17,774 research outputs found

    Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708: Constraints on the Hubble Constant

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    We present measurements of bi-directional motions in the jets of the radio galaxy 1946+708 at z=0.101. This is a Compact Symmetric Object with striking S-symmetry. Sensitive 15 GHz observations reveal a compact component at the center of symmetry with a strongly inverted spectrum, that we identify as the core. From five 4.9 GHz observations spread over 4 years we have determined the velocities of four compact jet components. If simple kinematic models can be applied then the inclination of the source and the bulk jet velocity can be directly determined for any assumed value of the Hubble constant. Conversely, the measurements already place constraints on the Hubble constant, and we show how further observations of 1946+708 can yield an increasingly accurate determination of H_0.Comment: in press at ApJ Letters, 12 page LaTex document includes 5 postscript figure

    Detection of Optical Synchrotron Emission from the Radio Jet of 3C279

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    We report the detection of optical and ultraviolet emission from the kiloparsec scale jet of the well-known quasar 3C~279. A bright knot, discovered in archival V and U band {\it Hubble Space Telescope} Faint Object Camera images, is coincident with a peak in the radio jet \sim0.6\arcsec from the nucleus. The detection was also confirmed in Wide Field Planetary Camera-2 images. Archival Very Large Array and MERLIN radio data are also analyzed which help to show that the high-energy optical/UV continuum, and spectrum, are consistent with a synchrotron origin from the same population of relativistic electrons responsible for the radio emission.Comment: 6 pages, 3 figs. accepted for publication in ApJL with minor revision

    Space VLBI Observations Show Tb>1012KT_b > 10^{12} K in the Quasar NRAO 530

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    We present here space-based VLBI observations with VSOP and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 GHz and 5 GHz. The brightness temperature of the core at 1.6 GHz is 5×10115 \times 10^{11} K. The size is near the minimum observable value in the direction of NRAO~530 due to interstellar scattering. The 5 GHz data show a single component with a brightness temperature of ∌3×1012\sim 3 \times 10^{12} K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit (Readhead 1994). This is strong evidence for relativistic motion in a jet requiring model-dependent Doppler boosting factors in the range 6 to 60. We show that a simple homogeneous sphere probably does not model the emission region accurately. We favor instead an inhomogeneous jet model with a Doppler boosting factor of 15.Comment: 12 pages, 2 figures. Accepted for publication in ApJ Letter

    The two sided parsec scale structure of the Low Luminosity Active Galactic Nucleus in NGC 4278

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    We present new Very Long Baseline Interferometry observations of the LINER galaxy NGC 4278. The observations were taken with the Very Long Baseline Array (VLBA) and a single antenna of the Very Large Array (VLA) at 5 GHz and 8.4 GHz and have a linear resolution of <0.1 pc. Our radio data reveal a two sided structure, with symmetric S-shaped jets emerging from a flat spectrum core. We fit the jet brightness with gaussian components, which we identify from a previous observation taken five years before. By comparing the positions of the components in the two epochs, we measure motions between 0.45 +/- 0.14 and 3.76 +/- 0.65 mas, corresponding to apparent velocities < 0.2c, and to ages in the range 8.3 - 65.8 years. Assuming that the radio morphology is intrinsically symmetric and its appearance is governed by Doppler beaming effects, we find that NGC4278 has mildly relativistic jets (beta ~ 0.75), closely aligned to the line-of-sight (2 degrees < theta < 4 degrees). Alternatively, the source could be oriented at a larger angle and asymmetries could be related to the jet interaction with the surrounding medium. We also present new simultaneous VLA observations between 1.4 and 43 GHz, and a 5 GHz light curve between 1972 and 2003. The radio spectrum can be fit by a relatively steep power-law (alpha = 0.54). We find significant variability at 5 GHz. All these arguments indicate that the radiation from NGC 4278 is emitted via the synchrotron process by relativistic particles accelerated by a supermassive black hole. Despite a much lower power, this is the same process that takes place in ordinary radio loud AGNs.Comment: 29 pages, 9 figures, ApJ accepte

    VLBA Imaging of NGC 4261: Symmetric Parsec-scale Jets and the Inner Accretion Region

