Eclipsing Stellar Flare on the Demon Star Algol Binary System Observed during the MAXI-NICER Follow-up Campaign in 2018

TL;DR

This study leverages an eclipsing X-ray flare on the Algol binary to constrain flare size and location using eclipse geometry, enabled by a rapid MAXI-triggered NICER follow-up under the MANGA program. The NICER data reveal a 5.8-hour X-ray eclipse during the flare decay, with multi-temperature spectral modeling indicating a high-temperature component near $T_H \approx 4.15$ keV and a cooler component near $T_L \approx 0.90$ keV, plus a variable $N_H$ suggesting absorbing material. The inferred flare occurs on Algol B at latitude $45^{\circ}$S with a loop height of $H \approx 0.8R_B$ and loop length $L \approx (0.4-2.6)R_B$, consistent with the observed eclipse timings ($\phi_{\rm orb} \approx 0.468$) and a peak luminosity of $\sim 6.2\times10^{32}$ erg s$^{-1}$. A possible coronal mass ejection is implicated by pre-eclipse $N_H$ increases and spectral evolution, highlighting the dynamic nature of stellar coronae and the value of eclipse-based diagnostics for unresolved flares.

Abstract

Algol is a well-known eclipsing binary hosting an active and variable star that exhibits frequent stellar flares. Here, we report our pre-planned and coordinated rapid X-ray follow-up observations of an eclipsing flare on Algol. The Monitor of All-sky X-ray Image (MAXI) detected a flare on Algol at 05:52 UT on 2018 July 4. Subsequently, we carried out a prompt X-ray monitoring with the Neutron star Interior Composition Explorer (NICER) starting at 19:45 UT on the same day, and the observation ended at 06:02 UT on 2018 July 6. During the decaying phase of the flare, we successfully detected a 5.8-hour-long eclipse, corresponding to the secondary eclipse in which Algol A blocks the line of sight to Algol B. During the eclipse, the 2-10 keV X-ray flux is decreased to 20% level from $1.9\times10^{-10}~ \mathrm{erg~cm^{-2}~s^{-1} }$ to $4.5\times10^{-11}~ \mathrm{erg~cm^{-2}~s^{-1} }$. We found a configuration of the flare size and location to explain the X-ray observations; e.g., the flare occurred at the latitude 45°S of the Algol B surface with a flare height of $1.9\times10^{11}~\mathrm{cm}$, corresponding to 0.8 times the stellar radius of Algol B, giving 80% obscuration of the flare loop by Algol A. The apparent absorption increase before the eclipse might originate from coronal mass ejection (CME) in the line of sight ejected during the flare.

Figures (5)

• Figure 1: NICER (blue circles, skyblue triangles) and MAXI (orange squares, red diamonds) count rates and luminosity in the 2.0--5.0 keV band of Algol, which are binned at 64 s and 211 s (each MAXI scan) in the left and right panels, respectively. The MAXI data are added after being converted to the corresponding NICER count rate by PIMMS, assuming that the MAXI spectrum can be represented by a power law and the temperture was 4 keV. The time zero of MJD 58300.0 corresponds to 2018 July 1, 00:00 UT. The blue dotted line and green dotted line show the 2.0--5.0 keV NICER count rate and luminosity in the quiescent state on 2018 August 11, $2.0~\mathrm{counts~s^{-1}}$ and $9.5\times10^{29}~\mathrm{erg~s^{-1}}$ in the left and right panels, respectively.
• Figure 2: Optical observation of Algol by TESS on MJD 58793.2--58795.8 (Photometer, 600--1000 nm wavelength, blue dot) folded at the orbital period of Algol ($P_\mathrm{{orb}}$ in Table \ref{['star_param']}). The inset figure shows this TESS observation covering the entire orbital period. The 2.0--10.0 keV absorbed flux from the NICER flare observations is overlaid. The red shaded areas show the eclipse start and end times of X-rays. The blue horizontal dotted line shows the 2.0-10.0 keV NICER flux at the quiescent level of $1.4\times10^{-11}~\mathrm{erg~cm^{-2}~s^{-1}}$. It should be noted that the TESS and NICER observations were not conducted simultaneously.
• Figure 3: Background-subtraced Algol flare spectra in the 0.3--8.0 keV band for the first NICER observation on MJD 58303.83 (ObsID 1200260101, exposure time 1068 sec, panel a), before the eclipse on MJD 58304.34 (ObsID 1200260102, exposure time 1104 sec, panel b), during the eclipse on MJD 58304.47 (ObsID 1200260102, exposure time 1104 sec, panel c), and quiescent on MJD 58341.76 (ObsID 1200260104, exposure time 1024 sec, panel d). The flare spectra are fitted with the $\mathrm{tbabs(vapec+vapec+vapec+vapec)}$ model. The quiescent spectrum is fitted with the two-temperature CIE plasma model, and its best-fit result is included in each flare fit with fixed parameters. The additional two collisionally-ionized components of the flare are fitted, with the best-fit models shown by the solid red line, composed of low-temperature (blue dashed), high-temperature (green dashed-dotted), and an added Gaussian at the Fe-L line (purple dotted). The residuals, $\mathrm{(data-model)/error}$, are shown in the bottom panels. The expected NICER background spectra (black dotted) are calculated with the SCORPEON model and shown for comparison.
• Figure 4: Time evolutions of the best-fit NICER spectral parameters. (a) Absorbed flux in the 0.3--2.0 keV (blue circle), 2.0--10.0 keV (red squere), and total (0.3--10.0 keV, orange diamond) bands. (b) Temperatures of the two plasma components. (c) Emission Measure (EM) calculated from the normalization of the vapec components, i.e., $EM=4\pi d^2k\times10^{14}$ where $d$ is the distance to the source (cm) assuming $d=28.8~\mathrm{pc}=8.9\times10^{19}~\mathrm{cm}$ for Algol and $k={10^{-14}\over{4\pi d^2}}\int n_en_HdV$, where $n_e$ and $n_H$ are the electron and hydrogen densities (cm$^{-3}$), respectively is the normalization derived in Xspec. (d) Hydrogen equivalent column density ($N_{\mathrm{H}}$). The last four points are not shown since the spectra are fitted well without the ISM absorption component.
• Figure 5: The observed configurations around and during the second eclipse shown in the line of sight of Algol and two possible examples of the flare. The upper panels a1--a3 show the flare occurring in the mid-latitude region of the southern hemisphere of Algol B (Example A), whereas the lower panels b1--b3 show the flare in the low-latitude region of the northern hemisphere (Example B). The green arrows indicate the moving orbital direction of Algol B. As the orbital motion progresses, the flare loop moves to the near side due to the tidally-locked rotation of Algol B. Panels a1 and b1 correspond to the eclipse start ($\phi_{\mathrm{orb}}=0.414$) when the flare loop begins to be obscured by Algol A. Panels a2 and b2 are at the center of the eclipse ($\phi_{\mathrm{orb}}=0.468$) when 80% of the flare loop is obscured. Panels a3 and b3 are the end of the eclipse ($\phi_{\mathrm{orb}}=0.522$), when the flare loop completely moves out of the region blocked by Algol A.