2,263 research outputs found
Observable QPOs produced by steep pulse profiles in Magnetar Flares
Strong quasi-periodic oscillations in the tails of the giant gamma-ray flares
seen in SGR 1806-20 and SGR 1900+14 are thought to be produced by starquakes in
the flaring magnetar. However, the large fractional amplitudes (up to ~20%)
observed are difficult to reconcile with predicted amplitudes of starquakes.
Here we demonstrate that the steeply pulsed emission profile in the tail of the
giant flare can enhance the observed amplitude of the underlying oscillation,
analogously to a beam of light oscillating in and out of the line of sight.
This mechanism will also broaden the feature in the power spectrum and
introduce power at harmonics of the oscillation. The observed strength of the
oscillation depends on the amplitude of the underlying starquake, the
orientation and location of the emission on the surface of the star, and the
gradient of the light curve profile. While the amplification of the signal can
be significant, we demonstrate that even with uncertainties in the emission
geometry, this effect is not sufficient to produce the observed QPOs. This
result excludes the direct observation of a starquake, and suggests that the
observed variations come from modulations in the intensity of the emission.Comment: 5 pages, 4 figures, accepted by ApJ
Inside Money, Procyclical Leverage, and Banking Catastrophes
We explore a model of the interaction between banks and outside investors in
which the ability of banks to issue inside money (short-term liabilities
believed to be convertible into currency at par) can generate a collapse in
asset prices and widespread bank insolvency. The banks and investors share a
common belief about the future value of certain long-term assets, but they have
different objective functions; changes to this common belief result in
portfolio adjustments and trade. Positive belief shocks induce banks to buy
risky assets from investors, and the banks finance those purchases by issuing
new short-term liabilities. Negative belief shocks induce banks to sell assets
in order to reduce their chance of insolvency to a tolerably low level, and
they supply more assets at lower prices, which can result in multiple
market-clearing prices. A sufficiently severe negative shock causes the set of
equilibrium prices to contract (in a manner given by a cusp catastrophe),
causing prices to plummet discontinuously and banks to become insolvent.
Successive positive and negative shocks of equal magnitude do not cancel;
rather, a banking catastrophe can occur even if beliefs simply return to their
initial state. Capital requirements can prevent crises by curtailing the
expansion of balance sheets when beliefs become more optimistic, but they can
also force larger price declines. Emergency asset price supports can be
understood as attempts by a central bank to coordinate expectations on an
equilibrium with solvency.Comment: 31 pages, 10 figure
On the dependence of X-ray burst rate on accretion and spin rate
Nuclear burning and its dependence on the mass accretion rate are fundamental
ingredients for describing the complicated observational phenomenology of
neutron stars in binary systems. Motivated by high quality burst rate data
emerging from large statistical studies, we report general calculations
relating bursting rate to mass accretion rate and neutron star rotation
frequency. In this first work we neglect general relativistic effects and
accretion topology, though we discuss where their inclusion should play a role.
The relations we derive are suitable for different burning regimes and provide
a direct link between parameters predicted by theory and what is to be expected
in observations. We illustrate this for analytical relations of different
unstable burning regimes that operate on the surface of an accreting neutron
star. We also use the observed behaviour of burst rate to suggest new
constraints on burning parameters. We are able to provide an explanation for
the long standing problem of the observed decrease of burst rate with
increasing mass accretion that follows naturally from these calculations: when
accretion rate crosses a certain threshold, ignition moves away from its
initially preferential site and this can cause a net reduction of the burst
rate due to the effects of local conditions that set local differences in both
burst rate and stabilization criteria. We show under which conditions this can
happen even if locally the burst rate keeps increasing with accretion.Comment: Accepted for publication on Ap
The "New" Science of Networks
In recent years, the analysis and modeling of networks, and also networked
dynamical systems, have been the subject of considerable interdisciplinary
interest, yielding several hundred papers in physics, mathematics, computer science,
biology, economics, and sociology journals (Newman 2003c), as well as a number of
books (Barabasi 2002, Buchanan 2002,Watts 2003). Here I review the major findings
of this emerging field and discuss briefly their relationship with previous work in the
social and mathematical sciences
The Structure of Information Pathways in a Social Communication Network
Social networks are of interest to researchers in part because they are
thought to mediate the flow of information in communities and organizations.
