A multi-wavelength study of the 2025 low state of the intermediate polar BG CMi

TL;DR

This study documents the first low state of BG CMi, revealing changes in accretion mode, X-ray properties, and timing signals, indicating a transition to disk-overflow accretion during the low state.

Contribution

It provides the first multi-wavelength observational analysis of BG CMi's low state, highlighting a shift in accretion mode and timing properties not previously documented.

Findings

Optical brightness decreased by ~0.5 mag for ~50 days.

X-ray observations showed decreased absorption and increased luminosity.

Timing analysis indicated a shift from orbital to double orbital frequency dominance.

Abstract

We present multi-wavelength observations of the first recorded low state of the intermediate polar BG CMi. Optical monitoring of the source by members of the American Association of Variable Star Observers reveals a decrease of ~0.5 mag that lasted ~50 d in early 2025. During the low state the optical timing properties imply that BG CMi underwent a change in the accretion mode, as power at the spin frequency $\omega$ dramatically dropped. An XMM-Newton observation revealed a substantial decrease in intrinsic absorption and a slight increase in intrinsic X-ray luminosity, compared to archival Suzaku data. Timing analysis of the X-ray light curves shows that power shifted from the orbital frequency $\Omega$ (prominent in Suzaku data) to $2\Omega$ in the low state XMM-Newton data, along with the strengthening of certain orbital sidebands. We suggest that BG CMi transitioned to disk-overflow accretion, where the white dwarf accreted matter via both a disk and a stream, the latter becoming more dominant during the low state due to a decrease in the mass and size of the disk.