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 Lx ≈ 4 × 10 LEdd. The Chandra, HST and MMT 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 ISM 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 an 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 Lx ∼ 10 LEdd. This spectrum is comprised 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 (Rtr ∼ 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 Lx ≈ 3.5 × 10 30 ergs s (0.3–7 keV); the bolometric luminosity of the optical/UV component is ≈ 20 times greater. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Observations reported here were obtained at the MMT Observatory, a facility operated jointly by the University of Arizona and the Smithsonian Institution. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; [email protected], [email protected], [email protected], [email protected] Department of Astronomy, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182; [email protected] Center for Space Research, MIT, Cambridge, MA 02139; [email protected]