MAXI J1543-564: Follow-up Swift observation and timing analysis

ATel #3336; J. A. Kennea (PSU), P. Esposito (INAF-OAC), G. L. Israel (INAF-OAR), N. Rea (CSIC-IEEC), P. A. Evans (UL), P. Romano (INAF-IASFPA), P. Curran (CEA-Saclay)
on 10 May 2011; 18:52 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 13:33UT on May 9th, 2011, Swift began a 5ks target of observation of MAXI J1543-564 (Negoro et al., ATEL #3330), in follow up to a previously reported upon observation by Swift (Kennea et al., ATEL #3331). The XRT observation was taken in Windowed Timing (WT) mode to obtain high resolution timing information, as well as to avoid issues of pile-up as the source is relatively bright (~6 XRT c/s).

We find that the flux and spectral parameters measured from the WT mode observation are consistent with the previous observation by XRT on May 8th (Kennea et al., ATEL #3331). The WT spectrum is well fit by an absorbed power-law model, with the N_H = 8.1 x 10²¹ cm^-2, and a photon index of 1.52 +/- 0.04. The measured WT flux is 3.8 +/- 0.1 x 10^-10 erg/s/cm² (errors are 90% confidence). The 5ks of WT mode data span 3 ~96 minute Swift orbits, and in that time the source flux is consistent with a constant level. MAXI J1543-564 therefore shows no evidence of having brightened between the two observations. We note that these data were taken starting ~12 hours before the RXTE observations reported by Altamirano et al (ATEL #3334), so later observations by Swift will be needed to confirm their reported brightening.

We performed timing analysis on the Swift XRT data and found a marginally significant (2.7-sigma) quasi-coherent signal at 0.75+/-0.03 Hz (1.35+\-0.03 s) with FWHM 0.30+\-0.12 Hz and rms 21+\-8 per cent (1-sigma uncertainties). The frequency is close to the QPO observed at 2.08 UT, May 10, 2011, by RXTE (Altamirano et al., ATel #3334). Although the detection is not significant per se, being close to the RXTE detection, we believe the signal is real. We note that there is a ~0.3 Hz frequency shift between our best peak frequency and the RXTE QPO peak, which might reflect a typical frequency shift often observed in the BH binaries QPO.

We have analyzed UVOT data taken for MAXI J1543-564, and no counterpart is detected, as expected given the expected extinction (A_V ~ 11) in the direction of the source. Based on the data taken on May 8th (segment 1) and May 9th (segment 2), we find the following upper limits at the XRT derived location of MAXI J1543-564:

 
Seg    Filter   Exp(s)   Magnitude  
---------------------------------- 
001    v      78.5       > 19.45   
001    b      78.5       > 20.44   
001    u      78.5       > 20.06   
001    uw1    157        > 20.16   
001    um2    249        > 20.15   
001    uw2    315        > 20.70   
002    um2    4901       > 22.29   

Continued monitoring of this source is planned by Swift.