105 research outputs found
The Galactic halo magnetic field revisited
Recently, Sun et al. (2008) published new Galactic 3D-models of magnetic
fields in the disk and halo of the Milky Way and the distribution of cosmic-ray
electron density by taking into account the thermal electron density model
NE2001 by Cordes & Lazio (2002, 2003). The models successfully reproduce
observed continuum and polarization all-sky maps and the distribution of
rotation measures of extragalactic sources across the sky. However, the model
parameters obtained for the Galactic halo, although reproducing the
observations, seem physically unreasonable: the magnetic field needs to be
significantly stronger in the Galactic halo than in the plane and the
cosmic-ray distribution must be truncated at about 1 kpc to avoid excessive
synchrotron emission from the halo. The reason for these unrealistic parameters
was the low scale-height of the warm thermal gas of about 1 kpc adapted in the
NE2001 model. However, this scale-height seemed well settled by numerous
investigations. Recently, the scale-height of the warm gas in the Galaxy was
revised by Gaensler et al. (2008) to about 1.8 kpc, by showing that the 1 kpc
scale-height results from a systematic bias in the analysis of pulsar data.
This implies a higher thermal electron density in the Galactic halo, which in
turn reduces the halo magnetic field strength to account for the observed
rotation measures of extragalactic sources. We slightly modified the NE2001
model for the new scale-height and revised the Sun et al. (2008) model
parameters accordingly: the strength of the regular halo magnetic field is now
2 microG or lower, and the physically unrealistic cutoff in z for the
cosmic-ray electron density is removed. The simulations based on the revised
3D-models reproduce all-sky observations as before.Comment: 11 pages, 6 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
Eski mektuplar
Ahmet Mithat'ın TercĂŒman-ı Hakikat'te tefrika edilen Eski Mektuplar adlı roman
Topic identification challenge
Merit, Expertise and Measuremen
The Global Magneto-Ionic Medium Survey (GMIMS): The brightest polarized region in the Southern sky at 75cm and its implications for Radio Loop II
Using the Global Magneto-Ionic Medium Survey (GMIMS) Low-Band South (LBS)
southern sky polarization survey, covering 300 to 480 MHz at 81 arcmin
resolution, we reveal the brightest region in the Southern polarized sky at
these frequencies. The region, G150-50, covers nearly 20deg, near
(l,b)~(150 deg,-50 deg). Using GMIMS-LBS and complementary data at higher
frequencies (~0.6--30 GHz), we apply Faraday tomography and Stokes QU-fitting
techniques. We find that the magnetic field associated with G150-50 is both
coherent and primarily in the plane of the sky, and indications that the region
is associated with Radio Loop II. The Faraday depth spectra across G150-50 are
broad and contain a large-scale spatial gradient. We model the magnetic field
in the region as an expanding shell, and we can reproduce both the observed
Faraday rotation and the synchrotron emission in the GMIMS-LBS band. Using
QU-fitting, we find that the Faraday spectra are produced by several Faraday
dispersive sources along the line-of-sight. Alternatively, polarization horizon
effects that we cannot model are adding complexity to the high-frequency
polarized spectra. The magnetic field structure of Loop II dominates a large
fraction of the sky, and studies of the large-scale polarized sky will need to
account for this object. Studies of G150-50 with high angular resolution could
mitigate polarization horizon effects, and clarify the nature of G150-50.Comment: 25 pages, 14 figures. Accepted for publication in MNRA
CpG-Methylation Regulates a Class of Epstein-Barr Virus Promoters
DNA methylation is the major modification of eukaryotic genomes and plays an essential role in mammalian gene regulation. In general, cytosine-phosphatidyl-guanosine (CpG)-methylated promoters are transcriptionally repressed and nuclear proteins such as MECP2, MBD1, MBD2, and MBD4 bind CpG-methylated DNA and contribute to epigenetic silencing. Methylation of viral DNA also regulates gene expression of Epstein-Barr virus (EBV), which is a model of herpes virus latency. In latently infected human B cells, the viral DNA is CpG-methylated, the majority of viral genes is repressed and virus synthesis is therefore abrogated. EBV's BZLF1 encodes a transcription factor of the AP-1 family (Zta) and is the master gene to overcome viral gene repression. In a genome-wide screen, we now identify and characterize those viral genes, which Zta regulates. Among them are genes essential for EBV's lytic phase, which paradoxically depend on strictly CpG-methylated promoters for their Zta-induced expression. We identified novel DNA recognition motifs, termed meZRE (methyl-Zta-responsive element), which Zta selectively binds in order to âreadâ DNA in a methylation- and sequence-dependent manner unlike any other known protein. Zta is a homodimer but its binding characteristics to meZREs suggest a sequential, non-palindromic and bipartite DNA recognition element, which confers superior DNA binding compared to CpG-free ZREs. Our findings indicate that Zta has evolved to transactivate cytosine-methylated, hence repressed, silent promoters as a rule to overcome epigenetic silencing
The Great American Crime Decline : Possible Explanations
This chapter examines the most important features of the crime decline in the United States during the 1990s-2010s but also takes a broader look at the violence declines of the last three centuries. The author argues that violent and property crime trends might have diverged in the 1990s, with property crimes increasingly happening in the online sphere and thus traditional property crime statistics not being reflective of the full picture. An important distinction is made between âcontact crimesâ and crimes that do not require a victim and offender to be present in the same physical space. Contrary to the uncertainties engendered by property crime, the declines in violent (âcontactâ) crime are rather general, and have been happening not only across all demographic and geographic categories within the United States but also throughout the developed world. An analysis of research literature on crime trends has identified twenty-four different explanations for the crime drop. Each one of them is briefly outlined and examined in terms of conceptual clarity and empirical support. Nine crime decline explanations are highlighted as the most promising ones. The majority of these promising explanations, being relative newcomers in the crime trends literature, have not been subjected to sufficient empirical scrutiny yet, and thus require further research. One potentially fruitful avenue for future studies is to examine the association of the most promising crime decline explanations with improvements in self-control
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