Optical photometry and polarimetry of GX 339-4 during its outburst rise

ATel #1962; D. M. Russell (Univ. of Amsterdam), F. Lewis (Las Cumbres Observatory Global Telescope Network (LCOGTN)/Open Univ.), P. Casella (Univ. of Amsterdam), M. L. Pretorius (South African Astronomical Observatory), R. P. Fender (Univ. of Southampton), P. Roche (LCOGTN/Cardiff Univ.), S. Clark (Open Univ.)
on 11 Mar 2009; 19:29 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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GX 339-4 is currently brightening at X-ray, UV, optical and radio frequencies (ATel #1945, #1954, #1960). Our monitoring campaign with the Faulkes Telescope South (ATel #1586) gives the following recent magnitudes (light curves are below; errors are ~ 0.02 mag):

2009-02-20 (MJD 54882.65): i' ~ 16.66 (and varying by ~ 0.2 mag in 10 minutes)
2009-02-21 (MJD 54883.76): V ~ 16.94, R ~ 16.46, i' ~ 16.68
2009-03-06 (MJD 54896.76): V ~ 16.37, R ~ 15.94, i' ~ 15.50
2009-03-07 (MJD 54897.76): V ~ 16.31, R ~ 15.89, i' ~ 15.65

The source brightened by 0.6, 0.5 and 1.2 mag in V, R and i' respectively in 13 days between 2009-02-21 and 2009-03-06. The V-band magnitude on 2009-03-07 differs from that measured 3 hours previously by UVOT on Swift by just 0.2 +- 0.1 mag (ATel #1954). The V-i' colour is becoming redder as the flux rises, suggesting the jet flux is rising faster than the irradiated disc flux (which presumably dominates the UVOT bands) at these frequencies, as was the case in the 2002 outburst (Homan et al. 2005, ApJ, 624, 295).

On 2009-02-22 we observed GX 339-4 with the HIgh speed Photo-POlarimeter (HIPPO; Potter et al. 2008, SPIE, 7014, 179) on the 1.9m telescope of the South African Astronomical Observatory (SAAO). Variability is present on timescales of tens of seconds and simultaneous I-band and clear (no filter) fluxes are clearly correlated. The r.m.s. variability of the flux is 9% in both I-band and clear during the 30-minute observation, which corresponds to 0.1 mag. For comparison, the magnitude varied by > 0.3 on timescales of minutes during the strong variability episode of June-September 2007. We find a linear polarization level of ~ 2% (polarized at the 3 sigma level; no filter) and no evidence for variability in the polarization. This is consistent with that measured in 2005 when the source was ~ 2.4 mag fainter and is likely due to interstellar dust (Russell & Fender 2008, MNRAS, 387, 713). In I-band the polarization is constrained to < 7% (3 sigma upper limit). If the jet indeed dominates the i'-band flux and if the jet synchrotron emission is optically thin, these low polarization levels are not consistent with a highly aligned magnetic field near the base of the jet.

We will continue to monitor the source as this outburst progresses. If a state transition is made, the optical/IR colour should become bluer as the X-ray spectrum softens, due to the jet fading (Homan et al. 2005).

DMR acknowledges Stephen Potter for help with the HIPPO data reduction. 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.

Faulkes and SAAO 1.9m/HIPPO light curves of GX 339-4