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VLBI Observations of SN 2011dh

ATel #3641; Michael F. Bietenholz (HartRAO/York University), Andreas Brunthaler (MPIfR/NRAO), Norbert Bartel (York University), Laura Chomiuk (Harvard-Smithsonian CfA), Michael P. Rupen (NRAO), Alicia Soderberg (CfA) and Bevin Zauderer (CfA)
on 9 Sep 2011; 04:20 UT
Credential Certification: Michael Bietenholz (mbieten@yorku.ca)

Subjects: Radio, Supernovae

We report on 22-GHz VLBI observations of SN 2011dh (ATEL #3398, #3399, #3400, #3401, #3402, #3405, #3406, #3411, #3413, #3414, #3420, #3428, #3431, #3432, #3433) in M51, using the NRAO VLBA and GBT, as well as the Effelsberg telescopes. The observations were carried out on 2011 Aug. 21, and were of 13 hours total length. The weather was poor at several stations, resulting in relatively high noise.

Nonetheless, SN 2011dh was clearly detected, at a position coincident with those determined from earlier VLA and EVN observations. The peak brightness was 630 micro-Jy/beam, and the image background rms was 92 micro-Jy/beam, where the beam was 0.66 x 0.35 mas (FWHM) at p.a. -12 degs. The true peak brightness was likely somewhat higher, as some loss of phase coherence is probable.

Fitting a spherical-shell model (see e.g., Bietenholz et al. 2010, ApJ, v725, p4) directly to the visibility measurements we find an outer angular radius of 0.11 (-0.11,+0.09) mas, corresponding to a 3-sigma upper limit of 0.38 mas. At an age of 82.3 days, this implies an average expansion velocity of 19,000 (-19,000,+16000) km/s (for distance = 8.4 Mpc). It is expected that the outer angular radius of the radio emission traces the forward shock front, therefore our expansion velocity represents an estimate of that of SN 2011dh's forward shock. As such it is consistent with the expectations of the forward shock velocity obtained from broadband lightcurve fitting by Soderberg et al. 2011, ArXiv, 1107.1876.

Further VLBI observations are planned.