145 research outputs found
Transparent Helium in Stripped Envelope Supernovae
Using simple arguments based on photometric light curves and velocity
evolution, we propose that some stripped envelope supernovae (SNe) show signs
that a significant fraction of their helium is effectively transparent. The
main pieces of evidence are the relatively low velocities with little velocity
evolution, as are expected deep inside an exploding star, along with
temperatures that are too low to ionize helium. This means that the helium
should not contribute to the shaping of the main SN light curve, and thus the
total helium mass may be difficult to measure from simple light curve modeling.
Conversely, such modeling may be more useful for constraining the mass of the
carbon/oxygen core of the SN progenitor. Other stripped envelope SNe show
higher velocities and larger velocity gradients, which require an additional
opacity source (perhaps the mixing of heavier elements or radioactive nickel)
to prevent the helium from being transparent. We discuss ways in which similar
analysis can provide insights into the differences and similarities between SNe
Ib and Ic, which will lead to a better understanding of their respective
formation mechanisms.Comment: Revised version accepted for publication in The Astrophysical Journal
Letters, 4 pages, 4 figure
Exploring the Potential Diversity of Early Type Ia Supernova Light Curves
During the first several days after explosion, Type Ia supernova light curves
probe the outer layers of the exploding star and therefore provide important
clues for identifying their progenitors. We investigate how both the shallow
Ni distribution and the presence of circumstellar material shape these
early light curves. This is performed using a series of numerical experiments
with parameterized properties for systematic exploration. Although not all of
the considered models may be realized in nature (and indeed there are arguments
why some of them should not occur), the spirit of this work is to provide a
broader exploration of the diversity of possibilities. We find that shallower
Ni leads to steeper, bluer light curves. Differences in the shape of the
rise can introduce errors in estimating the explosion time and thus impact
efforts to infer upper limits on the progenitor or companion radius from a lack
of observed shock cooling emission. Circumstellar material can lead to
significant luminosity during the first few days, but its presence can be
difficult to identify depending on the degree of nickel mixing. In some cases,
the hot emission of circumstellar material may even lead to a signature similar
to interaction with a companion, and thus in the future additional diagnostics
should be gathered for properly assessing early light curves.Comment: Revised version with additional figures and discussions. 8 pages, 15
figures, accepted for publication in The Astrophysical Journa
Resonant Shattering of Neutron Star Crusts
The resonant excitation of neutron star (NS) modes by tides is investigated
as a source of short gamma-ray burst (sGRB) precursors. We find that the
driving of a crust-core interface mode can lead to shattering of the NS crust,
liberating ~10^46-10^47 erg of energy seconds before the merger of a NS-NS or
NS-black hole binary. Such properties are consistent with Swift/BAT detections
of sGRB precursors, and we use the timing of the observed precursors to place
weak constraints on the crust equation of state. We describe how a larger
sample of precursor detections could be used alongside coincident gravitational
wave detections of the inspiral by Advanced LIGO class detectors to probe the
NS structure. These two types of observations nicely complement one another,
since the former constrains the equation of state and structure near the
crust-core boundary, while the latter is more sensitive to the core equation of
state.Comment: 5 pages, 2 figures. Accepted to PR
Reconciling ^(56)Ni Production in Type Ia Supernovae with Double Degenerate Scenarios
We combine the observed distribution of Type Ia supernova (SN Ia) ^(56)Ni yields with the results of sub-Chandrasekhar detonation and direct collision calculations to estimate what mass white dwarfs (WDs) should be exploding for each scenario. For collisions, the average exploding WD mass must be peaked at â0.75M_â, significantly higher than the average field WD mass
of â0.55â0.60M_â. Thus, if collisions produce most SNe Ia, then a mechanism must exist that favours higher mass WDs. On the other hand, in old stellar populations, collisions would naturally result in low-luminosity SNe Ia, and we suggest these may be related to 1991bg-like events. For sub-Chandrasekhar detonations, the average exploding WD mass must be peaked at â1.1M_â. This is similar to the average total mass in WDâWD binaries, but it is not
clear whether double degenerate mergers would synthesize sufficient ^(56)Ni to match observed yields. If not, then actual â1.1M_â WDs would be needed for sub-Chandrasekhar detonations. Since such high-mass WDs are produced relatively quickly in comparison to the age of SN Ia
