MAXI J1543-564: Swift/XRT confirmation of transition to soft state
ATel #3362; J. A. Kennea (PSU), A. P. Beardmore, P. A. Evans (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), P. Curran (CEA-Saclay), K. Yamaoka (AGU) and H. Negoro (Nihon U.)
on 18 May 2011; 17:49 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Transient
Swift has been performing approximately daily monitoring of the BHC transient MAXI J1543-564 (e.g. Negoro et al., ATEL #3300, Kennea et al., ATEL #3331, Munoz-Darias ATEL #3341). The source has recently been reported to have entered the soft state (Munoz-Darias, ATEL #3355). Here we report on the X-ray spectral and flux evolution of MAXI J1543-564 during the initial monitoring of the source since detection.
Swift observations began on MJD 55689.7, and between that time and MJD 55696.7 showed a steady rise from ~6 XRT ct/s to ~30 XRT ct/s. After MJD 55696.7 MAXI J1543-564 appears to have peaked, and the two most recent observations show the source count rate is dropping (~25.8 XRT ct/s on MJD 55698.6). The source also also showing significant softening, which can be likely attributed to a state transition from the hard to the soft state.
We have performed X-ray spectral fitting on each XRT observation in Windowed Timing mode taken by Swift so far. The spectra were fit by an absorbed power-law plus disk blackbody (diskbb) model to evaluate the strength of each component, as well as absorbed power-law and absorbed diskbb models to evaluate the need for both components. We have fixed absorption for all fits to NH = 2 x 1022 cm-2.
We found that for the data taken between MJD 55690.6 and MJD 55694.1 can be well fit by a simple absorbed power-law model. However, starting MJD 55695.1, the power-law fit becomes poor (reduced chi2 = 1.4) compared to a combined diskbb and powerlaw model (reduced chi2 = 0.92). After MJD 55697.7 the power-law component is no longer needed to fit the spectrum, as the disk component dominates. The results are reported in the table below. Note that for MJD 55693.6 and MJD 55694.1 we show the 2 component model fit, although for these observations the fit improvement over a simple power-law is marginal.
MJD Seg kT_diskbb Photon Index Red. Disk Model Flux
(keV) chi^2 Frac. (0.5-10 keV)
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55690.6 2 N/A 1.86 +/- 0.03 1.08 0.0% 0.7e-9 erg/s/cm^2
55692.1 3 N/A 2.27 +/- 0.04 0.99 0.0% 1.0e-9 erg/s/cm^2
55693.6 4 1.10 +/- 0.15 2.42 +/- 0.18 0.99 30.8% 1.6e-9 erg/s/cm^2
55694.1 5 1.00 +/- 0.13 2.34 +/- 0.15 0.92 37.7% 1.6e-9 erg/s/cm^2
55695.1 6 1.09 +/- 0.08 2.52 +/- 0.25 0.98 45.8% 2.0e-9 erg/s/cm^2
55696.7 7 1.27 +/- 0.07 1.88 +/- 0.93 1.01 84.1% 2.6e-9 erg/s/cm^2
55697.7 8 1.21 +/- 0.04 N/A 0.99 100% 2.5e-9 erg/s/cm^2
55698.6 9 1.23 +/- 0.02 N/A 0.88 100% 2.2e-9 erg/s/cm^2
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Note that the "Disk Frac." is the fraction of the total flux in the 0.5-10 keV band that is contributed by the disk component, "Seg" is the observation segment. Where a component is marked "N/A" it means that this component is not required for the fit. The model flux is corrected for absorption. As can be seen the Swift/XRT spectral fits confirm that the source entered the canonical soft state around MJD 55696.7, confirming the RXTE result of Munoz-Darias et al. (ATEL #
3355), and the spectrum has become completely dominated by emission from the accretion disk.
The BAT hard X-ray flux in the 15-50 keV band also showed a marked decrease over the time period when the spectrum of MAXI J1543-564 evolved from a hard to a soft state. The daily average rate dropped from 0.007 +/- 0.001 ct/s/cm2 on MJD 55697 to 0.002 +/- 0.001 ct/s/cm2 on MJD 55699.
Further monitoring observations of MAXI J1543-564 by Swift are planned.
BAT Transient Monitor page for MAXI J1543-564
