BeppoSAX Observations of Algol: The first observations of an X-ray flare
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X-ray light curve of Algol measured with the SAX MECS2-MECS3
detectors in the 1.6 - 10 keV band between Aug 30, 1997 3:04 UT and Sep 1, 1997,
20:32 UT with phase calculated from the ephemeris JD = 2445739.003 + 2.8673285 E
(E integer) for the times of primary minimum; a hundredth of a phase corresponds
to 41.3 minutes. The phasing is such that for Phi(E) = E + 0.5 the X-ray dark
early type primary is in front of the X-ray emitting late type secondary. Note
the huge flare starting at phase Phi = 1.0 with a rise time of about 8.3 hours
and peaking at Phi = 1.12. A rapid initial decay until at Phi = 1.25 is followed
by an exponential decay. A clear eclipse of the flaring plasma is seen at Phi =
1.5, when the early type primary is in front of the late-type secondary. The
dotted line indicates an estimate of the quiescent out-of-flare rate,
extrapolated from the observed ROSAT all-sky survey count rate.
Fig.2: a) Upper panel: MECS count rate vs. phase in the interval 1.3 - 1.7;
the solid line represents an exponential fit to the pre- and post eclipse light
curve. b) Lower panel: Count rate vs. phase in the interval 1.3 - 1.7 with
exponential decay (shown in panel a) removed; the zero line is explicitly shown.
The flare eclipse starts at Phi = 1.39 with a somewhat shallow decay, whence at
Phi = 1.451 a sharp decay starts (first contact). Totality begins at Phi = 1.488
(second contact) and ends at Phi = 1.505 (third contact). The light curve
increases fast until Phi = 1.545 (fourth contact) and then more slowly until Phi
= 1.605, when the flare eclipse is over. Below panel (b) the viewing geometry is
shown at phases Phi = 1.42, 1.45, 1.50, and 155; the filled circle represents
Algol A as it moves across Algol B.
Fig.3: 3D-Visualisation of the Algol system including the maximum volume
containing the flare. Shown are the two binary components Algol A (foreground)
and Algol B (background) as seen from our line of sight at phase Phi = 1.45. The
extension below the ``south'' pole of Algol B is the computed locus of all
points satisfying the following requirements: First, they must be totally
eclipsed by Algol A between second and third contact. Second, they must be not
eclipsed by Algol A and not occulted by Algol B itself before first and after
fourth contact. Third, they must be visible between first and second but not
between third and fourth contact or vice versa. And fourth, assuming that the
flaring plasma originates from the surface of Algol B, along any radial line of
sight from the surface of the Algol B to the point under consideration there
should be no more than twenty percent rotational modulation. These requirements
define the hose-like structure above the ``south'' pole of of Algol B, within
which the flare must have occurred. The flare is clearly associated with Algol B
and did not occur in the interbinary region.