75 research outputs found
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
Rapid formation of large dust grains in the luminous supernova SN 2010jl
The origin of dust in galaxies is still a mystery. The majority of the
refractory elements are produced in supernova explosions but it is unclear how
and where dust grains condense and grow, and how they avoid destruction in the
harsh environments of star-forming galaxies. The recent detection of 0.1-0.5
solar masses of dust in nearby supernova remnants suggests in situ dust
formation, while other observations reveal very little dust in supernovae the
first few years after explosion. Observations of the bright SN 2010jl have been
interpreted as pre-existing dust, dust formation or no dust at all. Here we
report the rapid (40-240 days) formation of dust in its dense circumstellar
medium. The wavelength dependent extinction of this dust reveals the presence
of very large (> 1 micron) grains, which are resistant to destructive
processes. At later times (500-900 days), the near-IR thermal emission shows an
accelerated growth in dust mass, marking the transition of the supernova from a
circumstellar- to an ejecta-dominated source of dust. This provides the link
between the early and late dust mass evolution in supernovae with dense
circumstellar media.Comment: 62 pages, 13 figures, 1 table. Author version of the Letter to
Nature, published online July 9 2014 (Nature, 511, 7509, pp. 326-329 (2014)),
prior to the final editorial changes to conform to Journal style; includes
Methods and Extended Data Figures and the Supplementary Information. See
published version
http://www.nature.com/nature/journal/v511/n7509/full/nature13558.htm
The interacting nature of dwarf galaxies hosting superluminous supernovae
(Abridged) Type I superluminous supernovae (SLSNe I) are rare, powerful
explosions whose mechanism and progenitors remain elusive. SLSNe I show a
preference for low-metallicity, actively star-forming dwarf galaxies. We
investigate whether the hosts of SLSNe I show increased evidence for
interaction. We use a sample of 42 SLSN I images obtained with
and measure the number of companion galaxies by counting the objects detected
within a given radius from the host. As a comparison, we used two Monte
Carlo-based methods to estimate the expected average number of companion
objects in the same images, as well as a sample of 32 galaxies that have hosted
long gamma-ray bursts (GRBs). About 50% of SLSN I hosts have at least one major
companion (within a flux ratio of 1:4) within 5 kpc. The average number of
major companions per SLSN I host galaxy is . Our Monte
Carlo comparison methods yield a lower number of companions for random objects
of similar brightness in the same image or for the SLSN host after randomly
redistributing the sources in the same image. The Anderson-Darling test shows
that this difference is statistically significant independent of the redshift
range. The same is true for the projected distance distribution of the
companions. The SLSN I hosts are, thus, found in areas of their images, where
the object number density is greater than average. SLSN I hosts have more
companions than GRB hosts ( companions per host
distributed over 25% of the hosts) but the difference is not statistically
significant. The difference between their separations is, however, marginally
significant. The dwarf galaxies hosting SLSNe I are often part of interacting
systems. This suggests that SLSNe I progenitors are formed after a recent burst
of star formation. Low metallicity alone cannot explain this tendency.Comment: Accepted for publication in A&A. In v2 replaced graphs with higher
quality PDF version
Helium in natal HII regions: the origin of the X-ray absorption in gamma-ray burst afterglows
Soft X-ray absorption in excess of Galactic is observed in the afterglows of
most gamma-ray bursts (GRBs), but the correct solution to its origin has not
been arrived at after more than a decade of work, preventing its use as a
powerful diagnostic tool. We resolve this long-standing problem and find that
He in the GRB's host HII region is responsible for most of the absorption. We
show that the X-ray absorbing column density (N_Hx) is correlated with both the
neutral gas column density and with the optical afterglow extinction (Av). This
correlation explains the connection between dark bursts and bursts with high
N_Hx values. From these correlations we exclude an origin of the X-ray
absorption which is not related to the host galaxy, i.e. the intergalactic
medium or intervening absorbers are not responsible. We find that the
correlation with the dust column has a strong redshift evolution, whereas the
correlation with the neutral gas does not. From this we conclude that the
column density of the X-ray absorption is correlated with the total gas column
density in the host galaxy rather than the metal column density, in spite of
the fact that X-ray absorption is typically dominated by metals. The strong
redshift evolution of N_Hx/Av is thus a reflection of the cosmic metallicity
evolution of star-forming galaxies. We conclude that the absorption of X-rays
in GRB afterglows is caused by He in the HII region hosting the GRB. While dust
is destroyed and metals are stripped of all of their electrons by the GRB to
great distances, the abundance of He saturates the He-ionising UV continuum
much closer to the GRB, allowing it to remain in the neutral or singly-ionised
state. Helium X-ray absorption explains the correlation with total gas, the
lack of strong evolution with redshift as well as the absence of dust, metal or
hydrogen absorption features in the optical-UV spectra.Comment: 10 pages, 4 figures, submitted to Ap
<紹介及批評>山口茂氏『流通經濟の貨幣的機構』
In this article, airfoil trailing edge bluntness noise is investigated using both computational aero-acoustic and semi-empirical approach. For engineering purposes, one of the most commonly used prediction tools for trailing edge noise are based on semi-empirical approaches, for example, the Brooks, Pope, and Marcolini airfoil noise prediction model developed by Brooks, Pope, and Marcolini (NASA Reference Publication 1218, 1989). It was found in previous study that the Brooks, Pope, and Marcolini model tends to over-predict noise at high frequencies. Furthermore, it was observed that this was caused by a lack in the model to predict accurately noise from blunt trailing edges. For more physical understanding of bluntness noise generation, in this study, we also use an advanced in-house developed high-order computational aero-acoustic technique to investigate the details associated with trailing edge bluntness noise. The results from the numerical model form the basis for an improved Brooks, Pope, and Marcolini trailing edge bluntness noise model
Polarimetry of the superluminous supernova LSQ14mo: no evidence for significant deviations from spherical symmetry
We present the first polarimetric observations of a Type I superluminous
supernova (SLSN). LSQ14mo was observed with VLT/FORS2 at five different epochs
in the V band, with the observations starting before maximum light and spanning
26 days in the rest frame (z=0.256). During this period, we do not detect any
statistically significant evolution (< 2) in the Stokes parameters. The
average values we obtain, corrected for interstellar polarisation in the
Galaxy, are Q = -0.01% ( 0.15%) and U = - 0.50% ( 0.14%). This low
polarisation can be entirely due to interstellar polarisation in the SN host
galaxy. We conclude that, at least during the period of observations and at the
optical depths probed, the photosphere of LSQ14mo does not present significant
asymmetries, unlike most lower-luminosity hydrogen-poor SNe Ib/c.
