311 research outputs found
Broadband modelling of short gamma-ray bursts with energy injection from magnetar spin-down and its implications for radio detectability
The magnetar model has been proposed to explain the apparent energy injection
in the X-ray light curves of short gamma-ray bursts (SGRBs), but its
implications across the full broadband spectrum are not well explored. We
investigate the broadband modelling of four SGRBs with evidence for energy
injection in their X-ray light curves, applying a physically motivated model in
which a newly formed magnetar injects energy into a forward shock as it loses
angular momentum along open field lines. By performing an order of magnitude
search for the underlying physical parameters in the blast wave, we constrain
the characteristic break frequencies of the synchrotron spectrum against their
manifestations in the available multi-wavelength observations for each burst.
The application of the magnetar energy injection profile restricts the
successful matches to a limited family of models that are self-consistent
within the magnetic dipole spin-down framework.We produce synthetic light
curves that describe how the radio signatures of these SGRBs ought to have
looked given the restrictions imposed by the available data, and discuss the
detectability of these signatures with present-day and near-future radio
telescopes. Our results show that both the Atacama Large Millimetre Array and
the upgraded Very Large Array are now sensitive enough to detect the radio
signature within two weeks of trigger in most SGRBs, assuming our sample is
representative of the population as a whole. We also find that the upcoming
Square Kilometre Array will be sensitive to depths greater than those of our
lower limit predictions.Comment: 15 pages, 4 figures, 6 tables, accepted for publication in MNRA
The Environments of the Most Energetic Gamma-Ray Bursts
We analyze the properties of a sample of long gamma-ray bursts (LGRBs)
detected by the Fermi satellite that have a spectroscopic redshift and good
follow-up coverage at both X-ray and optical/nIR wavelengths. The evolution of
LGRB afterglows depends on the density profile of the external medium, enabling
us to separate wind or ISM-like environments based on the observations. We do
this by identifying the environment that provides the best agreement between
estimates of , the index of the underlying power-law distribution of
electron energies, as determined by the behavior of the afterglow in different
spectral/temporal regimes. At 11 rest-frame hours after trigger, we find a
roughly even split between ISM-like and wind-like environments. We further find
a 2 separation in the prompt emission energy distributions of wind-like
and ISM-like bursts. We investigate the underlying physical parameters of the
shock, and calculate the (degenerate) product of density and magnetic field
energy (). We show that must be to avoid
implied densities comparable to the intergalactic medium. Finally, we find that
the most precisely constrained observations disagree on by more than would
be expected based on observational errors alone. This suggests additional
sources of error that are not incorporated in the standard afterglow theory.
For the first time, we provide a measurement of this intrinsic error which can
be represented as an error in the estimate of of magnitude .
When this error is included in the fits, the number of LGRBs with an identified
environment drops substantially, but the equal division between the two types
remains.Comment: 31 pages (+14 appendix), 9 figures, 6 tables. Accepted for
publication in Ap
Constraining properties of GRB magnetar central engines using the observed plateau luminosity and duration correlation
An intrinsic correlation has been identified between the luminosity and
duration of plateaus in the X-ray afterglows of Gamma-Ray Bursts (GRBs;
Dainotti et al. 2008), suggesting a central engine origin. The magnetar central
engine model predicts an observable plateau phase, with plateau durations and
luminosities being determined by the magnetic fields and spin periods of the
newly formed magnetar. This paper analytically shows that the magnetar central
engine model can explain, within the 1 uncertainties, the correlation
between plateau luminosity and duration. The observed scatter in the
correlation most likely originates in the spread of initial spin periods of the
newly formed magnetar and provides an estimate of the maximum spin period of
~35 ms (assuming a constant mass, efficiency and beaming across the GRB
sample). Additionally, by combining the observed data and simulations, we show
that the magnetar emission is most likely narrowly beamed and has 20%
efficiency in conversion of rotational energy from the magnetar into the
observed plateau luminosity. The beaming angles and efficiencies obtained by
this method are fully consistent with both predicted and observed values. We
find that Short GRBs and Short GRBs with Extended Emission lie on the same
correlation but are statistically inconsistent with being drawn from the same
distribution as Long GRBs, this is consistent with them having a wider beaming
angle than Long GRBs.Comment: MNRAS Accepte
A multi-messenger model for neutron star - black hole mergers
We present a semi-analytic model for predicting kilonova light curves from
the mergers of neutron stars with black holes (NSBH). The model is integrated
into the MOSFiT platform, and can generate light curves from input binary
properties and nuclear equation-of-state considerations, or incorporate
measurements from gravitational wave (GW) detectors to perform multi-messenger
parameter estimation. The rapid framework enables the generation of NSBH
kilonova distributions from binary populations, light curve predictions from GW
data, and statistically meaningful comparisons with an equivalent BNS model in
MOSFiT. We investigate a sample of kilonova candidates associated with
cosmological short gamma-ray bursts, and demonstrate that they are broadly
consistent with being driven by NSBH systems, though most have limited data. We
also perform fits to the very well sampled GW170817, and show that the
inability of an NSBH merger to produce lanthanide-poor ejecta results in a
significant underestimate of the early (< 2 days) optical emission. Our model
indicates that NSBH-driven kilonovae may peak up to a week after merger at
optical wavelengths for some observer angles. This demonstrates the need for
early coverage of emergent kilonovae in cases where the GW signal is either
ambiguous or absent; they likely cannot be distinguished from BNS mergers by
the light curves alone from ~2 days after the merger. We also discuss the
detectability of our model kilonovae with the Vera C. Rubin Observatory's
Legacy Survey of Space and Time (LSST).Comment: 14 pages, 6 figures, 2 tables. Accepted for publication in MNRAS.
