90 research outputs found
Quiescent Thermal Emission from the Neutron Star in Aql X-1
We report on the quiescent spectrum measured with Chandra/ACIS-S of the
transient, type-I X-ray bursting neutron star Aql X-1, immediately following an
accretion outburst. The neutron star radius, assuming a pure hydrogen
atmosphere and hard power-law spectrum, is =13.4{+5}{-4} (d/5 \kpc)
km. Based on the historical outburst record of RXTE/ASM, the quiescent
luminosity is consistent with that predicted by Brown, Bildsten and Rutledge
from deep crustal heating, lending support to this theory for providing a
minimum quiescent luminosity of transient neutron stars. While not required by
the data, the hard power-law component can account for 18+/-8% of the 0.5-10
keV thermal flux. Short-timescale intensity variability during this observation
is less than 15% rms (3 sigma; 0.0001-1 Hz, 0.2-8 keV). Comparison between the
Chandra spectrum and three X-ray spectral observations made between Oct 1992
and Oct 1996 find all spectra consistent with a pure H atmosphere, but with
temperatures ranging from 145--168 eV, spanning a factor of 1.87+/-0.21 in
observed flux. The source of variability in the quiescent luminosity on long
timescales (greater than years) remains a puzzle. If from accretion, then it
remains to be explained why the quiescent accretion rate provides a luminosity
so nearly equal to that from deep crustal heating.Comment: 15 pages, 1 figure, 2 tables; ApJ, accepte
Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars
Nuclear reactions occurring deep in the crust of a transiently accreting
neutron star efficiently maintain the core at a temperature >5e7 K. When
accretion halts, the envelope relaxes to a thermal equilibrium set by the flux
from the hot core, as if the neutron star were newly born. For the
time-averaged accretion rates typical of low-mass X-ray transients, standard
neutrino cooling is unimportant and the core thermally re-radiates the
deposited heat. The resulting luminosity has the same magnitude as that
observed from several transient neutron stars in quiescence. Confirmation of
this mechanism would strongly constrain rapid neutrino cooling mechanisms for
neutron stars. Thermal emission had previously been dismissed as a predominant
source of quiescent emission since blackbody spectral fits implied an emitting
area much smaller than a neutron star's surface. However, as with thermal
emission from radio pulsars, fits with realistic emergent spectra will imply a
substantially larger emitting area. Other emission mechanisms, such as
accretion or a pulsar shock, can also operate in quiescence and generate
intensity and spectral variations over short timescales. Indeed, quiescent
accretion may produce gravitationally redshifted metal photoionization edges in
the quiescent spectra (detectable with AXAF and XMM). We discuss past
observations of Aql~X-1 and note that the low luminosity X-ray sources in
globular clusters and the Be star/X-ray transients are excellent candidates for
future study.Comment: 5 pages, 2 ps figures, uses AASTEX macros. To appear in ApJ letters,
10 September 1998. Revised to conform with journal; minor numerical
correction
Crustal Emission and the Quiescent Spectrum of the Neutron Star in KS 1731-260
(Abridged). The type-I X-ray bursting low mass X-ray binary KS 1731-260 was
recently detected for the first time in quiescence by Wijnands et al.,
following an approximately 13 yr outburst which ended in Feb 2001. Unlike all
other known transient neutron stars, the duration of this recent outburst is as
long as the thermal diffusion time of the crust. The large amount of heat
deposited by reactions in the crust will have heated the crust to temperatures
much higher than the equilibrium core temperature. As a result, the thermal
luminosity currently observed from the neutron star is dominated not by the
core, but by the crust. Moreover, the level and the time evolution of quiescent
luminosity is determined mostly by the amount of heat deposited in the crust
during the most recent outburst. Using estimates of the outburst mass accretion
rate, our calculations of the quiescent flux immediately following the end of
the outburst agree with the observed quiescent flux to within a factor of a
few. We present simulations of the evolution of the quiescent lightcurve for
different scenarios of the crust microphysics, and demonstrate that monitoring
observations (with currently flying instruments) spanning from 1--30 yr can
measure the crust cooling timescale and the total amount of heat stored in the
crust. These quantities have not been directly measured for any neutron star.Comment: Submitted to ApJ; 7 text pages, 3 figures, uses emulateapj.sty and
apjfonts.st
Reaction rates and transport in neutron stars
Understanding signals from neutron stars requires knowledge about the
transport inside the star. We review the transport properties and the
underlying reaction rates of dense hadronic and quark matter in the crust and
the core of neutron stars and point out open problems and future directions.Comment: 74 pages; commissioned for the book "Physics and Astrophysics of
Neutron Stars", NewCompStar COST Action MP1304; version 3: minor changes,
references updated, overview graphic added in the introduction, improvements
in Sec IV.A.
Variable Thermal Emission from Aql X-1 in Quiescence
We obtained four Chandra/ACIS-S observations beginning two weeks after the
end of the November 2000 outburst of the neutron star (NS) transient Aql X-1.
