A differential astrometry technique is discussed. An improved proper motion and a parallax limit for pulsar 1929 + 10 is presented as well as a limit on the space velocity of the enigmatic object in SgrA

Backer, D. C.

English

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DIFFERENTIAL RADIO ASTRONOMY OF GALACTIC OBJECTS 
D. C. Backer 
Radio Astronomy Laboratory 
University of California 
INTRODUCTION 
In contrast to the very distant quasars and galaxies used for geodetic studies discussed so far, pul- 
sars, radio stars, and .other galactic continuum objects have large transverse motions from proper 
motion, parallax, and orbital effects which can be measured with recently developed microwave 
interferometers. We (the author in collaboration with R. A. Sramek of the National Radio Astron- 
omy Observatory (NRAO)) established a program in 1974 designed to measure these motions with 
the 35km interferometer in Green Bank, West Virginia, at an observing wavelength of 11.1 cm. 
Our first results (1976, &I, 81, 430) demonstrated the sensitivity of this instrument to the proper 
motions of pulsars: Motions of Oi’OS yr-l were detectable for these faint objects with several ob- 
serving sessions spaced over 1% years. We will discuss our differential astrometry technique and 
present an improved proper motion and a parallax limit for pulsar 1929 + 10 and a limit on the 
space velocity of the enigmatic object in SgrA. 
TECHNIQUE 
Most of you are familiar with the 35-km, radio-link interferometer developed by the NRA0 and cur- 
rently operated by the U.S. Naval Observatory (USNO). Three baselines, each with dual polariza- 
tion, are formed between a remote 45foot (13.7-meter) telescope and the local 85-foot (25.9-meter) 
telescopes in Green Bank. Six simultaneous interferometers are thus recorded providing necessary 
sensitivity and extremely useful redundancy. A small amount of tropospheric phase noise is also 
eliminated. We observe a sequence of reference (A) - program object - reference (B) - program 
object - etc., with references located on nearly opposite sides of the program object. A weighted 
average of reference data will yield the equivalent data of a pseudo-reference source as close as pos- 
sible to the program object. In principle, three references could be observed and the pseudo- 
reference placed coincident with the program object to remove phase fluctuations which vary 
linearly over the object/reference region of the sky. However, the sky density of catalogued com- 
pact objects, instrumental sensitivity, and temporal variations of the phase fluctuations conspire to 
limit our observations to two reference sources. 
The online phase residuals from Green Bank were first corrected for known errors in baseline, clock, 
atmosphere, and source position in Charlottesville, and then averaged to form an amplitude and 
phase for each baseline and polarization for each 5 to 10 minute scan. These data were taken to 
Berkeley for further analysis containing the following steps: 
187 
https://ntrs.nasa.gov/search.jsp?R=19800020313 2020-03-21T17:49:49+00:00Z
188 RADIO INTERFEROMETRY 
(1) position update for errors ascertained after initiating a standard reduction procedure in 
Charlottesville; 
(2) integration over polarization channels and three successive hour-angle scans to obtain un- 
ambiguous phase estimates on the faint program object; 
(3) removal of lobe ambiguities; 
(4) linear interpolation in time of the reference phases to the time of object observations and 
subtraction of weighted sum of these interpolated values from the object phases; 
(5) fitting of the above “first difference” phase for each baseline on each day and each epoch 
to a pseudo-position offset. The pseudo-position offsets for any program object may be 
modeled by 
ASPP = A& +kT +2(T) + orbital effects (T) 
where 42, is a constant and T is the interval of time from initiation of observations. 
Source structure effects may also enter the model. 
1929 + 10 RESULTS 
An example of the effectiveness of this technique is demonstrated in figure 1 for the pulsar 1929 + 
10. The pseudo-position offsets in milliarc seconds are shown during 1974 to 1976 for position 
axes (x, y) rotated 40” from the (a, 6) axes to coincide with the normal axes of the measurement 
errors. This results from our 4-hour observation providing limited W coverage in pa -50”. The 
results for each day and baseline are given separately. The agreement of the Y-offset measurements 
with a proper motion of 01’08/yr is excellent. This corresponds to a transverse velocity component 
