Caballero, I.Santangelo, A.Kretschmar, P.Staubert, R.Postnov, K.Klochkov, D.Camero-Arranz, A.Finger, M. H.Kreykenbohm, I.Pottschmidt, KatjaRothschild, R. E.Suchy, S.Wilms, J.Wilson, C. A.2023-10-042023-10-042008-01-25Caballero, I., A. Santangelo, P. Kretschmar, R. Staubert, K. Postnov, D. Klochkov, A. Camero-Arranz, et al. “The Pre-Outburst Flare of the A 0535+26 August/September 2005 Outburst.” Astronomy & Astrophysics 480, no. 2 (March 1, 2008): L17–20. https://doi.org/10.1051/0004-6361:20079310.https://doi.org/10.1051/0004-6361:20079310http://hdl.handle.net/11603/29957Aims.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 P= (-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 E꜀ᵧ꜀~ 50 keV during the pre-outburst flare, and E꜀ᵧ꜀ ~ 46 keV during the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.5 pagesen-USThis item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.Attribution 4.0 International (CC BY 4.0)https://creativecommons.org/licenses/by/4.0/Text