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Detection of colour variations from gravitational microlensing observations in the quadruple quasar HE0435-1223: Implications for the accretion disk

Christian Sorgenfrei, Robert W. Schmidt, Joachim Wambsganss

TL;DR

The paper addresses constraining quasar accretion-disk structure via chromatic microlensing in the quadruply lensed quasar HE0435-1223. Using a decade of Las Cumbres Observatory data in the $R$ and $V$ bands, the authors construct difference light curves to isolate microlensing and compare them with magnification-pattern simulations for Gaussian and Shakura–Sunyaev disks. They find a size ratio of $q_{R/V}=1.24^{+0.08}_{-0.20}$ (Gaussian) and $1.42^{+0.11}_{-0.22}$ (thin disk), consistent with standard thin-disk theory, and estimate absolute disk sizes around $r_{1/2} oughly 0.7$–$1.0$ Einstein radii, corresponding to $R_{2500} oughly 2.4 imes10^{16}$ cm. They also infer that image B undergoes caustic crossings at about once per year, highlighting the role of microlensing in probing inner quasar structure and motivating future multi-band surveys such as LSST for larger samples and tighter constraints.

Abstract

We present monitoring observations of quasar microlensing in the quadruple quasar HE0435-1223. The microlensing-induced light curves of the quasar images are chromatic, i.e. they depend on the applied filter band. Comparison with microlensing simulations allows us to infer properties of the accretion disk. We determine the R and V band light curves of the four images of HE0435-1223 from 79 and 80 epochs respectively, taken from 2014 to 2024 at the Las Cumbres Observatory using difference imaging analysis. We consider difference light curves to remove the intrinsic quasar variability. This reveals a prominent long-term chromatic microlensing event in image B. We use microlensing light curve simulations with both Gaussian and standard thin accretion disk brightness profiles to analyse this signal. The particularly strong signal observed in image B of HE0435-1223 makes it possible to detect a size ratio of the accretion disk in the R band compared to the V band of $1.24^{+0.08}_{-0.20}$ and $1.42^{+0.11}_{-0.22}$ for the Gaussian and the thin disk model, respectively. These values are in agreement with standard thin disk theory. For the absolute size we find large disk half-light radii of around 0.7 to 1.0 Einstein radii with an uncertainty of about 0.6 dex (depending on the filter bands and the models). Finally, our calculations show that image B undergoes caustic crossings about once per year.

Detection of colour variations from gravitational microlensing observations in the quadruple quasar HE0435-1223: Implications for the accretion disk

TL;DR

The paper addresses constraining quasar accretion-disk structure via chromatic microlensing in the quadruply lensed quasar HE0435-1223. Using a decade of Las Cumbres Observatory data in the and bands, the authors construct difference light curves to isolate microlensing and compare them with magnification-pattern simulations for Gaussian and Shakura–Sunyaev disks. They find a size ratio of (Gaussian) and (thin disk), consistent with standard thin-disk theory, and estimate absolute disk sizes around – Einstein radii, corresponding to cm. They also infer that image B undergoes caustic crossings at about once per year, highlighting the role of microlensing in probing inner quasar structure and motivating future multi-band surveys such as LSST for larger samples and tighter constraints.

Abstract

We present monitoring observations of quasar microlensing in the quadruple quasar HE0435-1223. The microlensing-induced light curves of the quasar images are chromatic, i.e. they depend on the applied filter band. Comparison with microlensing simulations allows us to infer properties of the accretion disk. We determine the R and V band light curves of the four images of HE0435-1223 from 79 and 80 epochs respectively, taken from 2014 to 2024 at the Las Cumbres Observatory using difference imaging analysis. We consider difference light curves to remove the intrinsic quasar variability. This reveals a prominent long-term chromatic microlensing event in image B. We use microlensing light curve simulations with both Gaussian and standard thin accretion disk brightness profiles to analyse this signal. The particularly strong signal observed in image B of HE0435-1223 makes it possible to detect a size ratio of the accretion disk in the R band compared to the V band of and for the Gaussian and the thin disk model, respectively. These values are in agreement with standard thin disk theory. For the absolute size we find large disk half-light radii of around 0.7 to 1.0 Einstein radii with an uncertainty of about 0.6 dex (depending on the filter bands and the models). Finally, our calculations show that image B undergoes caustic crossings about once per year.

Paper Structure

This paper contains 13 sections, 7 equations, 7 figures, 1 table.

Table of Contents

  1. Introduction
  2. From LCO data to microlensing signal
  3. Data reduction
  4. Light curves of HE0435-1223
  5. Difference curves and microlensing signal
  6. Microlensing Simulations
  7. Magnification patterns
  8. Light curve fitting
  9. Results and Discussion
  10. Size ratio and accretion disk temperature profile
  11. Accretion disk size estimates
  12. Number of caustic crossings
  13. Conclusion

Figures (7)

  • Figure 1: Light curves of HE0435-1223. This figure shows the time delay corrected light curves of the four images of HE0435-1223 in the $R$ band (left) and $V$ band (right) with the same time and magnitude ranges, with $1\sigma$ magnitude errors. Inside the left panel we have inserted the DIA reference image of HE0435-1223 in the $R$ band (linear in flux) as an example. The image size is $9arcsec\times9arcsec$, quasar image A is on the right and images B, C and D follow clockwise. Here, image B appears to be brightest, since the $R$ band reference image is combined mostly from images of the year 2022.
  • Figure 2: Difference curves of HE0435-1223. Shown are the six combinations of differences of the four light curves ($B-A$, $C-A$ and $D-A$ on the left side and $C-B$, $D-B$ and $D-C+1.0mag$ on the right side) both in the $R$ band (red circles, orange diamonds and brown squares) and the $V$ band (blue crosses, violet dots and green minuses) with $1\sigma$-uncertainties.
  • Figure 3: Magnification pattern with example tracks. Shown are the first 200 tracks (in red) on the unconvolved magnification map of image B as produced by Teralens for $\kappa_\star/\kappa=0.8$.
  • Figure 4: Best-fitting simulated light curves. We show here the best-fitting $R$ and $V$ simulated microlensing light curves (red and blue curves) from the 25 tracks with smallest $\chi^2=\chi^2_R+\chi^2_V$ for the Gaussian disk model (left) and the Shakura-Sunyaev thin disk model (right). The data points are the measured $R$ (orange circles) and $V$ (purple squares) $B-C$ difference curves (i.e. the two uppermost curves on the right side of Fig. \ref{['fig:diffcurves']} inverted and with adapted error bars as described in Sect. \ref{['sec:lcfit']}) as used for the $\chi^2$ calculation.
  • Figure 5: Disk size ratio distribution. Shown are the probability distributions of the size ratio $q_{R/V}$ (ratio of $R$ and $V$ band size) for the Gaussian (blue) and the thin disk model (red) with corresponding mean values (blue circle for the Gaussian and red diamond for the thin disk model, each with $1\sigma$-uncertainties; note that their position along the ordinate is arbitrary), as well as the dash-dotted black line showing the theoretically expected value.
  • ...and 2 more figures