Measuring Microlensing Using Spectra of Multiply Lensed Quasars

Abstract

We report on a program of spectroscopic observations of gravitationally lensed QSOs with multiple images. We seek to establish whether microlensing is occurring in each QSO image using only single-epoch observations. We calculate flux ratios for the cores of emission lines in image pairs to set a baseline for no microlensing. The offset of the continuum flux ratios relative to this baseline yields the microlensing magnification free from extinction, as extinction affects the continuum and the lines equally. When we find chromatic microlensing, we attempt to constrain the size of the QSO accretion disk. SDSSJ1004+4112 and HE1104-1805 show chromatic microlensing with amplitudes 0.2 < |Δm| < 0.6 and 0.2 < |Δm| < 0.4 mag, respectively. Modeling the accretion disk with a Gaussian source (I ∝ exp(−R 2 /2r 2 s )) of size r s ∝ λ p and using magnification maps to simulate microlensing, we find r s (λ3363) = 7 ± 3 lt-day(18.1 ± 7.8 × 10 15 cm) and p = 1.1 ± 0.4 for SDSS1004+4112, and r s (λ3363) = 6±2 lt-day(15.5±5.2×1015 cm) and p = 0.7±0.1 for HE1104-1805. For SDSSJ1029+2623, we find strong chromaticity of ∼0.4 mag in the continuum flux ratio, which probably arises from microlensing, although not all the available data fit within this explanation. For Q0957+561, we measure B − A magnitude differences of 0.4 mag, much greater than the ∼0.05 mag amplitude usually inferred from light-curve variability. It may substantially modify the current interpretations of microlensing in this system, likely favoring the hypothesis of smaller sources and/or larger microdeflectors. For HS0818+1227, our data yield possible evidence of microlensing.

Structure of the Accretion Disk in the Lensed Quasar Q2237+0305 From Multi-Epoch and Multi-Wavelength Narrowband Photometry

“…The study of the disk temperature profile is more complicated because multi-wavelength observations are needed to detect chromatic microlensing (Anguita et al 2008;Bate et al 2008;Eigenbrod et al 2008;Poindexter et al 2008;Floyd et al 2009;Blackburne et al 2011Blackburne et al , 2015Mediavilla et al 2011b;Muñoz et al 2011;Motta et al 2012;Rojas et al 2014). In this case, it also becomes more important to separate the contributions from strong emission lines.…”

Discovery of two gravitationally lensed quasars in the Dark Energy Survey

“…The microlensing effect on the multiple quasar images, induced by stars in the deflector, provides a quantitative handle on the stellar content of the lens galaxies (e.g., Schechter & Wambsganss 2002;Oguri, Rusu & Falco 2014;Schechter et al 2014;Jiménez-Vicente et al 2015), and can simultaneously provide constraints on the inner structure of the lensed quasar, both the accretion disk size and the thermal profile (e.g. Poindexter, Morgan & Kochanek 2008;Anguita et al 2008;Eigenbrod et al 2008;Motta et al 2012) as well as the geometry of the broad line region (e.g. Sluse et al 2011, Guerras et al 2013, Braibant et al 2014.…”

Deep optical imaging and spectroscopy of the lens system SDSS J1339+1310

“…2.2, and each of these three emissions is most likely generated in spatially different regions (e.g., Marziani et al 2010;Sluse et al 2011;Motta et al 2012). According to Motta et al (2012), the broad wings of a given emission line could be produced in a compact region, whereas its core is probably dominated by photons arising from the NLR and the outer parts of the BLR. Thus, large sources would yield the line cores, which would be unaffected by microlensing.…”

“…These macrolens (intrinsic) flux ratios are difficult to measure because the quasar light may be affected by differential microlensing magnification and dust extinction within the intervening galaxy (e.g., Wambsganss 1990;Falco et al 1999). Fortunately, one can use flux ratios of emission line cores to fit macrolens ratios and extinction properties (e.g., Wucknitz et al 2003;Mediavilla et al 2009;Motta et al 2012). The typical Einstein ring of stars (in the source plane) has a size similar to that of the accretion disk, which is the reason why microlenses (stars) strongly affect the X-ray, UV, and optical continuum emission (e.g., Yonehara et al 1998;Morgan et al 2012;Mosquera et al 2013).…”

“…Microlensing spectra (microlensing magnification ratios of the continuum at many wavelengths) allow astronomers to discuss the accretion disk structure (e.g., Mediavilla et al 2011;Motta et al 2012), while microlensing (extrinsic) variability in broadband light-curves of the quasar images often complicate the measuring of time delays between intrinsic variations. However, intensive microlensing simulations in the time-domain can separate both kinds of variability and yield accurate time delays (and sizes of variable sources; e.g., Hainline et al 2013).…”