721 research outputs found
Nuclear PDFs
We present the latest global QCD analysis of nuclear parton distribution functions. The emerging picture is one of consistency, with universal nuclear modification factors reproducing the main features of the data. Differences with
previous analyses are addressed
Development and analysis of spring plant phenology products: 36 years of 1-km grids over the conterminous US
Time series of phenological products provide information on the timings of recurrent biological events and on their temporal trends. This information is key to studying the impacts of climate change on our planet as well as for managing natural resources and agricultural production. Here we develop and analyze new long term phenological products: 1âŻkm grids of the Extended Spring Indices (SI-x) over the conterminous United States from 1980 to 2015. These new products (based on Daymet daily temperature grids and created by using cloud computing) allow the analysis of two primary variables (first leaf and first bloom) and two derivative products (Damage Index and Last Freeze Day) at a much finer spatial resolution than previous gridded or interpolated products. Furthermore, our products provide enough temporal depth to reliably analyze trends and changes in the timing of spring arrival at continental scales. Validation results confirm that our products largely agree with lilac and honeysuckle leaf and flowering onset observations. The spatial analysis shows a significantly delayed spring onset in the northern US whereas in the western and the Great Lakes region, spring onset advances. The mean temporal variabilities of the indices were analyzed for the nine major climatic regions of the US and results showed a clear division into three main groups: early, average and late spring onset. Finally, the region belonging to each group was mapped. These examples show the potential of our four phenological products to improve understanding of the responses of ecosystems to a changing climat
AX J1749.1-2733 and AX J1749.2-2725 - the close pair of X-ray pulsars behind the Galactic Center: an optical identification
Two faint X-ray pulsars, AX J1749.2-2725 and AX J1749.1-2733, located in the
direction to the Galactic Center, were studied in detail using data of
INTEGRAL, XMM-Newton and Chandra observatories in X-rays, the SOFI/NTT
instrument in infrared and the RTT150 telescope in optics. X-ray positions of
both sources were determined with the uncertainty better than ~1 arcsec, that
allowed us to identify their infrared counterparts. From the subsequent
analysis of infrared and optical data we conclude that counterparts of both
pulsars are likely massive stars of B0-B3 classes located behind the Galactic
Center at distances of 12-20 kpc, depending on the type, probably in further
parts of galactic spiral arms. In addition, we investigated the extinction law
towards the galactic bulge and found that it is significantly different from
standard one.Comment: 6 pages, 7 figures, will be published in MNRA
The Evolution of Helium and Hydrogen Ionization Corrections as HII Regions Age
Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic, HII regions provide the data used to infer the primordial helium
mass fraction, Y_P. In deriving abundances from observations, the correction
for unseen neutral helium or hydrogen is usually assumed to be absent; i.e.,
the ionization correction factor is taken to be unity (icf = 1). In a previous
paper (VGS), we revisited the question of the icf, confirming a "reverse"
ionization correction: icf < 1. In VGS the icf was calculated using more nearly
realistic models of inhomogeneous HII regions, suggesting that the published
values of Y_P needed to be reduced by an amount of order 0.003. As star
clusters age, their stellar spectra evolve and so, too, will their icfs. Here
the evolution of the icf is studied, along with that of two, alternate,
measures of the "hardness" of the radiation spectrum. The differences between
the icf for radiation-bounded and matter-bounded models are also explored,
along with the effect on the icf of the He/H ratio (since He and H compete for
some of the same ionizing photons). Particular attention is paid to the amount
of doubly-ionized helium predicted, leading us to suggest that observations of,
or bounds to, He++ may help to discriminate among models of HII regions ionized
by starbursts of different ages and spectra. We apply our analysis to the
Izotov & Thuan (IT) data set utilizing the radiation softness parameter, the
[OIII]/[OI] ratio, and the presence or absence of He++ to find 0.95 < icf <
0.99. This suggests that the IT estimate of the primordial helium abundance
should be reduced by Delta-Y = 0.006 +- 0.002, from 0.244 +- 0.002 to 0.238 +-
