34 research outputs found
Polycrystalline Crusts in Accreting Neutron Stars
The crust of accreting neutron stars plays a central role in many different
observational phenomena. In these stars, heavy elements produced by H-He
burning in the rapid proton capture (rp-) process continually freeze to form
new crust. In this paper, we explore the expected composition of the solid
phase. We first demonstrate using molecular dynamics that two distinct types of
chemical separation occur, depending on the composition of the rp-process
ashes. We then calculate phase diagrams for three-component mixtures and use
them to determine the allowed crust compositions. We show that, for the large
range of atomic numbers produced in the rp-process (--), the
solid that forms has only a small number of available compositions. We conclude
that accreting neutron star crusts should be polycrystalline, with domains of
distinct composition. Our results motivate further work on the size of the
compositional domains, and have implications for crust physics and accreting
neutron star phenomenology.Comment: 8 pages, 4 figures, Submitted to ApJ, this article supersedes
arXiv:1709.0926
Faraday rotation measures of northern-hemisphere pulsars using CHIME/Pulsar
Using commissioning data from the first year of operation of the Canadian
Hydrogen Intensity Mapping Experiment's (CHIME) Pulsar backend system, we
conduct a systematic analysis of the Faraday Rotation Measure (RM) of the
northern hemisphere pulsars detected by CHIME. We present 55 new RMs as well as
obtain improved RM uncertainties for 25 further pulsars. CHIME's low observing
frequency and wide bandwidth between 400-800 MHz contribute to the precision of
our measurements, whereas the high cadence observation provide extremely high
signal-to-noise co-added data. Our results represent a significant increase of
the pulsar RM census, particularly regarding the northern hemisphere. These new
RMs are for sources that are located in the Galactic plane out to 10 kpc, as
well as off the plane to a scale height of ~16 kpc. This improved knowledge of
the Faraday sky will contribute to future Galactic large-scale magnetic
structure and ionosphere modelling.Comment: 13 pages, 7 figures, accepted by MNRA
Revealing the Dynamic Magneto-ionic Environments of Repeating Fast Radio Burst Sources through Multi-year Polarimetric Monitoring with CHIME/FRB
Fast radio bursts (FRBs) display a confounding variety of burst properties
and host galaxy associations. Repeating FRBs offer insight into the FRB
population by enabling spectral, temporal and polarimetric properties to be
tracked over time. Here, we report on the polarized observations of 12
repeating sources using multi-year monitoring with the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) over 400-800 MHz. We observe significant
RM variations from many sources in our sample, including RM changes of several
hundred over month timescales from FRBs 20181119A,
20190303A and 20190417A, and more modest RM variability ( few tens rad m) from FRBs 20181030A, 20190208A, 20190213B and
20190117A over equivalent timescales. Several repeaters display a frequency
dependent degree of linear polarization that is consistent with depolarization
via scattering. Combining our measurements of RM variations with equivalent
constraints on DM variability, we estimate the average line-of-sight magnetic
field strength in the local environment of each repeater. In general, repeating
FRBs display RM variations that are more prevalent/extreme than those seen from
radio pulsars in the Milky Way and the Magellanic Clouds, suggesting repeating
FRBs and pulsars occupy distinct magneto-ionic environments
LOFAR Detection of 110-188 MHz Emission and Frequency-Dependent Activity from FRB 20180916B
FRB 20180916B is a well-studied repeating fast radio burst source. Its
proximity (~150 Mpc), along with detailed studies of the bursts, have revealed
many clues about its nature -- including a 16.3-day periodicity in its
activity. Here we report on the detection of 18 bursts using LOFAR at 110-188
MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are
seen down to the lowest-observed frequency of 110 MHz, suggesting that their
spectra extend even lower. These observations provide an order-of-magnitude
stronger constraint on the optical depth due to free-free absorption in the
source's local environment. The absence of circular polarization and nearly
flat polarization angle curves are consistent with burst properties seen at
300-1700 MHz. Compared with higher frequencies, the larger burst widths
(~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due
to scattering. We find ~2-3 rad/m^2 variations in the Faraday rotation measure
that may be correlated with the activity cycle of the source. We compare the
LOFAR burst arrival times to those of 38 previously published and 22 newly
detected bursts from the uGMRT (200-450 MHz) and CHIME/FRB (400-800 MHz).
Simultaneous observations show 5 CHIME/FRB bursts when no emission is detected
by LOFAR. We find that the burst activity is systematically delayed towards
lower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results
in the context of a model in which FRB 20180916B is an interacting binary
system featuring a neutron star and high-mass stellar companion.Comment: Accepted for publication by ApJ