15 research outputs found
A space-time high-order implicit shock tracking method for shock-dominated unsteady flows
High-order implicit shock tracking (fitting) is a class of high-order,
optimization-based numerical methods to approximate solutions of conservation
laws with non-smooth features by aligning elements of the computational mesh
with non-smooth features. This ensures the non-smooth features are perfectly
represented by inter-element jumps and high-order basis functions approximate
smooth regions of the solution without nonlinear stabilization, which leads to
accurate approximations on traditionally coarse meshes. In this work, we extend
implicit shock tracking to time-dependent problems using a slab-based
space-time approach. This is achieved by reformulating a time-dependent
conservation law as a steady conservation law in one higher dimension and
applying existing implicit shock tracking techniques. To avoid computations
over the entire time domain and unstructured mesh generation in higher
dimensions, we introduce a general procedure to generate conforming,
simplex-only meshes of space-time slabs in such a way that preserves features
(e.g., curved elements, refinement regions) from previous time slabs. The use
of space-time slabs also simplifies the shock tracking problem by reducing
temporal complexity. Several practical adaptations of the implicit shock
tracking solvers are developed for the space-time setting including 1) a
self-adjusting temporal boundary, 2) nondimensionalization of a space-time
slab, 3) adaptive mesh refinement, and 4) shock boundary conditions, which lead
to accurate solutions on coarse space-time grids, even for problem with complex
flow features such as curved shocks, shock formation, shock-shock and
shock-boundary interaction, and triple points.Comment: 35 pages, 20 figure
Multiwavelength Radio Observations of Two Repeating Fast Radio Burst Sources: FRB 121102 and FRB 180916.J0158+65
The spectra of fast radio bursts (FRBs) encode valuable information about the source's local environment, underlying emission mechanism(s), and the intervening media along the line of sight. We present results from a long-term multiwavelength radio monitoring campaign of two repeating FRB sources, FRB 121102 and FRB 180916.J0158+65, with the NASA Deep Space Network (DSN) 70 m radio telescopes (DSS-63 and DSS-14). The observations of FRB 121102 were performed simultaneously at 2.3 and 8.4 GHz, and spanned a total of 27.3 hr between 2019 September 19 and 2020 February 11. We detected two radio bursts in the 2.3 GHz frequency band from FRB 121102, but no evidence of radio emission was found at 8.4 GHz during any of our observations. We observed FRB 180916.J0158+65 simultaneously at 2.3 and 8.4 GHz, and also separately in the 1.5 GHz frequency band, for a total of 101.8 hr between 2019 September 19 and 2020 May 14. Our observations of FRB 180916.J0158+65 spanned multiple activity cycles during which the source was known to be active and covered a wide range of activity phases. Several of our observations occurred during times when bursts were detected from the source between 400 and 800 MHz with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope. However, no radio bursts were detected from FRB 180916.J0158+65 at any of the frequencies used during our observations with the DSN radio telescopes. We find that FRB 180916.J0158+65's apparent activity is strongly frequency-dependent due to the narrowband nature of its radio bursts, which have less spectral occupancy at high radio frequencies (≳ 2 GHz). We also find that fewer or fainter bursts are emitted from the source at high radio frequencies. We discuss the implications of these results for possible progenitor models of repeating FRBs
A Dual-band Radio Observation of FRB 121102 with the Deep Space Network and the Detection of Multiple Bursts
The spectra of repeating fast radio bursts (FRBs) are complex and time-variable, sometimes peaking within the observing band and showing a fractional emission bandwidth of about 10%–30%. These spectral features may provide insight into the emission mechanism of repeating FRBs, or they could possibly be explained by extrinsic propagation effects in the local environment. Broadband observations can better quantify this behavior and help to distinguish between intrinsic and extrinsic effects. We present results from a simultaneous 2.25 and 8.36 GHz observation of the repeating FRB 121102 using the 70 m Deep Space Network radio telescope, DSS-43. During the 5.7 hr continuous observing session, we detected six bursts from FRB 121102, which were visible in the 2.25 GHz frequency band. However, none of these bursts were detected in the 8.36 GHz band, despite the larger bandwidth and greater sensitivity in the higher-frequency band. This effect is not explainable by Galactic scintillation and, along with previous multi-band experiments, clearly demonstrates that apparent burst activity depends strongly on the radio frequency band that is being observed
A Dual-band Radio Observation of FRB 121102 with the Deep Space Network and the Detection of Multiple Bursts
The spectra of repeating fast radio bursts (FRBs) are complex and
time-variable, sometimes peaking within the observing band and showing a
fractional emission bandwidth of about 10-30%. These spectral features may
provide insight into the emission mechanism of repeating fast radio bursts, or
they could possibly be explained by extrinsic propagation effects in the local
environment. Broadband observations can better quantify this behavior and help
to distinguish between intrinsic and extrinsic effects. We present results from
