162 research outputs found
The Recurrent Eclipsing Nova U Sco: A Short Review
U Scorpii is the recurrent nova with the shortest inter-outburst period, only ten years. The last active phase took place at the beginning of 2010, and it provided a large amount of data from both ground-based and space observatories. This paper reviews some of the more relevant recent findings and points out some, still unanswered, questions
The relationship between soft X-rays and the 1640 Ă… feature fluxes in late-type stars
The λ 1640 feature has been observed in a sample of late type stars of different luminosity classes. The intensity was measured from IUE low dispersion spectra, and it has been compared with the observed X-ray fluxes, finding a relationship between both quantities for "solar type" stars. The X-ray fluxes derived from this relationship for a reduced sample of stars are consistent with the observed ones in the case of "solar type" stars. "Non solar type" stars exhibit discrepancies that could be explained assuming that the λ 1640 feature is formed by contriÂbuters other than He II, which supply an important fraction of this emission in "solar type" stars. The obtained empirical relationship has been used to derive the X-ray flux for some stars that have not been observed in the X-ray range
The INES System IV: The IUE Absolute Flux Scale
This paper deals with the definition of the input fluxes used for the
calibration of the IUE Final Archive. The method adopted consists on the
determination of the shape of the detector's sensitivity curves using IUE low
resolution observations with model fluxes of the DA white dwarf G191-B2B. A
scale factor was then determined so that the IUE observations of some bright
OAO-2 standards match the original measurements from Meade (1978) in the
spectral region 2100-2300 A. The ultraviolet fluxes of six standard stars used
as input for the Final Archive photometric calibration together with the model
fluxes of G191-B2B normalized to the OAO-2 scale are given. A comparison with
the independent FOS calibration shows that the IUE flux scale for the
Ultraviolet is 7.2% lower. We consider this mainly to be caused by the
different normalization procedures. It is shown that the present flux
calibration applies to spectra processed with the INES low resolution
extraction software.Comment: 17 pages, 8 figures. To appear in Astronomy and Astrophysic
IGR J17544-2619: A new supergiant fast X-ray transient revealed by optical/infrared observations
One of the most recent discoveries of the INTEGRAL observatory is the
existence of a previously unknown population of X-ray sources in the inner arms
of the Galaxy. IGR J17544-2619, IGR J16465-4507 and XTE J1739-302 are among
these sources. Although the nature of these systems is still unexplained, the
investigations of the optical/NIR counterparts of the two last sources,
combined with high energy data, have provided evidence of them being highly
absorbed high mass X-ray binaries with blue supergiant secondaries and
displaying fast X-ray transient behaviour. In this work we present our
optical/NIR observations of IGR J17544-2619, aimed at identifying and
characterizing its counterpart. We show that the source is a high mass X-ray
binary at a distance of 2-4 kpc with a strongly absorbed O9Ib secondary, and
discuss the nature of the system.Comment: 7 pages, 4 figures. Accepted for publication in A&
XMM-Newton observations of the INTEGRAL X-ray transient J17544-2619
On 2003 September 17 INTEGRAL discovered a bright transient source 3 degrees
from the Galactic Center, IGR J17544-2619. The field containing the transient
was observed by XMM-Newton on 2003 March 17 and September 11 and 17. A bright
source, at a position consistent with the INTEGRAL location, was detected by
the European Photon Imaging Camera (EPIC) during both September observations
with mean 0.5-10 keV unabsorbed luminosities of 1.1x10^35 and 5.7x10^35 erg s-1
for an (assumed) distance of 8 kpc. The source was not detected in 2003 March,
with a 0.5-10 keV luminosity of < 3.8x10^32 erg s-1. The September 11 and 17
EPIC spectra can be represented by a power-law model with photon indices of
2.25+/-0.15 and 1.42+/-0.17, respectively. Thus, the 0.5-10 keV spectrum
hardens with increasing intensity. The low-energy absorption during both
September observations is comparable to the interstellar value. The X-ray
lightcurves for both September observations show energy dependent flaring which
may be modeled by changes in either low-energy absorption or power-law index.Comment: Six pages, five figures; to appear in Astronomy and Astrophysic
Star formation in the nucleus of the galaxy NGC-5253
