142 research outputs found
Radio Supernovae in the Great Survey Era
Radio properties of supernova outbursts remain poorly understood despite
longstanding campaigns following events discovered at other wavelengths. After
~ 30 years of observations, only ~ 50 supernovae have been detected at radio
wavelengths, none of which are Type Ia. Even the most radio-loud events are ~
10^4 fainter in the radio than in the optical; to date, such intrinsically dim
objects have only been visible in the very local universe. The detection and
study of radio supernovae (RSNe) will be fundamentally altered and dramatically
improved as the next generation of radio telescopes comes online, including
EVLA, ASKAP, and MeerKAT, and culminating in the Square Kilometer Array (SKA);
the latter should be > 50 times more sensitive than present facilities. SKA can
repeatedly scan large (> 1 deg^2) areas of the sky, and thus will discover RSNe
and other transient sources in a new, automatic, untargeted, and unbiased way.
We estimate SKA will be able to detect core-collapse RSNe out to redshift z ~
5, with an all-redshift rate ~ 620 events yr^-1 deg^-2, assuming a survey
sensitivity of 50 nJy and radio lightcurves like those of SN 1993J. Hence SKA
should provide a complete core-collapse RSN sample that is sufficient for
statistical studies of radio properties of core-collapse supernovae. EVLA
should find ~ 160 events yr^-1 deg^-2 out to redshift z ~ 3, and other SKA
precursors should have similar detection rates. We also provided
recommendations of the survey strategy to maximize the RSN detections of SKA.
This new radio core-collapse supernovae sample will complement the detections
from the optical searches, such as the LSST, and together provide crucial
information on massive star evolution, supernova physics, and the circumstellar
medium, out to high redshift. Additionally, SKA may yield the first radio Type
Ia detection via follow-up of nearby events discovered at other wavelengths.Comment: 21 pages, 5 figures, accepted for publication in Ap
The Diffuse Gamma-ray Background from Type Ia Supernovae
The origin of the diffuse extragalactic gamma-ray background (EGB) has been
intensively studied but remains unsettled. Current popular source candidates
include unresolved star-forming galaxies, starburst galaxies, and blazars. In
this paper we calculate the EGB contribution from the interactions of cosmic
rays accelerated by Type Ia supernovae (SNe), extending earlier work which only
included core-collapse SNe. We consider Type Ia events in star-forming
galaxies, but also in quiescent galaxies that lack star formation. For
star-forming galaxies, consistently including Type Ia events makes little
change to the star-forming EGB prediction, so long as both SN types have the
same cosmic-ray acceleration efficiencies in star-forming galaxies. Thus, our
updated EGB estimate continues to show that star-forming galaxies can represent
a substantial portion of the signal measured by Fermi. For quiescent galaxies,
conversely, we find a wide range of possibilities for the EGB contribution. The
dominant uncertainty we investigated comes from the mass in hot gas, which
provides targets for cosmic rays; total gas masses are as yet poorly known,
particularly at larger radii. Additionally, the EGB estimation is very
sensitive to the cosmic-ray acceleration efficiency and confinement, especially
in quiescent galaxies. In the most optimistic allowed scenarios, quiescent
galaxies can be an important source of the EGB. In this case, star-forming
galaxies and quiescent galaxies together will dominate the EGB and leave little
room for other contributions. If other sources, such as blazars, are found to
have important contributions to the EGB, then either the gas mass or cosmic-ray
content of quiescent galaxies must be significantly lower than in their
star-forming counterparts. In any case, improved Fermi EGB measurements will
provide important constraints on hot gas and cosmic rays in quiescent galaxies.Comment: 29 pages, 4 figures, Accepted for publication in Ap
Occurrence cubes : a new paradigm for aggregating species occurrence data
In this paper we describe a method of aggregating species occurrence data into what we coined “occurrence cubes”. The aggregated data can be perceived as a cube with three dimensions - taxonomic, temporal and geographic - and takes into account the spatial uncertainty of each occurrence. The aggregation level of each of the three dimensions can be adapted to the scope. Built on Open Science principles, the method is easily automated and reproducible, and can be used for species trend indicators, maps and distribution models. We are using the method to aggregate species occurrence data for Europe per taxon, year and 1km2 European reference grid, to feed indicators and risk mapping/modelling for the Tracking Invasive Alien Species (TrIAS) project
Probing the Gamma-Ray Burst Rate with Trigger Simulations of the Swift Burst Alert Telescope
The long gamma-ray burst (GRB) rate is essential for revealing the connection
between GRBs, supernovae and stellar evolution. Additionally, the GRB rate at
high redshift provides a strong probe of star formation history in the early
universe. While hundreds of GRBs are observed by Swift, it remains difficult to
determine the intrinsic GRB rate due to the complex trigger algorithm of Swift.
