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 x 10^{-9} of the Eddington luminosity. 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 deg, 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 L_x = 4 x 10^{-8.5} of the Eddington luminosity. This spectrum is comprised 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 ~ 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 x 10^{30} erg/s (0.3-7 keV); the bolometric luminosity of the optical/UV component is 20 times greater.
Highlights
An X–ray nova (a.k.a. soft X–ray transient) typically brightens in X–rays by as much as 107 in a week and decays back into quiescence over the course of a year
We have presented a multiwavelength spectrum of the black hole primary in J1118 in its quiescent state at Lx ≈ 4 × 10−9LEdd (Fig. 10)
The following are the major conclusions that can be drawn from our results: (i) The spectrum of J1118 does not appear to be peculiar to this source or to this particular observation
Summary
An X–ray nova (a.k.a. soft X–ray transient) typically brightens in X–rays by as much as 107 in a week and decays back into quiescence over the course of a year (van Paradijs & McClintock 1995). The quiescent X–ray luminosities of short-period X–ray novae like J1118 are known to be extraordinarily low, Lx ∼ 1031ergs s−1 ∼ 10−8.5LEdd (McClintock, Horne & Remillard 1995; Garcia et al 2001), and the quality of the spectral data is severely limited by counting statistics We undertook these observations in order to make crucial tests of the advection–dominated accretion flow (ADAF) model (Narayan & Yi 1994, 1995b; Abramowicz et al 1995; see Narayan, Mahadevan & Quataert 1998 for a review).
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