinking. We suggest that an external pressure support from the inner part of radiatively-inefficient accretion flow may be dynamically important in shaping/confining the footprint of the magnetized jet. We also present the first VLBI 86 GHz polarimetric experiment for this source, where a highly polarized (∼20%) feature is detected near the jet base, indicating the presence of a well-ordered magnetic field. As a by-product, we additionally report a 43/86 GHz polarimetric result for our calibrator 3C 273 suggesting an extreme rotation measure near the core.

We investigated the detailed inner jet structure of M87 using the Very Long Baseline Array data at 2, 5, 8.4, 15, 23.8, 43, and 86 GHz, especially focusing on the multi-frequency properties of the radio core at the jet base. First, we measured a size of the core region transverse to the jet axis, defined as W c , at each frequency ν, and found a relation between W c and ν as W c (ν) ∝ ν −0.71±0.05 . Then, by combining W c (ν) and the frequency dependence of the core position r c (ν), which was obtained by our previous study, we have constructed a collimation profile of the innermost jet W c (r) down to ∼10 Schwarzschild radii (R s ) from the central black hole. We found that W c (r) smoothly connects with the width profile of the outer edge-brightened, parabolic jet, and then follows a similar radial dependence down to several tens of R s . Closer to the black hole, the measured radial profile suggests a possible change of the jet collimation shape from the outer parabolic one, in which the jet shape tends to become more radially-oriented. This could be related to a magnetic collimation process or/and interections with surrounding materials at the jet base. The present results shed light on the importance of higher-sensitivity/resolution imaging studies for M87 at 86, 43 and also 22 GHz, and should be examined more rigorously.

The M87 jet is extensively examined by utilizing general relativistic magnetohydrodynamic (GRMHD) simulations as well as the steady axisymmetric force-free electrodynamic (FFE) solution. Quasi-steady funnel jets are obtained in GRMHD simulations up to the scale of ∼ 100 gravitational radius (r g ) for various black hole (BH) spins. As is known, the funnel edge is approximately determined by the following equipartitions; i) the magnetic and rest-mass energy densities and ii) the gas and magnetic pressures. Our numerical results give an additional factor that they follow the outermost parabolic streamline of the FFE solution, which is anchored to the event horizon on the equatorial plane. We also identify the matter dominated, non-relativistic corona/wind play a dynamical role in shaping the funnel jet into the parabolic geometry. We confirm a quantitative overlap between the outermost parabolic streamline of the FFE jet and the edge of jet sheath in VLBI observations at ∼ 10 1 -10 5 r g , suggesting that the M87 jet is likely powered by the spinning BH. Our GRMHD simulations also indicate a lateral stratification of the bulk acceleration (i.e., the spine-sheath structure) as well as an emergence of knotty superluminal features. The spin characterizes the location of the jet stagnation surface inside the funnel. We suggest that the limb-brightened feature could be associated with the nature of the BH-driven jet, if the Doppler beaming is a dominant factor. Our findings can be examined with (sub-)mm VLBI observations, giving a clue for the origin of the M87 jet.

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We report the finding of kiloparsec (kpc)-scale radio structures in three radio-loud narrow-line Seyfert 1 (NLS1) galaxies from the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) of the Very Large Array (VLA), which increases the number of known radio-loud NLS1s with kpc-scale structures to six, including two γ-ray emitting NLS1s (PMN J0948+0022 and 1H 0323+342) detected by the Fermi Gamma-ray Space Telescope. The detection rate of extended radio emissions in NLS1s is lower than that in broad-line active galactic nuclei (AGNs) with a statistical significance. We found both core-dominated (blazar-like) and lobe-dominated (radio-galaxy-like) radio structures in these six NLS1s, which can be understood in the framework of the unified scheme of radio-loud AGNs that considers radio galaxies as non-beamed parent populations of blazars. Five of the six NLS1s have (i) extended radio luminosities suggesting jet kinetic powers of 10 44 erg s −1 , which is sufficient to make jets escape from hosts' dense environments, (ii) black holes of 10 7 M ⊙ , which can generate the necessary jet powers from near-Eddington mass accretion, and (iii) two-sided radio structures at kpc scales, requiring expansion rates of ∼ 0.01c-0.3c and kinematic ages of 10 7 years. On the other hand, most typical NLS1s would be driven by black holes of 10 7 M ⊙ in a limited lifetime of ∼ 10 7 years. Hence the kpc-scale radio structures may originate in a small window of opportunity during the final stage of the NLS1 phase just before growing into broad-line AGNs.

We present an investigation into how well the properties of the accretion flow onto a supermassive black hole may be coupled to those of the overlying hot corona. To do so, we specifically measure the characteristic spectral index, Γ, of a power-law energy distribution, over an energy range of 2 to 10 keV, for X-ray selected, broad-lined radio-quiet active galactic nuclei (AGN) up to z∼2 in COSMOS and E-CDF-S. We test the previously reported dependence between Γ and black hole mass, FWHM and Eddington ratio using a sample of AGN covering a broad range in these parameters based on both the Mg ii and Hα emission lines with the later afforded by recent near infrared spectroscopic observations using Subaru/FMOS. We calculate the Eddington ratios, λ Edd , for sources where a bolometric luminosity (L Bol ) has been presented in the literature, based on SED fitting, or, for sources where these data do not exist, we calculate L Bol using a bolometric correction to the X-ray luminosity, derived from a relationship between the bolometric correction, and L X /L 3000 . From a sample of 69 X-ray bright sources (> 250 counts), where Γ can be measured with greatest precision, with an estimate of L Bol , we find a statistically significant correlation between Γ and λ Edd , which is highly significant with a chance probability of 6.59×10 −8 . A statistically significant correlation between Γ and the FWHM of the optical lines is confirmed, but at lower significance than with λ Edd indicating that λ Edd is the key parameter driving conditions in the corona. Linear regression analysis reveals that Γ = (0.32 ± 0.05)log 10 λ Edd +(2.27 ± 0.06) and Γ = (−0.69 ± 0.11)log 10 (FWHM/km s −1 )+(4.44 ± 0.42). Our results on Γ-λ Edd are in very good agreement with previous results. While the Γ-λ Edd relationship means that X-ray spectroscopy may be used to estimate black hole accretion rate, considerable dispersion in the correlation does not make this viable for single sources, however could be valuable however for large X-ray spectral samples, such as those to be produced by eROSITA.