Summary. Optical and ultraviolet spectroscopic observations of the nucleus of the galaxy NGC5253 are analyzed. This galaxy presents the typical features of an elliptical system at large distances from its center. However, its nucleus is dominated by an emission complex composed by severa) giant H rr regions. The analysis of the optical spectra shows that the metallic abundances in the nucleus of the galaxy are below the solar values. The presence of O stars can be deduced from the numerous absorption Iines present in the UV spectrum. The UV emission lines indicate a high effective temperature for the ionizing star cluster. The comparison of the observations with evolutive models of W(Hp) and the slope of the UV continuum shows that the age of the brightest knot of the nucleus of NGC 5253 is less than three millions years. The exact age depends on the choice of the extinction law, not well known in this type of objects. An LMCÂ like law (consistent with the low metallicity of the object) leads to an age of 2.3 106 yr, and to an IMF similar or slightly flatter than that found by Salpeter for the solar neighbourhood, with an upper mass limit in the range 60 M 0 < M0P < 120 M 0
The symbiotic system AG Draconis. Soft X-ray bremsstrahlung from the nebulae
The modeling of UV and optical spectra emitted from the symbiotic system AG
Draconis, adopting collision of the winds, predicts soft X-ray bremsstrahlung
from nebulae downstream of the reverse shock with velocities > 150 km/s and
intensities comparable to those of the white dwarf black body flux. At
outbursts, the envelop of debris, which corresponds to the nebula downstream of
the high velocity shocks (700-1000 km/s) accompanying the blast wave, absorbs
the black body soft X-ray flux from the white dwarf, explains the broad
component of the H and He lines, and leads to low optical-UV-X-ray continuum
fluxes. The high optical-UV flux observed at the outbursts is explained by
bremsstrahlung downstream of the reverse shock between the stars. The depletion
of C, N, O, and Mg relative to H indicates that they are trapped into dust
grains and/or into diatomic molecules, suggesting that the collision of the
wind from the white dwarf with the dusty shells, ejected from the red giant
with about 1 year periodicity, leads to the U-band fluctuations during the
major bursts.Comment: 12 pages, 5 figures, 2 tables. New Astronomy, in pres
An XMM-Newton Study of the Bright, Nearby Supernova Remnant G296.1-0.5
We present a detailed study of the supernova remnant G296.1-0.5, performed
using observations with the EPIC and RGS instruments of the XMM-Newton
satellite. G296.1-0.5 is a bright remnant that displays an incomplete
multiple-shell morphology in both its radio and X-ray images. We use a set of
observations towards G296.1-0.5, from three distinct pointings of EPIC, in
order to perform a thorough spatial and spectral analysis of this remnant, and
hence determine what type of progenitor gave rise to the supernova explosion,
and describe the evolutionary state of the SNR. Our XMM-Newton observations
establish that the spectral characteristics are consistent across the X-ray
bright regions of the object, and are best described by a model of the emission
from a nonequilibrium ionization collisional plasma. The study reveals that the
emission from the shell is characterized by an excess of N and an
underabundance of O, which is typical of wind material from red supergiant
(RSG) and Wolf-Rayet (WR) stars. Additionally, we have detected transient X-ray
source 2XMMi J115004.8-622442 at the edge of the SNR whose properties suggest
that it is the result of stellar flare, and we discuss its nature in more
detail.Comment: 10 pages, 7 figures, 3 tables. Accepted for publication in Ap
The massive stellar content in NGC604 and its evolutionary state
The ultraviolet resonance wind stellar lines, the nebular optical emission
lines and the higher order terms of the Balmer series and HeI absorption lines
detected in the spectra of NGC 604 are interpreted using evolutionary models
optimized for young star forming regions. The evolutionary state and the
massive stellar content of the region is derived in a self-consistent way.
The three techniques applied suggest that the central ionizing cluster in NGC
604 is very young, 3 Myr old, and that the stars in the cluster were formed in
an instantaneous burst following a Salpeter or flatter IMF, having stars more
massive that 80 Msol. The stellar cluster is able to provide most of the
ionizing photons needed to photoionize the whole nebula, and the wind power to
form the central shell structure where the cluster core is located. The stellar
cluster is affected by an extinction similar to the average extinction that
affects the ionized gas. The estimated number of massive stars in the cluster
is also in agreement with that derived from previous studies based on the
detection of individual stars. The results that we present here support the use
of these techniques for the interpretation of the integrated light of more
distant star forming regionsComment: To be published in MNRAS. 17 pages and 17 figure
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