Complete and simultaneous spectral observations of the black hole X-ray nova XTE J1118 + 480

The X-ray nova XTE J1118 + 480 suffers minimal extinction (b = 62°) and therefore represents an outstanding opportunity for multiwavelength studies. Hynes et al. conducted the first such study, which was centered on 2000 April 8 using UKIRT, EUVE, HST, and RXTE. On 2000 April 18, the Chandra X-Ray Observatory obtained data coincident with a second set of observations using all of these same observatories. A 30 ks grating observation using Chandra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover ≈ 80% of the electromagnetic spectrum from the infrared to hard X-rays. The UV/X-ray spectrum of XTE J1118 + 480 consists of two principal components. The first of these is an ≈ 24 eV thermal component that is caused by an accretion disk with a large inner disk radius: ≳ 35R_Schw. The second is a quasi power-law component that was recorded with complete spectral coverage from 0.4 to 160 keV. A model for this two-component spectrum is presented in a companion paper by Esin et al.