[vsnet-obs 31124] The Mass Function of XTE J1118+480

[Jeff McClintock <jem@head-cfa.harvard.edu>]

A short while ago we submitted the following report to the editor
of the IAU Circulars:

XTE J1118+480
    J. McClintock, M. Garcia, P. Zhao, N. Caldwell, and E. Falco,
Harvard-Smithsonian Center for Astrophysics, report: "Spectroscopic
observations of this X-ray nova with the new 6.5-m MMT on Dec. 1 & 4
show that it contains a black hole primary of at least 6 solar masses.
Our prior photometric observations using the FLWO 1.2-m telescope
showed that the X-ray nova was near its pre-outburst brightness (IAUC
7390) at V = 18.8 on Oct. 26.  The quiescent state of the system was
confirmed on the nights of Nov. 30 & Dec. 1 by P. Garnavich who
observed a double-humped, ellipsoidal light curve using the VATT 1.8-m
(priv. comm.).  Two MMT spectra were obtained on Dec. 1 and six
additional spectra were obtained on Dec. 4.  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 of K = 709 +/- 11 km/s and an orbital period of P =
0.1700 +/- 0.0003 days (IAUC 7412,7418).  The large value of the mass
function, f(M) = 6.3 +/- 0.3 solar masses, is rivaled only by that of
V404 Cyg.  A time of maximum velocity occurred on 2000 Dec. 1.4725 +/-
0.0005 UT."

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