[vsnet-campaign 3] XTE J1118+480: possible growth of superhumps

[Taichi Kato <tkato>]

XTE J1118+480: possible growth of superhumps

Dear Colleagues,

    Thanks to all contributed to the (quasi-)simultaneous spectroscopic and
photometic runs of XTE J1118+480, we have able to detect a remarkable
change in the light modulation, which now shows steeper rise and slower
decline, characterstic to SU UMa-type superhumps.  The finding is consistent
with that reported by Lew Cook in [vsnet-alert 4733].  The hump amplitude
has now grown to 0.07 mag.  The best period determined from 5397 observations
between Apr. 21 and May 3 is 0.1704(3), which is slightly shorter than
the initially determined period (cf. 0.17078(4) d by Uemura et al. 2000).
The preliminary O-C analysis also suggests the existence of period shortening.
Altough the present uncertainty of the period, mainly due to the short
baseline, would make the conclusion still preliminary, the detectable
change in periodicity and the variation profile strongly suggest that
superhumps in black-hole transients are now indeed developing
(eg. Bailyn 1992, ApJ 391, 298; Mineshige et al. 1992, PASJ 44, L15;
O'Donoghue and Charles 1996, MNRAS 282, 191 etc.).  The result of more
detailed analysis, with continuously incoming and updated data, will be
presented later by Uemura-san.  We strongly encourage all observers to
continue observing this target.  We are planning to have a collaborative
multiwavelength campaign in the coming week, which will be soon announced.
Please keep up your excellent works on this remarkable target!

    We are very looking forward to receiving any 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-campaign@kusastro.kyoto-u.ac.jp].

Regards,
Makoto Uemura, Taichi Kato, Katsura Matsumoto, Lew Cook, Rudolf Novak,
Biran Martin, Arto Oksanen, Marko Moilanen, Yasuo Sano, Denis Buczynski,
Gianluca Masi, Alessia Cassetti, Masami Honkawa and Osaka Kyoiku University
team, Kesao Takamizawa, Katsumi Haseda, Gunma Astronomical Observatory team
on behalf of the VSNET Collaboration team

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