Multi-Color Diffusion Simulation of Dye Over Folded Fabrics

Creating a unique dyed shirt requires accurately simulating a fabric model, implementing the ability to fold the fabric, encoding the process of dye diffusion through the fabric, and allowing for different color channels of dye to mix. These goals require an adherence to the physical properties and laws that govern the process of diffusion and a suitable representational model that this diffusion is conducted on and through. This paper presents a model for the representation of fabric that is comprised of two layers woven together in a weave pattern. Using Fick’s Second Law of Diffusion and properties of the fabric and threads, we calculate the rate of diffusion for each cell of the fabric. This calculation is done over each color channel to allow for the full spectrum of dye colors to be realized. Using a relationship matrix, the fabric can be folded over itself, and the dye will diffuse over this fold into the layer on the other side. Most of the parameters involved in defining the type of fabric can be modified to allow for a large range of visual expression in the fabric. The results support the effectiveness and veracity of the model

How Reasoning Aims at Truth

Many hold that theoretical reasoning aims at truth. In this paper, I ask what it is for reasoning to be thus aim-directed. Standard answers to this question explain reasoning’s aim-directedness in terms of intentions, dispositions, or rule-following. I argue that, while these views contain important insights, they are not satisfactory. As an alternative, I introduce and defend a novel account: reasoning aims at truth in virtue of being the exercise of a distinctive kind of cognitive power, one that, unlike ordinary dispositions, is capable of fully explaining its own exercises. I argue that this account is able to avoid the difficulties plaguing standard accounts of the relevant sort of mental teleology

Jet simulations and Gamma-ray burst afterglow jet breaks

The conventional derivation of the gamma-ray burst afterglow jet break time uses only the blast wave fluid Lorentz factor and therefore leads to an achromatic break. We show that in general gamma-ray burst afterglow jet breaks are chromatic across the self-absorption break. Depending on circumstances, the radio jet break may be postponed significantly. Using high-accuracy adaptive mesh fluid simulations in one dimension, coupled to a detailed synchrotron radiation code, we demonstrate that this is true even for the standard fireball model and hard-edged jets. We confirm these effects with a simulation in two dimensions. The frequency dependence of the jet break is a result of the angle dependence of the emission, the changing optical depth in the self-absorbed regime and the shape of the synchrotron spectrum in general. In the optically thin case the conventional analysis systematically overestimates the jet break time, leading to inferred opening angles that are underestimated by a factor 1.32 and explosion energies that are underestimated by a factor 1.73, for explosions in a homogeneous environment.Comment: MNRAS submitted. 9 pages, 12 figure

The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts

A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently selected by Salvaterra et al. (2011). The sample has a high level of completeness in redshift (91%). We derive here the intrinsic absorbing X-ray column densities of these GRBs making use of the Swift X-ray Telescope data. This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is consistent with the distribution of the column densities derived from the total sample of GRBs with redshift. We find a mild increase of the intrinsic column density with redshift. This can be interpreted as due to the contribution of intervening systems along the line of sight. Making use of the spectral index connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the relation of the intrinsic column density and the GRB `darkness'. We find that there is a very tight correlation between dark GRBs and high X-ray column densities. This clearly indicates that the dark GRBs are formed in a metal-rich environment where dust must be present.Comment: MNRAS, 6 pages, 3 figures, 1 tabl

The normalisation of Food Aid: What happened to feeding people well?

In the UK, food poverty has increased in the last 15 years and the food aid supply chain that has emerged to tackle it is now roughly 10 years old. In this time, we have seen the food aid supply chain grow at a rate that has astounded many.  Recently that growth has been aided by a grant of £20m from a large supermarket chain. It appears institutionalisation is just around the corner, if not already here. It also appears that there is far greater emphasis on dealing with  the symptoms as opposed to solving the root causes of the problem. As an opinion piece, this paper reflects on some of the prevalent issues, and suggests some ways forward

Detailed optical and near-infrared polarimetry, spectroscopy and broadband photometry of the afterglow of GRB 091018: Polarisation evolution

