DISCOVERY OF THE VERY RED NEAR-INFRARED AND OPTICAL AFTERGLOW OF THE SHORT-DURATION GRB 070724A
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
- Department of Astronomy, University of California, Berkeley, CA 94720 (United States)
- Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802 (United States)
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 iJHK{sub s} observations starting 2.3 hr after the burst with K{sub s} = 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 K{sub s} band between 2.8 and 3.7 hr at a 4sigma significance level. The optical/near-IR spectral index, beta{sub O,NIR} approx -2, is much redder than expected in the standard afterglow model, pointing to either significant dust extinction, A {sup host} {sub V} approx 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 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 (mini-SN), leading to an estimated ejected mass of M approx 10{sup -4} M {sub sun} and a radioactive energy release efficiency of f approx 5 x 10{sup -3} (for v approx 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 (approx2'' 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.
- OSTI ID:
- 21367391
- Journal Information:
- Astrophysical Journal, Vol. 704, Issue 1; Other Information: DOI: 10.1088/0004-637X/704/1/877; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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