SIMBAD references

Swift/XRT has been performing regular monitoring observations of the black-hole candidate binary system MAXI J1543-564 (Negoro et al., ATEL #3330) since May 8th, 2010 (Kennea et al., ATEL #3331). The source was seen to go into the soft state, where the spectrum is dominated by thermal emission from the disk, around May 15th, 2011 (e.g. Kennea et al., ATEL #3362, Munoz-Darias et al., ATEL #3355).

The source remained in the soft state, and flux has been slowly dropping, with the spectrum characterized by a disk blackbody model that has been cooling (from ∼1 keV to ∼0.6 keV), with the absorption corrected 0.5-10 keV flux decreasing from peak of ∼1.6 x 10^-9 erg/s/cm² on May 15th, 2011 to the current value of ∼9 x 10^-11 erg/s/cm^-2 measured on Sept 26th, 2011. The most recent observations have shown that from September 23rd, 2011 the hardness ratio (4-10 keV count rate/ 0.5-4 keV count rate) has started to increase, signaling that the source is returning to the hard state.

Examining the combined spectra of an observations taken by Swift/XRT of MAXI J1543-564 taken on September 25th-26th, 2011, we find that the spectrum is well fit by an absorbed power-law model with the following parameters:

N_H = 1.4 ± 0.13 x 10²² cm<sup-2</sup>

Photon Index = 2.69 (+0.13/-0.13)

Flux (0.5-10 keV) = 8.3 ± 0.2 x 10^-11 erg/s/cm^-2

Although a disk component is not required to fit the spectrum, if we add this component to the fit, in the 0.5-10 keV range the disk fraction is ∼ 5%. By the standard definition (e.g. Remillard and McClintock, 2006), this is consistent with MAXI J1543-564 transitioning into the hard state (disk fraction < 20%).

MAXI J1543-564 also appears to be increasing in brightness in the BAT 15-50 keV band. The rate on Sep. 26, 2011 is 0.0027 =/- 0.0015 ct/s/cm² (∼12 mCrab). The source had been undetectable in the BAT since approximately July 12, 2011.

Monitoring of this source by Swift is continuing.

BAT Transient Monitor light curve of MAXI J1543-564: http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/MAXIJ1543-564