(click figures for larger versions)
XTE J1118+480 Figure 1.
Figure 1. Power spectrum
of the 30-night CBA light curve of XTE J1118+480 in unfiltered ("pink")
light. A strong signal at 5.856(4) c/d appears; there is no strong
evidence of period or amplitude change during the month (April 2000). The
little peaks at very low frequency arise from uncorrected differential
extinction, which is very hard to eliminate at such low signal strengths
XTE J1118+480 Figure 2.
Figure 2. Mean light curve of the 5.853 c/d (0.17085 d) signal. A
perfect sinusoid within measurement limits. However, it is noteworthy
that all individual-night light curves show vastly more scatter, up to
~0.4 mag. The reason is that the star flickers violently on timescales of
a few seconds or faster. Longer integrations smooth over this flickering,
which - remarkably! - does not seem to extend to much lower frequencies.
XTE J1118+480 Figure 3.
Figure 3. Light curve in unfiltered light (3500-9000 A) at 6 s time
resolution with the MDM 1.3 m telescope. Violent variations are evident,
and unresolved at this time resolution (the measurement error is only
0.02 mag/point). But the flickering power drops to essentially zero at
lower frequency, which is why long integration times can average over
dozens of flickers and achieve good signal-to-noise in this "noisiest star
in the Universe".
XTE J1118+480 Figure 4.
Figure 4. The 10-s quasiperiodic oscillation in the UMa transient.
Upper frame, average power spectrum from 36 12.5 minute runs with a
3.1 s cycle time (1 s integration, 2.1 s readout) during April 30-May 3.
Lower frame, log-log power spectrum after smoothing. All data from
CBA-East (Dave Skillman, 66 cm f/2.9 telescope). This signal appears to
be of ~0.15 mag amplitude --- the cause of the great scatter in the
high-speed light curves!