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

Abstract

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^-9L_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^-8.5L_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~10⁴ 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~3.5×10³⁰ ergs s^-1 (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 NAS 5-26555.