Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: High-resolution X-ray spectroscopy of the ultracompact LMXB pulsar 4U
  1626-67   [abridged] We report results from four recent observations of the
ultracompact LMXB pulsar 4U 1626-67. All the observations obtained
high-resolution X-ray spectra of the system, two from the Chandra X-ray
Observatory using the HETGS, and two from the XMM-Newton Observatory using the
RGS as well as the EPIC PN and MOS. These data allow us to study in detail the
prominent Ne and O emission line complexes which make 4U 1626-67 unique among
LMXBs. The observations were spaced over a period of 3 years for a total
observing time of 238 ks, allowing us to monitor the line regions as well as
the overall source flux, continuum spectrum, and timing properties. The
structure of the emission lines and the ratios of the components of the
helium-like Ne IX and O VII triplets support the hypothesis that they are
formed in the high-density environment of the accretion disk. We do not find
any significant changes in the line widths or ratios over this time period,
though we note that the line equivalent widths decrease. We are able to place
constraints on the strengths of the Ne K, Fe L, and O K photoelectric
absorption edges, and find that the data do not require an overabundance of Ne
or O in the system relative to the expected ISM values. We find that the pulsar
is still spinning down, and note that the pulse profile has changed
significantly from what was found prior to the torque reversal in 1990,
suggesting that this event may be linked to a change in the geometry of the
accretion column. The flux of 4U 1626-67 continues to decrease, in keeping with
the trend of the last approximately 30 years over which it has been observed.
Taking into consideration current theory on disk stability, we expect that 4U
1626-67 will enter a period of quiescence in 2-15 years.

36
The categories of this paper are: cmp-lg
33
The categories of this paper are: plasm-ph
35
The categories of this paper are: mtrl-th
2
The categories of this paper are: math
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
8
The categories of this paper are: hep-ex
37
The categories of this paper are: supr-con
16
The categories of this paper are: stat
15
The categories of this paper are: nucl-ex
28
The categories of this paper are: alg-geom
32
The categories of this paper are: atom-ph
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph