Letter to the Editor

The pre-outburst flare of the A 0535+26 August/September 2005 outburst

¹ Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany e-mail: isabel@astro.uni-tuebingen.de
² ISOC, European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ Sternberg Astronomical Institute, 119999 Moscow, Russia
⁴ GACE, Instituto de Ciencias de los Materiales, 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'Écogia, 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

Received: 21 December 2007
Accepted: 8 January 2008

Abstract

Aims.We study the spectral and temporal behavior of the High Mass X-ray Binary A 0535+26 during a “pre-outburst flare” which took place ~ 5 d before the peak of a normal (type I) outburst in August/September 2005. We compare the studied behavior with that observed during the outburst.

Methods.We analyse RXTEobservations that monitored A 0535+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.

Results.We present three main results: a constant period P = 103.3960(5) sis measured until periastron passage, followed by a spin-up with a decreasing period derivative of = (-1.69 ±0.04) 10^-8 ss^-1 at MJD 53 618, 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 ~ 50 keV during the pre-outburst flare, and ~ 46 keVduring the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.