Microlensing in the double quasar SBS1520+530

Abstract

We present the results of a monitoring campaign of the double quasar SBS1520+530 at Maidanak observatory from April 2003 to August 2004. We obtained light curves in V and R filters that show small-amplitude \Delta m~0.1 mag intrinsic variations of the quasar on time scales of about 100 days. The data set is consistent with the previously determined time delay of \Delta t=(130+-3) days by Burud et al. (2002). We find that the time delay corrected magnitude difference between the quasar images is now larger by (0.14+-0.03) mag than during the observations by Burud et al. (2002). This confirms the presence of gravitational microlensing variations in this system. Comment: 6 pages, 7 figures. Accepted for publication in A&A

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arXiv:astro-ph/0505489v1 24 May 2005
Astronomy & Astrophysics manuscript no. sbs1520
(DOI: will be inserted by hand later)
February 2, 2008
Microlensing in the double quasar SBS1520+530
E. R. Gaynullina1, R. W. Schmidt2, T. Akhunov1,3, O. Burkhonov3, S. Gottl¨ ober4, K. Mirtadjieva1,3, S. N.
Nuritdinov1,3, I. Tadjibaev1,3, J. Wambsganss2, and L. Wisotzki4
1National University of Uzbekistan, Physics Faculty, Tashkent, 700174, Uzbekistan
2Astronomisches Rechen-Institut, Zentrum f¨ ur Astronomie der Universit¨ at Heidelberg, M¨ onchhofstraße 12-14, 69120
Heidelberg, Germany
3Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences and Isaac Newton Institute of Chile, Uzbek Branch,
Astronomicheskaya 33, Tashkent, 700052, Uzbekistan
4Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
Draft February 2, 2008
Abstract. We present the results of a monitoring campaign of the double quasar SBS1520+530 at Maidanak observatory from
April 2003 to August 2004. We obtained light curves in V and R filters that show small-amplitude ∆m ≈ 0.1 mag intrinsic
variations of the quasar on time scales of about 100 days. The data set is consistent with the previously determined time delay
of ∆t = (130 ± 3) days by Burud et al. (2002). We find that the time delay corrected magnitude difference between the quasar
images is now larger by (0.14 ± 0.03) mag than during the observations by Burud et al. (2002). This confirms the presence of
gravitational microlensing variations in this system.
Key words. gravitational lensing – dark matter – quasars: individual: SBS1520+530 – cosmology: observations
1. Introduction
The broad absorption line (BAL) quasar SBS1520+530
(zq = 1.855) was discovered by Chavushyan et al. (1997)
as a gravitationally lensed double quasar with an angu-
lar separation of 1.′′56. The lensing galaxy was detected
by Crampton, Schechter & Beuzit (1998) using infrared adap-
tive optics imaging at the Canada-France-Hawaii Telescope.
Faure et al. (2002) observed the lensing galaxy with the
Hubble Space Telescope. Burud et al. (2002) (in the follow-
ing B02) finally succeeded in obtaining both the redshift
zgal = 0.717 (consistent with absorption lines first found by
Chavushyan et al. 1997) of the lensing galaxy with a Keck
obervatory spectrum and the time delay of ∆t = (130 ± 3)
days between the two quasar images using monitoring data
from the Nordic Optical Telescope (NOT). For this, B02 ob-
tained an almost gapless lightcurve of the object of about
800 days between February 1999 and May 2001. An almost
continuous coverage of the light curve of SBS1520+530 is
made possible by its high declination. Further photometry ob-
tained at Maidanak observatory on the system was published
by Zheleznyak, Sergeev & Burkhonov (2003).
Interestingly, B02 found that by simply shifting the light
curve of image B backward by 130 days and correcting for the
magnitude difference ∆m = 0.69 mag of the images made the
quasar light curves align only approximately. They obtained
Send offprint requests to: R. W. Schmidt, E-mail: rschmidt@ari.uni-
heidelberg.de
a better match by allowing for an additional linear trend, and
by correcting for faster variations using an iterative scheme
(Burud et al., 2001). B02 interpret these additional variations
to be probably due to microlensing variability.
SBS1520+530 thus is one of the prime targets for mi-
crolensing studies since it provides at once the prospects for
long, uninterrupted light curves with a known, relatively short
time delay and known microlensing variations. For this reason
we decided to continue the optical monitoring of this system.
