Gravitational Lens VLBI using the
Arecibo Telescope
Porcas (MPIfR, Germany), Garrett (JIVE, Netherlands), Patnaik (Canada)
& Nair (RRI, India) have attempted to make a ``definitive''
high-resolution study of the gravitational lens system, 2016+112, using
an 18-station Global VLBI Array run at 5 GHz, including the Arecibo
305-m telescope.
This gravitational lens system has always been something of a puzzle.
There are 3 image ``regions'' (A, B & C) spread over an area of 4
arcsec. Previous VLBI observations (e.g., Garrett et al., 1996,
MNRAS, 279, L7) reveal that A and B are both images of a single radio
source, produced by bending and delay of the ray paths as they traverse
the intense gravitational field of a massive galaxy a few arcsec to the
south. Region C breaks up into at least 4 distinct components, aligned
along a gently curving arc of length 200 milliarcsec. It is presumed
that this region contains the ``merging images'' of (almost) another 2
images of the same source, making this system a (nearly) 4-image
``quad'' system. However, the details of the relationship between
images A and B, and their partial counterparts in region C remain
unclear. With these VLBI observations (another observation at 1.6 GHz
was made on February 25th) this team will shed some light on the matter.
Figure 1: The c-band fringe amplitude
for 2016+112 on the
Arecibo-VLBA_Saint Croix baseline. The
bandwidth is 16 MHz, the
integration time per point is 1 min, and the two circular polarizations
are averaged. Note the beating between the three image regions.
All 18 stations (VLBA, phased-VLA, Arecibo and 6 stations of the EVN)
worked successfully during the observations. The calibration source
shows fringes on all 152 baselines, including all 17 baselines to
Arecibo for the 2.25 hr for which the source was trackable there
(Fig.~\ref{fig117}). There are fringes on the gravitational lens
source(s) too, but these are harder to see for two reasons. Firstly,
the total source flux density is around 10 times weaker than the
calibrator -- ~70 mJy. Secondly, this emission is distributed amongst
the 3 image regions, which causes very rapid beating in the visibility
function on the long, transatlantic baselines -- and bandwidth smearing
too if averaging across the 16-MHz bandwidth. Thus, to preserve
the total 4 arcsec field of view for the mapping process, the data must
be kept unaveraged in time (1-s samples) and bandwidth (500-kHz
samples) -- and this is not so easily viewed!
Due to the very weak signal expected on long baselines (even those to
the highly-sensitive Arecibo), the observations were made in
``phase-reference'' mode, so that the instrumental ``residuals'' (clock
errors, phase errors, etc.) could be determined from frequent
observations of the calibrator, 2029+121. A duty cycle of 6 or 7
min was used -- a compromise which ensured that Arecibo did not spend
most of the cycle driving between the program source and the
calibrator, whilst hopefully fast enough to track the wandering
instrumental phase throughout the observation.
Figure 2: A very preliminary 5-GHz Global VLBI map of 2016+112, region
C.
A first crude mas-resolution image of the gravitational lens has been
made from unedited, not-properly-calibrated, blind-phase-referenced
data, and is the start of a more laborious process of calibration and
editing refinement. Figure 2 is a crude representation of region C, in
which one caN see the 4 distinct components of region C - a thin band
of emission, stretched by the strong gravitational field of the galaxy
lens.
