We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A0535+26 with the Fermi/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of the rich historical data (since ∼1978) and discussed in terms of the neutron-star-Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70mHz X-ray quasi-periodic oscillation was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25keV. In the long term a strong optical/X-ray correlation was found for this system, however in the medium term the Hα equivalent width and the V-band brightness showed an anti-correlation after ∼2002 August. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the Hα showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the Hα line. These had a period of ∼25days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning ∼6 months following a giant outburst. © 2012. The American Astronomical Society. All rights reserved.
Publication Source (Journal or Book title)
Camero-Arranz, A., Finger, M., Wilson-Hodge, C., Jenke, P., Steele, I., Coe, M., Gutierrez-Soto, J., Kretschmar, P., Caballero, I., Yan, J., Rodríguez, J., Suso, J., Case, G., Cherry, M., Guiriec, S., & McBride, V. (2012). X-ray and optical observations of A0535+26. Astrophysical Journal, 754 (1) https://doi.org/10.1088/0004-637X/754/1/20