Fifth set of INTEGRAL observations of GX339-4: preliminary spectral fit results

ATel #1050; M. D. Caballero-Garcia (LAEFF/INTA), J. Miller (Univ. of Michigan), E. Kuulkers (ESA/ESAC), M. Diaz Trigo (ESA/ESAC), on behalf of a larger collaboration
on 5 Apr 2007; 22:06 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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We present the results of preliminary spectral fits to the fifth period of 150 ksec public INTEGRAL ToO observations of GX 339-4 (see ATels #980, #986, #1000, #1012, #1029, #1032, #1048), obtained between UT 2007, Mar 29, 04:40 and Mar 31, 01:42. We made joint spectral fits to spectra obtained with the JEM-X (5-20 keV) and IBIS/ISGRI (23-100 keV) instruments. The spectrum continued to soften, as observed in the previous observation (ATel #1032), with a contribution of the disk compared to the power-law emission (see below) of about 74%. A phenomenological model consisting of an absorbed multi-color disk with T_in = 0.98 ± 0.08 keV plus a power-law with a photon index of 2.6 ± 0.2 fits the spectrum fairly (chi-square per degrees of freedom, d.o.f., of 48/25). We find that, when using a cut-off power-law rather than a power-law, it does not improve the fit (chi-square per d.o.f of 51/23), and we do not find any break energy between 20 and 100 keV compatible with the data. We note that a fit with a compTT model gives slightly worse results (chi-square per d.o.f. of 54/24). The situation resembles that of our second and fourth INTEGRAL observations (ATels #1012 and #1032): approximately same photon index and no evidence for a break at high energies, with characteristics typical of a high state. The unabsorbed total luminosity in the 5-100 keV range, assuming a distance of 8.5 kpc, is about 1 E+37 erg/s. This corresponds to a decrease of about 50% in luminosity with respect to that observed in our fourth observation (ATel #1032). We also examined the JEM-X and ISGRI light curves with a time binning of 1000 sec, in the 3-6, 6-10, 10-15, 15-35, 20-40, 40-60, 60-100, 100-200 keV energy ranges. No strong variability or trends were seen. PostScript figures of the preliminary spectral fit results, as well as the JEM-X and IBIS/ISGRI light curves, can be found at http://isdc.unige.ch/~kuulkers/GX339-4/.