Discovery of the Very Red Near-Infrared and Optical Afterglow of the Short-Duration Grb 070724a

Abstract

We report the discovery of the near-infrared and optical afterglow of the short-duration gamma-ray burst GRB 070724A. The afterglow is detected in iJHKs observations starting 2.3 hr after the burst with Ks = 19.59± 0.16 mag and i = 23.79±0.07 mag, but is absent in images obtained 1.3 years later. Fading is also detected in the Ks-band between 2.8 and 3.7 hr at a 4σ significance level. The optical/near-IR spectral index, βO,NIR −2, is much redder than expected in the standard afterglow model, pointing to either significant dust extinction, A host V ≈ 2 mag, or a non-afterglow origin for the near-IR emission. The case for extinction is supported by a shallow optical to X-ray spectral index, consistent with the definition for “dark bursts”, and a normal nearIR to X-ray spectral index. Moreover, a comparison to the optical discovery magnitudes of all short GRBs with optical afterglows indicates that the near-IR counterpart of GRB 070724A is one of the brightest to date, while its observed optical emission is one of the faintest. In the context of a non-afterglow origin, the near-IR emission may be dominated by a mini-supernova, leading to an estimated ejected mass of M ∼ 10−4 M⊙ and a radioactive energy release efficiency of f ∼ 5×10−3 (for v ∼ 0.3c). However, the mini-SN model predicts a spectral peak in the UV rather than near-IR, suggesting that this is either not the correct interpretation or that the mini-SN models need to be revised. Finally, the afterglow coincides with a star forming galaxy at z = 0.457, previously identified as the host based on its coincidence with the X-ray afterglow position (∼ 2′′ radius). Our discovery of the optical/near-IR afterglow makes this association secure, and furthermore localizes the burst to the outskirts of the galaxy, with an offset of 4.8±0.1 kpc relative to the host center. At such a large offset, the possible large extinction points to a dusty environment local to the burst and rules out a halo or intergalactic origin.