5,865 research outputs found
Injective and Projective Model Structures on Enriched Diagram Categories
In the enriched setting, the notions of injective and projective model
structures on a category of enriched diagrams also make sense. In this paper,
we prove the existence of these model structures on enriched diagram categories
under local presentability, accessibility, and "acyclicity" conditions, using
the methods of lifting model structures from an adjunction introduced by
Garner, Hess, Kedziorek, Riehl, and Shipley.Comment: 21 page
The 1997 event in the Crab pulsar revisited
A complex event observed in the radio pulses from the Crab pulsar in 1997
included echoes, a dispersive delay, and large changes in intensity. It is
shown that these phenomena were due to refraction at the edge of a plasma cloud
in the outer region of the Crab Nebula. Several similar events have been
observed, although in less detail. It is suggested that the plasma cloud is in
the form of filaments with diameter around 3 x 10^11m and electron density of
order 10^4 cm-3Comment: 5 pages 4 figs Accepted by MNRA
[Review of] Mark Christian Thompson. Black Fascisms: African American Literature and Culture Between the Wars
In How Bigger Was Born, Richard Wright described the political choice available to young black men like Bigger Thomas as being between communism and fascism. A plethora of recent scholarship from critics like Barbara Foley, James Smethurst, and William Maxwell has articulated the complex relationship between black and red in the first half of the twentieth century. Mark Christian Thompson\u27s Black Fascisms begins to explore the other half of Wright\u27s binary, tracing the uses of fascist ideology in the work of Marcus Garvey, George S. Schuyler, Claude McKay, Zora Neale Hurston, and Richard Wright
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Impact of housing options to inform the development of the regional spatial strategy
The report summarises work undertaken by Experian for emda to assess the economic impact of the preferred housing option set out in the Regional Spatial Strategy (RSS) and examined alternate scenarios linked to population projections
‘Large complaints in little papers’ : negotiating Ovidian genealogies of complaint in Drayton's Englands Heroicall Epistles
Taking as its starting point Michael Drayton's reworking of a key Heroidean topos, the heroine's self-conscious reflection on letter-writing as an activity fraught with anxiety, this essay examines the cultural and literary factors that conspire to inhibit or facilitate the emergence of a distinctive feminine epistolary voice in Englands Heroicall Epistles. In particular it seeks to explain how Drayton's female letter-writers manage to negotiate the impediments to self-expression they initially encounter and thus go on to articulate morally and politically incisive forms of complaint. It argues that the participation of Drayton's fictional writers in the authorial business of revising Ovid for an altered historical context plays a crucial role in supporting that process. This allows Drayton's heroines to recover a degree of textual authority through an independent critical engagement, by turns resistant and identificatory, with his Ovidian sources, including the Metamorphoses as well as the Heroides. A comparative analysis of the ways in which intertextual allusions to these sources are deployed by his male and female writers reveals them to be governed by a different dynamic and used for different ends. It is primarily by means of their complex, intersecting dialogues with their male correspondents and with the Ovidian models upon which they draw that Drayton's heroines are able to formulate a compelling counter-perspective on the politics of love and history
Observations of four glitches in the young pulsar J1833-1034 and study of its glitch activity
We present the results from timing observations with the GMRT of the young
pulsar J1833-1034, in the galactic supernova remnant G21.5-0.9. We detect the
presence of 4 glitches in this pulsar over a period of 5.5 years, making it one
of a set of pulsars that show fairly frequent glitches. The glitch amplitudes,
characterized by the fractional change of the rotational frequency, range from
1 \times 10^-9 to 7 \times 10^-9, with no evidence for any appreciable
relaxation of the rotational frequency after the glitches. The fractional
changes observed in the frequency derivative are of the order of 10-5 . We show
conclusively that, in spite of having significant timing noise, the sudden
irregularities like glitches detected in this pulsar can not be modeled as
smooth timing noise. Our timing solution also provides a stable estimate of the
second derivative of the pulsar spin-down model, and a plausible value for the
braking index of 1.857, which, like the value for other such young pulsars, is
much less than the canonical value of 3.0. PSR J1833-1034 appears to belong to
a class of pulsars exhibiting fairly frequent occurrence of low amplitude
glitches. This is further supported by an estimate of the glitch activity
parameter, Ag = 1.53 \times 10^-15 s^-2, which is found to be significantly
lower than the trend of glitch activity versus characteristic age (or spin
frequency derivative) that a majority of the glitching pulsars follow. We
present evidence for a class of such young pulsars, including the Crab, where
higher internal temperature of the neutron star could be responsible for the
nature of the observed glitch activity.Comment: 10 pages, 16 figures, accepted for publication in MNRA
Unusual glitch activity in the RRAT J1819-1458: an exhausted magnetar?
