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 the X-ray nova XTE J1118+480 in quiescence at L-X approximate to 4 x 10(-9) L-Edd. The Chandra, Hubble Space Telescope, and Multiple Mirror Telescope spectroscopic observations were performed simultaneously on 2002 January 12 UT. Because this 4.1 hr binary is located at b = 62degrees, the transmission of the interstellar medium is very high (e.g., 70% at 0.3 keV). We present many new results for the quiescent state, such as the first far-UV spectrum and evidence for a 0.35 mag orbital modulation in the near-UV flux. However, the centerpiece of our work is the multiwavelength spectrum of XTE J1118+480, which we argue represents the canonical spectrum of a stellar-mass black hole radiating at L-X similar to 10(-8.5) L-Edd. This spectrum is composed of two apparently disjoint components: a hard X-ray spectrum with a photon index Gamma = 2.02 +/- 0.16 and an optical/UV continuum that resembles a 13,000 K disk blackbody spectrum punctuated by several strong emission lines. We present a model of the source in which the accretion flow has two components: (1) an X-ray-emitting interior region where the flow is advection-dominated and (2) a thin exterior accretion disk with a truncated inner edge (R-tr similar to 10(4) Schwarzschild radii) that is responsible for the optical/UV spectrum. For D = 1.8 kpc, the luminosity of the X-ray component is L-X approximate to 3.5 x 10(30) ergs s(-1) (0.3-7 keV); the bolometric luminosity of the optical/UV component is approximate to20 times greater.