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SMALL AUXILIARY ANTENNA AT ARECIBO


1. Introduction

The 305-m Arecibo radio telescope is the world's largest, most sensitive,
single-dish radio telescope. It is equipped with receivers between 47 MHz
and 10 GHz. In addition to its single-dish capabilities, the telescope also
participates in Very Long Baseline Interferometry (VLBI) observations with
the VLBA, HSA, EVN and Global VLBI networks.

In its quest for better sensitivity, VLBI observations have been evolving
over the years.
For a given VLBI array, this can be achieved by either increasing the
bandwidth or the integration time. While bandwidth limitation comes either
from equipments (in case of
continuum sources) or from the natural phenomena under study (spectral
line),
phase fluctuations due to propagation of signal through the Earth's
troposphere and ionosphere limits the basic coherence time in VLBI
observations. The technique of "phase-referencing" in VLBI allows the
possibility of correcting for these effects
and thereby increase the coherence time by factors of ten in some case.


1. VLBI -- Phase Referencing:

Phase referencing in VLBI observations has made it possible to study very
weak radio sources by increasing the effective coherence time from, at
maximum, a few minutes at a stretch to hours. Currently, some 50% of VLBI
observations are carried out using this technique.

Phase-referenced observations can be performed in two modes, nodding style,
and in-beam. In nodding style phase referencing, the antennas switch
between the target source and a nearby calibrator, called the phase
reference, every few minutes. The duration of one cycle of observing the
target and phase reference is called the cycle time, and is typically about
5 minutes or shorter. This procedure can be successfully carried out for
observations at 1 GHz and above. However, at frequencies below 1 GHz the
raw coherence times become very short due to ionospheric effects, requiring
the phase calibrator to lie within the (voltage) primary beam of all
antennas in the array.

However, such observations encounter limitations with the Arecibo 305-m
telescope since the Gregorian dome, located on a suspended platform, has
slow slew rates (24o/min in azimuth, 2o.4 /min in zenith angle.) Hence, in
a typical phase-referenced observation, where the calibrator could be
located 3o or more from the target, a significant amount of observing time,
often ™ 50%, is wasted slewing between the two sources, leading to severe
losses in signal-to-noise ratios. However, phase-referenced VLBI could be
performed using a smaller "auxiliary" telescope to track the phase
calibrator, while the 305-m antenna observes the target most of the time,
only occasionally moving to the calibrator. The effects due to
ionospheric/tropospheric phase fluctuations can then be derived from the
small-telescope data and applied to the target data from the 305-m dish.
While this technique has been successfully applied for observations with
the 220-km baseline MERLIN array in the UK, it will be new to VLBI.
Implementing its VLBI application will have to be developed.


2. Areas of Astronomical reseach benefiting from the small antenna in Phase
Referenced VLBI

2.1 Stellar (radio) Astrometry: In a white paper submitted to the NSF
ExoPlanet Task Force, Bower et al. (arXiv:astro-ph/0704.0238v1) explored
the possibility of "Radio Astrometric Detection and Characterization of
Extra-Solar Planets". Utilizing the better than 100-microarcsec positional
accuracy routinely achieved with the VLBA, they propose carrying out a
Radio Interferometric Planet search (RIPL) that will survey 29 low-mass,
active (radio-loud) M-dwarf stars over 3 years. This would have sub-Jovian
planet mass sensitivity at distances of ~1AU from the star. They note that,
"Radio astrometric planet searches occupy a unique volume in planet
discovery and characterization parameter space, which gives greater
sensitivity to planets at large radii than do radial velocity searches. For
the VLBA and the expanded VLBA, the targets of radio astrometric surveys
are by necessity nearby, low-mass, active stars, which cannot be studied
efficiently through the radial velocity method, coronography, or optical
interferometry."

The addition of Arecibo in such surveys would increase the detection
sensitivity by a factor of four, making it possible to study objects with
one third the mass of Jupiter as companions of starts of similar types. As
Arecibo's primary beam is much smaller than that of other telescopes, and
the slew rate slower, the availability of a small and fast antenna for
phase referencing would be highly beneficial for undertaking such studies .

