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Register a new userUVES/VLT high resolution spectroscopy of GRB 050730 afterglow: probing the features of the GRB environment
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We analyze high resolution spectroscopic observations of the optical afterglow of GRB050730, obtained with UVES@VLT about hours after the GRB trigger. The spectrum shows that the ISM of the GRB host galaxy at z = 3.967 is complex, with at least five components contributing to the main absorption system. We detect strong CII*, SiII*, OI* and FeII* fine structure absorption lines associated to the second and third component. For the first three components we derive information on the relative distance from the site of the GRB explosion. Component 1, which has the highest redshift, does not present any fine structure nor low ionization lines; it only shows very high ionization features, such as CIV and OVI, suggesting that this component is very close to the GRB site. From the analysis of low and high ionization lines and fine structure lines, we find evidences that the distance of component 2 from the site of the GRB explosion is 10-100 times smaller than that of component 3. We evaluated the mean metallicity of the z=3.967 system obtaining values about 0.01 of the solar metallicity or less. However, this should not be taken as representative of the circumburst medium, since the main contribution to the hydrogen column density comes from the outer regions of the galaxy while that of the other elements presumably comes from the ISM closer to the GRB site. Furthermore, difficulties in evaluating dust depletion correction can modify significantly these values. The mean [C/Fe] ratio agrees well with that expected by single star-formation event models. Interestingly the [C/Fe] of component 2 is smaller than that of component 3, in agreement with GRB dust destruction scenarios, if component 2 is closer than component 3 to the GRB site.
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GRB 050730 is a long duration high-redshift burst (z=3.967) discovered by Swift. The afterglow shows variability and is well monitored over a wide wavelength range. We present comprehensive temporal and spectral analysis of the afterglow of GRB 050730 including observations from the millimeter to X-rays. We use multi-wavelength afterglow data to understand the temporal and spectral decay properties with superimposed variability of this high redshift burst. Five telescopes were used to study the decaying afterglow of GRB 050730 in the B, V, r, R, i, I, J and K photometric pass bands. A spectral energy distribution was constructed at 2.9 hours post-burst in the K, J, I, R, V and B bands. X-ray data from the satellites Swift and XMM-Newton were used to study the afterglow evolution at higher energies. The early afterglow shows variability at early times and shows a steepening at ~0.1 days (8.6 ks) in the B, V, r, R, i, I, J and K passbands. The early afterglow light curve decayed with alpha_1 = -0.60+/-0.07 and alpha_2 = -1.71+/-0.06 based on R and I band data. A millimeter detection of the afterglow around 3 days after the burst shows an excess in comparison to predictions. The early X-ray light curve observed by Swift is complex and contains flares. At late times the X-ray light curve can be fit by a powerlaw alpha_x = -2.5+/-0.15 which is steeper than the optical light curve. A spectral energy distribution (SED) was constructed at ~2.9 hours after the burst. An electron energy index, p, of ~ 2.3 was calculated using the SED and the photon index from the X-ray afterglow spectra and indicates that the synchrotron cooling frequency nu_c is above observed frequencies.
We present early WHT ISIS optical spectroscopy of the afterglow of gamma-ray burst GRB 050730. The spectrum shows a DLA system with the highest measured hydrogen column to date: N(HI) = 22.1 +/- 0.1 at the third-highest GRB redshift z = 3.968. Our analysis of the Swift XRT X-ray observations of the early afterglow show X-ray flares accompanied by decreasing X-ray absorption. From both the optical and the X-ray spectra we constrain the dust and gas properties of the host galaxy. We find the host to be a low metallicity galaxy, with low dust content. Much of the X-ray absorbing gas is situated close to the GRB, whilst the HI absorption causing the DLA is most likely located further out.
We report on the photometric, spectroscopic and polarimetric, monitoring of the optical afterglow of Gamma-Ray Burst (GRB) 030328 detected by HETE-2. Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07 and a break at t_b = 0.48 +/- 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle theta_{jet} of about 3.2 degrees. An achromatic bump starting around 0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-alpha Absorber. The optical V-band afterglow appears polarized, with P= (2.4 +/- 0.6) % and theta = (170 +/- 7) degrees, suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope beta_{opt} = 0.47 +/- 0.15, and an X-ray slope beta_{X} = 1.0 +/- 0.2. The discussion of these results in the context of the fireball model shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium.
The Swift Gamma-Ray Burst Explorer, launched on 2004 November 20, is a multiwavelength, autonomous, rapid-slewing observatory for gamma-ray burst (GRB) astronomy. On 2004 December 23, during the activation phase of the mission, the Swift X-Ray Telescope (XRT) was pointed at a burst discovered earlier that day by the Swift Burst Alert Telescope. A fading, uncataloged X-ray source was discovered by the XRT and was observed over a period of about 3 hours, beginning 4.6 hours after the burst. The X-ray detection triggered a VLT observation of the optical/NIR counterpart, located about 1.1 arcseconds from the XRT position. The X-ray counterpart faded rapidly, with a power law index of -1.72 +/- 0.20. The average unabsorbed X-ray flux 4.6-7.9 hours after the burst was 6.5 x 10^{-12} erg cm^{-2} s^{-1} in the 0.5-10 keV band, for a power-law spectrum of photon index 2.02 +/- 0.13 with Galactic absorption. The NIR counterpart was observed at three epochs between 16 and 87 hours after the burst, and faded with a power-law index of -1.14 +/- 0.08 with a reddening-corrected SED power-law slope of -0.40 +/- 0.03. We find that the X-ray and NIR data are consistent with a two-component jet in a wind medium, with an early jet break in the narrow component and an underlying electron index of 1.8-2.0.
We present the observations of the afterglow of gamma-ray burst GRB 090102. Optical data taken by the TAROT, REM, GROND, together with publicly available data from Palomar, IAC and NOT telescopes, and X-ray data taken by the XRT instrument on board the Swift spacecraft were used. This event features an unusual light curve. In X-rays, it presents a constant decrease with no hint of temporal break from 0.005 to 6 days after the burst. In the optical, the light curve presents a flattening after 1 ks. Before this break, the optical light curve is steeper than that of the X-ray. In the optical, no further break is observed up to 10 days after the burst. We failed to explain these observations in light of the standard fireball model. Several other models, including the cannonball model were investigated. The explanation of the broad band data by any model requires some fine tuning when taking into account both optical and X-ray bands.
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