[Paper Review] Spectroscopic analysis of a super-hot giant flare observed on Algol by BeppoSAX on 30 August 1997

F. Favata, J. H. M. M. Schmitt|arXiv (Cornell University)|Sep 2, 1999

Solar and Space Plasma Dynamics24 citations

TL;DR

This study presents a detailed X-ray spectroscopic analysis of an exceptionally energetic flare on the Algol binary system using BeppoSAX data. By combining time-resolved spectroscopy with eclipse constraints, it demonstrates that the flare's decay is driven primarily by sustained heating rather than intrinsic loop cooling, resolving discrepancies in loop size estimates from traditional quasi-static models.

ABSTRACT

We present an X-ray observation of the eclipsing binary Algol, obtained with the BeppoSAX observatory. During the observation a huge flare was observed, exceptional both in duration as well as in peak plasma temperature and total energy release. The wide spectral response of the different BeppoSAX instruments, together with the long decay time scale of the flare, allowed us to perform a detailed time-resolved X-ray spectroscopic analysis of the flare. We derive the physical parameters of the emitting region together with the plasma density applying different methods to the observed flare decay. The X-ray emission from the flare is totally eclipsed during the secondary optical eclipse, so that the size of the emitting region is strongly constrained (as described in a companion paper) on purely geometrical arguments. The size of the flare thus derived is much smaller than the size derived from the analysis of the evolution of the spectral parameters using the quasi-static cooling formalism, showing that the time evolution of the flare is determined essentially from the temporal profile of the heating, with the intrinsic decay of the flaring loop having little relevance. The very high signal-to-noise of the individual spectra strongly constrains some of the derived physical parameters. In particular, very significant evidence for a three-fold increase in coronal abundance and for a large increase in absorbing column density during the initial phases of the flare evolution is present.

Motivation & Objective

• To analyze the X-ray emission of a super-hot giant flare on Algol observed by BeppoSAX on 30 August 1997.
• To determine the physical parameters of the flaring plasma, including temperature, density, and abundance evolution.
• To constrain the size of the flaring region using the total eclipse of the X-ray emission during the secondary eclipse.
• To test the validity of different flare decay models—particularly the quasi-static cooling formalism and the Reale et al. (1997) method—against geometric and spectral constraints.
• To investigate whether observed spectral changes during the flare are due to real abundance variations or non-equilibrium effects.

Proposed method

• Utilized time-resolved X-ray spectroscopy from multiple BeppoSAX instruments (MECS, PDS, LECS) to analyze the flare's spectral evolution.
• Applied the quasi-static cooling formalism of van den Oord & Mewe (1989) to estimate loop length and plasma density from the flare decay light curve.
• Employed the Reale et al. (1997) method for flare decay analysis, which accounts for time-dependent heating and is less sensitive to cooling assumptions.
• Used the total eclipse of the flare to geometrically constrain the flaring region size, assuming emission is confined to the K-type secondary's hemisphere.
• Compared derived loop lengths from spectral models with the eclipse-derived upper limit of 2.4×10¹¹ cm to assess model consistency.
• Assessed ionization equilibrium timescales to rule out non-equilibrium effects as the cause of observed spectral changes, favoring real abundance variations.

Experimental results

Research questions

• RQ1 What are the physical conditions (temperature, density, abundance) of the plasma during the super-hot giant flare on Algol?
• RQ2 How does the observed flare decay compare with predictions from the quasi-static cooling model, and what does this imply about the flaring loop's size?
• RQ3 Can the observed spectral changes—particularly the threefold increase in coronal abundance—be attributed to real abundance variations rather than non-equilibrium ionization?
• RQ4 Is the observed increase in absorbing column density during the flare's rise phase due to a coronal mass ejection or other transient absorbing material?
• RQ5 Which flare decay model—quasi-static cooling or time-dependent heating (Reale et al. 1997)—better explains the observed data in light of geometric constraints?

Key findings

• The flare exhibited a threefold increase in coronal abundance during its rise phase, with a rapid return to pre-flare levels, indicating real abundance variation rather than non-equilibrium effects.
• The plasma relaxation time to ionization equilibrium was estimated at less than 30 seconds, ruling out non-equilibrium ionization as the cause of spectral changes.
• A large absorbing column density of ~3×10²¹ cm⁻² was detected during the flare's initial phase, interpreted as evidence for a coronal mass ejection in the line of sight.
• The loop length derived from the quasi-static cooling model (18–28×10¹¹ cm) significantly exceeded the eclipse-derived upper limit of 2.4×10¹¹ cm, indicating the model overestimates size.
• The Reale et al. (1997) method yielded loop lengths still slightly larger than the eclipse constraint but in better agreement than the quasi-static model, suggesting sustained heating during the flare.
• The small intrinsic thermal decay time of the loop (due to high density and small size) implies that the observed X-ray light curve must closely reflect the heating profile, indicating that sustained heating drives the flare decay.