SN 2005ip: A LUMINOUS TYPE IIn SUPERNOVA EMERGING FROM A DENSE CIRCUMSTELLAR MEDIUM AS REVEALED BY X-RAY OBSERVATIONS

We report on the X-ray spectral evolution of the nearby Type IIn supernova (SN) 2005ip based on Chandra and Swift observations covering similar to 1-6 yr after explosion. X-ray spectra in all epochs are well fitted by a thermal emission model with kT greater than or similar to 7 keV. The somewhat high temperature suggests that the X-ray emission mainly arises from the circumstellar medium (CSM) heated by the forward shock. We find that the spectra taken two to three years after the explosion are heavily absorbed (N-H similar to 5 x 10(22) cm(-2)), but the absorption gradually decreases to the level of the Galactic absorption (N-H similar to 4 x 10(20) cm(-2)) at the final epoch. This indicates that the SN went off in a dense CSM and that the forward shock has overtaken it. The intrinsic X-ray luminosity stays constant until the final epoch, when it drops by a factor of similar to 2. The intrinsic 0.2-10 keV luminosity during the plateau phase is measured to be similar to 1.5 x 10(41) erg s(-1), ranking SN 2005ip as one of the brightest X-ray SNe. Based on the column density, we derive a lower limit of a mass-loss rate to be (M) over dot similar to 1.5 x 10(-2) (V-w/100 km s(-1)) M-circle dot yr(-1), which roughly agrees with that inferred from the X-ray luminosity, (M) over dot similar to 2 x 10(-2) (V-w/100 km s(-1)) M-circle dot yr(-1), where V-w is the circumstellar wind speed. Such a high mass-loss rate suggests that the progenitor star had eruptive mass ejections similar to a luminous blue variable star. The total mass ejected in the eruptive period is estimated to be similar to 15 M-circle dot, indicating that the progenitor mass is greater than or similar to 25 M-circle dot.