Swift XRT/UVOT observations of the supersoft X-ray source candidate MAXI J0158-744
ATel #3765; K. L. Li and A. K.H. Kong (NTHU, Taiwan)
on 15 Nov 2011; 05:09 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)
Subjects: Optical, Ultra-Violet, X-ray, Transient
We report two new Swift follow-up observations of the supersoft source (SSS) candidate MAXI J0158-744 (ATel #3756, #3759). The observations were performed with the XRT on 2011-11-12 17:05:31 UTC and 2011-11-14 01:08:14 UTC for durations of ~2ks each. Both observations were taken in the Window Timing (WT) mode.
Since the detection from MAXI (ATel #3756), the SSS candidate was still bright in X-ray with high count rates of 1.36 +/- 0.03 ct/s and 0.34 +/- 0.02 ct/s (0.2-2.0 keV) in the 2011-11-12 and the 2011-11-14 WT images, respectively. We investigated the energy spectra by fitting with an absorbed blackbody model. The best-fit temperatures are 60 (+4/-6) eV and 59 (+7/-7) eV, which is slightly cooler than the temperature on 2011-11-11 (ATel #3759). The best fit column densities are NH=5.8e21 (+1.5/-0.9) cm-2 and 1.9e21 (+0.8/-0.6) cm-2. These indicate that the spectral shape is changing. The unabsorbed 0.2-2 keV luminosity varied between 7.2e39 erg/s and 8.5e37 erg/s (assuming a distance of 61.3 kpc). We caution that the luminosity is very sensitive to the high absorption and low temperature. Careful modeling is required to confirm if the source is ultraluminous. We also re-investigated the reprocessed observations taken on 2011-11-11 (ATel #3756, #3759). By examining the hardness ratio (HR) evolution of the source (Soft band: 0.2-0.5keV; Hard band: 0.5-2.0keV), we found that the HRs were fluctuating between -0.37 to +1.00 in a short time scale (~ks). Similar features also appear in the new WT data. This feature further confirms that the spectral behavior was indeed changing. Detailed spectral analysis is now in progress to understand the spectral change.
The UVOT observed MAXI J0158-744 with its six UV/optical filters in this monitoring campaign. The magnitudes (in Vega system) are as follow:
On 2011-11-12:
V: 14.88 +/- 0.05 (stat) +/- 0.01 (sys)
B: 14.87 +/- 0.04 (stat) +/- 0.02 (sys)
U: 13.44 +/- 0.05 (stat) +/- 0.02 (sys)
UVW1: 13.10 +/- 0.05 (stat) +/- 0.03 (sys)
UVM2: 13.00 +/- 0.05 (stat) +/- 0.03 (sys)
UVW2: 12.93 +/- 0.06 (stat) +/- 0.03 (sys)
On 2011-11-14:
V: 15.06 +/- 0.05 (stat) +/- 0.01 (sys)
B: 14.97 +/- 0.04 (stat) +/- 0.02 (sys)
U: 13.55 +/- 0.05 (stat) +/- 0.02 (sys)
UVW1: 13.13 +/- 0.05 (stat) +/- 0.03 (sys)
UVM2: 12.93 +/- 0.05 (stat) +/- 0.03 (sys)
UVW2: 12.90 +/- 0.06 (stat) +/- 0.03 (sys)
It is clear that the magnitudes in all UV/optical bands were dropping and the changes were decreasing with the frequencies. By comparing with the USNO B1.0 counterpart with B=16.16 mag (ATel #3759), we confirmed that it is an optical variable associated with MAXI J0158-744.
We acknowledge the use of the quick look data from the Swift public archive.
