Newsletter Article:

       Call for Proposals for the High Sensitivity Array (HSA)

   Proposals for a high sensitivity VLBI array consisting of the VLBA,
Green Bank Telescope (GBT), phased VLA (Y27), and Arecibo (AR) are now
being accepted with priority.  A total of up to 100 hours in each
trimester (~25 hours/month) may be scheduled for highly rated projects
that can justify the increase in sensitivity.  By adding the GBT, Y27,
and AR to a VLBA experiment the sensitivity can be increased by
more than an order of magnitude.  This capability opens up promising
new avenues for scientific discovery.  In a 1 hour integration at 1.4
GHz the predicted image sensitivity is 6.5 microJy/beam, compared to
94 microJy/beam for the VLBA alone.  At 22 GHz, the above array
(without Arecibo which is limited to 10 GHz and below) the thermal
image noise in 1 hour is 43 microJy/beam compared to 275 microJy/beam
for the stand-alone VLBA.  For further estimates, observers can make
use of the EVN Sensitivity Calculator at
http://www.evlbi.org/cgi-bin/EVNcalc.  Trial runs with SCHED may also
be desirable in order to determine the mutual visibility of the array
on the target source.  In general a source that transits over Arecibo
(with transit time 1-2.75 hours for 0 < declination < 37 degrees) is
visible during an Arecibo transit by all other HSA antennas.

   The High Sensitivity Array (HSA) is available at frequencies of
0.33, 0.61, 1.4, 5, 8.4, 15, 22, and 43 GHz starting with the June 1,
2004 proposal deadline.  The proposal should be sent to NRAO only. To
request HSA time, the observer should indicate in item 11 of the
proposal coversheet on the line starting "VLBA" that the antennas
requested are "HSA = VLBA+GBT+Y27+AR".  If the celestial position,
or required frequencies, prohibit participation by Arecibo then the
desired subset should be described.  Proposals requesting less than 3
of the 4 instruments will not be considered for the HSA time.  It is
also assumed that the VLBA will participate.  Due to the coordinated
nature of the observations, dynamic scheduling is not possible.
Proposals to the HSA are limited in recording bandwidth to 256 Mbps,
but all continuum observations are encouraged to request the use of
this maximum bandwidth.

   Observers are reminded that with the extremely high gains of these
antennas, fields with an integrated flux density of more than ~1 Jy
will contribute substantially to the thermal noise.  The good news
though is that sources with even a few mJy of emission on
milliarcsecond scales may be suitable for self-calibration, and
phase-calibrators can be similarly weak.  Phase referencing by fast
switching to calibrators within 2 degrees of a faint target source
with cycle times of ~3 minutes are feasible.  All HSA observations
will be correlated in Socorro.  For further details consult the web
page at (http://www.nrao.edu/HSA) or direct your questions to the
undersigned.
 
                                       - Greg Taylor