Doubly imaged quasar SDSS J1515+1511: time delay and lensing galaxies

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by  Vyacheslav N. Shalyapin

Accepted for publicationin ApJ
PreprinttypesetusingLATEXstyleAASTeX6v.1.0
DOUBLY IMAGED QUASAR SDSS J1515+1511: TIME DELAY AND LENSING GALAXIES
Vyacheslav N. Shalyapin1,2 and Luis J. Goicoechea2
1
Institute for Radiophysics and Electronics, National Academy of Sciences of Ukraine, 12 Proskura St., 61085 Kharkov, Ukraine;
7 [email protected]
1 2DepartamentodeF´ısicaModerna,UniversidaddeCantabria,Avda. deLosCastross/n,39005Santander, Spain;[email protected]
0
2 ABSTRACT

n WeanalysenewopticalobservationsofthegravitationallenssystemSDSSJ1515+1511. Theseinclude
a
a 2.6–year photometric monitoring with the Liverpool Telescope (LT) in the r band, as well as a
J
spectroscopic follow–up with the LT and the Gran Telescopio Canarias (GTC). Our r–band LT light
6
1 curvescoveraquiescentmicrolensingperiodofthedoublyimagedquasaratzs =2.049,whichpermits
us to robustly estimate the time delay between the two imagesA and B:211± 5 days(1σ confidence

] interval; Ais leading). Unfortunately, the main lensing galaxy(G1) is so faintand closeto the bright
A
quasar that it is not feasible to accurately extract its spectrum through the GTC data. However,
G
assuming the putative redshift z = 0.742, the GTC and LT spectra of the distant quasar are used
G1
.
h to discuss the macrolens magnification, and the extinction and microlensing effects in G1. The new
p constraints on the time delay and macrolens magnification ratio essentially do not change previous
-
o findings on the mass scale of G1 and external shear, while the redshift of the lensing mass is found
r to be consistent with the assumed value of z . This is a clear evidence that G1 is indeed located at
t G1
s z =0.742. Fromthe GTC datawealsoobtainthe redshiftoftwoadditionalobjects(the secondary
G1
a
galaxy G2 and a new absorption system) and discuss their possible role in the lens scenario.
[

Keywords: gravitationallensing: strong — quasars: individual (SDSS J1515+1511)
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v
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1. INTRODUCTION
2
4 Optical light curves and spectra of gravitationally lensed quasars show different phases of microlensing activity
0
induced by stars in lensing galaxies, and each of these phases is relevant for certain astrophysical studies. Periods of
.
1 high microlensing activity were reported for several systems, including prominent flux variations (e.g., Udalski et al.
0
2006; Hainline et al. 2013), significant spectral distortions (e.g., Richards et al. 2004; Rojas et al. 2014), or both
7
1 (e.g., Goicoechea & Shalyapin 2016). Such high activity is related to stellar mass microlenses strongly affecting the
: accretion disk and the innermost part of the broad line region (BLR), while the narrow line region (NLR) remains
v
unaffected (e.g., Schneider & Wambsganss 1990; Abajas et al. 2002). Hence, a strong microlensing episode in a
i
X
lensedquasarcanbe usedtoprobethe structureofits accretiondiskandBLR,as wellasthe compositionofthe main
r lensinggalaxy(e.g.,Mosquera & Kochanek 2011,andreferencestherein). Unfortunately,whenanimportantextrinsic
a
(microlensing)variabilityispresentinthelightcurveofanimageofalensedquasar,itisdifficulttoaccuratelymeasure
the time delay between that image and any other. Only in some cases, time delays were measured to ∼ 5% precision
after monitoring during 5−10 year periods and using sophisticated techniques of analysis (e.g., Ofek & Maoz 2003;
Morgan et al. 2008; Hainline et al. 2013).
