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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint 10/08

The pre-outburst flare of the A0535+26 August/September 2005 outburst

I. Caballero (1), A. Santangelo (1), P. Kretschmar (2), R. Staubert (1) , K. Postnov (1,3), D. Klochkov (1), A. Camero-Arranz (4), M. H. Finger (5), I. Kreykenbohm (1,6), K. Pottschmidt (7,8), R. E. Rothschild (9), S. Suchy (9), J. Wilms (10), C. A. Wilson (5)

(1) Institut für Astronomie und Astrophysik, University of Tübingen, Sand 1, 72076 Tübingen, Germany
(2) ISOC, ESAC (ESA), Apartado 50727, 28080, Madrid, Spain
(3) Sternberg Astronomical Institute, Moscow University, 119992 Moscow, Russia
(4) GACE, Universidad de Valencia, PO Box 20085, 46071, Valencia, Spain
(5) National Space Science and Technology Center, 320 Sparkman Drive NW, Huntsville, AL, 35805, USA
(6) ISDC, 16 Ch. d'Ecogia, 1290 Versoix, Switzerland
(7) CRESST, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
(8) NASA Goddard Space Flight Center, Astrophysics Science Division,Code 661,Greenbelt, MD 20771,USA
(9) CASS, University of California at San Diego, La Jolla, CA 92093-0424, USA
(10) Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg,Sternwartstr. 7, 96049 Bamberg, Germany

A&A 480, L17 (2008)

Abstract. We study the spectral and temporal behavior of the High Mass X-ray Binary A0535+26 during a `pre-outburst flare' which took place ~5d before the peak of a normal (type I) outburst in August/September 2005. We compare the studied behavior with that observed during the outburst. We analyse RXTE observations that monitored A0535+26 during the outburst. We complete spectral and timing analyses of the data. We study the evolution of the pulse period, present energy-dependent pulse profiles both at the initial pre-outburst flare and close to outburst maximum, and measure how the cyclotron resonance-scattering feature (hereafter CRSF) evolves.We present three main results: a constant period P=103.3960(5)s is measured until periastron passage, followed by a spin-up with a decreasing period derivative of Pdot=(-1.69±0.04)e-8s/s at MJD 53618, and P remains constant again at the end of the main outburst. The spin-up provides evidence for the existence of an accretion disk during the normal outburst. We measure a CRSF energy of Ecyc~50keV during the pre-outburst flare, and Ecyc~46keV during the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.
Key words: X-rays: binaries - stars:magnetic fields -- stars:individual: A0535+26

Preprint (151 kb PDF file including figures)
Astrophysics (astro-ph): 0801.3167

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