3,322 research outputs found
How rapidly do neutron stars spin at birth? Constaints from archival X-ray observations of extragalactic supernovae
Traditionally, studies aimed at inferring the distribution of birth periods of neutron stars are based on radio surveys. Here we propose an independent method to constrain the pulsar spin periods at birth based on their X-ray luminosities. In particular, the observed luminosity distribution of supernovae (SNe) poses a constraint on the initial rotational energy of the embedded pulsars, via the correlation found for radio pulsars, and under the assumption that this relation continues to hold beyond the observed range. We have extracted X-ray luminosities (or limits) for a large sample of historical SNe observed with Chandra, XMM and Swift, which have been firmly classified as core-collapse SNe. We have then compared these observational limits with the results of Monte Carlo simulations of the pulsar X-ray luminosity distribution for a range of values of the birth parameters. We find that a pulsar population dominated by millisecond periods at birth is ruled out by the data
High Resolution Transmission Spectroscopy as a Diagnostic for Jovian Exoplanet Atmospheres: Constraints from Theoretical Models
We present high resolution transmission spectra of giant planet atmospheres
from a coupled 3-D atmospheric dynamics and transmission spectrum model that
includes Doppler shifts which arise from winds and planetary motion. We model
jovian planets covering more than two orders of magnitude in incident flux,
corresponding to planets with 0.9 to 55 day orbital periods around solar-type
stars. The results of our 3-D dynamical models reveal certain aspects of high
resolution transmission spectra that are not present in simple 1-D models. We
find that the hottest planets experience strong substellar to anti-stellar
(SSAS) winds, resulting in transmission spectra with net blue shifts of up to 3
km s, whereas less irradiated planets show almost no net Doppler shifts.
Compared to 1-D models, peak line strengths are significantly reduced for the
hottest atmospheres owing to Doppler broadening from a combination of rotation
(which is faster for close-in planets under the assumption of tidal locking)
and atmospheric winds. Finally, high resolution transmission spectra may be
useful in studying the atmospheres of exoplanets with optically thick clouds
since line cores for very strong transitions should remain optically thick to
very high altitude. High resolution transmission spectra are an excellent
observational test for the validity of 3-D atmospheric dynamics models, because
they provide a direct probe of wind structures and heat circulation.
Ground-based exoplanet spectroscopy is currently on the verge of being able to
verify some of our modeling predictions, most notably the dependence of SSAS
winds on insolation. We caution that interpretation of high resolution
transmission spectra based on 1-D atmospheric models may be inadequate, as 3-D
atmospheric motions can produce a noticeable effect on the absorption
signatures.Comment: Accepted to ApJ; 34 pages, 6 figures, 1 tabl
Deep VLT infrared observations of X-ray Dim Isolated Neutron Stars
X-ray observations have unveiled the existence of a family of radio-quiet
Isolated Neutron Stars whose X-ray emission is purely thermal, hence dubbed
X-ray Dim Isolated Neutron Stars (XDINSs). While optical observations have
allowed to relate the thermal emission to the neutron star cooling and to build
the neutron star surface thermal map, IR observations are critical to pinpoint
a spectral turnover produced by a so far unseen magnetospheric component, or by
the presence of a fallback disk. The detection of such a turnover can provide
further evidence of a link between this class of isolated neutron stars and the
magnetars, which show a distinctive spectral flattening in the IR.
Here we present the deepest IR observations ever of five XDINSs, which we use
to constrain a spectral turnover in the IR and the presence of a fallback disk.
The data are obtained using the ISAAC instrument at the VLT.
For none of our targets it was possible to identify the IR counterpart down
to limiting magnitudes H = 21.5 - 22.9. Although these limits are the deepest
ever obtained for neutron stars of this class, they are not deep enough to rule
out the existence and the nature of a possible spectral flattening in the IR.
We also derive, by using disk models, the upper limits on the mass inflow rate
in a fallback disk. We find the existence of a putative fallback disk
consistent (although not confirmed) with our observations.Comment: 6 pages, 2 figures, accepted by A&A on 26-06-200
Identifying the Environment and Redshift of GRB Afterglows from the Time-Dependence of Their Absorption Spectra
The discovery of Gamma-Ray Burst (GRB) afterglows revealed a new class of
variable sources at optical and radio wavelengths. At present, the environment
and precise redshift of the detected afterglows are still unknown. We show that
if a GRB source resides in a compact (<100pc) gas-rich environment, the
afterglow spectrum will show time-dependent absorption features due to the
gradual ionization of the surrounding medium by the afterglow radiation.
Detection of this time-dependence can be used to constrain the size and density
of the surrounding gaseous system. For example, the MgII absorption line
detected in GRB970508 should have weakened considerably during the first month
if the absorption occurred in a gas cloud of size <100pc around the source. The
time-dependent HI or metal absorption features provide a precise determination
of the GRB redshift.Comment: 13 pages, 4 figures, submitted to ApJ
New Archaeobotanical Data from the Roman Villa of Villamagna (Urbisaglia, Marche Region, Italy)
Population Synthesis of Isolated Neutron Stars with magneto-rotational evolution II: from radio-pulsars to magnetars
Population synthesis studies constitute a powerful method to reconstruct the
birth distribution of periods and magnetic fields of the pulsar population.
When this method is applied to populations in different wavelengths, it can
break the degeneracy in the inferred properties of initial distributions that
arises from single-band studies. In this context, we extend previous works to
include -ray thermal emitting pulsars within the same evolutionary model as
radio-pulsars. We find that the cumulative distribution of the number of X-ray
pulsars can be well reproduced by several models that, simultaneously,
reproduce the characteristics of the radio-pulsar distribution. However, even
considering the most favourable magneto-thermal evolution models with fast
field decay, log-normal distributions of the initial magnetic field
over-predict the number of visible sources with periods longer than 12 s. We
then show that the problem can be solved with different distributions of
magnetic field, such as a truncated log-normal distribution, or a binormal
distribution with two distinct populations. We use the observational lack of
isolated NSs with spin periods P>12 s to establish an upper limit to the
fraction of magnetars born with B > 10^{15} G (less than 1\%). As future
detections keep increasing the magnetar and high-B pulsar statistics, our
approach can be used to establish a severe constraint on the maximum magnetic
field at birth of NSs.Comment: 12 pages, 11 figures, 5 table
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