CLUSTER MAGNETIC FIELDS Carilli, C. L; Taylor, G. B
Annual review of astronomy and astrophysics,
01/2002, Volume:
40, Issue:
1
Journal Article
Peer reviewed
Magnetic fields in the intercluster medium have been measured using a
variety of techniques, including studies of synchrotron relic and halo radio
sources within clusters, studies of inverse Compton ...X-ray emission from
clusters, surveys of Faraday rotation measures of polarized radio sources both
within and behind clusters, and studies of cluster cold fronts in X-ray images.
These measurements imply that most cluster atmospheres are substantially
magnetized, with typical field strengths of order 1 μGauss with high areal
filling factors out to Mpc radii. There is likely to be considerable variation
in field strengths and topologies both within and between clusters, especially
when comparing dynamically relaxed clusters to those that have recently
undergone a merger. In some locations, such as the cores of cooling flow
clusters, the magnetic fields reach levels of 10-40 μG and may be
dynamically important. In all clusters the magnetic fields have a significant
effect on energy transport in the intracluster medium. We also review current
theories on the origin of cluster magnetic fields.
Using Chandra X-ray observations of nine nearby, X-ray luminous elliptical galaxies with good optical velocity dispersion measurements, we show that a tight correlation exists between the Bondi ...accretion rates calculated from the observed gas temperature and density profiles and estimated black hole masses, and the power emerging from these systems in relativistic jets. The jet powers, which are inferred from the energies and time-scales required to inflate cavities observed in the surrounding X-ray emitting gas, can be related to the accretion rates using a power-law model of the form log (PBondi/1043 erg s−1) =A+B log (Pjet/1043 erg s−1), with A= 0.65 ± 0.16 and B= 0.77 ± 0.20. Our results show that a significant fraction of the energy associated with the rest mass of material entering the Bondi accretion radius (2.2+1.0−0.7 per cent, for Pjet= 1043 erg s−1) eventually emerges in the relativistic jets. The data also hint that this fraction may rise slightly with increasing jet power. Our results have significant implications for studies of accretion, jet formation and galaxy formation. The observed tight correlation suggests that the Bondi formulae provide a reasonable description of the accretion process in these systems, despite the likely presence of magnetic pressure and angular momentum in the accreting gas. The similarity of the PBondi and Pjet values argues that a significant fraction of the matter entering the accretion radius flows down to regions close to the black holes, where the jets are presumably formed. The tight correlation between PBondi and Pjet also suggests that the accretion flows are approximately stable over time-scales of a few million years. Our results show that the black hole ‘engines’ at the hearts of large elliptical galaxies and groups can feed back sufficient energy to stem cooling and star formation, leading naturally to the observed exponential cut off at the bright end of the galaxy luminosity function.
Context.
According to radiative models, radio galaxies may produce
γ
-ray emission from the first stages of their evolution. However, very few such galaxies have been detected by the
Fermi
Large Area ...Telescope (LAT) so far.
Aims.
NGC 3894 is a nearby (
z
= 0.0108) object that belongs to the class of compact symmetric objects (CSOs, i.e., the most compact and youngest radio galaxies), which is associated with a
γ
-ray counterpart in the Fourth
Fermi
-LAT source catalog. Here we present a study of the source in the
γ
-ray and radio bands aimed at investigating its high-energy emission and assess its young nature.
Methods.
We analyzed 10.8 years of
Fermi
-LAT data between 100 MeV and 300 GeV and determined the spectral and variability characteristics of the source. Multi-epoch very long baseline array (VLBA) observations between 5 and 15 GHz over a period of 35years were used to study the radio morphology of NGC 3894 and its evolution.
Results.
NGC 3894 is detected in
γ
-rays with a significance >9
σ
over the full period, and no significant variability has been observed in the
γ
-ray flux on a yearly time-scale. The spectrum is modeled with a flat power law (Γ = 2.0 ± 0.1) and a flux on the order of 2.2 × 10
−9
ph cm
−2
s
−1
. For the first time, the VLBA data allow us to constrain with high precision the apparent velocity of the jet and counter-jet side to be
β
app, NW
= 0.132 ± 0.004 and
β
app, SE
= 0.065 ± 0.003, respectively.
Conclusions.
Fermi
-LAT and VLBA results favor the youth scenario for the inner structure of this object, with an estimated dynamical age of 59 ± 5 years. The estimated range of viewing angle (10° <
θ
< 21°) does not exclude a possible jet-like origin of the
γ
-ray emission.
The scattering of dark matter (DM) particles with sub-GeV masses off nuclei is difficult to detect using liquid xenon-based DM search instruments because the energy transfer during nuclear recoils is ...smaller than the typical detector threshold. However, the tree-level DM-nucleus scattering diagram can be accompanied by simultaneous emission of a bremsstrahlung photon or a so-called "Migdal" electron. These provide an electron recoil component to the experimental signature at higher energies than the corresponding nuclear recoil. The presence of this signature allows liquid xenon detectors to use both the scintillation and the ionization signals in the analysis where the nuclear recoil signal would not be otherwise visible. We report constraints on spin-independent DM-nucleon scattering for DM particles with masses of 0.4-5 GeV/c^{2} using 1.4×10^{4} kg day of search exposure from the 2013 data from the Large Underground Xenon (LUX) experiment for four different classes of mediators. This analysis extends the reach of liquid xenon-based DM search instruments to lower DM masses than has been achieved previously.
