6,408 research outputs found
A Chandra observation of GRO J1744-28: the bursting pulsar in quiescence
We present a Chandra/ACIS-I observation of GRO J1744-28. We detected a source
at a position of R.A = 17h 44m 33.09s and Dec. = -28degr 44' 27.0'' (J2000.0;
with a 1sigma error of ~0.8 arcseconds), consistent with both ROSAT and
interplanetary network localizations of GRO J1744-28 when it was in outburst.
This makes it likely that we have detected the quiescent X-ray counterpart of
GRO J1744-28. Our Chandra position demonstrates that the previously proposed
infrared counterpart is not related to GRO J1744-28. The 0.5-10 keV luminosity
of the source is 2 - 4 x 10^{33} erg/s (assuming the source is near the
Galactic center at a distance of 8 kpc). We discuss our results in the context
of the quiescent X-ray emission of pulsating and non-pulsating neutron star
X-ray transients.Comment: Accepted for publication in ApJ Letters, 20 February 200
On the Chandra X-ray Sources in the Galactic Center
Recent deep Chandra surveys of the Galactic center region have revealed the
existence of a faint, hard X-ray source population. While the nature of this
population is unknown, it is likely that several types of stellar objects
contribute. For sources involving binary systems, accreting white dwarfs and
accreting neutron stars with main sequence companions have been proposed. Among
the accreting neutron star systems, previous studies have focused on stellar
wind-fed sources. In this paper, we point out that binary systems in which mass
transfer occurs via Roche lobe overflow (RLOF) can also contribute to this
X-ray source population.
A binary population synthesis study of the Galactic center region has been
carried out, and it is found that evolutionary channels for neutron star
formation involving the accretion induced collapse of a massive ONeMg white
dwarf, in addition to the core collapse of massive stars, can contribute to
this population. The RLOF systems would appear as transients with quiescent
luminosities, above 2 keV, in the range from 10^31-10^32 ergs/s. The results
reveal that RLOF systems primarily contribute to the faint X-ray source
population in the Muno et al. (2003) survey and wind-fed systems can contribute
to the less sensitive Wang et al. (2002) survey. However, our results suggest
that accreting neutron star systems are not likely to be the major contributor
to the faint X-ray source population in the Galactic center.Comment: 12 pages, 3 figures, 1 table ApJ in press (Dec 2004). Substantial
change
A XMM-Newton observation during the 2000 outburst of SAX J1808.4-3658
I present a XMM-Newton observation of the accretion driven millisecond X-ray
pulsar SAX J1808.4-3658 during its 2000 outburst. The source was conclusively
detected, albeit at a level of only ~2 x 10^{32} erg/s. The source spectrum
could be fitted with a power-law model (with a photon index of ~2.2), a neutron
star atmosphere model (with a temperature of ~0.2 keV), or with a combination
of a thermal (either a black-body or an atmosphere model) and a power-law
component. During a XMM-Newton observation taken approximately one year later,
the source was in quiescence and its luminosity was a factor of ~4 lower. It is
possible that the source spectrum during the 2000 outburst was softer than its
quiescent 2001 spectrum, however, the statistics of the data do not allow to
make a firm conclusion. The results obtained are discussed in the context of
the 2000 outburst of SAX J1808.4-3658 and the quiescent properties of the
source.Comment: Accepted for publication in ApJ, 15 January 200
Quasi-Superradiant Soliton State of Matter in Quantum Metamaterials
Strong interaction of a system of quantum emitters (e.g., two-level atoms)
with electromagnetic field induces specific correlations in the system
accompanied by a drastic insrease of emitted radiation (superradiation or
superfluorescence). Despite the fact that since its prediction this phenomenon
was subject to a vigorous experimental and theoretical research, there remain
open question, in particular, concerning the possibility of a first order phase
transition to the superradiant state from the vacuum state. In systems of
natural and charge-based artificial atome this transition is prohibited by
"no-go" theorems. Here we demonstrate numerically a similar transition in a
one-dimensional quantum metamaterial - a chain of artificial atoms (qubits)
strongly interacting with classical electromagnetic fields in a transmission
line. The system switches from vacuum state with zero classical electromagnetic
fields and all qubits being in the ground state to the quasi-superradiant (QS)
phase with one or several magnetic solitons and finite average occupation of
qubit excited states along the transmission line. A quantum metamaterial in the
QS phase circumvents the "no-go" restrictions by considerably decreasing its
total energy relative to the vacuum state by exciting nonlinear electromagnetic
solitons with many nonlinearly coupled electromagnetic modes in the presence of
external magnetic field.Comment: 6 pages, 4 figure
XMM-Newton observations of the neutron star X-ray transient KS 1731-260 in quiescence
We report on XMM-Newton observations performed on 2001 September 13-14 of the
neutron star X-ray transient KS 1731-260 in quiescence. The source was detected
