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INTEGRAL and RXTE observations of A0535+26

ATel #2161; I. Caballero (CEA Saclay), C. Ferrigno (AIT/ISDC), P. Kretschmar (ISOC/ESAC), J. Wilms, I. Kreykenbohm (Dr. Karl Remeis Observatory, Bamberg/ECAP, Erlangen), K. Pottschmidt (CRESST/GSFC/UMBC), A. Santangelo (AIT) , S. Suchy (CASS/UCSD), F. Lebrun (CEA Saclay), P. Mandrou (CESR)
on 15 Aug 2009; 07:14 UT
Credential Certification: I. Caballero (isabel.caballero@cea.fr)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 2337, 2496, 6043

The Be/X-ray binary A0535+26 is currently undergoing an outburst, first reported by Fermi/GBM on July 26 (ATel #2142). After its last giant outburst in 2005 (ATel #504), the source has shown occasional outbursts associated with the periastron passage (ATel #1725). The current outburst is the fourth in a series of weaker outbursts close to periastron, but is unusual by starting at an earlier orbital phase (ATel #2142) and by a peculiar lightcurve. A first ``flare'' lasted about 9 days from MJD 55043, about 14 days before the periastron, reaching a flux of ~0.1 counts/s/cm^2 (~440 mCrab) in the 15-50 keV Swift/BAT lightcurve before decreasing again to less than 0.05 counts/s/cm^2. Since MJD 55055 the flux is rising again reaching ~0.1 counts/s/cm^2 (~440mCrab) in the Swift/BAT at MJD 55057, around the periastron.

Target of Opportunity observations have been triggered with RXTE and INTEGRAL. First results from the Near-Real Time data confirm the presence of a strong fundamental cyclotron line feature in the spectrum centered at 45.4+0.7/-0.6 keV (INTEGRAL) or 44.5+0.8/-1.1 keV (RXTE). The source flux in the 20-50 keV range during these first observations was measured as 4.3753e-9 erg/cm^2/s with RXTE (MJD 55047) and 3.9257e-9 erg/cm^2/s with IBIS/ISGRI (MJD 55055). The spin frequency measured by IBIS/ISGRI without correction for orbital motion is 0.0096625[5] Hz (103.493[5] s).

In addition to the targeted observations, the source is also detected in INTEGRAL Core Program observations of the Perseus Arm Region at a large off-axis angle (ATel #2146). First results from NRT data of these observations confirm the presence of a cyclotron line at ~45 keV.

Further TOO observations with INTEGRAL, RXTE and Suzaku are planned to follow the evolution of the outburst. INTEGRAL Core Program observations of the Perseus Arm Region will also continue until MJD 55063.