X-RAY AND OPTICAL OBSERVATIONS OF A 0535+26

Abstract

We present recent contemporaneous X-ray and optical observations of the\nBe/X-ray binary system A\\,0535+26 with the \\textit{Fermi}/Gamma-ray Burst\nMonitor (GBM) and several ground-based observatories. These new observations\nare put into the context of the rich historical data (since $\\sim$1978) and\ndiscussed in terms of the neutron star Be-disk interaction. The Be\ncircumstellar disk was exceptionally large just before the 2009 December giant\noutburst, which may explain the origin of the unusual recent X-ray activity of\nthis source. We found a peculiar evolution of the pulse profile during this\ngiant outburst, with the two main components evolving in opposite ways with\nenergy. A hard 30-70 mHz X-ray QPO was detected with GBM during this 2009\nDecember giant outburst. It becomes stronger with increasing energy and\ndisappears at energies below 25\\,keV. In the long-term a strong optical/X-ray\ncorrelation was found for this system, however in the medium-term the\nH$_\\alpha$ EW and the V-band brightness showed an anti-correlation after\n$\\sim$2002 Agust. Each giant X-ray outburst occurred during a decline phase of\nthe optical brightness, while the H$_\\alpha$ showed a strong emission. In late\n2010 and before the 2011 February outburst, rapid V/R variations are observed\nin the strength of the two peaks of the H$_\\alpha$ line. These had a period of\n$\\sim$\\,25 days and we suggest the presence of a global one-armed oscillation\nto explain this scenario. A general pattern might be inferred, where the disk\nbecomes weaker and shows V/R variability beginning $\\sim$\\,6 months following a\ngiant outburst.\n