7,510 research outputs found
Swift UVOT Grism Observations of Nearby Type Ia Supernovae - I. Observations and Data Reduction
Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) are useful tools
for understanding progenitor systems and explosion physics. In particular, UV
spectra of SNe Ia, which probe the outermost layers, are strongly affected by
the progenitor metallicity. In this work, we present 120 Neil Gehrels Swift
Observatory UV spectra of 39 nearby SNe Ia. This sample is the largest UV
(lambda < 2900 A) spectroscopic sample of SNe Ia to date, doubling the number
of UV spectra and tripling the number of SNe with UV spectra. The sample spans
nearly the full range of SN Ia light-curve shapes (delta m(B) ~ 0.6-1.8 mag).
The fast turnaround of Swift allows us to obtain UV spectra at very early
times, with 13 out of 39 SNe having their first spectra observed >~ 1 week
before peak brightness and the earliest epoch being 16.5 days before peak
brightness. The slitless design of the Swift UV grism complicates the data
reduction, which requires separating SN light from underlying host-galaxy light
and occasional overlapping stellar light. We present a new data-reduction
procedure to mitigate these issues, producing spectra that are significantly
improved over those of standard methods. For a subset of the spectra we have
nearly simultaneous Hubble Space Telescope UV spectra; the Swift spectra are
consistent with these comparison data.Comment: Accepted for publication in MNRA
NALNET book system: Cost benefit study
The goals of the NASA's library network system, NALNET, the functions of the current book system, the products and services of a book system required by NASA Center libraries, and the characteristics of a system that would best supply those products and services were assessed. Emphasis was placed on determining the most cost effective means of meeting NASA's requirements for an automated book system. Various operating modes were examined including the current STIMS file, the PUBFILE, developing software improvements for products as appropriate to the Center needs, and obtaining cataloging and products from the bibliographic utilities including at least OCLC, RLIN, BNA, and STIF. It is recommended that NALNET operate under the STIMS file mode and obtain cataloging and products from the bibliographic utilities. The recommendations are based on the premise that given the current state of the art in library automation it is not cost effective for NASA to maintain a full range of cataloging services on its own system. The bibliographic utilities can support higher quality systems with a greater range of services at a lower total cost
SN 2006bt: A Perplexing, Troublesome, and Possibly Misleading Type Ia Supernova
SN 2006bt displays characteristics unlike those of any other known Type Ia
supernova (SN Ia). We present optical light curves and spectra of SN 2006bt
which demonstrate the peculiar nature of this object. SN 2006bt has broad,
slowly declining light curves indicative of a hot, high-luminosity SN, but
lacks a prominent second maximum in the i band as do low-luminosity SNe Ia. Its
spectra are similar to those of low-luminosity SNe Ia, containing features that
are only present in cool SN photospheres. Light-curve fitting methods suggest
that SN 2006bt is reddened by a significant amount of dust; however, it
occurred in the outskirts of its early-type host galaxy and has no strong Na D
absorption in any of its spectra, suggesting a negligible amount of host-galaxy
dust absorption. C II is possibly detected in our pre-maximum spectra, but at a
much lower velocity than other elements. The progenitor was likely very old,
being a member of the halo population of a galaxy that shows no signs of recent
star formation. SNe Ia have been very successfully modeled as a one-parameter
family, and this is fundamental to their use as cosmological distance
indicators. SN 2006bt is a challenge to that picture, yet its relatively normal
light curves allowed SN 2006bt to be included in cosmological analyses. We
generate mock SN Ia datasets which indicate that contamination by similar
objects will both increase the scatter of a SN Ia Hubble diagram and
systematically bias measurements of cosmological parameters. However, spectra
and rest-frame i-band light curves should provide a definitive way to identify
and eliminate such objects.Comment: ApJ, accepted. 13 pages, 13 figure
Blood pressure and long-term mortality in United States hemodialysis patients: USRDS Waves 3 and 4 Study11The data reported here were supplied by the United States Renal Data System. Interpretation of these data is the responsibility of the authors, and in no way should be seen as an official policy or interpretation of the U.S. government.
Blood pressure and long-term mortality in United States hemodialysis patients: USRDS Waves 3 and 4 Study.BackgroundThe long-term prognostic associations of pre- and post-dialysis blood pressures, interdialytic weight gain, and antihypertensive use in hemodialysis patients are unclear.MethodsThe United States Renal Data System (USRDS) Dialysis Morbidity and Mortality Waves 3 and 4 Study, a randomly generated sample of 11,142 subjects receiving hemodialysis on December 31, 1993, was examined, with vital status followed until May 2000.ResultsPre- and post-dialysis blood pressure values, interdialytic weight gain and number of antihypertensives averaged 151.8/79.7, 137.0/74, 3.6% and 0.76, respectively. Prognostic discrimination was maximized by considering pre- and post-systolic and diastolic blood pressure values simultaneously, in a pattern suggesting that wide pulse pressures were associated with mortality (P < 0.0001). Comorbidity adjustment markedly affected associations, with low pre-dialysis diastolic (P < 0.05), low post-dialysis dialysis diastolic pressure (P < 0.05), high post-dialysis dialysis systolic pressure (P < 0.05), and high interdialytic weight gains (P = 0.005) associated with mortality. Each class of antihypertensive drug, except angiotensin-converting enzyme (ACE)-inhibitors, was associated with lower mortality in unadjusted models, an effect most pronounced for beta-blockers (hazards ratio 0.72, 95% CI 0.66 to 0.79, P < 0.0001). Comorbidity adjustment eliminated survival associations for each antihypertensive class except beta-blockers.ConclusionsPre- and post-dialysis blood pressure values have independent associations with mortality, in a way that implicates wide pulse pressures. Much of the adverse prognosis of wide pulse pressures probably reflects older age and cardiovascular comorbidity. Large interdialytic weight gains are associated with shorter survival when comorbidity is taken into account. Beta-blocker use shows a robust association with survival, and may be protective
Dipolar and scalar He and Xe frequency shifts in mm-sized cells
We describe a He-Xe comagnetometer operating in stemless
anodically bonded cells with a 6 mm volume and a Xe spin coherence
time of 300 sec. We use a Rb pulse-train magnetometer with co-linear
pump and probe beams to study the nuclear spin frequency shifts caused by spin
polarization of He. By systematically varying the cell geometry in a
batch cell fabrication process we can separately measure the cell shape
dependent and independent frequency shifts. We find that a certain aspect ratio
of the cylindrical cell can cancel the effects of He magnetization that
limit the stability of vapor-cell comagnetometers. Using this control we also
observe for the first time a scalar He-Xe collisional frequency
shift characterized by an enhancement factor .Comment: 4 pages, 4 figure
Evidence for a Photospheric Component in the Prompt Emission of the Short GRB120323A and its Effects on the GRB Hardness-Luminosity Relation
The short GRB 120323A had the highest flux ever detected with the Fermi/GBM.
