TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2586 SUBJECT: Type Ic SN2001em (off-axis GRB jet?), optical spectrum DATE: 04/05/07 16:54:40 GMT FROM: Alicia Soderberg at Caltech A. M. Soderberg, A. Gal-Yam and S. R. Kulkarni, report on behalf of a larger Caltech/Carnegie collaboration: "We report optical spectroscopy of the type Ic SN 2001em (Papenkova et al., IAUC 7722, Filippenko & Chornock, IAUC 7737) in UGC 11794 (d=84 Mpc). This supernova was recently shown to have a rising radio luminosity (Stockdale et al., IAUC 8282) and bright X-ray emission (Pooley & Lewin, IAUC 8323), both of which are atypical for a type Ic supernova at t > 2 years. An off-axis GRB jet was proposed as an explanation for this behavior (Granot & Ramirez-Ruiz, astro-ph/0403421). The optical spectrum is highly unusual for a type Ic SN, and is dominated by a broad (FWHM of 40 angstrom = 1800 km/s) H-alpha emission line. The strong nebular features of oxygen and calcium, characteristic of old (>1 year) SNe Ic, including SN 1998bw and SN 2002ap (e.g., Foley et al. 2003, PASP, 115, 1220), are not detected. The spectrum is suggestive of strong interaction with the circumstellar medium and is similar in shape to spectra of strongly interacting type IIn SNe at a similar age (e.g., SN 1998S, Pozzo et al. 2004, astro-ph/0404533). This interpretation is consistent with the report by Pooley & Lewin that the X-ray emission can be well fit with a thermal bremsstrahlung model. A dense circumstellar medium would also explain the bright radio emission, as was proposed by Soderberg, Frail & Wieringa (2004, ApJ, 607, L13). A comparison between our spectrum of SN 2001em and those of type IIn SN 1994Y (age 180 day; Filippenko 1997, ARA&A, 35, 309) and the type Ic SN 1998bw (age 376 days; Patat et al. 2001, ApJ, 555, 900), taken from the data base presented by Poznanski et al. (2002, PASP, 114, 833) and Gal-Yam et al. (2004, astro-ph/0403296) is posted at: http://www.astro.caltech.edu/~ams/SN2001em.gif " This message may be cited.