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

ATel #1029; 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 14 Mar 2007; 16:45 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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We present the results of preliminary spectral fits to the third period of 145.6 ksec public INTEGRAL ToO observations of GX 339-4 (see ATels #980, #986, #1000, #1012), obtained between UT 2007, Mar 4, 15:51 and Mar 6, 13:04. 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 hardened with respect to the second period of observations (ATel #1012), with a ratio of the soft (disk) to hard (power-law) X-ray emission being 26% (in contrast to the 50% and 90% of the first and second period observations, respectively).

An absorbed multi-color disk black-body, with T_in = 0.95 (+0.05, -0.04 keV), plus a cut-off power-law fits better (chi-square per degrees of freedom, dof, of 75/58) than adding a single power-law (chi-square per dof of 95/60). The obtained values for the photon index, cut-off and un-folded energies are 2.45 (+0.10, -0.13), 21 (+10, -6) keV and 157 (+60, -40) keV, respectively. A model containing a multi-color black-body plus a compTT model also fits the spectrum quite well (chi-square per dof of 79/59), implying a temperature of the electrons and an opacity of the corona of 53 (+80, -27) keV and 0.94 +/- 0.5, respectively. We do not see evidence for any iron-like emission feature in the residuals at 6-8 keV greater than 10% of the flux.

The implied unabsorbed luminosity of the source in the 5-300 keV band is L= 5.25 +/- 1.7 E+37 erg/s, assuming a distance to the source of 8.5 kpc. So, the unabsorbed luminosity increased by 70% with respect the second INTEGRAL observation.

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. The luminosity was slightly decreasing in the 20-60 keV energy range.

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/.