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

ATel #1032; 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 23 Mar 2007; 15:52 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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We present the results of preliminary spectral fits to the fourth period of 130 ksec public INTEGRAL ToO observations of GX 339-4 (see ATels #980, #986, #1000, #1012, #1029), obtained between UT 2007, Mar 16, 17:03 and Mar 18, 07:17. We made joint spectral fits to spectra obtained with the JEM-X (5-20 keV) and IBIS/ISGRI (23-200 keV) instruments. The spectrum has slightly softened with respect to the previous observation (ATel #1029), with a contribution of the disk compared to the power-law emission of 36%.

A phenomenological model consisting of an absorbed multi-color disk with T_in = 0.80 ± 0.05 keV plus a power-law with a photon index of 2.7 ± 0.1 fits the spectrum fairly (chi-square per d.o.f. of 70/38). 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 69/35), and we can not constrain the high-energy cut-off (values between 20 and 300 keV are compatible with the data). We note that a fit with a compTT model gives slightly worse results (chi-square per d.o.f. of 86/36). The situation resembles that of our second INTEGRAL observation (ATel #1012): approximately same photon index, unabsorbed luminosity and no evidence for a break up to about 300 keV, with characteristics typical of a high state.

The unabsorbed total luminosity in the 5-300 keV range, assuming a distance of 8.5 kpc, is 1.17-2.32 E+37 erg/s. This corresponds to a decrease of 67% in luminosity with respect to that observed in our third observation (ATel #1029).

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/.