The optical rebrightening of GRB100814A: an interplay of forward and reverse shocks?

We present a wide dataset of -ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of GRB100814 the best

The unusual 2006 dwarf nova outburst of GK Persei

The 2006 outburst of GK Persei differed significantly at optical and ultraviolet (UV) wavelengths from typical outbursts of this object. We present multiwavelength (X-ray, UV and optical) Swift and AAVSO data, giving unprecedented broad-band coverage of the outburst, allowing us to follow the evolution of the longer-than-normal 2006 outburst across these wavelengths. In the optical and UV we see a triple-peaked morphology with maximum brightness ∼1.5 mag lower than in previous years. In contrast, the peak hard X-ray flux is the same as in previous outbursts. We resolve this dichotomy by demonstrating that the hard X-ray flux only accounts for a small fraction of the total energy liberated during accretion, and interpret the optical/UV outburst profile as arising from a series of heating and cooling waves traversing the disc, caused by its variable density profile

The dust scattering model cannot explain the shallow X-ray decay in GRB afterglows

A dust scattering model was recently proposed to explain the shallow X-ray decay (plateau) observed prevalently in Gamma-Ray Burst (GRB) early afterglows. In this model, the plateau is the scattered prompt X-ray emission by the dust located close (about 10 to a few hundred pc) to the GRB site. In this paper, we carefully investigate the model and find that the scattered emission undergoes strong spectral softening with time, due to the model's essential ingredient that harder X-ray photons have smaller scattering angle thus arrive earlier, while softer photons suffer larger angle scattering and arrive later. The model predicts a significant change, that is Δβ∼ 2–3, in the X-ray spectral index from the beginning of the plateau towards the end of the plateau, while the observed data show close to zero softening during the plateau and the plateau-to-normal transition phase. The scattering model predicts a big difference between the harder X-ray light curve and the softer X-ray light curve, i.e. the plateau in harder X-rays ends much earlier than in softer X-rays. This feature is not seen in the data. The large scattering optical depths of the dust required by the model imply strong extinction in optical, A[subscript: V]≳ 10, which contradicts current findings of A[subscript: V]= 0.1–0.7 from optical and X-ray afterglow observations. We conclude that the dust scattering model cannot explain the X-ray plateaus

ON THE ELECTRON ENERGY DISTRIBUTION INDEX OF SWIFT GAMMA-RAY BURST AFTERGLOWS

The electron energy distribution index, p, is a fundamental parameter of the synchrotron emission from a range of astronomical sources. Here we examine one such source of synchrotron emission, gamma-ray burst (GRB) afterglows observed by the Swift satellite. Within the framework of the blast wave model, we examine the constraints placed on the distribution of p by the observed X-ray spectral indices and parameterize the distribution. We find that the observed distribution of spectral indices are inconsistent with an underlying distribution of p composed of a single discrete value but consistent with a Gaussian distribution centered at p = 2.36 and having a width of 0.59. Furthermore, accepting that the underlying distribution is a Gaussian, we find that the majority (≳94%) of GRB afterglows in our sample have cooling break frequencies less than the X-ray frequency

Highly variable AGN from the XMM-Newton slew survey

We investigate the properties of a variability-selected complete sample of active galactic nuclei (AGN) in order to identify the mechanisms which cause large amplitude X-ray variability on timescales of years. Methods. A complete sample of 24 sources was constructed, from AGN which changed their soft X-ray luminosity by more than one order of magnitude over 5-20 years between ROSAT observations and the XMM-Newton slew survey. Follow-up observations were obtained with the Swift satellite. We analysed the spectra of these AGN at the Swift and XMM observation epochs, where six sources had continued to display extreme variability. Multiwavelength data are used to calculate black hole masses and the relative X-ray brightness αOX. Results. After removal of two probable spurious sources, we find that the sample has global properties which differ little from a non-varying control sample drawn from the wider XMM-slew/ROSAT/Veron sample of all secure AGN detections. A wide range of AGN types are represented in the varying sample. The black hole mass distributions for the varying and non-varying sample are not significantly different. This suggests that long timescale variability is not strongly affected by black hole mass. There is marginal evidence that the variable sources have a lower redshift (2σ) and X-ray luminosity (1.7σ). Apart from two radio-loud sources, the sample sources have normal optical-X-ray ratios (αOX) when at their peak but are X-ray weak during their lowest flux measurements. Conclusions. Drawing on our results and other studies, we are able to identify a variety of variability mechanisms at play: tidal disruption events, jet activity, changes in absorption, thermal emission from the inner accretion disc, and variable accretion disc reflection. Little evidence for strong absorption is seen in the majority of the sample and single-component absorption can be excluded as the mechanism for most sources

Swift/X-ray Telescope monitoring of the candidate supergiant fast X-ray transient IGR J16418-4532

We report on the Swift monitoring of the candidate supergiant fast X-ray transient (SFXT) IGR J16418−4532, for which both orbital and spin periods are known (∼3.7 d and ∼1250 s, respectively). Our observations, for a total of ∼43 ks, span over three orbital periods and represent the most intense and complete sampling of the light curve of this source with a sensitive X-ray instrument. With this unique set of observations, we can address the nature of this transient. By applying the clumpy wind model for blue supergiants to the observed X-ray light curve, and assuming a circular orbit, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from ∼5 × 10[superscript: 16] to 10[superscript: 21] g. Our data suggest, based on the X-ray behaviour, that this is an intermediate SFXT