Time delays can be used to constrain the Hubble constant (and other cosmological parameters) and lensing mass
distributions (e.g., Refsdal 1964, 1966; Schneider et al. 2006). Therefore, lensed quasars with two or more images
in quiescent phases of microlensing activity are ideal targets to determine delays and study the Universe on different
scales. Knowingthe time delay betweentwoquiescentimagesofthe samequasar,if wedetect asharpvariationin the
optical flux of the leading image, a multiwavelength campaign can be planned to follow the variability of the trailing
one. Multiwavelength intrinsic variations in this trailing image are valuable tools to carry out reverberationmapping
studies (e.g., Gil-Merino et al. 2012; Dahle et al. 2015), and thus check the quasar structure derived from active
microlensing periods. Time delays from quiescent images and images showing smooth microlensing variations are
often measuredto ≤ 3%precision(e.g., Eulaers et al. 2013; Rathna Kumar et al. 2013), andsometimes to precision
levels as low as a few tenths of a percent (e.g., Kundi´c et al. 1997; Shalyapin et al. 2008).
2
The lensed quasar SDSS J1515+1511 consists of two optically bright images (A and B) separated by ∼ 2′′
(Inada et al. 2014). Although Inada et al. reported a source redshift z = 2.054, this relies on the SDSS spec-
s
trum of the A image taken in 2007, which has all its emission features at z = 2.049. It is also worth to note that
em
the SDSS/BOSS spectrum of A in 2012, as well as the new spectra of A and B in 2015 (see Section 2.2.1), contain
emission lines at a redshift of 2.049. We thus adopt z = z = 2.049 throughout this paper. Inada et al. also
s em
presented spectra of A and B that were obtained through a 900 s exposure with the DOLORES spectrograph on the
3.6m Telescopio NazionaleGalileo (TNG). They foundstrong Mgii absorptionat∼4900˚A in the spectrumof B (the
image closer to the main lensing galaxy G1). Such an absorption feature corresponds to intervening gas at a redshift
′
of 0.742, suggesting that z = 0.742. New Subaru Telescope adaptive–optics observations in the K –band improved
G1
the relative astrometry of the lens system and the morphological parameters of G1 (Rusu et al. 2016). Rusu et al.
also detected a secondary galaxy G2 southwest of the ABG1 system and predicted the time delay ∆t for three
AB
different lens models. To perform this model fitting, the macrolens magnification ratio ∆m was constrainedwithin
AB
an interval based on the K′–band magnitude difference B−A. The expected value of ∆t ranged from ∼ 145 to ∼
AB
216days,withthelongestdelayforthemostrealisticmodel: singularisothermalellipsoidplusexternalshear(SIE+γ).
Here we focus on observations of SDSS J1515+1511 during a quiescent phase of microlensing activity over the
2014−2016 period. The outline of the paper is the following: Section 2 presents a 2.6–year photometric monitoring
of both quasar images, and spectroscopic follow–up observations of the lens system. These last observations include
new long–slitspectra that are50 times deeper than the previous ones with the TNG (see above). Section 3 is devoted
to accurately measure ∆t , while we estimate ∆m in Section 4. In Section 4, we also discuss the dust extinction
AB AB
and microlensing magnification in G1. In Section 5, we update the SIE+γ lens model, confirm the tentative redshift
of G1, and discuss the role of G2 (and other intervening object) in the lensing phenomenon. Our main results and
conclusions appear in Section 6.
2. OBSERVATIONS AND DATA REDUCTION
2.1. Photometric monitoring
WeconductedaphotometricmonitoringcampaignofSDSSJ1515+1511fromearlyFebruaryof2014tomidSeptem-
ber of 2016, i.e., for 2.6 years with an average sampling rate of about two nights every week. All optical observations
were performed with the 2.0 m fully robotic Liverpool Telescope (LT) at the Roque de los Muchachos Observatory,
Canary Islands (Spain), using the IO:O CCD camera (pixel scale of 0′.′30). Eachobserving night, 2 × 300 s exposures
weretakeninthe r Sloanpassband,andthe correspondingframesweresubsequently passedthroughapre–processing
pipeline. The LT data reduction pipeline included bias subtraction, overscantrimming, and flat fielding. In addition,
we cleaned cosmic rays and interpolated over bad pixels using the bad pixel mask1.
After the basic reduction of frames, we assumed that the point–spread function (PSF) for each exposure is defined
by the surface brightness distribution of the unsaturated star with r = 16.770 mag in the vicinity of the lens system
(see the left panel of Figure 1). We also adopted this PSF star as the reference object for differential photometry.
