1,365 research outputs found
Path integral Monte Carlo simulations for rigid rotors and their application to water
In this work the path integral formulation for rigid rotors, proposed by
M\"user and Berne [Phys. Rev. Lett. {\bf 77}, 2638 (1996)], is described in
detail. It is shown how this formulation can be used to perform Monte Carlo
simulations of water. Our numerical results show that whereas some properties
of water can be accurately reproduced using classical simulations with an
empirical potential which, implicitly, includes quantum effects, other
properties can only be described quantitatively when quantum effects are
explicitly incorporated. In particular, quantum effects are extremely relevant
when it comes to describing the equation of state of the ice phases at low
temperatures, the structure of the ices at low temperatures, and the heat
capacity of both liquid water and the ice phases. They also play a minor role
in the relative stability of the ice phases.Comment: to appear in Molecular Physics (2011
Broad-band X-ray measurements of the black hole candidate XTE J1908+094
XTE J1908+094 is an X-ray transient that went into outburst in February 2002.
After two months it reached a 2-250 keV peak flux of 1 to 2 X 10-8 erg/s/cm2.
Circumstantial evidence points to an accreting galactic black hole as the
origin of the the X-radiation: pulsations nor thermonuclear flashes were
detected that would identify a neutron star and the spectrum was unusually hard
for a neutron star at the outburst onset. We report on BeppoSAX and RXTE All
Sky Monitor observations of the broad-band spectrum of XTE J1908+094. The
spectrum is consistent with a model consisting of a Comptonization component by
a ~40 keV plasma (between 2 and 60 keV this component can be approximated by a
power law with a photon index of 1.9 to 2.1), a multicolor accretion disk
blackbody component with a temperature just below 1 keV and a broad emission
line at about 6 keV. The spectrum is heavily absorbed by cold interstellar
matter with an equivalent hydrogen column density of 2.5 X 10+22 cm-2, which
makes it difficult to study the black body component in detail. The black body
component exhibits strong evolution about 6 weeks into the outburst. Two weeks
later this is followed by a swift decay of the power law component. The
broadness of the 6 keV feature may be due to relativistic broadening or Compton
scattering of a narrow Fe-K line.Comment: Accepted for publication in Astronomy & Astrophysic
IGR J17254-3257, a new bursting neutron star
The study of the observational properties of uncommonly long bursts from low
luminosity sources with extended decay times up to several tens of minutes is
important when investigating the transition from a hydrogen-rich bursting
regime to a pure helium regime and from helium burning to carbon burning as
predicted by current burst theories. IGR J17254-3257 is a recently discovered
X-ray burster of which only two bursts have been recorded: an ordinary short
type I X-ray burst, and a 15 min long burst. An upper limit to its distance is
estimated to about 14.5 kpc. The broad-band spectrum of the persistent emission
in the 0.3-100 keV energy band obtained using contemporaneous INTEGRAL and
XMM-Newton data indicates a bolometric flux of 1.1x10^-10 erg/cm2/s
corresponding, at the canonical distance of 8 kpc, to a luminosity about
8.4x10^35 erg/s between 0.1-100 keV, which translates to a mean accretion rate
of about 7x10^-11 solar masses per year. The low X-ray persistent luminosity of
IGR J17254-3257 seems to indicate the source may be in a state of low accretion
rate usually associated with a hard spectrum in the X-ray range. The nuclear
burning regime may be intermediate between pure He and mixed H/He burning. The
long burst is the result of the accumulation of a thick He layer, while the
short one is a prematurate H-triggered He burning burst at a slightly lower
accretion rate.Comment: 4 pages, 4 figures, 1 table; accepted for publication in A&A Letters.
1 reference (Cooper & Narayan, 2007) correcte
Six new candidate ultracompact X-ray binaries
Ultracompact X-ray binaries (UCXBs) appear able to sustain accretion onto the
compact accretor at rates lower than in wider X-ray binaries. This may be
understood by the smaller accretion disks in UCXBs: a lower X-ray luminosity
suffices to keep a disk completely ionized through irradiation and, thus, keep
the viscosity at a sufficiently high level to allow effective transport of
matter to the compact object. We employ this distinguishing factor on data from
RXTE and BeppoSAX to identify six new candidate UCXBs, thus increasing the
population by one quarter. The candidates are drawn from the population of
persistently accreting and type-I X-ray bursting low-mass X-ray binaries. The
X-ray bursts establish the low-mass X-ray binary nature and provide a handle on
the accretion rate. We find that the low accretion rates are supported by the
long burst recurrence times and the hard X-ray spectra of the persistent
emission as derived from the 2nd INTEGRAL catalog of soft gamma-ray sources. We
discuss the peculiar light curves of some new UCXB candidates.Comment: Section 2 corrected and improved thanks to comments by J.-P. Lasota.
