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Register a new userA Large Catalogue of Multi-wavelength GRB Afterglows I: Color Evolution And Its Physical Implication
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The spectrum of gamma-ray burst (GRB) afterglows can be studied with color indices. Here we present a large comprehensive catalogue of 70 GRBs with multi-wavelength optical transient data on which we perform a systematic study to find the temporal evolution of color indices. We categorize them into two samples based on how well the color indices are evaluated. The Golden sample includes 25 bursts mostly observed by GROND, and the Silver sample includes 45 bursts observed by other telescopes. For the Golden sample, we find that 95% of the color indices do not vary over time. However, the color indices do vary during short periods in most bursts. The observed variations are consistent with effects of (i) the cooling frequency crossing the studied energy bands in a wind medium (43%) and in a constant density medium (30%), (ii) early dust extinction (12%), (iii) transition from reverse shock to forward shock emission (5%), or (iv) an emergent supernova emission (10%). We also study the evolutionary properties of the mean color indices for different emission episodes. We find that 86% of the color indices in the 70 bursts show constancy between consecutive ones. The color index variations occur mainly during the late GRB-SN bump, the flare and early reversed-shock emission components. We further perform a statistical analysis of various observational properties and model parameters (spectral index $beta_{o}^{rm CI}$, electron spectral indices $p^{rm CI}$, etc.) using color indices. Overall, we conclude that $sim$ 90% of colors are constant in time and can be accounted for by the simplest external forward shock model, while the varying color indices call for more detailed modeling.
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GRB 100418A is a long burst at z=0.624 without detection of any associated supernova (SN). Its lightcurves in both the prompt and afterglow phases are similar to GRB 060614, a nearby long GRB without an associated SN. We analyze the observational data of this event and discuss the possible origins of its multi-wavelength emission. We show that its joint lightcurve at 1 keV derived from Swift BAT and XRT observations is composed of two distinguished components. The first component, whose spectrum is extremely soft (Gamma = 4.32), ends with a steep decay segment, indicating the internal origin of this component. The second component is a slowly-rising, broad bump which peaks at ~10^5 seconds post the BAT trigger. Assuming that the late bump is due to onset of the afterglow, we derive the initial Lorentz factor (Gamma_0) of the GRB fireball and find that it significantly deviates from the relation between the Gamma_0 and Eiso of typical GRBs. We also check whether it follows the same anti-correlation between X-ray luminosity and the break time observed in the shallow decay phase of many typical GRBs, which is usually regarded as a signal of late energy injection from the GRB central engine. However, we find that it does not obey this correlation. We propose that the late bump could be contributed by a two-component jet. We fit the second component with an off-axis jet model for a constant medium density and find the late bump can be represented by the model. The derived jet half-opening angle is 0.30 rad and the viewing angle is 0.315 rad. The medium density is 0.05 cm^-3, possibly suggesting that it may be from a merger of compact stars. The similarity between GRBs 060614 and 100418A may indicate that the two GRBs are from the same population and the late bump observed in the two GRBs may be a signal of a two-component jet powered by the GRB central engine.
We present the second Swift Ultra-Violet/Optical Telescope (UVOT) gamma-ray burst (GRB) afterglow catalog, greatly expanding on the first Swift UVOT GRB afterglow catalog. The second catalog is constructed from a database containing over 120,000 independent UVOT observations of 538 GRBs first detected by Swift, the High Energy Transient Explorer 2 (HETE2), the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), the Interplanetary Network (IPN), Fermi, and Astro-rivelatore Gamma a Immagini Leggero (AGILE). The catalog covers GRBs discovered from 2005 Jan 17 to 2010 Dec 25. Using photometric information in three UV bands, three optical bands, and a `white or open filter, the data are optimally co-added to maximize the number of detections and normalized to one band to provide a detailed light curve. The catalog provides positional, temporal, and photometric information for each burst, as well as Swift Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) GRB parameters. Temporal slopes are provided for each UVOT filter. The temporal slope per filter of almost half the GRBs are fit with a single power-law, but one to three breaks are required in the remaining bursts. Morphological comparisons with the X-ray reveal that approximately 75% of the UVOT light curves are similar to one of the four morphologies identified by Evans et al. (2009). The remaining approximately 25% have a newly identified morphology. For many bursts, redshift and extinction corrected UV/optical spectral slopes are also provided at 2000, 20,000, and 200,000 seconds.
We report on follow-up observations of 20 short-duration gamma-ray bursts performed in grizJHKs with the seven-channel imager GROND between mid-2007 and the end of 2010. This is one of the most comprehensive data sets on GRB afterglow observations of short bursts published so far. In three cases GROND was on target within less than 10 min after the trigger, leading to the discovery of the afterglow of GRB 081226A and its faint underlying host galaxy. In addition, GROND was able to image the optical afterglow and follow the light-curve evolution in further five cases, GRBs 090305, 090426, 090510, 090927, and 100117A. In all other cases optical/NIR upper limits can be provided on the afterglow magnitudes.
We investigate the afterglow of GRB 140713A, a gamma-ray burst (GRB) that was detected and relatively well-sampled at X-ray and radio wavelengths, but was not present at optical and near-infrared wavelengths, despite searches to deep limits. We present the emission spectrum of the likely host galaxy at $z = 0.935$ ruling out a high-redshift explanation for the absence of the optical flux detection. Modelling the GRB multi-wavelength afterglow using the radiative transfer hydrodynamics code BOXFIT provides constraints on physical parameters of the GRB jet and its environment, for instance a relatively wide jet opening angle and an electron energy distribution slope $p$ below 2. Most importantly, the model predicts an optical flux about two orders of magnitude above the observed limits. We calculated that the required host extinction to explain the observed limits in the $r$, $i$ and $z$ bands was $A^{rm host}_{V} > 3.2$ mag, equivalent to $E(B-V)^{rm host} > 1.0$ mag. From the X-ray absorption we derive that the GRB host extinction is $A^{rm host}_{rm V} = 11.6^{+7.5}_{-5.3}$ mag, equivalent to $E(B-V)^{rm host} = 3.7^{+2.4}_{-1.7}$ mag, which is consistent with the extinction required from our BOXFIT derived fluxes. We conclude that the origin of the optical darkness is a high level of extinction in the line of sight to the GRB, most likely within the GRB host galaxy.
GRB 080810 was one of the first bursts to trigger both Swift and the Fermi Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and UV/optical bands by Swift, in the optical by ROTSE and a host of other telescopes and was detected in the radio by the VLA. The redshift of z= 3.355 +/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100 s after the trigger; alternatively, this spectral evolution could be identified with the blackbody temperature of a quasithermal model shifting from ~60 keV to ~3 keV over the same time interval. The first optical detection was made at 38 s, but the smooth, featureless profile of the full optical coverage implies that this originated from the afterglow component, not the pulsed/flaring prompt emission. Broadband optical and X-ray coverage of the afterglow at the start of the final X-ray decay (~8 ks) reveals a spectral break between the optical and X-ray bands in the range 10^15 - 2x10^16 Hz. The decay profiles of the X-ray and optical bands show that this break initially migrates blueward to this frequency and then subsequently drifts redward to below the optical band by ~3x10^5 s. GRB 080810 was very energetic, with an isotropic energy output for the prompt component of 3x10^53 erg and 1.6x10^52 erg for the afterglow; there is no evidence for a jet break in the afterglow up to six days following the burst.
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