INTEGRAL observations of GX339-4: preliminary spectral fit results

ATel #1000; 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 11 Feb 2007; 09:48 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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We present the results of preliminary spectral fits to the first 130 ksec INTEGRAL ToO observation of GX 339-4 (see ATel #980, #986), obtained between 2007, Jan 30, 15:56 and Feb 01, 06:11. We made joint spectral fits to spectra obtained with the JEM-X (5-20 keV) and IBIS/ISGRI (23-300 keV) instruments.

The model that best phenomenologically fits the joint spectrum consists of an absorbed multi-color black-body disk plus a powerlaw with a high-energy cut-off at 22 +/- 5 keV and a folding energy of 68 +/- 5 keV. The presence of curvature between 20-30 keV and line-like residuals revealed between 6-8 keV, is likely due to disk reflection. We will explore fits with reflection models in future work.

The presence of the radiation coming from the disk is significant, and we find a color temperature of the inner disk of T = 1.5 +/- 0.1 keV. The powerlaw has a photon index of 1.67 +/- 0.07. These parameters are broadly consistent with rising hard states. The implied luminosity of the source in the 5-300 keV band was L = 1.3-1.7 E+38 erg/s, calculated taking into account a distance to the source of 8.5 kpc.

We examined the JEM-X and IBIS/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 source was slowly increasing its brightness since the beginning of the observation in the overall energy range of 3-200 keV.

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/.