Bulk motion of ultrarelativistic conical blazar jets

Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales, we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their flux-variability and proper-motion measurements. This is in accord with our earlier finding that such ultrarelativistic (Lorentz factor,  Γ > 30  ) conical jets can reconcile the relatively slow apparent motions of Very Long Baseline Interferometry (VLBI) knots in TeV blazars with the extremely fast flows implied by their rapid γ-ray variability. This jet geometry also implies that de-projected jet opening angles will typically be significantly underestimated from VLBI measurements. In addition, de-projected jet lengths will be considerably overestimated if high Lorentz factors and significant opening angles are not taken into account.     

Cassini between Earth and asteroid belt: first in-situ charge measurements of interplanetary grains

Dust particles in interplanetary space are expected to charge up to an electrostatic potential of about +5 V mostly by the solar UV (Horányi, 1996, Annu. Rev. Astrophys. 34, 383). Since the dynamics of charged grains may be quite different from neutral particles, the knowledge of the grain charge Q_d is highly desirable. In the last two decades, several detectors on spacecraft were flown to measure the dust charge in-situ, but the instrumentation was not capable of determining the dust charge unambiguously. The Cosmic Dust Analyser (CDA) on the Cassini spacecraft includes a charge sensitive entrance grid system (QP detector). While entering the instrument, sufficiently charged particles induce a characteristic charge feature onto the grid system, which allows a reliable determination of Q_d as well as of the impact speed v_d. Here we report the first successful in-situ measurement of charged interplanetary dust grains by CDA. Amongst 37 impacts by interplanetary grains registered between November 1999 and January 2000, we identified 6 impacts whose QP signals show a clear feature caused by charged grains, corresponding to Q_d between 1.3 and 5.4 fC. Knowledge of Q_d also allows us to estimate the grain mass m_d. Assuming a potential of φ_d≈+5 V and spheroidal grain morphologies with ratios of the maximum size to the minimum size of less than 2 the masses derived from Q_d were found to be in excess of 10⁻¹³ kg. The dynamics of such particles are dominated by the Sun's gravity. In the framework of the micro-meteoroid models of the Solar System these grains belong to the core population of interplanetary grains (Divine, 1993, J. Geophys. Res. 98, 17029). Furthermore, a rate of 6 impacts of grains with m_d?10⁻¹³ kg during 107 days is in good agreement with the predictions of the interplanetary dust environment model by Staubach et al. (1997, Adv. Space Res. 19, 301). This result demonstrates that charge detectors as the CDA QP system offer a reliable in-situ technique for determining simultaneously both the mass and velocity of big interplanetary grains. The primary CDA subsystem to determine m_d and v_d, however, is an impact ionisation detector. The majority of the 37 recorded dust impacts produced impact charges are well outside the calibrated range. Moreover, these impacts were usually characterised by impact ionisation signals which differ significantly from signals taken in calibration experiments. In this paper we took advantage of the fact that the measurement of Q_d is not affected by the subsequent impact of the grain with the detector. By relating m_d and v_d derived from Q_d of the 6 QP impactors to their corresponding ionisation signals we show that in many cases even for energetic impacts outside the calibrated range meaningful values for the dust mass can be obtained. The observed deviations of the ionisation signals from the calibration measurements are likely due to the large amount of plasma generated by such impacts. We discuss the implications of these findings on a meaningful reduction of impact ionisation signals caused by big particle impacts. A new scheme to identify and to evaluate such signals is presented. These finding are of great importance for future Cassini measurements in the saturnian system.     

85-GHz BIMA observations of the double-hotspot radio galaxy 3C 20

We present 85-GHz observations of the archetypal double-hotspot radio source 3C 20 made with the BIMA millimetre array. The resolution of BIMA allows us to separate the two components of the eastern hotspot. By comparing the BIMA observations with existing VLA data, we show that the spectra of the two hotspot components are very similar, despite the clear differences in their radio structure and their wide separation. We discuss the implications for models of double hotspot formation. Weak emission from the lobes of 3C 20 is detected at 85 GHz, at a level consistent with the predictions of standard spectral ageing models.     

Magnetic fields in the 3C 129 cluster

We present multifrequency VLA observations of the two radio galaxies 3C 129 and 3C 129.1 embedded in a luminous X-ray cluster. These radio observations reveal a substantial difference in the Faraday Rotation Measures (RMs) toward 3C 129.1 at the cluster centre and 3C 129 at the cluster periphery. After deriving the density profile from available X-ray data, we find that the RM structure of both radio galaxies can be fitted by a tangled cluster magnetic field with strength 6 μG extending at least 3 core radii (450 kpc) from the cluster centre. The magnetic field makes up a small contribution to the total pressure (5 per cent) in the central regions of the cluster. The radio morphology of 3C 129.1 appears disturbed on the southern side, perhaps by the higher pressure environment. In contrast with earlier claims for the presence of a moderately strong cooling flow in the 3C 129 cluster, our analysis of the X-ray data places a limit on the mass deposition rate from any such flow of < 1.2 M_⊙ yr⁻¹.     

Size and Physical Properties of the Comet Hale–Bopp (C/1995 O1) Nucleus

This article uses fractal and correlation analysis of solar radio emission for determining the solar coronal rotation. It is clear from this analysis that radio emissions are modulated by the solar rotation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Extending the observable zenith angle of FAST using an offset feed

The five hundred meter aperture spherical radio telescope is will use an active spherical reflector. When the zenith scan angle is changed, the illuminated part of the reflecting surface is made to fit a paraboloid of revolution in real time by active control. The maximum zenith scan angle |ψ_max| of FAST is 30^° under conditions of the geometry selected in order that the feed does not illuminate the ground. The result of this paper shows that the maximum zenith scan angle |ψ_max| can be extended to 69^° by offsetting the feed. This revised version was published online in July 2006 with corrections to the Cover Date.