En-Tzu Lin(NTHU); Albert K.H. Kong(NTHU)
On August 17, 2017, a binary neutron star merging event triggered a detected gravitational wave signal known as GW170817 accompanied by electromagnetic emissions. On-going monitoring to this source has revealed that the emissions are across the whole electromagnetic spectrum.The X-ray counterpart of GW170817 was discovered by the Chandra X-ray Observatory at 9 days post-merger. In our work, we used the archival data released by Chandra to perform spectral analysis. Instead of using the conventional way of spectral fitting with chi-square minimization, we employed Bayesian statistics to deal with the low signal-to-noise data. With an absorbed power-law spectral model of neutral hydrogen column density fixed at a Galactic value of 7.5 x 1020 cm-2, the photon index is ~1.6 between 15 days and 164 days post-merger, consistent with the results from other teams. However, the first data set observed at 9 days after merger showed a harder spectrum with a photon index of ~1 in our analysis. This may provide the first evidence of a possible spectral change during the early stage of the X-ray emission.