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On the source of the dust extinction in type Ia supernovae and the discovery of anomalously strong Na i absorption
No full textHigh-dispersion observations of the Na I D λλ5890, 5896 and K I λλ7665, 7699 interstellar lines, and the diffuse interstellar band at 5780 Å in the spectra of 32 Type Ia supernovae are used as an independent means of probing dust extinction. We show that the dust extinction of the objects where the diffuse interstellar band at 5780 Å is detected is consistent with the visual extinction derived from the supernova colors. This strongly suggests that the dust producing the extinction is predominantly located in the interstellar medium of the host galaxies and not in circumstellar material associated with the progenitor system. One quarter of the supernovae display anomalously large Na I column densities in comparison to the amount of dust extinction derived from their colors. Remarkably, all of the cases of unusually strong Na I D absorption correspond to "Blueshifted" profiles in the classification scheme of Sternberg et al. This coincidence suggests that outflowing circumstellar gas is responsible for at least some of the cases of anomalously large Na I column densities. Two supernovae with unusually strong Na I D absorption showed essentially normal K I column densities for the dust extinction implied by their colors, but this does not appear to be a universal characteristic. Overall, we find the most accurate predictor of individual supernova extinction to be the equivalent width of the diffuse interstellar band at 5780 Å, and provide an empirical relation for its use. Finally, we identify ways of producing significant enhancements of the Na abundance of circumstellar material in both the single-degenerate and double-degenerate scenarios for the progenitor system. © 2013. The American Astronomical Society. All rights reserved.
PTF10iya: A short-lived, luminous flare from the nuclear region of a star-forming galaxy
No full textWe present the discovery and characterization of PTF10iya, a short-lived (Δt≈ 10d, with an optical decay rate of ∼0.3magd -1), luminous (mag) transient source found by the Palomar Transient Factory. The ultraviolet/optical spectral energy distribution is reasonably well fitted by a blackbody with T≈ (1-2) × 10 4K and peak bolometric luminosity L BB≈ (1-5) × 10 44ergs -1 (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy (z= 0.22405 ± 0.00006) to within 350mas (99.7per cent confidence radius), or a projected distance of less than 1.2kpc. At first glance, these properties appear reminiscent of the characteristic 'big blue bump' seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early 'super-Eddington' phase of a solar-type star being disrupted by a ∼10 7M ⊙ black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS
Constraining gamma-ray burst emission physics with extensive early-time, multiband follow-up
No full textUnderstanding the origin and diversity of emission processes responsible for gamma-ray bursts (GRBs) remains a pressing challenge. While prompt and contemporaneous panchromatic observations have the potential to test predictions of the internal-external shock model, extensive multiband imaging has been conducted for only a few GRBs. We present rich, early-time, multiband data sets for two Swift events, GRB 110205A and GRB 110213A. The former shows optical emission since the early stages of the prompt phase, followed by the steep rising in flux up to 1000s after the burst (t -α with α = -6.13 ± 0.75). We discuss this feature in the context of the reverse-shock scenario and interpret the following single power-law decay as being forward-shock dominated. Polarization measurements, obtained with the RINGO2 instrument mounted on the Liverpool Telescope, also provide hints on the nature of the emitting ejecta. The latter event, instead, displays a very peculiar optical to near-infrared light curve, with two achromatic peaks. In this case, while the first peak is probably due to the onset of the afterglow, we interpret the second peak to be produced by newly injected material, signifying a late-time activity of the central engine. © 2011. The American Astronomical Society. All rights reserved
A putative early-type host galaxy for GRB 060502B: Implications for the progenitors of short-duration hard-spectrum bursts
No full textStarting with the first detection of an afterglow from a short-duration hard-spectrum γ-ray burst (SHB) by Swift last year, a growing body of evidence has suggested that SHBs are associated with an older and lower redshift galactic population than long-soft GRBs and, in a few cases, with large (≳10 kpc) projected offsets from the centers of their putative host galaxies. Here we present observations of the field of ORB 060502B, a SHB detected by Swift and localized by the X-Ray Telescope (XRT). We find a massive red galaxy at a redshift of z = 0.287 at an angular distance of 17.1″ from our revised XRT position. Using associative and probabilistic arguments, we suggest that this galaxy hosted the progenitor of GRB 060502B. If true, this offset would correspond to a physical displacement of 73 ± 19 kpc in projection , about twice the largest offset inferred for any SHB to date and almost an order of magnitude larger than a typical long-soft burst offset. Spectra and modeling of the star formation history of this possible host show it to have undergone a large ancient starburst. If the progenitor of GRB 060502B was formed in this starburst episode, the time of the GRB explosion since birth is τ ≈ 1.3 ± 0.2 Gyr and the minimum kick velocity of the SHB progenitor is vkick,min = 55 ± 15 km s-1. © 2007, The Astronomical Society. All right reserved
The chemical enrichment of long gamma-ray bursts nurseries up to z = 2
Get PDFAims. We investigate the existence of a metallicity threshold for the production of long gamma-ray bursts (LGRBs). Methods. We used the host galaxies of the Swift/BAT6 sample of LGRBs. We considered the stellar mass, star formation rate (SFR), and metallicity determined from the host galaxy photometry and spectroscopy up to z = 2 and used them to compare the distribution of host galaxies to that of field galaxies in the mass-metallicity and fundamental metallicity relation plane. Results. We find that although LGRBs also form in galaxies with relatively large stellar masses, the large majority of host galaxies have metallicities below log(O=H) ∼ 8:6. The extension to z = 2 results in a good sampling of stellar masses also above Log(M=M) ∼ 9:5 and provides evidence that LGRB host galaxies do not follow the fundamental metallicity relation. As shown by the comparison with dedicated numerical simulations of LGRB host galaxy population, these results are naturally explained by the existence of a mild (∼0:7 Z) threshold for the LGRB formation. The present statistics does not allow us to discriminate between different shapes of the metallicity cutoff, but the relatively high metallicity threshold found in this work is somewhat in disagreement to most of the standard single-star models for LGRB progenitors. © ESO, 201
A gamma-ray burst at a redshift of z approximate to 8.2
No full textLong-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars(1), and some are bright enough that they should be observable out to redshifts of z > 20 using current technology(2-4). Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy(5). Here we report that GRB 090423 lies at a redshift of z approximate to 8.2, implying that massive stars were being produced and dying as GRBs similar to 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy
A possible relativistic jetted outburst from a massive black hole fed by a tidally disrupted star
Get PDFGas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, but most MBHs are considered dormant. Occasionally, a star passing too near an MBH is torn apart by gravitational forces, leading to a bright tidal disruption flare (TDF). Although the high-energy transient Sw 1644+57 initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that observations suggest a sudden accretion event onto a central MBH of mass about 106 to 107 solar masses. There is evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 to a temporary smaller-scale blazar
Evaluating the effects of acoustic stimulation on fibroblast cell migration and muscle fibre formation.
