Multi-wavelength light curve between November 7, 2023 (MJD 60255), and January 16, 2024 (MJD 60325). The red line marks the XMM-Newton observation taking place on December 13, 2023 (MJD 60291), and the vertical gray bands highlight the IXPE observing windows. From top to bottom: MAGIC fluxes in the > 1 TeV (first panel) and 0.2–1 TeV (second panel) bands with nightly bins (the typical nightly exposure is ≈40 min). The gray dashed line depicts the average flux of Mrk 421 from Acciari et al. (2014); Fermi-LAT fluxes in three-day bins; X-ray fluxes binned per observation including Swift-XRT and XMM-Newton; Swift-UVOT and optical R-band data using the telescopes listed in Sect. 2; radio data from the SMAPOL and QUIVER programs; polarization degree and polarization angle observations in the X-rays from IXPE (in black and empty gray markers the data are binned over 12 h and 6 h bins, respectively), the optical R-band and the radio band. In the polarization angle panel, we plot with a horizontal blue band the average direction angle determined at 43 GHz by Weaver et al. (2022) (see text in Section 3 for further details).