196 research outputs found
POSSIS: predicting spectra, light curves and polarization for multi-dimensional models of supernovae and kilonovae
We present POSSIS, a time-dependent three-dimensional Monte Carlo code for
modelling radiation transport in supernovae and kilonovae. The code
incorporates wavelength- and time-dependent opacities and predicts
viewing-angle dependent spectra, light curves and polarization for both
idealized and hydrodynamical explosion models. We apply the code to a kilonova
model with two distinct ejecta components, one including lanthanide elements
with relatively high opacities and the other devoid of lanthanides and
characterized by lower opacities. We find that a model with total ejecta mass
and half-opening angle of the lanthanide-rich
component provides a good match to GW 170817 / AT 2017gfo for
orientations near the polar axis (i.e. for a system viewed close to face-on).
We then show how crucial is the use of self-consistent multi-dimensional models
in place of combining one-dimensional models to infer important parameters as
the ejecta masses. We finally explore the impact of and
on the synthetic observables and highlight how the relatively fast computation
times of POSSIS make it well-suited to perform parameter-space studies and
extract key properties of supernovae and kilonovae. Spectra calculated with
POSSIS in this and future studies will be made publicly available.Comment: 10 pages, 7 figures; accepted for publication in MNRAS; all modelled
spectra are made available at https://github.com/mbulla/kilonova_model
Modeling continuum polarization levels of tidal disruption events based on the collision-induced outflow mode
TDEs have been observed in the optical and UV for more than a decade but the
underlying emission mechanism still remains a puzzle. It has been suggested
that viewing angle effects could potentially explain their large photometric
and spectroscopic diversity. Polarization is indeed sensitive to the viewing
angle and the first polarimetry studies of TDEs are now available, calling for
a theoretical interpretation. In this study, we model the continuum
polarization levels of TDEs using the radiative transfer code POSSIS and the
collision-induced outflow (CIO) TDE emission scenario where unbound shocked gas
originating from a debris stream intersection point offset from the black hole,
reprocesses the hard emission from the accretion flow into UV and optical
bands. We explore two different cases of peak mass fallback rates M'p (~3 and
~0.3 Msol/yr) while varying the following geometrical parameters: the distance
R_int from the black hole (BH) to the intersection point, the radius of the
photosphere around the BH R_ph, on the surface of which the photons are
generated, and the opening angle Deltheta (anisotropic emission). For the high
mass fallback rate case, we find for every viewing angle polarization levels
below one (P<1%) and P<0.5% for 10/12 simulations. The absolute value of
polarization reaches its maximum (P_max) for equatorial viewing angles. For the
low mass fallback rate case, the maximum value predicted is P~8.8% and P_max is
reached for intermediate viewing angles. We find that the polarization depends
strongly on i) the optical depths at the central regions set by the different
M'p values and ii) the viewing angle. Finally, by comparing our model
predictions to polarization observations of a few TDEs, we attempt to constrain
their observed viewing angles and we show that multi-epoch polarimetric
observations can become a key factor in constraining the viewing angle of TDEs.Comment: Accepted for publication in Astronomy and Astrophysics journal; 20
page
Black Hole - Neutron Star mergers: using kilonovae to constrain the equation of state
The merging of a binary system involving two neutron stars (NSs), or a black
hole (BH) and a NS, often results in the emission of an electromagnetic (EM)
transient. One component of this EM transient is the epic explosion known as a
kilonova (KN). The characteristics of the KN emission can be used to probe the
equation of state (EoS) of NS matter responsible for its formation. We predict
KN light curves from computationally simulated BH-NS mergers, by using the 3D
radiative transfer code \texttt{POSSIS}. We investigate two EoSs spanning most
of the allowed range of the mass-radius diagram. We also consider a soft EoS
compatible with the observational data within the so-called 2-families scenario
in which hadronic stars coexist with strange stars. Computed results show that
the 2-families scenario, characterized by a soft EoS, should not produce a KN
unless the mass of the binary components are small (, ) and the BH is rapidly spinning
(). In contrast, a strong KN signal potentially
observable from future surveys (e.g. VRO/LSST) is produced in the 1-family
scenario for a wider region of the parameter space, and even for non-rotating
BHs () when and . We also provide a fit that allows for the calculation of the
unbound mass from the observed KN magnitude, without running timely and costly