Here we study the temporal dynamics of communication using on-line data,
including e-mail communication among the faculty and staff of a large
university over a two-year period. We formulate a temporal notion of "distance"
in the underlying social network by measuring the minimum time required for
information to spread from one node to another -- a concept that draws on the
notion of vector-clocks from the study of distributed computing systems. We
find that such temporal measures provide structural insights that are not
apparent from analyses of the pure social network topology. In particular, we
define the network backbone to be the subgraph consisting of edges on which
information has the potential to flow the quickest. We find that the backbone
is a sparse graph with a concentration of both highly embedded edges and
long-range bridges -- a finding that sheds new light on the relationship
between tie strength and connectivity in social networks.Comment: 9 pages, 10 figures, to appear in Proceedings of the 14th ACM SIGKDD
International Conference on Knowledge Discovery and Data Mining (KDD'08),
August 24-27, 2008, Las Vegas, Nevada, US
Discovery of Quasi-Periodic Oscillations in the Recurrent Burst Emission from SGR 1806-20
We present evidence for Quasi-Periodic Oscillations (QPOs) in the recurrent
outburst emission from the soft gamma repeater SGR 1806-20 using NASA's Rossi
X-ray Timing Explorer (RXTE) observations. By searching a sample of 30 bursts
for timing signals at the frequencies of the QPOs discovered in the 2004
December 27 giant flare from the source, we find three QPOs at 84, 103, and 648
Hz in three different bursts. The first two QPOs lie within 1
from the 92 Hz QPO detected in the giant flare. The third QPO lie within
9 from the 625 Hz QPO also detected in the same flare. The detected
QPOs are found in bursts with different durations, morphologies, and
brightness, and are vindicated by Monte Carlo simulations, which set a lower
limit confidence interval . We also find evidence for
candidate QPOs at higher frequencies in other bursts with lower statistical
significance. The fact that we can find evidence for QPOs in the recurrent
bursts at frequencies relatively close to those found in the giant flare is
intriguing and can offer insight about the origin of the oscillations. We
confront our finding against the available theoretical models and discuss the
connection between the QPOs we report and those detected in the giant flares.
The implications to the neutron star properties are also discussed.Comment: 5 pages, 2 figures, Accepted for publication in the Astrophysical
Journal Letters (ApJL
The diminishing state of shared reality on US television news
The potential for a large, diverse population to coexist peacefully is
thought to depend on the existence of a ``shared reality:'' a public sphere in
which participants are exposed to similar facts about similar topics. A
generation ago, broadcast television news was widely considered to serve this
function; however, since the rise of cable news in the 1990s, critics and
scholars have worried that the corresponding fragmentation and segregation of
audiences along partisan lines has caused this shared reality to be lost. Here
we examine this concern using a unique combination of data sets tracking the
production (since 2012) and consumption (since 2016) of television news content
on the three largest cable and broadcast networks respectively. With regard to
production, we find strong evidence for the ``loss of shared reality
hypothesis:'' while broadcast continues to cover similar topics with similar
language, cable news networks have become increasingly distinct, both from
broadcast news and each other, diverging both in terms of content and language.
With regard to consumption, we find more mixed evidence: while broadcast news
has indeed declined in popularity, it remains the dominant source of news for
roughly 50\% more Americans than does cable; moreover, its decline, while
somewhat attributable to cable, appears driven more by a shift away from news
consumption altogether than a growth in cable consumption. We conclude that
shared reality on US television news is indeed diminishing, but is more robust
than previously thought and is declining for somewhat different reasons
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