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Register a new userGRB 021004: a Massive Progenitor Star Surrounded by Shells
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Added by
Bradley E. Schaefer
Publication date
2002
fields
Physics
and research's language is
English
Authors
Bradley E. Schaefer
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We present spectra of the optical transient of GRB021004 obtained with the Hobby-Eberly telescope starting 15.48, 20.31 hours, and 4.84 days after the burst and a spectrum obtained with the H. J. Smith 2.7 m Telescope starting 14.31 hours after the burst. GRB021004 is the first afterglow whose spectrum is dominated by absorption lines from high ionization species with multiple velocity components separated by up to 3000 km/s. We argue that these lines are likely to come from shells around a massive progenitor star. The high velocities and high ionizations arise from a combination of acceleration and flash-ionization by the burst photons and the wind velocity and steady ionization by the progenitor. We also analyze the broad-band spectrum and the light curve. We distinguish six components along the line of sight: (1) The z~2.293 absorption lines arise from the wind of a massive star. For a mass loss rate of ~6 x 10^{-5} solar masses per year, this component also provides the external medium to create the afterglow light. (2) A second shell produces absorption lines with a relative velocity of 560 km/s, and this is associated with the shell created by the fast massive star wind blowing a bubble in the preceding slow wind at a radial distance of order 10 pc. (3) More distant clouds within the host galaxy lie between 30-2500 pc, where they have been ionized by the burst. (4-6) The massive star wind has clumps with radii and over-densities of 0.022, 0.063, and 0.12 parsecs and 50%, 10%, and 10% respectively. The immediate progenitor of the burster could either be a WC-type Wolf-Rayet star or a highly evolved star whose original mass was just too small for it to become a WN-type Wolf-Rayet star.
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We present spectra of the afterglow of GRB 021004 taken with WHT ISIS and VLT FORS1 at three epochs spanning 0.49--6.62 days after the burst. We observe strong absorption likely coming from the host galaxy, alongside absorption in HI, SiIV and CIV with blueshifts of up to 2900 km/s from the explosion centre which we assume originates close to the progenitor. We find no significant variability of these spectral features. We investigate the origin of the outflowing material and evaluate various possible progenitor models. The most plausible explanation is that these result in the fossil stellar wind of a highly evolved Wolf-Rayet star. However, ionization from the burst itself prevents the existence of HI, SiIV and CIV close to the afterglow surface where the fast stellar wind should dominate, and large amounts of blueshifted hydrogen are not expected in a Wolf-Rayet star wind. We propose that the Wolf-Rayet star wind is enriched by a hydrogen-rich companion, and that the GRB has a structured jet geometry in which the gamma rays emerge in a small opening angle within the wider opening angle of the cone of the afterglow. This scenario is able to explain both the spectral line features and the irregular light curve of this afterglow.
We present U,B,V,R_C,and I_C photometry of the optical afterglow of the gamma-ray burst GRB 021004 taken at the Nordic Optical Telescope between approximately eight hours and 30 days after the burst. This data is combined with an analysis of the 87 ksec Chandra X-ray observations of GRB 021004 taken at a mean epoch of 33 hours after the burst to investigate the nature of this GRB. We find an intrinsic spectral slope at optical wavelengths of beta_UH = 0.39 +/- 0.12 and an X-ray slope of beta_X = 0.94 +/- 0.03. There is no evidence for colour evolution between 8.5 hours and 5.5 days after the burst. The optical decay becomes steeper approximately five days after the burst. This appears to be a gradual break due to the onset of sideways expansion in a collimated outflow. Our data suggest that the extra-galactic extinction along the line of sight to the burst is between A_V = 0.3 and A_V = 0.5 and has an extinction law similar to that of the Small Magellanic Cloud. The optical and X-ray data are consistent with a relativistic fireball with the shocked electrons being in the slow cooling regime and having an electron index of p = 1.9 +/- 0.1. The burst occurred in an ambient medium that is homogeneous on scales larger than approximately 10e18 cm but inhomogeneous on smaller scales. The mean particle density is similar to what is seen for other bursts (0.1 < n < 100 cm^{-3}). Our results support the idea that the brightening seen approximately 0.1 days was due to interaction with a clumpy ambient medium within 10^{17} and 10^{18} cm of the progenitor. The agreement between the predicted optical decay and that observed approximately ten minutes after the burst suggests that the physical mechanism controlling the observed flux approximately ten minutes is the same as the one operating at t > 0.5 days.
High resolution spectroscopy of GRB 021004 revealed a wealth of absorption lines from several intermediate ionization species. The velocity structure of the absorber is complex and material with velocity up to >3000 km/s is observed. Since only the blueshifted component is observed, the absorber is very likely to be material closely surrounding the gamma-ray burst. We use a time-dependent photoionization code to track the abundance of the ions over time. Thanks to the presence of absorption from intermediate ionization states at long times, we can estimate the location and mass of the components of the absorber. We interpret those constraints within the hypernova scenario showing that the mass loss rate of the progenitor must have been ~10^{-4} solar masses per year, suggestive of a very massive star. In addition, the wind termination shock must lie at a distance of at least 100 pc, implying a low density environment. The velocity structure of the absorber also requires clumping of the wind at those large distances.
We present polarimetric observations of the afterglow of gamma-ray burst (GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among the observations shows a 45 degree change in the position angle from 9 hours after the burst to 16 hours after the burst, and comparison with published data from later epochs even shows a 90 degree change between 9 and 89 hours after the burst. The degree of linear polarization shows a marginal change, but is also consistent with being constant in time. In the context of currently available models for changes in the polarization of GRBs, a homogeneous jet with an early break time of t_b ~ 1 day provides a good explanation of our data. The break time is a factor 2 to 6 earlier than has been found from the analysis of the optical light curve. The change in the position angle of the polarization rules out a structured jet model for the GRB.
WeBo 1 (PN G135.6+01.0), a previously unrecognized planetary nebula with a remarkable thin-ring morphology, was discovered serendipitously on Digitized Sky Survey images. The central star is found to be a late-type giant with overabundances of carbon and s-process elements. The giant is chromospherically active and photometrically variable, with a probable period of 4.7 days; this suggests that the star is spotted, and that 4.7 days is its rotation period. We propose a scenario in which one component of a binary system became an AGB star with a dense stellar wind enriched in C and s-process elements; a portion of the wind was accreted by the companion, contaminating its atmosphere and spinning up its rotation. The AGB star has now become a hot subdwarf, leaving the optical companion as a freshly contaminated barium star inside an ionized planetary nebula.
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