8,246 research outputs found
Ambitious STS-7 mission to feature first landing at Kennedy
The STS-7 press briefing schedule, NASA select television schedule; launch preparations, countdown and liftoff; major countdown milestones; launch window; STS-7 flight sequence of events, landing timeline; STS-7 flight timeline; landing and post landing operations; flight objectives; Telesat's ANIK-C 2; PALAPA-B; STS-7 experiments; and spacecraft tracking and data network are presented
Is an obscured AGN at the centre of the disk galaxy IC 2497 responsible for Hanny's Voorwerp?
We present the results of VLBI and MERLIN observations of the massive disk
galaxy IC 2497. Optical observations of IC 2497 revealed the existence of a
giant emission nebula "Hanny's Voorwerp" in the proximity of the galaxy.
Earlier short-track 18 cm observations with e-VLBI at 18 cm, detected a compact
radio component (C1) at the centre of IC 2497. The brightness temperature of C1
was measured to be greater than 4E5 K. Deeper, long-track e-VLBI observations
presented here, re-confirm the existence of C1 but also reveal the existence of
a second compact component (C2) located about 230 milliarcseconds to the
North-East of C1. The brightness temperature of C2 is measured to be greater
than 1.4E5 K, suggesting that both components may be related to AGN activity
(e.g. a radio core and jet hotspot). Lower resolution 18cm MERLIN observations
show both components. C1 is shown to be compact with a slight elongation along
the direction of Hanny's Voorwerp, and C2 shows a lot of extended emission in
an almost perpendicular direction to the direction of the Voorwerp. Our results
continue to support the hypothesis that IC 2497 contains an Active Galactic
Nucleus (AGN), and that a jet associated with this AGN clears a path that
permits ionising radiation from the AGN to directly illuminate the emission
nebula.Comment: Presented at The 8th International e-VLBI Workshop: the Science and
Technology of Long Baseline Real-Time Interferometry, EXPReS09, June 22-26
2009 Madrid, Spain. 5 pages, 5 article
WSRT 1.4 GHz Observations of the Hubble Deep Field
We present WSRT 1.38 GHz observations of the Hubble Deep Field (and flanking
fields). 72 hours of data were combined to produce the WSRT's deepest image
yet, achieving an r.m.s. noise level of 8 microJy per beam. We detect radio
emission from galaxies both in the HDF and HFF which have not been previously
detected by recent MERLIN or VLA studies of the field.Comment: 2 pages, 1 figure, to appear in "The Universe at Low Radio
Frequencies", IAU Symposium 199. For colour figures, see
http://www.nfra.nl/~mag/hdf_wsrt.htm
Magnification Ratio of the Fluctuating Light in Gravitational Lens 0957+561
Radio observations establish the B/A magnification ratio of gravitational
lens 0957+561 at about 0.75. Yet, for more than 15 years, the optical
magnfication ratio has been between 0.9 and 1.12. The accepted explanation is
microlensing of the optical source. However, this explanation is mildly
discordant with (i) the relative constancy of the optical ratio, and (ii)
recent data indicating possible non-achromaticity in the ratio. To study these
issues, we develop a statistical formalism for separately measuring, in a
unified manner, the magnification ratio of the fluctuating and constant parts
of the light curve. Applying the formalism to the published data of Kundi\'c et
al. (1997), we find that the magnification ratios of fluctuating parts in both
the g and r colors agrees with the magnification ratio of the constant part in
g-band, and tends to disagree with the r-band value. One explanation could be
about 0.1 mag of consistently unsubtracted r light from the lensing galaxy G1,
which seems unlikely. Another could be that 0957+561 is approaching a caustic
in the microlensing pattern.Comment: 12 pages including 1 PostScript figur
Values of H_0 from Models of the Gravitational Lens 0957+561
The lensed double QSO 0957+561 has a well-measured time delay and hence is
useful for a global determination of H0. Uncertainty in the mass distribution
of the lens is the largest source of uncertainty in the derived H0. We
investigate the range of \hn produced by a set of lens models intended to mimic
the full range of astrophysically plausible mass distributions, using as
constraints the numerous multiply-imaged sources which have been detected. We
obtain the first adequate fit to all the observations, but only if we include
effects from the galaxy cluster beyond a constant local magnification and
shear. Both the lens galaxy and the surrounding cluster must depart from
circular symmetry as well.
