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Title:
The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs
Authors:
Mundell, C. G.; Melandri, A.; Guidorzi, C.; Kobayashi, S.; Steele, I. A.; Malesani, D.; Amati, L.; D'Avanzo, P.; Bersier, D. F.; Gomboc, A.; Rol, E.; Bode, M. F.; Carter, D.; Mottram, C. J.; Monfardini, A.; Smith, R. J.; Malhotra, S.; Wang, J.; Bannister, N.; O'Brien, P. T.; Tanvir, N. R.
Affiliation:
AA(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AB(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AC(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.; Dipartimento di Fisica, Universitá di Milano-Bicocca, Milan 20126, Italy.; INAF-Osservatorio Astronomico di Brera, Merate (LC) 23807, Italy.), AD(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AE(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AF(International School for Advanced Studies (SISSA-ISAS), Trieste 34014, Italy.), AG(INAF-IASF Bologna, Bologna, Italy.), AH(INAF-Osservatorio Astronomico di Brera, Merate (LC) 23807, Italy.; Università dell'Insubria, Dipartimento di Fisica e Matematica, Como I-22100, Italy.), AI(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AJ(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.; FMF, University in Ljubljana, Ljubljana 1000, Slovenia.), AK(Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK.), AL(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AM(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AN(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AO(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.; CNRS-CTBT, Grenoble 38000, France.), AP(Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.), AQ(School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404.), AR(Center for Astrophysics, University of Science and Technology of China, Hefei, Anhui 230026, China.), AS(Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK.), AT(Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK.), AU(Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK.)
Publication:
The Astrophysical Journal, Volume 660, Issue 1, pp. 489-495. (ApJ Homepage)
Publication Date:
05/2007
Origin:
UCP
ApJ Keywords:
Cosmology: Observations, Gamma Rays: Bursts, Radiation Mechanisms: Nonthermal, Shock Waves
DOI:
10.1086/512605
Bibliographic Code:
2007ApJ...660..489M

Abstract

We present a multiwavelength analysis of Swift GRB 061007. The 2 m robotic Faulkes Telescope South began observing 137 s after the onset of the γ-ray emission, when the optical counterpart was already decaying from R~10.3 mag, and continued observing for the next 5.5 hr. These observations begin during the final γ-ray flare and continue through and beyond a long, soft tail of γ-ray emission whose flux shows an underlying simple power-law decay identical to that seen at optical and X-ray wavelengths, with temporal slope α~1.7 (F~t-α). This remarkably simple decay in all of these bands is rare for Swift bursts, which often show much more complex light curves. We suggest the afterglow emission begins as early as 30-100 s and is contemporaneous with the ongoing variable prompt emission from the central engine, but originates from a physically distinct region dominated by the forward shock. The observed multiwavelength evolution of GRB 061007 is explained by an expanding fireball whose optical, X-ray, and late-time γ-ray emission is dominated by emission from a forward shock with typical synchrotron frequency, νm, that is already below the optical band as early as t=137 s and a cooling frequency, νc, above the X-ray band to at least t=105 s. In contrast, the typical frequency of the reverse shock lies in the radio band at early time. We suggest that the unexpected lack of bright optical flashes from the majority of Swift GRBs may be explained with a low νm originating from small microphysics parameters, ɛe and ɛB. Finally, the optical light curves imply a minimum jet opening angle θ=4.7deg, and no X-ray jet break before t~106 s makes GRB 061007 a secure outlier to spectral energy correlations.
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