Here we study its remarkable spectral properties and their evolution using two
spectral models: (i) a single emission component scenario, where the spectrum
is modeled by the empirical Band function, and (ii) a two component scenario,
where thermal (Planck-like) emission is observed simultaneously with a
non-thermal component (a Band function). We find that the latter model fits the
integrated burst spectrum significantly better than the former, and that their
respective spectral parameters are dramatically different: when fit with a Band
function only, the Epeak of the event is unusually soft for a short GRB, while
adding a thermal component leads to more typical short GRB values. Our
time-resolved spectral analysis produces similar results. We argue here that
the two-component model is the preferred interpretation for GRB 120323A, based
on: (i) the values and evolution of the Band function parameters of the two
component scenario, which are more typical for a short GRB, and (ii) the
appearance in the data of a significant hardness-intensity correlation,
commonly found in GRBs, when we employee two-component model fits; the
correlation is non-existent in the Band-only fits. GRB 110721A, a long burst
with an intense photospheric emission, exhibits the exact same behavior. We
conclude that GRB 120323A has a strong photospheric emission contribution,
first time observed in a short GRB. Magnetic dissipation models are difficult
to reconcile with these results, which instead favor photospheric thermal
emission and fast cooling synchrotron radiation from internal shocks. Finally,
we derive a possibly universal hardness-luminosity relation in the source frame
using a larger set of GRBs L,i=(1.59+/-0.84).10^50 (Epeak,i)^(1.33+/-0.07)
erg/s), which could be used as a possible redshift estimator for cosmology.Comment: 27 pages, 13 figures, Accepted by ApJ (April, 7th 2013
The differential impact of risk factors on mortality in hemodialysis and peritoneal dialysis
The differential impact of risk factors on mortality in hemodialysis and peritoneal dialysis.BackgroundWhile the survival ramifications of dialysis modality selection are still debated, it seems reasonable to postulate that outcome comparisons are not the same for all patients at all times. Trends in available data indicate the relative risk of death with hemodialysis (HD) compared to peritoneal dialysis (PD) varies by time on dialysis and the presence of various risk factors. This study was undertaken to identify key patient characteristics for which the risk of death differs by dialysis modality.MethodsAnalyses utilized incidence data from 398,940 United States Medicare patients initiating dialysis between 1995 and 2000. Proportional hazards regression identified the presence of diabetes, age, and the presence of comorbidity as factors that significantly interact with treatment modality. Stratifying by these factors, proportional and nonproportional hazards models were used to estimate relative risks of death [RR (HD:PD)].ResultsOf the 398,940 patients studied, 11.6% used PD as initial therapy, 45% had diabetes mellitus (DM), 51% were 65 years or older, and 55% had at least one comorbidity. Among the 178,693 (45%) patients with no baseline comorbidity, adjusted mortality rates in nondiabetic (non-DM) patients were significantly higher on HD than on PD [age 18–44: RR (95% CI) = 1.24 (1.07, 1.44); age 45–64: RR = 1.13 (1.02, 1.25); age 65+: RR = 1.13 (1.05, 1.21)]. Among diabetic (DM) patients with no comorbidity, HD was associated with a higher risk of death among younger patients [age 18–44: RR = 1.22(1.05, 1.42)] and a lower risk of death among older patients [age 45–64: RR = 0.92 (0.85, 1.00); age 65+: RR = 0.86 (0.79, 0.93)]. Within the group of 220,247 (55%) patients with baseline comorbidity, adjusted mortality rates were not different between HD and PD among non-DM patients [age 18–44: RR = 1.19 (0.94, 1.50); age 45–64: RR = 1.01 (0.92, 1.11); age 65+: RR = 0.96 (0.91, 1.01)] and younger DM patients [age 18–44: RR = 1.10 (0.92, 1.32)], but were lower with HD among older DM patients with baseline comorbidity [age 45–64: RR = 0.82 (0.77, 0.87); age 65+: RR = 0.80 (0.76, 0.85)].ConclusionValid mortality comparisons between HD and PD require patient stratification according to major risk factors known to interact with treatment modality. Survival differences between HD and PD are not constant, but vary substantially according to the underlying cause of ESRD, age, and level of baseline comorbidity. These results may help identify technical advances that will improve outcomes of patients on dialysis
- …