[en] We present a previously unused method for the detection of flares in gamma-ray burst (GRB) light curves and use this method to detect flares in the ultraviolet/optical. The algorithm makes use of the Bayesian Information Criterion to analyze the residuals of the fitted light curve, removing all major features, and to determine the statistically best fit to the data by iteratively adding additional ''breaks'' to the light curve. These additional breaks represent the individual components of the detected flares: T_start, T_stop, and T_peak. We present the detection of 119 unique flaring periods detected by applying this algorithm to light curves taken from the Second Swift Ultraviolet/Optical Telescope (UVOT) GRB Afterglow Catalog. We analyzed 201 UVOT GRB light curves and found episodes of flaring in 68 of the light curves. For those light curves with flares, we find an average number of ∼2 flares per GRB. Flaring is generally restricted to the first 1000 s of the afterglow, but can be observed and detected beyond 10⁵ s. More than 80% of the flares detected are short in duration with Δt/t of <0.5. Flares were observed with flux ratios relative to the underlying light curve of between 0.04 and 55.42. Many of the strongest flares were also seen at greater than 1000 s after the burst

[en] We present a correlation between the average temporal decay (${\mathrm{\alpha <!--\; ??\; -->}}_{\mathrm{X},\mathrm{a}\mathrm{v}\mathrm{g},><!--\; >\; -->200\mathrm{s}}$) and early-time luminosity ($L_{\mathrm{X},200\mathrm{s}}$) of X-ray afterglows of gamma-ray bursts as observed by the Swift X-ray Telescope. Both quantities are measured relative to a rest-frame time of 200 s after the γ-ray trigger. The luminosity–average decay correlation does not depend on specific temporal behavior and contains one scale-independent quantity minimizing the role of selection effects. This is a complementary correlation to that discovered by Oates et al. in the optical light curves observed by the Swift Ultraviolet Optical Telescope. The correlation indicates that, on average, more luminous X-ray afterglows decay faster than less luminous ones, indicating some relative mechanism for energy dissipation. The X-ray and optical correlations are entirely consistent once corrections are applied and contamination is removed. We explore the possible biases introduced by different light-curve morphologies and observational selection effects, and how either geometrical effects or intrinsic properties of the central engine and jet could explain the observed correlation.

[en] The brightest ultra-luminous X-ray source HLX-1 in the galaxy ESO 243-49 provides strong evidence for the existence of intermediate-mass black holes (IMBHs). As the luminosity and thus the mass estimate depend on the association of HLX-1 with ESO 243-49, it is essential to confirm its affiliation. This requires follow-up investigations at wavelengths other than X-rays, which in turn needs an improved source position. To further reinforce the IMBH identification, it is necessary to determine HLX-1's environment to establish whether it could potentially form and nourish a black hole at the observed luminosities. Using the High Resolution Camera on board Chandra, we determine a source position of R.A. = 01^h10^m28.^s3 and decl. = -46⁰04'22.''3. A conservative 95% error of 0.''3 was found following a boresight correction by cross-matching the positions of three X-ray sources in the field with the Two Micron All Sky Survey catalog. Combining all Swift UV/Optical Telescope uvw2 images, we failed to detect a UV source at the Chandra position down to a 3σ limiting magnitude of 20.25 mag. However, there is evidence that the UV emission is elongated in the direction of HLX-1. This is supported by archival data from GALEX and suggests that the far-UV emission is stronger than the near-UV. This could imply that HLX-1 may be situated near the edge of a star-forming region. Using the latest X-ray observations, we deduce the mass accretion rate of a 500 M _sun black hole with the observed luminosity and show that this is compatible with such an environment.

[en] The new and extreme population of gamma-ray bursts (GRBs) detected by the Fermi Large Area Telescope (LAT) shows several new features in high-energy gamma rays that are providing interesting and unexpected clues into GRB prompt and afterglow emission mechanisms. Over the last six years, it has been Swift that has provided the robust data set of UV/optical and X-ray afterglow observations that opened many windows into components of GRB emission structure. The relationship between the LAT-detected GRBs and the well-studied, fainter, and less energetic GRBs detected by the Swift Burst Alert Telescope is only beginning to be explored by multi-wavelength studies. We explore the large sample of GRBs detected by BAT only, BAT and the Fermi Gamma-ray Burst Monitor (GBM), and GBM and LAT, focusing on these samples separately in order to search for statistically significant differences between the populations, using only those GRBs with measured redshifts in order to physically characterize these objects. We disentangle which differences are instrumental selection effects versus intrinsic properties in order to better understand the nature of the special characteristics of the LAT bursts.

[en] We present observations of the optical afterglow of GRB 170817A, made by the Hubble Space Telescope, between 2018 February and August, up to one year after the neutron star merger GW170817. The afterglow shows a rapid decline beyond 170 days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for “failed-jet” scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with a Lorentz factor of Γ ≳ 100, forms a narrow core (∼5°) and is surrounded by a wider angular component that extends to ∼15°, which is itself relativistic (Γ ≳ 5). For a two-component model of this structure, the late-time optical decline, where F ∝ t ^−α, is α = 2.20 ± 0.18, and for a Gaussian structure the decline is α = 2.45 ± 0.23. We find the Gaussian model to be consistent with both the early ∼10 days and late ≳290 days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution, and its continued decline, indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster with a luminosity ≲4000 L _⊙ (M _F606W ≳ −4.3).

