Spectral and Temporal Behavior of the Black Hole Candidate XTE J1118+480 as Observed with BeppoSAX

Astrophysics, abstract
astro-ph/0304261

From: L. Amati <amati@tesre.bo.cnr.it>

Date: Mon, 14 Apr 2003 16:53:01 GMT   (370kb)

Authors:
F. Frontera,
L. Amati,
A. A. Zdziarski,
T. Belloni,
S. Del Sordo,
N. Masetti,
M. Orlandini,
E. Palazzi

Comments: 26 pages, 8 figures, accepted for publication in The Astrophysical
Journal

XTE J1118+480 is a well established black hole candidate with a mass estimate
in the range from 7 to 10 solar masses. With BeppoSAX we observed the source 4
times, from April to December 2000. Results of the firs t observation were
already reported (Frontera et al. 2001). Here we report spectral results of the
later observations, performed in May, June and December 2000 and compare them
with the results obtained from the 2000 April observation. We observe a
decrease of the column density from a value consistent with the Galactic N_H
obtained from radio measurement s to a value a factor 2 lower. The spectra are
well fit with a thermal Comptonization plus a blackbody model. The blackbody
luminosity decreases with time, while the electron temperature of the
Comptonizing electron s does not show significant changes. A Compton reflection
component is apparent and stable, although weak (mean value of Omega/2pi =
0.21[-0.04,+0.05]). The reflector shows a low metallicity (mean value of Z/Zsun
= 0.13[-0.04,+0.06]). On the basis of the spectral results, a hot central disk
appears the best scenar io for the high energy photons, while the temporal
properties point to a non therma l origin of a fraction of the soft X-ray
photons, likely synchrotron emission internal to the hot disk.

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