2. Observations
We report here on our observations of SBS1520+530 (Fig. 1)
with the 1.5m AZT-22 telescope in Maidanak, Uzbekistan
(Ehgamberdievet al., 2000), between April 25 and October 12
in 2003, and again between January 16 and August 22 in 2004.
Intheseintervalswe observedtheobjectalmostdailywhenever
the weather permitted it. The V and R-filters provide photom-
etry in the Johnson-Cousins system. We observed on average
four frames per night with an exposure of 3.5 min in V and 3
min in R with a median seeing of 1.07 arcsec.
Intotalweobtained116nightsintheV bandand191nights
in the R band. After selecting for good seeing (better than 1.4
arcsec full-width at half maximum) and low sky-background
we finally used 80 nights in the V-band and 123 nights in the
R-band. The observations were made with the BROCAM CCD
detector with a pixel scale of 0.26 arcsec. Bias correction and

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2 E. R. Gaynullina et al.: Microlensing in the double quasar SBS1520+530
Fig.1. R-band image of SBS1520+530 obtained on May 7,
2003. The quasar images, the reference stars S3 and S4, and
the star we use for the PSF are labelled. The field size is 2.8
arcmin×3.5 arcmin. The area marked with the box is shown in
Fig. 2.
flat fielding of the images were done using standard IRAF soft-
ware.
3. Photometry
A special property of the SBS1520+530 system is its location
within 14 arcseconds of a bright 12th magnitude star (Fig. 2).
This is great for adaptive optics studies of the system because
a bright reference star is at hand (Crampton et al., 1998). For
photometry,however,the brighthalo and the diffractionpattern
caused by the star on the CCD needs to subtracted carefully.
In order to do this, we followed the method described by
Zheleznyak et al. (2003); we extracted the western half of the
star and subtracted it from the eastern part where SBS1520 is
situated. This procedure also efficiently subtracts the light due
to the horizontaldiffractionspike that extends towards the dou-
ble quasar.
Photometry on the quasar components A and B was per-
formed using the DAOPHOT package (Stetson, 1987). We
chose this method because it is well-suited to the mildly asym-
metric and time-variable AZT-22 point spread function (PSF).
BecauseofthevariablePSFit wasnotpossibletousetheimage
subtraction technique (Alard & Lupton, 1998; Alard, 2000).
The quoted error bars are the standard 1σ errors determined
by DAOPHOT. Since there are four point sources (the quasar
Fig.2. R-band zoom of the central part of SBS1520+530. The
field size is 30 arcsec×25 arcsec. North is up and East is to the
left. Thebrightforegroundstar nextto the two quasarimagesA
and B is saturated in the centre. S1 and S2 are also foreground
stars. The lens galaxy (mR,gal ≈ 21.6 mag, Crampton et al.
1998) is too faint to be seen in this image.
components A and B, and the stars S1 and S2) crowded in
a rather small region, we had to fit the positions and magni-
tudes of all four components at the same time. Absolute quasar
magnitudes were calibrated using the brightness of the refer-
ence star S3 (see Fig. 1) in the V-band (mV = (17.37 ± 0.02)
mag) and R-band (mR = (17.18 ± 0.02) mag) determined by
Zheleznyak et al. (2003). The star we used as a template to
model the point spread function in DAOPHOT is marked with
“PSF” in Fig. 1.
Inouranalysis we ignorethe presenceof the lensinggalaxy
nearimage B because it is toofaint to be detectedin ourimages
(mR,gal= 21.6 mag, Crampton et al. 1998). If all of the galaxy
light contributes to our magnitude estimate of quasar image B,
the measured brightness would increase by a constant offset of
0.08 mag (assuming mR,B= 18.8 mag).
4. Results
4.1. Light curves
We present the results of the R-band photometry of
SBS1520+530 in Fig. 3 and Table 11. The light curves of
the two quasar components A and B are plotted together with
the light curve of an additional reference star S4 (see Fig. 1,
mR= 18.16 mag). This shows that both S3 (which is used for
the absolute magnitude calibration) and S4 do not vary.
Both quasar components show low-amplitude ∆m ≈ 0.1
mag variations on time scales of about 100 days. As we will
showinthenextsection,theoverallsimilarityofthetwoquasar
1Table 1 is being made available electronically at CDS. Columns
1-4 contain the A and B magnitudes with errors, Column 5 contains
the Julian Date of the observations.

