377 research outputs found
On the X-ray Spectra of Anomalous X-ray Pulsars and Soft Gamma Repeaters
We revisit the apparent correlation between soft X-ray band photon index and
spin-down rate nudot previously reported for Anomalous X-ray Pulsars (AXPs) and
Soft Gamma Repeaters (SGRs) by Marsden & White (2001). Our analysis, improved
thanks to new source discoveries, better spectral parameter measurements in
previously known sources, and the requirement of source quiescence for
parameter inclusion, shows evidence for the previously noted trend, although
with greater scatter. This trend supports the twisted magnetosphere model of
magnetars although the scatter suggests that factors other than nudot are also
important. We also note possible correlations involving the spectra of AXPs and
SGRs in the hard X-ray band. Specifically, the hard-band photon index shows a
possible correlation with inferred nudot and B, as does the degree of spectral
turnover. If the former trend is correct, then the hard-band photon index for
AXP 1E 1048.1-5937 should be ~0--1. This may be testable with long integrations
by INTEGRAL, or by the upcoming focussing hard X-ray mission NuSTAR.Comment: 15 pages, Accepted for publication in ApJ Letter
Five years of SGR 1900+14 observations with BeppoSAX
We present a systematic analysis of all the BeppoSAX data of the soft
gamma-ray repeater SGR 1900+14: these observations allowed us to study the long
term properties of the source quiescent emission. In the observation carried
out before the 1998 giant flare the spectrum in the 0.8-10 keV energy range was
harder and there was evidence for a 20-150 keV emission, possibly associated
with SGR 1900+14. This possible hard tail, if compared with the recent INTEGRAL
detection of SGR 1900+14, has a harder spectrum (power-law photon index ~1.6
versus ~3) and a 20-100 keV flux ~4 times larger. In the last BeppoSAX
observation (April 2002), while the source was entering the long quiescent
period that lasted until 2006, the 2-10 keV flux was ~25% below the historical
level. We also studied in detail the spectral evolution during the 2001 flare
afterglow. This was characterized by a softening that can be interpreted in
terms of a cooling blackbody-like component.Comment: Accepted for publication on Astronomy and Astrophysics on August 31,
200
Chandra and RXTE Observations of 1E 1547.0-5408: Comparing the 2008 and 2009 Outbursts
We present results from observations of the magnetar 1E 1547.0-5408 (SGR
J1550-5418) taken with the Chandra X-ray Observatory and the Rossi X-ray Timing
Explorer (RXTE) following the source's outbursts in 2008 October and 2009
January. During the time span of the Chandra observations, which covers days 4
through 23 and days 2 through 16 after the 2008 and 2009 events, respectively,
the source spectral shape remained stable, while the pulsar's spin-down rate in
the same span in 2008 increased by a factor of 2.2 as measured by RXTE. The
lack of spectral variation suggests decoupling between magnetar spin-down and
radiative changes, hence between the spin-down-inferred magnetic field strength
and that inferred spectrally. We also found a strong anti-correlation between
the phase-averaged flux and the pulsed fraction in the 2008 and 2009 Chandra
data, but not in the pre-2008 measurements. We discuss these results in the
context of the magnetar model.Comment: 4 figures, accepted for publication in Ap
Agricultural Turns, Geographical Turns: Retrospect and Prospect.
It is accepted that British rural geography has actively engaged with the ‘cultural turn’, leading to a resurgence of research within the sub-discipline. However, a reading of recent reviews suggests that the cultural turn has largely, if not completely, bypassed those geographers interested in the agricultural sector. Farming centred engagements with notions of culture have been relatively limited compared with those concerned with the non-agricultural aspects of rural space. Indeed, agricultural geography represents something of an awkward case in the context of the disciplinary turn to culture, a situation that demands further exposition. In seeking explanation, it becomes evident that research on the farm sector is more culturally informed than initially appears. This paper argues that there have been both interesting and important engagements between agricultural geography and cultural perspectives over the past decade. The paper elaborates four specific areas of research which provide evidence for concern about the ‘culture’ within agriculture. The future contribution that culturally informed perspectives in geographical research can bring to agricultural issues is outlined by way of conclusion
Chandra astrometry sets a tight upper limit to the proper motion of SGR 1900+14
The soft gamma-ray repeater (SGR) SGR 1900+14 lies a few arcminutes outside
the edge of the shell supernova remnant (SNR) G42.8+0.6. A physical association
between the two systems has been proposed - for this and other SGR-SNR pairs -
based on the expectation of high space velocities for SGRs in the framework of
the magnetar model. The large angular separation between the SGR and the SNR
center, coupled with the young age of the system, suggest a test of the
association with a proper motion measurement. We used a set of three
Chandra/ACIS observations of the field spanning 5 years to perform accurate
relative astrometry in order to measure the possible angular displacement of
the SGR as a function of time. Our investigation sets a 3-sigma upper limit of
70 mas/yr to the overall proper motion of the SGR. Such a value argues against
an association of SGR 1900+14 with G42.8+0.6 and adds further support to the
mounting evidence for an origin of the SGR within a nearby, compact cluster of
massive stars.Comment: Accepted for publication in The Astrophysical Journal. 4 pages in
emulate-apj styl
Spectral properties of anomalous X-ray pulsars
In this paper, the spectra of the persistent emission from anomalous X-ray
pulsars (AXPs) and their variation with spin-down rate is
considered. Firstly, based on an accretion-powered model, the influences of
both magnetic field and mass accretion rate on the spectra properties of AXPs
are addressed. Subsequently, the relation between the spectral property of AXPs
and mass accretion rate is investigated. The result shows that there
exists a linear correlation between the photon index and mass accretion rate,
and the spectral hardness increases with increasing . A possible
emission mechanism for the explanation of spectral properties of AXPs is also
discussed.Comment: 11pages, 3 figures, Chin. J. Astron. Astrophys. in pres
Discovery of 5.16s pulsations from the isolated neutron star RBS 1223
The isolated neutron star candidate RBS 1223 was observed with the Advanced
CCD Imaging Spectrometer aboard the Chandra X-ray observatory on 2000 June 24.
