2,900 research outputs found
The swansong in context: long-timescale X-ray variability of NGC 4051
On 9-11 May 1998, the highly-variable, low luminosity Seyfert 1 galaxy
NGC4051 was observed in an unusual low flux state by BeppoSAX (Guainazzi et al.
1998) RXTE and EUVE. We present fits of the 4-15 keV RXTE spectrum and BeppoSAX
MECS spectrum obtained during this observation, which are consistent with the
interpretation that the source had switched off, leaving only the spectrum of
pure reflection from distant cold matter. We place this result in context by
showing the X-ray lightcurve of NGC4051 obtained by our RXTE monitoring
campaign over the past two and a half years, which shows that the low state
lasted for ~150 days before the May observations (implying that the reflecting
material is > 10^17 cm from the continuum source) and forms part of a
lightcurve showing distinct variations in long-term average flux over
timescales > months. We show that the long-timescale component to X-ray
variability is intrinsic to the primary continuum and is probably distinct from
the variability at shorter timescales, possibly associated with variations in
the accretion flow of matter onto the central black hole. As the source
approaches the low state, the variability process becomes non-linear. NGC4051
may represent a microcosm of all X-ray variability in radio quiet active
galactic nuclei (AGNs), displaying in a few years a variety of flux states and
variability properties which more luminous AGNs may pass through on timescales
of decades to thousands of years.Comment: 5 pages, accepted for publication in MNRA
Short Timescale AGN X-ray Variability with EXOSAT: Black hole mass and Normalised Variability Amplitude
The old EXOSAT medium energy measurements of high frequency (HF) AGN power
spectral normalisation are re-examined in the light of accurate black hole mass
determinations which were not available when these data were first published
(Green et al 1993). It is found that the normalised variability amplitude
(NVA), measured directly from the power spectrum, is proportional to M^{beta}
where beta ~ -0.54 +/- 0.08. As NVA is the square root of the power, these
observations show that the normalisation of the HF power spectrum for this
sample of AGN varies very close to inversely with black hole mass. Almost the
same value of is obtained whether the quasar 3C273 is included in the
sample or not, suggesting that the same process that drives X-ray variability
in Seyfert galaxies applies also to 3C273. These observations support the work
of Gierlinski et al (2008) who show that an almost exactly linear
anticorrelation is required if the normalisations of the HF power spectra of
AGN and X-ray binary systems are to scale similarly. These observations are
also consistent with a number of studies showing that the short timescale
variance of AGN X-ray lightcurves varies approximately inversely with mass.Comment: 6 pages, 2 figures, accepted for publication in Monthly Notices as a
lette
Adult reading teachers’ beliefs about how less-skilled adult readers can be taught to read.
Despite large-scale interventions, significant numbers of adults worldwide continue to have problems with basic literacy, in particular in the area of reading. To be effective, adult reading teachers need expert knowledge at practitioner level. However, practices in adult reading education vary widely, often reflecting the individual beliefs of each teacher about how an adult can learn to read. In this study, phenomenographic analysis was used to identify categories of approaches to teaching adult reading, used by a group of 60 teachers in Western Australia and New Zealand. Four approaches were identified: reassurance, task-based, theory-based and responsive. It is argued that for teachers to become effective and consistent in responding to learner needs, they must understand their own beliefs and the consequences of these. The identification of different approaches in adult reading education is an important step in this process
An Elasticity Measure Of Welfare Loss In Symmetric Oligopoly
We derive a measure of welfare loss as a proportion of the value of sales under quantity-setting symmetric oligopoly in terms of the equilibrium industry price elasticity of demand, the number of firms in the industry and a conjectural variation term in the context of the standard linear model. This generalises the monopoly measure in James and McHardy (1997)
Airport Deregulation and Airline Competition
Liberalisation has affected all parts of the air travel industry, with airports as well as airlines increasingly run on commercial lines. This paper models interactions between airports and airlines to show that, for example, the potential benefits to passengers of increased competition between airlines may be (more than) absorbed by the unregulated airports through which they travel, and that effecting airport competition in one country may lead to the majority of the gains going abroad. The policy conclusion is that the (de)regulation of airlines and associated services should be fully co-ordinated and internationally coherent. Keywords: Airports, airlines, competition, deregulation
Measuring the Balance of Intra-Regional Trade
We introduce an original method of measuring the extent of the overall balance of migration among countries within a region which allows comparisons of the balance over time as well as between regions and various possible sub-regions
Airport Deregulation and Airline Competition
Liberalisation has affected all parts of the air travel industry, with airports as well as airlines increasingly run on commercial lines. This paper models interactions between airports and airlines to show that, for example, the potential benefits to passengers of increased competition between airlines may be (more than) absorbed by the unregulated airports through which they travel, and that effecting airport competition in one country may lead to the majority of the gains going abroad. The policy conclusion is that the (de)regulation of airlines and associated services should be fully co-ordinated and internationally coherent. Keywords: Airports, airlines, competition, deregulation
Secrecy versus patents: process innovations and the role of uncertainty
Whilst firms often prefer secrecy to patents and process innovations particu- larly lend themselves to secrecy, we establish a rationale for process innovators who patent. Using a simple two-period model, we show that under myopic op- timisation, the incentive to patent rather than pursue secrecy increases as the probability that the rival firm attaches to it being low-cost falls and as the pro- portion of the cost reduction due to the innovation, secured by the rival firm in the period after the patent has expired, falls. However, the gain to the innovating firm from patenting rather than secrecy strictly increases if the cost reduction due to the innovation is sufficiently small that the high-cost firm could profitably bluff that it is low-cost. Finally, allowing the low-cost firm the option of using an output signal in such cases, may make the patent strategy more or less attractive relative to the case of myopic optimisation
A Slower Superluminal Velocity for the Quasar 1156+295
As part of an ongoing effort to observe high energy gamma-ray blazars with
VLBI, we have produced 8 and 2 GHz VLBI images, at ten epochs spanning the
years 1988 to 1996, of the quasar 1156+295. The VLBI data have been taken from
the Washington VLBI correlator's geodetic database. We have detected detected
four components and have measured their apparent speeds to be 8.8 +/- 2.3, 5.3
+/- 1.1, 5.5 +/- 0.9, and 3.5 +/- 1.2 h^{-1}c from the outermost component
inwards. (H_{0}=100h km/(s Mpc), q_{0}=0.5 throughout paper). These velocities
contradict a previously published very high superluminal velocity of 26 h^{-1}c
for this source.Comment: 8 pages, 2 figures, accepted to ApJ Letter
On the problem of network monopoly
We introduce a new regulatory concept: the independent profit-maximising agent, as a model for regulating a network monopoly. The agent sets prices on cross-network goods taking either a complete, or arbitrarily small, share of the associated profit. We examine welfare and profits with and without each agent type under both network monopoly and network duopoly. We show that splitting up the network monopoly (creating network duopoly) may be inferior for both firm(s) and society compared with a network monopoly "regulated" by an agent and that society always prefers any of the four agent regimes over network monopoly and network duopoly
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