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E. R. Gaynullina et al.: Microlensing in the double quasar SBS1520+5303
Fig.3. R-bandlight curves of the two quasarimages A and B in
SBS1520+530 and the reference star S4. For clarity the mag-
nitude for image B was shifted by -0.4 mag, S4 was shifted by
+2.4 mag. The dotted vertical line indicates the day with the
Julian Date-2452000=880 when the mirror was cleaned. The
dashed horizontal line shows the magnitude of the reference
star S4 mS4= 18.16 mag.
Fig.4. V − R colour curve for the quasar images A (top) and B
(bottom).Thebest-fittingvalueforeachquasarimageis plotted
with a dashed line.
image light curves is due to a gradual brightness decrease of
the lensed quasar. On the day with Julian Date-2452000=880
the telescope mirror was cleaned. This led to a large sensitivity
improvement that visibly improved the accuracy of the quasar
brightness measurements, especially for the fainter image B.
In Fig. 4 and Table 22we show the difference between our
V-band and R-band light curves of SBS1520+530. We find an
average V − R colour mV− mR = 0.15 mag for image A and
mV−mR= 0.18 mag for image B. We do not find any evidence
for significant colour variations during our observing interval.
The small difference ∆(V − R) ≈ 0.03 mag of the V − R colour
between the quasar images indicates the presence of a small
level of differential reddening along the light paths.
4.2. Time delay
A wide variety of algorithms have been developed for the de-
termination of time delays in gravitational lens systems (e.g.,
Kundic et al., 1997; Burud et al., 2001; Gil-Merino et al.,
2002). We choose here a strightfoward linear interpolation
scheme because our light curve mainly consists of two fre-
quently sampled observing intervals. The unfrequently sam-
pled gap in the middle cannot be confidently interpolated with
any method. In detail we use the following recipe:
1. TheBlightcurveisshiftedbythetimedelay∆t tobetested.
2. One of the two light curvesis linearly interpolatedto match
the observing dates of the other light curve.
3. Only gaps that are less than 40 days (including the gaps
between Julian Date-2452000=1000 and 1100) are inter-
polated, no difference is calculated for larger gaps because
they can introduce a false signal due to sparse sampling of
the light curve.
4. For the remaining N days of overlap, the weighted differ-
ence ∆m =< mA−mB> is determined and the goodness of
fit estimator
χ2
ν=
1
N − 2
N ?
i=1
(mA(ti) − mB(ti+ ∆t) − ∆m)2
σ2
A+ σ2
B
(1)
(correspondingto the time delay ∆t) is calculated.
For the linear interpolation the errors are added in quadrature.
The number of degrees of freedom is ν = N − 2 because there
are two free paramaters: time delay and magnitude shift. The
1/(N − 2) factor penalizes solutions with a small number of
overlap days.
The best-fitting time delay can be determined by calculat-
ing χ2
time delay with the lowest χ2
associated measurementuncertaintieswe used 10000bootstrap
resamplings of the observed light curve, smoothed by a trian-
gular filter with a full width of 20 days (e.g., Kundic et al.,
1997). For each smoothed resampling the best-fitting time de-
lay was determined for time delays between 0 and 220 days
(image A leading). This procedure is robust with respect to the
size of the filter. We always interpolated image A because in
this case the sparse sampling of our light curve between Julian
Date-2452000=900and1100hasasmallerimpactonthedeter-
mined limits. We have verified that interpolating image B does
νvalues for a range of time delays, and by choosing the
νvalue. In order to determine the
2Table 2 is being made available electronically at CDS. Columns
1-4 contain the V −R colours for images A and B with errors, Column
5 contains the Julian Date of the observations.

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4 E. R. Gaynullina et al.: Microlensing in the double quasar SBS1520+530
Fig.5.
SBS1520+530lightcurve.Theprobabilitywascalculatedfrom
10000 Monte-Carlo realizations of the light curve, and by in-
terpolating quasar A each time. 95 per cent of the probability
is contained in the four separate regions above the dashed line.
Probability p for time delays based on our
not change the answers. It would only weaken the determined
constraints.
The result of the Monte-Carlo resampling is shown in
Fig. 5. For two-day bins of the time delay ∆t the number of
Monte-Carlo light curves with a best-fitting time delay in the
given range has been calculated. The probability p of each bin
was calculated by dividing by the total number of resamplings.
We find that there are several time delays that are consistent
with our data; the 95 per cent confidence region consists of
four separate regions. For these sub-regions we can calculate
the average time delay and the standard deviation, yielding
∆t = (115.4± 2.1) days, (130.5± 2.9) days, (146.3± 1.0) days
and (198.8 ± 1.1) days. The regions carry 33 per cent, 54 per
cent, 2 per cent and 6 per cent, respectively, of the statistical
weight.
The sub-region with the largest statistical weight is consis-
tent with the time delay ∆t = (130±3) days found by B02. For
this time delay our data require an offset of ∆m = −0.83 mag.
In the remainder of this paper we will use this time delay for
our data.
4.3. Microlensing
In the top panel of Fig. 6 we show the quasar A and B light
curves in one plot, where image B was shifted to the left by
the time delay of 130 days and up by the magnitude offset of
−0.83 mag (see Sect. 4.2). The composite light curve has no
large gaps. It shows that the quasar has been going through a
Fig.6.Top panel:R-bandlightcurvesofthetwoquasarimages
A (filled circles) and B (open circles). The B light curve has
been shifted by −130 days, as indicated by the arrow, and by
−0.83mag.Bottom panels: Differencelight curvebetweenthe
shifted and interpolated quasar images. This was calculated in
both ways: by interpolating image A or by interpolating image
B (see text).
series of three small ∆m ≈ 0.1 mag brightness variations that
each lasted about 100 days.
Microlensing in the lens galaxy would only affect one of
the light paths to the quasar and could thus be detected as a
residual light curve difference (e.g., Schmidt & Wambsganss,
1998; Wambsganss et al., 2000). In order to study whether mi-
crolensing variations are present in our data, we calculated the
difference between the two observed light curves for the time
delay ∆t = 130 days found by B02 and the magnitude offset of
∆m = −0.83 mag.
To calculate the differencelight curve,quasarB was shifted
in time and magnitude. The rest of the procedureis identical to
the one described in Sect. 4.2; we calculated the difference by
linearly interpolating the light curves of quasar A or B. The
light curves were interpolated whenever the gap was less than
40 days. No difference was calculated for larger gaps. The er-
ror bars were added in quadrature.The resulting two difference
light curves are plotted in the bottom two panels of Fig. 6.
It can be taken from these plots that we do not detect a sig-
nificant difference between the two quasar light curves.We can

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E. R. Gaynullina et al.: Microlensing in the double quasar SBS1520+5305
calculate the goodness-of-fit estimator for the null hypothesis
that there is no difference between the light curves on the N
daysforwhichthedifferencewas calculated(yieldingν = N−1
degrees of freedom):
χ2
ν=
1
N − 1
?
i
∆m2
σ2
i
i
.
(2)
This calculation yields χ2
or B is interpolated,indicatingthat there is excellentagreement
between the light curves. The data are formally compatible at
a probability of 47 per cent with the null hypothesis. This χ2
procedure ignores a possible temporal correlation of the data,
but there is also no evidence in Fig. 6 for such a correlation.
Assuming that the data have a Gaussian scatter aroundzero
we can determine the standard deviation
ν= 1.0 regardless of whether image A
ν-
σ2=
1
N − 1
?
i
∆m2
i.
(3)
This yields σ = 0.03 mag if image A is interpolated and
σ = 0.04 mag if image B is interpolated.These values are indi-
cated in Fig. 6 (dashed lines). They are of the same magnitude
as the error bars, again showing that the difference light curve
is consistent with beingentirely dueto measurementuncertain-
ties.
In their earlier data taken between February 1999 and May
2001, B02 did find a difference between the light curves of the
two quasar images in this system. Applying our procedure to
the light curves in their table 2, we can also calculate the differ-
ence light curveof their data. The result is shown with in Fig. 7
(open circles) together with our difference light curve (filled
circles) from the first of the two bottom panels in Fig. 6 (image
A was linearly interpolated).The differencecurve in this figure
is plotted without offset.
Fig. 7 shows that B02 observed a coherent and highly sig-
nificant difference light curve with a maximum amplitude of
∆m ≈ 0.08 mag. Since the time of the observations by B02 the
magnitude difference between the quasar images has increased
by 0.14±0.03 mag (see the discussion in Sect. 5). B02 already
identified a linear trend (dashed line) in their data. Our data
are consistent with this linear trend having continued until the
epoch of our observations.
