ATCA radio observations of MAXI J1543-564

ATel #3364; J. C.A. Miller-Jones (ICRAR - Curtin), A. K. Tzioumis (ATNF), P. G. Jonker (SRON). G. R. Sivakoff (U. Alberta), T. J. Maccarone (U. Southampton) & G. Nelemans (U. Nijmegen)
on 19 May 2011; 08:57 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)

Tweet

The new X-ray transient MAXI J1543-564 (ATel #3330) was recently identified as a black hole candidate X-ray binary in transition from a hard to a soft X-ray state (ATel #3341).

Following its identification as a black hole candidate, we observed MAXI J1543-564 with the Australia Telescope Compact Array (ATCA) on 2011 May 14 (16:51-18:02 UT; MJD 55695.73). We used the Compact Array Broadband Backend (CABB) to observe simultaneously at central frequencies of 5.5 and 9.0 GHz, with an observing bandwidth of 2048 MHz at each frequency. We detected a radio source at both frequencies at a position consistent with the Swift localization (ATel #3331) to within the 1-sigma error bars. Our best radio position (measured at 9.0 GHz) is:
RA (J2000) = 15h 43m 17.180s +/- 0.049s
Dec (J2000) = -56d 24' 49.611" +/- 0.775"

The measured flux densities were 0.38+/-0.09 mJy at 5.5 GHz and 0.27+/-0.08 mJy at 9.0 GHz. The spectral index, alpha (where S_nu propto nu^alpha), is poorly constrained at -0.68+/-0.79, so we cannot discriminate between steep and flat spectrum radio emission.

X-ray timing analyses (ATel #3341, ATel #3355) show that the source made a transition from the hard intermediate state to a soft state between MJD 55694.092 and 55696.65. Model fitting the X-ray spectra of Swift X-ray data (ATel #3362) showed that the disk fraction on MJD 55695.1 was 45.8%, although a power-law component was still required for the spectral fits until after MJD 55696.7. The significant and rising disc fraction by the date of our radio observations suggest that the source was en route from the hard intermediate to the soft state, and possibly in the soft intermediate state. This is consistent with the faint radio emission and possible optically thin spectrum, likely corresponding to the variable, quenched radio emission typically seen prior to a radio flare (e.g. Fender, Belloni & Gallo, 2004, MNRAS, 355, 1105), rather than emission from a steady, compact, flat-spectrum core jet. Now that the transition to the soft state has occurred, it is likely that a radio flare may be underway. Further radio observations are strongly encouraged.

We thank ATNF and Phil Edwards for rapid scheduling of these observations.