Alternatively, it is possible that we may have observed LSQ14mo from a special
viewing angle. Supporting spectroscopy and photometry confirm that LSQ14mo is a
typical SLSN I. Further studies of the polarisation of Type I SLSNe are
required to determine whether the low levels of polarisation are a
characteristic of the entire class and to also study the implications for the
proposed explosion models.Comment: ApJ Letters, 4 Figures, 3 Tables. The previous version was accepted.
This version contains minor modifications to match proofs (as much as
possible
SN 2007uy - metamorphosis of an aspheric Type Ib explosion
The supernovae of Type Ibc are rare and the detailed characteristics of these
explosions have been studied only for a few events. Unlike Type II SNe, the
progenitors of Type Ibc have never been detected in pre-explosion images. So,
to understand the nature of their progenitors and the characteristics of the
explosions, investigation of proximate events are necessary. Here we present
the results of multi-wavelength observations of Type Ib SN 2007uy in the nearby
( 29.5 Mpc) galaxy NGC 2770. Analysis of the photometric observations
revealed this explosion as an energetic event with peak absolute R band
magnitude , which is about one mag brighter than the mean value
() derived for well observed Type Ibc events. The SN is highly
extinguished, E(B-V) = 0.630.15 mag, mainly due to foreground material
present in the host galaxy. From optical light curve modeling we determine that
about 0.3 M radioactive Ni is produced and roughly 4.4
M material is ejected during this explosion with liberated energy
erg, indicating the event to be an energetic one.
Through optical spectroscopy, we have noticed a clear aspheric evolution of
several line forming regions, but no dependency of asymmetry is seen on the
distribution of Ni inside the ejecta. The SN shock interaction with the
circumburst material is clearly noticeable in radio follow-up, presenting a
Synchrotron Self Absorption (SSA) dominated light curve with a contribution of
Free Free Absorption (FFA) during the early phases. Assuming a WR star, with
wind velocity \ga 10^3 {\rm km s}^{-1}, as a progenitor, we derive a lower
limit to the mass loss rate inferred from the radio data as \dot{M} \ga
2.4\times10^{-5} M, yr, which is consistent with the results
obtained for other Type Ibc SNe bright at radio frequencies.Comment: 22 pages, 13 figures, accepted for publication in MNRA
Constraints on the neutron star equation of state from AT2017gfo using radiative transfer simulations
The detection of the binary neutron star GW170817 together with the
observation of electromagnetic counterparts across the entire spectrum
inaugurated a new era of multi-messenger astronomy. In this study we
incorporate wavelength-dependent opacities and emissivities calculated from
atomic-structure data enabling us to model both the measured lightcurves and
spectra of the electromagnetic transient AT2017gfo. Best-fits of the
observational data are obtained by Gaussian Process Regression, which allows us
to present posterior samples for the kilonova and source properties connected
to GW170817. Incorporating constraints obtained from the gravitational wave
signal obtained by the LIGO-Virgo Scientific Collaboration, we present a
upper bound on the mass ratio and a lower bound on the tidal
deformability of , which rules out sufficiently
soft equations of state. Our analysis is a path-finder for more realistic
kilonova models and shows how the combination of gravitational wave and
electromagnetic measurements allow for stringent constraints on the source
parameters and the supranuclear equation of state
The peculiar Type Ia supernova iPTF14atg: Chandrasekhar-mass explosion or violent merger?
iPTF14atg, a subluminous peculiar Type Ia supernova (SN Ia) similar to SN 2002es, is the first SN Ia for which a strong UV flash was observed in the early-time light curves. This has been interpreted as evidence for a single-degenerate (SD) progenitor system where such a signal is expected from interactions between the SN ejecta and the non-degenerate companion star. Here, we compare synthetic observables of multi-dimensional state-of-the-art explosion models for different progenitor scenarios to the light curves and spectra of iPTF14atg. From our models, we have difficulties explaining the spectral evolution of iPTF14atg within the SD progenitor channel. In contrast, we find that a violent merger of two carbon-oxygen white dwarfs with 0.9 and 0.76 M⊙, respectively, provides an excellent match to the spectral evolution of iPTF14atg from 10 d before to several weeks after maximum light. Our merger model does not naturally explain the initial UV flash of iPTF14atg. We discuss several possibilities like interactions of the SN ejecta with the circum-stellar medium and surface radioactivity from a He ignited merger that may be able to account for the early UV emission in violent merger models
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
- …