161 research outputs found
Diversity of supernovae Ia determined using equivalent widths of Si II 4000
Spectroscopic and photometric properties of low and high-z supernovae Ia (SNe
Ia) have been analyzed in order to achieve a better understanding of their
diversity and to identify possible SN Ia sub-types. We use wavelet transformed
spectra in which one can easily measure spectral features. We investigate the
\ion{Si}{II} 4000 equivalent width (EW_w\lbrace\ion{Si}{II}\rbrace). The
ability and, especially, the ease in extending the method to SNe at high- is
demonstrated. We applied the method to 110 SNe Ia and found correlations
between EW_w\lbrace\ion{Si}{II}\rbrace and parameters related to the
light-curve shape for 88 supernovae with available photometry. No evidence for
evolution of EW_w\lbrace\ion{Si}{II}\rbrace with redshift is seen. Three
sub-classes of SNe Ia were confirmed using an independent cluster analysis with
only light-curve shape, colour, and EW_w\lbrace\ion{Si}{II}\rbrace. SNe from
high- samples seem to follow a similar grouping to nearby objects. The
EW_w\lbrace\ion{Si}{II}\rbrace value measured on a single spectrum may point
towards SN Ia sub-classification, avoiding the need for expansion velocity
gradient calculations.Comment: 12 pages, 5 figure
Quantifying Spectral Features of Type Ia Supernovae
We introduce a new technique to quantify highly structured spectra for which
the definition of continua or spectral features in the observed flux spectra is
difficult. The method employs wavelet transformation which allows the
decomposition of the observed spectra into different scales. A procedure is
formulated to define the strength of spectral features so that the measured
spectral indices are independent of the flux levels and are insensitive to the
definition of continuum and also to reddening. This technique is applied to
Type Ia supernovae spectra, where correlations are revealed between the
luminosity and spectral features. The current technique may allow for
luminosity corrections based on spectral features in the use of Type Ia
supernovae as cosmological probe.Comment: 35 pages, 15 figure
NTT and NOT spectroscopy of SDSS-II supernovae
Context. The SDSS-II Supernova Survey, conducted between 2005 and 2007, was
designed to detect a large number of Type Ia supernovae (SNe Ia) around z~0.2,
the redshift "gap" between low-z and high-z SN searches. The survey has
provided multi-band photometric lightcurves for variable targets, and SN
candidates were scheduled for spectroscopic observations, primarily to provide
SN classification and accurate redshifts. We present SN spectra obtained in
2006 and 2007 using the NTT and the NOT. Aims. We provide an atlas of SN
spectra in the range z =0.03-0.32 that complements the well-sampled lightcurves
from SDSS-II in the forthcoming three-year SDSS SN cosmology analysis. The
sample can, for example, be used for spectral studies of SNe Ia, which are
critical for understanding potential systematic effects when SNe are used to
determine cosmological distances. Methods. The spectra were reduced in a
uniform manner, and special care was taken in estimating the uncertainties for
the different processing steps. Host-galaxy light was subtracted when possible
and the SN type fitted using the SuperNova IDentification code (SNID). We also
present comparisons between spectral and photometric dating using SALT
lightcurve fits to the photometry from SDSS-II, as well as the global
distribution of our sample in terms of the lightcurve parameters: stretch and
colour. Results. We report new spectroscopic data from 141 SNe Ia, mainly
between -9 and +15 days from lightcurve maximum, including a few cases of
multi-epoch observations. This homogeneous, host-galaxy subtracted, SN Ia
spectroscopic sample is among the largest such data sets and unique in its
redshift interval. The sample includes two potential SN 1991T-like SNe (SN
2006on and SN 2007ni) and one potential SN 2002cx-like SN (SN 2007ie). In
addition, the new compilation includes spectra from 23 confirmed Type II and 8
Type Ib/c SNe.Comment: Accepted for publication in A&
High-Velocity Features: a ubiquitous property of Type Ia SNe
Evidence of high-velocity features such as those seen in the near-maximum
spectra of some Type Ia Supernovae (eg SN 2000cx) has been searched for in the
available SNIa spectra observed earlier than one week before B maximum. Recent
observational efforts have doubled the number of SNeIa with very early spectra.
