Optical evolution of Swift J174510.8-262411 suggests the compact jet is fading, radio flare imminent?

ATel #4456; D. M. Russell (IAC, Tenerife), F. Lewis (Univ. of Glamorgan, Faulkes Telescope Project), C. G. Mundell (LJMU), A. Tripp (Faulkes Telescope Project), T. Belloni (INAF-OAB, Italy), P. Curran (ICRAR - Curtin), H. Krimm (NASA-GSFC), D. Maitra (Univ. of Michigan), J. C. A. Miller-Jones (ICRAR - Curtin), G. R. Sivakoff (U. Alberta)
on 6 Oct 2012; 10:27 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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We report on optical monitoring observations of the outburst of the black hole candidate X-ray binary Swift J174510.8-262411 (Sw J1725-26; e.g. GCN #13744; ATel #4383; #4393; #4394; #4450) with the Faulkes Telescope South (FTS; located at Siding Spring, Australia) in SDSS i'-band, Bessel R and Bessel V filters. The first FTS images were acquired just 19 minutes after the original Swift BAT trigger. The source was a suspected GRB initially, and several short (10 - 30 sec) exposures were taken under a GRB (afterglow) follow-up program. Images in the same filter were combined. A source was detected at the coordinates of the optical/infrared and radio counterparts (ATel #4380; #4394; the optical association was confirmed with the detection of broad H-alpha emission; ATel #4388), with the following magnitudes (using field stars in the USNO and NOMAD catalogues to calibrate the flux) on 2012-09-16 (starting at 09:34 UT) = MJD 56186.40: V = 19.59 +- 0.44; R = 18.53 +- 0.05; i' = 17.22 +- 0.07. There may be systematic errors in these magnitudes by up to a few tenths of a mag due to the typical inaccuracies of the catalogue field star magnitudes.

We subsequently observed the source on three further dates; 2012-09-21, 2012-09-25 and 2012-10-04 (MJD 56191.4, 56195.4 and 56204.5) with exposures of 100 - 200 sec per filter. The source brightened between 2012-09-16 and 2012-09-21, peaking at V = 19.36 +- 0.18; R = 18.20 +- 0.09; i' = 16.84 +- 0.04 on the latter date, before fading slightly since. Our most recent magnitudes, on 2012-10-04 were R = 18.31 +- 0.04; i' = 17.07 +- 0.04. The optical peak flux occurred around the time of the peak hard X-ray (Swift BAT and INTEGRAL IBIS) flux (ATel #4436, #4450). At this time, the radio flux was also very bright (ATel #4394, #4410) and the X-ray spectrum was starting to soften away from the low/hard state (ATel #4401, #4436, #4450).

The R-i' colour became slightly redder when it brightened, which is not expected for thermal emission, but has been reported when synchrotron emission from the jet makes a relatively stronger contribution near the peak of the low/hard state (e.g. Russell et al. 2011). The compact jet has been confirmed as a bright, flat spectrum radio source (ATel #4394, #4410). Since then the R-i' colour has evolved, becoming bluer by ~0.15 mag in the last 10 days. This is expected as the jet normally starts to fade at optical/infrared frequencies at this stage of the outburst (e.g. Homan et al. 2005; Coriat et al. 2009; Russell et al. 2007; 2011; Buxton et al. 2012; Dincer et al. 2012), before the radio. The disc may now be dominating the optical emission. Assuming the source continues to soften, we may expect the compact jet to be quenched and relativistic jet ejection(s) to be launched in the near future, as also suggested by the X-ray evolution (ATel #4450).

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.