Over the past decade, long-duration γ-ray bursts (GRBs)-including the subclass of X-ray flashes (XRFs)-have been revealed to be a rare variety of type Ibc supernova. Although all these events result ...from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 1048 erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.
Despite a rich phenomenology, γ-ray bursts (GRBs) are divided into two classes based on their duration and spectral hardness-the long-soft and the short-hard bursts. The discovery of afterglow ...emission from long GRBs was a watershed event, pinpointing their origin to star-forming galaxies, and hence the death of massive stars, and indicating an energy release of about 1051 erg. While theoretical arguments suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics and environments of these events remain elusive despite recent localizations. Here we report the discovery of the first radio afterglow from the short burst GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift z = 0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10-1,000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (> 1 Gyr) stellar population, strengthening earlier suggestions and providing support for coalescing compact object binaries as the progenitors.
Objective
To investigate the effects of Image Biomarker Standardisation Initiative (IBSI) compliance, harmonisation of calculation settings and platform version on the statistical reliability of ...radiomic features and their corresponding ability to predict clinical outcome.
Methods
The statistical reliability of radiomic features was assessed retrospectively in three clinical datasets (patient numbers: 108 head and neck cancer, 37 small-cell lung cancer, 47 non-small-cell lung cancer). Features were calculated using four platforms (PyRadiomics, LIFEx, CERR and IBEX). PyRadiomics, LIFEx and CERR are IBSI-compliant, whereas IBEX is not. The effects of IBSI compliance, user-defined calculation settings and platform version were assessed by calculating intraclass correlation coefficients and confidence intervals. The influence of platform choice on the relationship between radiomic biomarkers and survival was evaluated using univariable cox regression in the largest dataset.
Results
The reliability of radiomic features calculated by the different software platforms was only excellent (ICC > 0.9) for 4/17 radiomic features when comparing all four platforms. Reliability improved to ICC > 0.9 for 15/17 radiomic features when analysis was restricted to the three IBSI-compliant platforms. Failure to harmonise calculation settings resulted in poor reliability, even across the IBSI-compliant platforms. Software platform version also had a marked effect on feature reliability in CERR and LIFEx. Features identified as having significant relationship to survival varied between platforms, as did the direction of hazard ratios.
Conclusion
IBSI compliance, user-defined calculation settings and choice of platform version all influence the statistical reliability and corresponding performance of prognostic models in radiomics.
Key Points
• Reliability of radiomic features varies between feature calculation platforms and with choice of software version.
• Image Biomarker Standardisation Initiative (IBSI) compliance improves reliability of radiomic features across platforms, but only when calculation settings are harmonised.
• IBSI compliance, user-defined calculation settings and choice of platform version collectively affect the prognostic value of features.
The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the ...coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.
Over the past decade, our physical understanding of gamma-ray bursts (GRBs) has progressed rapidly, thanks to the discovery and observation of their long-lived afterglow emission. Long-duration (> 2 ...s) GRBs are associated with the explosive deaths of massive stars ('collapsars', ref. 1), which produce accompanying supernovae; the short-duration (< or = 2 s) GRBs have a different origin, which has been argued to be the merger of two compact objects. Here we report optical observations of GRB 060614 (duration approximately 100 s, ref. 10) that rule out the presence of an associated supernova. This would seem to require a new explosive process: either a massive collapsar that powers a GRB without any associated supernova, or a new type of 'engine', as long-lived as the collapsar but without a massive star. We also show that the properties of the host galaxy (redshift z = 0.125) distinguish it from other long-duration GRB hosts and suggest that an entirely new type of GRB progenitor may be required.
We present detailed optical, X-ray, and radio observations of the bright afterglow of the short gamma-ray burst 051221a obtained with Gemini, Swift XRT, and the Very Large Array, as well as optical ...spectra from which we measure the redshift of the burst, z = 0.5464. At this redshift the isotropic-equivalent prompt energy release was about 1.5 x 10 super(51) ergs, and using a standard afterglow synchrotron model, we find that the blast wave kinetic energy is similar, E sub(K,iso) -8.4 x 10 super(51) ergs. An observedjet break at t-5 days indicates that the opening angle is f sub(j) -7 and the total beaming-corrected energy is therefore -2.5 x 10 super(49) ergs, comparable to the values inferred for previous short GRBs. We further show that the burst experienced an episode of energy injection by a factor of 3.4 between t = 1.4 and 3.4 hr, which was accompanied by reverse shock emission in the radio band. This result provides continued evidence that the central engines of short GRBs maybe active significantly longer than the duration of the burst and/or produce a wide range of Lorentz factors. Finally, we show that the host galaxy is actively forming stars at a rate of about 1.6 M sub( )yr super(-1), yet exhibits evidence for an appreciable population of old stars (61 Gyr) and near-solar metallicity. These properties are intermediate between those of long GRB hosts and previous short burst hosts. The lack of bright supernova emission and the low circumburst density (n 610 super(-3) cm super(-3)), however, continue to support the idea that short bursts are not related to massive stellar death. Given that the total energy release is larger than the predicted yield for a neutrino annihilation mechanism, this suggests that magnetohydrodynamic processes may be required to power the burst.
