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ATEL # 2541; I. Caballero (CEA-AIM Saclay), A. Santangelo (IAAT), K. Pottschmidt (CRESST/GSFC/UMBC), D. Klochkov (IAAT), J. Rodriguez (CEA-AIM Saclay), J. Wilms, I. Kreykenbohm (Dr Karl Remeis Obs./ECAP), P. Kretschmar (ISOC/ESAC), C. Ferrigno (ISDC - University of Geneva), R. Rothschild, S. Suchy (CASS/UCSD)
on 7 Apr 2010; 14:54 UT
Password Certification: I. Caballero (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients
The Be/X-ray binary A0535+26 is currently undergoing a new outburst, associated with the periastron passage of the neutron star (MJD 55279.5, Finger et al., 2006). On April 3 2010 (MJD 55289) it reached a flux of ~1.18 Crab in the BAT light curve (15-50 keV). The outburst is remarkable since during the previous periastron passage on MJD 55168.4 a giant outburst that reached 5.14 Crab (15-50 keV) took place (ATel #2337). Optical observations of strong Halpha features had predicted the current outburst to be relatively bright (ATel #2497). To our knowledge, it is the first time that such a bright outburst takes place directly after a giant outburst.
TOO observations have been triggered with the major X-ray observatories INTEGRAL, RXTE, Suzaku, and Swift. Swift monitoring observations started at the onset of the outburst on March 25 2010 (MJD 55280) and are currently ongoing. RXTE monitoring observations started on April 3 2010. INTEGRAL performed one 100 ks observation that started on April 3 2010 (MJD 55289.8), and will also continue observing the outburst. One Suzaku observation has been performed on April 6 2010.
Preliminary analysis of INTEGRAL Near Real Time data and Swift Quick Look data lead to the following results: The source is detected with ISGRI at a flux of F(20-100 keV)=1.5e-08 erg cm-2 s-1 ~ 0.92 Crab (flux estimated with simultaneous Crab observation). A fundamental cyclotron line is detected in the ISGRI spectrum at E=47.6 +0.4/-0.3 keV (90% confidence), consistent with the value measured with INTEGRAL in a brightening of the source that took place about 3 weeks before the periastron (ATel#2496).
From the spectrum of one Swift/XRT observation performed on MJD 55280, the (2-10) keV flux is F=7.7e-10 erg cm-2 s-1. The data is best fit with an absorbed powerlaw (NH=(0.306 ± 0.08) e22 cm-2) plus a black body of kT=1.32 +0.09/-0.08 keV. Pulsations are detected in the XRT data at P=103.4 ± 0.8 s (no barycentric and orbital corrections applied).
We would like to thank all the scheduling teams, INTEGRAL/ISOC and Chris Winkler for planning the INTEGRAL observations right before Easter, the Swift team, Scott Barthelmy for setting BAT on evt-by-evt capture commands, the MAXI team for their trigger notification for Suzaku, the RXTE team for planning the monitoring observations, and the Suzaku team for scheduling the observation on the last day of the AO. We also would like thank Enrico Bozzo and the ISDC team for the quick analysis of the INTEGRAL NRT data.