The relative fluxes and magnitudes of both quasar images were then derived through PSF fitting. Our crowded–field
photometrypipelinereliedonIRAF2 tasksandtheIMFITFITSsoftware(McLeod et al. 1998),whilethephotometric
model consisted of two PSFs (A and B), an exponential profile convolved with the PSF (G1 galaxy located between
A and B), and a constant background. Inada et al. (2014) reported that the S´ersic index of G1 is consistent with n
= 1 (exponential profile), instead of n = 4 (de Vaucouleurs profile). Moreover, Rusu et al. (2016) found that G1 is
an edge–on disk–like galaxy. The ellipticity, orientation, and effective radius of G1, as well as the relative positions of
B and G1 (with respect to A), were taken from Rusu et al. (2016).
The galaxy brightness was estimated from the best frames in terms of signal–to–noise ratio (SNR) and full–width
at half–maximum (FWHM) of the seeing disc. These frames led to a galaxy–to–PSFstar ratio G1/PSF = 0.005, or
equivalently, r(G1) = 22.5 mag. We then applied our pipeline to the 310 usable frames, incorporating this G1/PSF
value as an additional constraint. As G1 is very faint in the r band, and the magnitude difference between G1 and
the faintest quasar image is ∼4 (B/G1∼ 40), we note that the adopted galaxy model does not play a critical role in
extracting quasar fluxes. We identified 19 frames producing outliers in the quasarlight curves,so these were removed
fromthefinaldatabase. Theremaining291frames(averageFWHMof∼1′.′4)allowedustoobtainr–SDSSmagnitudes
1 The pre–processed frames will be soon publicly available on the GLENDAMA archive at
http://grupos.unican.es/glendama/database/ (Goicoechea etal. 2015).
2 IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for
Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation. This software is available at
http://iraf.noao.edu/.
SDSS J1515+1511: time delay and lensing galaxies 3
of A and B at 150 nights (epochs). We also calculated the magnitudes of a field star at these epochs. The field star
has a brightness similar to that of A, and is located between the lens system and the PSF star (see the left panel of
Figure 1).
Figure 1. Left: First LT frame of SDSS J1515+1511 on 2014 February 4. This 300 s exposure in the r band was taken under
typicalseeingconditions(FWHM=1′.′3),justafewweeksaftertheannouncementofthediscoverybyN.Inadaandcoworkers.
Thefieldofviewcovers5′×5′,andincludestheblendedquasarimages(AandB),thePSFstar,andafieldstarthatliesabout
midway between the lensed quasar and the PSF star. Right: LT r–band light curves of A, B and the field star. The stellar
curveis shifted by +0.2 mag tofacilitate comparison.
Table 1. LT r–band light curvesof SDSSJ1515+1511AB.
MJD–50000 Aa σAa Ba σBa Sab σSab
6693.260 18.186 0.006 18.598 0.006 18.159 0.004
6696.282 18.183 0.007 18.582 0.007 18.156 0.005
6697.263 18.179 0.007 18.588 0.007 18.157 0.005
6700.232 18.191 0.007 18.581 0.007 18.123 0.006
6710.173 18.198 0.010 18.577 0.010 18.169 0.008
a
r–SDSSmagnitude.
b
Weuse S to denote thefield (control) star.
Note—Table1ispublishedinitsentiretyinthemachine-readablefor-
mat. A portion is shown here for guidance regarding its form and
content.
To estimate typical photometric errors in the light curves of A, B, and the star, we first determined the standard
deviations between magnitudes having time separations < 2.5 days, and then such deviations were divided by the
square root of 2 (e.g., Goicoechea & Shalyapin 2010). This procedure led to a typical uncertainty of 0.0085 mag in
the brightness of both quasar images, which is slightly larger than the typical error in the star (0.0065 mag). We also
considered that σ ∝ 1/SNR (e.g., Howell 2006) to infer nightly errors in the light curves. Table 1 includes the final
r
r–SDSS magnitudes and errors for A, B and the star. These brightness records are also shown in the right panel of
Figure 1. Apart from the two unavoidable seasonalgaps, the quasar variability is accurately traced.