A timing analysis of the data yielded a periodic modulation with a period
P=5.1571696^(+1.57*10^(-4) -1.36*10^(-4)s. Using ROSAT HRI archived
observations we detected a period P=5.1561274 \pm 4.4*10^(-4)s and determined
period derivative dP/dt=(0.7 - 2.0)*10^(-11) s*s^(-1). The detection of this
period and dP/dt indicates that RBS 12223 has a ``characteristic'' age of
6000-12000 years and huge magnetic field at the surface (B(dipole)~(1.7-
3.2)*10^(+14) G) typical for anomalous X-ray pulsars (AXPs).Comment: 7 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
Magnetars From Magnetized Cores Created by a Strong Interaction Phase Transition
We consider a model where the strong magnetic fields of magnetars arise from
a high baryon density, magnetized core. In this framework magnetars are
distinguished from pulsars by their higher masses and central density. For
magnetars, as core densities exceed a threshold, the strong interaction induces
a phase transition to a neutral pion condensate that aligns all magnetic
moments. The core magnetic field is initially shielded by the ambient high
conductivity plasma. With time the shielding currents dissipate transporting
the core field out, first to the crust and then breaking through the crust to
the surface of the star. Recent observations provide strong support for this
model which accounts for several properties of magnetars and also enables us to
identify new magnetars.Comment: 22 pages with 3 figures and 3 tables prepared in 'Science' forma
Rethinking the changing structures of rural local government - state power, rural politics and local political strategies?
Copyright © 2010 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Molecular and Biochemical Parasitology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Rural Studies, 2010, Vol. 26, Issue 3, pp 272–283 DOI: http://dx.doi.org/10.1016/j.jrurstud.2009.12.005There is a notable absence in contemporary rural studies - of both a theoretical and empirical nature - concerning the changing nature of rural local government. Despite the scale and significance of successive rounds of local government reorganisation in the UK, very little has been written on this topic from a rural perspective. Instead research on local political change has tended to concentrate on local governance and local partnerships – on the extra-governmental aspects of the governance system – rather than on local government itself. In contrast, this paper draws upon strategic relational state theory to explore the changing structures and institutions of rural local government, and analyse how these can be related to the changing state strategies of those groups which are politically powerful in rural areas. In this respect, the paper draws on current and previous rounds of local government reorganisation to illustrate how new objects of governance, new state strategies and new hegemonic projects are emerging as a consequence of such restructuring processes
Wind braking of magnetars
(adapted)Considering recent observations challenging the traditional magnetar
model, we explore the wind braking of magnetars. There is evidence for strong
multipole magnetic fields in active magnetars, but the dipole field inferred
from spin down measurements may be strongly biased by a particle wind. Recent
challenging observations of magnetars may be explained naturally in the wind
braking scenario: (1) The supernova energies of magnetars are of normal value;
(2) The non-detection in Fermi observations of magnetars; (3) The problem posed
by the low-magnetic field soft gamma-ray repeaters; (4) The relation between
magnetars and high magnetic field pulsars; (5) A decreasing period derivative
during magnetar outbursts. Transient magnetars may still be magnetic dipole
braking. This may explain why low luminosity magnetars are more likely to have
radio emissions. In the wind braking scenario, magnetars are neutron stars with
strong multipole field. For some sources, a strong dipole field may be no
longer needed. A magnetism-powered pulsar wind nebula and a braking index
smaller than three are the two predictions of the wind braking model.Comment: 34 pages, 1 table, 6 figures, accepted by Ap
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