21 research outputs found
Fast Shocks From Magnetic Reconnection Outflows
Magnetic reconnection is commonly perceived to drive flow and particle
acceleration in flares of solar, stellar, and astrophysical disk coronae but
the relative roles of different acceleration mecha- nisms in a given
reconnection environment are not well understood. We show via direct numerical
simulations that reconnection outflows produce weak fast shocks, when
conditions for fast recon- nection are met and the outflows encounter an
obstacle. The associated compression ratios lead to a Fermi acceleration
particle spectrum that is significantly steeper than the strong fast shocks
commonly studied, but consistent with the demands of solar flares. While this
is not the only acceleration mechanism operating in a reconnection environment,
it is plausibly a ubiquitous one
Jitter radiation as a possible mechanism for Gamma-Ray Burst afterglows. Spectra and lightcurves
The standard model of GRB afterglows assumes that the radiation observed as a
delayed emission is of synchrotron origin, which requires the shock magnetic
field to be relatively homogeneous on small scales. An alternative mechanism --
jitter radiation, which traditionally has been applied to the prompt emission
-- substitutes synchrotron when the magnetic field is tangled on a microscopic
scale. Such fields are produced at relativistic shocks by the Weibel
instability. Here we explore the possibility that small-scale fields populate
afterglow shocks. We derive the spectrum of jitter radiation under the
afterglow conditions. We also derive the afterglow lightcurves for the ISM and
Wind profiles of the ambient density. Jitter self-absorption is calculated here
for the first time. We find that jitter radiation can produce afterglows
similar to synchrotron-generated ones, but with some important differences. We
compare the predictions of the two emission mechanisms. By fitting
observational data to the synchrotron and jitter afterglow lightcurves, it can
be possible to discriminate between the small-scale vs large-scale magnetic
field models in afterglow shocks.Comment: 16 pages, 1 figur
Interaction of the magnetorotational instability with hydrodynamic turbulence in accretion disks
Accretion disks in which angular momentum transport is dominated by the
magnetorotational instability (MRI) can also possess additional, purely
hydrodynamic, drivers of turbulence. Even when the hydrodynamic processes, on
their own, generate negligible levels of transport, they may still affect the
evolution of the disk via their influence on the MRI. Here, we study the
interaction between the MRI and hydrodynamic turbulence using local MRI
simulations that include hydrodynamic forcing. As expected, we find that
hydrodynamic forcing is generally negligible if it yields a saturated kinetic
energy density that is small compared to the value generated by the MRI. For
stronger hydrodynamic forcing levels, we find that hydrodynamic turbulence
modifies transport, with the effect varying depending upon the spatial scale of
hydrodynamic driving. Large scale forcing boosts transport by an amount that is
approximately linear in the forcing strength, and leaves the character of the
MRI (for example the ratio between Maxwell and Reynolds stresses) unchanged, up
to the point at which the forced turbulence is an order of magnitude stronger
than that generated by the MRI. Low amplitude small scale forcing may modestly
suppress the MRI. We conclude that the impact of hydrodynamic turbulence on the
MRI is generically ignorable in cases, such as convection, where the additional
turbulence arises due to the accretion energy liberated by the MRI itself.
Hydrodynamic turbulence may affect (and either enhance or suppress) the MRI if
it is both strong, and driven by independent mechanisms such as self-gravity,
supernovae, or solid-gas interactions in multiphase protoplanetary disks.Comment: ApJ, in pres
Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817
The binary neutron star (BNS) merger GW170817 was the first astrophysical
source detected in gravitational waves and multi-wavelength electromagnetic
radiation. The almost simultaneous observation of a pulse of gamma-rays proved
that BNS mergers are associated with at least some short gamma-ray bursts
(GRBs). However, the gamma-ray pulse was faint, casting doubts on the
association of BNS mergers with the luminous, highly relativistic outflows of
canonical short GRBs. Here we show that structured jets with a relativistic,
energetic core surrounded by slower and less energetic wings produce afterglow
emission that brightens characteristically with time, as recently seen in the
afterglow of GW170817. Initially, we only see the relatively slow material
moving towards us. As time passes, larger and larger sections of the outflow
become visible, increasing the luminosity of the afterglow. The late appearance
and increasing brightness of the multi-wavelength afterglow of GW170817 allow
us to constrain the geometry of its ejecta and thus reveal the presence of an
off-axis jet pointing about 30 degrees away from Earth. Our results confirm a
single origin for BNS mergers and short GRBs: GW170817 produced a structured
outflow with a highly relativistic core and a canonical short GRB. We did not
see the bright burst because it was beamed away from Earth. However,
approximately one in 20 mergers detected in gravitational waves will be
accompanied by a bright, canonical short GRB.Comment: Models updated with new data and added references. Accepted for
publication in PRL, 8 pages, 7 figures and 1 table. A grid of models, jet
properties, and python interpolating routine is available at
http://www.science.oregonstate.edu/~lazzatid/cocoon.htm
Jitter radiation images, spectra, and light curves from a relativistic spherical blastwave
We consider radiation emitted by the jitter mechanism in a Blandford-McKee
self-similar blastwave. We assume the magnetic field configuration throughout
the whole blastwave meets the condition for the emission of jitter radiation
and we compute the ensuing images, light curves and spectra. The calculations
are performed for both a uniform and a wind environment. We compare our jitter
results to synchrotron results. We show that jitter radiation produces slightly
different spectra than synchrotron, in particular between the self-absorption
and the peak frequency, where the jitter spectrum is flat, while the
synchrotron spectrum grows as \nu^{1/3}. The spectral difference is reflected
in the early decay slope of the light curves. We conclude that jitter and
synchrotron afterglows can be distinguished from each other with good quality
observations. However, it is unlikely that the difference can explain the
peculiar behavior of several recent observations, such as flat X-ray slopes and
uncorrelated optical and X-ray behavior.Comment: 11 pages, 7 postscript figures. Accepted for publication in MNRA
Nanopore native RNA sequencing of a human poly(A) transcriptome
High-throughput complementary DNA sequencing technologies have advanced our understanding of transcriptome complexity and regulation. However, these methods lose information contained in biological RNA because the copied reads are often short and modifications are not retained. We address these limitations using a native poly(A) RNA sequencing strategy developed by Oxford Nanopore Technologies. Our study generated 9.9 million aligned sequence reads for the human cell line GM12878, using thirty MinION flow cells at six institutions. These native RNA reads had a median length of 771 bases, and a maximum aligned length of over 21,000 bases. Mitochondrial poly(A) reads provided an internal measure of read-length quality. We combined these long nanopore reads with higher accuracy short-reads and annotated GM12878 promoter regions to identify 33,984 plausible RNA isoforms. We describe strategies for assessing 3′ poly(A) tail length, base modifications and transcript haplotypes
Jitter radiation in gamma-ray bursts and their afterglows: emission and self-absorption
Relativistic electrons moving into a highly tangled magnetic field emit
jitter radiation. We present a detailed computation of the jitter radiation
spectrum, including self-absorption, for electrons inside Weibel-like shock
generated magnetic fields. We apply our results to the case of the prompt and
afterglow emission of gamma-ray bursts. We show that jitter emission can
reproduce most of the observed features with some important differences with
respect to standard synchrotron, especially in the frequency range between the
self-absorption and the peak frequency. We discuss the similarities and
differences between jitter and synchrotron and discuss experiments that can
disentangle the two mechanisms.Comment: 12 pages, 7 postscript figures. Figures, discussion, and references
updated. Accepted for publication in MNRA