Gravitational Lens System
monitoring program
Long-Term Optical Monitoring
Of The Selected Gravitational Lens Systems
A. Zheleznyak1,
V. Vakulik1, V. Dudinov1,
I. Chamitov2, B. Artamonov3
(July 1998)
(Reproduced with
permission).
Overwiew Of Planned Activity
The long-term photometric monitoring of selected GLS will
form the basis of the proposed Programme. The observations in BVRI spectral
bands will be carried out with the 1.5-meter telescope of the high-altitude
Maidanak observatory (Uzbekistan, Central Asia, elevation of 2600 m).
1. List of Objects
Six GLS were selected for investigation: Q0957+561, PG1115+080, Q2237+0305,
H1413+117, B1422+231 and B1600+434.
-
Q0957+561 Doubly imaged QSO with well established photometric
variability at optical wavelengths and in the radio. The time delay of
417 days at optical wavelengths is now considered to be accurate to better
than 1% (Kundic, Ap.J. 482, 1997; Pelt et al., 1996 and 1998), giving a
Hubble Constant of 63(12 km/(s Mpc) (2( error). Further homogeneous observational
data are needed to analyse microlensing effects which is particularly favourable
because the time delay is so well known (Pelt et al., 1998).
-
PG1115+080 This GLS consists of a quadruply imaged QSO; also
very promising for determining the Hubble constant. Monitoring of this
system by P. Schechter et al. (Ap.J 475, 1997) has led to approximate values
for the time delays of 9.4 days between B and A1+A2 and 23.7
days between B and C, but still only accurate to about 15 %. With
more extended and accurate light curves one expects considerable improvements.
-
H1413+117 ("Clover Leaf"). This quadruply imaged quasar shows
optical variability as well as radio variability. No lensing galaxy has
yet been found. Several systems of absorption lines were discovered, some
of which may originate in the lensing galaxy (Angonin et al. 1990, Afanas'ev
et al. 1996b).
-
B1422+231. The radio source B1422+231 was discovered by Patnaik
et al. (1992) and found to have four unresolved components within a region
of 1.3 arcsec. A similar configuration has been observed optically (Remy
et al. 1993). Ground based observations showed significant variability
of the components and the lensing group was identified (Yee and Bechtold,
AJ 111, p.1007, 1996).
-
B1600+434. This system was recently discovered (Jackson et al. MNRAS
274, 1995) in the CLASS radio survey. The double image of a distant quasar
(z=1.61) is produced by an edge-on spiral galaxy. Variability in brightness
and colour has been reported for the components (Jaunsen & Hjorth 1997).
-
Q2237+030 ("Einstein Cross"). This quadruply imaged quasar (z=1.695)
is a perfect system for studying microlensing since the expected time delays
very small (< 1 day) and the lensing Sb-galaxy at z=0.039 is by far
the closest one among all presently known GLS. The brightness and colour
variations of the lensed components will be investigated.
Any newly discovered GLS with promising parameters will be included
into the list and observed during the Programme run.
 |
| The
Einstein Cross in three different epochs, from left to right: Sept. 17,
1995, Aug. 30, 1997, and Aug 30, 1998,
R filter. The picture demonstrates noticeable
mutual brightness variations of the quasar
components. |
2. Methodology
The observations are assumed the direct CCD imaging in
the BVRI spectral bands in a series consisting of about 10 CCD-frames per
night/per object/per filter with a sampling rate of 3-5 days. Special
investigation of the properties of the used CCD detector as well as of
the photometric system parameters will be carried out at the first phase
of the Programme. This will ensure a reliability and homogeneity of the
observational data in the phase of their detecting, thus providing the
possibility of examining the intranight and night-to -night variability.
As was mentioned above, the extreme spatial compactness
of the most of GLS causes certain observational difficulties. Even
more important difficulties arise in the phase of photometric image
processing. For such objects, there is a danger to obtain the incorrect
brightness estimates for the individual quasar components. Therefore, great
attention will be paid to elaboration of special algorithms of image processing,
which would ensure the most reliable and unbiased brightness estimates
for all features of the observed image.
3. The Teams Involved
The observations in the framework of the Programme will be carried
out by three groups of observers from :
-
the Astronomal Observatory of the Kharkov University, headed
by Prof. Vladinir Dudinov (Ukraine),
-
Sternberg Astronomical Institute of the Moscow University, headed
by Dr. Boris Artamonov (Russia), and
-
Ulug Beg Astronomical Institute, headed by Dr. Irek Khamitov (Tashkent,
Uzbekistan).
All three groups consist of good specialists in observations
with a high angular resolution, in creating, examining and operating
the light detectors and other facilities.
The team from the Astronomical Observatory of the
Kharkov University is known by its great experience in image
processing aimed at achieving high angular resolution in ground-based observations.
Speckle interferometry of stars with the 6-meter BTA telescope, the reconstruction
of diffraction-limited images of the asteroid Vesta, as well as elaboration
of the efficient algorithm for obtaining the unbiased estimates of magnitudes
of the individual GLS components should be mentioned as the most
impotant works of this team.
The team from the Sternberg Astronomical Institute
has great experience in CCD observations of quasars, star-burst regions
in other galaxies, in studying the seeing conditions on Maidanak, and recently,
in collaboration with the Kharkov Astronomical Observatory - in observations
of GLS. The team leader was among those who since 1975 took part in pioneer
investigation of the seeing conditions on the Maidanak mountain and, further,
in developing the observational base for the Sternberg Astronomical Institute
on Maidanak. The team members took part in mounting the 1.5-meter
telescope and can therefore ensure the technical support for the observations.
The team from the Ulugh Beg Astronomical Institute
has great experience in CCD observations with the 1.5-m Maidanak telescope,
in photometry of stellar objects, in analysis and processing of the temporal
data sequences, and, recently, in image processing, which has been earlier
applied to obtaining high-resolution images of the solar surface features.
Suffice it to say, that photometric data obtained on Maidanak according
to the ROTOR international program (determining the rotational periods
of stars younger than the Main sequence), are as many as 60% of the total
world database.
1
Astronomical Observatory of Kharkov University, Ukraine
2Ulugh
Beg Astronomical Institute, Republic of Uzbekistan
3Sternberg
Astronomical Institute, Russia