ABSTRACT
The diffusive shock acceleration of cosmic rays by supernova remnants depends upon the generation of magnetic fluctuations by cosmic rays upstream of the shock. Strongly driven, ...non‐resonant, nearly purely growing modes grow more rapidly than the resonant Alfvén waves usually considered. Non‐linear simulation shows that the magnetic field can be amplified from its seed value by orders of magnitude. The consequences for the maximum attainable cosmic ray energy in supernova remnants are explored.
Galactic cosmic-ray (CR) acceleration to the knee in the spectrum at a few PeV is only possible if the magnetic field ahead of a supernova remnant (SNR) shock is strongly amplified by CRs escaping ...the SNR. A model formulated in terms of the electric charge carried by escaping CRs predicts the maximum CR energy and the energy spectrum of CRs released into the surrounding medium. We find that historical SNRs such as Cas A, Tycho and Kepler may be expanding too slowly to accelerate CRs to the knee at the present time.
Abstract
The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand ...the characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ∼15,000 deg
2
of high Galactic latitude (∣
b
∣ > 10°) southern sky to a 5
σ
depth of
g
,
r
,
i
,
z
∼ 23.5 mag. In addition, DELVE will cover a region of ∼2200 deg
2
around the Magellanic Clouds to a depth of
g
,
r
,
i
∼ 24.5 mag and an area of ∼135 deg
2
around four Magellanic analogs to a depth of
g
,
i
∼ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. We also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ∼520 million astronomical sources covering ∼5000 deg
2
of the southern sky to a 5
σ
point-source depth of
g
= 24.3 mag,
r
= 23.9 mag,
i
= 23.3 mag, and
z
= 22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform.
Astronomical wide-field imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new ...wide-field interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependences of casa's w-projection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarization correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2–12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.
Mars thermosphere as seen in MAVEN accelerometer data Zurek, R. W.; Tolson, R. A.; Bougher, S. W. ...
Journal of geophysical research. Space physics,
March 2017, 2017-03-00, 20170301, Letnik:
122, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The Mars thermosphere (above approximately 120 km) has been probed in situ for one Mars year using accelerometers on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. This region ...is affected by radiation and energy deposition from the Sun and by energy and momentum from the lower atmosphere. Densities derived from measurements made during the nominal science orbits (periapsis > 140 km) show consistent trends with solar zenith angle and Sun‐Mars distance, reflecting direct and indirect heating of the thermosphere, although orbit‐to‐orbit variability is still significant. The six Deep Dip campaigns that MAVEN has conducted (with periapsis dropping below ~135 km) significantly extend the vertical profiles of the densities derived from accelerometer data. These show complex structure and high variability, both dependent on season, local time, location, and lower atmosphere activity, including dust storms and wave propagation from a dynamic lower atmosphere. In particular, the terminators are a region of convoluted structure and high variability, which may be greatest in the postmidnight, predawn hours of the sol. This space‐time regime was not sampled by previous orbiters at Mars. While initial comparisons with thermospheric general circulation modes show broad areas of agreement, these terminator transition regions are not simulated well by current models. Judicious choice of the timing of these Deep Dip campaigns during the remaining MAVEN mission, as periapsis continues to precess through local time, latitude, and longitude in both hemispheres and in different seasons, should help clarify the processes at work in this complicated region.
Key Points
MAVEN accelerometer data fill major gaps in space‐time coverage of Mars in situ measurements of thermospheric bulk densities
Comparisons with models revealed the largest differences in the lower thermosphere (120‐160 km) near the terminators, especially predawn
Density variations are largest at the lowest (and coldest) altitudes, above the equator at midnight, and over the predawn midlatitudes
Purpose
To determine whether early goal-directed therapy (EGDT) reduces mortality compared with other resuscitation strategies for patients presenting to the emergency department (ED) with septic ...shock.
Methods
Using a search strategy of PubMed, EmBase and CENTRAL, we selected all relevant randomised clinical trials published from January 2000 to January 2015. We translated non-English papers and contacted authors as necessary. Our primary analysis generated a pooled odds ratio (OR) from a fixed-effect model. Sensitivity analyses explored the effect of including non-ED studies, adjusting for study quality, and conducting a random-effects model. Secondary outcomes included organ support and hospital and ICU length of stay.
