[vsnet-alert 4599] XTE J1118+480: remarkable change in the optical variability?

[Taichi Kato <tkato>]

XTE J1118+480: remarkable change in the optical variability?

    As a by-product of the intensive multiwavelength campaign of the X-ray
nova XTE J1118+480, we have detected an unprecedented 6-hour monotonous rise
on Apr. 8.  The trend has been confirmed on independent CCD photometry
runs by the Kyoto team and by Yasuo Sano.  The previously reported 4.1-hour
periodicity (IACU 7397) was hardly detectable.  The analysis of the recent
fragment (Apr. 7 - 8) of the data submitted to the VSNET Collaboration team
has yielded only slower modulations.  The present finding may suggest either
the superposition of more erratic (or flare-like -- if any, such a phenomenon
should have surely been recorded by the multiwavelength data!) behavior,
or the possible presence or development of a different underlying
periodicity.  Although the 4.1-hour periodicity previously well explains
many of the early data submitted, several runs seem to be equally well
explained by other candidate periods, such as 0.347 d, 0.514 d etc., which
are close to the mutiples of 4.1-hour.  More intensive, long time-series
observations as well as spectroscopic confirmation, are most urgently
requested.

    We are very looking forward to receiving data for further analysis
[when submitting data to us, please send them to both addresses
uemura@kusastro.kyoto-u.ac.jp (optical campaign coordinator) and
tkato@kusastro.kyoto-u.ac.jp (on behalf of the VSNET managing team) or
directly to vsnet-obs@kusastro.kyoto-u.ac.jp].

    A more detailed analysis will be later posted after finishing reducing
several other available runs.

Regards,
Makoto Uemura, Taichi Kato, Katsura Matsumoto, Lew Cook, Yasuo Sano,
Denis Buczynski, Gianluca Masi, Alessia Cassetti, Kesao Takamizawa,
Katsumi Haseda, Gunma Astronomical Observatory team
on behalf of the VSNET Collaboration team

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