Simultaneous INTEGRAL, RXTE, Swift and FT South observations of the transition of GX 339-4

ATel #2573; M. Cadolle Bel, E. Kuulkers, A. Ibarra, M. Diaz Trigo (ESAC Madrid, Spain), J. Tomsick (SSL/UC Berkeley, USA), J. Rodriguez, L. Prat, S. Corbel (CEA-AIM Saclay, France), D. M. Russell, D. Altamirano (University of Amsterdam), F. Lewis (Faulkes Telescope Project, University of Glamorgan, Open University), E. Bozzo, M. Turler, C. Ferrigno (ISDC, Geneva)
on 19 Apr 2010; 15:33 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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As GX 339-4 has started to leave the Low/Hard State (ATel #2545), we triggered the 2nd part of our INTEGRAL campaign on GX 339-4 in the declining phase of the hard X-ray band, as well as contemporaneous Swift observations. The source was observed by INTEGRAL for 158 ks between April 15 (01:57 UTC) and 17 (01:04 UTC), while Swift observed it four times 2 ks during that period (program #30943). During the same period, RXTE observed the source two times, 528 and 672 s (program #95409), plus once before (976 s) and once after the INTEGRAL campaign (1744 s), respectively. During this period GX 339-4 was extremely bright in the Swift/XRT (> 400 cts/s). Therefore, the data have been corrected for pile-up effects. During these dates, a simple cut-off powerlaw fit to the Swift/XRT+INTEGRAL/JEM-X+ISGRI data yields a photon index of (2.4 +/- 0.1), while the disc component reaches the inner temperature as high as ~0.6 keV, with a hydrogen column density of (6.0 +/- 0.1) e21/cm^2. The JEM-X fluxes were around 370 mCrab and 240 mCrab in, respectively, the 3-10 and 10-20 keV energy bands, while the ISGRI fluxes were varying around 190 mCrab and 130 mCrab in 20-40 keV and 40-80 keV bands, respectively. We measured the following unabsorbed fluxes:

Swift/XRT (0.5-10 keV): 2.8e-8 erg/cm^2/s
RXTE/PCA (3-25 keV): 1.0e-8 erg/cm^2/s
INTEGRAL/ISGRI (20-200 keV): 4.6e-9 erg/cm^2/s

The source is increasing in intensity in soft X-rays and it keeps declining in hard X-rays (as announced in ATels #2523 and #2556), exiting the hard state in which it was in our previous observations in March (ATels #2455 and #2549) and going towards a softer state.

This behavior is consistent with light curves of the instruments on board Swift, RXTE and MAXI (compiled at http://staff.science.uva.nl/~diego/GX339-4/ ).

The QPOs seen in the PCA keep evolving to higher frequencies since ATel #2556. In the last observation performed on Saturday 17th (14:29 UTC), we find QPOs at 5.7 and 11.8 Hz with rms amplitudes of 5.5 and 3.4%, respectively.

We also have simultaneous optical Faulkes data on April 15 (between 13:33 and 14:23 UTC), and we derived the following magnitudes: V=(16.28 +/- 0.02), R=(15.83 +/- 0.01) and i'=(15.80 +/- 0.01). We detected a large drop in optical rms variability amplitude over the first part of the transition (on longer timescales, hundreds of seconds). From our Faulkes data i'-band, we find that the rms went down from ~0.5 mJy to < 0.1 mJy (not de-reddened) in 5 days between April 08 and 13 and, on April 15, the rms was < 0.05 mJy (not de-reddened). In magnitudes, it went down from 0.09 mag on April 08 (ATel #2547) to (0.014 +/- 0.006) mag (the variability is significant at the 4 sigma level) on April 15.

A discrete radio ejection usually occurs around this time in a black hole transient outburst, but we think that it has not yet occurred in the case of GX 339-4. Multiwavelength follow-ups of the source are strongly encouraged.

We thank ISDC for providing the results of their quick look analysis and the Swift team for their prompt and positive reply for our TOO request.