2.2 A Broad-impact VLBI Measurement of Trigonometric Parallax of Star
Clusters: In an impressive work using the VLBA at 8 GHz, Menten et al.
(2007, A&A, 474, 515) have determined the trigonometric parallax of several
stars in the Orion BN/KL region, allowing them to derive the most accurate
value to date (414± 7pc) for the distance to this region. This is about an
order of magnitude better than the previous value of 361+168-87 pc
determined by Hipparcos from the optical parallax measurement of a single
star in this complex. Luminosity-based distance estimates of star-forming
regions may be adversely affected by poorly known extinction. The new radio
technique is an important way to improve the estimation of distances, and
hence luminosities, with subsequent impact on star-formation theories.
Once again, the inclusion of Arecibo would permit the extension of these
studies to fainter, more distant, star-forming regions.

2.3 Pulsar Astrometry: High-precision astrometry of pulsars over multiple
epochs can provide their basic astrometric parameters: positions, proper
motions, and annual trigonometric parallaxes. Due to the weakness of most
pulsars, with duty cycles of typically <10%, the participation of Arecibo
and phase referencing is vital to the success of this exercise. In respect
of positional measurements, we note that VLBI estimations are tied to the
reference frame of the distant quasars, rather than the Solar-system frame
employed by pulsar timing positional estimates. This allows fundamental
reference frame ties between the Solar-system and extragalactic (ICRF)
frames via measurements of recycled pulsars, which are highly stable
rotators.

Proper motion estimates allow pulsars to be traced back to their birth
sites and, for very young pulsars, associations with progenitor supernova
remnants (SNRs) can be established, providing independent age estimates for
SNRs. Combined with pulsar distance estimates, proper motion measurements
lead to estimates of space velocities, allowing a study of the natal kicks
imparted to pulsars at the time of their birth. When a parallax measurement
is possible, this yields a model-independent estimate for the distance (and
hence velocity) of the neutron star. Such measurements, (i) calibrate
models of the Galactic electron distribution, (ii) constrain SN core
collapse using the velocity estimates, and (c) provide photospheric sizes
for hot neutron stars with optically observed thermal surface radiation,
which in turn, constrains the equation of state of matter at extreme
pressures and densities.

2.4 Detection Experiments: 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 radio 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.

2.5 VLBI imaging of molecular gas in ULIRGs: Arecibo and the GBT are
currently searching for cm-wavelength lines of prebiotic and other
molecules in Ultra-Luminous InfraRed Galaxies (ULIRGs). The project has
been inspired by the recent Arecibo detection of the prebiotic molecule,
methanimine (CH2NH), in the protypical ULIRG/megamaser galaxy, Arp 220
(Salter et al. 2008, AJ, 136, 389). These galaxies are considered to be
``extreme mergers" and are heavily obscured at optical wavelengths.
Molecular lines from them often show wide velocity widths, caused by line
blending due to spatial and velocity overlaps. Detailed studies of maser
emission and molecular absorption lines from these objects require phase
referenced VLBI observations, and the presence of Arecibo's sensitivity in
the VLBI array.

3. Astronomy Using the Small telescope as an Independent Single Dish

3.1 Full-Stokes Galactic Plane Continuum Surveys: The small telescope,
together with existing Arecibo backends, will enable full-Stokes, continuum
surveys using the cooled dual-channel receivers that will be built for use
with the dish. Full-Stokes continuum surveys of the wider Galactic plane at
high frequencies with HAAT can provide unique databases in a number of
ways. Firstly, they can yield full spatial frequency, full-Stokes mapping
at previously unmapped wavelengths, with competitive resolution for such
extended features as the Galactic background emission, HII complexes, and
middle-aged and old SNRs. Comparison with existing lower frequency surveys
would allow accurate estimation of spectral index distributions over these
features, providing the ability to perform accurate thermal-nonthermal
separation on angular scales between 1o and <10 arcmin which allows the
study of energy injection to the ISM, energy losses for relativistic
particles associated with SNRs, and the mechanisms of vertical transport
and diffusion of energy from the disk of the Galaxy into the halo and
intergalactic space.

Linear polarization measurements are of especial importance. The
appearance of the polarized sky at ? > 21 cm is complex. Westerbork at 327
MHz (for high Galactic latitudes) and the 1.4-GHz Canadian Galactic Plane
Survey have shown that there is little relationship between total intensity
and polarization structures; for the diffuse Galactic synchrotron emission,
the bulk of the area of the Galactic Plane imaged on arcminute scales at L-
band reveals highly structured polarization features with no counterparts
in Stokes I. The accepted interpretation of this is that, although the
Galactic synchrotron emission is intrinsically quite smooth, differential
Faraday rotation in the intervening magneto-ionic medium, (the Faraday
Screen), imposes fine structure on the polarized emission. In other words,
the low-frequency polarized sky is dominated by propagation effects rather
than by intrinsic emission structure

