GX 339-4 is rising into outburst, and a deep optical quiescent magnitude observed by Faulkes Telescope South

ATel #4162; F. Lewis (Univ. of Glamorgan, Faulkes Telescope Project), D. M. Russell (IAC), T. Shahbaz (IAC)
on 11 Jun 2012; 19:14 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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We report a gradual (~ 1 mag in 50 days) optical re-brightening of the black hole X-ray binary, GX 339-4. We have been monitoring the source with the 2-m Faulkes Telescope (FT) South (located at Siding Spring, Australia) in SDSS i'-band, Bessel R and Bessel V filters (200-second exposures; see e.g. Cadolle Bel et al. 2011). The images are reduced using the FT pipeline. Since the source entered a low-luminosity state in 2011 May (ATel #3383), it has remained fainter than V > 19; R > 18; i' > 18 mag (below is a link to our light curve).

Since 2012-05-11 (MJD 56058), GX 339-4 has appeared brighter, with magnitudes measured on six dates in the range V = 18.1-18.8; R = 17.8-18.1; i' = 17.7-18.1 (our latest observation was on 2012-06-08 = MJD 56086). These values represent an increase of ~ 1 magnitude on the mean faint (approaching quiescence) values measured prior to 2012-05-11. This indicates GX 339-4 is entering a new outburst. It is not yet clear if this outburst will be bright (like in 2010-2011) or a mini-outburst (like those of 2008 and 2009). For monitoring of previous outbursts see Cadolle Bel et al. (2011); Buxton et al. (2012); Dincer et al. (2012).

Presently, there are no recent X-ray detections from Swift BAT; MAXI appears to show a possible slight increase of activity in some recent pointings. Pointed X-ray, and multiwavelength observations are encouraged during this early rise phase.

GX 339-4 lies 1 arcsec from a blend of two very close line-of-sight stars, with a combined magnitude similar to that of GX 339-4 in its low-luminosity state (Shahbaz et al. 2001). On inspecting our images during low-luminosity, we found that in a few images GX 339-4 was much fainter than the close blend. On 2012-04-16 (MJD 56033) the magnitudes from aperture photometry (which includes flux from the blend) were V = 19.79 +- 0.08; R = 19.00 +- 0.04; i' = 18.78 +- 0.03. We performed PSF photometry (using DAOPHOT in IRAF) on the R-band image taken on this date and subtracted the blend close to GX 339-4. A faint excess is visible in the resulting image at a position consistent with GX 339-4 as measured during outburst (see link to images below). Its magnitude is R = 21.50 +- 0.30 and the signal-to-noise ratio is S/N = 4.8.

This is the faintest magnitude ever reported for GX 339-4, and may represent the magnitude of the companion star, which has not been detected at low luminosity before (see also Remillard & McClintock 1987; Shahbaz et al. 2001). The distance to the source, and the companion mass have been derived by Hynes et al. (2004) and Munoz-Darias et al. (2008) to be 6-15 kpc and 0.17-1.1 M_sol, respectively. If the companion is a normal main sequence star, given these constraints, R = 21.50 +- 0.30 would imply a companion spectral type of G1 - G9. Spectral types earlier than G are unlikely, but later spectral types cannot be ruled out because accretion activity may prevail even at this faintest magnitude. Alternatively, the companion may be a stripped subgiant, as proposed by Munoz-Darias et al. (2008).

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008). The Faulkes Telescope South is maintained and operated by Las Cumbres Observatory Global Telescope Network. FL acknowledges support from the Dill Faulkes Educational Trust.

Faulkes light curves and low-luminosity images of GX 339-4