Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~tghosh/vlbiao/VLBI.doc Дата изменения: Tue Nov 25 22:44:03 2008 Дата индексирования: Sun Apr 10 02:42:01 2016 Кодировка: Поисковые слова: arp 220

High Angular Resolution Science: VLBI at Arecibo
By Murray Lewis

Arecibo's global internet connectivity increased steadily in 2008. In May,
Arecibo participated in the first-ever simultaneous four-continent eVLBI
run where fringes were detected between all telescopes. This involved
Arecibo from North America, TIGO at Concepcion in Chile, Hartebeesthoek in
South Africa; and Effelsberg (Germany), Westerbork (Netherlands), Medicina
(Italy) & Onsala (Sweden) from Europe. The Arecibo data rate was solid at
the required 256 Mbps. The run was also used as a realtime demonstration
at the annual meeting of TERENA (Trans-European Research and Education
Networking Association.

On September 9, 2008 another eVLBI milestone was reached. Transatlantic
fringes were obtained between Arecibo and other EVN telescopes for the
first time for a data rate of 512 Mbps.

Also in May 2008 , two Mark5B recorders and a Digital Back-End were brought
to Arecibo from Haystack for a special Ultra-wideband VLBI experiment
organized by Shep Doleman. This equipment allowed the entire 1-GHz IF from
our receivers to be recorded in a VLBI run for the first time, at an
unprecedented 4 Gbps. An equivalent system was also deployed at the GBT,
and in addition the 10 VLBA antennas were used at their standard data rate
of 256 Mbps. The targets for this search were the weak central images of 8
gravitational lenses. While data reduction is still underway, the signal-
to-noise ratio in a 10-sec integration on the Arecibo-GBT baseline was
1700:1 for a 122 mJy compact source, making this the most sensitive radio
interferometery ever.

VLBI experiments have diverse motivations. Three examples follow.

Forbrich et al. attempted to detect non-thermal coronal radio emission from
four proto stars with sub-AU spatial resolution and µJy sensitivity using
Arecibo in the HSA. They detected 8.4-GHz radio emission at the 9-sigma
level from the Class-I proto star YLW-VLA2 in the Rho Ophiucus molecular
cloud, constraining its coronal region to be <0.4 x 0.1 AU. The noise
level was ~15 µJy/beam.

Lonsdale et al. recorded parsec-resolution spectral-line data for two
epochs separated by 15 months from the innermost regions of Arp 220. Many
of the supernova-related radio continuum point sources in their spectral
maps exhibit OH absorption or maser emission from intervening gas. This
provides a sampling of conditions along very narrow and specific lines of
sight through the nuclear environment. There is evidence for variability
in the OH properties on timescales of ~1yr.

Paragi et al. detected a compact radio component at the position of SN
2001em, which was thought to be associated with an off-axis Gamma Ray
Burst event showing late X-ray and Radio brightening. These types of GRBs
are usually thought to be misaligned relativistic jets. From subsequent
maps, the supernova was found to be unresolved 6 years after the
explosion, with no evidence for the existence of a jet. This suggests that
the source of the radio continuum emission in this instance is interaction
of SN ejecta with a dense circumstellar medium.

VLBI developments

While charting the course of VLBI science in the US for the decade of 2010,
the Taylor and Lonsdale Committee (2004) noted, "Provided certain highly
cost-effective steps are taken, the scientific reach of the VLBI technique
will lengthen dramatically within 10 years. In practical terms, this means
that a steadily wider range of targets, from stars to masers to star-
forming galaxies and gamma ray burst afterglows, will become accessible to
this powerful technique". The report also produced a pathway to technical
developments that it considered part of the "cost-effective steps". Over
the past three years, both we at NAIC and our colleagues at NRAO have been
following this path in terms of equipment procurement and technique
development. This has been with the help of highly successful
collaborations with MIT-Haystack and the European VLBI Network (EVN). The
conversion to totally disk-based operations has already occurred, and the
planned upgrade to routine data recording speeds of up to 4 Gbps during the
next three years will provide us with a 4-fold increase in sensitivity from
today's standard VLBA+Y27+GBT+Ar observations (at 256 Mbps rate).

The phase referencing technique in VLBI observations makes it possible to
study weak radio sources by increasing the effective coherence time from a
few minutes to hours at a stretch. About 50% of VLBI experiments now use
this approach, though it encounters limitations at Arecibo as the telescope
has slow slew rates. Hence, in a typical phase-referenced observation,
where the calibrator may be located 3o or more from the target, often ™ 50%
of the observing time is lost in slewing between the two source positions,
which leads to severe losses in the signal-to-noise ratio. However, phase-
referenced VLBI experiments can be made more efficient if a smaller
"auxiliary" telescope were to be available in proximity to the 305 m.
This would be used to continuously track the phase calibrator while the
large antenna observes the target only occasionally moving to the
calibrator. The corrections for ionospheric/tropospheric phase
fluctuations are then derived from the small-telescope data and applied to
data obtained with the 305-m dish. A 12-m class antenna would be adequate
for this task. We are seeking to purchase a modern, off the shelf, antenna
using funds made available to us from the Puerto Rican Bond issue.

Present-day VLBI offers the highest sensitivity radio astronomical
observations yet achieved, with noise levels presently approaching 1
?Jy/beam for arrays using the world's most sensitive telescopes. Hence,
the 305-m Arecibo telescope is being increasingly used in experiments to
detect radio emission from very weak, very compact, astronomical targets
such as X-ray stars, distant supernovae and their remnants, Gamma-Ray
Bursts, and red-dwarf and other stars. For these sensitivity levels to be
reached for targets of very low intensity, it is essential that phase-
referencing be used.

Deploying Arecibo in a VLBI network increases both its sensitivity, and the
attainable astrometric precision. In particular a wider range of stellar
targets become detectable. This can be exploited in various ways. One
such is to look for extra-solar planets from the modulation they induce in
the position of the central star: thus the VLBA is presently being used to
look for planets around M-dwarf stars. With the presence of Arecibo this
could be extended to appreciably earlier, more Sun-like systems.
Similarly, a 414±7 pc distance has been estimated from the parallaxes of a
few stars in the Orion BN/KL region derived from 8-GHz observations: the
number and stellar type of objects, as well as the range of clusters
suitable for such measurements, would increase if Arecibo were to be
deployed. Likewise, high-precision astrometry of pulsars over multiple
epochs can provide their positions, proper motions, and annual
trigonometric parallaxes. Due to the weakness of most pulsars, with duty
cycles of typically <10%, the participation of Arecibo with phase
referencing is vital.