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Spectropolarimetry of SN2011fe (PTF11kly) in M101 at Early Times

ATel #3612; Paul Smith, Nathan Smith, Peter Milne, and Grant Williams (Steward Observatory, U. Arizona) and Buell Jannuzi (NOAO)
on 31 Aug 2011; 00:41 UT
Credential Certification: Nathan Smith (nathans@as.arizona.edu)

Subjects: Optical, Supernovae, Transient

We have obtained spectropolarimetry of the young Type Ia supernova SN2011fe (PTF11kly) using the SPOL spectropolarimeter mounted on the Bok 90-inch telescope at Kitt Peak. The data were obtained on the night of August 29 UT, covering the wavelength range 4000--7550 A with roughly 20 A resolution. The measured degree of polarization is wavelength-dependent. The red continuum has a polarization of approximately 0.2%, while higher degrees of polarization of 0.2%-0.3% occur in parts of the spectrum also exhibiting broad absorption troughs, including Si II 6355. The observed wavelength dependence of polarization and its position angle - particularly regions of the spectrum associated with broad spectral features - ensure that the observed polarized radiation is intrinsic to SN2011fe and not interstellar polarization within M101 or an instrumental residual. The polarization position angle changes significantly; at blue wavelengths between 4675 and 4875 A the position angle is about 145 degrees, but beyond 6000 A the position angle is approximately 55 degrees. Two unpolarized standards were observed to allow removal of any instrumental signatures on the measured spectrum of the polarized radiation, and two field stars were observed to correct for interstellar polarization.