Over the five month span in quiescence, the X-ray spectra are consistent with
thermal emission from a NS with a pure hydrogen photosphere and
R_{\infty}=15.9+{0.8}-{2.9} (d/5 kpc) km at the optically implied X-ray column
density. We also detect a hard power-law tail during two of the four
observations. The intensity of Aql X-1 first decreased by 50+/-4% over three
months, then increased by 35+/-5% in one month, and then remained constant (<6%
change) over the last month. These variations in the first two observations
cannot be explained by a change in the power-law spectral component, nor in the
X-ray column density. Presuming that R_{\infty} is not variable and a pure
hydrogen atmosphere, the long-term changes can only be explained by variations
in the NS effective temperature, from kT_{eff, \infty}=130+3-5 eV, down to
113+3-4 eV, finally increasing to 118+9-4 eV for the final two observations.
During one of these observations, we observe two phenomena which were
previously suggested as indicators of quiescent accretion onto the NS:
short-timescale (<1e4 sec) variability (at 32+8-6% rms), and a possible
absorption feature near 0.5 keV. The possible absorption feature can
potentially be explained as due to a time-variable response in the ACIS
detector. Even so, such a feature has not been detected previously from a NS,
and if confirmed and identified, can be exploited for simultaneous measurements
of the photospheric redshift and NS radius.Comment: Accepted, ApJ. 9 Figure
X-ray Spectral Identification of Three Candidate Quiescent Low-Mass X-ray Binaries in the Globular Cluster NGC 6304
We report the search for low-mass X-ray binaries in quiescence (qLMXBs) in
the globular cluster NGC 6304 using XMM observations. We present the spectral
analysis leading to the identification of three candidate qLMXBs within the
field of this globular cluster (GC), each consistent with the X-ray spectral
properties of previously identified qLMXBs in the field and in other globular
clusters -- specifically, with a hydrogen atmosphere neutron star with radius
between 5--20\km. One (source 4, with R=11.7^{+8.3}_{-0.4} (D/5.97 kpc) km and
kT_eff=117^{+59}_{-44} eV) is located within one core radius (r_c) of the
centre of NGC 6304. This candidate also presents a spectral power-law component
contributing 49 per cent of the 0.5-10 keV flux. A second one (source 9 with
R=15.3^{+11.2}_{-6.5} (D/5.97 kpc) km and kT_eff=100^{+24}_{-19} eV) is found
well outside the optical core (at 32 r_c) but still within the tidal radius.
From spatial coincidence, we identify a bright 2MASS infrared counterpart
which, at the distance of NGC 6304, seems to be a post-asymptotic giant branch
star. The third qLMXB (source 5 with R=23^{+38}_{-14} (D/5.97 kpc) km and
kT_eff=70^{+28}_{-20} eV) is a low signal-to-noise candidate for which we also
identify from spatial coincidence a bright 2MASS infrared counterpart, with
99.916 per cent confidence. Three qLMXBs from this GC is marginally consistent
with that expected from the encounter rate of NGC 6304. We also report a low
signal-to-noise source with an unusually hard photon index
(\alpha=-2.0^{+1.2}_{-2.2}). Finally, we present an updated catalogue of the
X-ray sources lying in the field of NGC 6304, and compare this with the
previous catalogue compiled from ROSAT observations.Comment: 28 pages, 9 figures, 6 tables. Accepted to MNRAS. In Pres
Small, Dense Quark Stars from Perturbative QCD
As a model for nonideal behavior in the equation of state of QCD at high
density, we consider cold quark matter in perturbation theory. To second order
in the strong coupling constant, , the results depend sensitively on
the choice of the renormalization mass scale. Certain choices of this scale
correspond to a strongly first order chiral transition, and generate quark
stars with maximum masses and radii approximately half that of ordinary neutron
stars. At the center of these stars, quarks are essentially massless.Comment: ReVTeX, 5 pages, 3 figure
The Quiescent X-Ray Spectrum of the Neutron Star in Cen X-4 Observed with Chandra/ACIS-S
We report on spectral and intensity variability analysis from a
Chandra/ACIS-S observation of the transient, type-I X-ray bursting low-mass
X-ray binary Cen X-4. The quiescent X-ray spectrum during this observation is
statistically identical to one observed previously with Beppo/SAX, and close,
but not identical, to one observed previously with ASCA. The X-ray spectrum is
best described as a pure Hydrogen atmosphere thermal spectrum plus a power-law
component that dominates the spectrum above 2 keV. The best-fit radius of the
neutron star is r=12.9+/-2.6 (d/1.2 kpc) km if the interstellar absorption is
fixed at the value implied by the optical reddening. Allowing the interstellar
absorption to be a free parameter yields r=19+45-10 (d/1.2 kpc) km (90%
confidence). The thermal spectrum from the neutron star surface is inconsistent
with a solar metallicity. We find a 3sigma upper-limit of root-mean-square
variability <18% (0.2-2.0 keV; 0.0001-1 Hz) during the observation. On the
other hand, the 0.5-10.0 keV luminosity decreased by 40+/-8% in the 4.9 years
between the Asca and Chandra observations. This variability can be attributed
to the power-law component. Moreover, we limit the variation in thermal
temperature to <10% over these 4.9 years. The stability of the thermal
temperature and emission area radius supports the interpretation that the
quiescent thermal emission is due to the hot neutron star core.Comment: 25 pages, 1 figure; ApJ, Accepted for April 10 2001 Issu
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