0.003.Comment: 27 double-spaced pages, 11 figures, 5 equations; revised to match the
version accepted for publication in the Ap
A possible magnetar nature for IGR J16358-4726
We present detailed spectral and timing analysis of the hard X-ray transient IGR J16358-4726 using multisatellite archival observations. A study of the source flux time history over 6 yr suggests that lower luminosity transient outbursts can be occurring in intervals of at most 1 yr. Joint spectral fits of the higher luminosity outburst using simultaneous Chandra ACIS and INTEGRAL ISGRI data reveal a spectrum well described by an absorbed power-law model with a high-energy cutoff plus an Fe line. We detected the 1.6 hr pulsations initially reported using Chandra ACIS also in the INTEGRAL ISGRI light curve and in subsequent XMM-Newton observations. Using the INTEGRAL data, we identified a spin-up of 94 s ( = 1.6 Ă 10-4), which strongly points to a neutron star nature for IGR J16358-4726. Assuming that the spin-up is due to disk accretion, we estimate that the source magnetic field ranges between 1013 and 1015 G, depending on its distance, possibly supporting a magnetar nature for IGR J16358-4726
Suzaku Observations of Four Heavily Absorbed HMXBs
We report on Suzaku observations of four unidentified sources from the
INTEGRAL and Swift BAT Galactic plane surveys. All the sources have a large
neutral hydrogen column density and are likely members of an emerging class of
heavily absorbed high mass X-ray binary (HMXB) first identified in INTEGRAL
observations. Two of the sources in our sample are approximately constant flux
sources, one source shows periodic variation and one source exhibited a short,
bright X-ray outburst. The periodicity is transient, suggesting it is produced
by a neutron star in an elliptical orbit around a stellar wind source. We
analyze the flaring source in several segments to look for spectral variation
and discuss the implications of the findings for the nature of the source. We
conclude that all four sources in our sample can be identified with the
emerging class of highly absorbed HMXBs, that one is a newly identified
transient X-ray pulsar and that at least one is a newly identified supergiant
fast X-ray transient (SFXT).Comment: 22 pages, 5 figures, submitted to Ap
A study of the photometric variability of the peculiar magnetic white dwarf WD1953-011
We present and interpret simultaneous new photometric and spectroscopic
observations of the peculiar magnetic white dwarf WD1953-011. The flux in the
V-band filter and intensity of the Balmer spectral lines demonstrate
variability with the rotation period of about 1.45 days. According to previous
studies, this variability can be explained by the presence of a dark spot
having a magnetic nature, analogous to a sunspot. Motivated by this idea, we
examine possible physical relationships between the suggested dark spot and the
strong-field magnetic structure (magnetic "spot", or "tube") recently
identified on the surface of this star. Comparing the rotationally-modulated
flux with the variable spectral observables related to the magnetic "spot" we
establish their correlation, and therefore their physical relationship.
Modeling the variable photometric flux assuming that it is associated with
temperature variations in the stellar photosphere, we argue that the
strong-field area and dark, low-temperature spot are comparable in size and
located at the same latitudes, essentially overlapping each other with a
possible slight longitudinal shift. In this paper we also present a new,
improved value of the star's rotational period and constrain the
characteristics of the thermal inhomogeneity over the degenerate's surface.Comment: accepted to the Ap
Swift J1753.5-0127: The Black Hole Candidate with the shortest orbital period
We present time-resolved photometry of the optical counterpart to the black
hole candidate Swift J1753.5-0127, which has remained in the low/hard X-ray
state and bright at optical/IR wavelengths since its discovery in 2005. At the
time of our observations Swift J1753.5-0127 does not show a decay trend but
remains stable at R=16.45 with a night to night variability of ~0.05 mag. The
R-band light curves, taken from 2007 June 3 to August 31, are not sinusoidal,
but exhibit a complex morphology with remarkable changes in shape and
amplitude. The best period determination is 3.2443+-0.0010 hours. This
photometric period is likely a superhump period, slightly larger than the
orbital period. Therefore, Swift J1753.5-0127 is the black hole candidate with
the shortest orbital period observed to date. Our estimation of the distance is
comparable to values previously published and likely places Swift J1753.5-0127
in the Galactic halo.Comment: Accepted for publication in Ap
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