Remarkably, all SNeIa with early data (7 in our RTN sample and 10 from other
programmes) show signs of such features, to a greater or lesser degree, in CaII
IR, and some also in SiII 6255A line. High-velocity features may be interpreted
as abundance or density enhancements. Abundance enhancements would imply an
outer region dominated by Si and Ca. Density enhancements may result from the
sweeping up of circumstellar material by the highest velocity SN ejecta. In
this scenario, the high incidence of HVFs suggests that a thick disc and/or a
high-density companion wind surrounds the exploding white dwarf, as may be the
case in Single Degenerate systems. Large-scale angular fluctuations in the
radial density and abundance distribution may also be responsible: this could
originate in the explosion, and would suggest a deflagration as the more likely
explosion mechanism. CSM-interaction and surface fluctuations may coexist,
possibly leaving different signatures on the spectrum. In some SNe the HVFs are
narrowly confined in velocity, suggesting the ejection of blobs of burned
material.Comment: 12 pages, 2 figures, ApJ Letters in pres
A Study of Carbon Features in Type Ia Supernova Spectra
One of the major differences between various explosion scenarios of Type Ia
supernovae (SNe Ia) is the remaining amount of unburned (C+O) material and its
velocity distribution within the expanding ejecta. While oxygen absorption
features are not uncommon in the spectra of SNe Ia before maximum light, the
presence of strong carbon absorption has been reported only in a minority of
objects, typically during the pre-maximum phase. The reported low frequency of
carbon detections may be due to low signal-to-noise data, low abundance of
unburned material, line blending between C II 6580 and Si II 6355, ejecta
temperature differences, asymmetrical distribution effects, or a combination of
these. However, a survey of published pre-maximum spectra reveals that more SNe
Ia than previously thought may exhibit C II 6580 absorption features and relics
of line blending near 6300 Angstroms. Here we present new SN Ia observations
where spectroscopic signatures of C II 6580 are detected, and investigate the
presence of C II 6580 in the optical spectra of 19 SNe Ia using the
parameterized spectrum synthesis code, SYNOW. Most of the objects in our sample
that exhibit C II 6580 absorption features are of the low-velocity gradient
subtype. Our study indicates that the morphology of carbon-rich regions is
consistent with either a spherical distribution or a hemispheric asymmetry,
supporting the recent idea that SN Ia diversity may be a result of off-center
ignition coupled with observer line-of-sight effects.Comment: 10 papges, 9 figures, 3 table
Factors associated with SARS-CoV-2 infection risk among healthcare workers of an italian university hospital
We report the results of a study on the cumulative incidence of SARS-CoV-2 infections in about 6000 workers of the University Hospital of Modena, Northern Italy, in the period March 2020–January 2021, and the relations with some individual and occupational factors. Overall, in healthcare workers (HCW) the cumulative incidence of COVID-19 during the period was 13.8%. Results confirm the role of overweight and obesity as significant risk factors for SARS-CoV-2 infection. Chronic respiratory diseases, including asthma, also proved to be significantly associated with the infection rate. Considering occupational factors, the COVID-19 risk was about threefold (OR: 2.7; 95% CI 1.7–4.5) greater in nurses and nurse aides than in non-HCW, and about double (OR: 1.9; 95% CI 1.2–3.2) in physicians. Interestingly, an association was also observed between infection risk and nightshifts at work (OR: 1.8; 95% CI 1.4–2.3), significantly related to the total number of shifts in the whole eleven-month period. Even if the vaccination campaign has now greatly modified the scenario of SARS-CoV-2 infections among HCW, the results of this study can be useful for further development of health and policy strategies to mitigate the occupational risk related to the new variants of coronavirus, and therefore the evolution of the pandemic
PHotometry Assisted Spectral Extraction (PHASE) and identification of SNLS supernovae
Aim: We present new extraction and identification techniques for supernova
(SN) spectra developed within the Supernova Legacy Survey (SNLS) collaboration.