No full textCells are known to interact and respond to external mechanical cues and recent work has shown that application of mechanical stimulation, delivered via acoustic vibration, can be used to control complex cell behaviours. Much of the work in this area has focused on the effects of mechanical stimulation upon bone cells. Very little has been done to investigate the effects of mechanical stimulation on other cell types. Therefore, could mechanical stimulation be used to accelerate the wound healing process? Could mechanical stimulation be used to encourage muscle cell growth? The data generated in this thesis will be completely new knowledge which will help us develop our understanding of mechanotransduction and help us develop new approaches and devices to address medical challenges such as accelerating wound healing process, increasing muscle mass in diseases associated with muscle degeneration and aging. The aim of this research was to investigate the effects of mechanical stimulation upon the migratory and morphological properties of three different fibroblast cells and muscle cells namely; human lung fibroblast cells (LL24), subcutaneous areolar/adipose mouse fibroblast cells (L929), Human dermal fibroblast cells (HDF) and C2C12P12 muscle cells. Using a speaker-based system, the effects of mechanical stimulation (0-1600Hz for 5 and 20 minutes) on wound/scratch assays, the mean cell migration distance (µm), actin and vinculin organisation, cell viability, muscle fibre diameters and the effects of temperature were investigated. The results show that 100Hz acoustic vibration can enhance cell migration, wound healing in LL24 and L929 cells and possible increase muscle fibre diameters although there was no significance, the trends were repeatable. Mechanical stimulation was also found to promote changes to the morphology of LL24 and L929 cell lines, particularly the formation of lamellipodia and filopodia. Overall lamellipodia was the most prominent actin structure displayed by the lung cell (LL24), whereas filopodia was the most prominent actin feature displayed by the fibroblast derived from subcutaneous areolar/adipose tissue. Vinculin appeared to be more punctate in L929 cells compared to LL24 cells. Mechanical stimulation at all the frequencies used here was found not to affect cell viability. However, as the frequency increased, the temperature also increased. These results suggest that low-frequency acoustic vibration may be used as a tool to manipulate the mechano-sensitivity of cells to control cell migration and to enhance the diameters of muscle fibres
Semblances of ‘War Tourism’ in Sarajevo, Post-2005
No full textWar creates negative place images, so cities, regions and countries often attempt to (re) create an image after tragic events. In some cases, post-war, a place will use the recent events as a way of educating tourists. After turbulent events cities often attempt to create a new image and fade memories of war and focus on the future as a means of removing the negative imaginations associating a place with its past. Sarajevo has often been referred to as a ‘War Tourism’ destination and this paper critically assesses the city in relation to destination image of war tourism since 2005. After the war ended, people associated Sarajevo with war, so the city strategically attempted to embrace this part of their recent history as a way of educating visitors about what really happened and not only what was presented to the outside world through the media. The purpose of the following commentary is to revisit the focus of tourism in Sarajevo ten years after the signing of the Dayton Peace Accords until present day. Newspaper content from international (English language) newspapers were used to excavate content on articles discussing, Sarajevo, war and tourism. The analysis section acknowledges that emphasis is still put on ‘war tourism in Sarajevo.’ As the discussion section will highlight, post-conflict, destinations can be conceptualized as being ‘permanent’ or ‘temporary’ war tourism destinations. It is important to recognize future directions concerning how tourism managers want to replace and fade memories of war in an attempt to revert back to the city’s former image-as an ideal place where different groups resided together
Experimental Studies and Effective Finite Element Modelling of Foot Deformation in Standing
Get PDFIn this work, a full scale subject specific FE foot model is developed to simulate the deformation of human foot under a standing position similar to a Navicular Drop Test. The model used a full bone structure and effective embedded structure method to increase the modelling efficiency. Navicular drop tests have been performed and the displacement of the navicular bone is measured using 3D image analysing system. The experimental results show a good agreement with the numerical models and published data. The model is verified by comparing the numerical data for simple standing against subject specific navicular drop test. The detailed deformation of the navicular bone and factors affecting the navicular bone displacement and measurement are discussed
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