GX 339-4 Approaching Quiescence ?

ATel #2270; F. Lewis (Open University, University of Glamorgan), D. M. Russell (University of Amsterdam)
on 28 Oct 2009; 12:01 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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The black hole X-ray binary (BHXB) GX 339-4 continues to fade in the optical, and has now reached its faintest optical flux since (at least) the start of its 2006-7 outburst (ATels #968, #1027). In this outburst, the source reached peak magnitude values of V ~ 15 and I ~ 14. On the decline of the outburst the source twice re-brightened for several months and displayed large amplitude variability (ATels #1586, #1588, #1945, #1954, #1960, #1962). Several months ago the source finally faded below the X-ray detection limits of the RXTE ASM and PCA Bulge Scan, as well as Swift BAT.

We have been monitoring the optical counterpart of GX 339-4 in V, R and i'-bands with the Faulkes Telescope South (located at Siding Spring, Australia) since September 2007 (ATels #1586, #1962) and note the following recent magnitudes (errors are <= 0.1 mag):

2009-07-20 -- MJD 55032.4 -- V = 17.5, R = 17.0, i' = 17.1
2009-08-11 -- MJD 55054.5 -- V = 18.7, R = 18.1, i' = 18.0
2009-10-14 -- MJD 55118.4 -- V = 19.5, R = 18.6, i' = 18.4
2009-10-22 -- MJD 55126.4 -- V = 19.6, R = 18.8, i' = 18.6

Our light curve is linked below. The listed magnitudes include flux in the aperture from two close interloper stars (Shahbaz et al. 2001, A&A, 376, L17).

Our recent measurements suggest that the system is now fading towards quiescence for the first time in almost three years. GX 339-4 has quasi-regular outbursts and is commonly in an active state. It is currently uncertain whether GX 339-4 ever reaches true quiescence; most BHXBs have X-ray luminosities of the order 10^(30-32) erg/s. The faintest X-ray luminosity of GX 339-4 recorded to date (as far as we are aware) is 4.6e31 * (d/kpc)^2 erg/s (0.4-11 keV) in 2003 (ATel #196), which is > 1e33 erg/s assuming d > 6 kpc (Hynes et al. 2004, ApJ, 609, 317); more luminous than typical quiescent BHXBs.

The faintest optical magnitude reported, r = 20.1 (Shahbaz et al. 2001) is similar to the combined magnitude of the two close interloper stars. From visual inspection of our most recent images we see that GX 339-4 is most likely now fainter than the combined flux of the two interlopers (in R and i'-bands), implying the source could now be fainter than that reported in Shahbaz et al. (2001). Deeper, higher resolution imaging is required to confirm this.

If GX 339-4 continues to fade into quiescence, we encourage multi-wavelength observations. The correlated multi-wavelength behaviour is now well studied in this source (e.g. Corbel et al. 2003, A&A, 400, 1007; Coriat et al. 2009, MNRAS, in press, arXiv:0909.3283) but never before at these very low luminosities.

The Faulkes Telescope observations are part of an ongoing monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0712.2751). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). FL acknowledges support from the Dill Faulkes Educational Trust. DMR acknowledges support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship.

GX 339 light curves