We present an analysis of regular timing observations of the
high-magnetic-field Rotating Radio Transient (RRAT) J18191458 obtained using
the 64-m Parkes and 76-m Lovell radio telescopes over the past five years.
During this time, the RRAT has suffered two significant glitches with
fractional frequency changes of and .
Glitches of this magnitude are a phenomenon displayed by both radio pulsars and
magnetars. However, the behaviour of J18191458 following these glitches is
quite different to that which follows glitches in other neutron stars, since
the glitch activity resulted in a significant long-term net decrease in the
slow-down rate. If such glitches occur every 30 years, the spin-down rate, and
by inference the magnetic dipole moment, will drop to zero on a timescale of a
few thousand years. There are also significant increases in the rate of pulse
detection and in the radio pulse energy immediately following the glitches.Comment: accepted for publication in MNRAS, 7 pages, 7 figures, 1 tabl
The geometry of the double-pulsar system J0737-3039 from systematic intensity variations
The recent discovery of J0737-3039A & B-two pulsars in a highly relativistic
orbit around one another - offers an unprecedented opportunity to study the
elusive physics of pulsar radio emission. The system contains a rapidly
rotating pulsar with a spin period of 22.7 ms and a slow companion with a spin
period of 2.77 s, hereafter referred to as 'A' and 'B', respectively. A unique
property of the system is that the pulsed radio flux from B increases
systematically by almost two orders-of-magnitude during two short portions of
each orbit. Here, we describe a geometrical model of the system that
simultaneously explains the intensity variations of B and makes definitive and
testable predictions for the future evolution of the emission properties of
both stars. Our model assumes that B's pulsed radio flux increases when
illuminated by emission from A. This model provides constraints on the spin
axis orientation and emission geometry of A and predicts that its pulse profile
will evolve considerably over the next several years due to geodetic precession
until it disappears entirely in 15-20 years
Testing Models of Magnetic Field Evolution of Neutron Stars with the Statistical Properties of Their Spin Evolutions
We test models for the evolution of neutron star (NS) magnetic fields (B).
Our model for the evolution of the NS spin is taken from an analysis of pulsar
timing noise presented by Hobbs et al. (2010). We first test the standard model
of a pulsar's magnetosphere in which B does not change with time and magnetic
dipole radiation is assumed to dominate the pulsar's spin-down. We find this
model fails to predict both the magnitudes and signs of the second derivatives
of the spin frequencies (). We then construct a phenomenological
model of the evolution of , which contains a long term decay (LTD) modulated
by short term oscillations (STO); a pulsar's spin is thus modified by its
B-evolution. We find that an exponential LTD is not favored by the observed
statistical properties of for young pulsars and fails to explain
the fact that is negative for roughly half of the old pulsars. A
simple power-law LTD can explain all the observed statistical properties of
. Finally we discuss some physical implications of our results to
models of the B-decay of NSs and suggest reliable determination of the true
ages of many young NSs is needed, in order to constrain further the physical
mechanisms of their B-decay. Our model can be further tested with the measured
evolutions of and for an individual pulsar; the decay
index, oscillation amplitude and period can also be determined this way for the
pulsar.Comment: To appear in ApJ. 20 pages, 10 figures, first submitted to ApJ on May
14, 2012; referee comments incorporated and re-submitted; typos corrected and
one reference added; additional minor comments from the referee incorporate
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