radiative transfer simulations. Findings presented in this paper will be used
to interpret light curves anticipated during the fourth observing run (O4), of
the advanced LIGO, advanced Virgo and KAGRA interferometers and thus to
constrain the EoS of NS matter.Comment: 14 pages, 16 figures, 2 table
Impact of jets on kilonova photometric and polarimetric emission from binary neutron star mergers
A merger of binary neutron stars creates heavy unstable elements whose
radioactive decay produces a thermal emission known as a kilonova. In this
paper, we predict the photometric and polarimetric behaviour of this emission
by performing 3-D Monte Carlo radiative transfer simulations. In particular, we
choose three hydrodynamical models for merger ejecta, two including jets with
different luminosities and one without a jet structure, to help decipher the
impact of jets on the light curve and polarimetric behaviour. In terms of
photometry, we find distinct color evolutions across the three models. Models
without a jet show the highest variation in light curves for different viewing
angles. In contrast, to previous studies, we find models with a jet to produce
fainter kilonovae when viewed from orientations close to the jet axis, compared
to a model without a jet. In terms of polarimetry, we predict relatively low
levels (<~0.3-0.4%) at all orientations that, however, remain non-negligible
until a few days after the merger and longer than previously found. Despite the
low levels, we find that the presence of a jet enhances the degree of
polarization at wavelengths ranging from 0.25 to 2.5\micron, an effect that is
found to increase with the jet luminosity. Thus, future photometric and
polarimetric campaigns should observe kilonovae in blue and red filters for a
few days after the merger to help constrain the properties of the ejecta (e.g.
composition) and jet.Comment: 11 pages, 7 figures, accepted for publication and in pres
An asymmetric electron-scattering photosphere around optical tidal disruption events
A star crossing the tidal radius of a supermassive black hole will be
spectacularly ripped apart with an accompanying burst of radiation. A few tens
of such tidal disruption events (TDEs) have now been identified in the optical
wavelengths, but the exact origin of the strong optical emission remains
inconclusive. Here we report polarimetric observations of three TDEs. The
continuum polarization is independent of wavelength, while emission lines are
partially depolarized. These signatures are consistent with optical photons
being scattered and polarized in an envelope of free electrons. An almost
axisymmetric photosphere viewed from different angles is in broad agreement
with the data, but there is also evidence for deviations from axial symmetry
before the peak of the flare and significant time evolution at early times,
compatible with the rapid formation of an accretion disk. By combining a
super-Eddington accretion model with a radiative transfer code we generate
predictions for the degree of polarization as a function of disk mass and
viewing angle, and we show that the predicted levels are compatible with the
observations, for extended reprocessing envelopes of 1000 gravitational
radii. Spectropolarimetry therefore constitutes a new observational test for
TDE models, and opens an important new line of exploration in the study of
TDEs.Comment: Author's version of paper to appear in Nature Astronomy. In the
journal version the detailed discussion on the ISP determination will be
moved from the Methods section to a Supplementary Information section. 58
pages in double spacing format, including 5 Figures, 10 Extended Data Figures
and 2 Table
DECam-GROWTH Search for the Faint and Distant Binary Neutron Star and Neutron Star-Black Hole Mergers in O3a
Synoptic searches for the optical counterpart to a binary neutron star (BNS) or neutron star-black hole (NSBH) merger can pose significant challenges towards the discovery of kilonovae and performing multi-messenger science. In this work, we describe the advantage of a global multi-telescope network towards this end, with a particular focus on the key and complementary role the Dark Energy Camera (DECam) plays in multi-facility follow-up. We describe the Global Relay of Observatories Watching Transients Happen (GROWTH) Target-of-Opportunity (ToO) Marshal, a common web application we built to ingest events, plan observations, search for transient candidates, and retrieve performance summary statistics for all of the telescopes in our network. Our infrastructure enabled us to conduct observations of two events during O3a, S190426c and S190510g. Furthermore, our analysis of deep DECam observations of S190814bv conducted by the DESGW team, and access to a variety of global follow-up facilities allowed us to place meaningful constraints on the parameters of the kilonova and the merging binary. We emphasize the importance of a global telescope network in conjunction with a power telescope like DECam in performing searches for the counterparts to gravitational-wave sources
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