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    We observed the nuclear region of NGC 4261 (3C270) with the VLBA at two frequencies (1.6 and 8.4 GHz) simultaneously. We find that the position angle of the parsec-scale radio axis agrees, within the errors, with the position angle of the VLA-scale jet. Our observations also reveal basically symmetric radio structures at both 1.6 and 8.4 GHz. Analysis of these images shows that most of the central 10 pc of this source is not significantly affected by free-free absorption, even though HST images show that the nucleus contains a nearly edge-on disk of gas and dust on larger scales. Our highest angular resolution image shows a very narrow gap in emission, which we interpret as an absorption feature, just east of the radio core. This suggests that there may be a small, dense inner accretion disk whose width is less than 0.1 pc. If the inclination of this inner disk is close to that of the larger-scale HST disk it becomes optically thin to 8.4 GHz radiation at a deprojected radius of about 0.8 pc. The brightness of the pc-scale jets falls off very rapidly on both sides of the core, suggesting that the jets are rapidly expanding during the the first several pc of their travel. It appears that there is a small dense inner disk centered on the radio core (the base of the jets; less than 1 pc), a low density bubble filling most of the the inner several pc of the nucleus (within which the radio jets expand rapidly; ~10 pc), and a surrounding cool, higher density region (of which the HST absorption disk is part; tens to hundreds of pc) within which the transverse expansion of the radio jets, as implied by the rate of decrease in jet brightness, is nearly halted.Comment: Accepted by the Astrophysical Journa

    Study of the tidal variations in mesospheric temperature at low and mid latitudes from WINDII and potassium lidar observations

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    Zonal mean daytime temperatures from the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) and nightly temperatures from a potassium (K) lidar are employed in the study of the tidal variations in mesospheric temperature at low and mid latitudes in the Northern Hemisphere. The analysis is applied to observations at 89km height for winter solstice, December to February (DJF), at 55° N, and for May and November at 28° N. The WINDII results are based on observations from 1991 to 1997. The K-lidar observations for DJF at KĂŒhlungsborn (54° N) were from 1996–1999, while those for May and November at Tenerife 28° N were from 1999. To avoid possible effects from year-to-year variability in the temperatures observed, as well as differences due to instrument calibration and observation periods, the mean temperature field is removed from the respective data sets, assuming that only tidal and planetary scale perturbations remain in the temperature residuals. The latter are then binned in 0.5h periods and the individual data sets are fitted in a least-mean square sense to 12-h and 8-h harmonics, to infer semidiurnal and terdiurnal tidal parameters. Both the K-lidar and WINDII independently observed a strong semidiurnal tide in November, with amplitudes of 13K and 7.4K, respectively. Good agreement was also found in the tidal parameters derived from the two data sets for DJF and May. It was recognized that insufficient local time coverage of the two separate data sets could lead to an overestimation of the semidiurnal tidal amplitude. A combined ground-based/satellite data set with full diurnal local time coverage was created which was fitted to 24h+12h+8h harmonics and a novel method applied to account for possible differences between the daytime and nighttime means. The results still yielded a strong semidiurnal tide in November at 28° N with an amplitude of 8.8K which is twice the SD amplitude in May and DJF. The diurnal tidal parameters were practically the same at 28° N and 55° N, in November and DJF, respectively, with an amplitude of 6.5K and peaking at ~9h. The diurnal and semidiurnal amplitudes in May were about the same, 4K, and 4.6K, while the terdiurnal tide had the same amplitudes and phases in May and November at 28° N. Good agreement is found with other experimental data while models tend to underestimate the amplitudes.&lt;br&gt;&lt;br&gt;&lt;b&gt; Key words.&lt;/b&gt; Atmospheric composition and structure (pressure, density and temperature) – Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides

    Resolved CO(1-0) Nuclei in IRAS 14348-1447: Evidence for Massive Bulge Progenitors to Ultraluminous Infrared Galaxies

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    High-resolution, CO(1-0) interferometry of the ultraluminous infrared galaxy IRAS 14348-1447 is presented. The merger system has a molecular gas mass of \~3x10^10 solar masses and a projected nuclear separation of 4.8 kpc (3.5"), making it one of the most molecular gas-rich galaxies known and an ideal candidate for studying the intermediate stages of an ultraluminous merger event. The CO morphology shows two molecular gas components associated with the stellar nuclei of the progenitors, consistent with the idea that the molecular disks are gravitationally bound by the dense bulges of the progenitor galaxies as the interaction proceeds. In contrast, less luminous infrared galaxies observed to date with projected nuclear separations of ~<5 kpc show a dominant CO component between the stellar nuclei. This discrepancy may be an indication that the progenitors of mergers with lower infrared luminosity do not possess massive bulges, and that the gas is stripped during the initial encounter of their progenitors. A comparison of the CO and radio luminosities of the NE and SW component show them to have comparable radio and CO flux ratios of f(NE)/f(SW) ~0.6, possibly indicating that the amount of star-forming molecular gas in the progenitors is correlated with the supernovae rate. The estimate of molecular gas masses of the nuclei and the extent of the radio emission are used to infer that the nuclei of IR 14348-1447 have gas densities comparable to the cores of elliptical galaxies.Comment: LaTex, 5 pages with 1 postscript and 1 jpg figure, ApJ Letters, in pres
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