Current studies usually approximate the Swift trigger algorithm by a single
detection threshold. However, unlike the previously flown GRB instruments,
Swift has over 500 trigger criteria based on photon count rate and additional
image threshold for localization. To investigate possible systematic biases and
explore the intrinsic GRB properties, we developed a program that is capable of
simulating all the rate trigger criteria and mimicking the image trigger
threshold. We use this program to search for the intrinsic GRB rate. Our
simulations show that adopting the complex trigger algorithm of Swift increases
the detection rate of dim bursts. As a result, we find that either the GRB rate
is much higher than previously expected at large redshift, or the luminosity
evolution is non-negligible. We will discuss the best results of the GRB rate
in our search, and their impact on the star-formation history.Comment: 6 pages, 3 figures, 7th Huntsville Gamma-Ray Burst Symposium, GRB
2013: paper 35 in eConf Proceedings C130414
Probing the Cosmic Gamma-Ray Burst Rate with Trigger Simulations of the Swift Burst Alert Telescope
The gamma-ray burst (GRB) rate is essential for revealing the connection
between GRBs, supernovae and stellar evolution. Additionally, the GRB rate at
high redshift provides a strong probe of star formation history in the early
universe. While hundreds of GRBs are observed by Swift, it remains difficult to
determine the intrinsic GRB rate due to the complex trigger algorithm of Swift.
Current studies of the GRB rate usually approximate the Swift trigger algorithm
by a single detection threshold. However, unlike the previously flown GRB
instruments, Swift has over 500 trigger criteria based on photon count rate and
additional image threshold for localization. To investigate possible systematic
biases and explore the intrinsic GRB properties, we develop a program that is
capable of simulating all the rate trigger criteria and mimicking the image
threshold. Our simulations show that adopting the complex trigger algorithm of
Swift increases the detection rate of dim bursts. As a result, our simulations
suggest bursts need to be dimmer than previously expected to avoid
over-producing the number of detections and to match with Swift observations.
Moreover, our results indicate that these dim bursts are more likely to be high
redshift events than low-luminosity GRBs. This would imply an even higher
cosmic GRB rate at large redshifts than previous expectations based on
star-formation rate measurements, unless other factors, such as the luminosity
evolution, are taken into account. The GRB rate from our best result gives a
total number of 4571^{+829}_{-1584} GRBs per year that are beamed toward us in
the whole universe.
SPECIAL NOTE (2015.05.16): This new version incorporates an erratum. All the
GRB rate normalizations () should be a factor of 2 smaller
than previously reported. Please refer to the Appendix for more details. We
sincerely apologize for the mistake.Comment: 52 pages, 17 figures, published in ApJ 783, 24L (2014). An erratum is
included. A typo in Eq. 8 is fixed in this versio
Evidence for a strong 19.5 Hz flux oscillation in Swift BAT and Fermi GBM gamma-ray data from GRB 211211A
The gamma-ray burst (GRB) GRB~211211A is believed to have occurred due to the
merger of two neutron stars or a neutron star and a black hole, despite its
duration of more than a minute. Subsequent analysis has revealed numerous
interesting properties including the possible presence of a ~Hz
quasiperiodic oscillation (QPO) during precursor emission. Here we perform
timing analysis of Fermi and Swift gamma-ray data on GRB~211211A and, although
we do not find a strong QPO during the precursor, we do find an extremely
significant 19.5~Hz flux oscillation, which has higher fractional amplitude at
higher energies, in a ~second segment beginning ~seconds
after the start of the burst. After presenting our analysis we discuss possible
mechanisms for the oscillation.Comment: 16 pages, 7 figures, 2 table
Modeling the Swift BAT Trigger Algorithm with Machine Learning
To draw inferences about gamma-ray burst (GRB) source populations based on Swift observations, it is essential to understand the detection efficiency of the Swift burst alert telescope (BAT). This study considers the problem of modeling the Swift BAT triggering algorithm for long GRBs, a computationally expensive procedure, and models it using machine learning algorithms. A large sample of simulated GRBs from Lien et al. (2014) is used to train various models: random forests, boosted decision trees (with AdaBoost), support vector machines, and artificial neural networks. The best models have accuracies of approximately greater than 97% (approximately less than 3% error), which is a significant improvement on a cut in GRB flux which has an accuracy of 89:6% (10:4% error). These models are then used to measure the detection efficiency of Swift as a function of redshift z, which is used to perform Bayesian parameter estimation on the GRB rate distribution. We find a local GRB rate density of eta(sub 0) approximately 0.48(+0.41/-0.23) Gpc(exp -3) yr(exp -1) with power-law indices of eta(sub 1) approximately 1.7(+0.6/-0.5) and eta(sub 2) approximately -5.9(+5.7/-0.1) for GRBs above and below a break point of z(sub 1) approximately 6.8(+2.8/-3.2). This methodology is able to improve upon earlier studies by more accurately modeling Swift detection and using this for fully Bayesian model fitting. The code used in this is analysis is publicly available online
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