Photometric Modulations on Intermediate Timescales in the Symbiotic Binaries

Abstract

We present a multi-band study of three symbiotic binaries using combined ground- and space-based monitoring that spans up to 14 years. These datasets enable a systematic investigation of variability on intermediate timescales (tens of days) and the detection of shorter-period signals. All systems display coherent photometric modulations that are distinct from the orbital cycles. In AX Per, a dominant 75-day signal and its 37-day harmonic are identified, which we interpret as pulsations of the cool giant. CI Cyg exhibits a stable modulation between 70 and 74 days, which likely arises from a combination of pulsation and circumstellar or disk-related variability. For Z And, we confirm a persistent modulation between 55 and 60 days, consistent with semiregular pulsations of the cool component. Additionally, space-based data reveal further short-period variability, including coherent signals at 26.7 and 66.6 minutes in Z And and CI Cyg, respectively, and a quasi-periodic modulation near 0.95 days in AX Per. These detections suggest the presence of rapid activity driven by accretion or rotation, superposed on the intermediate timescale behavior. Our results show that the observed variability in these symbiotic binaries reflects the combined effects of cool giant pulsation, circumstellar or disk activity, and possible rotation of the hot component. The multi-timescale behavior revealed here offers new constraints on mass transfer and activity cycles in interacting binaries.

Figures (17)

• Figure 1: The light variations of all-bands data from ASAS-SN, KWS, S19 and T60 for AX Per. The red vertical bands span the TESS observation Sector(s), and the blue dashed lines correspond to the ephemeris of the inferior spectroscopic conjunction of the red giant according to Fekel00_II. The offset values with the same color codes are on the outside of the graph.
• Figure 2: TUG T60 $U-B$, $B-V$ and $V-R$ colors for AX Per. T60 V-band is also shown by offset by -8.5 mag for easy comparisons with the colors.
• Figure 3: The expected minima locations for AX Per. The blue dotted lines indicate the positions of the observed minima based on the fit line, while the red dashed lines correspond to their positions calculated according to the ephemeris of Fekel00_II.
• Figure 4: TESS observations and Lomb-Scargle Periodogram of AX Per. From top to bottom, the raw TESS light curve, the detrended light curve, and the periodogram of the detrended light curve with phase-folded light curve in the mini panel, respectively. At the bottom panel, the vertical green dashed line marks the period with the highest amplitude, shown correspond to the period of $P = 0.9541\,\mathrm{d}$ ($f = 1.048\,\mathrm{d^{-1}}$).
• Figure 5: Lomb-Scargle Periodogram of AX Per for all observations. The vertical dashed line denotes the orbital frequency ($f_{\rm orb}$) based on the spectroscopic period of Fekel00_II. The horizontal dashed lines, according to the color codes, indicate $5\sigma$ S/N levels of corresponding observations.