TX Col Rapidly Vary Its Accretion Flow (Planetary Science)

Magnetic cataclysmic variables (MCVs) are semidetached interacting binary systems, in which the primary white dwarf (WD) accretes matter from a Rochelobe-filling secondary red dwarf star. Based on the magnetic field strength of the primary, MCVs are classified into two classes: polars and intermediate Polars (IPs). Polars are synchronous systems for which the magnetic field strength of the WD is more than 10 MG. The high magnetic field of WD prevents the formation of an accretion disk and the matter is accreted through magnetic field lines to the pole caps, while IPs are asynchronous systems and generally have magnetic field strength in the range of 1-10 MG. Hence, the material from the secondary star is accreted either through an accretion disk or an accretion stream, or a combination of both. For the majority of IPs, rotation period (Pω) is typically one-tenth of the orbital period (PΩ); i.e. Pω ∼ 0.1 PΩ.

In the IPs, three main accretion mechanisms are believed to occur, viz disk-fed (DF), stream-fed (SF), and disk-overflow (DO) accretion. In the DF accretion, the magnetic field strength of the WD truncates the inner edge of the accretion disk, forms “accretion curtains,” and dumps all the material to the magnetic poles of the WD. In this case, the magnetosphere is smaller than the radius of the minimum approach of the freefalling accretion stream and the stream could then circle the white dwarf and form a disk, ignoring the feeble magnetic field. In the disk-less or SF accretion, the magnetic field is so strong that an accretion disk does not form, and material flows along the magnetic field lines to the pole caps. Whereas in the case of a disk-overflow accretion, disk is present, but a part of the accretion stream skips the disk and directly interacts with the magnetosphere, leading to both disk-fed and stream-fed accretion simultaneously.

Figure 1. Phased light curves of TX Col for the beat and spin periods. Left panels (a) and (c) are observed, whereas the right panels (b) and (d) are obtained after the best-fit sinusoidal curves. © Nikita Rawat et al.

TX Col (=1H0542-407) was discovered by the HEAO-1 satellite and identified as an IP by Tuohy et al. According to Rawat and colleagues, TX Col is an interesting IP to study further as it changes its accretion geometry frequently. So, in their recent paper, they presented a detailed investigation of the optical photometry of TX Col as observed from the Transiting Exoplanet Survey Satellite (TESS) mission, spanning a range of 52 days with a cadence of 2 minutes.

“The beauty lies in the capabilities of the continuous short-cadence observations by TESS. For the first time, the continuous observations allowed us to probe the system’s accretion geometry in a very short cadence of one day.”

— told Nikita Rawat, first author of the study

The power spectra of almost 52 days continuous time-series data revealed the orbital period of 5.691 hr, spin period of 1909.5 s, and beat period of 2105.76 s, which is consistent with the earlier results.

They have also found the presence of quasi-periodic oscillations (QPOs) for a few days with a period of 5850-5950 s, which appears to be due to the beating of the Keplerian period with the spin period of the white dwarf. The continuous data allowed them to look thoroughly at the day-wise evolution of the system’s accretion geometry.

Figure 2. Ratio of spin to beat amplitudes obtained from the fitting for each day © Nikita Rawat et al.

In addition, they reported that the TX Col changes its accretion mechanism even on a time-scale of one day, confirming its variable disk-overflow accretion nature. For the majority of the time, it was found to be disk-overflow system with stream-fed dominance, however, pure disk-fed and pure stream-fed accretions cannot be ruled out during the observations.

“TX Col is shown to accrete via disk-overflow accretion for ∼ 89% of the total observing time and the other 11% of the time either pure disk-fed or pure stream-fed. It exhibits disk-overflow with stream-fed dominance for 71.1%, disk-overflow with equal dominance for 13.3%, and disk-overflow with disk-fed dominance with only 4.4% of the observing time. It also appears that TX Col has switched its accretion mechanism from disk-overflow to pure disk-fed accretion for two occasions during the observations. For the first time, a pure stream-fed accretion is also found to take place in this system for three different days.”

— wrote authors of the study

From the observations, authors concluded that the disk-dominant accretion and stream-dominant accretion repeat after every 13-16 days cycle. However, more observations are required to confirm this result.

Featured image: (a) TESS light curve of TX Col, where the red dots represent the mean counts s¯1 of each day. (b) Lomb-Scargle power spectra (top panel) and CLEANed power spectra (bottom panel). The horizontal dashed line in the top panel represents the 90% confidence level. © Nikita et al.

Reference: Nikita Rawat, J. C. Pandey, Arti Joshi, “TESS observations of TX Col: Rapidly varying accretion flow”, Arxiv, pp. 1-13, 2021. https://arxiv.org/abs/2104.06944

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