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INTEGRAL detects pre-outburst flares in EXO 2030+375

ATel #3052; D. Klochkov, A. Santangelo (IAAT), M. Turler, C. Ferrigno, E. Bozzo, (ISDC - University of Geneva), K. Postnov (SAI, Moscow), I. Caballero (CEA Saclay), P. Kretschmar (ESA, Madrid), J. Wilms, V. Grinberg (Dr. Karl Remeis-Observatory), K. Pottschmidt, D. Marcu (UMBC/NASA GSFC)
on 23 Nov 2010; 13:17 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

Subjects: X-ray, Gamma Ray, Binary, Neutron Star, Pulsar

During the observations of the Cygnus X region (MJD: 51519-55521, UTC: 2010-11-18T16:58 - 2010-11-21T08:57), INTEGRAL serendipitously monitored the onset of a normal outburst of the Be/X-ray binary EXO 2030+375, associated to a periastron passage. The IBIS/ISGRI light curve in the 20-40 keV energy range before MJD 55520 is characterized by a continuous series of flares with intensity of ~100 mCrab. The typical duration and the recurrence time of the flares are ~0.2-0.3 days.

After MJD 55520, the source flux displayed a remarkable increase, raising from ~70 to ~100 mCrab. At this time, the recurrence time between the flares increased, reaching 0.7 days between the last two flares with peak fluxes of ~150 mCrab. The relative intensity of the flares with respect to the underlying flux level decreased. Thus, the flaring activity seems to decline as the outburst develops.

The observed flaring behavior is very similar to that observed in the Be/XRB A0535+26 during the normal outburst in 2005, both in the characteristic time scales and relative amplitude of the flares. This phenomenon was interpreted by Postnov et al. (2008, A&A 408, L21) as a manifestation of unstable accretion from the layer accumulated at the magnetospheric boundary during quiescence. Such pre-outburst flaring behavior is observed for the first time in EXO 2030+375 and it might indicate similar instabilities at the magnetospheric radius of the pulsar.

Further INTEGRAL observations of the region are scheduled on 2010-11-24 16:30 and will continue for about six days, thus covering a significant fraction of this normal outburst.