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 degrees) an d therefore represents an outstanding opportunity for multiwavelength studi es. 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 Ch andra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover approximate to 8 0% 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 Drst of these is an approximate to 24 eV thermal component that is caused b y an accretion disk with a large inner disk radius : greater than or simila r to 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 twoc omponent spectrum is presented in a companion paper by Esin et al.