XTE J1118+480 is a well-established black hole candidate with a mass estimate in the range of 7-10 Msolar. With BeppoSAX we observed the source four times, from 2000 April to December. Results of the first observation have already been reported. Here we report spectral results of the later observations, performed in 2000 May, June, and December 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 NGH obtained from radio measurements to a value a factor of 2 lower. The spectra are well fitted with a thermal Comptonization plus blackbody model. The blackbody luminosity decreases with time, while the electron temperature of the Comptonizing electrons does not show significant change. A Compton reflection component is apparent and stable, although weak (mean value of Ω/2π=0.21+0.05-0.04). The reflector shows a low metallicity (mean value of Z/Zsolar=0.13+0.06-0.04). On the basis of the spectral results, a hot central disk appears to be the best scenario for the high-energy photons, while the temporal properties point to a nonthermal origin of a fraction of the soft X-ray photons, likely synchrotron emission internal to the hot disk.
Spectral and Temporal Behavior of the Black Hole Candidate XTE J1118+480 as Observed with BeppoSAX
FRONTERA, Filippo;
2003
Abstract
XTE J1118+480 is a well-established black hole candidate with a mass estimate in the range of 7-10 Msolar. With BeppoSAX we observed the source four times, from 2000 April to December. Results of the first observation have already been reported. Here we report spectral results of the later observations, performed in 2000 May, June, and December 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 NGH obtained from radio measurements to a value a factor of 2 lower. The spectra are well fitted with a thermal Comptonization plus blackbody model. The blackbody luminosity decreases with time, while the electron temperature of the Comptonizing electrons does not show significant change. A Compton reflection component is apparent and stable, although weak (mean value of Ω/2π=0.21+0.05-0.04). The reflector shows a low metallicity (mean value of Z/Zsolar=0.13+0.06-0.04). On the basis of the spectral results, a hot central disk appears to be the best scenario for the high-energy photons, while the temporal properties point to a nonthermal origin of a fraction of the soft X-ray photons, likely synchrotron emission internal to the hot disk.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
