The UK Grocery e-commerce industry is amongst the most developed in the world with an estimated value of £11.4Billion in 2018. Assisted by technological developments, the market has experienced ...dramatic growth over the past two decades. Grocery retailers have invested in online infrastructure and home delivery networks. As a result, consumers have a range of options which enable them to shop interchangeably between in-store and online channels. The proliferation of new grocery shopping channels such as home delivery or ‘click and collect’ (collect in store or collection from a non-store collection point), coupled with changing consumer behaviour, provides new challenges for retailers in understanding consumer dynamics in this market. This paper explores consumer behaviour and preferences for e-commerce in the grocery retail industry in Great Britain (GB) using a survey of 19,033 respondents from the major market research company YouGov. Respondents were asked a range of questions around uptake and channel usage in the grocery e-commerce sector. The survey presents a novel opportunity to analyse self-reported consumer behaviour in GB, with survey responses attached to key sociodemographic and locational information. We find that sociodemographics are an important driver of groceries e-commerce usage and channel choice, with females, more affluent households and those in the 25–44 age group most likely to use home delivery, corroborating previous research. Contrasting previous research, we also find statistically significant evidence of relatively high values of home delivery use among over-55s. Whilst overall usage is lower, we find a particular affinity to collection facilities among males and skilled manual workers. Spatially, we find evidence of both the innovation-diffusion theory and the efficiency theory at both a national and local authority district level, using a brief case study of Yorkshire and the Humber. These insights can support grocery retailers as they further develop costly localised infrastructure to support e-commerce. It could also assist retailers in understanding the localised drivers of channel choice as they seek to shift demand from home delivery (with high costs faced by the retailer in relation to the ‘last mile’) to click and collect (in which the customer faces the costs associated with the last mile).
Abstract
We have conducted long-slit spectroscopic observations and analyzed archival radio data for the ultraluminous X-ray source (ULX) NGC 4861 X-1. Our spectral line analysis unveils NGC 4861 X-1 ...as the fourth ULX situated within an X-ray photoionized nebula, following three previous findings made approximately two decades ago. Remarkably, we discover NGC 4861 X-1 also possesses a radio nebula emitting optically thin synchrotron radiation, which contradicts its X-ray photoionization and raises doubts about the four ULXs being a mere coincidence. Instead of gradually accumulating from different bands bit by bit, our multiband discovery is made all at once. Moreover, we tentatively perceive a faint continuum spectrum of the optical nebula. Further observations are needed to ascertain its radio structures and verify the optical continuum.
Abstract
We model the intermediate-mass black hole HLX-1, using the Hubble Space Telescope, XMM–Newton and Swift. We quantify the relative contributions of a bluer component, function of X-ray ...irradiation, and a redder component, constant and likely coming from an old stellar population. We estimate a black hole mass ${\approx } (2^{+2}_{-1}) \times 10^4 \,\mathrm{M}_{{\odot }}$, a spin parameter a/M ≈ 0.9 for moderately face-on view and a peak outburst luminosity ≈0.3 times the Eddington luminosity. We discuss the discrepancy between the characteristic sizes inferred from the short X-ray time-scale (R ∼ a few 1011 cm) and from the optical emitter ($R \sqrt{\cos \theta } \approx 2.2 \times 10^{13}$ cm). One possibility is that the optical emitter is a circumbinary disc; however, we disfavour this scenario because it would require a very small donor star. A more plausible scenario is that the disc is large but only the inner annuli are involved in the X-ray outburst. We propose that the recurrent outbursts are caused by an accretion-rate oscillation driven by wind instability in the inner disc. We argue that the system has a long-term-average accretion rate of a few per cent Eddington, just below the upper limit of the low/hard state; a wind-driven oscillation can trigger transitions to the high/soft state, with a recurrence period ∼1 yr (much longer than the binary period, which we estimate as ∼10 d). The oscillation that dominated the system in the last decade is now damped such that the accretion rate no longer reaches the level required to trigger a transition. Finally, we highlight similarities between disc winds in HLX-1 and in the Galactic black hole V404 Cyg.
