Context. Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to separate the various possible emission processes and scenarios proposed to interpret the ...complex gamma-ray burst phenomenology. Aims. We collected a large dataset on GRB 060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow. Methods. Data from Swift-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/near-infrared and millimeter telescopes allowed us to follow the afterglow evolution after about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions. Results. The prompt emission of GRB 060908 was characterised by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterised by a rather flat optical/near-infrared spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution. Conclusions. GRB 060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios, showing that a satisfactorily modelling of this event is challenging. In the future, similar efforts will enable us to obtain optical/near-infrared coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research.
The blazar PKS 0537-441 has been observed with all instruments of the Swift satellite between the end of 2004 and 2005 November. The BAT monitored it recurrently for a total of 2.7 Ms, and the XRT ...and UVOT pointed it on seven occasions for a total of 67 k. The automatic optical and near-infrared telescope REM has monitored simultaneously the source at all times. In 2005 January-February, PKS 0537-441 was detected at its brightest in optical and X-rays: more than a factor of 2 brighter in X-rays and about a factor of 60 brighter in the optical than observed in 2004 December. The 2005 July observation recorded a fainter X-ray state, albeit still brighter than the historical average. The simultaneous optical state is comparable to the one recorded in early 2005 January, before the outburst. In 2005 November, the source subsided both in X-rays and optical to a quiescent state. The optical and X-ray variations are well correlated, with no measurable time lag larger than about 1 month. On intraday timoscales there is no obvious correlation between X-ray and optical variations, but the former tend to be more pronounced, opposite to what is observed on monthly timescales. The widely different amplitude of the long-term variability in optical and X-rays is very unusual. The spectral energy distributions are interpreted in terms of the synchrotron and inverse Compton mechanisms within a jet where the plasma radiates via internal shocks and the dissipation depends on the distance of the emitting region from the central engine.
Context.Spectral variability is the main tool for constraining emission models of BL Lac objects. Aims.By means of systematic observations of the BL Lac prototype PKS 2155-304 in the infrared-optical ...band, we explore variability on scales of months, days, and hours. Methods.We made our observations with the robotic 60 cm telescope REM located at La Silla, Chile, and VRIJHK filters were used. Results.PKS 2155-304 was observed from May to December 2005. The wavelength interval explored, the total number of photometric points, and the short integration time render our photometry substantially superior to previous ones for this source. On the basis of the intensity and colour, we distinguish three different states of the source, each lasting months, which include all those described in the literature. In particular, we report the highest state ever detected in the H band. The source varied by a factor of 4 in this band, much more than in the V band (a factor ≈2). The source softened with increasing intensity, in contrast to the general pattern observed in the UV-X-ray bands. On five nights in November we had nearly continuous monitoring for 2–3 h. A variability episode on a time scale of $\tau\approx$ 24 h is well-documented, and a much more rapid flare with τ = 1–2 h, is also apparent, but is supported by relatively few points. Conclusions.The overall spectral energy distribution of PKS 2155-304 is commonly described by a synchrotron-self-Compton model. The optical infrared emission is in excess of the expectation of the model in its original formulation. This can be explained by a variation in the frequency of the synchrotron peak, which is not unprecedented in BL Lacs.
Context.Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Aims.Currently, only lower limits were securely set to the initial fireball Lorentz factor ...$\Gamma_0$. We aim to provide a direct measure of $\Gamma_0$. Methods.The early-time afterglow light curve carries information about $\Gamma_0$, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607A with the REM robotic telescope. Results.For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of $\Gamma_0\sim$ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be $R_{\rm dec} \approx$ 1017 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB.
The flat-spectrum radio quasar 3C 454.3 is well known to be a highly active and variable source with outbursts occurring across the whole electromagnetic spectrum over the last decades. In ...spring 2005, 3C 454.3 has been reported to exhibit a strong optical outburst which subsequently triggered multi-frequency observations of the source covering the radio up to γ-ray bands. Here, we present first results of our near-IR/optical (V, R, I, H band) photometry performed between May 11 and August 5, 2005 with the Rapid Eye Mount (REM) at La Silla in Chile and the Automatic Imaging Telescope (AIT) of the Perugia University Observatory. 3C 454.3 was observed during an exceptional and historical high state with a subsequent decrease in brightness over our 86 days observing period. The continuum spectral behaviour during the flaring and declining phase suggests a synchrotron peak below the near-IR band as well as a geometrical origin of the variations e.g. due to changes in the direction of forward beaming.
Context. Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to separate the various possible emission processes and scenarios proposed to interpret the ...complex gamma-ray burst phenomenology. Aims. We collected a large dataset on GRB060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow. Methods. Data from Swift-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/near-infrared and millimeter telescopes allowed us to follow the afterglow evolution after about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions. Results. The prompt emission of GRB060908 was characterised by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterised by a rather flat optical/near-infrared spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution. Conclusions. GRB060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios, showing that a satisfactorily modelling of this event is challenging. In the future, similar efforts will enable us to obtain optical/near-infrared coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research.
Context: Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to disentangle the various possible emission processes and scenarios proposed to interpret the ...complex gamma-ray burst phenomenology. Aims: We collected a large dataset on GRB060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow. Methods: Data from Swift-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/NIR and millimeter telescopes allowed us to follow the afterglow evolution from about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions. Results: The prompt emission of GRB060908 was characterized by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterized by a rather flat optical/NIR spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution. Conclusions: GRB060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios showing that a satisfactorily modeling of this event is challenging. In the future, similar efforts will enable us to obtain optical/NIR coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research.
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