We present the discovery of radio afterglow emission from the high-redshift (z = 6.295) burst GRB 050904. The peak flux density for this burst is similar to typical low-redshift gamma-ray bursts ...(GRBs). We further show that beyond a redshift of order unity, the flux densities of radio afterglows are largely insensitive to redshift, consistent with predictions. By combining the existing X-ray, near-infrared, and radio measurements, we derive estimates for the kinetic energy and opening angle of the blast wave and for the density of the circumburst medium into which it expands. Both the kinetic and radiated energy indicate that GRB 050904 was an unusually energetic burst (10 super(52) ergs). More importantly, we are able to make an in situ measurement of the density structure of the circumburst medium. We conclude that GRB 050904 exploded into a constant-density medium with n sub(0) = 680 cm super(-3), which is 2 orders of magnitude above the nominal value for low-redshift GRBs. The next generation of centimeter (EVLA) and millimeter radio instruments (ALMA) will be able to routinely detect events such as GRB 050904 and use them to study magnetic fields and the atomic and molecular gas in the high-redshift universe.
On 2006 May 5, a 4 s duration, low-energy, similar to 10 super(49) erg, gamma-ray burst (GRB) was observed, spatially associated with a z = 0.0804 galaxy. Here we report the discovery of the GRB ...optical afterglow and observations of its environment using Gemini South, the Hubble Space Telescope (HST), Chandra, Swift, and the Very Large Array. The optical afterglow of this GRB is spatially associated with a prominent star-forming region in the Sc-type galaxy 2dFGRS S173Z112. Its proximity to a star-forming region suggests that the progenitor delay time, from birth to explosion, is smaller than similar to 10 Myr. Our HST deep imaging rules out the presence of a supernova brighter than an absolute magnitude of about -11 (or -12.6 in the case of maximal extinction) at about 2 weeks after the burst and limits the ejected mass of radioactive super(56)Ni to be less than about 2 x 10 super(-4) M unk (assuming no extinction). Although it was suggested that GRB 060505 may belong to a new class of long-duration GRBs with no supernova, we argue that the simplest interpretation is that the physical mechanism responsible for this burst is the same as that for short-duration GRBs.
Our work was motivated by discoveries of prokaryotic communities that survive with little nutrient in ice and permafrost, with implications for past or present microbial life in Martian permafrost ...and European ice. We compared the temperature dependence of metabolic rates of microbial communities in permafrost, ice, snow, clouds, oceans, lakes, marine and freshwater sediments, and subsurface aquifer sediments. Metabolic rates per cell fall into three groupings: (i) a rate, μ g(T), for growth, measured in the laboratory at in situ temperatures with minimal disturbance of the medium; (ii) a rate, μ m(T), sufficient for maintenance of functions but for a nutrient level too low for growth; and (iii) a rate, μ s(T), for survival of communities imprisoned in deep glacial ice, subsurface sediment, or ocean sediment, in which they can repair macromolecular damage but are probably largely dormant. The three groups have metabolic rates consistent with a single activation energy of ≈110 kJ and that scale as μ g(T): μ m(T): μ s(T)≈ 106: 103: 1. There is no evidence of a minimum temperature for metabolism. The rate at -40°C in ice corresponds to ≈10 turnovers of cellular carbon per billion years. Microbes in ice and permafrost have metabolic rates similar to those in water, soil, and sediment at the same temperature. This finding supports the view that, far below the freezing point, liquid water inside ice and permafrost is available for metabolism. The rate μ s(T) for repairing molecular damage by means of DNA-repair enzymes and protein-repair enzymes such as methyltransferase is found to be comparable to the rate of spontaneous molecular damage.
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