Several early childhood obesity prediction models have been developed, but none for New Zealand's diverse population. We aimed to develop and validate a model for predicting obesity in 4-5-year-old ...New Zealand children, using parental and infant data from the Growing Up in New Zealand (GUiNZ) cohort. Obesity was defined as body mass index (BMI) for age and sex ≥ 95th percentile. Data on GUiNZ children were used for derivation (n = 1731) and internal validation (n = 713). External validation was performed using data from the Prevention of Overweight in Infancy Study (POI, n = 383) and Pacific Islands Families Study (PIF, n = 135) cohorts. The final model included: birth weight, maternal smoking during pregnancy, maternal pre-pregnancy BMI, paternal BMI, and infant weight gain. Discrimination accuracy was adequate AUROC = 0.74 (0.71-0.77), remained so when validated internally AUROC = 0.73 (0.68-0.78) and externally on PIF AUROC = 0.74 0.66-0.82) and POI AUROC = 0.80 (0.71-0.90). Positive predictive values were variable but low across the risk threshold range (GUiNZ derivation 19-54%; GUiNZ validation 19-48%; and POI 8-24%), although more consistent in the PIF cohort (52-61%), all indicating high rates of false positives. Although this early childhood obesity prediction model could inform early obesity prevention, high rates of false positives might create unwarranted anxiety for families.
Abstract
Approximately one-third of existing
γ
-ray sources identified by the Fermi Gamma-Ray Space Telescope are considered to be unassociated, with no known counterpart at other ...frequencies/wavelengths. These sources have been the subject of intense scrutiny and observational effort during the observatory’s mission lifetime, and here we present a method of leveraging existing radio catalogs to examine these sources without the need for specific dedicated observations, which can be costly and complex. Via the inclusion of many sensitive low-frequency catalogs we specifically target steep-spectrum sources such as pulsars. This work has found steep-spectrum radio sources contained inside 591 Fermi unassociated fields, with at least 21 of them being notable for having pulsar-like
γ
-ray properties as well. We also identify a number of other fields of interest based on various radio and
γ
-ray selections.
A search for sub-GeV dark matter produced from collisions of the Fermilab 8 GeV Booster protons with a steel beam dump was performed by the MiniBooNE-DM Collaboration using data from 1.86 × 1020 ...protons on target in a dedicated run. The MiniBooNE detector, consisting of 818 tons of mineral oil and located 490 meters downstream of the beam dump, is sensitive to a variety of dark matter initiated scattering reactions. Three dark matter interactions are considered for this analysis: elastic scattering off nucleons, inelastic neutral pion production, and elastic scattering off electrons. Multiple data sets were used to constrain flux and systematic errors, and time-of-flight information was employed to increase sensitivity to higher dark matter masses. No excess from the background predictions was observed, and 90% confidence level limits were set on the vector portal and leptophobic dark matter models. New parameter space is excluded in the vector portal dark matter model with a dark matter mass between 5 and 50 MeV c−2 . The reduced neutrino flux allowed to test if the MiniBooNE neutrino excess scales with the production of neutrinos. No excess of neutrino oscillation events were measured ruling out models that scale solely by number of protons on target independent of beam configuration at 4.6σ.
The atmosphere of Pluto as observed by New Horizons Gladstone, G. Randall; Stern, S. Alan; Ennico, Kimberly ...
Science (American Association for the Advancement of Science),
03/2016, Volume:
351, Issue:
6279
Journal Article
Peer reviewed
Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. Whereas the lower atmosphere (at altitudes of less than 200 kilometers) is ...consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are abundant minor species and likely feed the production of an extensive haze that encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state--over seasonal or geologic time scales.
We present constraints on weakly interacting massive particles (WIMP)-nucleus scattering from the 2013 data of the Large Underground Xenon dark matter experiment, including 1.4×10^{4} kg day of ...search exposure. This new analysis incorporates several advances: single-photon calibration at the scintillation wavelength, improved event-reconstruction algorithms, a revised background model including events originating on the detector walls in an enlarged fiducial volume, and new calibrations from decays of an injected tritium β source and from kinematically constrained nuclear recoils down to 1.1 keV. Sensitivity, especially to low-mass WIMPs, is enhanced compared to our previous results which modeled the signal only above a 3 keV minimum energy. Under standard dark matter halo assumptions and in the mass range above 4 GeV c^{-2}, these new results give the most stringent direct limits on the spin-independent WIMP-nucleon cross section. The 90% C.L. upper limit has a minimum of 0.6 zb at 33 GeV c^{-2} WIMP mass.
Notes that while prediction models can estimate an infant’s risk of developing obesity at a later point in early childhood, caregiver receptiveness to such information is largely unknown. Aims to ...assess the acceptability of these models to New Zealand caregivers. Source: National Library of New Zealand Te Puna Matauranga o Aotearoa, licensed by the Department of Internal Affairs for re-use under the Creative Commons Attribution 3.0 New Zealand Licence.
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