at an unabsorbed 0.5-10 keV flux of only 4 - 8 x 10^{-14} erg/s, depending on
the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10
keV X-ray luminosity of approximately 2 - 5 x 10^{32} erg/s. The September 2001
quiescent flux of KS 1731-260 is lower than that observed during the Chandra
observation in March 2001. In the cooling neutron star model for the quiescent
X-ray emission of neutron star X-ray transients, this decrease in the quiescent
flux implies that the crust of the neutron star in KS 1731-260 cooled down
rapidly between the two epochs, indicating that the crust has a high
conductivity. Furthermore, enhanced cooling in the neutron star core is also
favored by our results.Comment: Accepter for publication in ApJ Letters, 22 May 200
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
A model for the interaction of high-energy particles in straight and bent crystals implemented in Geant4
A model for the simulation of orientational effects in straight and bent
periodic atomic structures is presented. The continuum potential approximation
has been adopted.The model allows the manipulation of particle trajectories by
means of straight and bent crystals and the scaling of the cross sections of
hadronic and electromagnetic processes for channeled particles. Based on such a
model, an extension of the Geant4 toolkit has been developed. The code has been
validated against data from channeling experiments carried out at CERN
Chandra observations of the bursting X-ray transient SAX J1747.0-2853 during low-level accretion activity
We present Chandra/ACIS observations of the bursting X-ray transient SAX
J1747.0-2853 performed on 18 July 2001. We detected a bright source at the
position of R.A = 17^h 47^m 02.60^s and Dec. = -28 52' 58.9'' (J2000.0; with a
1 sigma error of ~0.7 arcseconds), consistent with the BeppoSAX and ASCA
positions of SAX J1747.0-2853 and with the Ariel V position of the transient GX
+0.2,-0.2, which was active during the 1970's. The 0.5-10 keV luminosity of the
source during our observations was ~3 x 10^{35} erg/s (assuming a distance of 9
kpc) demonstrating that the source was in a low-level accretion state. We also
report on the long-term light curve of the source as observed with the all sky
monitor aboard the Rossi X-ray Timing Explorer. After the initial 1998
outburst, two more outbursts (in 2000 and 2001) were detected with peak
luminosities about two orders of magnitude larger than our Chandra luminosity.
Our Chandra observation falls in-between those two outbursts, making the
outburst history for SAX J1747.0-2853 complex. Those bright 2000 and 2001
outbursts combined with the likely extended period of low level activity
in-between those outbursts strongly suggest that the classification of SAX
J1747.0-2853 as a faint X-ray transient was premature. It might be possible
that the other faint X-ray transients also can exhibit bright, extended
outbursts which would eliminate the need for a separate sub-class of X-ray
transients. We discuss our results also in the context of the behavior of X-ray
binaries accreting at low levels with luminosities around 10^{35} erg/s, a
poorly studied accretion rate regime.Comment: Accepte for publication in ApJ, 11 July 200
Coupled-Cluster Approach to Electron Correlations in the Two-Dimensional Hubbard Model
We have studied electron correlations in the doped two-dimensional (2D)
Hubbard model by using the coupled-cluster method (CCM) to investigate whether
or not the method can be applied to correct the independent particle
approximations actually used in ab-initio band calculations. The double
excitation version of the CCM, implemented using the approximate coupled pair
(ACP) method, account for most of the correlation energies of the 2D Hubbard
model in the weak () and the intermediate regions (). The error is always less than 1% there. The ACP approximation gets
less accurate for large () and/or near half-filling.
Further incorporation of electron correlation effects is necessary in this
region. The accuracy does not depend on the system size and the gap between the
lowest unoccupied level and the highest occupied level due to the finite size
effect. Hence, the CCM may be favorably applied to ab-initio band calculations
on metals as well as semiconductors and insulators.Comment: RevTeX3.0, 4 pages, 4 figure
Free field approach to diagonalization of boundary transfer matrix : recent advances
We diagonalize infinitely many commuting operators . We call these
operators the boundary transfer matrix associated with the quantum
group and the elliptic quantum group. The boundary transfer matrix is related
to the solvable model with a boundary. When we diagonalize the boundary
transfer matrix, we can calculate the correlation functions for the solvable
model with a boundary. We review the free field approach to diagonalization of
the boundary transfer matrix associated with and
. We construct the free field realizations of the
eigenvectors of the boundary transfer matrix . This paper includes new
unpublished formula of the eigenvector for . It is thought that
this diagonalization method can be extended to more general quantum group
and elliptic quantum group .Comment: To appear in Group 28 : Group Theoretical Method in Physic
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