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Register a new userThe earliest spectroscopy of the GRB 030329 afterglow with 6-m telescope
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The earliest BTA (SAO RAS 6-m telescope) spectroscopic observations of the GRB 030329 optical transient (OT) are presented, which almost coincide in time with the first break ($tsim 0.5$ day after the GRB) of the OT light curve. The beginning of spectral changes are seen as early as $sim 10-12$ hours after the GRB. So, the onset of the spectral changes for $t<1$ day indicates that the contribution from Type Ic supernova (SN) into the OT optical flux can be detected earlier. The properties of early spectra of GRB 030329/SN 2003dh can be consistent with a shock moving into a stellar wind formed from the pre-SN. Such a behavior (similar to that near the UV shock breakout in SNe) can be explained by the existence of a dense matter in the immediate surroundings of massive stellar GRB/SN progenitor). The urgency is emphasized of observation of early GRB/SN spectra for solving a question that is essential for understanding GRB physical mechanism: {it Do all} long-duration gamma-ray bursts are caused by (or physically connected to) {it ordinary} core-collapse supernovae? If clear association of normal/ordinary core-collapse SNe (SN Ib/c, and others SN types) and GRBs would be revealed in numbers of cases, we may have strong observational limits for gamma-ray beaming and for real energetics of the GRB sources.
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We report the earliest detection of an extremely bright optical afterglow of the gamma-ray burst (GRB) 030329 using a 30cm-telescope at Tokyo Institute of Technology (Tokyo, JAPAN). Our observation started 67 minutes after the burst, and continued for succeeding two nights until the afterglow faded below the sensitivity limit of the telescope (approximately 18 mag). Combining our data with those reported in GCN Circulars, we find that the early afterglow light curve of the first half day is described by a broken power-law (t^{- alpha}) function with indices alpha_{1} = 0.88 +/- 0.01 (0.047 < t < t_{b1} days), alpha_{2} = 1.18 +/- 0.01 (t_{b1} < t < t_{b2} days), and alpha_{3} = 1.81 +/- 0.04 (t_{b2} < t < 1.2 days), where t_{b1} ~ 0.26 days and t_{b2} ~ 0.54 days, respectively. The change of the power-law index at the first break at t ~ 0.26 days is consistent with that expected from a ``cooling-break when the cooling frequency crossed the optical band. If the interpretation is correct, the decay index before the cooling-break implies a uniform ISM environment.
We present the earliest optical imaging observations of GRB 030329 related to SN 2003dh. The burst was detected by the HETE-2 satellite at 2003 March 29, 11:37:14.67 UT. Our wide-field monitoring started 97 minutes before the trigger and the burst position was continuously observed. We found no precursor or contemporaneous flare brighter than $V=5.1$ ($V=5.5$) in 32 s (64 s) timescale between 10:00 and 13:00 UT. Follow-up time series photometries started at 12:51:39 UT (75 s after position notice through the GCN) and continued for more than 5 hours. The afterglow was $Rc= 12.35pm0.07$ at $t=74$ min after burst. Its fading between 1.2 and 6.3 hours is well characterized by a single power-law of the form $f{rm(mJy)} = (1.99pm0.02{rm (statistic)}pm0.14{rm (systematic)}) times (t/1 {rm day})^{-0.890pm 0.006 {rm (statistic)}pm 0.010 {rm (systematic)}}$ in $Rc$-band. No significant flux variation was detected and upper limits are derived as $(Delta f/f)_{rm RMS} = 3-5$% in minutes to hours timescales and $(Delta f/f)_{rm RMS} = 35-5$% in seconds to minutes timescales. Such a featureless lightcurve is explained by the smooth distribution of circumburst medium. Another explanation is that the optical band was above the synchrotron cooling frequency where emergent flux is insensitive to the ambient density contrasts. Extrapolation of the afterglow lightcurve to the burst epoch excludes the presence of an additional flare component at $t<10$ minutes as seen in GRB 990123 and GRB 021211.
The best-sampled afterglow light curves are available for GRB 030329. A distinguishing feature of this event is the obvious rebrightening at around 1.6 days after the burst. Proposed explanations for the rebrightening mainly include the two-component jet model and the refreshed shock model, although a sudden density-jump in the circumburst environment is also a potential choice. Here we re-examine the optical afterglow of GRB 030329 numerically in light of the three models. In the density-jump model, no obvious rebrightening can be produced at the jump moment. Additionally, after the density jump, the predicted flux density decreases rapidly to a level that is significantly below observations. A simple density-jump model thus can be excluded. In the two-component jet model, although the observed late afterglow (after 1.6 days) can potentially be explained as emission from the wide-component, the emergence of this emission actually is too slow and it does not manifest as a rebrightening as previously expected. The energy-injection model seems to be the most preferred choice. By engaging a sequence of energy-injection events, it provides an acceptable fit to the rebrightening at $sim 1.6$ d, as well as the whole observed light curve that extends to $sim 80$ d. Further studies on these multiple energy-injection processes may provide a valuable insight into the nature of the central engines of gamma-ray bursts.
We report 31 polarimetric observations of the afterglow of GRB 030329 with high signal-to-noise and high sampling frequency. The data imply that the afterglow magnetic field has small coherence length and is mostly random, probably generated by turbulence.
The X-ray afterglow of the Gamma-Ray Burst GRB 030329, associated to SN2003dh at z=0.1685, has been observed with XMM-Newton 258 days after the burst explosion. A source with flux of (6.2 +/- 2.3) 10^{-16} erg cm^{-2} s^{-1} (0.5-2 keV) has been detected at the GRB position. This measurement, together with a re-analysis of the previous X-ray observations, indicates a flattening of the X-ray light curve ~40 days after the burst. This is in remarkable agreement with the scenario invoking the presence of two jets with different opening angles. The wider jet should be responsible for the observed flattening due to its transition into the non-relativistic Sedov-Taylor phase.
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