Detection of spin-modulated circular polarisation and radial velocity variations in the long period Intermediate Polar 1RXS J080114.6-462324

TL;DR

This study investigates 1RXS J080114.6-462324, an intermediate polar with a long orbital period, to understand magnetic accretion geometry. The authors detect spin period $P_{spin} = 1307.5179 ± 0.0527 s$ and orbital period $P_{orb} = 11.8026 ± 0.0004 h$ from photometry and TESS data. Circular polarization up to ~5% modulated at the spin supports cyclotron emission from accretion curtains, with SALT spectropolarimetry confirming ~+4% and constraining B ≤ 10 MG. The combination of spin-modulated circular polarisation and redshifted spin-modulated absorption indicates a disc-fed, low-inclination geometry, making this system a valuable benchmark for studying magnetic CV evolution toward synchronism at long orbital periods.

Abstract

We present a comprehensive photometric, spectroscopic, and polarimetric study of the intermediate polar (IP) 1RXS J080114.6-462324, using observations from the South African Astronomical Observatory (SAAO) 1.0-m and 1.9-m telescopes and the Southern African Large Telescope (SALT), complemented by archival TESS photometry. Photometric and photo-polarimetric data reveal a coherent modulation at the white dwarf (WD) spin period. TESS confirms periodicities of 1307.517 s (spin) and 11.803 h (orbital). Photopolarimetric and circular spectropolarimetric measurements show circular polarisation reaching ~+5%, modulated with the WD spin period, consistent with cyclotron emission from an accreting magnetic pole. Time-resolved optical spectra display prominent Balmer (H$γ$, H$β$, and H$α$) and He\textsc{ii} $λ$4686 emission features and additional He\textsc{i} emissions, all exhibiting minimal radial-velocity variations. We detect red-shifted absorption dips adjacent to the He\textsc{ii} and H$β$ lines, modulated at the WD spin period; periodogram analysis of the emission lines also yields spin modulation. These observations indicate that the system is a disc-fed, low-inclination IP. The combination of circular polarisation and spin-modulated absorption by infalling accretion curtain material supports this classification. Its comparatively long orbital period among IPs and the detection of polarised emission render 1RXS J080114.6-462324 an appealing candidate for evolutionary studies, potentially offering insight into how magnetic accretion systems evolve toward synchronism at longer orbital periods.

Figures (15)

• Figure 1: TESS light curve of 1RXS J080114.6–462324. The top panel shows data from Sector 8; the middle panel shows Sectors 61 and 62, including a zoomed-in region from [BJD – 2,400,000] = 59983.90 to 59984.68 that highlights a short and long periodic variability. The bottom panel presents the light curve from Sectors 88 and 89.
• Figure 2: Lomb–Scargle periodogram of 1RXS J080114.6–462324, based on TESS photometric data from Sectors 88 and 89. The main panel shows the full power spectrum, with the symbols $\Omega$, $2\Omega$, $\omega$, and $2\omega$ indicating the orbital frequency, its second harmonic, the WD spin frequency, and its second harmonic, respectively. The insets highlight (left) the orbital frequency and its second harmonic, (middle) the spin frequency, and (right) the spin second harmonic. Revealing a prominent WD spin period of 1307.517 s and a possibly orbital period of approximately 11.803 h for the binary system.
• Figure 3: Dynamical Lomb–Scargle trailed spectra of TESS Sectors 88 and 89 for 1RXS J080114.6–462324, generated using a sliding 3-day window. Revealing a periodic signal at the frequency of 2.03 cycles d$^{-1}$ and 4.06 cycles d$^{-1}$.
• Figure 4: Phase-folded TESS light curves of 1RXS J080114.6–462324 from Sector 88 and 89 folded on frequencies of 2.0336 cycles d$^{-1}$ (top panel) and 4.0644 cycles d$^{-1}$ (bottom panel).
• Figure 5: Photometric light curves of 1RXS J080114.6–462324. Top panel: Light curve obtained using the SAAO 1.0-m telescope. Second panel: Light curve obtained using the SAAO 1.9-m telescope. Bottom three panels: Simultaneous photometric light curves obtained over three nights using the SAAO 1.9-m telescope and TESS. Of these, the top panel shows unfiltered observations from the SAAO 1.9-m telescope alongside TESS data in the optical/near-infrared band. The middle panel shows r-band (red) photometry with simultaneous TESS observations. The bottom panel shows g-band (blue) photometry with simultaneous TESS observations.