3,370 research outputs found
PESSTO monitoring of SN 2012hn: further heterogeneity among faint type I supernovae
We present optical and infrared monitoring data of SN 2012hn collected by the
Public ESO Spectroscopic Survey for Transient Objects (PESSTO). We show that SN
2012hn has a faint peak magnitude (MR ~ -15.7) and shows no hydrogen and no
clear evidence for helium in its spectral evolution. Instead, we detect
prominent Ca II lines at all epochs, which relates this transient to previously
described 'Ca-rich' or 'gap' transients. However, the photospheric spectra
(from -3 to +32 d with respect to peak) of SN 2012hn show a series of
absorption lines which are unique, and a red continuum that is likely intrinsic
rather than due to extinction. Lines of Ti II and Cr II are visible. This may
be a temperature effect, which could also explain the red photospheric colour.
A nebular spectrum at +150d shows prominent CaII, OI, CI and possibly MgI lines
which appear similar in strength to those displayed by core-collapse SNe. To
add to the puzzle, SN 2012hn is located at a projected distance of 6 kpc from
an E/S0 host and is not close to any obvious starforming region. Overall SN
2012hn resembles a group of faint H-poor SNe that have been discovered recently
and for which a convincing and consistent physical explanation is still
missing. They all appear to explode preferentially in remote locations offset
from a massive host galaxy with deep limits on any dwarf host galaxies,
favouring old progenitor systems. SN 2012hn adds heterogeneity to this sample
of objects. We discuss potential explosion channels including He-shell
detonations and double detonations of white dwarfs as well as peculiar
core-collapse SNe.Comment: 14 pages, 14 figures, accepted to MNRAS on 14/10/201
The Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by Strong Pi Pulses
In spectroscopy, it is conventional to treat pulses much stronger than the
linewidth as delta-functions. In NMR, this assumption leads to the prediction
that pi pulses do not refocus the dipolar coupling. However, NMR spin echo
measurements in dipolar solids defy these conventional expectations when more
than one pi pulse is used. Observed effects include a long tail in the CPMG
echo train for short delays between pi pulses, an even-odd asymmetry in the
echo amplitudes for long delays, an unusual fingerprint pattern for
intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments
that set limits on possible extrinsic causes for the phenomena are reported. We
find that the action of the system's internal Hamiltonian during any real pulse
is sufficient to cause the effects. Exact numerical calculations, combined with
average Hamiltonian theory, identify novel terms that are sensitive to
parameters such as pulse phase, dipolar coupling, and system size.
Visualization of the entire density matrix shows a unique flow of quantum
coherence from non-observable to observable channels when applying repeated pi
pulses.Comment: 24 pages, 27 figures. Revised from helpful referee comments. Added
new Table IV, new paragraphs on pages 3 and 1
The Supernova Legacy Survey 3-year sample: Type Ia Supernovae photometric distances and cosmological constraints
We present photometric properties and distance measurements of 252 high
redshift Type Ia supernovae (0.15 < z < 1.1) discovered during the first three
years of the Supernova Legacy Survey (SNLS). These events were detected and
their multi-colour light curves measured using the MegaPrime/MegaCam instrument
at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four
one-square degree fields in four bands. Follow-up spectroscopy was performed at
the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and
to measure their redshifts. Systematic uncertainties arising from light curve
modeling are studied, making use of two techniques to derive the peak
magnitude, shape and colour of the supernovae, and taking advantage of a
precise calibration of the SNLS fields. A flat LambdaCDM cosmological fit to
231 SNLS high redshift Type Ia supernovae alone gives Omega_M = 0.211 +/-
0.034(stat) +/- 0.069(sys). The dominant systematic uncertainty comes from
uncertainties in the photometric calibration. Systematic uncertainties from
light curve fitters come next with a total contribution of +/- 0.026 on
Omega_M. No clear evidence is found for a possible evolution of the slope
(beta) of the colour-luminosity relation with redshift.Comment: (The SNLS Collaboration) 40 pages, 32 figures, Accepted in A&
TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro
A survey of Galactic gamma-ray sources at a median energy of ~20 TeV has been
performed using the Milagro Gamma Ray Observatory. Eight candidate sources of
TeV emission are detected with pre-trials significance in the
region of Galactic longitude and latitude
. Four of these sources, including the Crab nebula
and the recently published MGRO J2019+37, are observed with significances
after accounting for the trials involved in searching the 3800
square degree region. All four of these sources are also coincident with EGRET
sources. Two of the lower significance sources are coincident with EGRET
sources and one of these sources is Geminga. The other two candidates are in
the Cygnus region of the Galaxy. Several of the sources appear to be spatially
extended. The fluxes of the sources at 20 TeV range from ~25% of the Crab flux
to nearly as bright as the Crab.Comment: Submitted to Ap
Milagro Constraints on Very High Energy Emission from Short Duration Gamma-Ray Bursts
Recent rapid localizations of short, hard gamma-ray bursts (GRBs) by the
Swift and HETE satellites have led to the observation of the first afterglows
and the measurement of the first redshifts from this type of burst. Detection
of >100 GeV counterparts would place powerful constraints on GRB mechanisms.