[en] 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

[en] GRB 090926A was detected by both the Gamma-ray Burst Monitor and Large Area Telescope (LAT) instruments on board the Fermi Gamma-ray Space Telescope. Swift follow-up observations began ∼13 hr after the initial trigger. The optical afterglow was detected for nearly 23 days post trigger, placing it in the long-lived category. The afterglow is of particular interest due to its brightness at late times, as well as the presence of optical flares at T0+10⁵ s and later, which may indicate late-time central engine activity. The LAT has detected a total of 16 gamma-ray bursts; nine of these bursts, including GRB 090926A, also have been observed by Swift. Of the nine Swift-observed LAT bursts, six were detected by UVOT, with five of the bursts having bright, long-lived optical afterglows. In comparison, Swift has been operating for five years and has detected nearly 500 bursts, but has only seen ∼30% of bursts with optical afterglows that live longer than 10⁵ s. We have calculated the predicted gamma-ray fluence, as would have been seen by the Burst Alert Telescope (BAT) on board Swift, of the LAT bursts to determine whether this high percentage of long-lived optical afterglows is unique, when compared to BAT-triggered bursts. We find that, with the exception of the short burst GRB 090510A, the predicted BAT fluences indicate that the LAT bursts are more energetic than 88% of all Swift bursts and also have brighter than average X-ray and optical afterglows.

[en] We present some of the earliest UV observations of a Type IIn supernova (SN)—SN 2007pk, where UV and optical observations using Swift's Ultra-Violet/Optical Telescope began 3 days after discovery or ∼5 days after shock breakout. The SN observations commence at approximately maximum light in the UV and u-band filters, suggesting that the UV light curve peaks begin very rapidly after the initial explosion, and subsequently exhibit a linear decay of 0.20, 0.21, 0.16 mag day^–1 in the UVOT uvw2, uvm2, uvw1 (λ_c = 1928, 2246, 2600 Å) filters. Meanwhile the b- and v-band light curves begin approximately seven days before v-band peak and exhibit a shallow rise followed by a subsequent decay. A series of optical/near-IR spectra taken with the Hobby-Eberly Telescope at days 3-26 after discovery show spectra similar to that of the peculiar Type IIn 1998S. The emission from 2007pk falls below detection ∼20 days after discovery in the UV and 50 days in the optical, showing no sign of the long duration emission seen in other Type IIn SNe. We examine the physical and spectral characteristics of 2007pk and compare its UV light curve and decay rate with other Type II SNe.

[en] GRB 100418A is a long gamma-ray burst (GRB) at redshift z = 0.6235 discovered with the Swift Gamma-ray Burst Explorer with unusual optical and X-ray light curves. After an initial short-lived, rapid decline in X-rays, the optical and X-ray light curves observed with Swift are approximately flat or rising slightly out to at least ∼7 x 10³ s after the trigger, peak at ∼5 x 10⁴ s, and then follow an approximately power-law decay. Such a long optical plateau and late peaking is rarely seen in GRB afterglows. Observations with Rapid Eye Mount during a gap in the Swift coverage indicate a bright optical flare at ∼2.5 x 10⁴ s. The long plateau phase of the afterglow is interpreted using either a model with continuous injection of energy into the forward shock of the burst or a model in which the jet of the burst is viewed off-axis. In both models the isotropic kinetic energy in the late afterglow after the plateau phase is ≥10² times the 10⁵¹ erg of the prompt isotropic gamma-ray energy release. The energy injection model is favored because the off-axis jet model would require the intrinsic T₉₀ for the GRB jet viewed on-axis to be very short, ∼10 ms, and the intrinsic isotropic gamma-ray energy release and the true jet energy to be much higher than the typical values of known short GRBs. The non-detection of a jet break up to t ∼ 2 x 10⁶ s indicates a jet half-opening angle of at least ∼14⁰, and a relatively high-collimation-corrected jet energy of E_jet ≥ 10⁵² erg.

[en] We present the UV, optical, X-ray, and radio properties of the Type IIb SN 2008ax discovered in NGC 4490. The observations in the UV are one of the earliest of a Type IIb supernova (SN). On approximately day 4 after the explosion, a dramatic upturn in the u and uvw1 (λ_c = 2600 A) light curves occurred after an initial rapid decline which is attributed to adiabatic cooling after the initial shock breakout. This rapid decline and upturn is reminiscent of the Type IIb SN 1993J on day 6 after the explosion. Optical/near-IR spectra taken around the peak reveal prominent Hα, He I, and Ca II absorption lines. A fading X-ray source is also located at the position of SN 2008ax, implying an interaction of the SN shock with the surrounding circumstellar material and a mass-loss rate of the progenitor of M-dot=(9±3)x10^-6M_odotyr^-1. The unusual time evolution (14 days) of the 6 cm peak radio luminosity provides further evidence that the mass-loss rate is low. Combining the UV, optical, X-ray, and radio data with models of helium exploding stars implies the progenitor of SN 2008ax was an unmixed star in an interacting binary. Modeling of the SN light curve suggests a kinetic energy (E_k ) of 0.5 x 10⁵¹ erg, an ejecta mass (M _ej) of 2.9 M _sun, and a nickel mass (M _Ni) of 0.06 M _sun.