2007A&A...465L..21C

In this Letter we present results from INTEGRAL xspaceand RXTE observations of the spectral and timing behavior of the High Mass X-ray Binary A0535+26 during its August/September 2005 normal (type I) outburst with an average flux F_(5–100)keV∼400mCrab. The search for cyclotron resonance scattering features (fundamental and harmonic) is one major focus of the paper. Our analysis is based on data from INTEGRAL xspaceand RXTE Target of Opportunity Observations performed during the outburst. The pulse period is determined. X-ray pulse profiles in different energy ranges are analyzed. The broad band INTEGRAL xspaceand RXTE pulse phase averaged X-ray spectra are studied. The evolution of the fundamental cyclotron line at different luminosities is analyzed. The pulse period P is measured to be 103.39315(5)s at MJD 53614.5137. Two absorption features are detected in the phase averaged spectra at E₁∼45keV and E₂∼100keV. These can be interpreted as the fundamental cyclotron resonance scattering feature and its first harmonic and therefore the magnetic field can be estimated to be B∼4x10¹²G.