Algol Eclipsing Flare Event

• Eclipsing flare on Algol is a transient magnetic reconnection event on the active K-type subgiant, providing direct spatial and energetic insights.
• X-ray observations show an 80% flux drop during secondary eclipse, constraining the flare’s location, loop height, and associated CME dynamics.
• Detailed spectral and geometrical modeling from the MAXI–NICER campaign offers a robust framework to study magnetic activity and mass transfer in binary systems.

An eclipsing flare on Algol is a transient event in which a magnetically induced stellar flare occurs on the active component of the Algol binary system and is partially or fully occulted by its companion during eclipse. Such phenomena provide direct insights into the spatial distribution, energetics, and magnetic activity of close binary stars with strong tidal interactions and mass transfer. Algol (“Beta Persei”) itself is archetypal: a hierarchical triple system whose inner pair is a semi-detached binary with a Roche-lobe-filling K-type subgiant (Algol B) and a more massive, hotter A-type primary (Algol A). Flares observed on Algol are linked to magnetic reconnection events on the convective secondary, which experiences strong tidal locking and enhanced dynamo action. When a flare occurs close to eclipse geometry, its X-ray and multiwavelength signature can be partially obscured, offering unique constraints on the flare’s location, size, and energetics. The synoptic MAXI–NICER X-ray campaign of July 2018 represents the most detailed modern case paper.

1. Observational Signatures and Temporal Evolution

The MAXI detected an X-ray flare from Algol on 2018 July 4 at 05:52 UT, followed by rapid NICER follow-up beginning at 19:45 UT and ending at 06:02 UT on July 6. During the decaying phase of the flare, a distinct 5.8-hour eclipse was observed—concurrent with the system’s secondary eclipse, wherein Algol A occults Algol B (the active flaring star) (Nakayama et al., 16 Oct 2025). The reduction in 2–10 keV X-ray flux from $1.9\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$ outside eclipse to $4.5\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$ during eclipse (an 80% drop) demonstrates that the bulk of the flare emission originates from Algol B and is spatially adjacent to the occulted stellar disk. Modeling of the eclipse ingress and egress employs piecewise error functions:

$$f(t) = A_1[1 + \text{erf}\left(\frac{t-\mu_1}{\sigma_1}\right)] + A_2[1 - \text{erf}\left(\frac{t-\mu_2}{\sigma_2}\right)] + \text{constant},$$

where $\mu_1$, $\mu_2$ mark the centers of transitions and $\sigma_1$, $\sigma_2$ their widths. The best-fit timings resolve the eclipse start and end to within $\sim0.13$ days.

Spectral evolution is marked by decreased plasma temperatures and emission measures: the hot component shifts from $\sim$3.96 keV to 3.26 keV and EM drops from $19 \times 10^{53}$ cm$^{-3}$ to $3.9 \times 10^{53}$ cm$^{-3}$. These changes directly reflect the occultation of the flare site; the spectral fits robustly track the eclipse geometry and flare decay.

2. Flare Geometry and Location on Algol B

Geometric modeling constrains both the flare location and the vertical extent above Algol B’s surface. The preferred solution situates the flare at latitude $45^\circ$ South, with the flare loop modeled as a perpendicular cylinder attached to the stellar surface. The fitted height is

$$H = 1.9 \times 10^{11} \ \text{cm} \approx 0.8 R_B,$$

where $R_B$ is the radius of Algol B. This configuration yields 80% obscuration at mid-eclipse (orbital phase $\phi \sim 0.468$), fully consistent with the observed flux drop. Alternate geometries (e.g., a lower latitude with $H \sim 0.6 R_B$) are less compatible with pre-eclipse light curve constraints.