a simultaneous 2.25 and 8.36 GHz observation of the repeating FRB 121102 using
the 70 m Deep Space Network (DSN) radio telescope, DSS-43. During the 5.7 hr
continuous observing session, we detected 6 bursts from FRB 121102, which were
visible in the 2.25 GHz frequency band. However, none of these bursts were
detected in the 8.36 GHz band, despite the larger bandwidth and greater
sensitivity in the higher-frequency band. This effect is not explainable by
Galactic scintillation and, along with previous multi-band experiments, clearly
demonstrates that apparent burst activity depends strongly on the radio
frequency band that is being observed.Comment: 8 pages, 3 figures, 1 table. Accepted for publication in ApJL on 2020
June 8. v2: Updated to match published versio
Bright X-ray and Radio Pulses from a Recently Reactivated Magnetar
Magnetars are young, rotating neutron stars that possess larger magnetic fields (B ≈ 10¹³-10¹⁵G) and longer rotational periods (P ≈ 1-12 s) than ordinary pulsars. In contrast to rotation-powered pulsars, magnetar emission is thought to be fueled by the evolution and decay of their powerful magnetic fields. They display highly variable radio and X-ray emission, but the processes responsible for this behavior remain a mystery. We report the discovery of bright, persistent individual X-ray pulses from XTE J1810-197, a transient radio magnetar, using the Neutron star Interior Composition Explorer (NICER) following its recent radio reactivation. Similar behavior has only been previously observed from a magnetar during short time periods following a giant flare. However, the X-ray pulses presented here were detected outside of a flaring state. They are less energetic and display temporal structure that differs from the impulsive X-ray events previously observed from the magnetar class, such as giant flares and short X-ray bursts. Our high frequency radio observations of the magnetar, carried out simultaneously with the X-ray observations, demonstrate that the relative alignment between the X-ray and radio pulses varies on rotational timescales. No correlation was found between the amplitudes or temporal structure of the X-ray and radio pulses. The magnetar's 8.3 GHz radio pulses displayed frequency structure, which was not observed in the pulses detected simultaneously at 31.9 GHz. Many of the radio pulses were also not detected simultaneously at both frequencies, which indicates that the underlying emission mechanism producing these pulses is not broadband. We find that the radio pulses from XTE J1810-197 share similar characteristics to radio bursts detected from fast radio burst (FRB) sources, some of which are now thought to be produced by active magnetars
Bright X-ray and Radio Pulses from a Recently Reactivated Magnetar
Magnetars are young, rotating neutron stars that possess larger magnetic fields (B ≈ 10¹³-10¹⁵G) and longer rotational periods (P ≈ 1-12 s) than ordinary pulsars. In contrast to rotation-powered pulsars, magnetar emission is thought to be fueled by the evolution and decay of their powerful magnetic fields. They display highly variable radio and X-ray emission, but the processes responsible for this behavior remain a mystery. We report the discovery of bright, persistent individual X-ray pulses from XTE J1810-197, a transient radio magnetar, using the Neutron star Interior Composition Explorer (NICER) following its recent radio reactivation. Similar behavior has only been previously observed from a magnetar during short time periods following a giant flare. However, the X-ray pulses presented here were detected outside of a flaring state. They are less energetic and display temporal structure that differs from the impulsive X-ray events previously observed from the magnetar class, such as giant flares and short X-ray bursts. Our high frequency radio observations of the magnetar, carried out simultaneously with the X-ray observations, demonstrate that the relative alignment between the X-ray and radio pulses varies on rotational timescales. No correlation was found between the amplitudes or temporal structure of the X-ray and radio pulses. The magnetar's 8.3 GHz radio pulses displayed frequency structure, which was not observed in the pulses detected simultaneously at 31.9 GHz. Many of the radio pulses were also not detected simultaneously at both frequencies, which indicates that the underlying emission mechanism producing these pulses is not broadband. We find that the radio pulses from XTE J1810-197 share similar characteristics to radio bursts detected from fast radio burst (FRB) sources, some of which are now thought to be produced by active magnetars
Intermediate-mass dilepton spectra and the role of secondary hadronic processes in heavy-ion collisions
We carry out a study of intermediate-mass (between 1 and 2.5 GeV) dilepton
spectra from hadronic interactions in heavy-ion collisions. The processes
considered are , , , , , and . The elementary cross sections for those are obtained
from chiral Lagrangians involving pseudoscalar, vector, and axial-vector
mesons. The respective electromagnetic form factors are determined by fitting
to experimental data for the reverse processes of . Based on
this input we calculate cross sections and thermal dilepton emission rates and
compare our results with those from other approaches. Finally we use these
elementary cross sections with a relativistic transport model and calculate
dilepton spectra in S+W collisions at SPS energies. The comparison of our
results with experimental data from the HELIOS-3 collaboration indicates the
importance of the secondary hadronic contributions to the intermediate-mass
dilepton spectra.Comment: 25 pages, including 20 postscript figure