36 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
A CHIME/FRB Study of Burst Rate and Morphological Evolution of the Periodically Repeating FRB 20180916B
FRB 20180916B is a repeating fast radio burst (FRB) with a 16.3 day periodicity in its activity. In this study, we present morphological properties of 60 FRB 20180916B bursts detected by CHIME/FRB between 2018 August and 2021 December. We recorded raw voltage data for 45 of these bursts, enabling microseconds time resolution in some cases. We studied variation of spectro-temporal properties with time and activity phase. We find that the variation in dispersion measure (DM) is ≲1 pc cm−3 and that there is burst-to-burst variation in scattering time estimates ranging from ∼0.16 to over 2 ms, with no discernible trend with activity phase for either property. Furthermore, we find no DM and scattering variability corresponding to the recent change in rotation measure from the source, which has implications for the immediate environment of the source. We find that FRB 20180916B has thus far shown no epochs of heightened activity as have been seen in other active repeaters by CHIME/FRB, with its burst count consistent with originating from a Poissonian process. We also observe no change in the value of the activity period over the duration of our observations and set a 1σ upper limit of 1.5 × 10−4 day day−1 on the absolute period derivative. Finally, we discuss constraints on progenitor models yielded by our results, noting that our upper limits on changes in scattering and DM as a function of phase do not support models invoking a massive binary companion star as the origin of the 16.3 day periodicity.</p