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A recent Swift/XRT observation of GX 339-4 during its faint optical flux

ATEL # 2281; T.-C. Yen, A.K.H. Kong (NTHU, Taiwan)
on 5 Nov 2009; 9:23 UT
Password Certification: Albert Kong (

Subjects: X-ray, Binaries, Black Holes, Transients, Variables

We report on the X-ray luminosity of the black hole X-ray binary GX 339-4, which was reported to fade in the optical and reach its lowest optical flux since the start of its 2006-7 outburst (ATels #968, #1027, #2270). A Swift XRT observation in PC mode was performed on 2009-10-30.35 UT with an exposure of 2984 seconds. The source appeared to be weak with about 25 net counts in the 0.3-10 keV band; it is substantially fainter than previous XRT observations in March 2009. The very low counts makes spectral fitting and timing analysis impossible; we therefore employ the WebPIMMS provided by HEASARC to estimate the X-ray flux. Assuming an absorbed power-law with a photon index of 2 (see ATel #196), we estimate the unabsorbed 0.3-10 keV flux of GX 339-4 to be 8e-13 erg sec^-1 cm^-2 which is a factor of 2 higher comparing to the faintest record in 2003 (ATel #196). For a distance of > 6 kpc (see Hynes et al. 2004, ApJ, 609, 317), the estimated 0.3-10 keV flux gives an associated luminosity of >3.4e33 erg sec^-1, which is at the upper end of the quiescent state for black hole X-ray binaries (Gallo et al. 2008, ApJ, 683, L51). It is possible that the source is now in a transition from the low/hard state to the quiescent state.

The correlated X-ray and optical observations are suggestive that GX 339-4 is fading into quiescence; observations at other wavelengths are encouraged.

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