of 40 km/s at a distance of 100 pc. The sine wave in figure 1 is the parallactic shift for an object at 
a distance of 50 pc - clearly 1929 + 10 is not closer than 50 pc. 
In figure 2, we compare absolute positions for 1929 + 10 from our program and from timing meas- 
urements which effectively employ an interferometer with a baseline of 1 AU but which has a delay 
resolution of only 10 to 1000 psec. The right ascension measurements are in excellent agreement. 
Our declination disagrees slightly with the timing measurements which disagree with each other in 
PS. 
In 1975, we began observing the compact object in our galactic center, SgrA. If this enigmatic ob- 
ject is at rest in the gravitational barycenter of our galaxy, then observations of SgrA with respect 
MOVEMENTS TERRESTRIAL AND CELESTIAL 
800 
t 
0 0 
. . 
A 
1974 1975 1976 1977 
DATE 
Figure 1. Pseudoposition offsets along axes at position angle of 40’ 
showing proper motion of pulsar 1929 + 10. Offsets are given in 
milliarc seconds. The sinusoid in the lower section is the expected 
offset from trigonometric parallax at a distance of 50 pc. Open 
circles designate data with large errors. 
189 
190 RADIO INTERFEROMETRY 
‘ij 51.X 
s! 
g 
i3 3 51.92 
z 
z 
!z 
s 5’.88 
04.0 
iii 
3 
b a. 
g 03.5 
m 
Y 
i5 
i= 
ZQ 3 03.0 
E 
i- I I I I I 
* 
I I I I I 
1970 1972 1974 1976 1978 
DATE 
Figure 2. Absolute position and proper motion of 1929 + lO.from our inter- 
ferometer measurements (A), and from timing measurements (0 = Helfand et al. 
1979, Ap. J., in press; 0 = Gullahorn and Rankin 1978, A. J., 83, 1219). The 
right ascension is 19h29m; the declination is 10’53’. 
to an extragalactic frame would directly measure the rotation rate of the galaxy. It is likely, how- 
ever, that SgrA is not at rest - the infrared studies of Lacy and Townes indicate a dynamical center 
not coincident with this radio object and velocities of order 300 km s-r. In this case, we would 
remove an assumed galactic rotation rate from a SgrA measurement and hope to learn more about 
the nature of this object. 
Figure 3 shows, unfortunately, that measurements with the 35-km instrument using reference 
sources 2” and 3” away can only place an upper limit of I-(, lOl’O1 yr-l (in 2 years) at a position 
angle roughly parallel to the galactic plane. The galactic rotation rate is OI’OOS yr1 in this plane. 
The limit corresponds to a transverse velocity limit of 1000 km s-l at 10 kpc - moderately interest- 
ing, but we would like to do 10 times better. 
0 
- -100 
t3 
E 
L 
E? 
b 
-200 
> 
- 300 
MOVEMENTS TERRESTRIAL AND CELESTIAL 
0 
0 
0 
A 
0 x 0, 
3’ 00 0 0 ?O 00 A 
h 0 
OA 
A A 
in 0 O 0 A 
n u A 
B 
0 A 
0 
A 0 
0 
A 
- YU Y - 
IV V VI VII VIII IX x 
I I I 
.--- .--- 
1976 1977 
DATE 
1978 
Figure 3. Pseudoposition offsets along an axis nearly parallel to the galactic plane for 
SgrA. Systematic differences between offsets from the three baselines (0 = 14, q = 34, 
A = 24) probably result from confusion in the vicinity of SgrA. The measurements 
were taken at epochs IV through X. 
We seem to be affected by structure which causes a 01’030 shift of the 14 baseline offset relative 
to 24. Tests indicate the shift is not consistent with a first difference lobe error. The shift may 
well result from structure of, or confusion near, SgrA. * Estimates of interstellar scattering for SgrA 
would predict an apparent size of 01’300 at 11 .l cm X. 
We note that the sun passes near SgrA in December. Estimates of the differential deflection pre- 
dicted by general relativity give 0!‘025 - a factor which must be removed before combining these 
data for formal proper motion limits. 
I should emphasize that figure 3 contains pseudo-position offsets for every day of every epoch in- 
dependent of the often variable weather in Green Bank. This plot then gives an indication of the 
reliability for absolute astrometry at declinations of -30” (set z >, 3). 
*This shift has been identified as the result of comparing Y offsets on slightly different frames of reference for the three baselines. 
An improved analysis of these observations will be presented elsewhere (Backer and Sramek, 1980, in preparation). 
192 RADIO INTEWEROMETRY 
Closer reference sources, a longer time base and, of course, a more southerly site are required to 
make further progress on this object. 
My thanks to Fred Crews and Jim Coe at NRA0 for making every epoch a success and to Tom 
Clark for support while at GSFC. 


Differential radio astronomy of galactic objects

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