15,952 research outputs found
A Thin HI Circumnuclear Disk in NGC4261
We report on high sensitivity, spectral line VLBI observations of the HI
absorption feature in the radio galaxy NGC4261. Although absorption is only
detectable on the most sensitive baseline, it can be unambiguously associated
with the counterjet and is interpreted to originate in a thin atomic
circumnuclear disk. This structure is probably a continuation of the dusty
accretion disk inferred from HST imaging, which could be feeding the massive
black hole. HI column densities in front of the counterjet of the order of
10^{21}(T_sp/100 K) cm^{-2} are derived, consistent with X-ray data and VLBI
scale free-free absorption. The data presented here are the result of the first
scientific project processed on the new EVN MkIV data processor.Comment: 4 pages, 3 postscript figures, Astronomy and Astrophysics Letters, in
pres
Multi-Frequency Synthesis of VLBI Images Using a Generalized Maximum Entropy Method
A new multi-frequency synthesis algorithm for reconstructing images from
multi-frequency VLBI data is proposed. The algorithm is based on a generalized
maximum-entropy method, and makes it possible to derive an effective spectral
correction for images over a broad frequency bandwidth, while simultaneously
reconstructing the spectral-index distribution over the source. The results of
numerical simulations demonstrating the capabilities of the algorithm are
presented.Comment: 17 pages, 8 figure
Climate and Livelihoods in Rural Ningxia: Final Report. The Impacts of Climate Change on Chinese Agriculture - Phase II National Level Study final report
Lobe Advance Velocities in the Extragalactic Compact Symmetric Object 4C 31.04
We report on the results of a two epoch study of the low power Compact
Symmetric Object 4C 31.04. Observations performed with the VLBA at 5 GHz in
1995 and 2000 have yielded images of this source at milliarcsecond angular
resolution. A central core is detected, with bright compact hot spots and
extended lobes on both sides. Model-fitting and other analysis of the data
(brightness profile, difference map) clearly indicate that the source is
expanding. We estimate the velocity of this expansion to be (0.085 +/- 0.016)
mas/yr, i.e. (0.33 +/- 0.06) c in both hot spots. Assuming a constant expansion
velocity, we estimate the kinematic age of the source at 550 yrs. We also study
the spectral index using VLBA observations at 1.3 GHz and MERLIN at 22 GHz. The
derived spectral age is 3000-5000 years in equipartition conditions. The two
estimates are discussed and found to be in agreement, given present
uncertainties.Comment: 9 pages, 8 .eps figures; accepted for publication in Astronomy &
Astrophysic
The nature of supernovae 2010O and 2010P in Arp 299 - II. Radio emission
We report radio observations of two stripped-envelope supernovae (SNe), 2010O
and 2010P, which exploded within a few days of each other in the luminous
infrared galaxy Arp 299. Whilst SN 2010O remains undetected at radio
frequencies, SN 2010P was detected (with an astrometric accuracy better than 1
milli arcsec in position) in its optically thin phase in epochs ranging from ~1
to ~3yr after its explosion date, indicating a very slow radio evolution and a
strong interaction of the SN ejecta with the circumstellar medium. Our
late-time radio observations toward SN 2010P probe the dense circumstellar
envelope of this SN, and imply a mass-loss rate (Msun/yr) to wind velocity (in
units of 10 km/s) ratio of (3.0-5.1)E-05, with a 5 GHz peak luminosity of
~1.2E+27 erg/s/Hz on day ~464 after explosion. This is consistent with a Type
IIb classification for SN 2010P, making it the most distant and most slowly
evolving Type IIb radio SN detected to date.Comment: 14 pages, 8 tables and 7 figures. Accepted for publication in MNRA
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