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 ≈ 4 × 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 = 62°, 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 ~ 10 -8.5 L Edd . This spectrum is composed of two apparently disjoint components: a hard X-ray spectrum with a photon index Γ = 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 ~ 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 ≈ 3.5 × 10 30 ergs s -1 (0.3-7 keV); the bolometric luminosity of the optical/UV component is ≈20 times greater.