Four Swift searches for transient sources of high-energy neutrinos

We present results of the first four Swift satellite follow-up campaigns seeking to identify transient or variable X-ray or UV/optical sources that might be associated with individual candidate high-energy cosmic muon neutrinos detected by the IceCube Neutrino Observatory. Real-time public alerts providing coordinates and arrival times of likely-cosmic neutrinos have been provided by IceCube, via the Astrophysical Multimessenger Observatory Network, since April 2016. Subsequent Swift X-ray observations of four likely-cosmic neutrinos (events 160731A, 161103A, 170312A, and 170321A) reveal multiple X-ray sources in the targeted 90%-containment regions, most of which have been previously identified, and none of which are considered likely sources of high-energy neutrinos. Observations exclude association with the brightest 30% to 65% of Swifttype γ-ray burst X-ray afterglows over the observed regions. Contemporaneous Swift UV/optical observations, providing reduced coverage of the event localizations, also reveal no candidate transient or variable UV/optical counterparts. We discuss the results of these campaigns and our plans for further follow-up of likely-cosmic high-energy neutrinos from IceCube

Confirmation of the supergiant fast X-ray transient nature of AX J1841.0-0536 from Swift outburst observations

Swift observed an outburst from the supergiant fast X-ray transient (SFXT) AX J1841.0−0536 on 2010 June 5, and followed it with X-ray Telescope (XRT) for 11 d. The X-ray light curve shows an initial flare followed by a decay and subsequent increase, as often seen in other SFXTs, and a dynamical range of ∼1600. Our observations allow us to analyse the simultaneous broad-band (0.3–100 keV) spectrum of this source, for the first time down to 0.3 keV, which can be fitted well with models usually adopted to describe the emission from accreting neutron stars in high-mass X-ray binaries, and is characterized by a high absorption (NH∼ 2 × 1022 cm−2), a flat power law (Γ∼ 0.2) and a high-energy cut-off. All of these properties resemble those of the prototype of the class, IGR J17544−2619, which underwent an outburst on 2010 March 4, whose observations we also discuss. We show how well AX J1841.0−0536 fits in the SFXT class, based on its observed properties during the 2010 outburst, its large dynamical range in X-ray luminosity, the similarity of the light curve (length and shape) to those of the other SFXTs observed by Swift and the X-ray broad-band spectral properties

Jet breaks and energetics of swift gamma-ray burst X-ray afterglows

We present a systematic temporal and spectral study of all Swift-X-ray Telescope observations of gamma-ray burst (GRB) afterglows discovered between 2005 January and 2007 December. After constructing and fitting all light curves and spectra to power-law models, we classify the components of each afterglow in terms of the canonical X-ray afterglow and test them against the closure relations of the forward shock models for a variety of parameter combinations. The closure relations are used to identify potential jet breaks with characteristics including the uniform jet model with and without lateral spreading and energy injection, and a power-law structured jet model, all with a range of parameters. With this technique, we survey the X-ray afterglows with strong evidence for jet breaks (~12% of our sample), and reveal cases of potential jet breaks that do not appear plainly from the light curve alone (another ~30%), leading to insight into the missing jet break problem. Those X-ray light curves that do not show breaks or have breaks that are not consistent with one of the jet models are explored to place limits on the times of unseen jet breaks. The distribution of jet break times ranges from a few hours to a few weeks with a median of ~1 day, similar to what was found pre-Swift. On average, Swift GRBs have lower isotropic equivalent γ-ray energies, which in turn result in lower collimation corrected γ-ray energies than those of pre-Swift GRBs. Finally, we explore the implications for GRB jet geometry and energetics

Multi-wavelength observations of the Be/X-ray binary IGR J01217−7257 (=SXP 2.16) during outburst

We present simultaneous, multi-wavelength observations of the Small Magellanic Cloud Be/XRB IGR J01217−7257 (=SXP 2.16) during outbursts in 2014, 2015 and 2016. We also present the results of RXTE observations of the Small Magellanic Cloud during which the source was initially discovered with a periodicity of 2.1652±0.0001 seconds which we associate with the spin period of the neutron star. A systematic temporal analysis of long term Swift/BAT data reveals a periodic signal of 82.5±0.7 days, in contrast with a similar analysis of long base line OGLE I-band light curves which reveals an 83.67±0.05 days also found in this work. Interpreting the longer X-ray periodicity as indicative of binary motion of the neutron star, we find that outbursts detected by INTEGRAL and Swift between 2014 and 2016 are consistent with Type I outbursts seen in Be/XRBs, occurring around periastron. Comparing these outbursts with the OGLE data, we see a clear correlation between outburst occurrence and increasing I-band flux. A periodic analysis of subdivisions of OGLE data reveals three epochs during which short periodicities of ∼1 day are significantly detected which we suggest are non-radial pulsations (NRPs) of the companion star. These seasons immediately precede those exhibiting clear outburst behaviour, supporting the suggested association between the NRPs, decretion disk growth and the onset of Type I outbursts