The signals produced by the Faraday Screen are rather weak, and the limited
surface brightness sensitivity of interferometers samples only the
strongest. Also, the derived rotation measures (RMs) are noisy due to low
signal-to-noise per channel, while missing zero-spacings in
interferometric observations lead to complications in interpretation. The
high brightness sensitivity of HAAT, coupled with its few arcminute beam
size at high frequencies, promises major advances in the study of the
magneto-ionic medium. At these frequencies (™ 5 GHz), the effects of
Faraday rotation become tiny (?? ? ?-2) and HAAT polarization position
angles will essentially be those intrinsic to the emission, providing both
intrinsic directions of magnetic fields and a database against which lower
frequency polarization distributions can be definitively interpreted.

In existing studies of the Faraday Screen, the spectral signatures of the
polarized intensity have been examined to seek only a single RM value per
image pixel. Such a value corresponds to the RM of the dominant polarized
emission component along any given sight-line. However, the Faraday Screen
is spread out in depth along each line of sight, with regions of polarized
emission at different distances along the sight-line contributing to the
observed spectrum with their corresponding foreground Faraday rotation
signature. With an appropriate combination of observing frequency,
bandwidth and spectral resolution, it should be possible to perform Faraday
tomography, wherein the spectral polarized intensity modulations along a
given sight-line can be transformed to a set of polarized intensities as a
function of Faraday depth (i.e. RM). Thus, it should be possible to derive
a polarized-intensity data cube (quite like a spectral-line data cube) with
two dimensions being the sky coordinates and the third being RM. High-
frequency images from HAAT would be invaluable in pursuing this endeavor.

Away from the Galactic plane, the high latitude regions contain several
well-known non-thermal emission structures, notably the North Polar Spur
(Loop 1), an object that contains rich small-scale structure, both on its
main arc and in internal ridging. Above b = 45 deg, low resolution
measurements of this nearby (~100 pc distant), old SNR show >70% linear
polarization at 1.4 GHz. Higher frequency, higher resolution HAAT images
will directly reveal detailed magnetic field directions in this object.

We specifically mention the L-band Arecibo GALFA Continuum Transit Survey
(GALFACTS), which is being made by an international consortium of
astronomers led by Prof. Russ Taylor (U. Calgary). This full-Stokes survey
of the whole sky observable with the 305-m telescope covers 1225 - 1525
MHz, with 8192 frequency channels. At L-band, the Faraday rotation effects
on the linearly polarized radiation are considerable, and a continuum
survey at much higher frequency, but similar resolution, (HPBW ~ 4 arcmin
for GALFACTS), would allow thermal-nonthermal separation, and aid Faraday
tomography when combined with GALFACTS. The same situation exists for a
large part of the Southern Galactic Plane L-band continuum survey being
made with the Parkes radio telescope (HPBW ~ 15 arcmin; Haverkorn et al
2006). HAAT surveys would also provide vital low-spatial frequency data for
future interferometric full-Stokes surveys.

Synergy with GLAST: ?-ray emission from our Galaxy is believed to be
produced by, a) brehmsstrahlung from the interaction of cosmic-ray
electrons and the interstellar gas, and b) the decay of neutral pions
produced in interactions between the gas and cosmic-ray protons and heavier
nuclei. The former is thought to dominate at <1 MeV, the latter at higher
energies. Similar distributions of ?-rays are found at low latitudes in
both energy ranges, suggesting that the cosmic-ray heavy particle-to-
electron ratio is constant over the Galaxy. If so, the ?-ray emissivity,
??, is proportional to the product of the cosmic-ray intensity and the
total (i.e. neutral and ionized atomic, plus molecular) gas density, ? ;
?? ? ?0 ?, where the cosmic-ray energy distribution is given by, N(E) dE =
N0 E-? dE. Now, for the synchrotron component of the Galactic radio
emission, the emissivity is, ? ? ?0 ?? (?+1)/2, where ?? is the
magnetic field strength perpendicular to the line of sight.

The Galactic distributions of the three quantities, ?0, ?, ?, are all of
great astrophysical interest. Arecibo will contribute significantly to a
knowledge of ? over the accessible sky, GALFA consortia providing, a) the 2-
dimensional distribution of HI, while the thermal-nonthermal separation of
the continuum emission mapped by HAAT and GALFACTS surveys will provide the
2-D distributions of, b) the thermal emission from HII and c) the non-
thermal synchrotron emission. The 2-D distribution of the molecular gas is
already available from CO surveys of similar resolution. Hence, combining
Arecibo HAAT and ALFA work with other radio data and the high-fidelity
GLAST ?-ray background images will provide the information needed to
"unfold" the 2-D distributions and derive the Galactic distributions of ?0,
?, & ?. This would represent a major contribution to our understanding of
the detailed distribution of the magnetic field and cosmic rays in the
Galactic disk.

