skip to main content

Search for: All records

Creators/Authors contains: "Schatz, H."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract

    The observation of X-rays during quiescence from transiently accreting neutron stars provides unique clues about the nature of dense matter. This, however, requires extensive modeling of the crusts and matching the results to observations. The pycnonuclear fusion reaction rates implemented in these models are theoretically calculated by extending phenomenological expressions and have large uncertainties spanning many orders of magnitude. We present the first sensitivity studies of these pycnonuclear fusion reactions in realistic network calculations. We also couple the reaction network with the thermal evolution codedStarto further study their impact on the neutron star cooling curves in quiescence. Varying the pycnonuclear fusion reaction rates alters the depth at which nuclear heat is deposited although the total heating remains constant. The enhancement of the pycnonuclear fusion reaction rates leads to an overall shallower deposition of nuclear heat. The impurity factors are also altered depending on the type of ashes deposited on the crust. These total changes correspond to a variation of up to 9 eV in the modeled cooling curves. While this is not sufficient to explain the shallow heat source, it is comparable to the observational uncertainties and can still be important for modeling the neutron star crust.

    more » « less
  2. ABSTRACT Rare, energetic (long) thermonuclear (Type I) X-ray bursts are classified either as intermediate-duration or ‘supern’ bursts, based on their duration. Intermediate-duration bursts lasting a few to tens of minutes are thought to arise from the thermonuclear runaway of a relatively thick (≈1010 g cm−2) helium layer, while superbursts lasting hours are attributed to the detonation of an underlying carbon layer. We present a catalogue of 84 long thermonuclear bursts from 40 low-mass X-ray binaries, and defined from a new set of criteria distinguishing them from the more frequent short bursts. The three criteria are: (1) a total energy release longer than 1040 erg, (2) a photospheric radius expansion phase longer than 10 s, and (3) a burst time-scale longer than 70 s. This work is based on a comprehensive systematic analysis of 70 bursts found with INTEGRAL, RXTE, Swift, BeppoSAX, MAXI, and NICER, as well as 14 long bursts from the literature that were detected with earlier generations of X-ray instruments. For each burst, we measure its peak flux and fluence, which eventually allows us to confirm the distinction between intermediate-duration bursts and superbursts. Additionally, we list 18 bursts that only partially meet the above inclusion criteria, possibly bridging the gap between normal and intermediate-duration bursts. With this catalogue, we significantly increase the number of long-duration bursts included in the MINBAR and thereby provide a substantial sample of these rare X-ray bursts for further study. 
    more » « less
    Free, publicly-accessible full text available March 23, 2024
  3. Abstract A promising astrophysical site to produce the lighter heavy elements of the first r -process peak ( Z = 38 − 47) is the moderately neutron-rich (0.4 < Y e < 0.5) neutrino-driven ejecta of explosive environments, such as core-collapse supernovae and neutron star mergers, where the weak r -process operates. This nucleosynthesis exhibits uncertainties from the absence of experimental data from ( α , xn ) reactions on neutron-rich nuclei, which are currently based on statistical model estimates. In this work, we report on a new study of the nuclear reaction impact using a Monte Carlo approach and improved ( α , xn ) rates based on the Atomki-V2 α optical model potential. We compare our results with observations from an up-to-date list of metal-poor stars with [Fe/H] < −1.5 to find conditions of the neutrino-driven wind where the lighter heavy elements can be synthesized. We identified a list of ( α , xn ) reaction rates that affect key elemental ratios in different astrophysical conditions. Our study aims to motivate more nuclear physics experiments on ( α , xn ) reactions using the current and new generation of radioactive beam facilities and also more observational studies of metal-poor stars. 
    more » « less
  4. Free, publicly-accessible full text available May 1, 2024
  5. Abstract

    Nuclear reactions heat and cool the crust of accreting neutron stars and need to be understood to interpret observations of X-ray bursts and long-term cooling in transiently accreting systems. It was recently suggested that previously ignored neutron transfer reactions may play a significant role in the nuclear processes. We present results from full nuclear network calculations that now include these reactions and determine their impact on crust composition, crust impurity, heating, and cooling. We find that a large number of neutron transfer reactions indeed occur and impact crust models. In particular, we identify a new type of reaction cycle that brings a pair of nuclei across the nuclear chart into equilibrium via alternating neutron capture and neutron release, interspersed with a neutron transfer. While neutron transfer reactions lead to changes in crust model predictions and need to be considered in future studies, previous conclusions concerning heating, cooling, and compositional evolution are remarkably robust.

    more » « less