A hard-to-soft state transition is going on in neutron star transient EXO 1745-248
ATel #7430; Zhen Yan, Jie Lin, Wenfei Yu, Wenda Zhang, Hui Zhang (Shanghai Astronomical Observatory)
on 22 Apr 2015; 07:16 UT
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)
Subjects: X-ray, Binary, Neutron Star
A new outburst has been detected in the globular cluster Terzan 5 (ATel #7240), which was identified as an outburst of the neutron star transient EXO 1745-248 (ATel #7242, ATel #7262). The X-ray count rate from Swift/BAT (15-50 keV) has started to decrease since MJD 57129 (2015-04-17). A sudden count rate decrease by a factor of six is seen in the period between MJD 57131 (2015-04-19) and 57133 (2015-04-21), while the hardness ratio between 15-50 keV and 2-10 keV decreased by a factor of 10.
Our detailed analysis of the 0.5-10 keV X-ray spectrum from Swift/XRT on 2015-04-13 21:13:59 UT shows the spectrum can be fit well with a powerlaw model with an index of 0.94+-0.06, which is consistent with that at the beginning of this outburst (ATel #7242). The fractional root mean square (RMS) in 0.1-10 Hz was 0.37+-0.02. But the X-ray spectrum on 2015-04-20 05:16:59 UT can only be fit well by including a blackbody component. Using a blackbody plus a powerlaw model, we obtained the temperature of the blackbody was about 1.4+-0.1 keV, and the powerlaw index was about 1.6+-0.1. Using a disk black body plus black body model, we obtained the temperatures as 1.3+-0.2 and 2.2+-0.2 for the disk black body and black body, respectively. The fractional RMS in 0.1-10 Hz was 0.09+-0.01, significantly lower than that measured a week ago. The corresponding unabsorbed X-ray luminosity (0.5-10 keV) was about 8e37 ergs/s at a distance of 5.9 kpc (Valenti et al. 2007), which reached nearly 50% of Eddington luminosity for a 1.4 solar mass neutron star. We conclude that this source is probably on the way to transit to the soft state.