environments, this would require either accretion on to lower mass WDs prior to ignition or a long time-scale between formation of the â1.1M_â WD and ignition
Radio Transients from the Accretion-induced Collapse of White Dwarfs
It has long been expected that in some scenarios when a white dwarf (WD) grows to the Chandrasekhar limit, it can undergo an accretion-induced collapse (AIC) to form a rapidly rotating neutron star. Nevertheless, the detection of such events has so far evaded discovery, likely because the optical, supernova-like emission is expected to be dim and short-lived. Here we propose a novel signature of AIC: a transient radio source lasting for a few months. Rapid rotation along with flux freezing and dynamo action can grow the WD's magnetic field to magnetar strengths during collapse. The spin-down of this newly born magnetar generates a pulsar wind nebula (PWN) within the ~10^(â3)-10^(â1) M_â of ejecta surrounding it. Our calculations show that synchrotron emission from the PWN may be detectable in the radio, even if the magnetar has a rather modest magnetic field of ~2 Ă 10^(14) G and an initial spin period of ~10 ms. An all-sky survey with a detection limit of 1 mJy at 1.4 GHz would see ~4(Æ/10^(â2)) above threshold at any given time, where f is the ratio of the AIC rate to Type Ia supernova rate. A similar scenario may result from binary neutron stars if some mergers produce massive neutron stars rather than black holes. We conclude with a discussion of the detectability of these types of transient radio sources in an era of facilities with high mapping speeds
A versatile source of polarisation entangled photons for quantum network applications
We report a versatile and practical approach for generating high-quality
polarization entanglement in a fully guided-wave fashion. Our setup relies on a
high-brilliance type-0 waveguide generator producing paired photon at a telecom
wavelength associated with an advanced energy-time to polarisation transcriber.
The latter is capable of creating any pure polarization entangled state, and
allows manipulating single photon bandwidths that can be chosen at will over
five orders of magnitude, ranging from tens of MHz to several THz. We achieve
excellent entanglement fidelities for particular spectral bandwidths, i.e. 25
MHz, 540 MHz and 100 GHz, proving the relevance of our approach. Our scheme
stands as an ideal candidate for a wide range of network applications, ranging
from dense division multiplexing quantum key distribution to heralded optical
quantum memories and repeaters.Comment: 5 figure
Unstable Nonradial Oscillations on Helium Burning Neutron Stars
Material accreted onto a neutron star can stably burn in steady state only
when the accretion rate is high (typically super-Eddington) or if a large flux
from the neutron star crust permeates the outer atmosphere. For such situations
we have analyzed the stability of nonradial oscillations, finding one unstable
mode for pure helium accretion. This is a shallow surface wave which resides in
the helium atmosphere above the heavier ashes of the ocean. It is excited by
the increase in the nuclear reaction rate during the oscillations, and it grows
on the timescale of a second. For a slowly rotating star, this mode has a
frequency of approximately 20-30 Hz (for l=1), and we calculate the full
spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The
short period X-ray binary 4U 1820--30 is accreting helium rich material and is
the system most likely to show this unstable mode,especially when it is not
exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally
stable atmosphere shows that nonradial perturbations have a different stability
criterion than the spherically symmetric thermal perturbations that generate
type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14
figure
Fast Optical Transients from Stellar-Mass Black Hole Tidal Disruption Events in Young Star Clusters
Observational evidence suggests that the majority of stars may have been born in stellar clusters or associations. Within these dense environments, dynamical interactions lead to high rates of close stellar encounters. A variety of recent observational and theoretical indications suggest stellar-mass black holes may be present and play an active dynamical role in stellar clusters of all masses. In this study, we explore the tidal disruption of main sequence stars by stellar-mass black holes in young star clusters. We compute a suite of over 3000 independent N-body simulations that cover a range in cluster mass, metallicity, and half-mass radii. We find stellar-mass black hole tidal disruption events (TDEs) occur at an overall rate of up to roughly 300 Gpcâ»Âł yrâ»Âč in young stellar clusters in the local universe, with the majority occurring through binary--mediated dynamical encounters. These TDEs are expected to have several characteristic features, namely fast rise times of order a day, peak X-ray luminosities of at least 10âŽâŽ erg sâ»Âč, and bright optical luminosities (roughly 10âŽÂčâ10âŽâŽ erg sâ»Âč) associated with reprocessing by a disk wind. In particular, we show these events share many features in common with the emerging class of Fast Blue Optical Transients
- âŠ