4
2.2. Spectroscopic follow–up
2.2.1. Deep long–slit spectra
Deep long–slit spectra of SDSS J1515+1511 were obtained using the R500B and R500R grisms in the OSIRIS
spectrograph on the 10.4 m Gran Telescopio Canarias (GTC)3. The slit was oriented along the line joining both
quasar images, and its width was 1′.′23 (∼5 pixel). We took a single 1800 s exposure with the blue grism (R500B) on
′′
2015April15undergoodobservingconditions: darkandphotometricnight,FWHM=0.95atthecentralwavelength
λ = 4745 ˚A, and airmass of 1.04. We also observed the lens system with the red grism (R500R) on 2015 April 16.
c
This second grey night, 3 × 1800 s exposures were taken under variable seeing (FWHM = 1′.′19, 0′.′89, and 0′.′79 at λ
c
= 7165 ˚A) and airmass (1.04, 1.06, and 1.10). Hence, the new data cover a wavelength range of 3570−9250 ˚A with
a resolving power of ∼ 300−400. Observations of the spectrophotometric standard star Ross 640 (Oke 1974) were
′′
performedwith a wider slit of 2.52,which forcedus to carryout a careful flux calibrationfrom photometric data (see
below).
Foreachgrism,throughastandarddatareductionwithIRAF,weobtainedasky–subtracted,wavelength–calibrated
spectrumforeach∼5pixelslicealongtheslit. However,theextractionoftheindividualspectraofA,B,andG1isnot
a simple task. We deal with three sources next to each other. Moreover, there is high brightness contrast between B
andG1(e.g.,B/G1∼40inther band),only0′.′4apart. ThesecomplicationspreventeddetectionofG1usingtheblue
grism data. Among the known extraction techniques (e.g., Fassnacht & Cohen 1998; Ofek et al. 2006; Sluse et al.
2007;Shalyapin & Goicoechea 2014),aftersometests,wechoseamethodsimilartothatofSluse et al. (2007). They
derived spectra for three close point–like sources along a slit by fitting three 1D Moffat profiles for each wavelength
bin. Although G1 is an extended source,it has a verylargeellipticity with positionangle almostperpendicular to the
slit axis, and its effective radius in the spatial direction is ∼ 0′.′13 (Rusu et al. 2016). Thus, in the spatial direction,
this very faint galaxy can be treated as a point–like object (we checked the validity of this approach). For the red
grism,ourinitialmodelconsistedofthree1DMoffatprofiles(A+B+G1)ateachwavelength,whileweonlyconsidered
two components (A+B) when analysing the blue grism data.
Figure 2. GTC–OSIRIS–R500R fluxes along the slit. Left: Normalized spatial profile as a result of combining the 1D flux
distributions for all wavelength bins. The measured fluxes (data) are compared with fitted fluxes, using a model that consists
of three 1D Moffat profiles (A+B+G1) at each wavelength. Right: Differences between the true PSF and the Moffat function.
Theseresiduals (PSF−Moffat) comefrom thecentralregion andtheleft wingofA(redcircles), andtherightwingofB(blue
circles). Thesmoothed behaviour(purpleline) is the LUTcorrection (see main text).
From now on, we are going to describe in detail the extraction of the individual spectra of A, B, and G1, via the
red grismdata. We used the Rusu et al.’s astrometryto set the positions of B and G1 with respect to A. In addition,
the 1D Moffat function is characterized by three parameters: centroid, slope, and width, and after preliminary data
analysis, we also fixed a global value (the same at all wavelengths) for the slope (Jahnke et al. 2007). Thus, in a
3 TheOSIRISUserManual(v3.1)byA.Cabrera-Laversisavailableathttp://www.gtc.iac.es/instruments/osiris.
SDSS J1515+1511: time delay and lensing galaxies 5
first iteration, only the position (centroid) of A, the width of the Moffat function, and the amplitudes of the three
components were allowed to vary at each wavelength. The first fits to the multi–wavelength 1D flux distributions
generatedwavelength–dependentvalues ofthe position ofA and the width of the 1D PSFmodel, which were fitted to
smooth polynomial functions. In a second iteration, these two position–structure parameters were evaluated by their
polynomiallaws,leavingonlytheamplitudesasfreeparameters. WethenobtainedthefluxesofA,B,andG1through
the fitted amplitudes and the observational priors for the structure parameters of the PSF model.
