Candidate optical counterparts of MAXI J1543-564

ATel #3359; D. M. Russell (Univ. of Amsterdam), F. Lewis, P. Roche (Univ. of Glamorgan, Faulkes Telescope Project, Open Univ.), D. Altamirano (Univ. of Amsterdam)
on 18 May 2011; 11:56 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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We report on optical observations of the field of the black hole candidate X-ray binary MAXI J1543-564 (ATel #3330, #3331, #3334, #3336, #3341, #3355) with the 2-m Faulkes Telescope South (located at Siding Spring, Australia). Three images were acquired in SDSS i'-band on 2011 May 11th, 14th and 15th (MJD 55692.585, 55695.757 and 55696.752). Exposures were 200 seconds on each date. Preliminary flux calibration of the images was achieved using three i'-band observations of the Landolt standard star field RU149 on 2011 May 15 (six stars in this field were used). Conditions during this night were photometric. The airmass of the target and standard were 1.45 and 1.57, respectively.

One faint star is detected within the 1.8" Swift error circle reported in ATel #3331. Below is a link to our finding chart. The star is not variable between the three dates within errors (~0.1 mag), and we measure i' = 19.1 +- 0.2 for this star. There may be systematic errors of a few tenths of a magnitude because these estimates are calculated from one standard star field. Variability measurements are more precise (relative errors are much smaller). Over the 4.2 days covered by our observations, the X-ray count rate (3 - 20 keV) increased by a factor ~ 2 - 3 (ATel #3355), so we may expect changes in the optical flux from the accretion disc. The i' ~ 19.1 mag star is therefore probably not the optical counterpart to the X-ray binary, but further observations as the outburst progresses are required to confirm this.

Several stars are detected within a few arcsec of the 1.8" error circle, but none within ~ 5" are variable between the three dates, within errors. The brighter stars are also seen in DSS images. The non-detection of MAXI J1543-564 in optical is consistent with a large absorption column towards the source. N_H ~ (8.1 - 9.0) x 10^21 cm^-2 was measured (ATel #3331, #3336), which corresponds (Predehl & Schmitt 1995, A&A, 293, 889) to a visual extinction of A_V ~ 4.5 - 5.0. We note that no source was detected in optical or UV with Swift UVOT, to a limiting magnitude of v > 19.45 (ATel #3336). We encourage near-infrared observations, which suffer less from extinction, and multi-wavelength observations of this black hole candidate in the coming months.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0712.2751). The Faulkes Telescope South is maintained and operated by Las Cumbres Observatory Global Telescope Network. FL acknowledges support from the Dill Faulkes Educational Trust.

MAXI J1543-564 finding chart