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The radio luminous SN 2007bg: evidence for circumstellar interaction

ATEL # 2066; Alicia M. Soderberg (Harvard-Smithsonian CfA)
on 31 May 2009; 23:19 UT
Distributed as an Instant Email Notice (Supernova)
Password Certification: Alicia M. Soderberg (ams@astro.caltech.edu)

Subjects: Radio, Gamma-Ray Bursts, Nova, Supernova

Through our on-going Very Large Array program dedicated to the study of radio emission from Type Ibc supernovae, we have obtained extensive multi-frequency observations of SN2007bg throughout the two years since its optical discovery (Quimby et al. CBET #927). Over this timescale, the SN shows detectable radio emission with a luminosity somewhat higher than that of other radio luminous SNe, including the Type Ibc SN 2003L (Soderberg et al. 2005) and the Type IIn SN1988Z (Williams et al. 2002).

The overall temporal and spectral evolution of the radio emission is complex, characterized by a late-time pronounced transition from optically-thin to optically-thick (self-absorbed) emission between 1.4 and 22 GHz, associated with an increase in brightness. This behavior can be naturally explained by the interaction of the SN blastwave with a significant circumstellar density enhancement, leading to an associated increase in the self-absorption frequency and the brightness. Similar behavior has been observed in other radio SNe (Ibc, IIL, IIn, IIb), inferred to have density-induced flux modulations (SN1979C: Weiler et al. 1992; 2001ig: Ryder et al. 2004; 2001em: Stockdale et al. 2004, Chugai & Chevalier 2006; and SN2003bg: Soderberg et al. 2006), and this is not unexpected from massive star progenitors.

We further note that each of these radio SNe also showed spectroscopic evidence for Hydrogen features, including even the Type Ibc SNe 2001em (Soderberg, Gal-Yam, & Kulkarni, GCN #2586) and 2003bg (Hamuy, Phillips, & Thomas-Osip, IAUC # 8088 ). Along this line, we note that SN2007bg also shows evidence for broad H-alpha in its photospheric spectra (Harutyunyan et al. CBET #948).

We therefore stress that neither radio luminosity nor brightness modulations alone can be used as reliable proxies for an off-axis GRB jet, as has been proposed for SN2001em (Granot & Ramirez 2004) and most recently for SN2007bg (Prieto, Watson & Stanek, ATEL #2065). In the former case Very Long Baseline Interferometry (VLBI) observations clearly ruled out the presence of an off-axis GRB jet (Stockdale et al. 2005; Bietenholz & Bartel 2005; Bietenholz & Bartel 2007; Schinzel et al. 2009). Likewise, we have recently obtained VLBI observations of SN2007bg (PI Soderberg) to determine the projected size of the blastwave (analysis of these data is in progress).

To summarize, the preponderance of existing observations of SN2007bg can be simply explained as the result of circumstellar interaction with an irregular progenitor wind; an off-axis GRB jet is not required at present.


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