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تسجيل مستخدم جديدEvidence for Supernova-Synthesised Dust from the Rising Afterglow of GRB 071025 at z~5
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We present observations and analysis of the broadband afterglow of Swift GRB 071025. Using optical and infrared (RIYJHK) photometry, we derive a photometric redshift of 4.4 < z < 5.2; at this redshift our simultaneous multicolour observations begin at ~30 s after the GRB trigger in the host frame and during the initial rising phase of the afterglow. We associate the light curve peak at 580 s in the observer frame with the formation of the forward shock, giving an estimate of the initial Lorentz factor Gamma_0 ~ 200. The red spectral energy distribution (even in regions not affected by the Lyman-alpha break) provides secure evidence of a large dust column. However, the inferred extinction curve shows a prominent flat component between 2000-3000 Angstroms in the rest-frame, inconsistent with any locally observed template but well-fit by models of dust formed by supernovae. Time-dependent fits to the extinction profile reveal no evidence of dust destruction and limit the decrease in the extinction column to Delta A_3000 < 0.54 mag after t = 50 s in the rest frame. Our observations provide evidence of a transition in dust properties at z~5, in agreement with studies of high-z quasars, and suggest that SN-formed dust continues to dominate the opacity of typical galaxies at this redshift.
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We present broadband observations and analysis of Swift gamma-ray burst (GRB) 120119A. Our early-time afterglow detections began under 15 s after the burst in the host frame (redshift z = 1.73), and they yield constraints on the burst energetics and
local environment. Late-time afterglow observations of the burst show evidence for a moderate column of dust (A_V ~ 1.1 mag) similar to, but statistically distinct from, dust seen along Small Magellanic Cloud sightlines. Deep late-time observations reveal a dusty, rapidly star-forming host galaxy. Most notably, our early-time observations exhibit a significant red-to-blue colour change in the first ~200 s after the trigger at levels heretofore unseen in GRB afterglows. This colour change, which is coincident with the final phases of the prompt emission, is a hallmark prediction of the photodestruction of dust in GRB afterglows. We test whether dust-destruction signatures are significantly distinct from other sources of colour change, namely a change in the intrinsic spectral index {beta}. We find that a time-varying power-law spectrum alone cannot adequately describe the observed colour change, and allowing for dust destruction (via a time-varying A_V) significantly improves the fit. While not definitively ruling out other possibilities, this event provides the best support yet for the direct detection of dust destruction in the local environment of a GRB.
We present optical photometry of the GRB 060912A afterglow obtained with ground-based telescopes, from about 100 sec after the GRB trigger till about 0.3 day later, supplemented with the Swift optical afterglow data released in its official website.
The optical light curve (LC) displays a smooth single power-law decay throughout the observed epochs, with a power-law index of about -1 and no significant color evolution. This is in contrast to the X-ray LC which has a plateau phase between two normal power-law decays of a respective index of about -1 and -1.2. It is shown by our combined X-ray and optical data analysis that this asynchronous behavior is difficult to be reconciled with the standard afterglow theory and energy injection hypothesis. We also construct an optical-to-X-ray spectral energy distribution at about 700 sec after the GRB trigger. It displays a significant flux depression in the B-band, reminding us of the possibility of a host-galaxy (at z=0.937) 2175-A dust absorption similar to the one that characterizes the Milky Way extinction law. Such an identification, although being tentative, may be confirmed by our detailed analysis using both template extinction laws and the afterglow theory. So far the feature is reported in very few GRB afterglows. Most seem to have a host galaxy either unusually bright for a GRB, just like this one, or of an early type, supporting the general suggestion of an anti-correlation between the feature and star-forming activities.
Context: GRB afterglows are excellent probes of gas and dust in star-forming galaxies at all epochs. It has been posited that dust in the early Universe must be different from dust at lower z. To date two reports directly support this contention, one
of which is based on the spectral shape of GRB 050904 at z = 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We address the claimed evidence for unusual (SN-origin) dust in its host galaxy by simultaneously examining the X-ray and optical/NIR spectrophotometric data. Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4 days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn photometric data, the spectroscopic and z-band photometric data at ~3 days from the Subaru telescope, as well as the critical UKIRT Z-band photometry at 0.47 days, upon which the claim of dust detection largely relies. Results: We find no evidence of dust extinction in the SED. We compute flux densities at lambda_rest = 1250 AA directly from the observed counts at all epochs. In the earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3 +- 0.2 mag) than previously reported and statistically insignificant (<1.5 sigma). Furthermore we find that the photometry of this band is unstable and difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that there is no evidence for dust extinction -- the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type extinction curve. We conclude that there is no evidence of any extinction in the afterglow of GRB 050904 and that the presence of SN-origin dust in the host of GRB 050904 must be viewed skeptically. [abridged]
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 t
he 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.
We present a suite of 34 high-resolution cosmological zoom-in simulations consisting of thousands of halos up to M_halo~10^12 M_sun (M_star~10^10.5 M_sun) at z>=5 from the Feedback in Realistic Environments project. We post-process our simulations wi
th a three-dimensional Monte Carlo dust radiative transfer code to study dust extinction, dust emission, and dust temperature within these simulated z>=5 galaxies. Our sample forms a tight correlation between infrared excess (IRX=F_IR/F_UV) and ultraviolet (UV)-continuum slope (beta_UV), despite the patchy, clumpy dust geometry shown in our simulations. We find that the IRX-beta_UV relation is mainly determined by the shape of the extinction curve and is independent of its normalization (set by the dust-to-gas ratio). The bolometric IR luminosity (L_IR) correlates with the intrinsic UV luminosity and the star formation rate (SFR) averaged over the past 10 Myr. We predict that at a given L_IR, the peak wavelength of the dust spectral energy distributions for z>=5 galaxies is smaller by a factor of 2 (due to higher dust temperatures on average) than at z=0. The higher dust temperatures are driven by higher specific SFRs and SFR surface densities with increasing redshift. We derive the galaxy UV luminosity functions (LFs) at z=5-10 from our simulations and confirm that a heavy attenuation is required to reproduce the observed bright-end UVLFs. We also predict the IRLFs and UV luminosity densities at z=5-10. We discuss the implications of our results on current and future observations probing dust attenuation and emission in z>=5 galaxies.
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