Method: The new spectral extraction method takes full advantage of
photometric information from the Canada-France-Hawai telescope (CFHT) discovery
and reference images by tracing the exact position of the supernova and the
host signals on the spectrogram. When present, the host spatial profile is
measured on deep multi-band reference images and is used to model the host
contribution to the full (supernova + host) signal. The supernova is modelled
as a Gaussian function of width equal to the seeing. A chi-square minimisation
provides the flux of each component in each pixel of the 2D spectrogram. For a
host-supernova separation greater than <~ 1 pixel, the two components are
recovered separately and we do not use a spectral template in contrast to more
standard analyses. This new procedure permits a clean extraction of the
supernova separately from the host in about 70% of the 3rd year ESO/VLT spectra
of the SNLS. A new supernova identification method is also proposed. It uses
the SALT2 spectrophotometric template to combine the photometric and spectral
data. A galaxy template is allowed for spectra for which a separate extraction
of the supernova and the host was not possible.
Result: These new techniques have been tested against more standard
extraction and identification procedures. They permit a secure type and
redshift determination in about 80% of cases. The present paper illustrates
their performances on a few sample spectra.Comment: 27 pages, 18 Figures, 1 Table. Accepted for publication in A&
Spectra of High-Redshift Type Ia Supernovae and a Comparison with their Low-Redshift Counterparts
We present spectra for 14 high-redshift (0.17 < z < 0.83) supernovae, which
were discovered by the Supernova Cosmology Project as part of a campaign to
measure cosmological parameters. The spectra are used to determine the redshift
and classify the supernova type, essential information if the supernovae are to
be used for cosmological studies. Redshifts were derived either from the
spectrum of the host galaxy or from the spectrum of the supernova itself. We
present evidence that these supernovae are of Type Ia by matching to spectra of
nearby supernovae. We find that the dates of the spectra relative to maximum
light determined from this fitting process are consistent with the dates
determined from the photometric light curves, and moreover the spectral
time-sequence for SNe Type Ia at low and high redshift is indistinguishable. We
also show that the expansion velocities measured from blueshifted CaHK are
consistent with those measured for low-redshift Type Ia supernovae. From these
first-level quantitative comparisons we find no evidence for evolution in SNIa
properties between these low- and high-redshift samples. Thus even though our
samples may not be complete, we conclude that there is a population of SNe Ia
at high redshift whose spectral properties match those at low redshift.Comment: Accepted for publication in AJ. Also available at
http://supernova.lbl.gov
Restframe I-band Hubble diagram for type Ia supernovae up to redshift z ~0.5
We present a novel technique for fitting restframe I-band light curves on a
data set of 42 Type Ia supernovae (SNe Ia). Using the result of the fit, we
construct a Hubble diagram with 26 SNe from the subset at 0.01< z<0.1. Adding
two SNe at z~0.5 yields results consistent with a flat
Lambda-dominated``concordance universe''
()=(0.25,0.75). For one of these, SN 2000fr, new near
infrared data are presented. The high redshift supernova NIR data are also used
to test for systematic effects in the use of SNe Ia as distance estimators. A
flat, Lambda=0, universe where the faintness of supernovae at z~0.5 is due to
grey dust homogeneously distributed in the intergalactic medium is disfavoured
based on the high-z Hubble diagram using this small data-set. However, the
uncertainties are large and no firm conclusion may be drawn. We explore the
possibility of setting limits on intergalactic dust based on B-I and B-V colour
measurements, and conclude that about 20 well measured SNe are needed to give
statistically significant results. We also show that the high redshift
restframe I-band data points are better fit by light curve templates that show
a prominent second peak, suggesting that they are not intrinsically
underluminous.Comment: Accepted for publication in A&A (01/04/2005
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