Swift detects MAXI J1543-564 transition into hard state
ATel #3662; J. A. Kennea (PSU), S. Motta (INAF/Brera), P. Curran (CEA-Saclay), P. A. Evans (U Leicester). H. A. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), K. Yamaoka (AGU) and H. Negoro (Nihon U.)
on 26 Sep 2011; 17:40 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Transient
Referred to by ATel #: 3664
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:
NH = 1.4 +/- 0.13 x 1022 cm
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