Although the seconditerationproduces accurate spectra of the quasarimages,the spectrum of G1 is not as smooth
as would be desired. The PSF shape is not perfectly described with a Moffat function, and residuals (data − model)
are comparable to the 1D fluxes of G1. Because of the existence of this important residuals–inducednoise, we refined
the PSF model to try to remove/minimize residuals with an empirical look–up–table (LUT) correction (e.g., Møller
2000; Jahnke et al. 2007). Following Møller (2000), the ∼1000measuredspatialprofiles (1D flux distributions) were
combined into the single normalized distribution that is shown in the left panel of Figure 2. The differences between
measured and fitted fluxes (purple circles) display a regular spatial pattern, and we focused on a region around and
to the left of the centroid of A, as well as on a region to the right of the centroid of B. Both regions basically include
the residuals of interest, i.e., differences between the true PSF and the Moffat function. In the right panel of Figure
2, we present properly scaled residuals for a PSF centered at x = 0. These were smoothed in order to reduce noise
(see the purple line inthe rightpanelof Figure 2), andthen incorporatedinto a refinedPSFmodel: analyticalMoffat
function plus empirical LUT.
In a third iteration, we used Moffat+LUT profiles to describe the contributions of the three sources (quasar images
and main lensing galaxy) at each wavelength, and thus obtained the final instrumental spectra of A, B, and G1.
From the instrumental spectrum of the standard star, we also built the spectral response function, and corrected the
instrumentalfluxesofthequasarandthegalaxy. Unfortunately,thestandardstarwasobservedwitharelativelywide
slit (see above),which did not allow us to accurately calibrate the flux scale. In order to calibrate the quasarspectra,
weusedr–bandfluxesofAandBfromLTframestakenon2015April16(seeSection2.1). Tofixthe fluxscaleofG1,
we also considered the i–band flux of the galaxy by Inada et al. (2014). Our final flux–calibrated spectra of the lens
system (blue and red grisms) are shown in Figure 3. Tables 2 and 3 include the spectra of the lens system from the
blue and red grism data, respectively. Additionally, all raw and reduced frames in FITS format are publicly available
at the GTC archive4.
Table 2. GTC–OSIRIS–R500B
spectra of SDSS J1515+1511AB.
λa F (A)b F (B)b
λ λ
3567.142 48.233 32.704
3570.736 47.358 30.125
3574.330 48.826 29.310
3577.924 46.260 28.706
3581.518 48.972 32.073
aObserved wavelength in ˚A.
bFlux in 10−17 erg cm−2 s−1 ˚A−1.
Note—Table 2 is published in its
entirety in the machine-readable
format. A portion is shown here
for guidance regarding its form
and content.
4 http://gtc.sdc.cab.inta-csic.es/gtc/index.jsp
6
Figure 3. GTC–OSIRISspectraofSDSSJ1515+1511ABG1 in2015. Verticaldottedlinesindicateemission linesatzs =2.049,
while grey highlighted regions are associated with atmospheric artefacts. Top: R500B grism. The spectrum of B includes Fei,
Feii, and Mgii absorption at zabs = 0.742, and the spectrum of A contains a notable Fe/Mg absorption at zabs = 1.166 (see
Sections 4.2 and 5). Bottom: R500R grism. The top sub–panel displays a zoomed–in version of the G1 spectrum along with
thered–shifted (z = 0.742) spectral template of an S0 galaxy (Kinney et al. 1996).
SDSS J1515+1511: time delay and lensing galaxies 7
Table 3. GTC–OSIRIS–R500R spectra of
SDSSJ1515+1511ABG1.
λa F (A)b F (B)b F (G1)b
λ λ λ
4846.622 26.253 18.984 0.466
4851.437 23.692 16.945 0.551
4856.251 23.546 15.886 0.300
4861.066 23.258 14.193 0.598
4865.880 23.847 14.016 0.268
aObserved wavelength in ˚A.
bFlux in 10−17 erg cm−2 s−1 ˚A−1.
Note—Table 3 is published in its entirety in
the machine-readable format. A portion is
shown here for guidance regarding its form
and content.
Figure 4. GTC–OSIRIS–R500R spectrumofthesecondarygalaxy G2. Wealsodisplay severalexpectedabsorption features at
zG2 = 0.541 (vertical dashed lines), and use crosses and grey rectangles to highlight the riz–SDSS fluxes and spectral regions
associated with atmospheric artefacts, respectively. The observational configuration appears in the top sub–panel. This shows
thefour sources A,B, G1, and G2 within theslit edges (two parallel dashed lines).
