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Limits on thermal variations in a dozen quiescent neutron stars over a decade | | Arash Bahramian
; Craig O. Heinke
; Nathalie Degenaar
; Laura Chomiuk
; Rudy Wijnands
; Jay Strader
; Wynn C. G. Ho
; David Pooley
; | | Date: |
14 Jul 2015 | | Abstract: | In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the
origin of the thermal X-ray component may be either release of heat from the
core of the neutron star, or continuing low-level accretion. In general, heat
from the core should be stable on timescales $<10^4$ years, while continuing
accretion may produce variations on a range of timescales. While some quiescent
neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal
components on a range of timescales, several others, particularly those in
globular clusters with no detectable nonthermal hard X-rays (fit with a
powerlaw), have shown no measurable variations. Here, we constrain the spectral
variations of 12 low mass X-ray binaries in 3 globular clusters over $sim10$
years. We find no evidence of variations in 10 cases, with limits on
temperature variations below 11% for the 7 qLMXBs without powerlaw components,
and limits on variations below 20% for 3 other qLMXBs that do show non-thermal
emission. However, in 2 qLMXBs showing powerlaw components in their spectra
(NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline in
temperature, suggesting the presence of continuing low-level accretion. This
work adds to the evidence that the thermal X-ray component in quiescent neutron
stars without powerlaw components can be explained by heat deposited in the
core during outbursts. Finally, we also investigate the correlation between
hydrogen column density (N$_H$) and optical extinction (A$_V$) using our sample
and current models of interstellar X-ray absorption, finding $N_H ({
m
cm}^{-2}) = (2.81pm0.13) imes10^{21} A_V$. | | Source: | arXiv, 1507.3994 | | Services: | Forum | Review | PDF | Favorites | |
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