Although the exposuretimes for the Maidanak and the B02
data are similar, the error bars of the Maidanak data are larger
than the error bars obtained by B02. The main reason for this
are the different telescope apertures (1.5m at the AZT-22 vs
2.5m at the NOT). In addition, however, the AZT-22 transmis-
sion was reduced by about one magnitude before the mirror
was cleaned on Julian Date-2452000=880(see Fig. 3).
We note that the long-term variation of the difference light
curve may even be slightly larger than shown in this plot be-
cause some light from the lensing galaxy could be included
in our magnitude estimate of image B (see Sect. 3). If all of
the lensing galaxy light were included, the Maidanak differ-
ence light curve would have to be shifted upward by a constant
offset of -0.08mag to correct for the galaxy contribution.
Fig.7. Composite of the difference light curves based on our
Maydanak data (filled circles, see Fig. 6) and the data pub-
lished by Burud et al. (2002) (open circles, the linear trend de-
termined by them is shown with a dashed line). In both cases
image A was linearly interpolated.
5. Summary and discussion
We have presented V-band and R-band photometry of the
gravitational lens system SBS1520+530 taken at Maidanak
Observatory in the years 2003 (April to October) and 2004
(January to August). During the ≈ 500 day observation period
with 80 data points in V and 123 data points in R we find small
amplitude intrinsic variations (∆m ≈ 0.1 mag) on time scales
of about 100 days in both quasar images. The V − R colour of
thequasaris consistentwithbeingconstantduringtheobserved
period.
Using linear interpolation of the quasar light curves, we
have determined the 95 per cent confidence region of time
delays for our data set. Due to gaps in the light curve, our
data allow four separate values of the time delay, the best
one of which agrees with the time delay of (130 ± 3) days
found by B02. Image A is leading, which is also consis-
tent with lens models of the system (Asano 2000; Faure et al.
2002;B02;Zheleznyak et al. 2003).
Usingthe B02 timedelay,we calculatedthedifferencelight
curve between the two quasar images. This shows that within
the statistical uncertainty the two quasar light curves are iden-
tical during our observing interval (Fig. 6). In the observations
taken by B02 between 1999 and 2001, a highly significant and
variable difference with an amplitude of ∆m ≈ 0.08 mag was
present for a fraction of the observing interval. Since then, the
overallR-bandmagnitudedifferencebetween the A and B light
curves has changed by 0.14 ± 0.03 mag, the difference being
larger in our data (Fig. 7).
Any variable difference between the light curves of
SBS1520+530 can be interpreted as gravitational microlens-
ing because other changes of the source would be visible
in both quasar images, delayed by the time delay. In addi-
tion to the microlensing variability on short time-scales (≈
100 days and less) found by B02, our data show that there

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6 E. R. Gaynullina et al.: Microlensing in the double quasar SBS1520+530
are also variations on longer time-scales of ∆t ≈ (100 −
1000) days. This overall level of microlensing variations in
SBS1520+530 appears comparable to variations seen in other
lens systems (e.g., Hjorth et al., 2002; Wyithe & Loeb, 2002;
Schechter et al., 2003).
An exciting prediction for the microlensing effect of
quasars is the colour-dependence of the microlensing light
curve in the vicinity of caustics (Wambsganss & Paczynski,
1991). In such a situation the difference between the V and the
R light curve could provide valuable clues to the source struc-
ture of the quasar. We will continue to observe SBS1520+530
from Maidanak observatory because frequent sampling of the
source remains crucial to derive limits on microlensing vari-
ability. If colour variations associated with microlensing could
be proven in this system, there would be a strong case for par-
allel spectral observations of the quasar (see also B02).
Since microlensing currently remains the only technique
with the promise to scan the continuum emission regions of
quasars on microarcsecond scales, SBS1520+530 should be
viewed as a prime target because of the combination of known
quasar variability and the at least occasional occurence of mi-
crolensing diagnostics at the same time.
Acknowledgements. We thank B. P. Artamonov and V. N. Dudinov
for useful advice on the realization of our observations. We thank
the former German Ambassador to Uzbekistan, Dr. Martin Hecker,
for his support of our collaboration. The Uzbek team thanks the
AIP and the University of Potsdam for hospitality during visits. SG
and RWS thank the Ulugh Begh Astronomical Institute for hospital-
ity. This project was supported by the German Research Foundation
(DFG), grant 436 USB 113/5/0-1. We also acknowledge support by
the European Community’s Sixth Framework Marie Curie Research
Training Network Programme, Contract No. MRTN-CT-2004-505183
”ANGLES”.
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