Multiwavelength observations revealing the outbursts of the two soft X-ray transients XTE J1859+226 and XTE J1118+480

S. Chaty, The Open University
C. A. Haswell, The Open University
R. I. Hynes, The Open University
C. R. Shrader, The Open University
W. Cui, The Open University
C. W. Mauche, The Open University
A. J. Norton, The Open University
J. E. Solheim, The Open University
R. Ostensen, The Open University
T. R. Geballe, The Open University
Z. Ioannou, The Open University
A. R. King, The Open University

Abstract

We report here on multiwavelength observations of the two new soft X-ray transients (SXTs) XTE J1859+226 and XTE J1118+480, which we observed with HST/RXTE/UKIRT. For XTE J1118+480 we also used EUVE since it is located at a very high galactic latitude and suffers from very low extinction. The two sources exhibited very different behaviour. XTE J1859+226 seems quite normal and therefore a good object for testing the accretion mechanisms in place during the outbursts, XTE J1118+480 is much more unusual because it exhibits i) a low X-ray to optical ratio and ii) a strong non-thermal contribution in the radio to optical domain, which is likely to be due to synchrotron emission. We concentrate here on the near-infrared (NIR) and optical observations of these two systems.