Accepted for publication in Astronomy and Astrophysic
Discovery of SAX J1753.5-2349 and SAX J1806.5-2215: two X-ray bursters without detectable steady emission
We report the discovery with BeppoSAX-WFC of two new X-ray sources that were
only seen during bursts: SAX J1753.5-2349 and SAX J1806.5-2215. For both
sources, no steady emission was detected above an upper limit of 5 mCrab (2 to
8 keV) for 3 10**5 s around the burst events. The single burst detected from
SAX J1753.5-2349 shows spectral softening and a black body color temperature of
2.0 keV. Following the analogy with bursts in other sources the burst very
likely originates in a thermonuclear flash on a neutron star. The first of two
burst detected from SAX J1806.5-2215 does not show spectral softening and
cannot be confirmed as a thermonuclear flash.Comment: 4 pages, 4 figures, to be published in Proceedings of the Symposium
"The Active X-Ray Sky: Results from BeppoSAX and Rossi-XTE", Rome, Italy,
21-24 October, 1997, Nuclear Physics B Proceedings Supplements. Eds. L.
Scarsi, H. Bradt, P. Giommi, and F. Fior
A comparative study of the X-ray afterglow properties of optically bright and dark GRBs
We have examined the complete set of X-ray afterglow observations of dark and
optically bright GRBs performed by BeppoSAX until February 2001. X-ray
afterglows are detected in 90% of the cases. We do not find significant
differences in the X-ray spectral shape, in particular no higher X-ray
absorption in GRBs without optical transient (dark GRBs) compared to GRBs with
optical transient (OTGRBs). Rather, we find that the 1.6-10 keV flux of OTGRBs
is on average about 5 times larger than that of the dark GRBs. A K-S test shows
that this difference is significant at 99.8% probability. Under the assumption
that dark and OTGRB have similar spectra, this could suggest that the first are
uncaught in the optical band because they are just faint sources. In order to
test this hypothesis, we have determined the optical-to-X ray flux ratios of
the sample. OTGRBs show a remarkably narrow distribution of flux ratios, which
corresponds to an average optical-to-x spectral index 0.794\pm 0.054. We find
that, while 75% of dark GRBs have flux ratio upper limits still consistent with
those of OT GRBs, the remaining 25% are 4 - 10 times weaker in optical than in
X-rays. The significance of this result is equal to or higher than 2.6 sigma.
If this sub-population of dark GRBs were constituted by objects assimilable to
OTGRBs, they should have shown optical fluxes higher than upper limits actually
found. We discuss the possible causes of their behaviour, including a possible
occurrence in high density clouds or origin at very high redshift and a
connection with ancient, Population III stars.Comment: 19 pages, 5 figures. To be published in The Astrophysical Journa
Simultaneous BeppoSAX and Rossi X-ray Timing Explorer observations of 4U1812-12
4U1812-12 is a faint persistent and weakly variable neutron star X-ray
binary. It was observed by BeppoSAX between April 20th and 21st, 2000 in a hard
spectral state with a bolometric luminosity of ~2x10^36 ergs/s. Its broad band
energy spectrum is characterized by the presence of a hard X-ray tail extending
above ~100 keV. It can be represented as the sum of a dominant hard Comptonized
component (electron temperature of ~36 keV and optical depth ~3) and a weak
soft component. The latter component which can be fitted with a blackbody of
about 0.6 keV and equivalent radius of ~2 km is likely to originate from the
neutron star surface. We also report on the first measurement of the power
density spectrum of the source rapid X-ray variability, as recorded during a
simultaneous snapshot observation performed by the Rossi X-ray Timing Explorer.
As expected for a neutron star system in such hard spectral state, its power
density spectrum is characterized by the presence of a ~0.7 Hz low frequency
quasi-periodic oscillation together with three broad noise components, one of
which extends above ~200 Hz.Comment: 6 pages, 3 figures, accepted for publication in A&
The supergiant fast X-ray transient IGRJ18483-0311 in quiescence: XMM-Newton, Swift, and Chandra observations
IGR J18483-0311 was discovered with INTEGRAL in 2003 and later classified as
a supergiant fast X-ray transient. It was observed in outburst many times, but
its quiescent state is still poorly known. Here we present the results of
XMM-Newton, Swift, and Chandra observations of IGRJ18483-0311. These data
improved the X-ray position of the source, and provided new information on the
timing and spectral properties of IGR J18483-0311 in quiescence. We report the
detection of pulsations in the quiescent X-ray emission of this source, and
give for the first time a measurement of the spin-period derivative of this
source. In IGRJ18483-0311 the measured spin-period derivative of
-(1.3+-0.3)x10^(-9) s/s likely results from light travel time effects in the
binary. We compare the most recent observational results of IGRJ18483-0311 and
SAXJ1818.6-1703, the two supergiant fast X-ray transients for which a similar
orbital period has been measured.Comment: Accepted for publication in MNRA
Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721
We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an
X-ray transient that in 2008 underwent a six month long outburst, starting
(unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts
throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range
of inferred accretion rates (between <1% and about 20% of the Eddington
accretion rate m-dot_Edd) spanned during the outburst allows us to study
changes in the nuclear burning processes and to identify up to seven different
phases. The burst rate increased gradually with the accretion rate until it
dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few
days before the outburst peak, after which bursts were not detected for a
month. As the persistent emission subsequently decreased, the bursting activity
resumed with a much lower rate than during the outburst rise. This hysteresis
may arise from the thermal effect of the accretion on the surface nuclear
burning processes, and the timescale is roughly consistent with that expected
for the neutron star crust thermal response. On the other hand, an undetected
superburst, occurring within a data gap near the outburst peak, could have
produced a similar quenching of burst activity.Comment: 18 pages, 12 figures, 1 table, accepted for publication in MNRA
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