Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Quiescent X-ray emission from Cen X-4: a variable thermal component   The nearby neutron star low-mass X-ray binary, Cen X-4, has been in a
quiescent state since its last outburst in 1979. Typically, quiescent emission
from these objects consists of thermal emission (presumably from the neutron
star surface) with an additional hard power-law tail of unknown nature.
Variability has been observed during quiescence in Cen X-4 on both timescales
as short as hundreds of seconds and as long as years. However, the nature of
this variability is still unknown. Early observations seemed to show it was all
due to a variable hard X-ray tail. Here, we present new and archival
observations that contradict this. The most recent Suzaku observation of Cen
X-4 finds it in a historically low state, a factor of 4.4 fainter than the
brightest quiescent observation. As the spectrum during the brightest
observation was comprised of approximately 60% from the thermal component and
40% from the power-law component, such a large change cannot be explained by
just power-law variability. Spectral fits with a variable thermal component fit
the data well, while spectral fits allowing both the column density and the
power-law to vary do not, leading to the conclusion that the thermal component
must be variable. Interestingly, we also find that the thermal fraction remains
consistent between all epochs, implying that the thermal and power-law fluxes
vary by approximately the same amount. If the emitting area remains unchanged
between observations, then the effective surface temperature must change.
Alternatively, if the temperature remains constant, then the emitting area must
change. The nature of this thermal variability is unclear, but may be explained
by variable low-level accretion.

34
The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio
36
The categories of this paper are: cmp-lg
15
The categories of this paper are: nucl-ex
3
The categories of this paper are: physics
19
The categories of this paper are: eess
17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
0
The categories of this paper are: hep-ph
11
The categories of this paper are: hep-lat
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
8
The categories of this paper are: hep-ex