TY - JOUR
T1 - Discovery of the very red near-infrared and optical afterglow of the short-duration GRB 070724A
AU - Berger, E.
AU - Cenko, S. B.
AU - Fox, D. B.
AU - Cucchiara, A.
PY - 2009
Y1 - 2009
N2 - We report the discovery of the near-infrared and optical afterglow of the short-duration gamma-ray burst GRB070724A. 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 yr 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 near-IR 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 GRB070724A 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 (mini-SN), 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.
AB - We report the discovery of the near-infrared and optical afterglow of the short-duration gamma-ray burst GRB070724A. 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 yr 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 near-IR 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 GRB070724A 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 (mini-SN), 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.
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U2 - 10.1088/0004-637X/704/1/877
DO - 10.1088/0004-637X/704/1/877
M3 - Article
AN - SCOPUS:70549111243
SN - 0004-637X
VL - 704
SP - 877
EP - 882
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
ER -