X-ray, UV, and optical observations of supernova 2006bp with Swift: Detection of early X-ray emission

We present results on the X-ray and optical/UV emission from the Type H-P supernova (SN) 2006bp and the interaction of the SN shock with its environment, obtained with the X-Ray Telescope (XRT) and UV/Optical Telescope (UVOT) on board Swift. SN 2006bp is detected in X-rays at a 4.5 a level of significance in the merged XRT data from days 1 to 12 after the explosion. If the 0.2-10 keV band X-ray luminosity of L_0.2-10 = (1.8 ±0.4) x 10³⁹ ergs s^-1 is caused by interaction of the SN shock with circumstellar material (CSM) deposited by a stellar wind from the progenitor's companion star, a mass-loss rate of M ≈(1 × 10^-5 M_⊙ yr ^-1)(v_w/10 km s^-1) is inferred. The mass-loss rate is consistent with the nondetection in the radio with the VLA on days 2, 9, and 11 after the explosion and is characteristic of a red supergiant progenitor with a mass of ≈12-15 M⊙ prior to the explosion. The Swift data further show a fading of the X-ray emission starting around day 12 after the explosion. In combination with a follow-up XMM-Newton observation obtained on day 21 after the explosion, an X-ray rate of decline of L_xα t^-n with index n = 1.2 ±0.6 is inferred. Since no other SN has been detected in X-rays prior to the optical peak, and since Type II-P SNe have an extended "plateau" phase in the optical, we discuss the scenario that the X-rays might be due to inverse Compton scattering of photospheric optical photons offrelativistic electrons produced in circumstellar shocks. However, due to the high required value of the Lorentz factor (≈10-100), which is inconsistent with the ejecta velocity inferred from optical line widths, we conclude that inverse Compton scattering is an unlikely explanation for the observed X-ray emission.