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The Nature of GRB 980425 and the Search for Off-Axis Gamma-Ray Burst Signatures in Nearby Type Ib/c Supernova Emission

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Eli Waxman1

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The identification of Type Ib/c supernovae (SNe Ib/c) as gamma-ray burst (GRB) progenitors is motivated by the association of GRB 980425 with SN 1998bw and of GRB 030329 with SN 2003dh. While the gamma-ray luminosity of GRB 030329 was typical to cosmological GRBs, the luminosity of the nearby (40 Mpc) GRB 980425 was ~5 orders of magnitude lower. The large luminosity difference is commonly explained by hypothesizing either that SNe Ib/c produce two different classes of GRBs or that GRB 980425 was a typical cosmological GRB jet viewed off-axis. In the latter scenario, strong radio emission, Lν ~ 1030νimg1.gif ergs s-1 Hz-1, is expected at ~1 yr delay as a result of jet deceleration to subrelativistic speed, as observed from GRB 970508. The radio luminosity of SN 1998bw was 3 orders of magnitude lower than this value. We show that the low radio flux may be consistent with the off-axis jet interpretation, if the density of the wind surrounding the progenitor is lower than typically expected, img2.gif ≡ (img3.gif/10-5 Msun yr-1)/(vw/103 km s-1) simeq 0.1 instead of img2.gif gtrsim 1. The lower value of img2.gif is consistent with the observed radio emission from the SN shock driven into the wind. This interpretation predicts transition to subrelativistic expansion at ~10 yr delay, with current ≈1 mJy 10 GHz flux and mV ≈ 23 optical flux, and with ≈10 mas angular source size. It also implies that in order to search for the signature of off-axis GRBs associated with nearby SNe Ib/c, follow-up observations should be carried out on a multiyear timescale.


Subject headings

gamma rays: bursts; gamma rays: theory; radio continuum: general; supernovae: general


Dates

Issue 2 (2004 February 20)

Received 2003 October 14, accepted for publication 2003 November 7



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