3.2 Pulsar timing: The majority of pulsars are too weak to be
satisfactorily timed with a 12-m dish. However, timing observations of the
strong Crab and Vela pulsars would compliment dedicated monitoring of the
rotational state of these neutron stars undertaken at other observatories.
The traditional aim has been to detect glitches and timing noise in these
objects, which is scientifically important by itself (e.g. Lyne et al. 1996
Nature, 381, 497 & Lyne et al. 1993, MNRAS, 265, 1003). This has become
even more relevant now with the launch of Gamma-ray satellites such as
AGILE and GLAST, which require continuous monitoring of these pulsars in
order to fold the many gamma-ray pulsar candidates with a temporal and hour
angle coverage that are impossible to achieve with the 305-m telescope, and
that complement other telescopes: for instance, we can time the Vela pulsar
while it is below the horizon of the dedicated telescope at Mt. Pleasant in
Tasmania.

Another application of this antenna will be on the statistical properties
of the giant pulses of the Crab and other pulsars. These have fluxes of the
order of MJy, which are very easily detectable with the HAAT. This would
have several important ramifications. Paulo Freire is part of an
international collaboration that is using several millisecond pulsars to
detect very low-frequency gravitational waves. One of the main issues of
this project has to do with the calibration of the time delays of all the
telescopes and back-ends used for this international project. The giant
pulses from the Crab detected by the 12-m dish can be used as an
international time reference for all telescopes in the Western Hemisphere,
giving us a calibration of all the instrumental delays of all the
telescopes and back-ends involved in this global high-precision timing
project.

The signals received by the 12-m antenna will be piped back to the control
room for processing with existing Arecibo back-ends, which have well-known
time characteristics. This avoids any duplication in the processing
hardware or software, although it will add to the present data storage and
processing requirements.

3.3 Seven-Beam Interferometry with ALFA:

At L-band, the 7 ALFA beams of the 305-m dish would all lie near the peak
of the (voltage) primary beam of a small (12-m class). As the voltage
beams have essentially double the sky coverage of their power patterns, and
overlap close to the half power points, one can foresee ``mosaicing'' fair-
sized pieces of sky from the small telescope - ALFA interferometry. The
future addition of more elements at other sites around P.R. could
enhance this possibility, providing both uv-coverage, and phase and/or
amplitude self-calibration. Again, the necessary cross correlator for small
telescope - ALFA interferometry, i.e. 7 baselines, could likely be provided
by an existing backend, or be built collaboratively? Software correlators,
currently under development for eMERLIN and the VLBA could also be grafted
in at AO for this purpose. This wide-field interferometric mode of
operation would appear relevant to development work for the Square
Kilometer Array (SKA).


4. Geodatic use of the small antenna:

The ``International VLBI Service for Geodesy & Astrometry'' (IVS) recently
laid down a set of guide lines for the next-generation of geodetic VLBI
measurements of station positions and earth orientation parameters. These
are called ``VLBI 2010''.The aim is to achieve 1~mm positional accuracy on
intercontinental baselines with a 24-hr turnaround for results. However,
there is insufficient funding for an international network at present, and
the geodetic community is interested in any telescope that could
participate. As is being planned for the small telescope, the antenna
needs to have minimal horizon obstruction, and be sited on good bed-rock.

The VLBI 2010 requirements specify a telescope design of 12 meter diameter
with a system temperature of about TR = 45 K, SEFD < 2500 Jy. A frequency
coverage of 2 - 18 GHz will ultimately be needed. Dual-frequency operations
are a requirement to allow for ionospheric correction. An eventual data
recording rate of 8 - 16 Gbps is being set as a goal, and is consistent
with NAIC development plans for its VLBI backend. Recording at a rate of
4~Gbps was recently made with the 305-m telescope using a borrowed digital
backend, and dual state-of-the-art Mk5 recorders. Much of the data transfer
is envisaged as being over the internet (e-transfer), and Arecibo is
already the player in N. American astronomical eVLBI.

