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IAUC 7617: XTE J1118+480

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                                                  Circular No. 7617
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1118+480
     J. Casares, C. Zurita, T. Shahbaz, and P. Rodriguez-Gil,
Instituto de Astrofiisica de Canarias (IAC); P. Charles and R.
Hynes, University of Southampton; R. M. Wagner, LBT Observatory; E.
Ryan, University of Arizona; C. Foltz, MMT Observatory; and S.
Starrfield, Arizona State University, report on the preliminary
analysis of long-term R-band CCD photometry of the blackhole x-ray
transient XTE J1118+480 (cf. IAUC 7389):  "Observations obtained on
41 nights from Dec. 2000 to Apr. 2001 with the IAC-80 and 1-m
Optical Ground Station telescopes at Tenerife, the 2.56-m Nordic
Optical Telescope and 1-m Jacobus Kapteyn Telescope at La Palma,
and the MDM 1.3-m telescope at Kitt Peak reveal that XTE J1118+480
is still fading from its early 2000 outburst (IAUC 7390) at a rate
of about 0.003 mag/day, and it is currently at R = 18.6 (2001 Apr.
25).  After subtraction of a nightly mean magnitude, the Fourier
power spectrum of these data is dominated by the orbital period,
P_orb, with P_orb/2 = 0.084968(1) day.  Additionally, large night-
to-night variations (amplitude 0.08 mag) are clearly detected in
the depth of the minima and the overall lightcurve shape,
indicative of a short-timescale distorting wave.  After subtraction
of the orbital (ellipsoidal) modulation, a phase-dispersion-
minimization analysis (cf. Stellingwerf 1978, Ap.J. 224, 953)
reveals prominent peaks at a period that we intrepret as the
superhump period (caused by a precessing eccentric disk), P_sh =
0.17049(1) day, as well as at P_sh/2 and P_sh/4.  With this P_sh
(and consistent with that seen in outburst; cf. IAUC 7418), the
disk precession period must therefore be about 52 days, and the
power spectrum of our original data indeed shows a low-frequency
peak at about 45 days.  Extrapolating the superhump period-excess-
versus-mass-ratio relation (Osaki 1985, A.Ap. 144, 376) for
cataclysmic variables, our value of P_sh = 1.0033P_orb implies the
most extreme mass ratio in this class with M_2/M_1 about 0.008;
however, we note that for this extreme ratio, the 2:1 resonance
rather than the classical 3:1 resonance might be the dominant
tidal instability (Whitehurst and King 1991, MNRAS 249, 25).  Our
spectroscopic database (see Wagner et al., http://xxx.lanl.gov/
abs/astro-ph/?0104032) shows nightly bulk motions in H-alpha of a
few hundred km/s that may represent an elliptical precessing disk.
Further spectroscopy and photometry is urged in order to monitor
this behavior as the disk decays."

                      (C) Copyright 2001 CBAT
2001 May 2                     (7617)            Daniel W. E. Green

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