Letter to the Editor

SDSS J1001+5027: Strong microlensing-induced chromatic variation caught in the act

¹ Instituto de Física de Cantabria (CSIC-UC), Avda. de Los Castros s/n, E-39005 Santander, Spain
² Departamento de Física Moderna, Universidad de Cantabria, Avda. de Los Castros s/n, E-39005 Santander, Spain
³ O.Ya. Usikov Institute for Radiophysics and Electronics, National Academy of Sciences of Ukraine, 12 Acad. Proscury St., UA-61085 Kharkiv, Ukraine

^⋆ Corresponding authors: goicol@unican.es; vshal@ukr.net

Received: 21 July 2025
Accepted: 25 August 2025

Abstract

We conducted long-term monitoring of the doubly imaged gravitationally lensed quasar SDSS J1001+5027 consisting of spectro-photometric observations separated by ∼120 days (time delay between both quasar images), as well as test and auxiliary data. This monitoring approach allowed us to reliably find a strong microlensing-induced chromatic variation of the quasar continuum in the period 2022–2025. The ongoing microlensing event has caused the delay-corrected spectral flux ratio in 2025 to have a dramatic changing look, opening the door to very promising observations of the system in the coming years. These future follow-up observations of such a rare event are expected to provide critical information to discuss, among other things, the structure of the inner accretion flow towards the central supermassive black hole in SDSS J1001+5027.