538 research outputs found
The distance and luminosity probability distributions derived from parallax and flux with their measurement errors with application to the millisecond pulsar PSR J0218+4232
We use a Bayesian approach to derive the distance probability distribution
for one object from its parallax with measurement uncertainty for two spatial
distribution priors, viz. a homogeneous spherical distribution and a
galactocentric distribution - applicable for radio pulsars - observed from
Earth. We investigate the dependence on measurement uncertainty, and show that
a parallax measurement can underestimate or overestimate the actual distance,
depending on the spatial distribution prior. We derive the probability
distributions for distance and luminosity combined, and for each separately,
when a flux with measurement error for the object is also available, and
demonstrate the necessity of and dependence on the luminosity function prior.
We apply this to estimate the distance and the radio and gamma-ray luminosities
of PSR J0218+4232. The use of realistic priors improves the quality of the
estimates for distance and luminosity, compared to those based on measurement
only. Use of a wrong prior, for example a homogeneous spatial distribution
without upper bound, may lead to very wrong results.Comment: 10 pages, 9 figures, accepted 27-04-2016 to Astronomy and
Astrophysic
Linear Two-Dimensional MHD of Accretion Disks: Crystalline structure and Nernst coefficient
We analyse the two-dimensional MHD configurations characterising the steady
state of the accretion disk on a highly magnetised neutron star. The model we
describe has a local character and represents the extension of the crystalline
structure outlined in Coppi (2005), dealing with a local model too, when a
specific accretion rate is taken into account. We limit our attention to the
linearised MHD formulation of the electromagnetic back-reaction characterising
the equilibrium, by fixing the structure of the radial, vertical and azimuthal
profiles. Since we deal with toroidal currents only, the consistency of the
model is ensured by the presence of a small collisional effect,
phenomenologically described by a non-zero constant Nernst coefficient (thermal
power of the plasma). Such an effect provides a proper balance of the electron
force equation via non zero temperature gradients, related directly to the
radial and vertical velocity components.
We show that the obtained profile has the typical oscillating feature of the
crystalline structure, reconciled with the presence of viscosity, associated to
the differential rotation of the disk, and with a net accretion rate. In fact,
we provide a direct relation between the electromagnetic reaction of the disk
and the (no longer zero) increasing of its mass per unit time. The radial
accretion component of the velocity results to be few orders of magnitude below
the equatorial sound velocity. Its oscillating-like character does not allow a
real matter in-fall to the central object (an effect to be searched into
non-linear MHD corrections), but it accounts for the out-coming of steady
fluxes, favourable to the ring-like morphology of the disk.Comment: 15 pages, 1 figure, accepted for publication on Modern Physics
Letters
Identification of the LMXB and Faint X-ray Sources in NGC 6652
We have detected three new x-ray point sources, in addition to the known
low-mass x-ray binary (LMXB) X1832-330, in the globular cluster NGC 6652 with a
Chandra 1.6 ksec HRC-I exposure. Star 49 (M_{V}~4.7), suggested by Deutsch et
al.(1998) as the optical candidate for the LMXB, is identified (<0.3") not with
the LMXB, but with another, newly detected source (B). Using archival HST
images, we identify (<0.3") the LMXB (A) and one of the remaining new sources
(C) with blue variable optical counterparts at M_{V}~3.7 and 5.3 respectively.
The other new source (D) remains unidentified in the crowded cluster core. In
the 0.5-2.5 keV range, assuming a 5 keV thermal bremsstrahlung spectrum and
N_{H}=5.5*10^{20}, source A has intrinsic luminosity L_{X}~5.3*10^{35} ergs/s.
Assuming a 1 keV thermal bremsstrahlung spectrum, B has L_{X}~4.1*10^{33}
ergs/s, while C and D have L_{X}~8*10^{32}$ ergs/s. Source B is probably a
quiescent LMXB, while source C may be either a luminous CV or quiescent LMXB.Comment: 14 pages, 3 figures, accepted by Astrophysical Journa
New BeppoSAX-WFC results on superbursts
Presently seven superbursters have been identified representing 10% of the
total Galactic X-ray burster population. Four superbursters were discovered
with the Wide Field Cameras (WFCs) on BeppoSAX and three with the All-Sky
Monitor and Proportional Counter Array on RXTE. We discuss the properties of
superbursters as derived from WFC observations. There are two interesting
conclusions. First, the average recurrence time of superbursts among X-ray
bursters that are more luminous than 10% of the Eddington limit is 1.5 yr per
object. Second, superbursters systematically have higher alpha values and
shorter ordinary bursts than most bursters that have not exhibited superbursts,
indicating a higher level of stable thermonuclear helium burning. Theory
predicts hitherto undetected superbursts from the most luminous neutron stars.
We investigate the prospects for finding these in GX~17+2.Comment: Submitted in January 2004 for the Proceedings of the meeting 'X-Ray
Timing 2003: Rossi and Beyond', eds. P. Kaaret, F. K. Lamb, & J. H. Swank
(Melville, NY: American Institute of Physics
Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components
We report on an RXTE/PCA observation of Aql X-1 during its outburst in March
1997 in which, immediately following a Type-I burst, the broad-band 2-10 keV
flux decreased by about 10% and the kilohertz QPO frequency decreased from
813+-3 Hz to 776+-4 Hz. This change in kHz QPO frequency is much larger than
expected from a simple extrapolation of a frequency-flux correlation
established using data before the burst. Meanwhile a very low frequency noise
(VLFN) component in the broad-band FFT power spectra with a fractional
root-mean-square (rms) amplitude of 1.2% before the burst ceased to exist after
the burst. All these changes were accompanied by a change in the energy
spectral shape. If we characterize the energy spectra with a model composed of
two blackbody (BB) components and a power law component, almost all the
decrease in flux was in the two BB components. We attribute the two BB
components to the contributions from a region very near the neutron star or
even the neutron star itself and from the accretion disk, respectively.Comment: 12 pages with 4 figures, accepted for publication in ApJ Letters,
typos corrected and references update
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
Optical Observations of the Binary Pulsar System PSR B1718-19: Implications for Tidal Circularization
We report on Keck and Hubble Space Telescope optical observations of the
eclipsing binary pulsar system PSR B1718-19, in the direction of the globular
cluster NGC 6342. These reveal a faint star (; Vega
system) within the pulsar's 0\farcs5 radius positional error circle. This may
be the companion. If it is a main-sequence star in the cluster, it has radius
\rcomp\simeq0.3 \rsun, temperature \teff\simeq3600 K, and mass
\mcomp\simeq0.3 \msun. In many formation models, however, the pulsar (spun up
by accretion or newly formed) and its companion are initially in an eccentric
orbit. If so, for tidal circularization to have produced the present-day highly
circular orbit, a large stellar radius is required, i.e., the star must be
bloated. Using constraints on the radius and temperature from the Roche and
Hayashi limits, we infer from our observations that \rcomp\simlt0.44 \rsun
and \teff\simgt3300 K. Even for the largest radii, the required efficiency of
tidal dissipation is larger than expected for some prescriptions.Comment: 10 pages, 2 figures, aas4pp2.sty. Accepted for publication in Ap
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