Searching for rotational X-ray modulation on TIC 277539431

TL;DR

This study investigates whether high-latitude coronal loops on the fast-rotating fully convective M7 dwarf TIC 277539431 produce detectable X-ray modulation due to rotational occultation. By combining TESS photometry to refine the stellar rotation period to $P_{\text{rot}}=273.593$ min with XMM-Newton X-ray timing, the authors search for phase-locked variability using a grid of sinusoidal models and a reduced chi-square test. They find no evidence of rotational X-ray modulation; the best-fit model is effectively flat with $\chi^2_{\mathrm{red}}\approx1.34$ and $A=0$, suggesting either no stable high-latitude loops during the observation or modulation below the data’s sensitivity. The work demonstrates the feasibility of using X-ray modulation to probe coronal structures on fully convective stars and highlights the need for more sensitive future missions (e.g., NewAthena) to detect such signals and empirically constrain loop heights.

Abstract

TIC 277539431, a fast rotating M7 dwarf, was detected to host the highest latitude flare to date at $81^\circ$. Magnetic activity like stellar flares occurring at high latitude indicate occurrence of coronal loops at these latitudes on fully-convective M dwarfs. In contrast, sunspots usually occur below $30^\circ$. In our study we look for modulation on the X-ray signal occurring due to occultation of coronal loops by the star due to stellar rotation. We report an updated rotation period for this star as $P_{\text{rot}}=273.593$ min based on TESS sectors 12, 37, 39, 64 and 65. We conducted $χ^2_{\textrm{red}}$ fits by varying the amplitude and the phase of a sinusoidally modulated signal derived from the new rotation period. We find no evidence of rotational modulation in the X-ray signal. This could be due to multiple scenarios, such as lack of a stable coronal loop during observation or the modulated signal being too weak, however given the dataset, individual scenarios cannot be distinguished.

Figures (3)

• Figure 1: Selected excerpts of TESS (PDCSAP_FLUX) lightcurves from sectors 12, 37 and 65. Our model using the $P_{\text{rot}}=273.593$ min matches the modulation in all TESS sectors in contrast to the previously reported $P_{\text{rot}} = 273.618\pm 0.007$ min.
• Figure 2: X-ray lightcurve data from the PN detector. Light gray area represents the removed flare. Purple curve is representative of the modulating signal we expect in X-ray based on the period estimated from TESS ($180^\circ$ phase shift). Orange curve represents the best $\chi^2_{\text{red}}$ fit, i.e a straight line.
• Figure 3: Reduced chi-square fit ($\chi^2_{\text{red}}$) of the X-ray observation to the modelled grid of varying fluxes. The lower the $\chi^2_{\text{red}}$ value, the better the fit. The best fit ($\chi^2_{\text{red}} = 1.34$) corresponds to a wave with zero amplitude, i.e a straight line.