Giant outburst of A0535+26 monitored with RXTE

ATel #2337; I. Caballero (CEA Saclay), P. Kretschmar (ISOC/ESAC), K. Pottschmidt (CRESST/GSFC/UMBC), J. Wilms, I. Kreykenbohm (Dr. Karl Remeis Observatory/ECAP), S. Suchy, R. Rothschild (CASS/UCSD), C. Ferrigno (ISDC), A. Santangelo, D. Klochkov, R. Staubert (AIT)
on 7 Dec 2009; 23:16 UT
Credential Certification: I. Caballero (isabel.caballero@cea.fr)

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A0535+26 is currently undergoing a giant outburst, having reached a flux of ~3.7 Crab in the 15-80 keV range measured with Swift-BAT (ATel #2324, #2336). The outburst is taking place around periastron (MJD 55168.4, Finger et al. 2006). During the three previous periastron passages normal outbursts have been observed, the last of them showing a peculiar double-peaked light curve (ATel #2161). A similar behavior was observed prior to the 1994 giant outburst (Finget et al. 1996ApJ...459..288F).

We are monitoring the source daily with RXTE. As the giant outburst of the source in 2005 could not be observed due to Sun constraints (ATel #504), the current RXTE observations provide the first detailed study of a giant outburst of the source. Unfortunately, and again due to solar angle constraints, INTEGRAL and Suzaku are currently not able to observe A0535+26.

Preliminary analysis of the RXTE observations reveal the presence of a fundamental cyclotron line at E = 45.1 +1.1/-1.0 keV (90% confidence) using HEXTE data, from one observation performed December 5 at MJD 55170.69. The flux of that observation is F(20-100 keV) = 2.97 x10^-8 erg cm^-2 s^-1, which implies a luminosity of L_X(20-100 keV) = 1.4 x 10³⁷ erg s^-1 assuming a distance of 2 kpc. The cyclotron line was measured at E = 46.7+0.6/-0.7 keV with RXTE and E = 45.9 ± 0.3 keV with INTEGRAL during a normal outburst in 2005 (Caballero et al. 2007A&A, 465, L21C).