[Abridged] A number of phenomena have been observed in GRB afterglows that defy explanation by simple versions of the standard fireball model, leading to a variety of new models. Polarimetry can be a major independent diagnostic of afterglow physics, probing the magnetic field properties and internal structure of the GRB jets. In this paper we present the first high quality multi-night polarimetric light curve of a Swift GRB afterglow, aimed at providing a well calibrated dataset of a typical afterglow to serve as a benchmark system for modelling afterglow polarisation behaviour. In particular, our dataset of the afterglow of GRB 091018 (at redshift z=0.971) comprises optical linear polarimetry (R band, 0.13 - 2.3 days after burst); circular polarimetry (R band) and near-infrared linear polarimetry (Ks band). We add to that high quality optical and near-infrared broadband light curves and spectral energy distributions as well as afterglow spectroscopy. The linear polarisation varies between 0 and 3%, with both long and short time scale variability visible. We find an achromatic break in the afterglow light curve, which corresponds to features in the polarimetric curve. We find that the data can be reproduced by jet break models only if an additional polarised component of unknown nature is present in the polarimetric curve. We probe the ordered magnetic field component in the afterglow through our deep circular polarimetry, finding P_circ < 0.15% (2 sigma), the deepest limit yet for a GRB afterglow, suggesting ordered fields are weak, if at all present. Our simultaneous R and Ks band polarimetry shows that dust induced polarisation in the host galaxy is likely negligible.Comment: 20 pages, 14 figures, 3 tables. Accepted for publication in MNRAS. Some figures are reduced in quality to comply with arXiv size requirement

Pre-ALMA observations of GRBs in the mm/submm range

GRBs generate an afterglow emission that can be detected from radio to X-rays during days, or even weeks after the initial explosion. The peak of this emission crosses the mm/submm range during the first hours to days, making their study in this range crucial for constraining the models. Observations have been limited until now due to the low sensitivity of the observatories in this range. We present observations of 10 GRB afterglows obtained from APEX and SMA, as well as the first detection of a GRB with ALMA, and put them into context with all the observations that have been published until now in the spectral range that will be covered by ALMA. The catalogue of mm/submm observations collected here is the largest to date and is composed of 102 GRBs, of which 88 had afterglow observations, whereas the rest are host galaxy searches. With our programmes, we contributed with data of 11 GRBs and the discovery of 2 submm counterparts. In total, the full sample, including data from the literature, has 22 afterglow detections with redshift ranging from 0.168 to 8.2. GRBs have been detected in mm/submm wavelengths with peak luminosities spanning 2.5 orders of magnitude, the most luminous reaching 10^33erg s^-1 Hz^-1. We observe a correlation between the X-ray brightness at 0.5 days and the mm/submm peak brightness. Finally we give a rough estimate of the distribution of peak flux densities of GRB afterglows, based on the current mm/submm sample. Observations in the mm/submm bands have been shown to be crucial for our understanding of the physics of GRBs, but have until now been limited by the sensitivity of the observatories. With the start of the operations at ALMA, the sensitivity will be increased by more than an order of magnitude. Our estimates predict that, once completed, ALMA will detect up to 98% of the afterglows if observed during the passage of the peak synchrotron emission.Comment: 23 pages, 14 figures, 5 tables (one big one!), Accepted for publication in A&A. Includes the first observation of a GRB afterglow with ALM

