Strong radio emission from SN 2007bg one year after the explosion - detection of spreading, off-axis GRB jet?

ATel #2065; J. L. Prieto, L. C. Watson, K. Z. Stanek (Ohio State University)
on 30 May 2009; 15:25 UT
Credential Certification: Jose Prieto (prieto@astronomy.ohio-state.edu)

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In their study of supernova hosts using SDSS data (Prieto, Stanek & Beacom 2008, ApJ, 673, 999), the broad-lined SN Ic 2007bg at z=0.034 (Quimby et al. 2007, #CBET 927; Harutyunyan et al. 2007, CBET #948; Soderberg & Immler 2007, ATEL #1142), was identified as a good candidate for an off-axis GRB. The likely host of SN 2007bg is an extremely low-luminosity galaxy with M_B ~ -12, one of the least-luminous supernova hosts of any kind ever observed. Since such a low-luminosity host is almost certain to be very metal poor (~1/20 solar; e.g., Lee et al. 2006, ApJ, 647, 970), this broad-line SN Ic supernova is a very good candidate for association with an off-axis GRB (e.g., Stanek et al. 2006, AcA, 56, 333). Given that, Prieto, Stanek & Beacom (2007, GCN #6618) encouraged follow-up observations of SN 2007bg to look for an off-axis GRB jet.

Following up on their suggestion, we have analyzed some of the publicly available radio data obtained at the VLA at several epochs in several bands (Proposal #AS0887; PI A. Soderberg), and have detected a very strong radio source at the position of SN 2007bg:

  
UT               | days after SN disc. | nu (GHz) | flux (mJy) | flux_error (mJy) | Luminosity density (erg/s/Hz)  
Apr. 19.2, 2007            3.1             8.46        0.15           0.04                   3.6e27  
Aug. 18.9, 2007          124.7             8.46        0.58           0.04                   1.4e28  
Feb. 25.2, 2008          315.0             8.46        1.77           0.10                   4.4e28  

This makes SN 2007bg one of the strongest radio supernovae ever observed, in fact >4 times more luminous at approx. 1 year after explosion compared with SN 2003L (Soderberg et al. 2005, ApJ, 621, 908) and SN 2003bg (Soderberg et al. 2006, ApJ, 651, 1005), which were among the most luminous SN Ib/c observed in radio (see more details and figures at the link below). In fact, such strong radio emission a year after the explosion makes SN 2007bg an even better candidate for an off-axis GRB (e.g., Paczynski 2001, AcA, 51, 1), given that the only other broad-line SNe Ic ever observed in low metallicity enviroments have been associated with GRBs (Stanek et al. 2006, AcA, 56, 333; Modjaz et al. 2008, AJ, 135, 1136).

Further follow-up observations at all wavelengths are greatly encouraged.

Webpage with circular and plots of SN 2007bg