Revisiting a Quasar Microlensing Event Towards AGN~J1249+3449

TL;DR

The paper reassesses the optical bump toward AGN J1249+3449 as a potential electromagnetic counterpart to GW190521 by leveraging the ZTF data release 23 (2018–2024) and fitting three microlensing models (PSPL, FSPL, USBL) with pyLIMA. Parallax effects are found to be negligible, and the PSPL model provides the best description of the light curve, yielding a lens mass of around $0.1\,M_\odot$ (consistent with a brown dwarf or low-mass star) located in the host galaxy. FSPL does not significantly improve the fit, and USBL is disfavored due to unrealistic mass configurations and higher $\chi^2$. The results strengthen the microlensing interpretation of the event, illustrating the ability of microlensing to probe very low-mass objects in distant galaxies without invoking an electromagnetic counterpart to the gravitational wave event.

Abstract

The gravitational wave event GW190521 seems to be the only BH merger event possibly correlated with an electromagnetic counterpart, which appeared about 34 days after the GW event. This work aims to confirm that the electromagnetic bump towards the Active Galactic Nucleus (AGN) J1249+3449 can be explained within the framework of the gravitational microlensing phenomenon. In particular, considering the data of the Zwicky Transient Facility (ZTF), what emerges from a detailed analysis of the observed light curve using three fitting models (Point Source Point Lens, Finite Source Point Lens, Uniform Source Binary Lens) is that the optical bump can be explained as a microlensing event caused by a lens with mass {$\sim\,$0.1 $M_{\odot}$}, lying in the host galaxy of the AGN in question.} %MDPI: Please confirm if the bold formatting is necessary; if not, please remove it.

Figures (7)

• Figure S1: (Left Panel): standard microlensing geometry; the bold curve shows the path of the light from the source (S) to the observer (O) being deflected by the lens (L) of mass $M_L$. The image (I) is displaced from the source by the angular Einstein radius $\theta_E$, which, projected onto the source plane, corresponds to a physical distance $\hat{r}_E$. (Right Panel): natural microlensing geometry; mostly the same as the left panel except that the Einstein radius is now projected onto the observer plane as $\tilde{r}_E$ rather than onto the source plane as $\hat{r}_E$. Figure adapted from ApJ 1.
• Figure S2: The geometry of a binary lens system. The lens plane is defined by the coordinate system $(x,y)$, with the $x$-axis passing through the projection of the two masses onto the plane itself, with the central point between the lens objects. The source plane is, instead, expressed by the coordinate system $(u,v)$, presenting its origin at the intersection point between the plane and the optical axis. $\xi_1$ and $\xi_2$ are the vector positions of the projection points of the masses ($M_1$ and $M_2$) in the lens plane geometry; $\alpha$ represents the deflection angle, $\beta$ indicates the angular position of the point $\eta (u, v)$ in the source plane in which the light ray originates, and $\theta$ is the angular position of the intersection point $\xi (x, y)$ between the light ray and the lens plane. The distances are expressed as follows: $D_L$ is the distance between the observer and the lenses, $D_{LS}$ stands for the distance between the binary lens and the source, and $D_S$ indicates the distance between the observer and the source. Figure adapted from Geosci. 1.
• Figure S3: The $g-r$ color for the AGN J1249+3449 light curve is shown on the vertical axis, while the horizontal axis represents the time expressed in MJD. For this figure, the $r$ band magnitudes have been interpolated at the time of the $g$ band measurements. The range of time ends at approximately 60,000 since it includes the whole microlensing event and does not consider the last group of points, where the concentration of $r$ band data is very low. The magnitude remains constant for the entire graph.
• Figure S4: Dependence of the lens mass $M_L$ in $M_{\odot}$ (for PSPL and FSPL models) and of the total lens mass system $M_{L,\,TOT}$ in $M_{\odot}$ (for the USBL model) with respect to the lens--source distance $D_{LS}$ (in kpc). The y-axis, rescaled logarithmically, represents the lens mass, while the distance is graphed in the linear x-axis. These two parameters are linked by the equation for the calculation of the Einstein time of the event $t_E$ (Equation \ref{['formula_EA']}), which can be rewritten as follows (considering $D_L \simeq D_S$): $M_L = \frac{(t_E\, c \,v_{\perp})^2}{4 G D_{LS}}$. The PSPL model is characterized by a continuous line, the FSPL model by a dash-dotted line, and the USBL model by a dashed line.
• Figure S5: (Upper panel): Light curve of the AGN J1249+3449 fitted with the USBL model and the model residuals with respect to the best fit, with the vertical axis showing the magnitude of the source. (Lower panel): Close-up of the light curve and the model residuals, with the magnitude expressed on the vertical axis. For both panels, the horizontal axis represents the time expressed in MJD. The green points indicate the data in the $g$ band, the red ones the data in the $r$ band, and the blue points represent the data in the $i$ band. All the magnitudes are intrinsically corrected, with respect to a common baseline, by pyLIMA.