[Paper Review] One Planet or Two Planets? The Ultra-sensitive Extreme-magnification Microlensing Event KMT-2019-BLG-1953

Han, Cheongho, Kim, Doeon+49 more|ResearchSpace (University of Auckland)|Feb 13, 2020

Stellar, planetary, and galactic studies63 references15 citations

TL;DR

This study analyzes the ultra-high-magnification microlensing event KMT-2019-BLG-1953, which reached a peak magnification of ~900. Using high-cadence KMTNet and MOA survey data, a binary lens model (2L1S) with a planet of mass $ M_p = 0.64^{+0.76}_{-0.35}~{}M_{\rm J} $ is robustly detected, while a potential second planet remains ambiguous due to insufficient peak sampling. The host star is estimated at $ M_{\rm host} = 0.31^{+0.37}_{-0.17}~{}M_{\odot} $, located at $ D_{\rm L} = 7.04^{+1.10}_{-1.33}~{}\text{kpc} $ toward the Galactic bulge.

ABSTRACT

We present the analysis of a very high-magnification ($A\sim 900$) microlensing event KMT-2019-BLG-1953. A single-lens single-source (1L1S) model appears to approximately delineate the observed light curve, but the residuals from the model exhibit small but obvious deviations in the peak region. A binary lens (2L1S) model with a mass ratio $q\sim 2 imes 10^{-3}$ improves the fits by $Δχ^2=181.8$, indicating that the lens possesses a planetary companion. From additional modeling by introducing an extra planetary lens component (3L1S model) and an extra source companion (2L2S model), it is found that the residuals from the 2L1S model further diminish, but claiming these interpretations is difficult due to the weak signals with $Δχ^2=16.0$ and $13.5$ for the 3L1S and 2L2L models, respectively. From a Bayesian analysis, we estimate that the host of the planets has a mass of $M_{ m host}=0.31^{+0.37}_{-0.17}~M_\odot$ and that the planetary system is located at a distance of $D_{ m L}=7.04^{+1.10}_{-1.33}~{ m kpc}$ toward the Galactic center. The mass of the securely detected planet is $M_{ m p}=0.64^{+0.76}_{-0.35}~M_{ m J}$. The signal of the potential second planet could have been confirmed if the peak of the light curve had been more densely observed by followup observations, and thus the event illustrates the need for intensive followup observations for very high-magnification events even in the current generation of high-cadence surveys.

Motivation & Objective

• To investigate the nature of the ultra-high-magnification microlensing event KMT-2019-BLG-1953, which reached a peak magnification of ~900.
• To determine whether the observed light curve deviations indicate a single planet, multiple planets, or a binary source.
• To assess the limitations of current high-cadence surveys in resolving planetary signals during extreme magnification events.
• To evaluate the importance of intensive follow-up observations for confirming planetary systems in such rare events.

Proposed method

• Modeling the light curve using a single-lens single-source (1L1S) framework with finite-source effects to establish a baseline fit.
• Testing a binary lens single-source (2L1S) model to detect planetary companions, with a mass ratio $ q \sim 2 \times 10^{-3} $, which improves the fit by $ \Delta\chi^2 = 181.8 $.
• Exploring additional models: a three-lens single-source (3L1S) and a two-lens two-source (2L2S) model to test for secondary planetary or stellar companions.
• Applying Bayesian analysis to the measured Einstein timescale $ t_{\rm E} $ and angular Einstein radius $ \theta_{\rm E} $ to estimate physical parameters of the lens system.
• Assessing the feasibility of terrestrial parallax measurements using baseline separation between KMTNet sites, given the short expected time shifts.
• Evaluating the impact of cadence and observational coverage on the detectability of planetary signals, especially near the light curve peak.

Experimental results

Research questions

• RQ1 Does the light curve of KMT-2019-BLG-1953 exhibit unambiguous signatures of a planetary companion beyond the 1L1S model?
• RQ2 Can a second planetary companion be securely detected given the current observational cadence and data quality?
• RQ3 What are the physical parameters (mass, distance) of the host star and the detected planet in the system?
• RQ4 How does the lack of dense peak coverage affect the ability to confirm multiplanetary systems in extreme microlensing events?
• RQ5 To what extent can parallax measurements be used to constrain the lens distance and mass in such high-magnification events?

Key findings

• A robust 2L1S model with a planet of mass $ M_p = 0.64^{+0.76}_{-0.35}~{}M_{\rm J} $ is detected, improving the fit by $ \Delta\chi^2 = 181.8 $ over the 1L1S model.
• The host star has a mass of $ M_{\rm host} = 0.31^{+0.37}_{-0.17}~{}M_{\odot} $, consistent with a low-mass M-dwarf.
• The system is located at a distance of $ D_{\rm L} = 7.04^{+1.10}_{-1.33}~{}\text{kpc} $ toward the Galactic bulge.
• Residuals from the 2L1S model suggest a possible second planet or binary source, but the signals are weak ($ \Delta\chi^2 = 16.0 $ for 3L1S, $ 13.5 $ for 2L2S), making confirmation difficult.
• The event illustrates that even high-cadence surveys like KMTNet may miss critical planetary signals if peak coverage is insufficient, especially for extreme magnification events.
• Parallax measurements are unlikely to succeed due to short time shifts (~2 seconds) between widely separated telescopes, limiting distance and mass constraints without dense sampling.