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In these proceedings we present a systematic and statistical comparison of 26 well sampled optical/UV light curves with their X-ray counterparts. We use three statistical methods to compare their behaviour and to determine the average properties of the sample and the properties of the individual GRBs. We show that the X-ray and optical/UV light curves are remarkably different during the first 500 s after the BAT trigger, while they become more similar during the middle phase of the afterglow, i.e. between 2000 s and 20000 s. From 500 s onwards, we find that the average properties of the optical/UV and X-ray light curves are consistent with the forward-shock model assuming a constant density external medium. However, the properties of the individual GRBs indicate that some of the individual afterglows are more complex and require different or additional assumptions, such as the presence of energy injection. We also discuss the implications of a number of GRBs in our sample having chromatic breaks and whether these breaks can be accommodated by the forward-shock model. [ABSTRACT FROM AUTHOR]

The brightest Ultra-Luminous X-ray source HLX-1 in the galaxy ESO 243–49 currently provides strong evidence for the existence of intermediate mass black holes. Here we present the latest multi-wavelength results on this intriguing source in X-ray, UV and radio bands. We have refined the X-ray position to sub-arcsecond accuracy. We also report the detection of UV emission that could indicate ongoing star formation in the region around HLX-1. The lack of detectable radio emission at the X-ray position strengthens the argument against a background AGN. [ABSTRACT FROM AUTHOR]

In these proceedings we describe how we measured the mean GRB host galaxy dust extinction law using a sample of 21 GRBs, and present our results. This GRB extinction law not only provides a significantly improved model with which to fit the SEDs of other GRBs, but also holds information on the grain properties of the dust in the GRB surrounding environment. [ABSTRACT FROM AUTHOR]

An increasing sample of Swift GRBs show evidence of ‘chromatic breaks’, i.e. breaks that are present in the X-ray but not in the optical. Modelling the X-ray and optical data, we find that in a significant fraction of these GRB afterglows the component producing the X-ray emission cannot be responsible for the optical emission too. We propose that these afterglow lightcurves are the result of a two-component jet, in which both components are energy-injected for the whole observation and the X-ray break is due to a jet break in the narrow outflow. Bursts with chromatic breaks provide us with an explanation for another surprising finding, the paucity of late achromatic breaks. We show that our model may explain the behaviour of GRB emission in both X-ray and optical bands. Therefore, it can be a radical and noteworthy alternative to the current interpretation for the ‘canonical’ XRT and UVOT lightcurves, bearing fundamental implications for GRB physics. [ABSTRACT FROM AUTHOR]

In these proceedings we present and examine 26 GRB optical afterglows. We discuss the early afterglow, examine the luminosity lightcurves and compare the UVOT canonical model with the XRT canonical model. [ABSTRACT FROM AUTHOR]

The Swift Ultra-Violet/Optical Telescope has observed ∼200 gamma-ray bursts (GRBs) in its first two-and-a-half years of operation. From this collection of observations we have obtained ∼40 well sampled light curves. Using this dataset we present general optical/UV properties of GRBs, including filter dependent temporal slopes and color-color relationships. We also show that correlations exist between both the 11 hour X-ray flux (0.3–10 keV) and the prompt gamma-ray fluence (15–150 keV) and the v-band magnitude at 2000 s. [ABSTRACT FROM AUTHOR]

There is an increasing sample of Swift GRBs that manifest evidence of chromatic breaks, i.e. breaks that are present in the X-ray but absent in the optical. Modelling the X-ray and optical data, we find that, in a significant fraction of these GRB afterglows, the component producing the X-ray emission cannot be the same component responsible for the optical emission. We propose that these afterglow lightcurves are produced by a two-component jet model, in which both components are energy-injected for the whole observation and the X-ray break is a jet break in the narrow outflow. This study has important consequences for the GRB community, since bursts with chromatic breaks provide us with an explanation for another surprising finding, the paucity of achromatic breaks at late times. We show that our model can explain the behaviour of both X-ray and optical band emissions and can therefore be a noteworthy and radical alternative to the current interpretation for the canonical X-ray lightcurve, with fundamental implications for GRB collimation. [ABSTRACT FROM AUTHOR]

GRB 061007 had the brightest optical afterglow observed by UVOT so far and a highly unusual afterglow light curve. It had an usually smooth and steep panchromatic temporal decay, which, from the start of the Swift observations, decayed as a power with a slope of α = 1.65±0.02 in the X-ray and UV/optical bands. If we interpret the data in terms of a spherical blastwave, the energy budget that is implied is enormous, challenging the possible models for the progenitors. The alternative explanation is that with GRB 061007 we are witnessing the most collimated outflow ever observed. If this is correct, if implies the occurrence of a jet break within 80 s of the prompt emission. This event has therefore remarkable consequences for the GRB communicate, since it may imply that jet breaks can occur much earlier in the afterglow light curves than hitherto expected. [ABSTRACT FROM AUTHOR]

The prompt emission of gamma-ray bursts (GRBs) is generally thought to be produced by internal shocks within the relativistic outflow, while the longer-wavelength afterglow is produced by external shocks resulting from the interaction between the outflow and external medium. This picture has gained support from the clear separation between the prompt and afterglow phases seen in most Swift GRBs. However, no separation is seen in GRB 070318. The prompt X-ray emission extrapolated from the observed Swift BAT flux matches well the early X-ray afterglow emission in both temporal and spectral properties, and the combined 0.3–10 keV light curve can be fit with a single power-law with an index typical of late-time afterglows. The UV-optical light curve displays a similar power-law decay after the broad maximum. In addition, the gamma-ray emission displays a single, FRED(fast rise, exponential decay)-like peak and unusually smooth light curve, consistent with a single impulsive event interacting with the external medium. [ABSTRACT FROM AUTHOR]

An understanding of GRB host galaxy properties is pivotal to determining the progenitor stars, and is critical in identifying the effect of the GRB local environment on our observations. The imprint left by dust and gas absorption on GRB X-ray and optical afterglows provides an effective probe to the immediate surroundings, and for this well-sampled, multi-wavelength afterglow observations are imperative. Swift's capabilities to obtain simultaneous X-ray and UV/optical data make it ideal to study the dust and gas content in the local environment of GRBs. In these proceedings we further the work from [1], and present the results of analysis on the combined Swift and ground-based spectra of 24 GRB afterglows, which is the largest sample of GRB afterglow spectral energy distributions thus far studied. [ABSTRACT FROM AUTHOR]

Swift very fast follow up observations of Gamma-Ray Bursts have allowed us to discover a new feature of GRB afterglows: a phase of shallow decay, usually attributed to energy injection in the burst ejecta, which can last for several thousands of seconds both in the X-ray and optical bands. Here I shall discuss this phase in the cases of Swift GRBs 050401, 050801 and 050802. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present observations of GRB 060108, the 100th burst detected by Swift. The optical afterglow was found to be below UVOT detection limits within the first 100 s, and the ratio of X-ray and R band measurements at ∼ 1000 s after the trigger gives a spectral index βOX = 0.45, which makes this burst “dark” according to the Jakobbson classification. We derive a maximum photometric redshift of z<3.2, which excludes a high redshift as a possible cause of the observed darkness, and we find that the most likely explanation is a combination of intrinsic faintness, a hard optical to X-ray spectrum and dust extinction. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]