101 research outputs found
Revival of the magnetar PSR J1622-4950: observations with MeerKAT, Parkes, XMM-Newton, Swift, Chandra, and NuSTAR
New radio (MeerKAT and Parkes) and X-ray (XMM-Newton, Swift, Chandra, and
NuSTAR) observations of PSR J1622-4950 indicate that the magnetar, in a
quiescent state since at least early 2015, reactivated between 2017 March 19
and April 5. The radio flux density, while variable, is approximately 100x
larger than during its dormant state. The X-ray flux one month after
reactivation was at least 800x larger than during quiescence, and has been
decaying exponentially on a 111+/-19 day timescale. This high-flux state,
together with a radio-derived rotational ephemeris, enabled for the first time
the detection of X-ray pulsations for this magnetar. At 5%, the 0.3-6 keV
pulsed fraction is comparable to the smallest observed for magnetars. The
overall pulsar geometry inferred from polarized radio emission appears to be
broadly consistent with that determined 6-8 years earlier. However, rotating
vector model fits suggest that we are now seeing radio emission from a
different location in the magnetosphere than previously. This indicates a novel
way in which radio emission from magnetars can differ from that of ordinary
pulsars. The torque on the neutron star is varying rapidly and unsteadily, as
is common for magnetars following outburst, having changed by a factor of 7
within six months of reactivation.Comment: Published in ApJ (2018 April 5); 13 pages, 4 figure
Presupernova Structure of Massive Stars
Issues concerning the structure and evolution of core collapse progenitor
stars are discussed with an emphasis on interior evolution. We describe a
program designed to investigate the transport and mixing processes associated
with stellar turbulence, arguably the greatest source of uncertainty in
progenitor structure, besides mass loss, at the time of core collapse. An
effort to use precision observations of stellar parameters to constrain
theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory
Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics
and Space Science, submitted for peer review: 7 pages, 3 figure
Model Independent Determination of the Solar Neutrino Spectrum with and without MSW
Besides the opportunity for discovering new neutrino physics, solar neutrino
measurements provide a sensitive probe of the solar interior, and thus a
rigorous test of solar model predictions. We present model independent
determinations of the neutrino spectrum by using relevant flux components as
free parameters subject only to the luminosity constraint. (1) Without the
Mikheyev-Smirnov-Wolfenstein (MSW) effect, the best fit for the combined data
is poor. Furthermore, the data indicate a severe suppression of the Be flux
relative to the B, contradicting both standard and nonstandard solar models
in general; the flux takes its maximum value allowed by the luminosity
constraint. This pathology consistently appears even if we ignore any one of
the three data. (2) In the presence of the two-flavor MSW effect, the current
constraint on the initial B flux is weak, but consistent with the SSM and
sufficient to exclude nonstandard models with small B fluxes. No meaningful
constraint is obtained for the other fluxes. In the future, even allowing MSW,
the B and Be fluxes can be determined at the (15 -- 20)\% level,
making competing solar models distinguishable. We emphasize that the neutral
current sensitivity for Be neutrinos in BOREXINO, HELLAZ, and HERON is
essential for determining the initial fluxes. The constraints on the MSW
parameters in the model independent analysis are also discussed.Comment: Revtex 3.0, 61 pages including 23 figures, uuencoded ps file
attached. Easy way: compressed ps file of entire paper in landscape format
available by anonymous
ftp://upenn5.hep.upenn.edu/pub/hata/papers/model_ind.ps.
Improvements in the Canadian Earth System Model (CanESM) through systematic model analysis: CanESM5.0 and CanESM5.1
The Canadian Earth System Model version 5.0 (CanESM5.0), the most recent major version of the global climate model developed at the Canadian Centre for Climate Modelling and Analysis (CCCma) at Environment and Climate Change Canada (ECCC), has been used extensively in climate research and for providing future climate projections in the context of climate services. Previous studies have shown that CanESM5.0 performs well compared to other models and have revealed several model biases. To address these biases, the CCCma has recently initiated the “Analysis for Development” (A4D) activity, a coordinated analysis activity in support of CanESM development. Here we describe the goals and organization of this effort and introduce two variants (“p1” and “p2”) of a new CanESM version, CanESM5.1, which features important improvements as a result of the A4D activity. These improvements include the elimination of spurious stratospheric temperature spikes and an improved simulation of tropospheric dust. Other climate aspects of the p1 variant of CanESM5.1 are similar to those of CanESM5.0, while the p2 variant of CanESM5.1 features reduced equilibrium climate sensitivity and improved El Niño–Southern Oscillation (ENSO) variability as a result of intentional tuning of the atmospheric component. The A4D activity has also led to the improved understanding of other notable CanESM5.0 and CanESM5.1 biases, including the overestimation of North Atlantic sea ice, a cold bias over sea ice, biases in the stratospheric circulation and a cold bias over the Himalayas. It provides a potential framework for the broader climate community to contribute to CanESM development, which will facilitate further model improvements and ultimately lead to improved climate change information.</p
Periodic activity from a fast radio burst source
Fast radio bursts (FRBs) are bright, millisecond-duration radio transients
originating from extragalactic distances. Their origin is unknown. Some FRB
sources emit repeat bursts, ruling out cataclysmic origins for those events.
Despite searches for periodicity in repeat burst arrival times on time scales
from milliseconds to many days, these bursts have hitherto been observed to
appear sporadically, and though clustered, without a regular pattern. Here we
report the detection of a day periodicity (or possibly a
higher-frequency alias of that periodicity) from a repeating FRB
180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment
Fast Radio Burst Project (CHIME/FRB). In 38 bursts recorded from September
16th, 2018 through February 4th, 2020, we find that all bursts arrive in a
5-day phase window, and 50% of the bursts arrive in a 0.6-day phase window. Our
results suggest a mechanism for periodic modulation either of the burst
emission itself, or through external amplification or absorption, and disfavour
models invoking purely sporadic processes
CHIME/FRB Detection of Eight New Repeating Fast Radio Burst Sources
We report on the discovery of eight repeating fast radio burst (FRB) sources
found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME)
telescope. These sources span a dispersion measure (DM) range of 103.5 to 1281
pc cm. They display varying degrees of activity: six sources were
detected twice, another three times, and one ten times. These eight repeating
FRBs likely represent the bright and/or high-rate end of a distribution of
infrequently repeating sources. For all sources, we determine sky coordinates
with uncertainties of 10. FRB 180916.J0158+65 has a
burst-averaged DM = pc cm and a low DM excess over the
modelled Galactic maximum (as low as 20 pc cm); this source also
has a Faraday rotation measure (RM) of rad m, much
lower than the RM measured for FRB 121102. FRB 181030.J1054+73 has the lowest
DM for a repeater, pc cm, with a DM excess of 70
pc cm. Both sources are interesting targets for multi-wavelength
follow-up due to their apparent proximity. The DM distribution of our repeater
sample is statistically indistinguishable from that of the first 12 CHIME/FRB
sources that have not repeated. We find, with 4 significance, that
repeater bursts are generally wider than those of CHIME/FRB bursts that have
not repeated, suggesting different emission mechanisms. Our repeater events
show complex morphologies that are reminiscent of the first two discovered
repeating FRBs. The repetitive behavior of these sources will enable
interferometric localizations and subsequent host galaxy identifications.Comment: 40 pages, 11 figures; accepted by ApJL on 28 September 2019; added
analysis of correlation between width and max. flux densit
Recommended from our members
A verification framework for interannual-to-decadal predictions experiments
Decadal predictions have a high profile in the climate science community and beyond, yet very little is known about their skill. Nor is there any agreed protocol for estimating their skill. This paper proposes a sound and coordinated framework for verification of decadal hindcast experiments. The framework is illustrated for decadal hindcasts tailored to meet the requirements and specifications of CMIP5 (Coupled Model Intercomparison Project phase 5). The chosen metrics address key questions about the information content in initialized decadal hindcasts. These questions are: (1) Do the initial conditions in the hindcasts lead to more accurate predictions of the climate, compared to un-initialized climate change projections? and (2) Is the prediction model’s ensemble spread an appropriate representation of forecast uncertainty on average? The first question is addressed through deterministic metrics that compare the initialized and uninitialized hindcasts. The second question is addressed through a probabilistic metric applied to the initialized hindcasts and comparing different ways to ascribe forecast uncertainty. Verification is advocated at smoothed regional scales that can illuminate broad areas of predictability, as well as at the grid scale, since many users of the decadal prediction experiments who feed the climate data into applications or decision models will use the data at grid scale, or downscale it to even higher resolution. An overall statement on skill of CMIP5 decadal hindcasts is not the aim of this paper. The results presented are only illustrative of the framework, which would enable such studies. However, broad conclusions that are beginning to emerge from the CMIP5 results include (1) Most predictability at the interannual-to-decadal scale, relative to climatological averages, comes from external forcing, particularly for temperature; (2) though moderate, additional skill is added by the initial conditions over what is imparted by external forcing alone; however, the impact of initialization may result in overall worse predictions in some regions than provided by uninitialized climate change projections; (3) limited hindcast records and the dearth of climate-quality observational data impede our ability to quantify expected skill as well as model biases; and (4) as is common to seasonal-to-interannual model predictions, the spread of the ensemble members is not necessarily a good representation of forecast uncertainty. The authors recommend that this framework be adopted to serve as a starting point to compare prediction quality across prediction systems. The framework can provide a baseline against which future improvements can be quantified. The framework also provides guidance on the use of these model predictions, which differ in fundamental ways from the climate change projections that much of the community has become familiar with, including adjustment of mean and conditional biases, and consideration of how to best approach forecast uncertainty
- …