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

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 $\ddot{I}‰$ 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 $\hat{I}\text{c◯}$ (prominent in Suzaku data) to $2\hat{I}\text{c◯}$ 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.

20 pages, 10 figures, 1 table. Accepted for publication in ApJ. v2 fixes some footnotes and adds a note in the Acknowledgements section. v3 corrects a misspelled co-author's name