Results
From 2395 initially eligible abstracts, five randomised clinical trials (
n
= 4735 patients) met all criteria and generally scored high for quality except for lack of blinding. There was no effect on the primary mortality outcome (EGDT: 23.2 % 495/2134 versus control: 22.4 % 582/2601; pooled OR 1.01 95 % CI 0.88–1.16,
P
= 0.9, with heterogeneity
I
2
= 57 %;
P
= 0.055). The pooled estimate of 90-day mortality from the three recent multicentre studies (
n
= 4063) also showed no difference pooled OR 0.99 (95 % CI 0.86–1.15),
P
= 0.93 with no heterogeneity (
I
2
= 0.0 %;
P
= 0.97). EGDT increased vasopressor use (OR 1.25 95 % CI 1.10–1.41;
P
< 0.001) and ICU admission OR 2.19 (95 % CI 1.82–2.65);
P
< 0.001. Including six non-ED randomised trials increased heterogeneity (
I
2
= 71 %;
P
< 0.001) but did not change overall results pooled OR 0.94 (95 % CI 0.82 to 1.07);
P
= 0.33.
Conclusion
EGDT is not superior to usual care for ED patients with septic shock but is associated with increased utilisation of ICU resources.
A large flux of cosmic rays streaming through a magnetized plasma creates cavities of low plasma density and low magnetic field. The magnetic field focuses the cosmic ray trajectories into the ...cavities with the possible formation of filaments or beams of high-energy cosmic rays.
Relativistic charged particles (CRs for cosmic rays) produced by supernova explosion of the first generation of massive stars that are responsible for the reionization of the universe escape into the ...intergalactic medium, carrying an electric current. Charge imbalance and induction give rise to a return current, , carried by the cold thermal plasma which tends to cancel the CR current. The electric field, , required to draw the collisional return current opposes the outflow of low-energy CRs and ohmically heats the cold plasma. Owing to inhomogeneities in the resistivity, Delta *h(T), caused by a structure in the temperature, T, of the intergalactic plasma, the electric field possesses a rotational component which sustains Faraday's induction. It is found that a magnetic field is robustly generated throughout intergalactic space at a rate of 10--17 to 10--16 G Gyr--1, until the temperature of the intergalactic medium is raised by cosmic reionization. The magnetic field may seed the subsequent growth of magnetic fields in the intergalactic environment. The role of CR-driven instabilities is discussed, and nonlinear effects are briefly considered.
We present a sample of 1483 sources that display spectral peaks between 72 MHz and 1.4 GHz, selected from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. The GLEAM ...survey is the widest fractional bandwidth all-sky survey to date, ideal for identifying peaked-spectrum sources at low radio frequencies. Our peaked-spectrum sources are the low-frequency analogs of gigahertz-peaked spectrum (GPS) and compact-steep spectrum (CSS) sources, which have been hypothesized to be the precursors to massive radio galaxies. Our sample more than doubles the number of known peaked-spectrum candidates, and 95% of our sample have a newly characterized spectral peak. We highlight that some GPS sources peaking above 5 GHz have had multiple epochs of nuclear activity, and we demonstrate the possibility of identifying high-redshift (z > 2) galaxies via steep optically thin spectral indices and low observed peak frequencies. The distribution of the optically thick spectral indices of our sample is consistent with past GPS/CSS samples but with a large dispersion, suggesting that the spectral peak is a product of an inhomogeneous environment that is individualistic. We find no dependence of observed peak frequency with redshift, consistent with the peaked-spectrum sample comprising both local CSS sources and high-redshift GPS sources. The 5 GHz luminosity distribution lacks the brightest GPS and CSS sources of previous samples, implying that a convolution of source evolution and redshift influences the type of peaked-spectrum sources identified below 1 GHz. Finally, we discuss sources with optically thick spectral indices that exceed the synchrotron self-absorption limit.
The self-consistent interaction between energetic particles and self-generated hydromagnetic waves in a cosmic ray pressure dominated plasma is considered. Using a three-dimensional hybrid ...magnetohydrodynamics (MHD)-kinetic code, which utilizes a spherical harmonic expansion of the Vlasov-Fokker-Planck equation, high-resolution simulations of the magnetic field growth including feedback on the cosmic rays are carried out. It is found that for shocks with high cosmic ray acceleration efficiency, the magnetic fields become highly disorganized, resulting in near isotropic diffusion, independent of the initial orientation of the ambient magnetic field. The possibility of sub-Bohm diffusion is demonstrated for parallel shocks, while the diffusion coefficient approaches the Bohm limit from below for oblique shocks. This universal behaviour suggests that Bohm diffusion in the root-mean-squared field inferred from observation may provide a realistic estimate for the maximum energy acceleration time-scale in young supernova remnants. Although disordered, the magnetic field is not self-similar suggesting a non-uniform energy-dependent behaviour of the energetic particle transport in the precursor. Possible indirect radiative signatures of cosmic ray driven magnetic field amplification are discussed.
PDF