This is the author's final submitted version. The model code is available
through MOSFiT at https://github.com/guillochon/MOSFi
AT2023fhn (the Finch): a Luminous Fast Blue Optical Transient at a large offset from its host galaxy
Luminous Fast Blue Optical Transients (LFBOTs) - the prototypical example
being AT 2018cow - are a rare class of events whose origins are poorly
understood. They are characterised by rapid evolution, featureless blue spectra
at early times, and luminous X-ray and radio emission. LFBOTs thus far have
been found exclusively at small projected offsets from star-forming host
galaxies. We present Hubble Space Telescope, Gemini, Chandra and Very Large
Array observations of a new LFBOT, AT2023fhn. The Hubble Space Telescope data
reveal a large offset (greater than 3.5 half-light radii) from the two closest
galaxies, both at a redshift of 0.24. The isolated environment of AT 2023fhn is
in stark contrast with previous events, is challenging to explain with most
LFBOT progenitor models, and calls into question the homogeneity of LFBOTs as a
class.Comment: Submitted to MNRASL. 7 pages, 4 figures, 2 table
AT2023fhn (the Finch):a luminous fast blue optical transient at a large offset from its host galaxy
Luminous fast blue optical transients (LFBOTs) – the prototypical example being AT 2018cow – are a rare class of events whose origins are poorly understood. They are characterized by rapid evolution, featureless blue spectra at early times, and luminous X-ray and radio emission. LFBOTs thus far have been found exclusively at small projected offsets from star-forming host galaxies. We present Hubble Space Telescope, Gemini, Chandra, and Very Large Array observations of a new LFBOT, AT 2023fhn. The Hubble Space Telescope data reveal a large offset (>3.5 half-light radii) from the two closest galaxies, both at redshift z ∼ 0.24. The location of AT 2023fhn is in stark contrast with previous events, and demonstrates that LFBOTs can occur in a range of galactic environments
Evidence for the Gompertz Curve in the Income Distribution of Brazil 1978-2005
This work presents an empirical study of the evolution of the personal income
distribution in Brazil. Yearly samples available from 1978 to 2005 were studied
and evidence was found that the complementary cumulative distribution of
personal income for 99% of the economically less favorable population is well
represented by a Gompertz curve of the form , where
is the normalized individual income. The complementary cumulative
distribution of the remaining 1% richest part of the population is well
represented by a Pareto power law distribution . This
result means that similarly to other countries, Brazil's income distribution is
characterized by a well defined two class system. The parameters , ,
, were determined by a mixture of boundary conditions,
normalization and fitting methods for every year in the time span of this
study. Since the Gompertz curve is characteristic of growth models, its
presence here suggests that these patterns in income distribution could be a
consequence of the growth dynamics of the underlying economic system. In
addition, we found out that the percentage share of both the Gompertzian and
Paretian components relative to the total income shows an approximate cycling
pattern with periods of about 4 years and whose maximum and minimum peaks in
each component alternate at about every 2 years. This finding suggests that the
growth dynamics of Brazil's economic system might possibly follow a
Goodwin-type class model dynamics based on the application of the
Lotka-Volterra equation to economic growth and cycle.Comment: 22 pages, 15 figures, 4 tables. LaTeX. Accepted for publication in
"The European Physical Journal B
Exact Solution of an Evolutionary Model without Ageing
We introduce an age-structured asexual population model containing all the
relevant features of evolutionary ageing theories. Beneficial as well as
deleterious mutations, heredity and arbitrary fecundity are present and managed
by natural selection. An exact solution without ageing is found. We show that
fertility is associated with generalized forms of the Fibonacci sequence, while
mutations and natural selection are merged into an integral equation which is
solved by Fourier series. Average survival probabilities and Malthusian growth
exponents are calculated indicating that the system may exhibit mutational
meltdown. The relevance of the model in the context of fissile reproduction
groups as many protozoa and coelenterates is discussed.Comment: LaTeX file, 15 pages, 2 ps figures, to appear in Phys. Rev.
Infrared molecular hydrogen lines in GRB host galaxies
Molecular species, most frequently H_2, are present in a small, but growing,
number of gamma-ray burst (GRB) afterglow spectra at redshifts z~2-3, detected
through their rest-frame UV absorption lines. In rare cases, lines of
vibrationally excited states of H_2 can be detected in the same spectra. The
connection between afterglow line-of-sight absorption properties of molecular
(and atomic) gas, and the observed behaviour in emission of similar sources at
low redshift, is an important test of the suitability of GRB afterglows as
general probes of conditions in star formation regions at high redshift.
Recently, emission lines of carbon monoxide have been detected in a small
sample of GRB host galaxies, at sub-mm wavelengths, but no searches for H_2 in
emission have been reported yet. In this paper we perform an exploratory search
for rest-frame K band rotation-vibrational transitions of H_2 in emission,
observable only in the lowest redshift GRB hosts (z<0.22). Searching the data
of four host galaxies, we detect a single significant rotation-vibrational H_2
line candidate, in the host of GRB 031203. Re-analysis of Spitzer mid-infrared
spectra of the same GRB host gives a single low significance rotational line
candidate. The (limits on) line flux ratios are consistent with those of blue
compact dwarf galaxies in the literature. New instrumentation, in particular on
the JWST and the ELT, can facilitate a major increase in our understanding of
the H_2 properties of nearby GRB hosts, and the relation to H_2 absorption in
GRBs at higher redshift.Comment: 7 pages, 4 figures, MNRAS accepte
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