Flare loop sizes were independently cross-validated using X-ray spectral parameters and reconnection scaling laws:

$$L = 10^9 \left( \frac{\text{EM}}{10^{48} \ \text{cm}^{-3}} \right)^{3/5} \left( \frac{n_0}{10^9 \ \text{cm}^{-3}} \right)^{-2/5} \left( \frac{T}{10^7 \ \text{K}} \right)^{-8/5} \ \text{cm},$$

where EM is the emission measure, $T$ is the plasma temperature, and $n_0$ the pre-flare electron density. With typical values (EM $\sim 3.7 \times 10^{55}$ cm$^{-3}$, $T \sim 7 \times 10^7$ K, $n_0 \sim 10^{10}$–$10^{12}$ cm$^{-3}$), derived loop lengths are $L \sim (1.0$–$6.2) \times 10^{11}$ cm, i.e., $0.4$–$2.6 R_B$, supporting the geometric inference.

3. Absorption Events and Coronal Mass Ejections

A notable, transient increase in hydrogen column density $N_H$ was observed immediately before the eclipse, peaking at $(3.10 \pm 0.74) \times 10^{20}$ cm$^{-2}$ before dropping after eclipse. This absorption is plausibly attributed to a coronal mass ejection (CME) ejected during the flare event (Nakayama et al., 16 Oct 2025). Similar $N_H$ increases have been documented in prior Algol flare observations.

The simultaneous flare and CME event is consistent with the scenario proposed for previous Algol superflares (Moschou et al., 2017), where the temporal decay of X-ray absorption is modeled as a self-similarly expanding CME. The $t^{-2}$ decay law observed for $N_H$ in those events arises naturally from volume dilution and radial expansion in a spherical shell:

$N_H(t) = N_a /(4\pi r(t)^2) \propto r(t)^{-2}.$

This absorption signature provides an independent probe of ejected mass and CME kinematics.

4. Physical Interpretation and Mechanisms

Eclipsing flares observed on Algol are traced to the interplay of strong tidal forces, mass transfer, and magnetic activity on the Roche-lobe-filling secondary. The flare occurs within a large, magnetically structured loop whose scale can be directly inferred due to spatial occultation by the primary. The extent and location are tightly coupled to the observed X-ray flux drop.

The observed flare energetics, size, and recurrence are connected to persistent starspot coverage and enhanced dynamo activity induced by tidal locking and rapid (synchronous) rotation. The maximum available magnetic energy in starspots on Algol B (assuming fields of 1–2 kG in regions up to 10% of the surface, extending several $10^{11}$ cm radially) is of order $10^{39}$ erg, adequate to power the largest observed stellar flares and associated CMEs.

5. Broader Significance and Comparison with Other Binary Flares

Eclipsing flare events on Algol—especially when accompanied by multiwavelength, high-cadence monitoring—deliver spatial information on flare morphology unavailable in non-eclipsing flaring stars. The detailed modeling of the eclipse constrains the latitude, vertical loop extent, and physical parameters of the magnetic reconnection event. Such benchmarks provide strong calibration for extrapolating flare–CME relations from the solar regime to more active, tidally-distorted binaries (Moschou et al., 2017).

This direct spatial constraint is crucial for theoretical models addressing energy transport, reconnection topology, and CME-induced angular momentum loss. The absorption enhancement (CME signature) also validates global magnetohydrodynamic scenarios for mass ejection and energy partitioning in close binaries.

6. Conclusions and Future Directions

The 2018 MAXI–NICER campaign established the first direct geometric and energetic constraints on an eclipsing flare in Algol using time-resolved X-ray light curves and spectra. The event lasted $\sim$5.8 hours and was modeled as a flare loop at $45^\circ$ South with height $1.9\times10^{11}$ cm ($0.8R_B$), experiencing 80% obscuration during eclipse. The X-ray spectral evolution and increase in $N_H$ prior to eclipse link the flare to a co-temporal CME. This synthesis of geometric, spectral, and eclipse data provides a rigorous template for interpreting stellar flares in tidally locked binaries.

Continued multiwavelength time-series observation, coordinated with orbital phase, remains essential for quantitative paper of binary star magnetic activity, energy budgets, and feedback onto binary orbital evolution.