A deep search for the host galaxies of GRBs with no detected optical afterglow

Gamma-Ray Bursts can provide information about star formation at high redshifts. Even in the absence of a optical/near-infrared/radio afterglow, the high detection rate of X-ray afterglows by swift/XRT and its localization precision of 2-3 arcsec facilitates the identification and study of GRB host galaxies. We focus on the search for the host galaxies of a sample of 17 bursts with XRT error circles but no detected long-wavelength afterglow. Three of these events can also be classified as truly dark bursts: the observed upper limit on the optical flux of the afterglow was less than expected based on the X-ray flux. Our study is based on deep R and K-band observations performed with ESO/VLT instruments, supported by GROND and NEWFIRM. To be conservative, we searched for host galaxies in an area with a radius twice the 90% swift/XRT error circle. For 15 of the 17 bursts we find at least one galaxy inside the doubled XRT error circle. In seven cases we discover extremely red objects in the error circles. The most remarkable case is the host of GRB 080207 which as a colour of R-K~4.7 mag (AB), one of the reddest galaxies ever associated with a GRB. As a by-product of our study we identify the optical afterglow of GRB 070517A. Optically dim afterglows result from cosmological Lyman drop out and dust extinction, but the former process is only equired for a minority of cases (<1/3). Extinction by dust in the host galaxies might explain all other events. Thereby, a seemingly non-negligible fraction of these hosts are globally dust-enshrouded, extremely red galaxies. This suggests that bursts with optically dim afterglows trace a subpopulation of massive starburst galaxies, which are markedly different from the main body of the GRB host galaxy population, namely the blue, subluminous, compact galaxies.Comment: 29 pages, 31 figures, accepted for publication in A&

The dark GRB080207 in an extremely red host and the implications for GRBs in highly obscured environments

[Abridged] We present comprehensive X-ray, optical, near- and mid-infrared, and sub-mm observations of GRB 080207 and its host galaxy. The afterglow was undetected in the optical and near-IR, implying an optical to X-ray index <0.3, identifying GRB 080207 as a dark burst. Swift X-ray observations show extreme absorption in the host, which is confirmed by the unusually large optical extinction found by modelling the X-ray to nIR afterglow spectral energy distribution. Our Chandra observations obtained 8 days post-burst allow us to place the afterglow on the sky to sub-arcsec accuracy, enabling us to pinpoint an extremely red galaxy (ERO). Follow-up host observations with HST, Spitzer, Gemini, Keck and the James Clerk Maxwell Telescope (JCMT) provide a photometric redshift solution of z ~1.74 (+0.05,-0.06) (1 sigma), 1.56 < z < 2.08 at 2 sigma) for the ERO host, and suggest that it is a massive and morphologically disturbed ultra-luminous infrared galaxy (ULIRG) system, with L_FIR ~ 2.4 x 10^12 L_solar. These results add to the growing evidence that GRBs originating in very red hosts always show some evidence of dust extinction in their afterglows (though the converse is not true -- some extinguished afterglows are found in blue hosts). This indicates that a poorly constrained fraction of GRBs occur in very dusty environments. By comparing the inferred stellar masses, and estimates of the gas phase metallicity in both GRB hosts and sub-mm galaxies we suggest that many GRB hosts, even at z>2 are at lower metallicity than the sub-mm galaxy population, offering a likely explanation for the dearth of sub-mm detected GRB hosts. However, we also show that the dark GRB hosts are systematically more massive than those hosting optically bright events, perhaps implying that previous host samples are severely biased by the exclusion of dark events.Comment: 13 pages, 6 figures, accepted for publication in MNRA

GRB 080319B: A Naked-Eye Stellar Blast from the Distant Universe

Long duration gamma-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of a massive star. Over the last forty years, our understanding of the GRB phenomenon has progressed dramatically; nevertheless, fortuitous circumstances occasionally arise that provide access to a regime not yet probed. GRB 080319B presented such an opportunity, with extraordinarily bright prompt optical emission that peaked at a visual magnitude of 5.3, making it briefly visible with the naked eye. It was captured in exquisite detail by wide-field telescopes, imaging the burst location from before the time of the explosion. The combination of these unique optical data with simultaneous gamma-ray observations provides powerful diagnostics of the detailed physics of this explosion within seconds of its formation. Here we show that the prompt optical and gamma-ray emissions from this event likely arise from different spectral components within the same physical region located at a large distance from the source, implying an extremely relativistic outflow. The chromatic behaviour of the broadband afterglow is consistent with viewing the GRB down the very narrow inner core of a two-component jet that is expanding into a wind-like environment consistent with the massive star origin of long GRBs. These circumstances can explain the extreme properties of this GRB.Comment: 43 pages, 18 figures, 3 tables, submitted to Nature May 11, 200