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A very-high-energy component deep in the Gamma-ray Burst afterglow
Get PDFGamma-ray bursts (GRBs) are brief flashes of gamma rays, considered to be the most energetic explosive phenomena in the Universe. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow -- produced by the interaction between the ejected matter and the circumburst medium -- slows down, and a gradual decrease in brightness is observed. GRBs typically emit most of their energy via gamma-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elussive. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow -ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and gamma-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies
The definition of mass in asymptotically de Sitter space-times
Get PDFAn invariant definition of mass in asymptotically de-Sitter space-times is given that relies on the existence of a time-like Killing vector on a sphere surrounding the mass but does not require going to an asymptotic region. In particular the mass can be calculated exactly on a sphere inside the cosmological horizon. The formalism requires varying the background metric solution by a perturbation that satisfies the linearized equations of motion but need not share the Killing symmetry of the solution and is therefore ideally suited to calculating masses in stationary space-times perturbed by a gravitational wave without going beyond the cosmological horizon
Thermal emission from bow shocks I: 2D hydrodynamic models of the Bubble Nebula
Get PDFThe Bubble Nebula (or NGC 7635) is a parsec-scale seemingly spherical wind-blown bubble around the relatively unevolved O star
BD+60◦2522. The young dynamical age of the nebula and significant space velocity of the star suggest that the Bubble Nebula might
be a bow shock. We ran 2D hydrodynamic simulations to model the interaction of the wind of the central star with the interstellar
medium (ISM). The models cover a range of possible ISM number densities of n = 50−200 cm−3
and stellar velocities of v∗ =
20−40 km s−1
. Synthetic Hα and 24 µm emission maps predict the same apparent spherical bubble shape with quantitative properties
similar to observations. The synthetic maps also predict a maximum brightness similar to that from the observations and agree that the
maximum brightness is at the apex of the bow shock. The best-matching simulation had v∗ ≈ 20 km s−1
into an ISM with n ∼ 100 cm−3
,
at an angle of 60◦ with respect to the line of sight. Synthetic maps of soft (0.3−2 keV) and hard (2−10 keV) X-ray emission show that
the brightest region is in the wake behind the star and not at the bow shock itself. The unabsorbed soft X-rays have a luminosity of
∼1032−1033 erg s−1
. The hard X-rays are fainter: ∼10^30−10^31 erg s−1
, and may be too faint for current X-ray instruments to successfully
observe. Our results imply that the O star creates a bow shock as it moves through the ISM and in turn creates an asymmetric bubble
visible at optical and infrared wavelengths and predicted to be visible in X-rays. The Bubble Nebula does not appear to be unique; it
could simply be a favourably oriented, very dense bow shock. The dense ISM surrounding BD+60◦2522 and its strong wind suggest
that it could be a good candidate for detecting non-thermal emission
The definition of mass in asymptotically de Sitter space-times
Get PDFAn invariant definition of mass in asymptotically de-Sitter space-times is given that relies on the existence of a time-like Killing vector on a sphere surrounding the mass but does not require going to an asymptotic region. In particular the mass can be calculated exactly on a sphere inside the cosmological horizon. The formalism requires varying the background metric solution by a perturbation that satisfies the linearized equations of motion but need not share the Killing symmetry of the solution and is therefore ideally suited to calculating masses in stationary space-times perturbed by a gravitational wave without going beyond the cosmological horizon
Review of Clarke, Howard B.; Johnson, Ruth, eds., The Vikings in Ireland and Beyond: Before and After the Battle of Clontarf.
Get PDFA book review of Clarke, Howard B.; Johnson, Ruth, eds., The Vikings in Ireland and Beyond: Before and After the Battle of Clontarf
Modelling of electromagnetic signatures of global ocean circulation: physical approximations and numerical issues
No full textEstimating lateral and vertical resolution in receiver function data for shallow crust exploration
Get PDFIn order to test the horizontal and vertical resolution of teleseismic receiver functions, we perform a complete receiver function analysis and inversion using data from the La Barge array. The La Barge Passive Seismic Experiment was a seismic deployment in western Wyoming, recording continuously between November 2008 and June 2009, with 55 instruments deployed 250m apart—up to two orders of magnitude closer than in typical receiver function studies.
We analyse each station separately. We calculate receiver functions and invert them using a Bayesian algorithm. The inversion results are in agreement with measurements from nearby wells, and from other studies using the same data set. The resulting posterior probability distributions (PPDs), obtained for each station, are compared to each other by computing the Bhattacharyya coefficients, which quantify the overlap between two PPDs. Our results indicate that (a) the lateral resolution of 8 Hz receiver functions is approximately equal to the width of their first Fresnel zone, (b) minimum investigable depth is about 400m at 8 Hz, (c) lateral resolution depends on the local geology as expected and (d) velocity inversion in the shallow-crust can be resolved in the first few kilometres, even in case of dipping interfaces
Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE γ-ray observations with H.E.S.S.
Get PDFThe flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100 MeV 100 GeV) γ-ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with 8.7σ significance above an energy threshold of 66 GeV. No VHE variability was detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of 4.2 ± 0.3. The H.E.S.S. data along with a detailed and contemporaneous multiwavelength data set provide constraints on the physical parameters of the emission region. The minimum distance of the emission region from the central black hole was estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at r ≳ 1.7e17 cm from the black hole, that is beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models were used to describe the evolution of the 2015 flare. Neither model can fully reproduce the observations, despite testing various parameter sets. Furthermore, the H.E.S.S. data were used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279
SN-CAST: seismic network capability assessment software tool for regional networks-examples from Ireland
Get PDFEvent detection capability plays an important role in the operation of seismic observatories and temporary networks. The magnitude threshold for the detection of seismic events with a given network geometry is frequently derived from the observed magnitude of completeness. However, the latter might be unknown for regions that have not been monitored previously or where the observed seismicity rate is low. We present the open-source Python program SN-CAST with which the geographical distribution of event detection capability can be calculated as a function of station coordinates and station ambient noise amplitudes. The method employs the local magnitude scale, and hence is mainly applicable to regional networks with an aperture of less than about 1000 km. The attenuation characteristics of the study region
need to be derived independently or be known a priori.
SN-CAST can easily be employed to determine network performance in quasi real-time if station data streams are available. It can also be used for designing the geometry of new networks or assessing the effect of adding or removing stations from an existing network. We present examples from the Irish National Seismic Network (https://www.insn.ie), which operates in a region of low seismicity and large variations in ocean and wind-generated seismic noise. The seismicity in Ireland is too low to allow the calculation of a magnitude of completeness for comparison with the derived capability maps. However, the maps are in good agreement with the location and magnitude of detected local and regional earthquakes demonstrating that SN-CAST is a reliable tool for assessing the detection capability of seismic networks