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

ATel #1012; 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 26 Feb 2007; 13:05 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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We present the results of preliminary spectral fits to the second period of 115 ksec public INTEGRAL ToO observations of GX 339-4 (see ATels #980, #986, #1000), obtained between UT 2007, Feb 17, 18:53 and Feb 19, 04:39. 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 softened considerably with respect to our first set of INTEGRAL ToO observations (ATel #1000).

The model that best phenomenologically fits the joint spectrum consists of an absorbed multi-color black-body disk plus an unbroken power-law up to about 200 keV. Contrary to our first set of observations, we do not see any reflection feature in the spectrum during the second period. Also, we do not see any line-like feature the residuals in the range of 6-8 keV greater than 10%.

The relative flux of the disk compared to the power-law emission is about 0.9 (i.e., it has almost doubled with respect to our first set of observations). We find a color temperature of the inner disk of T=0.91 (+0.02-0.03) keV; the power-law has a photon index of 2.68+/-0.08. These parameters are broadly consistent with that seen during soft states in black-hole (candidate) binaries. The implied unabsorbed luminosity of the source in the 5-200 keV band was L=(2.3-3.7)E+37 erg/s, assuming a distance to the source of 8.5 kpc.

We also examined the JEM-X and ISGRI light curves with time bins of 1000 sec, in the 3-6, 6-10, 10-15, 15-35, 20-40, 40-60, 60-100, 100-200 keV energy ranges. Strong flaring activity with a peak up to about 70 mCrab was seen in the 20-60 keV band near the end of the observation.

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/.