2,049 research outputs found
Constraints on the Emission and Viewing Geometry of the Transient Anomalous X-ray Pulsar XTE J1810-197
The temporal decay of the flux components of Transient Anomalous X-ray Pulsar
XTE J1810-197 following its 2002 outburst presents a unique opportunity to
probe the emission geometry of a magnetar. Toward this goal, we model the
magnitude of the pulsar's modulation in narrow spectral bands over time.
Following previous work, we assume that the post-outburst flux is produced in
two distinct thermal components arising from a hot spot and a warm concentric
ring. We include general relativistic effects on the blackbody spectra due to
gravitational redshift and light bending near the stellar surface, which
strongly depend on radius. This affects the model fits for the temperature and
size of the emission regions. For the hot spot, the observed temporal and
energy-dependent pulse modulation is found to require an anisotropic,
pencil-beamed radiation pattern. We are able to constrain an allowed range for
the angles that the line-of-sight (psi) and the hot spot pole (xi) make with
respect to the spin-axis. Within errors, this is defined by the locus of points
in the xi-psi plane that lie along the line (xi+beta(R))(psi+beta(R)) ~
constant, where beta(R) is a function of the radius R of the star. For a
canonical value of R=12 km, the viewing parameters range from psi=xi=37 deg to
(psi,xi)=(85 deg,15 deg). We discuss our results in the context of magnetar
emission models.Comment: 8 pages, accepted to Ap
Scavi e ricerche nell'antica Plestia
in particolare pp. 213-231, 252-27
A Comparison of PCA-LDA and PLS-DA Techniques for Classification of Vibrational Spectra
Vibrational spectroscopies provide information about the biochemical and structural environment of molecular functional groups inside samples. Over the past few decades, Raman and infrared-absorption-based techniques have been extensively used to investigate biological materials under different pathological conditions. Interesting results have been obtained, so these techniques have been proposed for use in a clinical setting for diagnostic purposes, as complementary tools to conventional cytological and histological techniques. In most cases, the differences between vibrational spectra measured for healthy and diseased samples are small, even if these small differences could contain useful information to be used in the diagnostic field. Therefore, the interpretation of the results requires the use of analysis techniques able to highlight the minimal spectral variations that characterize a dataset of measurements acquired on healthy samples from a dataset of measurements relating to samples in which a pathology occurs. Multivariate analysis techniques, which can handle large datasets and explore spectral information simultaneously, are suitable for this purpose. In the present study, two multivariate statistical techniques, principal component analysis-linear discriminate analysis (PCA-LDA) and partial least square-discriminant analysis (PLS-DA) were used to analyse three different datasets of vibrational spectra, each one including spectra of two different classes: (i) a simulated dataset comprising control-like and exposed-like spectra, (ii) a dataset of Raman spectra measured for control and proton beam-exposed MCF10A breast cells and (iii) a dataset of FTIR spectra measured for malignant non-metastatic MCF7 and metastatic MDA-MB-231 breast cancer cells. Both PCA-LDA and PLS-DA techniques were first used to build a discrimination model by using calibration sets of spectra extracted from the three datasets. Then, the classification performance was established by using test sets of unknown spectra. The achieved results point out that the built classification models were able to distinguish the different spectra types with accuracy between 93% and 100%, sensitivity between 86% and 100% and specificity between 90% and 100%. The present study confirms that vibrational spectroscopy combined with multivariate analysis techniques has considerable potential for establishing reliable diagnostic models
UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers
We study the Gamma Ray Burst (GRB) environment and intervening absorbers by
analyzing the optical absorption features produced by gas surrounding the GRB
or along its line of sight. We analyzed high resolution spectroscopic
observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken
with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum
illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which
has at least four components in the main absorption system. We detect strong
FeII, SiII, and NiII excited absorption lines associated with the bluemost
component only. In addition to the host galaxy, at least two more absorbers
lying along the line of sight to the afterglow have been detected in the
redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost
component in the host galaxy, we derive information about its distance from the
site of the GRB explosion. We do so by assuming that the excited absorption
lines are produced by indirect UV pumping, and compare the data with a time
dependent photo-excitation code. The distance of this component is found to be
280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify
two additional MgII absorbers, one of them with a rest frame equivalent width
larger than 1A. The distance between the GRB and the absorber measured in this
paper confirms that the power of the GRB radiation can influence the conditions
of the interstellar medium up to a distance of at least several hundred pc. For
the intervening absorbers, we confirm the trend that on average one strong
intervening system is found per afterglow, as has been noted in studies
exhibiting an excess of strong MgII absorbers along GRB sightlines compared to
quasars.Comment: 8 Pages, 7 ps figures, A&A in pres
Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197
We report on observations aimed at searching for flux variations from the
proposed IR counterpart of the Anomalous X-ray Pulsar XTE J1810-197. These
data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the
ESO VLT, show that the candidate proposed by Israel et al. (2004) was fainter
by Delta H=0.7+/-0.2 and Delta Ks=0.5+/-0.1 with respect to October 2003,
confirming it as the IR counterpart of XTE J1810-197. We also report on an
XMM-Newton observation carried out the day before the VLT observations. The
0.5-10 keV absorbed flux of the source was 2.2x10^-11 erg/s/cm^2, which is less
by a factor of about two compared to the previous XMM-Newton observation on
September 2003. Therefore, we conclude that a similar flux decrease took place
in the X-ray and IR bands. We briefly discuss these results in the framework of
the proposed mechanism(s) responsible for the IR variable emission of Anomalous
X-ray Pulsars.Comment: accepted by A&A Letter
Are HI Supershells the Remnants of Gamma-Ray Bursts?
Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances
from the most powerful explosions in the Universe. If GRBs are not beamed then
the distribution of their number as a function of Gamma-ray flux implies that
they occur once per (0.3-40) million years per bright galaxy and that they
deposit >10^{53} ergs into their surrounding interstellar medium. The blast
wave generated by a GRB explosion would be washed out by interstellar
turbulence only after tens of millions of years when it finally slows down to a
velocity of 10 km/s. This rather long lifetime implies that there could be up
to several tens of active GRB remnants in each galaxy at any given time. For
many years, radio observations have revealed the enigmatic presence of
expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and
in other nearby galaxies. The properties of some supershells cannot be easily
explained in terms of conventional sources such as stellar winds or supernova
explosions. However, the inferred energy and frequency of the explosions
required to produce most of the observed supershells agree with the above GRB
parameters. More careful observations and analysis might reveal which fraction
of these supershells are GRB remnants. We show that if this link is
established, the data on HI supershells can be used to constrain the energy
output, the rate per galaxy, the beaming factor, and the environment of GRB
sources in the Universe.Comment: 8 pages, final version, ApJ Letters, in pres
Photometry of the Oort Cloud comet C/2009 P1(Garradd): pre-perihelion observations at 5.7 and 2.5 AU
The aim of this paper is to contribute to the characterization of the general properties of the Long Period Comets (LPCs) family, and in particular to report on the dust environment of comet C/2009 P1 (Garradd).
The comet was observed at two epochs pre-perihelion, at ~6 AU and at ~2.5 AU: broad-band images have been used to investigate its coma morphology and properties and to model the dust production rate.
Comet C/2009 P1 (Garradd) is one of the most active and “dust producing” LPCs ever observed, even at the large heliocentric distance rh~6 AU. Its coma presents a complex morphology, with subtle structures underlying the classical fan-shaped tail, and, at rh~2.5 AU, also jet-like structures and spiralling outflows. In the reference aperture of radius ρ=5°×104 km, the R-Afρ is 3693±156 cm and 6368±412 cm, in August 2010 (rh~6 AU) and July 2011 (rh~2.5 AU), respectively. The application of a first order photometric model, under realistic assumptions on grain geometric albedo, power-law dust size distribution, phase darkening function and grain dust outflow velocity, yielded a measure of the dust production rate for the two epochs of observation of Qd=7.27×102 kg/s and Qd=1.37×103 kg/s, respectively, for a reference outflow dust velocity of vsmall=25 m/s for small (0.1–10 µm) grains and vlarge=1 m/s for large (10 µm–1 cm) grains.
These results suggest that comet Garradd is one of the most active minor bodies observed in recent years, highly contributing to the continuous replenishment of the Interplanetary Dust Complex also in the outer Solar System, and pose important constraints on the mechanism(s) driving the cometary activity at large heliocentric distances
A route to benzodithiophene systems by exploiting a ligand-free Suzuki-Miyaura coupling reaction in deep eutectic solvents
Benzo[1,2-b:4,3-b\u2019]dithiophene (BDT) and its derivatives belong to an interesting class of thiophene-based aromatic \uf070-conjugated compounds that are widely studied as functional organic materials inserted, for instance, as units in mono and polydisperse oligomers [1], or as \uf070-spacers in push-pull organic chromophores for photovoltaic applications [2]. Moreover, BDTs are key intermediates for the synthesis of inherent chiral tetrathia[7]helicenes, which are an attractive class of heterohelicenes with unique physicochemical and chiroptical properties due to their helix-like structure [3]. Thus, BDT is a key starting molecule which can allow access to more complex and interesting systems through a selective and judicious functionalization of the \uf061 and \uf062-positions of the terminal thiophene rings. Building on our recent studies on the synthesis and functionalization of BDTs [4], we questioned whether a novel class of 2,7-diarylsubstituted BDTs 1 (Figure 1) could be synthesized via a palladium-catalysed Suzuki-Miyaura reaction between heteroaryl halides 2 and organoboron derivatives 3 in Deep Eutectic Solvents (DESs), which have proven to be effective as sustainable and environmentally responsible reaction media in several transition-metal-catalyzed reactions [5].In this communication, we report our preliminary results on the preparation of diarylsubstituted BDTs 1, and discuss the substrate scope of the proposed protocol. Some of the compounds so far obtained display interesting photophysical properties, which are currently under investigation
A search for pulsations from the compact object of GRB 060218
A fraction of massive stars are expected to collapse into compact objects
(accreting black holes or rapidly rotating neutron stars) that successfully
produce gamma-ray bursts (GRBs). We examine the possibility of directly
observing these gamma-ray burst compact objects (GCOs) using post-explosion
observations of past and future GRB sites. In particular, we present a search
for early pulsations from the nearby (z=0.0335) gamma-ray burst GRB 060218,
which exhibited features possibly consistent with a rapidly spinning neutron
star as its underlying GCO. We also consider alternative techniques that could
potentially achieve a detection of GCOs either in the Local Volume or near the
plane of our own Galaxy.Comment: 4 pages, 3 figures. Revised version, accepted for publication in
Astronomy and Astrophysic
- …