To confirm or not the tentative redshift of the main lensing galaxy (0.742; Inada et al. 2014), we analysed the G1
spectrum in the top sub–panel inside the bottom panel of Figure 3. In spite of all our efforts with the GTC data, G1
appears as a very faint source with significant contamination by light of B. This contamination distorts its spectral
shape by producing artefact peaks (e.g., Mgii residual flux at ∼8500 ˚A) and an almost flat continuum. Indeed, the
spectrumofG1doesnotdisplayanoticeabledropoffinfluxat∼7000˚A,whichwouldcorrespondtoa4000˚Abreakin
therestframeofadisk–likegalaxywithoutemissionlinesataredshiftof0.742(seethemagentaline). Verticaldashed
8
lines indicate the positions of some expected absorption features at z = 0.742, but unfortunately, these positions
G1
coincide with flux decrements that are indistinguishable from noise or are located in a sky absorption region. As a
consequence, we were not successful in an unambiguous spectroscopic determination of z . However, we were lucky
G1
because ourGTC–OSIRIS–R500Rlong–slitexposuresincluded lightfromthe secondarygalaxyG2 (see Section1 and
the top sub–panel inside Figure 4), and thus, we extracted the G2 spectrum and measured its redshift. Since G2
is ∼ 16′′ away from the A image, we used the IRAF/APEXRACT package to get an initial spectrum, and then the
riz–SDSS magnitudes of the galaxy to peform an accurate flux calibration. The final spectrum is consistent with an
early–type galaxy at z = 0.541 (see Figure 4).
G2
2.2.2. Spectroscopic monitoring
We are conducting a robotic monitoring of a small sample of ∼10 lensed quasars with images brighter than r =
20 mag and visible from the Northern Hemisphere (Goicoechea et al. 2015). Taking the spatial resolution of the
LT spectrographs into account, we only consider spectroscopic monitoring campaigns of double quasars with image
separation ≥ 2′′. For each such doubles, we obtain spectroscopic observations separated by the time delay between
its two images, which allows us to study magnification ratios for the continuum and the emission lines at different
wavelengths. Thesedelay–correctedratios(i.e.,atthesameemissiontime)arekeytoolstodiscussthedifferentialdust
extinction, the macrolens magnification ratio, and microlensing effects in the system (e.g., Schneider et al. 2006).
We observed SDSS J1515+1511 with the SPRAT long–slit spectrograph on 2015 August 16. The 1′.′8 (∼4 pixel)
wide slit was oriented along the line joining A and B, and we used the red grating mode. The grating may be set
to two different configurations which are optimized for the blue or red regions of the 4000−8000 ˚A wavelength range
(resolvingpowerof∼350at6000˚A).This firstnight,wetook5× 600sexposuresunder normalobservingconditions:
FWHM∼1′.′5at6000˚Aandairmassof∼1.3. Wealsoobtained5×600sLT–SPRAT–blueexposureson2015August
18 to check the best grating mode for this lens system. After getting the earliest SPRAT spectra in 2015 August, a
secondstepwasto decide ona samplingtime basedonsometime delayestimation. While currentlensmodels predict
delays ranging from 5 to 7 months (Rusu et al. 2016), we used our two first photometric monitoring seasons (see
Sections 2.1 and 3) to set the sampling time to 7 months. Thus, we have re–observed the system on 2016 March 17
to obtain 5 × 600 s LT–SPRAT–blue exposures under acceptable seeing conditions (FWHM ∼ 1′.′7 at 6000 ˚A) with
a low airmass of ∼1.05. We also observed the spectrophotometric standard star BD+33d2642 (Oke 1990) on each of
the three monitoring nights in 2015−2016.
Figure 5. LT–SPRAT spectra of SDSS J1515+1511AB. Vertical dotted lines indicate emission lines at zs = 2.049, while grey
highlighted regions are associated with atmospheric artefacts. Left: 2015 August (total exposure time = 6 ks). Right: 2016
March (total exposure time = 3 ks).
