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GX 339-4 remains in the hard state

ATel #5417; T. M. Belloni (INAF-OAB, Italy), S. Motta (ESAC/ESA, Spain), P. Casella (INAF-OAR, Italy)
on 25 Sep 2013; 10:19 UT
Credential Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

Subjects: X-ray, Binary, Black Hole

Referred to by ATel #: 5594

The recurrent black-hole transient GX 339-4 was reported to have entered a new outburst on the basis of optical/NIR observations on 2013 August 1st (ATel #5244). The outburst was confirmed by a Swift/XRT observation, which found its X-ray flux at the level of 1.6e-10 erg/cm2/s (0.3-10 keV), well above the quiescent level (ATel #5252). Follow up observations in the radio and X-rays were reported, with the X-ray flux (Swift/XRT) at 2e-10 erg/cm2/s (3-9 keV) and a hard power law spectral index (1.55). The ongoing outburst is observed by the MAXI all-sky monitor (2-10 keV), the large FoV Swift/BAT instrument (15-150 keV), and a series of pointed observations with the Swift/XRT telescope (0.3-10 keV). An updated light curve in all three instruments can be found in http://www.brera.inaf.it/utenti/belloni/BHTransients/GX339.html . Both the hard and soft X-ray flux increased linearly with time until around MJD 56535 (2013 Aug 13th), after which they leveled off to a constant. The rising part was accompanied by softening of the XRT spectrum, as shown by the hardness ratio (see above link). The latest Swift/XRT spectrum (taken on 2013 September 24th, ~1200 s exposure) can be fitted (chi2=584/536) by a model consisting of a disk blackbody and a power law, with interstellar absorption fixed at 5e21 cm^-2 (see Motta et al. 2011, MNRAS, 418, 2292). The inner disk temperature of 0.24+/-0.02 keV and the photon index of 1.44+/-0.04 indicate that the source is still in the hard state. The value for the disk normalization, assuming a distance of 10 kpc and an inclination of 30 degree, is around 100 (-17 +27) km, consistent with a receded inner disk. The 0.3-10 keV unabsorbed flux is 2.1e-9 erg/cm2/s, to which the disk component contributes 13%. A Power Density spectrum of the XRT data shows strong band-limited noise with a 0.01-60 Hz fractional rms of 48%, typical of a hard state. The current outburst is similar to the behavior of GX 339-4 in its 2004 outburst, when after an initial rise the flux stalled then decreased, still in the hard state.