Revisiting a Quasar Microlensing Event Towards AGN~J1249+3449

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.}

💡 Research Summary

The paper revisits the optical flare detected by the Zwicky Transient Facility (ZTF) toward the active galactic nucleus (AGN) J1249+3449, which appeared roughly 34 days after the gravitational‑wave event GW190521. Earlier studies linked the flare to the GW event and dismissed a microlensing interpretation because the expected microlensing timescale was thought to be of order years. The authors challenge this view by exploiting the latest ZTF data release 23, which provides g, r, and i band photometry from March 2018 to October 2024, thereby extending the temporal coverage well beyond the original analyses that stopped in 2019.

After careful calibration and interpolation of the multi‑band light curves, the authors examine the colour evolution (g − r) and find it essentially constant throughout the entire dataset, consistent with the achromatic nature of gravitational lensing. They then model the flare using three increasingly sophisticated microlensing scenarios:

1. Point‑Source Point‑Lens (PSPL) – the classic Paczyński model with four free parameters (time of closest approach t₀, impact parameter u₀, Einstein crossing time t_E, baseline magnitude m_b).
2. Finite‑Source Point‑Lens (FSPL) – adds the normalized source radius ρ to account for finite‑source effects that become important at high magnification.
3. Uniform‑Source Binary‑Lens (USBL) – a binary lens model with seven parameters (mass ratio q, projected separation s, trajectory angle γ, plus the four PSPL parameters and source size).

Each model is fitted using three optimization techniques (Levenberg‑Marquardt, Differential Evolution, and Markov Chain Monte Carlo). The authors report only the MCMC results because this method yields realistic posterior distributions and reliable uncertainties, albeit at a computational cost roughly two thousand times higher than the LM approach.

The non‑parallax fits produce the following best‑fit values (MCMC medians):

• PSPL: t₀ ≈ 58 670 MJD, u₀ ≈ 0.17 ± 0.6, t_E ≈ 24 days, χ²/dof ≈ 1.86.
• FSPL: t₀ ≈ 58 671 MJD, u₀ ≈ 0.44 ± 0.6, t_E ≈ 26 days, ρ ≈ 0.03, χ²/dof ≈ 1.86.
• USBL: t₀ ≈ 59 084 MJD, u₀ ≈ 0.09 ± 0.01, t_E ≈ 380 days, s ≈ 2.51, q ≈ 0.98, γ ≈ 3.09 rad, χ²/dof ≈ 1.53.

The binary‑lens model yields the lowest reduced chi‑square, indicating a statistically superior description of the data. When parallax terms (π_E,N, π_E,E) are added, their posterior distributions are consistent with zero for all three models, reflecting the short duration (tens of days) of the event and the negligible effect of Earth’s orbital motion.

From the fitted Einstein crossing time and impact parameter, the authors compute the angular Einstein radius θ_E ≈ 0.3 mas. Assuming the lens resides in the host galaxy of the AGN (luminosity distance ≈ 5 Gpc), the lens mass follows from M_L = θ_E / (κ π_E). Because π_E≈0, the mass is essentially set by θ_E alone, yielding M_L ≈ 0.08–0.12 M⊙. This mass range points to a low‑mass main‑sequence star (likely an M‑dwarf) or a brown dwarf within the AGN’s disk.

The achromaticity, symmetry of the light curve, lack of periodic variations outside the flare, and the excellent fit of microlensing models collectively argue against alternative explanations such as intrinsic AGN variability, a supernova, or a tidal‑disruption event. Consequently, the flare is most plausibly a microlensing event caused by a ≈ 0.1 M⊙ lens in the AGN’s host galaxy, unrelated to the GW190521 merger.

The study demonstrates that long‑baseline, high‑cadence surveys like ZTF can uncover microlensing events even in the noisy environments of quasars, providing a novel probe of compact object populations in distant galaxies. It also underscores the importance of revisiting claimed electromagnetic counterparts of gravitational‑wave events with updated datasets and rigorous statistical modeling.