The standard data reduction with IRAF included the same tasks that we used to process the GTC observations
in Section 2.2.1. The spectra of the two quasar images were extracted by fitting two 1D Gaussian profiles with fixed
separation. In a first iteration, the position of A, the width of the 1D PSF model, and the amplitudes of the two
components were allowed to vary at each wavelength. In a second iteration, we only considered the multi–wavelength
SDSS J1515+1511: time delay and lensing galaxies 9
instrumental fluxes of A and B as free parameters (see Section 2.2.1). From these instrumental spectra, the spectral
response functions, and r–band fluxes from LT frames (taken on 2015 August 19 and 2016 March 18), we derived
flux–calibrated spectra of A and B at the three observing epochs. The quasar spectra at the two first close epochs
werethencombinedto maketwospectralenergydistributions (AandB)with lowernoise (see the left panelofFigure
5). All final spectra are shown in Figure 5, and are also available in a tabular format (Tables 4 and 5). These contain
Siiv+Oiv], Civ, and Ciii] emission lines. It is also evident that the B spectrum varied appreciably on a timescale
equal to the time delay of the lens system (see Section 3).
Table 4. LT–SPRAT–blue/red
spectra of SDSS J1515+1511AB in
2015 August.
λa F (A)b F (B)b
λ λ
3980.459 38.616 13.170
3985.095 37.200 16.612
3989.731 30.862 20.126
3994.368 32.590 18.687
3999.004 35.608 17.765
aObserved wavelength in ˚A.
bFlux in 10−17 erg cm−2 s−1 ˚A−1.
Note—Table 4 is published in its
entirety in the machine-readable
format. A portion is shown here
for guidance regarding its form
and content.
Table 5. LT–SPRAT–blue spec-
traofSDSSJ1515+1511ABin2016
March.
λa F (A)b F (B)b
λ λ
3968.467 25.244 24.423
3973.122 35.824 22.400
3977.776 40.208 24.188
3982.431 36.540 25.976
3987.086 34.417 26.696
aObserved wavelength in ˚A.
bFlux in 10−17 erg cm−2 s−1 ˚A−1.
Note—Table 5 is published in its
entirety in the machine-readable
format. A portion is shown here
for guidance regarding its form
and content.
3. TIME DELAY
10
Assuming thatthe magnitude fluctuationsinthe rightpanelofFigure1 mainlyarisefromintrinsicvariationsin the
source quasar, we focused on two standard techniques to measure the time delay between A and B. First, we carried
out a reduced chi–square (χ2) minimization. This χ2 minimization was based on a comparison between the light
r r
curve of A and the time–shifted light curve of B, using different lags and bins with semisize α in B (e.g., Ull´an et al.
2006). For α ∼ 10 days, we found best solutions ∆t ∼ 210 days (χ2 ∼ 1), which are related with deep minima
AB r
in χ2–lag relationships (see the top panel of Figure 6). The uncertainties in ∆t and the r–band magnitude offset
r AB
(∆r ) were derived from 1000 repetitions of the experiment (pairs of synthetic light curves based on the observed
AB
records). To obtain synthetic curves for A and B,we modified the observedmagnitudes by adding randomquantities.
These random quantities were realizations of normal distributions around zero, with standard deviations equal to the
measured errors. We obtained 5000 delays and magnitude offsets by applying the χ2 minimization (α = 8, 9, 10, 11
r
and12days)tothe1000pairsofsyntheticcurves,andthecorrespondingdistributionsareshowninthebottompanels
of Figure 6.
Figure 6. Top: χ2–lag relationships for α = 8 days (black), α = 9 days(red), α = 10 days (green), α = 11 days(blue), and α
r
=12days(cyan). Bottom: Histograms from1000 pairsofsyntheticcurvesandtheχ2 minimization (α=8−12days). Theleft
r
and right panels display thebest solutions of thetime delay and themagnitude offset, respectively.
From the histograms in the bottom panels of Figure 6, we inferred the 1σ measurements (68% confidence intervals)
in the first result row of Table 6. The time delay is 211+4 days (A is leading), while the r–band magnitude offset
−5
(magnificationratio)is 0.3650± 0.0025mag. We also used the dispersionminimization to estimate the delay and the
offset. Thus, the D2 estimator (Pelt et al. 1996) with decorrelationlengths δ = 8−12days produceda minimum at
4,2
211 days. Additionally, when applying the D2 minimization (δ = 8−12 days) to the 1000 pairs of synthetic curves,
4,2