X-RAY EMISSION FROM SUPERNOVAE IN DENSE CIRCUMSTELLAR MATTER ENVIRONMENTS: A SEARCH FOR COLLISIONLESS SHOCKS

(Abridged). The optical light curve of some SNe may be powered by the outward\ndiffusion of the energy deposited by the explosion shock in optically thick\ncircumstellar matter (CSM). Recently, it was shown that the radiation-mediated\nand -dominated shock in an optically thick wind must transform into a\ncollisionless shock and can produce hard X-rays. The X-rays are expected to\npeak at late times, relative to maximum visible light. Here we report on a\nsearch, using Swift and Chandra, for X-ray emission from 28 SNe that belong to\nclasses whose progenitors are suspected to be embedded in dense CSM\n(IIn/Ibn/SLSN-I). Two SNe in our sample have X-ray properties that are roughly\nconsistent with the expectation for X-rays from a collisionless shock in\noptically thick CSM. Therefore, we suggest that their optical light curves are\npowered by shock breakout in CSM. We show that two other events were too X-ray\nbright during the SN maximum optical light to be explained by the shock\nbreakout model. We conclude that the light curves of some, but not all,\ntype-IIn/Ibn SNe are powered by shock breakout in CSM. For the rest of the SNe\nin our sample, including all the SLSN-I events, our X-ray limits are not deep\nenough and were typically obtained at too early times to conclude about their\nnature. We argue that the optical light curves of SNe, for which the X-ray\nemission peaks at late times, are likely powered by the diffusion of shock\nenergy from a dense CSM. We comment about the possibility to detect some of\nthese events in radio.\n