ABSTRACT
We have studied the unusual time variability of an ultraluminous X-ray source in M 101, 4XMM J140314.2 + 541806 (henceforth, J1403), using Chandra and XMM-Newton data. Over the last two ...decades, J1403 has shown short-duration outbursts with an X-ray luminosity ∼1–3 × 1039 erg s−1, and longer intervals at luminosities ∼0.5–1 × 1038 erg s−1. The bimodal behaviour and fast outburst evolution (sometimes only a few days) are more consistent with an accretor/propeller scenario for a neutron star than with the canonical outburst cycles of stellar-mass black holes. If this scenario is correct, the luminosities in the accretor and propeller states suggest a fast spin (P ≈ 5 ms) and a low surface magnetic field (B ∼ 1010 G), despite our identification of J1403 as a high-mass X-ray binary. The most striking property of J1403 is the presence of strong ∼600-s quasi-periodic oscillations (QPOs), mostly around frequencies of ≈1.3–1.8 mHz, found at several epochs during the ultraluminous regime. We illustrate the properties of such QPOs, in particular their frequency and amplitude changes between and within observations, with a variety of techniques (Fast Fourier Transforms, Lomb–Scargle periodograms, weighted wavelet Z-transform analysis). The QPO frequency range <10 mHz is an almost unexplored regime in X-ray binaries and ultraluminous X-ray sources. We compare our findings with the (few) examples of very low frequency variability found in other accreting sources, and discuss possible explanations (Lense–Thirring precession of the inner flow or outflow; radiation pressure limit-cycle instability; marginally stable He burning on the neutron star surface).
Discovery of two eclipsing X-ray binaries in M 51 Wang, Song; Soria, Roberto; Urquhart, Ryan ...
Monthly notices of the Royal Astronomical Society,
07/2018, Letnik:
477, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Abstract
We discovered eclipses and dips in two luminous (and highly variable) X-ray sources in M 51. One (CXOM51 J132943.3+471135) is an ultraluminous supersoft source, with a thermal spectrum at a ...temperature of about 0.1 keV and characteristic blackbody radius of about 104 km. The other (CXOM51 J132946.1+471042) has a two-component spectrum with additional thermal-plasma emission; it approached an X-ray luminosity of 1039 erg s−1 during outbursts in 2005 and 2012. From the timing of three eclipses in a series of Chandra observations, we determine the binary period (52.75 ± 0.63 h) and eclipse fraction ($22 \pm 0.1\hbox{ per cent}$) of CXOM51 J132946.1+471042. We also identify a blue optical counterpart in archival Hubble Space Telescope images, consistent with a massive donor star (mass of ∼20–35 M⊙). By combining the X-ray light-curve parameters with the optical constraints on the donor star, we show that the mass ratio in the system must be $M_2/M_1 \gtrsim 18$ and therefore the compact object is most likely a neutron star (exceeding its Eddington limit in outburst). The general significance of our result is that we illustrate one method (applicable to high-inclination sources) of identifying luminous neutron star X-ray binaries, in the absence of X-ray pulsations or phase-resolved optical spectroscopy. Finally, we discuss the different X-ray spectral appearance expected from super-Eddington neutron stars and black holes at high viewing angles.
Forecast growth in e-commerce home-delivery demand provides retailers with opportunities for expansion and increased levels of investment. To maximise these growth opportunities, retailers face ...operational and logistical challenges related to order fulfilment and the last mile. In contrast to other sectors, many grocers operate a store-based delivery model rather than a separate distribution channel. Under this model, orders are picked from store shelves and store-based fleets of delivery vehicles are used for last-mile fulfilment (delivery to the consumers’ home). With very rare access to commercial data from Sainsbury’s, the second largest grocer in Great Britain, we analyse the geographical variations of online groceries fulfilment capacity at store, region, and rural–urban geography levels, exploring the interrelated impact of capacity constraints related to storage and delivery in limiting the further growth of these services. The spatial extent of delivery service areas are found to considerably vary between stores and the existing store network presents barriers to further capacity expansion in some regions. We argue factors associated with the last mile are an important capacity constraint/enabler in the e-groceries sector and suggest that the effective expansion of these services requires further research into online service area delineation to maximise delivery efficiency and capacity. The approach used here is readily transferable to other online service delivery providers in both GB and elsewhere.
Abstract
The conditions under which accreting neutron stars launch radio-emitting jets and/or outflows are still poorly understood. The ultracompact X-ray binary X1850–087, located in the globular ...cluster NGC 6712, is a persistent atoll-type X-ray source that has previously shown unusual radio-continuum variability. Here we present the results of a pilot radio-monitoring program of X1850–087 undertaken with the Karl G. Jansky Very Large Array, with simultaneous or quasi-simultaneous Swift/XRT data obtained at each epoch. The binary is clearly detected in the radio in two of the six new epochs. When combined with previous data, these results suggest that X1850–087 shows radio emission at a slightly elevated hard-state X-ray luminosity of
L
X
≳ 2 × 10
36
erg s
−1
, but no radio emission in its baseline hard state
L
X
∼ 10
36
erg s
−1
. No clear X-ray spectral changes are associated with this factor of ≳10 radio variability. At all detected epochs, X1850–087 has a flat to inverted radio spectral index, more consistent with the partially absorbed optically thick synchrotron of a compact jet rather than the evolving optically thick to thin emission associated with transient expanding synchrotron-emitting ejecta. If the radio emission in X1850–087 is indeed due to a compact jet, then it is plausibly being launched and quenched in the hard state on timescales as short as a few days. Future radio monitoring of X1850–087 could help elucidate the conditions under which compact jets are produced around hard-state accreting neutron stars.