Arecibo would be attractive as it is situated on the Caribbean techtonic
plate. This plate is very complicated, and accurate measurements of
velocities in a global frame are most valuable. Currently, the 25-m VLBA
antenna on St. Croix is available to the geodetic array, but only on 6
occasions per year. For each geodetic session, the geodetic community would
need 24 hr participation. The minimum involvement required is once per
month, though once per week would be welcomed. While the geodetic
community intend extending their frequency coverage to Ka band eventually,
this would not be necessary for some while. They would require 2-bit
recording, and early recoding rates of (say) 2 Gbps.

To participate in geodetic VLBI a cable-delay system (i.e. a``round-trip'
phase measurement) will be needed as system delays need to be tracked at
the mm-level. Diurnal effects should be monitored, and the receiver has be
as stable as possible. A dual-frequency GPS receivers should exist close
to the antenna.

Geodetic operations are the responsibility of the individual antennas.
Analysis centers exist around the World, with some being based at
universities. It is likely that UPR could become involved? We note that the
analysis of VLBI data is simple compared to that for GPS data. Further,
while GPS is good for measuring regional motions (over ~1000 km), VLBI can
provide the tie to more distant points with better accuracy than GPS


5. Technical requirement for the small telescope:

Describe what is needed to achieve the above objectives.
1) For 100mJy at 5 sigma, what size is needed
2) Frequency coverage (where phase ref. is needed, + VLBI 2010)
3) Slewing speed - Az-El coverage
4)

State that we have the possibilities of getting the DSS33 or a Patriot
antenna.

Make a table comparing their pros and cons.


------------------------------- section below could be deleted if nothing
of it is needed --------

1.1.2. Is HAAT big enough for Phase Referencing? : The effective area of
HAAT yields a point-source sensitivity of ~0.026 K/Jy. The antenna is
expected to work with full efficiency up to 15 GHz, covering the complete
frequency range of the 305-m telescope (327 MHz to 10 GHz). For a system
temperature of T deg K, this would give a system equivalent flux density of
~38.6 T Jy. If HAAT were to have a similar Tsys to a VLBA telescope at
these wavelengths, then the baseline sensitivity for a HAAT to VLBA antenna
(1-?) would be ~20 mJy/beam. This implies that, at the 5-? level, sources
brighter than about 100 mJy/beam will be suitable for phase-referencing,
thus including the majority of sources from the VLBA Calibrator list.

4.1.1.3. Technical Collaboration for the HAAT: One of the aspects of this
collaboration is the construction of a C-band receiver. This project will
involve students from Humacao's Associate Degree Program in Electronics
Technology who will be in residence at Arecibo for at least one or two
summer seasons under the guidance of an electrical engineer from Humacao
working on the assembly of this receiver. The faculty at the Humacao campus
includes four electrical engineers. Funds are requested for the summer
salary and release time during two academic years for one engineer from
Humacao to supervise the construction of the device in coordination with
NAIC staff. Salary is also requested for one electronics technician for
helping with the building of equipment and maintenance of the HAAT.

We will commense operations, including providing proof of concept for the
phase-referencing technique, using the 6 cm receiver, the wavelength most
requested for Arecibo VLBI observations. In due course, the four standard
VLBI frequency bands below 10 GHz will need to be serviced by the HAAT. An
ultra wide-band receiver will be developed at a later date via other
funding sources. We expect HAAT to share the Observatory's maser
time/frequency standard and spare backends. Thus, NAIC's involvement in the
development of HAAT would be a contribution from it to the training of
Puerto Rican scientists, engineers and technicians, as well as in enhancing
the options it offers its user base for astronomy and planetary physics.
The UPR-Humacao's contribution will be to provide the material and
students' assistance in building this receiver, and develop the particular
form of phase-referencing technique (under the guidance of the NAIC staff)
via the current proposal.



[[ this section may have to be excluded --- All observing time on the 305-
m telescope is granted via a highly competitive peer review process, open
to researchers worldwide. Although not unknown, opportunities for
undergraduates to utilize the instrument, or gain hands-on experience in
radio sciences by building equipment for it, have been limited.

In order to broaden the range of scientific possibilities it offers to its
users, NAIC is working to acquire a small auxiliary radio telescope to be
sited near its 305-m dish. Such an instrument would have scientific
capabilities that are both complementary to, and independent of, the 305-m
telescope. We foresee the antenna being scheduled in support of "phase-
referencing VLBI" with the 305-m telescope for a maximum of ~20 % of its
time. All other time (apart from maintenance) would be available for
integration into the educational and research needs of UPR, and Public
Outreach ventures in collaboration with the UPR and the Observatory's
Visitor Center. ]]


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