Lens models which are consistent with observations to 95% CL indicate
H0=104^{+31}_{-23}(1-\kthirty) km/s/Mpc. Previous weak lensing measurements
constrain the mean mass density within 30" of G1 to be kthirty=0.26+/-0.16 (95%
CL), implying H0=77^{+29}_{-24}km/s/Mpc (95% CL). The best-fitting models span
the range 65--80 km/s/Mpc. Further observations will shrink the confidence
interval for both the mass model and \kthirty.
The range of H0 allowed by the full gamut of our lens models is substantially
larger than that implied by limiting consideration to simple power law density
profiles. We therefore caution against use of simple isothermal or power-law
mass models in the derivation of H0 from other time-delay systems. High-S/N
imaging of multiple or extended lensed features will greatly reduce the H0
uncertainties when fitting complex models to time-delay lenses.Comment: AASTEX, 48 pages 4 figures, 2 tables. Also available at:
http://www.astro.lsa.umich.edu:80/users/philf/www/papers/list.htm
High-z radio starbursts host obscured X-ray AGN
We use Virtual Observatory methods to investigate the association between
radio and X-ray emission at high redshifts. Fifty-five of the 92 HDF(N) sources
resolved by combining
MERLIN+VLA data were detected by Chandra, of which 18 are hard enough and
bright enough to be obscured AGN. The high-z population of microJy radio
sources is dominated by starbursts an order of magnitude more active and more
extended than any found at z<1 and at least a quarter of these simultaneously
host highly X-ray-luminous obscured AGN.Comment: 4 pages, 2 figures, To appear in the proceedings of 'At the Edge of
the Universe' (9-13 October 2006, Sintra, Portugal
Hybridization gap versus hidden order gap in URuSi as revealed by optical spectroscopy
We present the in-plane optical reflectance measurement on single crystals of
URuAs. The study revealed a strong temperature-dependent spectral
evolution. Above 50 K, the low frequency optical conductivity is rather flat
without a clear Drude-like response, indicating a very short transport life
time of the free carriers. Well below the coherence temperature, there appears
an abrupt spectral weight suppression below 400 cm, yielding evidence
for the formation of a hybridization energy gap arising from the mixing of the
conduction electron and narrow f-electron bands. A small part of the suppressed
spectral weight was transferred to the low frequency side, leading to a narrow
Drude component, while the majority of the suppressed spectral weight was
transferred to the high frequency side centered near 4000 cm. Below the
hidden order temperature, another very prominent energy gap structure was
observed, which leads to the removal of a large part of the Drude component and
a sharp reduction of the carrier scattering rate. The study revealed that the
hybridization gap and the hidden orger gap are distinctly different: they occur
at different energy scales and exhibit completely different spectral
characteristics.Comment: 5 page
A Deep WSRT 1.4 GHz Radio Survey of the Spitzer Space Telescope FLSv Region
The First Look Survey (FLS) is the first scientific product to emerge from
the Spitzer Space Telescope. A small region of this field (the verification
strip) has been imaged very deeply, permitting the detection of cosmologically
distant sources. We present Westerbork Synthesis Radio Telescope (WSRT)
observations of this region, encompassing a ~1 sq. deg field, centred on the
verification strip (J2000 RA=17:17:00.00, DEC=59:45:00.000). The radio images
reach a noise level of ~ 8.5 microJy/beam - the deepest WSRT image made to
date. We summarise here the first results from the project, and present the
final mosaic image, together with a list of detected sources. The effect of
source confusion on the position, size and flux density of the faintest sources
in the source catalogue are also addressed. The results of a serendipitous
search for HI emission in the field are also presented. Using a subset of the
data, we clearly detect HI emission associated with four galaxies in the
central region of the FLSv. These are identified with nearby, massive galaxies.Comment: 9 pages, 6 figures (fig.3 in a separate gif file). Accepted for
publication in A&A. The full paper and the related material can be downloaded
from http://www.astron.nl/wsrt/WSRTsurveys/WFLS
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