Multiwavelength Spectrum of the Black Hole XTE J1118+480 in Quiescence

We present an X-ray/UV/optical spectrum of the black hole primary in theX-ray nova XTE J1118+480 in quiescence at L_x = 4 x 10^{-9} of the Eddingtonluminosity. The Chandra, HST and MMT spectroscopic observations were performedsimultaneously on 2002 January 12 UT. Because this 4.1-hr binary is located atb = 62 deg, the transmission of the ISM is very high (e.g., 70% at 0.3 keV). Wepresent many new results for the quiescent state, such as the first far-UVspectrum and evidence for an 0.35 mag orbital modulation in the near-UV flux.However, the centerpiece of our work is the multiwavelength spectrum of XTEJ1118+480, which we argue represents the canonical spectrum of a stellar-massblack hole radiating at L_x = 4 x 10^{-8.5} of the Eddington luminosity. Thisspectrum is comprised of two apparently disjoint components: a hard X-rayspectrum with a photon index Gamma = 2.02 +/- 0.16, and an optical/UV continuumthat resembles a 13,000 K disk blackbody spectrum punctuated by several strongemission lines. We present a model of the source in which the accretion flowhas two components: (1) an X-ray-emitting interior region where the flow isadvection-dominated, and (2) a thin, exterior accretion disk with a truncatedinner edge (R_tr ~ 10^4 Schwarzschild radii) that is responsible for theoptical/UV spectrum. For D = 1.8 kpc, the luminosity of the X-ray component isL_x = 3.5 x 10^{30} erg/s (0.3-7 keV); the bolometric luminosity of theoptical/UV component is 20 times greater.Comment: 52 pages, 16 figures, accepted by Ap