ABSTRACT
Owing to their quiet evolutionary histories, nearby dwarf galaxies (stellar masses $M_\star \lesssim 3 \times 10^9 \, \mathrm{M}_\odot$) have the potential to teach us about the mechanism(s) ...that ‘seeded’ the growth of supermassive black holes, and also how the first stellar mass black holes formed and interacted with their environments. Here, we present high spatial resolution observations of three dwarf galaxies in the X-ray (Chandra), the optical/near-infrared (Hubble Space Telescope), and the radio (Karl G. Jansky Very Large Array). These three galaxies were previously identified as hosting candidate active galactic nuclei on the basis of lower resolution X-ray imaging. With our new observations, we find that X-ray sources in two galaxies (SDSS J121326.01+543631.6 and SDSS J122111.29+173819.1) are off-nuclear and lack corresponding radio emission, implying they are likely luminous X-ray binaries. The third galaxy (Mrk 1434) contains two X-ray sources (each with LX ≈ 1040 erg s−1) separated by 2.8 arcsec, has a low metallicity 12 + log(O/H) = 7.8, and emits nebular He ii λ4686 line emission. The northern source has spatially coincident point-like radio emission at 9.0 GHz and extended radio emission at 5.5 GHz. We discuss X-ray binary interpretations (where an ultraluminous X-ray source blows a ‘radio bubble’) and active galactic nucleus interpretations (where an $\approx 4\times 10^5 \, \mathrm{M}_\odot$ black hole launches a jet). In either case, we find that the He ii emission cannot be photoionized by the X-ray source, unless the source was ≈30–90 times more luminous several hundred years ago.
Little is known about the properties of the accretion flows and jets of the lowest-luminosity quiescent black holes. We report new, strictly simultaneous radio and X-ray observations of the nearby ...stellar-mass black hole X-ray binary GS 2000+25 in its quiescent state. In deep Chandra observations we detect the system at a faint X-ray luminosity of erg s−1 (1-10 keV). This is the lowest X-ray luminosity yet observed for a quiescent black hole X-ray binary, corresponding to an Eddington ratio LX/LEdd ∼ 10−9. In 15 hours of observations with the Karl G. Jansky Very Large Array, no radio continuum emission is detected to a 3 limit of <2.8 Jy at 6 GHz. Including GS 2000+25, four quiescent stellar-mass black holes with LX < 1032 erg s−1 have deep simultaneous radio and X-ray observations and known distances. These sources all have radio to X-ray luminosity ratios generally consistent with, but slightly lower than, the low-state radio/X-ray correlation for stellar-mass black holes with LX > 1032 erg s−1. Observations of these sources tax the limits of our current X-ray and radio facilities, and new routes to black hole discovery are needed to study the lowest-luminosity black holes.
Abstract
We report the discovery of Pavo, a faint (
M
V
= −10.0), star-forming, irregular, and extremely isolated dwarf galaxy at
D
≈ 2 Mpc. Pavo was identified in Dark Energy Camera Legacy Survey ...imaging via a novel approach that combines low surface brightness galaxy search algorithms and machine-learning candidate classifications. Follow-up imaging with the Inamori-Magellan Areal Camera and Spectrograph on the 6.5 m Magellan Baade telescope revealed a color–magnitude diagram (CMD) with an old stellar population, in addition to the young population that dominates the integrated light, and a tip of the red giant branch distance estimate of
1.99
−
0.22
+
0.20
Mpc. The blue population of stars in the CMD is consistent with the youngest stars having formed no later than 150 Myr ago. We also detected no H
α
emission with SOAR telescope imaging, suggesting that we may be witnessing a temporary low in Pavo’s star formation. We estimate the total stellar mass of Pavo to be
log
M
*
/
M
⊙
=
5.6
±
0.2
and measure an upper limit on its H
i
gas mass of 1.0 × 10
6
M
⊙
based on the HIPASS survey. Given these properties, Pavo’s closest analog is Leo P (
D
= 1.6 Mpc), previously the only known isolated, star-forming, Local Volume dwarf galaxy in this mass range. However, Pavo appears to be even more isolated, with no other known galaxy residing within over 600 kpc. As surveys and search techniques continue to improve, we anticipate an entire population of analogous objects being detected just outside the Local Group.
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