1. University of California, San Diego, Center for Astrophysics and Space Sciences, 9500 Gilman Dr, La Jolla, CA 92093-0424, USA
2. Visva-Bharati University, Santiniketan, Bolpur 731235, West Bengal, India
3. European Space Research and Technology Centre, Keplerlaan 1, NL-2201 AZ Noordwijk, Netherlands
4. Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland