A 0535+26 in the August/September 2005 outburst observed by RXTE and INTEGRAL

Abstract

Aims. In this Letter we present results from INTEGRAL and RXTE observations of the spectral and timing behavior of the High Mass X-ray Binary A 0535+26 during its August/September 2005 normal (type I) outburst with an average flux F (5−100) keV ∼ 400 mCrab. The search for cyclotron resonance scattering features (fundamental and harmonic) is one major focus of the paper. Methods. Our analysis is based on data from INTEGRAL and 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 and RXTE pulse phase averaged X-ray spectra are studied. The evolution of the fundamental cyclotron line at different luminosities is analyzed. Results. 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 E 1 ∼ 45 keV and E 2 ∼ 100 keV. 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 ∼ 4 × 10 12 G.

X-Ray and Optical Observations of a 0535+26

“…A similar overall shape has been found in a wide range of observations over several decades in time and outbursts of quite different peak brightness (e.g., Bradt et al 1976 and more recently Kretschmar et al 2005, Finger et al 2006, Camero-Arranz 2007, and Caballero et al 2007.…”

“…A possible explanation was bursts of wind-like accretion from capture of low angular momentum material, while the higher angular momentum material forms an accretion disk, which feed the main part of the outburst. Caballero et al (2007Caballero et al ( , 2008 found a change of the cyclotron line energy up to ∼50 keV during this period and no significant change in the spin frequency up to the time the main outburst began. Postnov et al (2008) proposed that magnetospheric instabilities developing at the onset of the accretion were the origin of the flaring activity.…”

“…RXTE data of the 2005 September outburst showed in addition that the pulse profile changed shape around the energy of the fundamental cyclotron line at 45 keV (Camero-Arranz 2007). Moreover, the pulse shape behavior during the preoutburst peak in that outburst was observed to be different than during the main peak (Camero-Arranz 2007; Caballero et al 2007). This has not been observed in other occasions.…”

“…The first peak was smaller, shorter, and totally detached from the main outburst peak (see the right panel of Figure 2). During the 2005 pre-outburst phase the pulse profile changed dramatically (Camero-Arranz 2007;Caballero et al 2007), as did the value of the center of the cyclotron line energy (Caballero et al 2008). However, in the 2009 and 2010 normal outbursts no changes have been found neither with RXTE/PCA nor GBM (see Section 5.3).…”

Tidal interaction in High‐Mass X‐ray Binaries

“…The source reappeared in a Type II outburst in May/June 2005 and was detected by Swift (Tueller et al 2005) and RHESSI (Smith et al 2005). It was subsequently seen to undergo a Type I outburst in August 2005 (Kretschmar et al 2006;Caballero et al 2007).…”