Seventeen short duration (< 5 s) GRBs detected by satellites occurred within
the field of view of the Milagro gamma-ray observatory between 2000 January and
2006 December. We have searched the Milagro data for >100 GeV counterparts to
these GRBs and find no significant emission correlated with these bursts. Due
to the absorption of high-energy gamma rays by the extragalactic background
light (EBL), detections are only expected for redshifts less than ~0.5. While
most long duration GRBs occur at redshifts higher than 0.5, the opposite is
thought to be true of short GRBs. Lack of a detected VHE signal thus allows
setting meaningful fluence limits. One GRB in the sample (050509b) has a likely
association with a galaxy at a redshift of 0.225, while another (051103) has
been tentatively linked to the nearby galaxy M81. Fluence limits are corrected
for EBL absorption, either using the known measured redshift, or computing the
corresponding absorption for a redshift of 0.1 and 0.5, as well as for the case
of z=0.Comment: Accepted for publication in the Astrophysical Journa
The Complete Star Formation History of the Universe
The determination of the star-formation history of the Universe is a key goal
of modern cosmology, as it is crucial to our understanding of how structure in
the Universe forms and evolves. A picture has built up over recent years,
piece-by-piece, by observing young stars in distant galaxies at different times
in the past.
These studies indicated that the stellar birthrate peaked some 8 billion
years ago, and then declined by a factor of around ten to its present value.
Here we report on a new study which obtains the complete star formation history
by analysing the fossil record of the stellar populations of 96545 nearby
galaxies. Broadly, our results support those derived from high-redshift
galaxies elsewhere in the Universe. We find, however, that the peak of star
formation was more recent - around 5 billion years ago. Our study also shows
that the bigger the stellar mass of the galaxy, the earlier the stars were
formed. This striking result indicates a very different formation history for
high- and low-mass formation.Comment: Accepted by Nature. Press embargo until publishe
The high-energy gamma-ray fluence and energy spectrum of GRB 970417a from observations with Milagrito
Evidence of TeV emission from GRB970417a has been previously reported using
data from the Milagrito detector. Constraints on the TeV fluence and the energy
spectrum are now derived using additional data from a scaler system that
recorded the rate of signals from the Milagrito photomultipliers. This analysis
shows that if emission from GRB970417a has been observed, it must contain
photons with energies above 650 GeV. Some consequences of this observation are
discussed.Comment: Submitted to the Astrophysical Journa
Discovery of Localized Regions of Excess 10-TeV Cosmic Rays
An analysis of 7 years of Milagro data performed on a 10-degree angular scale
has found two localized regions of excess of unknown origin with greater than
12 sigma significance. Both regions are inconsistent with gamma-ray emission
with high confidence. One of the regions has a different energy spectrum than
the isotropic cosmic-ray flux at a level of 4.6 sigma, and it is consistent
with hard spectrum protons with an exponential cutoff, with the most
significant excess at ~10 TeV. Potential causes of these excesses are explored,
but no compelling explanations are found.Comment: Submitted to PhysRevLet
Search for very high energy gamma-rays from WIMP annihilations near the Sun with the Milagro Detector
The neutralino, the lightest stable supersymmetric particle, is a strong
theoretical candidate for the missing astronomical ``dark matter''. A profusion
of such neutralinos can accumulate near the Sun when they lose energy upon
scattering and are gravitationally captured. Pair-annihilations of those
neutralinos may produce very high energy (VHE, above ) gamma-rays.
Milagro is an air shower array which uses the water Cherenkov technique to
detect extensive air showers and is capable of observing VHE gamma-rays from
the direction of the Sun with an angular resolution of . Analysis
of Milagro data with an exposure to the Sun of 1165 hours presents the first
attempt to detect TeV gamma-rays produced by annihilating neutralinos captured
by the Solar system and shows no statistically significant signal. Resulting
limits that can be set on gamma-ray flux due to near-Solar neutralino
annihilations and on neutralino cross-section are presented
Observation of TeV Gamma Rays from the Crab Nebula with Milagro Using a New Background Rejection Technique
The recent advances in TeV gamma-ray astronomy are largely the result of the
ability to differentiate between extensive air showers generated by gamma rays
and hadronic cosmic rays. Air Cherenkov telescopes have developed and perfected
the "imaging" technique over the past several decades. However until now no
background rejection method has been successfully used in an air shower array
to detect a source of TeV gamma rays. We report on a method to differentiate
hadronic air showers from electromagnetic air showers in the Milagro gamma ray
observatory, based on the ability to detect the energetic particles in an
extensive air shower. The technique is used to detect TeV emission from the
Crab nebula. The flux from the Crab is estimated to be 2.68(+-0.42stat +-
1.4sys) x10^{-7} (E/1TeV)^{-2.59} m^{-2} s^{-1} TeV^{-1}, where the spectral
index is assumed to be as given by the HEGRA collaboration.Comment: 22 pages, 11 figures, submitted to Astrophysical Journa
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