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INTEGRAL/JEM-X detects a bright X-ray flare from Algol (bet Per)

ATel #7848; M. Tuerler, D. Eckert, E. Bozzo (ISDC, Univ. of Geneva, Switzerland)
on 28 Jul 2015; 09:09 UT
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

Subjects: Star

The JEM-X instrument aboard the INTEGRAL satellite observed serendipitously a bright X-ray flare from the star Algol (bet Per) on 25 July 2015 during observations of NGC 1275 in the Perseus cluster. The transient was detected with high significance in two consecutive pointings of about 45 minutes each on 2015-07-25 from 15:22 to 16:52 (UTC), corresponding to an effective exposure of 4.6 ks. The measured flux in the 3-10 keV band is of 5.8 x 10-10 erg/cm2/s. Excluding these two pointings, we obtained only a very marginal (2.6 sigma) signal of the order of 1 x 10 -11 erg/cm2/s with intermittent coverage between 2015-07-25 11:29 and 2015-07-27 04:44 (UTC). The flare is not detected by the IBIS/ISGRI detector in the 20-40 keV band. The corresponding 3-sigma upper limit is of 6 x 10-11 erg/cm2/s (7 mCrab).

The JEM-X lightcurve is consistent with a roughly symmetrical rise and decay over the 90 minutes of the detection period. The source spectrum is well fitted (reduced χ2 = 0.2 for 7 d.o.f.) by a simple power-law model with a photon index of 3.8 ± 0.8 (90% CL). The spectrum can also be fitted (reduced χ2 = 0.8 for 7 d.o.f.) by an APEC thermal plasma model with a temperature of kT = 4.1 -1.9 +3.1 keV (90% CL), resulting in a slightly reduced 3-10 keV flux of 3.1 x 10-10 erg/cm2/s.

Algol consists of an eclipsing binary system plus a third star. It is a known X-ray emitting source and was already observed to flare up in Aug. 1997 with BeppoSAX (Schmitt & Favata 1999, Nature 401, 44). In that occasion, the giant outburst lasted the whole binary orbit of 2.9 days. Its peak luminosity in the 1.6-10 keV band was 3 x 1032 erg/s, which is about a factor five stronger than the luminosity of 5.6 x 1031 corresponding to the flare observed by JEM-X (at a distance of 28.5 pc). Three earlier X-ray flares from Algol have been detected in the past by EXOSAT in Aug. 1983, Ginga in Jan. 1989, and ROSAT in Aug. 1992 (see Favata et al. 2000, A&A 362, 628).