Message 7542: From IAUC mailing list Subject: IAUC 7542: XTE J1118+480 (58 lines) Circular No. 7542 XTE J1118+480 J. McClintock, M. Garcia, P. Zhao, N. Caldwell, and E. Falco, Harvard-Smithsonian Center for Astrophysics, write: "Spectroscopic observations of this x-ray nova were made with the new 6.5-m MMT on Dec. 1 (two spectra) and 4 (six spectra). As expected, the spectra contain both broad Balmer emission lines and the numerous absorption features characteristic of a late-type dwarf. Robust velocity measurements were obtained by cross-correlating the individual spectra (range 490-650 nm) against the spectra of several bright dwarfs (K5-M2). The eight velocities provide good phase coverage and yield a velocity amplitude K = 698 +/- 11 km/s, an orbital period P = 0.1701 +/- 0.0003 day (IAUC 7412, 7418), and a time of maximum velocity on 2000 Dec. 1.4776 +/- 0.0005 UT. The large value of the mass function, f(M) = 6.0 +/- 0.3 solar masses, is rivaled only by that of V404 Cyg. Our Oct. 26 observations (V = 18.8; Whipple Observatory 1.2-m telescope) showed that the nova was near its pre-outburst brightness (IAUC 7390). See also the simultaneous Nov. 30-Dec. 1 photometry given below." R. M. Wagner, LBT Observatory; C. B. Foltz, MMT Observatory; S. G. Starrfield, Arizona State University; and P. Hewett, Institute of Astronomy, Cambridge, report: "Thirteen optical spectra (range 410-750 nm; resolution 0.35 nm) of XTE J1118+480 were obtained with the 6.5-m MMT on Nov. 20.40-20.48 and 30.36- 30.51 UT. Convolution of the average flux-calibrated spectrum obtained on Nov. 20 with a V-band filter gives V = 19.0, comparable to its pre-outburst quiescent brightness (IAUC 7390). The spectrum exhibits broad (FWHM about 2400 km/s), double-peaked, Balmer emission lines superposed on the spectrum of an M0-M1V star, similar to other x-ray novae in their quiescent states. Cross- correlation of the individual spectra with three bright, late-type dwarf star spectra yields a sinusoidal variation in radial velocity of the absorption lines with the following orbital parameters: K = 695 +/- 16 km/s, P = 0.1699 +/- 0.0001 day, T_0 = 2000 Nov. 20.392 UT +/- 0.045 day (corresponding to absolute phase 0.0 or inferior conjunction of the dwarf star). The inferred mass function is 5.9 +/- 0.4 solar masses, suggesting that the compact object is a black hole." P. Garnavich, University of Notre Dame, monitored XTE J1118+480 via 2-min I-band exposures for a total of 6 hr with the 1.8-m Vatican Advanced Technology Telescope on Nov. 30 and Dec. 1 UT: "The star was near quiescence at V = 18.8 (P. Groot, priv. comm.). The light curve, phased to a period of 4.1 hr (Uemura et al. 2000, PASJ 52, 15), shows a double sinusoidal variation with a peak-to-peak amplitude of 0.15 mag, plus some additional flux at phase 0.55 away from minimum light." (C) Copyright 2000 CBAT 2000 December 15 (7542) Daniel W. E. Green