SN 2007bg

J. L. Prieto, L. C. Watson & K. Z. Stanek (Ohio State University)

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
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 ) 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) | 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

http://www.astronomy.ohio-state.edu/~prieto/sn2007bg/

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.

J. L. Prieto, K. Z. Stanek & J. F. Beacom (Ohio State) report:

In a study of supernova hosts (Prieto, Stanek & Beacom 2007,
arXiv:0707.0690v1 [astro-ph]), the recent broad-lined SN Ic 2007bg at
z=0.034 (Quimby et al. 2007, CBET 927; IAUC 8834;  Harutyunyan et al.
2007, CBET 948), was identified as a good candidate for an off-axis GRB.

The supernova, discovered on April 16 2007 (CBET 927, IAUC 8834), is
located at 1.6 arcsec North and 4.0 arcsec East of a source identified in
the SDSS-DR6 catalog, SDSS J114925.74+514920.2, which has model magnitudes
g=21.82 +/- 0.07, r=21.12 +/- 0.07 (uncorrected for Galactic extinction;
Ag=0.08, Ar=0.06) and estimated photometric redshift between 0.3 and 0.4.
There is a fainter source at ~2" from the position of the supernova
clearly detected in the SDSS g-band image, and which is not identified in
the SDSS-DR6 catalog.  This fainter source is most likely the host galaxy
of SN 2007bg (CBET 948). A 1x1 arcmin SDSS g-band image around the
position of the supernova can be found here:

http://www.astronomy.ohio-state.edu/~prieto/sn07bg_host.gif

We used Sextractor to measure a difference of 1.7 mag in g-band between
the likely host and SDSS J114925.74+514920.2. At the redshift of the
supernova, z=0.034 (CBET 927), this makes the likely host an extremely
low-luminosity galaxy with M_B ~ -12, one of the least-luminous supernova
hosts ever observed. Such a low-luminosity host is certain to be very
metal poor (~1/20 solar; Lee et al. 2006, ApJ, 647, 970L), and therefore
this supernova is a very good candidate for association with an off-axis
GRB (e.g., Stanek et al. 2006, AcA, 56, 333). We encourage follow-up
observations to look for an off-axis GRB jet.

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